Scanning tunneling microscopy and its applications

Fx)

1 (x > 0) (x::; 0) .

o

x

(2.9)

exp(ik · p) tl AI/2 XEx(X),

XE (x)

{

(2. lOa)

f/2

~+Ex 2m x

Ve(x)

{+

f/2k 2

(2.7)

The direction nonnal to the planar surface of the metal-vacuum contact is taken to be the x axis and the potential V e (x) is that indicated in Figs. 2.IIa,b. The Fermi energy of the free-electron model denoted by t and

x2 (E x )

Inserting the potential function into (2.7) gives the relevant solutions

2.2.1 Tunneling Current

[ -

xl(E x )

Fig. 2.11. (a) Schematic illustration of the potential energy at a metal-vacuum interface. The Fermi energy of the free-electron metal is denoted by f (b) Classical turning points xI (Ex) and x2 (Ex) of an electron energy Ex < Vm incident on the potential barrier at a metallic interface

2m (k 11 2

+ kx2 ),

(2. lOb)

exp(ikxx) + Rexp(-ikxx) (x < 0) T[B i (7/) + iA j (7/)]

(x

>

0)

(2.11 )

with _ [ 7/ - - K F X

+

Ex-~-
,KF

_ [2mF]I/3

-

f/2

.

In (2.10) A denotes the area of the planar contact, p is a vector in the plane of the contact, and the functions Ai (7/) and Bi (7/) are the two linearly independent solutions to Airy's equation. These functions oscillate in the region x > Xl (Ex) (Fig. 2.lla) with the asymptotic form A j (7/) ~ (-n-- 2 17/D-I/4 s in(-Y+7r/4),

(2.8)

Bi (7/)

~

(7r217/D-I/4cos(r+7r/4),

(2.12a) (2.12b)

with with 18

19

'Y

=

(2/3)( MD312

(2.12c)

.

Therefore, X shows the asymptotic behaviour for x

»

XE (x) ~ T(-1l, z l77D- I/4 exp[i('Y+?r/4)],

XI (Ex) (2.13)

x

which represents an outgoing wave. The probability current normal to the interface is expressed as

1

iii [d'.j;*

d1/;)

2m

dx

= -1/;--1/;*dx

liK F =-ITlz. ?rm

(2.14)

The transmission probability D of the barrier is defined as the ratio of the current 1 to the probability current 10 = (lik/m) of the "incident" plane wave Xo = exp(ikx). For the triangular barrier discussed D(Ex,k ll )

=

D(E x)

D(EJ

=

= llJ o '

I

+ Xz, Z)

=

B j (77o) - vA/(77o) ,

(2.15c)

Xz

=

Ai (770) + VW(77o) ,

(2.15d)

with 770

=

Kp(cp +~-EJ/F.

The usual exponential barrier penetration probabilities may be obtained from (2.15) by using the asymptotic forms for the Airy functions valid when 770 » 1. It is given by [2.10]

D(E x)

=

4(~

+ cP

- E

)I/Z

x

~+cP

20

'Y(770)

=

~ (I

K

F

(2.19)

,

x

=

(2s+l)eI 3

(2?r) Ii

[aE] dk

f(E) D(E x) ak

x

Z xd k II '

(2.20)

supposing that all electrons (of spin s and charge -e) which escape from the metal are swept away by the external field so that no space charge accumulates near the interface. For the free-electron model, the dependence of D upon Ex can be utilized conveniently by noting that, as

aE ak x

-z"o

(2.16)

e,

aE x =

ak x

00

J

=

exp[(E-~)!KT]

for this model, (2.20) can be written as

E I/ z

in which 'Yo is defined by substitution of 770 in (2.12c)

'Yo

I +

(2.15b)

XI

Kp/k,

=

where K is Boltzmann's constant, and T denotes the temperature. Therefore, the current per unit area may be written as

1

=

(2.18)

For example, from the phenomenological point of view, EII(k ) = liZ z ll xk ll /2m*, and m* in the metal is not equal to m in the vacuum. Hence, the dependence of D upon E and k ll (rather than upon Ex alone) must be recognized explicitly in calculations of the emission current. The general expression for the emission current is obtained by summing the expectation values of the current operator in the eigenstates of (2.7) over the occupied electronic states. The thermal occupation probability of a state with energy E relative to the minimum of the conduction band is

(2.15a)

where

v

Ex = E - E11(k ll ) .

f(E)

and 4v -(X ?r I Z

A major consequence of the free-electron model is the dependence of D only on Ex· More generally, D is taken to be a function of k , both of ll which are conserved in the tunneling transition and related to Ex via

(~ +; -

Ex)

r

I

~

e

fa OrE,) N(E,)dE, '

(2.2Ia)

z (2.17)

21

I I 00

+

2s- 3 1 -

N(EX>

(2 7r ) fJ

=

(2s +

_

dk y

00

00

Fig. 2.12. Schemalic picture of tunnel ing geomelry. The probe tip has an arbilrary shape, but il is assumed 10 be locally spherical with a radius of curvature R where it comes c10sesl to lhe surface (shaded). The dislance of neareSl approach is d. Cemer of curvature of lhe lip is labeled as rO [2.8]

dkzf(E)

_ 00

1)27rm[~:}n[1 + eXP[~=TEx]]

.

(2.21b)

The quantity N(E x) is calIed the supply function, and the product

P n (Ex>

=

(2.22)

D(E x) N(Ex> '

is referred to as the normal energy distribution. Now let us discuss the relation between the result of the metal-vacuum structure and the general equations (2.5,6). The crux of the matter is what to assume for the wave functions ¢til. and ¢tv appearing in (2.6). In the metal-vacuum structure discussed before, we obtain the wave function by solving (2.7) with the condition that the form of the potential barrier Ve (x) in (2.7) is assumed to be in the form of (2.8), and ¢til. and ¢tv are combined as one function, namely (2.10). Then the general expression of the emission current is obtained by summing the expectation values of the current operator in the eigenstates of (2.7) over the occupied electronic states.

2.2.2 Practical Tip and Surface Wave Functions The forms of the wave functions of both the tip and the surface are important. The sample wave function may be expressed by the expansion [2.8]

¢tv

= 0;1/2

L aG exp[-(k2 + Ik G I2)1/2

Z]

exp(ikG'x),

proaches closest to the surface, as illustrated in Fig.2.12. R is the local center of the radius of curvature, and d is the distance of the closest approach to the surface. In the region of interest, the wave functions of the tip are taken to have the asymptotic spherical form

=

(If

I

[

r - ro

I

exp(-klr-rol),

(2.24)

where Or is the probe volume, and k is defined as above. (For simplicity, the work function lp for the tip is assumed to be equal to that of the surface). Nonnalization requires that the parameter C[ (which is determined by the tip geometry, the detailed electronic structure, and the tip-vacuum boundary conditions) is of the order 1. The possible angular dependence of ¢til. is neglected in (2.24). Since

e- k Ir I

(2.23)

I

2 d qb(q) exp[ -(k 2 +q2)1/2/ z

klrl

G

which represent a general expression for ¢t in the region of negligible potential. Here Os is the sample volume, k = fJ- I (2mlp))/2 is the minimum inverse decay length for the wave functions in vacuum, lp is the work function, and kG = kll+G, k ll being the surface Bloch wave vector of the state, and G a surface reciprocal-lattice vector. The first few factors a G are typicalIy of the order unity. For a non-periodic surface the sum over G becomes an integral. Since the microscopic structure of the tip is not yet known, we choose a form for the tip wave function given by Tersojj and Hamann [2.8]. They assumed that the tip structure is 10calIy a spherical potential welI which ap-

kRe kR

o-I/2 C

¢tIL

IJ eiq'x ,

(2.25)

with b(q)

=

_1_[1 + q2]_112 27rk 2 k2

(2.26)

the wave function of the tip, (2.24), can be expanded into the form of the wave function of the sample surface, (2.23), by using (2.25 and 26). Then the matrix element can be evaluated almost trivialIy. Substituting the surface and the tip wave functions into (2.6) one finds

22 23

M IlV

=

112 1 n- 1/ 2kRe kR .J. (c ) -41rk2m ( 'f'v 0

(2.27)

'

with

21r

-,;-e

2

y

L IMllvI2o(Ev-EF)O(EIl-EF)'

(2.28)

where a is measured in 0- 1 , the distances in a.u., and the energy in eY. Since

It/tv (CO)/2

Substituting (2.27) into (2.28) yields the desired result

L It/t v (c )/2 o(E v -E O

F) ,

(2.29)

(2.30b)

II

/l,1I

321r 3 11- 1 e2 Y (f'2 D t (E F )R2 k- 4 e 2kR

L It/tv(co)12o(Ev~EF)'

8(co ,E F )

where Co is the position of the center of curvature of the tip. If the applied bias and the temperature are very small, (2.5) can be written as

ex e- 2k (R +
we see from (2.30) that a ex e- 2kd as expected, and the other factors in this formula can be regarded as a coefficient. Due to the conductance's exponential dependence on distance, it is not essential to know the coefficient in (2.30) accurately.

II

where D( is the density of states per unit volume of the probe tip. Note that (2.28) does not imply that the value of the surface wave function t/t v at Co is physically relevant. The matrix element is determined by an integral entirely confined to within the gap region. However, because of the analytic properties of (2.23,24), the formal evaluation of t/t v at the distance R+d correctly describes the lateral average due to the finite tip size. The spherical-tip approximation has only entered into the normalization, see (2.24). The crucial matter was the evaluation of the matrix element for an s-wave tip wave function. The q dependence of b(q) in (2.26) then cancels that of the z derivative in the matrix element of (2.6), so that (2.29) involves only undistorted wave functions at the surface. For tip wave functions with angular dependence (L ;r. 0), it is sufficient to include the corresponding angular quantum number M = 0 term (other M's give a node towards the surface). In this case the terms in the Fourier expansion of t/t are weighted by a factor ex (l +q2/k 2)1/2 in the matrix element, which for relevant values of q can be approximated by one for small L (in the example below the relevant q2/k 2 :::::: 0.1). The tip model therefore becomes less accurate for large R, where higher L values become more important. An exact treatment would probably be far less useful since it would require more specific information about the tip wave functions, and would not reduce to an explicit equation such as (2.29 or 30) below. Substituting typical values for a metal into (2.29), one obtains for the tunneling conductance a :::::: 0.IR2e2kR8(co,EF)

(2.30a)

2.3 Tip-Surface Interaction Model 2.3.1 Tunneling Current

If the distance d between the tip and the surface is not very large (d :::::: several Angstroms) and the bias voltage is small, the tunnel Hamiltonian method [2.7J is valid for the description of the tunneling process [2.11]. In the independent-particles model, the tunneling current I is approximated as

I

=

21re

h

f

dE[f(E)- f(E+eY)JA(R,E,E+eY),

(2.31)

A (R,E, E')

: : : f dp f 0T

dp'YT(p)YT(p')gS(p+R,p'+R;E)gT(p',p;E')

(2.32)

0T

where gS and g T are the imaginary part of the Green's function of the surface and the tip, respectively:

S

gS(c,c';E) :::::

L t/t1l(c)t/t:(c')O(E-EI')

(2.33)

/l 24

25

Fig. 2.13. Geometry of the sample surface/tip system and potentials V s , V T defining the eigenfunctions 1/;JJ.' 1/;. of each system [2. 11)

so....... ----

/'

/ /

.-r

Tip

tion for the surface electronic state, the current contribution A(R, E, E + eV)dE from E to E+dE is expressed as

/

L L [ L C;p(R)Cjq *(R)o(E +eV -E.)] G(p,jq(E)

A(R,E,E +eV)

z Qr

ij pq

lJ

(2.35) with

--

G(p,jq(E)

V,Ol. I

(1/;p (r- R)I o(E- Hs )l1/;q (r-Rj ))

(2.36)

dp1/;:(p+R-R)V T (p)1/;.(p).

(2.37)

z

and

C~p(R)

E~ f\ f\ ~ t=l~===-~~ r----

=

j

°T

In the formulation above 1/;p (r-R j ) is the p-th atomic orbital on the site i. The tunneling matrix element C ip (R) reflects the geometrical relationship between the tip and the surface. The tunnel current is determined to by all the terms being the product of the tunneling matrix elements and the imaginary part of the surface Green's function, as displayed in Fig.2.14. For a large distance d between the tip and the surface, the tunnel current I is given by a large number of surface atoms whereas in the case of small d, I results only from a few atoms. This is due to the fact that the

1jI~

Vr z

E.

Tip

and T

gT(r,r';E)

=

L 1/;. (r)1/;:(r ' )o(E-E.) .

(2.34)

lJ

The integration of (2.32) is performed over the volume 0T of the tip, p and pi are the internal coordinates of the tip measured from the fixed point R, describing the relative position of the tip with respect to the surface. VT (p) represents the potential within the tip region. In (2.33,34), 1/;JJ.' EJJ. (1/;. ,E.) are, respectively, the eigenfunction and the eigenenergy of the electron on the surface (tip). They are defined in the potential field without the attractive part of the tip (surface), as shown in Fig. 2.13. A(R,E,E+eV) is calculated by solving (2.32) over the integral region E < E < E +eV (V > 0 case) to get the tunnel current for the tip posiF F tion R. In the LCAO (Linear Combination of Atomic Orbitals) representa-

~////~

\

~/)(E'-E.)/

\

/

vUrY

\

Q

/

r

/

\ \, e·(R)

,\:J

"

e·'(R) /

/

J/

Fig. 2.14. Schematic representation of factors composing the terms of A(R, E, E') [2. 11) 27

26

0

d(A) z

Lx

(0)1

c

~

T' : ' ' ' J t •

,-,

8

~

•

IC

i

I

J

L

I

2.3.2 Tunnel Conductance 7

5

3

i

I

I

(b)., 0.2 1

The derivative of (2.31) with respect to the bias voltage reads

:>

0

dI dV

1\ /]-02;

I

W ,

I

=

...,-0.4 I

I

21re 2 IiA(R, E F -e V, E F ) 2

- 21re Ii

I

J

E -eV F

dE

aA(R, E', E+eV) aE

EF

j

(d)

IE'=E

(2.38)

or dI

. . .. ...... .. .. ..

--

•

.... .. .. . . . .. . . . . . . . . .. . . .

I

I

~

,

d

dV

=

21re 2 IiA(R, E F , E F +eV)

E -eV

I

Fig. 2.15. (a) Contour plot of constant local density of states(dashed line) and contour of total charge density (solid line). Filled circles indicate the positions of carbon atoms of the top two layers. (b) Potential used for the interaction of tip and surface (schematic). (c) Compress and (d) expansion of graphite for the tip at points A and C of (a), respectively [2.131.

resolution of an STM device reduces with increasing d. In numerical simulation, more and more distant sites are involved in the summation in (2.35) with an increase of d. Then, the LCAO construction of the surface wave function becomes inappropriate. Methods should be developed to calculate the surface wave function in the far-tail region more efficiently. It is worth mentioning here that another important aspect of tip-sample interaction involves forces of various origins, which could lead to variations of the measured corrugations at high tunneling current, and induce photon emissions for some materials. Clark et al.[2.12] carefully examined the tipsample interactions on Cu(lOO) surfaces and concluded that the interaction has atomic site dependence. In addition, the impact of the interacting force on the corrugation measurement is more pronounced on soft material surfaces, such as graphite [2.13] (Fig. 2 .15).

+ 21re

2

aA(R, E', E+eV)

F

1i

JEF

dE

aE' E'=E

(2.39)

If g T , the imaginary part of the Green's function of the tip, does not highly depend on energy, the second term of (2.38) can be neglected. Then the tunnel conductance is given by 21re 2 IiA(R, E F -eV, E ), which stems from F the electronic states of the surface and the tip, where the energy is measured with respect to the Fermi level (Fig. 2 .16). It is in proportion to the surface Density of States (DOS) 9(R, E F -eV). For simple cases, see (2.30),

Sample surface

peR, EF-eV)J----t~ --=--::'~~E~= EF-eV E F .1-:/,,-,-,-,-,..--,

PT (E) P (E)

Fig. 2.16. Offset of the Ferm i level of the surface (E ) and that of the tip (E -eV) [2.111 F F 28

29

dI dV oc

4.0 ,

(2.40)

8(R,E F -eV).

I

(a)

i

I

I

.I"-..

I

30

If a cluster of atoms is used as the model of a protrusion on the tip, the width of the cluster energy levels should be introduced to take into account the embedding effect of the cluster on the remaining part of the tip. Thus the () function in (2.35) is replaced by the Lorentzian function

'

1.0

(2.41)

in the simulation. For a more sophisticated treatment the width ~(Ey) should be determined by the Green's function theory, but for most cases a constant value ~ may be substituted. Kobayashi and Tsukada [2.14] used a W 10 cluster as a model for the (Ill) protrusion of a tungsten tip and simulated STM/STS on a graphite surface. If the width ~ is assumed larger than 1 eV, good overall agreement between the calculated and the observed STM/STS data is obtained. In this case the simulated STM image shows the normal triangular lattice pattern [2.15]. This would mean that the tip has unlocalized electronic states on its protrusion for normal cases. On the other hand, if the width ~ is reduced below 0.1 eV the (dIldV)/(l/V) curve shows rather complicated structures reflecting the discrete levels of the cluster [2.14]. The remarkable point of this model is that various types of the abnormal patterns often observed by STM of graphite are reproduced. Presumably a localized bound states exists on the protrusion of the tip in cases where abnormal STM images are noted. Simultaneous STS measurement should be performed to clarify the nature of the tip state. Recent STS studies revealed characteristic resonance peaks in dIldV versus V curves due to bcc and fcc W (Ill) surfaces, respectively [2.16]. This observation points to the differences in tip geometry effects to image recording and spectroscopic analysis (Fig. 2.17). Similar discussions have been seen in the analysis of imaging atomic structures with an AFM tip in non-contact mode [2.17]. Covalent bonding is proposed to be responsible for enhancing the imaging contrast.

2.3.3 Tunneling Active Orbital at the Tip It is an important problem to clarify what kind of tip orbital dominantly contributes to the tunnel current. Ohnishi and Tsukada [2.18] performed the LDA (Local Density functional Approximation) calculation for the elec-

~

2O

0.0 I ·1.0

~l7r

{)(E+eV-E y) =* (E+eV-E y)2 + ~2

i f'J I

'l

I

!

·0.5 0.0 0.5 sample voltage (V)

I 1.0

15.0

j (b)

~ 10.0 oS

>

J2 '0

5.0

0.0

0.5

1.0

Fig. 2. 17a, b. Experimental differential conductivity for Cu (111). (a) and (b) represent two kinds of spectra that are reproducibly obtained after cleaning the W tip by field emission corresponding to a tip apex with a bcc and a fcc pyramid, respectively [2. 16]

(a)

(b)

Fig. 2. 18. Single-state charge-density contour map for the tunneling active orbital of (a) W 4 (HOMO) and (b) W5 (level just below HOMO) cluster [2. II]

tronic states of the W 4' W 5 and W 8 cluster modes of the tip. The results indicated that tipe states with the large d/ component of the tip's apex atom mainly determine the tunnel current. They are the Highest-Occupied Molecular Orbital (HOMO) for the W 4 cluster and the level just below HOMO for the W 5 cluster. The contour maps of these states are displayed in Fig.2.18. The shape of the tip orbital is sensitively reflected by the STM image. This is, for example, found for the image of C H chemisorbed on a sub6 6 strate, as shown in Fig. 2 .19. The tunnel current is calculated by the level just below HOMO of the W5 cluster. The simulated STM image does not show any distinct shape of individual atoms, but rather the 1r electron cloud swelled over the whole molecule. Contour lines around the neighbouring carbon atoms behave somewhat differently. This is because the orbital of

30 31

,

Fig.2.19. STM current image of C 6 H 6 molecule scanned by a W5 tip. The height of the apex atom of the W5 cluster is fixed at 0.4 nm above the molecular plane [2. 11]

Tip I I I

°1

Surface Fig. 2.20. Geometry of a tip with two protrusions [2.llJ

A(R,E,E') the tip is not axially symmetric, and therefore the two C sites are not equivalent in the whole C6 H6 /W 5 system. With the increase of the cluster size the component of the particular tunneling-active orbital becomes distributed over many levels. The width of these lev~ls governs the features of the STS, as discussed in the previous subsection. For example, if the energy spreading of the tunneling-active orbital is very narrow, the STS spectrum is proportional to the derivative of the surface DOS. On the other hand, if the component of the tunneling-active orbital is distributed over a wide energy region, a simple simulation of (2.40) is appropriate.

2.3.4 Double-Tip and Interference Effects

If the tip has two protrusions contributing equally to the tunnel current, the STM image will be distorted considerably. For distant protrusions, the current image would be the overlap of the two images contributed by the respective protrusions [2.19]. This might correspond to images with ghosts which were often reported in the literature. If the distance between the two protrusions is short, a significant interference effect is expected. To see this we notice that the current contribution A(R, E, E')dE = (E' = E-eV) can be written as

I

=

J

i =1 , 2

OJ

J

dp'V T CP+T)V T (p'+T)Y(Pz;E)y(p;;E)

OJ

X

gS(p+Rj ,p' +R j ;E)g T (p' +Tj,P+Ti ;E')

+

[J

X

dp

O}

J

02

dp'V T (P+T1)V T (P'+T2)Y(Pz;E)y(p;;E)

gS(p+R 1 ,p' +R 2 ;E)gT (p' +T2 ,P+T} ;E') + (1- 2)J .(2.42)

Here, T[, T2 are the relative coordinates to the center of the mass; R} and R2 are the coordinates of these centers-of-mass points with respect to the surface (Fig.2.20). The weight factor used for the definition of the center of the mass is the product of the tip potential VT by J/;p (E-E p) and the decay function y(pz;E) = ex p [- : zY2m IE

I] .

(2.43)

In (2.42) 0i is the spatial region assigned to the protrusion i and gS is defined as gS(p+R j ,p' +Rj ;E)

32

dp

_

gS(p+Rj,p'+Rj;E)

-

y(pz;E)y(p; ;E)

(2.44)

33

The first term of (2.42) is just the simple overlap of the current images contributed by the relevant protrusions. If one takes the zeroth-order terms of the moment expansion [2.20] around the point R j and sums up, we obtain

(a)

(b)

}

r\

'V

V

A(R,E,E') =

L

I

/,

(2.45)

C j (E')8(R j ,E),

\ 1\

i=I,2 as discussed in [2.19]. The above result is just the extension of the theory of Tersoff and Hamann for the noninterfering double-tips case. The second term of (2.42) represents the interference effect. The order of magnitude of this term is estimated as Ainrerference :::: ,,2IvT/2 [gS(R 1 ,R 2 ;E)gT(T2,Tl;E')

+ (1-2)]

" (c)

/,

r\

(d)

.(2.46)

If there is no coherence between the electron wave functions on the two protrusions, the interference term vanishes. This case assumes that the tunneling-active orbital at one of the protrusions does not extend to the other protrusion. _ A numerical simulation of the STM current image of graphite by the antibonding orbital of the H2 molecule, as the tip orbital, distinctively shows the drastic effect of the interference. As depicted in Fig.2.21a, the STM image formed by the tip orbital with an axis in the x direction exhibits an abnormal ridgelike shape running in the y direction. The tip orbital is placed 0.6 om above the graphite surface. Such an unusual image is due to the interference effect of the current components through each tip hydrogen atom, and it is verified by looking at the tilt-angle dependence of the STM image. The images of Fig.2.21b-d correspond to the tip orbital tilt angle 15°,30°,90°, respectively. It is found that the image changes to that of a normal triangular lattice with increasing tilt angle. This is explained by the weakening of the interference, because the current due to the raised hydrogen atom is reduced by the increasing tilt angle. Kobayashi and Tsukada [2.14] demonstrated that the STM image of Fig.2.21a changes drastically when the tip orbital axis is rotated from the x direction parallel to the plane of the graphite surface. This fact is also an evidence of the interference effect, as shown analytically in [2.14]. It is important to note that the STM images (Fig.2.21) are often observed in experiments.

Fig. 2. 21a-d. The STM image of graphite using the antibonding orbital of H as the tunnel2 ing active orbital of the tip. The tilt angle of the axis of the tip orbital is 0 0 . 15 ° . 30 ° and 90° for (a), (b), (c) and (d), respectively [2.11)

34

35

3. Spectroscopy, and Spectroscopic Imaging

In the last chapter, the theoretical concepts of STM were reviewed. The previous description focused on the capability of an STM device to scan a surface and to provide data on its atomic-level topography. This capability is only part of the story; STM also opens up new possibilities in spectroscopy. To first approximation, measurements of the electron density across a surface correspond to the topography of the surface, showing the actual positions of atoms or atomic steps on surfaces. However, this description merely serves as a useful starting point. On an atomic scale, surface atoms are not hard spheres with distinct boundaries. While the STM image usually corresponds to surface corrugations or contours of atoms, STM measurements are actually based on the electron density of states. Electrons exist at specific values of energy, called electronic states. Since the tunneling current reflects the Local Density Of States (LDOS) of the sample's surface, STM can be utilized for atomically-resolved spectroscopy, that is, Scanning Tunneling Spectroscopy (STS). Richer than a mere topographic profile, tunneling spectroscopy provides a wealth of information about which electron energy states are occupied (filled) as well as unoccupied (empty). Detailed information that can be inferred from these measurements includes data on chemical composition, bonding, the energy gap, band-bending effects and adsorption at the surfaces or objects under investigation. The main merits of tunneling spectroscopy performed with an STM device are the following: (i) It is local (here, "local" is equivalent to "atomic ") and can be used to probe the electronic properties ranging from individual adatoms on a surface to spatial properties of vortex states of superconductors; (ii) it can be performed at preselected positions by using the scanning ability of the STM; (iii) it can be performed under well-defined conditions. For example, perfect surfaces with known composition can be examined under ultra-highvacuum conditions; (iv) it can be combined with other methods; and (v) it can provide spectroscopic images.

37

3.1 Concepts of Tunneling Spectroscopies 3.1.1 Solid-State-Barrier Tunneling Historically tunneling spectroscopy with the Metal-Insulator-Metal (MIM) tunneling junctions was first demonstrated by Giaever [3.1]. A common feature of solid-state-barrier tunneling structures is that the application of a bias voltage Y leads to tunneling of electrons with a well-defined range of energies 0 < E < 1 eY. This feature makes possible several versions of classical spectroscopy of the solid electrodes and of the barrier with energy resolution of a few times the thermal energy kT. These versions include spectroscopy of the superconducting state, which probes the details of both the energy-gap structure and the phonon spectrum that produces the pairedelectron system. The second major area of spectroscopy, known as lETS (Inelastic Electron Tunneling Spectroscopy), involves the measurement of inelastic excitations of the electrodes (usually in the normal state) and of the barrier. An example is the threshold for phonon generation in an Esaki diode at a bias of Y = nw/e, which is detected by an accompanying step in dIldY and thus a peak in dI2 Idy2. In a similar fashion, energies of plasmons, of spin waves, of spin-flip transitions of paramagnetic ions. and of a variety of other excitations occurring in or near the barrier in various tunneling structures, have been measured. A related area of particular activity in lETS is the measurement of vibrational frequencies of molecules, including large organic molecules, utilized in a MIM tunneling junction by adsorption to the barrier oxide. Consider an idealized tunneling junction in which the molecules with vibrational level spacing nw are sandwiched between two metal electrodes. If we measure the current as a function of voltage, we will find two components: (i) A steadily increasing current due to elastic electron tunneling, and (ii) a current which has the threshold voltage nw/e and increases steadily thereafter due to inelastic electron tunneling. This threshold is set by the requirement that the electrons must give up the energy nw to excite the molecular vibration. Figure 3.1 shows the total current I which is the sum of the currents through the elastic and inelastic tunneling channels. It has a kink at Y = nw/e which becomes a step in dIldY versus Y, and a peak in dI2/dy2. A plot of dI2/dy2 versus Y is called a tunneling spectrum. Thus, measurements of the spectrum yield a series of peaks appearing at bias voltages simply related to the phonon frequencies by the threshold relation eY = nw. This provides information about the bonding of molecules to solid surfaces and about the interactions or reactions, which adsorbed molecules may undergo. The third class of spectroscopy in junction tunneling is associated with the distribution of electron energy states either in the final electrode or, oc-

....- ...-

Fig. 3.1. The current versus voltage curve has a kink in it when the inelastic electron tunneling channel opens up. This kink becomes a step in the first derivative and a peak in the second derivative

T-r .~~ :

dIldV

o

d'"dV'1 o

I

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~

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: V

casionally, in the barrier. In classical planar MIM tunneling, the electronic states of a metal electrode may be observed in the current-voltage spectrum characteristic of the tunneling junction. This occurs because the current is determined by the availability of electronic states within the metallic electrodes; specifically bulk states such as the energy band, quantum size effects and surface states with wave functions which are confined mainly to the immediate surface of the metal. A similar dependence of the currentvoltage spectrum characteristic on surface electron structure is expected for the STM geometry. Useful spectroscopic information about energy-band positions in semiconductors and metals can be provided by this class of spectroscopy. Measurements of the current-voltage characteristics of tunneling junctions outlined above have had an important history in solid-state physics. A major disadvantage of conventional planar structures is that the desired spectroscopic information is spatially averaged over the full size of the junction. While junctions as small as 1200 x 3000 A2 have been fabricated, this is still considerably larger than the characteristic length over which the properties of interest might be expected to vary. For the case of superconductors, the relevant length scale is set by the coherence length, which can range from thousands to tens of Angstroms. For resonant or inelastic tunneling, large variations can be expected to occur over a single molecule. Conventional tunneling junctions also suffer from the disadvantage that the interpretation of results can be confused by unwanted effects of the nonideal solid tunneling barrier.

38

39

Fig.3.2. Graph of tunneling currenl vs. distance

4

Fig.3.3. Calculaled vertical lip displacemenl for a Na tip scanning over aNa, S and He alom, respeclively [3.2J

(different bias) and lhe calculaled lunneling currenl 105

(solid lines)

3


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stant tunneling current. He is different in that it screens the DOS of the electrode substrate and forces the tip to approach the sample more closely in order to maintain a constant tunneling current. The theoretical studies also show that the quantity d(lnI)/d(ln V) closely resembles the combined DOS of sample and tip in the tunneling region. Figure 3.4 depicts the density of states induced on a planar metal electrode by the adsorption of Na and Ca. The energy scale for Na is reversed so that it can be interpreted as the tip. Figure 3.4b (solid curve) presents a calculation of d(lnI)/ d(ln V) versus V for the Ca/Na tunneling system. The peaks

3.1.2 Metal-Vacuum-MetaI Tunneling The demonstrated capability of forming control1able vacuum-gap junctions between any two conductors using STM has led to a number of interesting new results. STM brings a unique advantage to spectroscopic studies. Where other spectroscopic methods provide results averaged over an area of the surface, STM can provide spectroscopic data with a lateral resolution approaching atomic dimensions. This enables close studies of surface irregularities, defects, dopants (used in semiconductors) and other local surface phenomena of a smal1 scale where spatial1y averaged data are not useful. STM provides spectroscopic data by utilizing its sensitivity to the electron energy states of the sample. In examining these data, it is useful to sort out characteristics that do not depend directly on the electron Density Of States (DOS) so that sample-specific structure in the sample DOS can be extracted. In principle, the tunneling current at an arbitrary voltage (below the field-emission threshold) can be calculated from (2.5 and 6). Figure 3.2 shows how a change in bias voltage affects the calculated tunneling current value for the same sample. By modeling the tip as a Na atom weakly adsorbed on a jel1ium surface, Lang [3.2] calculated tunneling current in the vacuum region between two planar metal electrodes, on each of which is an adsorbed atom. The tunneling image of the STM is generated numerically. Scanning of one atom (taken as the tip) past the other (the sample) permits plotting of tip displacement versus lateral separation for constant current. The result of such a study is depicted in Fig.3.3, where a Na tip scans over various sample atoms (Na, S and He) at a small bias voltage. Na has a DOS at the Fermi level higher than S and gives rise to_a larger displacement of the tip at con-

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40 41

observed in this spectrum bear a close relationship to the peaks seen in Fig. 3.4a. There are two important implications. First, the tunneling signal contains information that is, to a large degree, representative of the electronic structure of the tunneling electrodes. Secondly, the tip electronic structure is just as prominent in the data as in the sample's electronic structure. This result shows clearly that the microscope does not know which is tip and which is sample, and all wave functions are equal. This is one of the major reasons for the irreproducibility of spectroscopic data. We can conclude from the above discussion that current-voltage spectra may be obtained by varying the tunneling voltage at specific points on the surface. Peaks in d(1nl)/d(1n V) vs V in the low-voltage limit are interpreted as corresponding roughly to resonances in the tip and sample's densities of states, although their exact heights and positions also depend on the gap distance. At higher voltages, however, the current depends nonlinearly on the voltage. Further, electronic-structure effects and inelastic processes may introduce rich structure into the current-voltage characteristics. Alternatively, STM images obtained in the topographic mode at different voltages (described in Chap. I) contain geometric information about the sample (such as the presence of atomic steps), but also contain information on surface electronic structure (such as the presence of localized surface states, band bending, the energy-band gap of semiconductors or superconductors, or work-function variations). Spectroscopic images (or voltage derivatives of images) taken at specific bias voltages may be used to highlight the spatial distribution of particular states. The theoretical results derived from (2.40) for graphite at several bias voltages have subsequently been confirmed experimentally. This theoretical study assumed that the voltage dependence of the transmission coefficient through the barrier can be neglected relative to the voltage dependence of the density of states, and that the density of states at the tip is sufficiently featureless in the low-bias-voltage limit. Of course, the precise relationship between dlldV and the spectra or images depends on the experimental mode employed in taking the data.

3.2 Experimental Modes One can precisely vary the bias voltage and tip-to-sample distance of an STM for measurement at any point. Therefore, an STM device can be used in four related spectroscopy-like modes: (1) Current-voltage spectroscopy (1-V curve), (2) current-separation characteristics, (3) constant-current topography, and (4) current imaging tunneling spectroscopy.

3.2 .1 Current-Voltage Characteristics 1-V curves for fixed separations are obtained by measuring the variation of the tunneling current as a function of voltage at a constant tip-sample separation. This measurement requires the addition of a sample-and-hold circuit to the feedback loop, as discussed in ChapA. This circuit holds the tip position stable for a given time of the measurement and its response time should be fast enough. The feedback loop is interrupted for a few milliseconds by closing the integrator gate without resetting the integrator. Then the tunneling bias voltage is ramped under computer control while the tunneling current is simultaneously measured. In this scheme the sample-tip distance does not change with voltage and the tunneling current is allowed to vanish because the feedback circuit is inactive. The tunneling bias voltage is then reset to its original value and the integrator gate is opened, re-establishing feedback. Typically, a few hundred such current-voltage spectra are averaged to increase the signal-to-noise ratio, and the dlldV curve can be constructed afterwards. Normalizing the dynamic conductance dlldV to the DC conductance I/V can be used to eliminate the dependence of the tunneling signal on the value of the DC tunneling conductance. It is also possible to measure individual I-V spectra at regular intervals during a scan and correlate the I-V characteristics with the scan. Such spatially resolved I-V curves have been used to elucidate the electronic structure of some semiconductor and metal surfaces. For small voltages, < I V, the I-V curves show a linear voltage-dependence characteristic of ohmic behavior. For larger voltage, the exponential dependence dominates the I-V characteristic, and no current density is observed at low voltage for separations larger than 7 A. The implications of this distance dependence on acquiring spectroscopic data are that the data can only be obtained over a limited voltage range for a fixed separation. To achieve a wide dynamic range of tunneling current and conductivity, the tip-sample separation can be varied by adjusting the initial reference tunneling voltage while the feedback is active, in which the tunneling current at low bias voltage is amplified by reducing the separation. Subsequent normalization of the spectrum is performed to remove the effects of varying the tip-sample separation. The I-V curves for the Si(III)-2x I surface [3.4,5] have been acquired in this fashion, i.e. a series of I-V curves is obtained at various fixed tip-sample separations, which are then normalized. Another STM operation mode measures dlldV by adding a small AC component to the DC bias voltage. The frequency w is chosen to be sufficiently high that the feedback cannot respond to it. The tunneling current will contain an in-phase modulation of the frequency w that is the derivative of I with respect to V, at the DC bias voltage V at which the feedback circuit operates. By sweeping V, dlldV can be measured as a function of V. 43

42

Care has to be taken to pick up only the in-phase modulation of the tunneling current with a lock-in amplifier. Displacement currents can give rise to strongly phase-shifted signals. There are three problems with this scheme: (i) The sample-tip separation varies with V, because I is kept constant by the feedback circuit. With increasing voltage and distance, the lateral resolution is degraded; (ii) the DC tunneling resistance is not constant during the measurement, again because I is kept constant. At decreasing bias voltages the DC conductance increases (by decreasing the tip-sample separation) and diverges as the zero bias voltage is approached. Likewise dl/dV diverges. In practice, useful data have not been obtained below 1 V, which for many structures is the most interesting region; (iii) this approach requires that a tunneling current can be maintained over the voltage range of interest. This is not always possible. If there is a band gap, a stable tunneling current can often not be maintained if the bias voltage is set within the gap, and the tip will crash into the sample. Nevertheless, this method has been used to study the electronic structure of surfaces such as Si (111)-7 x7, etc. The third method for improving the measuring procedure is to first set the bias voltage at a relatively large value, typically ±2 V, for inducing a sizeable tunneling current while the tip is stabilized by the active feedback loop. Then the feedback is disabled, and a V-shaped voltage is added to the Z piezo voltage, synchronized with the bias ramp. The tunneling current as a function of the bias voltage is recorded. Using a modulation frequency of about 1 kHz with a lock-in amplifier leads to a measurement time for the spectrum of a few seconds. Thus, this method is only possible for a very stable mechanical system and a very long holding time of the Z position. Generally, the residual thermal or piezoelectric drift rate of the STM should be less than 0.1 A/s. An example of this type of data is the spectroscopic measurements on a monolayer of Sb on the GaAs(l10) surface [3.6).

3.2.2 Current-Separation and Separation-Voltage Characteristics Equation (1.1) may be considered a first approximation for the tunneling current. Making use of the variables in the equation, the local work function or, correctly speaking, the effective tunneling barrier height between the tip and the sample can theoretically be derived from tunneling current I(s) at constant V or from the separation s(V) (we use s instead of d to represent the tip-sample separation hereafter) at constant I. Experimentally, d(1nI)/ds can be measured in STM experiments by a modulation of the gap separation with phase-sensitive detection of the current at the modulation

The modulation signal divided by the current is, neglecting logarithmic terms in s, - d(lnl)/ds

= V + (lIzV <1»

xd/ds .

(3.1)

The effective barrier height in (3.1) generally differs from the work function. The former is some average over the actual barrier height across the gap between the tip and the sample. The latter is equal to this actual height only at infinite distance from the surface, i.e., at the distance s where the second term in (3.1) becomes negligible. The work function of a metal surface is defined as the minimum energy required to remove an electron from the bulk to the vacuum level. The barrier height is a local electronic parameter. It is closely related to the local electronic charge, i.e., the barrier is enhanced if excess electronic charge is present and lowered if there is a charge deficency. The image potential [3.7], a general aspect of an electron moving to or from the surface, has an influence on d(1nl)/ds, but this influence can be reduced by performing the experiment at a separation larger than 4 A [3.8]. We should like to stress at this point that "barrier-height images" also contain topographic features, and the magnitude of the experimentally determined values are often considerably smaller than expected. Some questions relating to barrier height have so far remained unexplained, so great care needs to be taken in performing experiments to measure refined changes of tunneling barriers [3.9]. A related method for measuring spectroscopic data is to keep the feedback loop active while varying the bias voltage at a fixed tunneling current. In this way the tip-sample separation is constantly adjusting itself, and the tip is following a separation-vs-voltage curve at constant current, getting the s-V data. When the applied potential exceeds the work function, there is a transition from the vacuum-tunneling regime to the field-emission regime where the electron experiences a positive kinetic energy. In this range, quantum interference effects can occur, leading to barrier resonances in the electron transmission at certain energies. These transmission resonances show up as wiggle's in the s-V data or as kinks in the constant-distance I-V data [3.4,5]. The position and strength of the resonances are sensitive to the shape of the potential barrier at the sample surface. Due to the difficulty in realistic calculations of the tunneling process, it is difficult to extract quantitative characteristics of the barrier potential from the barrier resonance [3.10]. Barrier-height images and barrier resonances have been used to study some biomaterials [3.11,12], and metal and semiconductor surfaces [3.5, 13, 14).

frequency. 45 44

3.2.3 Constant-Current Topography Constant-Current Topographs (CCTs) are obtained by keeping the tunneling current constant and measuring the voltages controlling the tip height at different bias voltages. This method for revealing real-space surface-state images relies on the fact that at many bias voltages only the electronic states between the Fermi levels of the tip and the sample contribute to the tunneling current. The CCT method is experimentally very simple and often provides a quick way to assess if there are any interesting differences between different polarities. As a spectroscopic tool, however, it is rather limited.

3.2.4 Current-Imaging Tunneling Spectroscopy Combining the spectroscopic capabilities and the scanning ability of an STM device allows for high-spatial-resolution spectroscopy measurements, essentially on an atomically-resolved basis. There are two methods for obtaining Current-Imaging Tunneling Spectroscopic (CITS) data. One of them is measuring a full I-V curve at each pixel of a topographic image, A tunneling bias voltage is applied between the sample and the tip for about 0.1 ms, During this time the feedback circuit is activated by opening a gate. In the next 0.4 ms the feedback circuit is interrupted and the position of the tip held fixed. The bias voltage is ramped under computer control while the tunneling current is measured at a number of voltages during this ramp. Thus, one can create an image of the tunneling current at different voltages, while the STM progress along the raster scan with the tunneling current is stabilized at the preset bias voltage. The I-V curves can be seen in real time on an oscilloscope and may display very strong variations with position, The variation of tunneling current at a specific bias voltage can be visualized as an image. This method can provide sufficient data to construct real-space images of the surface states. Another method for obtaining CITS is measuring dIldV at the fixed bias voltage V in a raster scan. A small voltage modulation is superimposed on the DC bias at a frequency w, which is too fast for the feedback circuit to follow. The current modulation at frequency w is measured with a lock-in amplifier and used to form an image of dlldV. Two disadvantages of this method are: (i) Spectroscopic information is obtained at a single bias voltage only, If one wants spectroscopic information over a larger range of bias voltages. the experiment has to be repeated for every bias voltage. Problems associated with drift of the sample during the measurement and with variations of the geometric and electronic struc46

tures of the tip tend to make this experiment very troublesome, (ii) The tip follows a different contour for each bias voltage, because different sample and tip states contribute to the tunneling current at different voltages. This may affect the lateral resolution, in particular for features close to the resolution limit of the experiment. In addition, it has been shown that, in the absence of any electronic structure variation across the sample, the dIldV image is not featureless but presents an inverted topographic image, This is because the transmission-probability density depends on the local radius of the curvature of the electrodes, The major advantage of the CITS technique is that all spectrosopic information is obtained simultaneously, thus eliminating problems associated with sample drift and tip instabilities. Also, spectroscopic images at different bias voltages are obtained along identical spatial contours of the tip, thus removing ambiguities arising from possible variations in lateral resolution with tip-sample distance. The CITS technique demands a laboratory automation system to drive the microscope. At present it is possible to record a 256-point I-V curve in each pixel of a 256 X 256 topographic image. In each pixel the computer triggers a transient recorder, which acquires the I-V information, provides hardware averaging of multiple I-V curves at each location, and then sends the data to a second computer which stores the results directly on a highdensity disc. The data can be analyzed either on-line or off-line on a larger mainframe computer,

3.3 Energy Resolution

The resolution of the STM, operated in the spectroscopic mode, involves several factors as follows: • There will be a thermal limit determined by the width of the edge of the Fermi distribution of the electrons in the metal electrodes. This amounts to approximately 2kT (about 50meV at room temperature), and is small enough to allow resolution of most features of interest on metals. For an inelastic process the limit should be 5.4 kT, or about 150 meV at room temperature, which should be adequate for many electronic excitations. This resolution would not be sufficient, however, for superconductivity and other inelastic processes associated with photons or excitation of vibrational modes of adsorbed molecues, which have a width of about 5 meV. It is necessary to use low-temperature STMs for reducing thermal smearing to an acceptable level.

47

E'

Fig. 3. Sa, b. Band structure of nearly free-electron dispersion for a two-band model. The band structure along the k perpendicular direction is shown in (a) with energy resolution. The measured densities of states for a full tip (broken line) and a sharp tip (solid line) are shown on (b) [3.3]

(a)

Vg

0

sharp lip

1

I

/

n/a

with a dull tip. Figure 3.5b depicts the measured densities of states using dull and sharp tips for a two-band model of nearly free-electron dispersion. For a sharp tip, the measured density of states does not reveal a detailed structure due to its poor energy resolution. Kuk and Silverman [3.15] concluded therefore that a sharp tip is desirable for topographic scanning but undesirable for tunneling spectroscopy on a surface with a dispersive band structure. Most successful STM-spectroscopic measurements have been done with surfaces of single crystals prepared in high vacuum. This is in part due to the difficulty of obtaining stable STM operation free of excess noise in the presence of impurities and unstable surface layers and oxides.

k

N(E)' (b)

3.4 Examples

E

• A modulation amplitude of sufficient size is required to observe a derivative spectrum, a consideration closely related to the time needed to acquire a reasonable noise-free spectrum. Typically, a modulation of at least 0.1 V is required to obtain a spectrum in a few minutes. • Electronic states in metals are lifetime-broadened by an amount which is energy dependent but can be several tenths of an eV or more in width for energies of one electron volt and larger. • The uncertainty principle may play a role in determining the energy resolution of the spectroscopy since the area through which the tunneling current passes is so small, typically of the order of one nanometer or less. Assuming that the radius and the lateral resolution of a tunneling tip are roughly equal, the relation LlkLlx :::::: 1 indicates that a tip of radius 2 A will have a momentum uncertainty of 0.5 A-I. (Although the crystal momentum in a solid is not a physical momentum, a resolution limit can be defined). Figure 3.5a shows the energy-band structure of nearly free electrons in a periodic potential. A sharp tip is indicated by the heavy line spanning the first and the second Brillouin zone; poor momentum resolution results in poor energy resolution around the probing energy V g' The broken line in the figure indicates the better momentum and energy resolution attained

48

Above we have reviewed some ways of obtaining spectroscopic information with STM. In this section we will give some examples of the most notable achievements of scanning tunneling spectroscopy. Classical spectroscopies are conveniently classified by the energy range as follows [3.16]: • Superconducting energy-gap spectroscopy (0-7-20meV). • Electrode phonon spectroscopy (5-7-50meV). • Barrier and semiconductor phonon spectroscopy (30 -7- 100 meV). • Molecular vibrational spectroscopy (30 -7- 500 meV). • Molecular electronic spectroscopy (500meV -7-2eV). • Electrode plasmon spectroscopy (1.5-7-5eV). For a bias voltage of no more than a few volts, electrons tunnel directly through the vacuum space from one electrode to the other. This mode is clearly analogous to the classical tunneling experiments in which a MIM thin-film structure is employed and tunneling takes place between the metals through the thin oxide insulator. With STM, however, tunneling occurs through a thin vacuum gap. In contrast, STM-spectroscopic experiments, in which image states and other electronic reflection effects at surfaces are observed, operate in the voltage range of about 5 eV. For this large-voltage case electrons tunnel through the barrier immediately in front of the emitting surface and can occupy real states in the gap between tip and metal surface.

49

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Fig. 3.6. dI/dV spectra measured in the two halves of the Si (111)-7 x:J unit cell. Also shown is the variation of the sample-tip spacing with bias voltage, while the DC tunnel ing current is held constant. The solid line represents the tip height vs bias voltage during the conductance measurements. The dashed curve shows the theoretical calculations of the contribution of standing-wave states to the tunneling conductance [3.17]

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While an STM is capable of atomic-resolution images and of spectroscopy of surface electronic structure, the imaging of surface states in real space has provided a major, nontrivial challenge. Figure 3.6 displays two dIldV curves obtained in the two halves of the Si (111)-7 x 7 unit cell. Between 1 and 3 V (no data were obtained below 1V), differences are seen between the two curves due to differences in the electronic structure in the two halves of the unit cell. Also shown is the variation of tip height above the surface, which increases with increasing voltage, as expected. The regular series of peaks observed at higher voltages are not due to surface electronic structure, but are due to standing-wavelike states between tip and sample. The broken curve exhibits a theoretical prediction of these states, in good agreement with the data. By combining scanning-tunneling microscopy and spectroscopy, surface states of the Si(111)-7x7 with specific atomic sites have been identified. In the first step of the analysis the I-V characteristics were plotted at a number of different positions inside the (7 x 7) unit cell (Fig.3.7). Strong differences are seen between the various locations. For instance, a strong 50

~

-2

-I

o

2

Energy leV] Fig. 3.7. (a) Tunneling conductance (IIV) measured at several locations inside the (7 X7) un it cell: ( ) adatom; (.) rest atom ; ( X) center position . (b) Surface states on Si (lll)-7 X7 observed with photoemission (solid line) and inverse phocoemission (dashed line). (c) Areaaveraged tunnel ing spectra [3. 18]

increase in conductance at -0.8 V observed on the rest atoms (rest-atom sites are dangling bonds in the second layer of the crystal) is absent on the adatoms, even though they are separated by only 0.443 nm. The steps in the DC conductance correspond directly with the surface states long known from photoemission and inverse photoemission experiments [3.19]. Thus, this measurement can be used to identify these states with specific features of the structure of the (7 x 7) surface. Real-space images of these states 51

were subsequently obtained by taking the difference between current images just above and below the observed onsets in conductance. Such difference images, between 0 and -0.35 V and between -0.6 and -1.0 V, are depicted in Fig.3.8, together with a schematic view of the Dimer-AdatomStacking (DAS) fault model in the 7 x 7 symmetry (Fig. 3.8a) and a topographic image obtained at +2 V (Fig.3.8b). A different image centered at 0.35 V (Fig.3.8c) reveals an electronic state which is localized on the 12 adatoms but which has a different DOS on each of four types of adatoms. The state is stronger in the faulted than in the unfaulted half of the cell, and in each half the stage is stronger on the three adatoms adjacent to a corner hole than on the three. Figure 3.8d depicts a difference image centered around -0.8 V. This state is localized at three spots within each half of the unit cell, exactly where the DAS model predicts dangling bonds on the "rest atoms" in the first full atomic layer. The current images contain information not only on the electronic structure for the voltage at which they are obtained, but also on the electronic states contributing to the tunneling current at the feedback voltage. Current images of the Si (Ill) -7 x 7 surface obtained at - 2 and +2 V applied bias (tip ground) will look different because the tip follows a different contour. An image obtained at +2 V reveals mirror symmetry of the unit cell across the short diagonal (all bumps look the same), whereas an image obtained at -2 V indicates a pronounced asymmetry with one half of the unit cell apparently higher than the other half, and the bumps near the corner holes higher than the bumps between them. Thus, in general, it is difficult to interpret a current image directly. Trump et al. [3.20] eased this problem substantially by taking the difference between two current images and normalising this difference image to the DC conductance. The created d(1nI)/ d(1n V) image by this method would be qualitatively similar due to the very pronounced nature of the surface states. Figure 3.9 shows the constant-current image of the GaAs(l10) surface obtained simultaneousl at (a) + 1. 9 V and (b) -1.9 V, representing empty and filled states of GaAs (110)-1 x 2 surface, respectively. On this surface the filled states are localized on the As atoms; the empty states are localized on the Ga atoms. As a result, either the Ga atoms or the As atoms are imaged, depending on the bias voltage. Figure 3.9a represents an image of the Ga states, Fig.3.9b of the As states. Ga and As form zig-zag chains, alternating between Ga and As. By comparing the measured images with theoretical predictions, the buckling angles of the Ga and As bonds was estimated. Figure 3.9c illustrates a schematic top view of the surface atoms. Si (111)2 x 1 surface structures characterized by quasi-one-dimensional zig-zag chains of Si atoms in the so-called 7r-bonded chain model. The voltage-dependent images [3.22] show that a maximum in the image taken at + 1 V appears as a minimum at -1 V. Considering the electronic struc52

~

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.'~~'~~"'~""~1.," ®.~.

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- ••' ~ ®e>.. ~~ •.O~~.'. ~ 1'5";0. .•, w• • ~~~~~~W~". ~ \SJ

..

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(a)

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~

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0

•

Adatom ReSlalom layer Second layer Bulk layer

Fig. 3. 8. (a) A schematic view of the DimerAdatom-Slacking (DAS) model in the 7 X 7 symmelry. (b) Topographic image obtained at +2 V. (c) Adatom Slale al -0.35 V. And (d) dangling-bond state at-0.8 V [3.18] 53

!

~~(;~~~--j o

20

Distance

(c)

.~

.0

• 0

<[liOI

o I

S

80

(A)

40

Its

2.0"'

:.

60

80

~

'Ii

t

0.5

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[001]

.0 Fig. 3.9. (a) Empty-state (+ I. 9V) and (b) filled-state (-I. 9V) topographies of GaAs (110) taken simultaneously. Positions of surface atoms are schematically shown (c), where open circles indicate As atoms and closed circles represent Ga atoms [3.21]

ture of the 71"-bonded chain model one can understand that the maxima do not correspond simply to atomic locations, but are intimately related to the details of the bonding between the atoms and, hence, the surface band structure. Tunneling at positive voltages accesses the empty 71"* band, while at negative voltage the tunneling current is probing the filled 71" band. At low voltages the states primarily contributing to the tunneling process are from states near the surface Brillouin zone edge. This method relies on the fact that at any bias voltage only the electronic states between the Fermi levels of the tip and the sample contribute to the tunneling current. Thus one obtains implicitly an image of those states, as is apparently the case in Fig.3.9. However, it seems to be somewhat more problematic if both geometric and electronic structures contribute to both images, such as in the case of both current images of the Si (1ll) -7 x 7 surface obtained at - 2 V and +2 V applied bias. This would also be true if one images a region containing an atomic step where, in addition to the electronic structure, one would observe the purely geometric features associated with the step. The surface states of low miller index surfaces of a number of metals, such as Au(1ll), Fe(OOl), Cr(OOl), etc, have also been identified by STS method [3.23-25]. Moreover, quantizations in the surface state spectrum, seen as oscillatory behavior in conductance curves, was observed at the

54

20

60

40

8

25

>-

E!'

50

75

100

125

Distance (A)

(J)

c

W

Fig. 3. lOa, b. Distribution of the confined electron states of a 36 AAu (Ill) terrace. (a) 3D map of dIldV as a function of the distance perpendicular to the steps and of the vollage appl· ied to the sample(energy). The bias ranges from -0.47 V (bottom) to +0.38 V (top). (b) Individual dIldV line scans for different sample voltages. The dOlled line is a constant-current STM Iine scan, step edge peaks are marked by S [3.26]

monatomic step sites [3.26]. In this case, the steps sites are served as the dimensional restrains and resulted in the energy levels for electronic states (Fig.3.1O).

55

3.4.2 Adsorbate-Covered Surfaces Measurement of I-V curves at constant tip-sample separation has found wide application in spatially-resolved studies of adsorbate-covered surfaces. The most notable cases are the study of the interaction of NH 3 [3.27], Cl [3.28] and 0 [3.29] with Si (111) and the study of localized atomic scale defects. STM images of a clean Si(111)-7x7 surface clearly show two inequivalent types of surface Si adatoms, called "corner" adatoms and "center" adatoms. Upon exposure to NH" individual adatoms disappear from the images as their dangling bonds are passivated by reaction with NH 2 or H. Curves of d(lnI)/d(ln V) were measured on a dense grid, allowing a detailed study of the interaction (Fig. 3.10). Spectroscopic measurements provide further insight into charge transfer and energy-level shifts associated with the reaction. It is found that the so-called rest-atom sites are the most reactive of all. Reaction with H gives rise to significant charge transfer to neighboring adatoms, which react next. Results from the Si(111)-(V3 x V3)AI surface are exhibited in Fig. 3.11. Images in Fig.3.11a,b were obtained with +2 V and -2 V bias, respectively, and show the (V3 XV3) lattice, with some defects. The defects appear as reductions in Fig. 3. 11 a and as elevations in Fig. 3.11 b. Figure 3.11c displays d(lnI)/d(ln V) curves taken on a regular lattice position and on a defect position. In a detailed comparison with theory, Hamers et al. [3.30] demonstrated that the regular lattice positions correspond with an Al atom adsorbed on the top of three-layer Si atoms. The three Al valenceelectron pair with the three Si dangling bonds. These back-bonds are imaged in Fig. 3.11 b. An empty P z orbital protrudes into the vacuum and is imaged in Fig.3.11a. The defects are due to Si atoms substituting for Al atoms. Si has one more valence electron than AI, and the Pz orbital is therefore not empty, giving rise to the extra electronic state at -0.5 eV seen in Fig.3.11c. In the STS measurements of oxygen adsorption on GaAs (110) J .A. Stroscio and co-workers observed both short-range effects due to adsorbateinduced changes in the local density of states as well as long-range effects due to surface band bending. It was noted that the oxygen-GaAs complex displays a positive STM contour when sampling the filled electronic state, while a negative STM contour is observed when sampling the unoccupied states. This striking voltage dependence observed in the STM contours results from the energy dependence of the local density of states. The oxygen data verified the prediction from a theoretical analysis based on a negatively-charged adsorbate. The negative charge can be associated with a filled surface acceptor state on p-type material. The oxygen appears neutral in the

56

--;;;;-"" ~

~

-2

-1

0

1

2

Energy reV]

-2

-I

0

1

2

Energy reV]

Fig. 3.11. (a) Empty-state topography of the Si (111)-7 X7 surface. The spatially resolved tunneling spectra shown below were obtained while in the positions indicated by arrows for the restatom site (A), an adatom next 10 the corner hole (B) and a center adatom (C). (b) Empty-state topography of the Si (111)-7 X7 surface after reaction with NH 3 . Curve A was laken over a reacted restatom, curve B (broken) over a reacted corner adatom, curve B (solid) over an unreacled corner ada 10m and curve C over an unreacted center atom. The reaction leads to readily observable changes in local electronic structure at reacted sites; compare spectra B (left) and B (right, dashed line). In addition, the effect at unreacted sites as a result of a reaction at neighboring sites is apparent; compare spectra C (left) and C (right) [3.27]

images, indicative of the acceptor state being unfilled, which agrees with the lack of band bending observed in spectroscopic measurements.

3.4.3 Superconductivity One of the many promising applications of the combined STM and spectroscope is in the field of superconductivity, especially since there is the possibility of spatially resolving the energy gap in superconductors on the scale of a few Angstroms. Real-space images of variations of the superconducting energy gap have been obtained from Nb foils, NbN films, Nb 2 Sn, Nb 3 Sn, organic su57

perconductor (BEDT-TTFh Cu(NCSh and a number of high-T c oxide superconductors. The comparison of the surface topography and gap distribution permits the detection of possible correlations between local variations in superconductivity and topographic features. One example of the applications of spectroscopic methods to the surface electronic states of superconductors is to clarify the origin of the conductance plane of Bi 2 Sr 2Ca . CU 2 8 , From the results obtained by resonant photoemission (PES), Inverse PhotoEmission Spectroscopy (IPES), X-ray absorption and electron energy loss spectroscopies, it has been proposed that the electronic states at the Fermi level consist of mainly 2p atomic orbitals with x-y (in plane) symmetry. A critical problem in modeling the high-T c mechanism based on these experimental results is which plane in the crystal, BiO or Cu0 2 , provides the 02p orbitals at the Fermi level. Figure 3 .12a shows the normalized differential conductance curve [(dIldV)/(I1V)] of the surface of Bi 2 Sr 2 Ca' CU 2 8 , This curve has a gap of about 0.3 eV at E F , suggesting a nonmetallic nature of the BiO plane (STM and TEM studies of a cleaved surface of a single-crystal sample indicate that the topmost cleaved surface is a BiO plane). Figure 3.12b displays the PES and IPES spectra of the same sample which represent the occupied and unoccupied electronic density of states, respectively, the PES and IPES spectra reveal a clear Fermi edge leading to a substantial density of states at E F , taking into account the effects of energy resolution: 0.2 eV for PES and 0.4 eV for IPES. Considering the differ-

°

°

81

f

~

I

I

AI

AI

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-1

~ 6

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°

1

2

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2

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4

°. 2 (c)

·1

0 Energy leV]

3

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Fig.3.13. (a) Tunneling spectrum of a cleaved surface of a Bi 2 CaSr2 CU2 08 single crystal. The STS spectral intensity (dIldV)/(lIV) represents the local density of states and (dIldV)/(I1V) = 1 corresponds to zero density of states. (b) Photoemission and inverse photoemission spectra in the vicinity of the Ferm i level (E F ) [3.31)

STS

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ence in probing depth between the photoemission (probing a few atomic layers) and the tunneling spectroscopy (probing a topmost atomic layer), the tunneling spectrum presents the electronic density of state of the BiO plane, while PES and IPES spectra probably correspond to the electronic density of states involving both the BiO and the Cu0 2 planes. Thus, Tanaka et al. [3.31] concluded that the BiO planes have a nonmetallic nature and the Fermi liquid states observed by PES and IPES are spread on Cu0 2 planes. A Bi 2 Sr 2 CaCu2 08 crystal may be composed of alternating stacking of metallic (Cu0 2) and nonmetallic (BiO) planes, as sketched schematically in Fig.3.13. This mode is also supported by resonant Raman spectroscopy data. In addition, an energy gap of 20 meV was identified in association with CuO chains in YBa 2 CU 3 7 , The origin was related to either CDW transition or proximity coupled superconductivity [3.32]. Using STM has enabled researchers to find the quasiparticle bound state in type-II superconductor 2H-NbSe 2 [3.33]. It has been further proposed that STM could be used to probe the impurity-induced resonances and the superconducting order parameter symmetry [3.34].

°

Fig.3.12. Si (111)-(''; 3 X V 3)AI: (a) empty-state image; (b) filled-state image; (c) (dIldV)/(IIV) taken on an AI adalOm (top) and a Si adalOm (boltom) [3.30) 58

59

3.4.4 Outlook As outlined above, the capability to measure the spatial variation of the tunneling spectra with the resolution of STM has proved especially fruitful. However, there are still some problems to be resolved in the future.

a) Influence of the Tip The size, shape and chemical identity of the tip influence not only the resolution and shape of a STM scan but also the measured electronic structure. It was shown that the wave functions of well-prepared, clean and stable tips (for instance, prepared by FIM) are apparently sufficiently featureless so as to be indistinguishable in the data. However, such well prepared tips have not been utilized in most STM and STS experiments. In general, the tip DOS does not resemble that of a free-electron metal. To have meaningful STS measurements, the tip DOS must be determined independently. Two methods have been proposed for determining the energy spectrum of a sharp metal tip. For a free-electron metal tip, the Field Emission Spectroscopy (FES) is described by the Young formula [3.35]. A deviation from the formula indicates a deviation from a free-electron metal behavior [3.36]. The other method is known as the marching method, measuring the dynamic conductasnces on the same sample using a free-electron-metal tip and another tip of unknown DOS [3.36].

b) Interpretation of Spectroscopy Results There is no sure and simple way to understand spectroscopic information. For example, the image of the Si (111)-("v'3 XV3) Ag surface contains two maxima in the unit cell, with the maxima arranged in a honeycomb structure. Loenen et al. [3.37] argued on spectroscopic grounds that these have to be Si atoms, but Wilson et al. argued on the basis of essentially identical data that the maxima must be Ag atoms [3.38]. The simple fact suggests, no matter how many I-V curves one measure, that STM is not going to tell us whether the bumps are Ag or Si.

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The other note of warning is that, although the (dIJdV)/ (IN) (or d(ln 1)/ d(ln V» corresponds closely to the sample density of states, it is not related directly to the density of states of both sample and tip. Remember that the tunneling current is also determined by the tunneling transmission probability density. If the sample has a large density of states, but these states do not overlap with the tip, these states are inaccessible in a tunneling experiment. One example may be the d band on Ag. In a photoemission experiment, the d orbitals will be the dominant feature in the electron energy distribution curve. However, the d orbitals are too small to be visible in a tunneling experiment. In addition, one has to be careful that the apparent location of an electronic orbital is not the same as the location of the atomic core. In most cases it would be difficult to figure out what is going on without the help of theory [3.39]. In order to fully understand the STM results, Ding et al. [3.40] have investigated this surface using first-principles total-energy calculations. The lowest-energy configuration consists of a top layer of Ag atoms arranged as honeycomb-chained trimers lying above a distorted "missing-top-layer" Si (111) substrate. It is found that the honeycomb structure observed in STM images arises not from the top-layer atomic positions but rather from the electronic charge density of an empty surface band near the Fermi level. The maxima of the electronic distribution for the empty states occur at the center of the Ag trimers, and are situated over the fourth Si layer, in agreement with the result obtained by Wilson and Chiang [3.41]. In this chapter we have reviewed the concept of tunneling spectroscopy and some ways to obtain spectroscopic information with STM. In spite of some limitations mentioned above, scaning tunneling microscopy and spectroscopy will continue to make many important contributions to surface science in the years to come.

_7.

-~ Non·metal

Non-metal

/

~Non-metal

Fig. 3.14. Schematic view of alternating metallic Cu02 and nonmetallic BiO planes derived from Fig. 3. 12 61

4. STM Instrumentation

Even though the principle of a scanning tunneling microscope is not very complicated, many factors must be taken into consideration in the design to ensure a stable and reliable instrument. Optimum functioning of an STM device requires tip-to-sample position control with picometer precision, a rough and fine positioning capability in three dimensions, a scanning speed as high as possible, and also, preferably, simplicity of operation. These requirements have to be satisfied in the presence of building vibrations with up to micrometer-size amplitudes, electric noise, thermal drift, creep and hysteresis of piezoelectric translation elements and other perturbations. Some design rules are discussed in this chapter with emphasis on the vibration isolation, stiffness, electrical control circuitry, tip preparation and computer control system.

4.1 The Vibration Isolation System The frame of the instrument is always subjected to vibrations transmitted from the ground or the air. As indicated in (1.1), the tunneling current depends exponentially on the gap between tip and sample, even the smallest vibrations, such as those caused by sound in air and by people walking around a building, can affect the stability of the instrument. For many materials, especially metals, the atomic corrugations observed in the constantcurrent STM mode will typically be 0.01 nm. Therefore, a good vibrationisolation system is very important for a well-functioning STM, and the changes of the gap distance caused by vibrations must be kept less than 0.001 nm [4.1]. There are two types of disturbance from which an STM device must be isolated: vibration and shock. Vibrations are generally repetitive and continuous, while shock is defined as a transient condition whereby kinetic energy is transferred to a system in a short period of time. Of them, vibration isolation is most critical. Machines running at or near laboratories excite vibrations of the building at frequencies typically between 10 and 100 Hz. The frame, walls and 63

floors of a building undergo mostly shear and bending vibrations which usually resonate at frequencies between 15 and 25 Hz. Vibrations originating from ventilation ducts, transformers and motors are at frequencies between 6 and 65 Hz, while those caused by people walking and working in laboratories typically are in the range of 1 to 3 Hz. Hence, in the process of minimization of the sensitivity of an STM to building vibrations, primary attention has to be given to the frequency range between 1 and 100 Hz. Increasing the inherent resonance frequency of the STM body and employing a vibration damping system are two major ways to isolate vibrations. Viton, metal springs and magnetic eddy-current damping can be utilized for damping components. For an UHV-STM assembly a widely used vibration-isolation scheme which adopts these components is illustrated in Fig.4.1. The vibration isolation consists of two sets of stages which, suspended from springs, nestle within the stainless-steel cylindrical frame of the microscope. The inner stage slips into the outer stage, from which it is suspended by three springs. The inner stage carries the multi-stacked metal plates separated by Viton spacers. The heart of the microscope containing both the scanning tip and the sample sits on the top of the plates. When the

entire microscope resides in vacuum, air resistance is minimum, and the outer and inner stages could, if they were disturbed, bounce up and down almost indefinitely. To stop this motion, permanent magnets mounted on the bottom of the outer stage slide between copper plates attached to the support frame and the inner stage. As each plate slides up and down, the magnetic field induces an eddy current. The interaction between the eddy current and the magnetic field retards the motion of the plate and thereby damps the STM. The Viton spacers between the metal plates are used mainly for the purposes of damping the vibrations propagating along the springs and reducing the effects caused by a large-amplitude shock, although they exhibit pronounced creep. They are substantially incompressible. When strained under compression, their compression stiffness (equivalent to the spring constant) is often large, resulting in a rather high (> 10 -:-100Hz) resonance frequency. However, they have inherent damping of a tenth or a hundredth of the critical damping. Metal springs can have smaller spring constants, resulting in low resonance frequencies (5 -:- 0.5 Hz), but they provide little damping and require additional damping components. The eddy-current damping systems, on the other hand, can easily have varied damping coefficients and be compatible with an UHV system. The eddy-current damping coefficient [g/sJ is equal to 'Y = 9.69' 10- 9 B2 eslp ,

D

Stainless steel

CJ

Magnet

~ Copper

I!'JI

First stage 64

Viton

Fig. 4. l. Schematic diagram of the assembly of a STM instrument and its vibration isolation system.

(4.1)

where B is the magnetic field [GaussJ, e is the thickness (in the direction of B) of the conductor [cm], S is the cross-sectional area [cm2 ], and p is the resistivity [0' cm]. In general, the eddy-current damping with permanent magnets is useful only in the case that the mass and natural frequency of the system are relatively low. Commercially available vibration-isolation systems employing pneumatic springs are often used as the support for a STM. Some of them can have a resonant frequency as low as 0.5 Hz with a variable damping constant. The more rigid the construction of an STM is, the fewer requirements it has for the external vibration-isolation system. Hansma et al. [4.2J described a vibration isolation system that consists of only a concrete block hung from the ceiling with rubber tubes which are at the same time springs and dampers. The very rigid STM unit is placed directly on the concrete block. Vibrations are dissipated by hysteresis losses due to the inherent structural damping of a rigid body. In the construction of a rigid STM body, the resonant frequency of piezo drivers as well as joints tightened by screws, epoxy junctions, three-point contacts and loose spring connectors will have serious effects on the resonant frequency of the constructed STM and the quality factor Q' of the tip-sample junction. 65

The vibration amplitude transfer expression proposed by Kuk and Silverman [4.3] for a spring-viscous damping system under the condition of structural damping P~ » Po (p~ being the resonant frequency of a rigidly constructed STM, and Po is the resonant frequency of a vibration isolation system) is given by

T

=

1 + (~plpo)2 ] 1/2 [ (1-1' 2 /1' 2)2 + (2~plpo)2 [(1-1'2 0

(plp~)2

Ip~2)2 + (plp~Q')2]112 .

(

4.2

)

The first term represents the amplitude-transfer term of a spring's viscous damping system and the second term is the damping-transfer function for a hysteresis loss due to the inherent structural damping of a rigid body. In addition, I' is the external exciting frequency, ~ (= y/y c) is the damping ratio, y is the damping coefficient of the system, and Yc (= 4m7rp o) is the critical damping coefficient. The amplitude transfers calculated for four STMs are shown in Fig. 4.2. The solid line corresponds to a system with Po = 2 Hz, P~ = 2 kHz, ~ = 0.4, and Q' = 10. With a floor-vibration amplitude of a few thousand Angstroms, the gap stability for this system will be better than 1 A. For a very rigid STM assembly (shown by a dotted line in the figure where P~ = 12kHz and Q' = 50), the amplitude transfer is still too large (higher than 1 A. at 200 Hz) for a stable STM junction. With the addition of a vibrationisolation table (po = 1Hz and ~ = 0.4), the amplitude transfer (shown by 10°

"

,,

10- 2 ~

~ '"c::

g "0 '" i= .

10- 4

./' ./"

10- 6

"

E

-<

./ 10- 8

~,

1,/ 10- 1

',

f' 10- 10

./"./"

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/

/\

/

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10°

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10 2 10 3 Frequeocy [Hz]

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Fig. 4.2. Total amplitude transfer for four systems with different vibration isolations and structural dampings [4.3] 66

the broken line) is < 10- 6 and the tunneling gap will be stable to <0.01 A. at 200 Hz. For a two-stage vibration isolation system: one with an internal spring system (po = 1.0Hz, ~ = 0.4) and another with an external table (po = 1.1 Hz, ~ = 0.5), the amplitude transfer (shown as a chained line) indicates that the estimated vibration amplitude is less than 0.001 A. in most of the frequency range when the structural damping for the STM assembly is P~ = 2 kHz and Q' = 10.

4.2 Mechanical Designs The following criteria must be satisfied in the design of an ideal STM [4.4]: • • • •

•

The z-scanning range is at least 1 /-Lm with an accuracy of 0.001 nm. The x- and y-scanning capability must be of the same range with an accuracy of 0.01 nm. The scan window (x, y) should be selectable in the same range. The coarse sample-approach system should be smooth without rocking motion or backlash, and should move in the 1 mm to 100 nm range with <0.1 /-Lm accuracy, or at least less than the z-piezoelectric driver range, depending on the driving voltage and the piezoelectric expansion coefficient. The gap between the tip and the sample should be as rigid as possible, i.e., a high mechanical resonance frequency and a low Q-factor of the tip-sample junction.

The first two requirements result from the need for a three-dimensional tip-translation capability that is necessary to record surface topography with atomic resolution in a reasonable scanning range. For samples which need a large scan range (for example, O.lmmxO.lmm), different scanners should be used due to the fact that the same three-dimensional scanner is not capable of achieving atomic resolution over such a large-scan range. The third requirement is for the purpose of selecting an area of interest on the sample surface, for example, to localize the details of surface defects or adsorbed atoms and molecules which have been found in a largearea scan. The fourth requirement emerges from the need for the capability of convenient sample loading and tip exchange, for efficient reduction of tip-to-sample separation from macroscopic to tunneling distances and to avoid accidental contact of the tip with the sample. The last requirement, as described previously, is for dissipating external vibrations. In general, the trend has been toward compactness and simplicity since it has become clear that increasing the rigidity of microscopes can be more conveniently achieved than perfecting a vibration isolation system. 67

An air-STM is often used for inert samples, and the construction and operation is simpler compared to the UHV-STM which is required for accurate and reliable studies of clean surfaces. The design concepts of the two instruments are quite similar but the desorption rate of the materials, a capability for in-situ tip exchanging and sample loading, and for convenient sample treatments must be taken into consideration in the UHV-5TM design. The rather large devices for mounting and changing the tip and sample in the UHV-STM reduce the mechanical rigidity of the microscope and lower the mechanical resonant frequencies. The design of an UHVSTM is further complicated by the backe-out that is required for outgassing the entire vacuum system and by the sample-cleaning procedure that usually involves heating and sputtering. The mechanical design of the STM for operating at low temperature also represents a considerably greater challenge because of the difficulties of vibration isolation, in-situ surface preparation and characterization, the problems of dimensional contraction, and reduced piezoelectric response under cryogenic conditions.

high-temperature applications. The PZT-5A type has a high sensitivity, a high dielectric constant and a good time stability; it can be used in transmitters. Compared with PZT-5A, PZT-5H with a higher dielectric constant can find its best application at low frequencies, but functions only in a limited temperature range. Both of the PZT-5 types have high mechanical motion sensitivity, therefore they are suitable for three-dimensional scanners in the STM for operation in room temperature. The PZT ceramics are available from many manufacturers in a variety of shapes and configurations, including strips, plates, bimorphs, stacks and tubes, as shown in Fig.4.3. The directions are identified by the subscripts 1,2,3, representing the cartesian coordinates x,y,z, respectively. By convention, the polarization direction is 3, pointing from the positive to the negative electrode. For a strip of length eand thickness t, a displacement in the I-direction is produced as a result of the electric field V in the 3-direction. The magnitude of the displacement is given by (4.3)

M = (f/t)d 31 V

4.2.1 Piezoelectric Ceramics where d [mlY] is the piezoelectric strain coefficient for a particular materiAs discussed above, for high-performance STMs, a resolution is sought of about 0.01 om normal to the sample surface (z direction), and about 0.1 nm laterally (x, y directions). This results in very narrow tolerance requirements for controlling the tip, which cannot be satisfied by using the usual mechanical devices. Piezoelectric ceramics are therefore widely employed as an electromechanical transducer to obtain the accurate x,y,z motion and the tip approach. A piezoelectric actuator is an electromechanical device that undergoes a dimensional change when an electric voltage is applied. Single crystals of many compounds, quartz for instance, possess piezoelectric properties. More recently, polycrystalline ceramic materials such as lead zirconate titanate [Pb(Ti. Zr)03] (PZT) and barium titanate have been extensively developed for their piezoelectric properties. Piezoelectric ceramics differ from crystals in that they must undergo a poling procedure for the piezoelectric phenomenon to occur. These materials can convert electric potential signals of 1 mV to 1000 V into mechanical motion in the range from less than one Angstrom to a few micrometers. PZT piezoelectric ceramic material is available in a variety of properties and compositions with different dopants. PZT-4 and PZT-8 types of piezoceramics can be applied to high-power acoustic transmission and highvoltage generators for their high-power capability. At cryogenic temperatures, PZT-8 has the lowest temperature variations. In addition, PZT-8 has a high depoling field and a high Curie temperature, which is suitable for 68

Ca) PZT strip Contraction

1/

tL pI

r

.

~+

,-

"

Cd) Bimorph

----

.-/

+. I I

pI pI

./

./ ....... /\. ./ )

./

;>

..... .-.-

./

.-

(b) PZT laminate Contraction

f:lW (c) PZT stack assembly . - - - - Insulator

(e) PZT tube

I~+

Fig. 4.3. Several shapes of PZT piezoelectric ceramics 69

ai, expressing the ratio of strain developed along one axis to the electric field strength along the same or a different axis, assuming all external stresses are constant. The first subscript of d indicates the direction of the applied electric field and the second is the direction of the induced strain. The strain coefficient d is negative representing contraction perpendicular to the field; and positive for stain measured along the 3-direction (along which the thickness t is measured) representing expansion parallel to the electric field direction: (4.4)

Lit = d 33 V .

Although there are many ceramic compositions used today, most can be placed into two general categories: hard and soft PZT materials. Typical d coefficients for hard PZT materials are

Stack extension: d 33 Nd V

where Nd is the number of disks. As implied by the equation above, large displacements can be achieved by a small stack as long as each disk is sufficiently thin. As indicated in (4.3) for a strip geometry, the largest extension per one volt is obtained when the ratio elt is maximized. Unfortunately, making very thin strips results in actuators that are compliant and prone to buckling when loaded lengthwise. To improve their strength and rigidity, strips can be bonded side by side with opposing poling directions to form laminated structures (Fig.4.3) with the same extension per one volt as the individual strips. The changes along its length of such a laminated structure can be expressed by Lix

d 33

250.10- 12 m/V, d 31

=

= -

110.10- 12

600'10- 12 m/V, d 31

=

- 270 .10- 12 mlY .

d 31 = - 273.10- 12 mlY .

A bimorph represents a sandwich structure with a thin metal strip bonded in between two PZT strips, as shown in Fig.4.3. When held at one end in an external electric field, bimorphs bend with a displacement at the other end, reaching 100 14m due to the lengthwise expansion or contraction of the PZT elements. However, such devices lack mechanical strength and inherently have low resonant frequencies. Increased mechanical strength is possible by supporting the bimorph at both ends and utilizing the displacement at the center of the strip. In this case, the expression becomes Lix

=

(3/8)d 31 V(elt)2 .

(4.5)

It is apparent from (4.4) that the change in thickness of a PZT material in any of the three directions is only a function of the voltage applied. No matter what thickness, the absolute extension Lit depends only on the voltage V for a given d constant. Therefore, to obtain a larger displacement in the direction in which either t or e is measured, it is necessary to assemble a stack of disk-shaped or strip piezoelectric elements, as sketched in Fig.4.3. The total extension is thereby the sum of the individual elements and given by

70

d 31 Vee/t) .

(4.7)

The choice between a strip or laminate assembly for a particular application will be determined by the need for the load-bearing capability if a thin strip is required to achieve sufficient extension. A tube with electrodes on its inner and outer surfaces is a very common structure for a single piezoelectric element, and the poling direction is right through the wall. Consequently, the axial extension is given by

For PZT-5H d 33 = 593.10- 12 m/V,

=

m/V;

and for soft PZT materials d 33

(4.6)

ve Lie

=

d31 OD _ ID

(4.8)

where e is the length along tube axis, OD and ID are the outer and the inner diameters of the tube, respectively. The radial extension is Llr

=

d 33 V .

(4.9)

A tubular piezoelectric element is required to have higher uniformity of the wall thickness. Otherwise, unequal wall thickness around the perimeter of the tube will cause a bending motion as one side of the tube extends farther than the other.

4.2.2 Three-Dimensional Scanners The 3-dimensional scanner is a device that moves the tunneling tip across the sample surface (x,y) and controls the tip-sample separation (z). BasicalIy, the designs of 3-dimensional scanners made of piezoelectric ceramic materials include the tripod, the single tube and a cross combined with a 71

(3)

Jf

(b)

________l z offset voltage

----r

Teipod

controls tip height

z Electrode

Common

z + YElectrode

_ y Electrode

+ YElectrode

+ y Offset

- x Electrode (hidden)

x Offset

+ x Electrode + x Electrode

x

Fig. 4.5. lIJustrating the voltages applied to the electrodes of the single-tube scanner

x

+ Fig.4.4. Common three-dimensional scanners: (a) tripod, (b) single tube, and (c) a cross combined with a single tube

single tube. The earliest design of a scanner is a tripod of three orthogonal piezo "sticks" (Fig.4.4a). The STM tip is placed on the top of the tripod, and the independent extension and contraction of the three "sticks" make the tip move along the x,y,z directions. The response of the tripod made of three PZT-5H piezo sticks being 13 mm long, 2 mm high and 2 mm wide is generally 1.5 nm per volt in each direction, and the maximum mechanical resonant frequency is 5 kHz. Replacement of the "sticks" by tubes can reduce the cross talk and raises the resonant frequency. As a simple piezoelectric element, the big advantage of tubes over strips is their structural rigidity. A compact, single-tube scanner was first employed in an STM by Binnig and Smith [4.5]. The outer electrode of a PZT tube is divided into four equal quadrants, the inside electrode being not separated. By applying different voltages to opposite quadrants, the tube is made to bend perpendicular to the tube axis. Orthogonal x-y motion is obtained by controlling voltages on quadrants 90 degrees apart, while the contraction of the whole tube along the tube axis by applying a voltage on the inner electrode leads to the movement in the z direction. The other two outer electrodes can be either at ground, or a bias DC voltage can be applied for the purpose of scan-window selection. Figure 4.5 illustrates several schemes for applying voltages onto the electrodes of the tube scanner. The tube scanner exhibits large displacements, a low cross talk and a high resonant frequency. The resonant frequency [Hz] of the single-tube design can be estimated by 72

f r = 1.08·10 5 e- 2

Jro 2 + r2 j

(4.10)

where r i , r a are the radius of the inner and outer wall, respectively, and e is the length of the tube in cm. For a PZT-5H tube of 12.7 mm in length, 6.35 mm in diameter and 0.51 mm in wall thickness, the response in the x, yand z directions is 5 nm/V, and the inherent resonant frequencies parallel and perpendicular to the tube axis are 40 and 8 kHz, respectively. This design has become popular owing to its small size and high resonant frequency. Unequally divided outer electrodes of the tube, unequal wall thickness and the deviation of the tip position from the center of the tube axis will result in nonorthoganl x-y motion. Therefore, only accurately manufactured piezoelectric ceramic tubes can be used for the scanner. There is another design of an x-y scanner employing a cross-shaped piezoelectric ceramic material. The motion in the z direction is accomplished by a piezoelectric tube mounted at the center of the cross. The controlling voltages with the same magnitude but opposite signs are applied to the x,-x and y,-y electrodes, respectively (Fig.4.6). This design with sym-

Fig. 4.6. Illustrating the voltages applied to the electrodes of a cross single-tube scanner 73

metrically arranged piezo blocks has been successful in reducing the effects of thermal drift. The primary design goal is for a scanner to be as rigid as possible for a given scan range. Specifically, the figures of merit are the resonant frquencies of the scanner in each orthogonal direction. It is important to have the resonant frequencies high, not only because they fix the scanning-speed (feedback performance), but also because they determine its rigidity against vibration. Other requirements of a good scanner are high resolution, orthogonality and linearity - the amount of movement should be proportional to the applied voltage. The problems of nonlinearity, hysteresis and creep become more serious for large area scans, where the electric field is large. These effects can be minimized by controlling the total charge applied to the piezoelectric rather than the voltage [4.6], by taking an image of a standard sample to find the amount of distortion and re-map the image with software in a way that removes the distortion, or by using independent position sensors for the x and y axes, such as an optical sensor or a capacitance probe [4.7,8].

age between MF and GP, and are released when the voltage is removed. Elongating and contracting the body of the louse with the appropriate clamping sequence of the feet moves the louse in any direction in steps between 10 nm and I !-,m, and up to 30 steps/so In this way, sample and tip can be approached vibration-free to within the working range of the piezo drivers to avoid accidental contact of tip and sample. The rough drive also serves for separating the sample (ca. I cm) in the cleaning procedures and to compensate for thermal expansions when working at elevated temperatures. Burleigh Instruments has developed a unique piezoelectric linear motor called the Inchworm Motor. The principle of operation is depicted in Fig. 4.8. The Inchworm Motor consists of a piezoelectric ceramic tube and a shaft that closely fits the tube. The outer electrode of the tube is divided perpendicularly to its length into three PZT elements. Two of them serve as clamps and a center one is the PZT actuator that does not touch the shaft. When a voltage is applied to the end element 1 or 3, it is made to clamp tightly to the shaft due to the contraction of the circumference of the tube governed by d 33 , and to release when the voltage is removed. When a voltage is applied to the first PZT element, it clamps the shaft. Then, a vari-

4.2.3 Coarse Sample Positioning The z-piezo range is limited in the STM for which the recognition of individual surface atoms is the key design objective. Therefore, a means of reducing the gap distance between tip and sample to within the working range of the z piezo, known as the coarse-approach system, is required. Such a device also needs to be able to move the sample far enough away from the tip to allow sample transfer. A coarse approach to such a device is called "louse" sketched in Fig. 4.7. It was used in the early STM designed by G. Binnig and H. Rohrer at the IBM ZOrich Laboratory. The louse (L) body consists of a piezoplate with a sample holder on top and resting on three metal feet (MF), separated by high-dielectric-constant insulators from the metal ground plates (GP). The feet are clamped electrostatically to the ground plate by applying a volt-

I

+ 100 -IOOOY

Extend 2

74

I

Cf ==l:=J L..F

I

I

•

I

3 JD § JD ~

Clnmpll

~

I

I

p

Contract 2 «

Unclamp 3 Fig. 4.7. Sketch of the "louse"

I_.,...=::=~=!....­

Clnmp I

Unclnmp I

~GP

l~

off

Clnmp 3

2

~

I

I

I

I

I

Fig. 4. 8. The principle of operation of the piezoelectric Inchworm Motor 75

able-rate staircase voltage is applied to the center PZT element causing it to change length in discrete steps of approximately 4 nm each. The staircase may be stopped or reversed at any step. At the end of the staircase a voltage is applied to the third PZT element causing it to grip the shaft. Then, the voltage is removed from the first PZT element, releasing the shaft. The staircase starts downwards until it reaches its lower limit, at which point the first PZT element is activated again, the third PZT element released, and the staircase starts again. If the center PZT element is fixed to a support, the above manipulation will make the shaft move along one direction. This sequence can be repeated any number of times or reversed to make the shaft move in the opposite direction. The total travel distance of the shaft is limited only by the length of the shaft. By employing piezoelectric ceramic materials with different stress coefficients, changing the amplitude and frequency of the voltage applied, the precision and speed of the shaft motion can well be controlled. The Inchworm Motor has successfully been utilized in the sample coarse-approach system of the STM. There are several factors to be considered in selecting a sampleapproach mechanism: reliability, geometry, rigidity and speed. The selection of any kind of sample-positioning device should be based upon the unique requirements of the particular microscope design. Three useful techniques for coarse sample positioning are: •

• •

A walker which moves by piezo expansion and using electrostatic, mechanical or magnetic clamping. Both the louse and the Inchworm Motor belong to this category. A differential screw micrometer pushing on a reduction lever. The differential screw can be driven either manually or by a stepping motor. A lead screw pushing against a differential spring.

Each design has its own advantages: the first design is often used in vacuum chambers, the second in the atmosphere, and the third at low temperature. Lever or differential screw mechanisms have been employed for a fast approach and easy manipulation of the sample holder, which can usually be detached or flipped away from the STM assembly.

4.2.4 STMs for Operation in Various Environments Due to different three-dimensional scanners and sample coarse-approach mechanisms employed in STM, hundreds of types of STMs have been designed to date. The smallest STM is only 1000 x 200 x 8 ~m3 in size; the maximum scanning range can reach 200 ~m. Some of them are designed for operating in an UHV chamber equipped with standard surface characterization tools such as AES, LEED, HM, SEM, etc. (Chap. I). 76

(b)

(3)

Rod connected to linear motion feedthrough Coarse adjust beam (B) .....--'"11'""

Steel ball (B)

I

Slide (S)

I

;'

'II

I

I

I I

Flexible joints Shifter (Sh) connected to linear motion feed through

I

Piezo-tube for z motion

I

t . = ;:::l I

Tunneling tip Cantilever support plate (Csp)

I llr J

-tjf;1J~ UI 'CI

Cantilever Spring loaded screw (Sc) Fig. 4.9. The mechanical adjusting mechanism of the low-temperature UHV AFM/STM

[4.9J

Giessibl et al. [4.9] built an UHV atomic force/scanning AFM/STM which can be operated at 4.2 K. The microscope, of only 20 x20 x70 mm 3 in size, is incorporated into a very small chamber (100ml) which can be evacuated and baked to UHV within a few hours via a specially designed valve. Figure 4.9 illustrates the principle of the AFM/STM system. Figure 4.9a is a sketch of the approach mechanism for the z distance between sample and AFM cantilver. The slide (S) carrying the sample is pressed onto a steel ball (B) and an axle (A) by a spring (Sp) which acts on a ball bearing. The slide is held in position by friction between the slide, the axle (A) and the block that holds the entire instrument. Thus, by turning the axle (A) the slide is rolling gently in the vertical direction. The approach is done as follows: The shifter (Sh) is moved upwards until the lower end of its gap (ca. 4mm wide) touches the axle (A). Then, the entire slide is moved upwards until the sample is about 200 ~m away from the cantilever. Thereafter, the shifter is pulled downwards, setting the axle (A) free and turning the lever (L). By turning the lever, the axle (A) is rotating and thereby further approaching the slide (S) towards the cantilever. The lever and axle are made of I mm steel wire, the length of L is 10 mm, so the motion of the shifter is demagnified by a factor of 10. The flexible joints are made by spot-welded sheet metal. Figure 4.9b depicts a sketch of the approach mechanism for the z distance between the cantilever and the tunneling tip. The tunneling tip sits at the end of a piezo-tube. The microfabricated cantilever is held on its sup77

port plate (Csp) and pivots around a spring-loaded screw (Sc). The coarseadjust beam (B) demagnifies the motion of the rod which is connected to one of the linear feedthroughs. By pulling the linear feedthrough, the coarse-adjust beam pulls the cantilever support plate against the tunneling tip. The cantilever is plated with chrome and gold; the tunneling tip is made of tungsten. The z distance between the lever and sample, and the z distance between the tunneling tip and lever can be controlled mechanically from the outside while the instrument can be operated in UHV and submersed into the liquid-helium dewar. To reduce the effect of thermal drift, the AFM itself is made mainly of Invar. Because the sensitivity (displacement per volt) and thus the full-range motion of the piezoelectric-tube scanner at 4.2 K is only about 25 % compared to that at 300 K, the precision of the mechanical approach has to be even better at liquid-helium temperature than at room temperature. Owing to the small size of the instrument and the use of Invar the cantilever and the tunneling tip remain aligned as the instrument is cooled to liquid-helium temperature. However, stress relief in the tunneling-tip mount during bakeout can move the tunneling tip up to 50 /Lm sideways, so care has to be taken when mounting the tunneling tip. PZT-5A is chosen as piezoelectric material for this design because it has a fairly high Curie temperature (i.e., the temperature at which it becomes unpolarized and therefore loses sensitivity) and thus can be soldered. The tube scanner and the piezoelectric tube that controls the distance between the tunneling tip and cantilever are soldered to metal plates at one end with a lead-free solder. The other end is soldered to a slab of metalcoated alumina. This AFM/STM has been tested by imaging Highly Oriented Pyrolytic Graphite (HOPG) in the STM mode and KBr in the AFM mode at 4.2 K and 300 K in UHV. Atomic resolution has been achieved on HOPG, and both the potassium and the bromium ions on the KBr (00 1) cleavage plane have been resolved. Apart from the criteria listed at the beginning of this section, special requirements have to be satisfied in the design of an STM for in-situ electrochemical studies. First, only the very end of the tunneling tip and the sample can take part in chemical react ions with solut ions. Secondly, unwanted Faradaic current has to be minimized because the large Faradaic leakage current will cause difficulties to the feedback control. In order to reduce the Faradaic leakage current, the ideal tip should have a chemically and electrochemically inert insulation except for the very end of the tip. In the next section, some simple techniques for coating the tip with insulating materials are introduced.

78

Single tube xyz~~

translater

Sample ~ /'0.---"

Fig. 4. 10. Schematic diagram of the STM proposed by Sonnenfeld and Hansma for the operation in solution [4.10)

Stepper motor Insulated sample wire -----..' Vibration isolation: I

MACOR-fJr base

samPle~

Pytex sample holder

~

Pyrex base Teflon base

E

Teflon

~

''''''';00

u

I

Bias wire

I It Piezo-tube

~MACOR

. RTV _ O-ring Insulallon Brass'·:- .•..:.. MACOR Tip

Fig. 4. 11. Schematic diagram of the STM capable for the automatic approach [4.11). The insulated tip is shown

Two designs of the STM that can be operated in solutions are depicted in Figs.4.10 and 11. Both of them use piezoelectric ceramic tubes as the three-dimensional scanner. The later design adopts an automatic approach mechanism, permitting the approach in a glove box under N2 atmosphere.

79

4.3 Tip Preparation A frustrating aspect of STM operation is the reliable formation of tunneling probe tips. The size, shape and chemical identity of the tunneling tip influence not only the resolution and shape of an STM scan, but also the measured electronic structure. Three features of probe tips seem to be most important for reliable STM operation [4.12]. First, blunt macrostructuring leads to high flexural resonant frequencies, thus minimizing phase hysteresis and allowing higher data rates to be achieved. Secondly, the atomic microstructure of the tip is the key to image resolution because the tunneling current depends exponentially on the gap distance. It is necessary for stable operation to have a single site of closest atomic approach, which is well supported, i.e., not a whisker. Anomalous imaging artifacts will appear when simultaneous tunneling occurs through multiple atoms on the tip. This is commonly referred to as double-tip imaging. Finally, tip purity is important so that a series barrier is not present. For example, the effective resistance of a tungsten-oxide layer can easily be much higher than the desired tunneling gap resistance. Therefore, mechanical contact of tip and sample would occur before the required tunneling current can be obtained. A nonmetallic tip may also make the STM tunneling spectrum not represent the true electronic structure of the sample surface. STM tips are typically fabricated from metal wires of tungsten (W), platinum-iridium (Pt-Ir), or gold (Au) and sharpened by mechanical grinding, cutting with a wire cutter or razor blade, "controlled" crashing, field emission/evaporation, ion milling, fracture, or electrochemical etching. W tips which fulfill the requirement of being stiff, have been used to a great extent to image specimens. However, an oxide layer may exist on the surface of the tip and sometimes makes it difficult to acquire STM images. Platinum, although a soft metal, is a material preferred over W because it is inert to oxidation. The addition of Ir to form a Pt/Ir alloy adds stiffness while maintaining a chemical inert material. Pt-Ir tips are widely employed, too, particularly in atmospheric and electrochemical environments.

(a)

(b)

-

OW

:N.OH

~

-Stainle~- ~

/(~l

_steel __-

OW

WO~- flow

Fig.4.12. (a) Schematic diagram of the electrochemical cell showing the tungsten wire (anode) being etched in NaOH. The cathode consists of a stainless-steel cylinder which surrounds the anode. (b) Sketch of the etching mechanism showing the "flow" of the tungstate anion down the sides of wire in solution (4. 13]

potential. Each procedure gives a different tip shape; the AC etched tips have a conical shape and much larger cone angles than the DC etched tips. The DC etched tips in the shape of a hyperboloid, on the other hand, are much sharper than AC etched tips and are preferable for high-resolution STM imaging. Figure 4.12 shows the details of the electrochemical cell used in DC etching, which contains 100 ml of 2M NaOH or KOH. The W wire to be etched is placed in the center of the cell and serves as the anode. It is mounted on a micrometer so that its position relative to the surface of the electrolyte can be adjusted more precisely. The counter electrode (or cathode) consists of a stainless-steel cylinder which surround the anode. When a DC voltage of 13 V is applied to the anode, bubbles are observed emerging on the cathode/solution interface. The overall electrochemical reaction is

W(s)

+ 6e+ 80H-

anode:

W(s)

+ 20H- + 2H 2 0 ~ WO~ + 3H2 (g)

4.3.1 Preparation of Tungsten Tips

80

-- - I - I I

-

cathode: 6H 2 0

The preferred method for preparing tungsten tips is the electrochemical etching method which was developed for preparing samples for FIM and FES (Field Electron Spectroscopy). The electrochemical etching procedure usually involves the anodic dissolution of the metal electrode; tungsten in this case. There are two ways in which this can be done: An AlternatingCurrent (AC) etch or a Direct-Current (DC) etch according to the applied

w

w

+

~

+ 60H+ 4H2 0 + 6e-

3H2 (g)

~ WO~

SRP = - 2.45 V SOP

= + 1.05 V

EO = - 1.43 V .

The above reaction involves the oxidation dissoluation of W to soluble tungstate (WO~-) anions at the anode, and the reduction of water to form bubbles of hydrogen gas and OH- ions at the cathode. EO is the standard elec81

trode potential given by the sum of the Standard Reduction Potential (SRP) for water and the Standard Oxidation Potential (SOP) for tungsten. Actually, the reaction mechanism is much complexer than indicated by the above equations, and the potential required to drive an electrochemical reaction is usually higher than that calculated from standard electrode potentials. The excess potential is called the electrochemical polarization, which is affected by changes in the concentration of the reactants and products, and other mass transfer processes; the rate of the reaction or the current density is an exponential function of the polarization. Several factors affecting the etching process have been studied by [be et al. [4.13]. Due to the surface tension of the aqueous solution, a meniscus is formed around the wire once it is placed into the electrolyte, as sketched in Fig.4.l2b. It is primarily the shape of the meniscus which determines the aspect ratio and overall shape of the tip. The shorter the meniscus is, the smaller the aspect ratio becomes. A low aspect ratio is important in reducing vibration in the tip during scanning. With the reaction going on, the change in the surface area of the wire and in the fluid disturbances may result in the variation of the meniscus height. To avoid oddly shaped tips, the meniscus height should be kept at the same position by adjusting the micrometer during the etching. The current density of the reaction is limited by the surface area of the working electrode (tungsten) and is also dependent on the concentration and activity of OH- ions. The portion of the tip below the meniscus would normally be etched away without the denser tungstate layer flowing down along the sides of the wire to protect the lower end. Therefore, due to the protection of the tungstate layer over the W wire below the meniscus, the DC drop-off method will, in fact, produce two tips simultaneously: the part dropping off to the cell bottom after being etched, and the part above the meniscus. Usually, a quick automatic cutoff circuit is used to cut off the potential to avoid over-etching because any part of the wire (usually the very end of the tip) remaining in the solution will continue top-etch as long as there is an applied voltage. Figure 4.13 illustrates a simple automatic cutoff circuit, where "Load" denotes the electrochemical cell (Fig.4.l2). A variable resistor R b has a significant effect on the potential across the load. The voltage Vb is compared to a reference voltage for the accurate setting of a low cutoff voltage. The cutoff time of the etch circuit has a significant effect on the radius of curvature of the tip: the shorter the cutoff time, the smaller both the radius of curvature and the cone angle of the etched tip; i.e., the faster the cutoff time, the sharper the tip. The etching circuit shown in Fig.4.13 has a minimum cutoff time of 500 ns. If the part of the W wire above the drop-off point is desired, the tip should be raised up quickly and rinsed in distilled water to remove the residual etchant solution after etching. If, on the other hand, the drop-off part is desired, protecting 82

~ + 12 V <>-=-i--il G

I

Optional capacitor

R ,Vb 50~ + 5V

SW I

~

1.

V'ef

I

Comparator

I

Etch status LEDs Green=ON Red=OFF

~+12V

1

Rref Etch (lOkQ) enable/disable switch

13kQ

Fig. 4.13. Block diagram of the electronic control circuit to minimize the electrochemical reaction cutoff time following dropoff. R Joad denotes the electrochemical cell illustrated in Fig. 4.12 (4.13]

solutions (for example, trichloromethane) should be placed below the electrolyte to avoid further etching by the electrolyte. It was found that the length of the wire in the solution has a direct effect on the tip, with a longer wire causing the stub to drop off much sooner due to the increased weight. When the weight of the stub exceeds the tensile strength of the neck or the drop-off portion, the stub breaks off, leaving a rough surface or a slight recoil at the end of the tip as the result. These tips have a larger radii of curvature than those of a shorter length in the solution. If the tip above the meniscus is desired, the length of the wire in solution in the range of 1--:-3 mm is most appropriate for a 0.25 mm diameter W wire. The electrolyte concentration is another factor affecting the etching process. Because OH- is consumed in the reaction, it is necessary to replace NaOH solution periodically. The drop-off time increases with the decrease of the OH- concentration. Figure 4.14 exhibits the detailed electrochemical cell used in AC etching. In this method, the AC voltage is much lower than the DC voltage, and a shorter drop-off time can be expected. Either the upper portion or the lower portion which drops off, can be adopted as the tip. If the latter is desired, a shielding solution is also needed to protect the tip.

83

Fig. 4.14. Schematic diagram of the electrochemical cell used in AC etching

Tip holder

T shaped stainless steel fixture Tip

2V

AC

Gold wire loop 2 mm in diameter Fig. 4.16. Schematic diagram of a micropolishing setup to etch Pt-Ir tips by volume: 60% saturated CaCI 2 , 36% H2 0, and 4% HCI. Etching conditions: 2 V rms AC against aAu electrode [4. 14) Micropositioner

4.3.2 Preparation of Pt-Ir Tips Mechanical shearing is a common approach for fabricating Pt-Ir tips. In spite of the variation in shape, many experiments have proved that atomic resolution can be reached by using the mechanically fabricated Pt-Ir tips. Additionally, Pt-Ir tips can electrochemically be etched in several solutions: CaCI 2 /H 2 O/HCI, NaCN/NaOH, KCIIH 2 0/HCI, NaCN/KOH, and molten NaNO) /NaCI. Although resolution requirements are usually not as stringent for highly topographic samples, wide-area scans place unique restrictions on tip morphology. For such samples, symmetric, controlled-geometry tips with small radii of curvature and high aspect ratios are necessary to minimize the convolution of the tip shape into the acquired image. For example, Fig.4.15 illustrates the effect of the tip geometry on the imaged profile of grooves 0.75 f-tm wide and 1 f-tm deep. As indicated in the figure above, a tip of 50 nm in radius and a 15 0cone half angle is shown schematically to be too broad to resolve the square bottomed sample feature; while a more accurate profile of the samesize groove is given by using a tip of 50 nm in radius and 50 cone half

(a)

VIS'

---=-=-=,\ \

"n----\

I

\ I I

\

\ \

I

(b)

.----.-----

\

----.\

t~·_ , ,

I

II \

I \

I

\_'

1

1__

J

I I I

Fig. 4. 15a, b. Effect of tip geometry on the measured STM profile of grooves 0.75 f-tm wide and 1 f-tm deep using: (a) a tip with a 50 nm radius of curvature and 15 0 cone hal f angle, (b) a tip with a50 nm radius of curvature and 50 cone half angle 84

angle. From this example it is clear that a proper tip geometry is crucial to obtain the STM image that represents the sample surface. In order to fabricate specially shaped tips suitable for observing narrow and deep grooves on surfaces, Musselman and Russel [4.14] have developed a technique to fabricate Pt-Ir tips having small radii of curvate and high aspect ratios in a two-step process. In the first etching step, a 1.25 cm long piece of 0.2 mm 80:20 Pt/lr wire is etched in bulk etchant solution consisting of saturated CaCI 2 /H 2 0/HCl (60%/36%/4% by volume) against a C rod at 25 V rms AC for about 5 minutes, producing a tip comprised of a rigid structure with a long slender region just prior to the tip end. The second step involves precision micropolishing of the tip in a thin film of etchant held in a Au wire loop (Fig.4.16). With the help of a stereo-microscope and a mechanical micropositioner, the long slender region at the tip end can be thinned by moving it through the film, or sharpened by making brief contact with the film. The tip fabricated in this way can have a small radius of curvature «50nm), a high aspect ratio (8 0 cone half angle) and a smooth surface (Fig.4.17). Figure 4.19 depicts STM images of an Au-coated PMMA lithographic test pattern obtained by using a chemically etched Pt-Ir tip in the two-step process described above and a mechanically cut Pt-Ir tip, an image of which is presented in Fig. 4 .18. The grooves are 0.75 f-tm wide and 1 f-tm deep. The top view and line scan (Fig.4.19a, b) acquired by an electrochemically etched Pt-Ir tip illustrate that the widths of the groove (top and bottom) are approximately equal. In contrast, the top view and line scan (Fig.4.19c, d) acquired using a mechanically cut Pt-Ir tip demonstrate that although the cut tip can reach the groove bottom, providing an accurate measurement of groove depth, the irregular shape and broadness of the cut tip is clearly incorporated into the STM image of the test pattern, and the width at the bottom of the groove is only one third of the actual width of 0.75 f-tm. This striking comparison of STM images acquired from a highly topographic sample using Pt-Ir tips with dramatically different geometries clearly illus85

Fig. 4.17. SEM image of a Pt-Ir tip with a radius of curvature <50 nm and a cone half angle of 8 0 fabricated in the two-step process [4.14)

Fig. 4.19a-d. STM image of an Au-coated PMMA lithographic test pattern. Grooves are 0.75 p.m wide and I p.m deep. (a) Three-
the contamination following etching. With limited success, the thickness of the C layer can be reduced using an ion mill or an oxidizing oxygen acetylene flame. Unfortunatley, ion milling is a slow process requiring at least 30 minutes per tip. Even with careful calibration of the flame temperature, it is difficult to consistently oxidize the C layer without altering the tip shape via melting and recrystallization. Thus, carbon contamination must be reduced as much as possible during the electrochemical etching process. Carbon dioxide, which readily dissolves in water to form carbonic acid, has a higher vapor pressure than water and can therefore be eliminated by boiling and subsequently cooling all water used to rinse the tips and to prepare the etchant. During the micropolishing step, the contact of CO 2 in the air with the tip and with the thin film of etchant makes it necessary to polish in a glove box under N2 atmosphere. Fig. 4.18. SEM images of a mechan ically cut Pt-Ir tip.

4.3.3 Other Ways to Prepare STM Tips trates the importance of using a controlled geometry tip for the acquisition of reliable and reproducible images. An analysis of the surface chemical composition of either electrochemically etched or mechanically cut tips indicates that the tip surface is often covere.d with a carbon contamination layer which is tens of Angstroms thick. Therefore, the initial fabrication procedure should be carefully done to improve the surface chemistry either by inhibiting the formation of carbon contamination layers during electrochemical etching or by removing 86

Other methods for preparing STM tips have been reported. Reproducible sharply-pointed Mo tips can be fabricated by controlling the applied AC voltage, wave shape, phase angle, frequency, and number of waves. Ion milling and FIM techniques have been used to prepared tips from singlecrystal W. Tips capable of achieving atomic resolution on graphite include pencil lead [4.15] and colloidal graphite-coated tungsten. A gold wire is usually etched in an etchant containing 50 % HCI, 25 % high-purity ethyl alcohol and 25 % glycerin, where the gold wire acts as the anode and another 87

piece of gold wire or stainless steel as the cathode. A DC voltage of 7 V is applied to the electrodes. In the case of operating the STM in an electrochemical environment to study solid/liquid interfaces, the overall current between the tip and the sample mainly consists of Faradaic charge-transfer current. The ideal tip for electrochemical studies should have a chemically and electrochemically inert insulation except for the very end of the tip. Good insulation reduces the Faradaic leakage current and, consequently, noise. The leakage current depends on such factors as tip bias, and the nature and the pH value of the solution. The use of glass, poly(a-methylstyrene) and Apiezon wax as the insulating materials have been reported. Heben et al. [4.16] put glass on a Pt wire shaped as a tear drop of 1.5 mm inner diameter and heated it to the melted state. They then inserted the electrochemically etched Pt-Ir tip into the melted glass and obtained the glass-insulated tip by controlling the melting temperature and the inserting speed. The area of the bare metal at the very end is < I nm 2 . In the same way, the tip can also be insulated when poly(a-methylstyrene) is heated to its melting point or above (483K). They tested the insulation of the tip in 1.0 M NaCI solution and 1.0 M NaCI aqueous solution containing 0.1 M(Fe)CN~- and 0.1 M(Fe)CN~-, and found that the Faradaic leakage current was efficiently reduced. Atomically resolved images of graphite were also obtained under solutions. However, both of these materials crack easily, resulting in increased Faradaic current. In addition, glass-insulated tips cannot be employed in concentrated alkali solutions. Another promising insulating material appears to be Apiezon wax [4. I7]. The apparatus/technique for the tip insulation with this material is illustrated in Fig A .20. A 1 cm 2 copper plate (1.5mm thick), held horizontally, as shown in FigA.20, is heated by means of soldering iron to melt the Apiezon wax. A 1 mm wide rectangular slit extends from one side to the center of the copper plate, providing a temperature gradient (colder at the open end and hotter at the closed end) for the melted wax. A electrochemically etched tip is brought from underneath the slit by means of a manipulator. The tip is first moved slowly into the hot wax and allowed to attain a thermal equilibrium and uniform welting. The tip is then raised through the wax and allowed to break the top surface of the melt. If the tip breaks the surface at too hot a region, it is mostly bare. If the tip breaks at the colder region of the melt, it raises a blob of wax above it. In between these two regions exists an optimum point where the wax insulates the tip almost completely. The insulated tip is moved sideways out of the groove so as to leave the very end of the tip unperturbed. These wax-coated tips are much less fragile than glass or polystyrene-coated tips and appear to be inert in most electrolytes. 88

2

~a 4

5

~b

Fig.4.20a-c. II1ustration of the apparatus used for tip insulation. (I - soldering iron, 2 - copper plate, 3 - I mm wide sl it, 4 - STM tip, and 5 - holder for manipulator). The point where the tip penetrates the Apiezon wax along the slit determines how much of the tip gets insulated. (a) At a too cold region, the tip is completely covered with Apiezon wax. (b) Optimum point allowing only the extreme end of the tip to be exposed. (c) At a too hot region, the tip receives little insulation and is thus mostly bare [4. 18)

~c

Only 100 pA of Faradaic leakage current are detected in 0.1 M HCI0 4 with a tip bias of 0.1 V. Other types of tips have greatly enriched the probing capability of STM. In fact, novel tips can lead to substantially new research directions. For example, the wiskers of Cr0 2 provided tunneling electrons with selected spins [4.19], making it possible to retrive informations associated with spins of electrons such as spin density waves which is a very interesting topic. The breakthroughs in the research of nanotubes have provided an alternative for preparing tips for SPM [4.20]. By attaching a single nanotube to the probe apex, one can truly have a multifunctional tip with high reproducibility, which can also be used to study the electrical conductivity, flexibility of the nanotube, etc. (Fig.4.2l).

4.304 Tip Treatment Almost all experiments to date have been done with metal tips sharpened by electrochemical polishing/etching or by mechanical shearing. These tips sometimes produce atomic-resolution images with no further treatment indicating that there was at least one atomically sharp tip projecting out from the (relatively dull) end. However, most tips would not provide atomic resolution immediately. A further tip treatment can be done during actual tunneling. The effect of the in-situ tip treatment can be verified by immediate imaging. Several in-situ tip treatments were already used at the birth of STM by the inventors (G. Binnig and H. Rohrer): by gently touching the tip 89

"'8/

Fig.4.21. TEM images showing a single nanotube attached to the pyramidal tip of a silicon cantilever for scanning force microscopy [4.20]

O.2nm [001]

O.4nm [001]

[11~1O Inm I nm

[110]%10] Inm Inm

IJI1l

0.2

0.1

o

with the sample surface, the resolution was often improved. By exposing the tip to high electric fields, of the order of 10 8 V fcm, the tips are often sharpened. After the tip is formed by an in-situ treatment it is prudent to move it laterally to a new region of the sample since these methods for tip forming could modify the surface directly under the tip. The high-electric-field and high-tunneling-current techniques are supposed to either clean or sharpen the tip. Clearly, however, asymmetric or double tips are often formed, resulting in misleading sample topographs. Several groups are working with instruments to do tip characterization with Field Ion Microscopy (FIM) and STM in the same vacuum system (called FI-STM). The oriented tips (single-crystal wires) can be sharpened by annealing in the presence of a high electric field, producing a pyramid shape of certain facets. These monatomic tips are so stable that the tips can be used continuously in vacuum for several months. By taking the FIM image of the tip before and after a STM scan, the character of the STM topograph 90

(e)

Fig. 4.22. (a) FIM image of a well-defined W (100) tip before and after the STM scan. The arrow indicates the scanning direction with respect to the tip. Filled circ1~s are the first layer atoms. (b) STM topograph of Au (00 I). Rows along the close-packed [110] are separated by 1.4 nm. Scales along [110], [110] and [001] indicate I, I and 0.2 nm. respectively. (c) FIM image of a blunt W (100) tip. (d) STM topograph of the same sample scanned by the tip shown in (c) [4.21]

can be correlated to the tip structure. The dependence of the measured corrugation amplitude on the size of the tunneling tip has experimentally been examined by Kuk and Silverman [4.21]. Figure 4.22a shows a FIM picture of the tunneling tip used in the STM scan of a clean Au (00 1)-5 x 1 surface with a corrugation width of 1.44 nm (Fig.4.22b). The corrugation amplitude can be estimated theoretically by ~

oc

e-13(R

+ d)

(4.11)

where {3 "" 'A G2 fK, R is the radius of curvature of the tip (being approximately a hemisphere), d is the gap distance, K -I is the electron decay length in vacuum, and G is the smallest surface reciprocal wave vector (27f f ai' a j being the largest corrugation width). Figure 4.23 depicts the average measured corrugations of the Au (001)-5 x 1 and the Au (110)-1 x 2 reconstructions as a function of the tip size. The measured {3's for the Au (00 1) and the Au (110) are in good agreement with the values predicted by (4.11). The tip size versus measured corrugation for several other surfaces is shown. Two broken horizontal lines in Fig.4.23 indicate STM noise levels, typically due to mechanical vibration or electronic noise, for two hypothetical STMs. The significance of the tip size is now apparent from Fig.4.23. For a noise level corresponding to 0.05 nm, the tip must be terminated by a single atom in order to detect any reconstruction. On the other hand, the 91

In the constant-current scan mode, the gap separation between tip and sample is set by comparing the tunneling current with the set-point current (reference current); thereafter it is regulated by a negative feedback loop. Since the STM is a sub-Angstrom servo device, the electronic design should closely be coupled to the mechanical design. Kuk and Silverman [4.3] have performed a feedback-circuit analysis by using the following relations for the response function of each component:

0.1

E

" 0.05 ;;' .S c;;

"". ..... .....

00

t

\.. ' '\

o

u

-0

" ~ "'"

.

0.01 ,

\l,

:E 0.005

,

"

..... ,

\..

--"'l(.--.....

'\-

,

'

~MOS~\--- .~-~~~_~;Ymetals

o

G I (s)

Kp Srp + 1 '

(4.12)

Gz(s)

Ki Sri +

(4.13)

_

\Si(l I I) - (Zx 1) ',<"Si(l I I) - (7x7) Z

3

1'

Size of cluster on tip [nm]

Fig. 4.23. Dependence of measured corrugrations on the size of the tip for Au (001)- 5 X I (solid and filled circle), Au(iIO)-IX2 (broken line and triangle), Si(lII)-7X7 (_. -), Si (I I 1)-2 XI (dotted line), and most metals without reconstruction (- .. -) [4.21]

G 3 (s)

K hv

W

corrugations of the Au (00 1), Au (110), and Si (111)-7 X 7 surfaces could be detected by a 2 nm tip with the STM noise of 0.005 nm. Most metal surfaces with small reconstruction unit cells or 1 X 1 surfaces require a very sharp tip and a low STM noise level, making atomic resolution difficult to achieve without special treatment of the tip. Recently, it has been proposed that a molecular chain could be jointed to the STM tip made of a single crystal with the expectation of controlling precisely chemical and mechanical properties of the tip.

HI (s)

Basically, the electronics for an STM device consits of the following equipment: (1) A source for the x and y raster voltages, (2) high-voltage amplifiers to drive the piezoelectric elements, (3) a voltage source and current sensing amplifier to establish and sense the tunneling current between the tip and the sample, (4) an error amplifier to amplify and (usually) integrate the difference between the tunneling current and some set-point current, and (5) a display device for the images. Most of the pieces are commercially available. However, many STMs have used a computer for (1) and (5), and also for determining the bias voltage, tunneling current, controlling tip advance, even for replacing the analog feedback by a digital one. 92

n

zK T

(4.15)

+ wns/Q + wzn '

SZ

K£ Hz(s) = Sr£

(4.16)

+1

The resultant transfer function T s for the feedback system depicted in Fig. 4.24 can be expressed as

Ts 4.4 Electronics

(4.14)

Srhv + 1 '

+ GZ (s)]G 3 (s) 1 + [G I (s) + Gz (s) HI (s) Hz (s)] , [G I (s)

(4.17)

z-Piezo position

Hz

Gap distance -i

Fig. 4.24. Block diagram of the STM negative feedback-control system 93

3I

I

, Tip p05ilion ADC

I

I

A=1000

,.

-----,

Current

..I ~.M I I ADC

,.

Prop amp

'" z

~ o

~ ~

c

.~ c

'" ~

STM lip

driver Tip I vollage

or o

I

I

I

Z 3 Time [x 10- 3 5]

I Z? I

I

I

I

4

5

Fig.4.25. Transient response of lhe STM control system depicled in Fig. 4.23. The resull for lhree different loop gains are presented [4.3] Fig. 4.26. Block diagram of a compuler-conrrolled STM

where G 1 , G z , G 3 are the transfer functions of the proportional feedback (subscript p), the integral feedback (subscript i) and the high-voltage amplifier (subscript hv), respectively. HI and Hz are the transfer functions of the tunneling gap and the preamplifier (including a logarithmic amplifier if there is one), respectively. K and T stand for the gain and time constants of each system. The resonance frequency and the Q factor of the tunneling gap can be measured experimentally. Although the settling time of a feedback system can be shortened with a proportional feedback component, K p = 0 for many existing STMs. From (4.17), Kuk and Silverman calculated the transient respone of the feedback system for an STM with wn "'" 1.2' 10 4 , Q "'" 10, Te "'" 3,10- 4 , and Ti "'" 0.11. The results shown in Fig.4.24 indicate three cases with three different loop gains. The response is overdamped with a gain of 100, critically damped with 200, and underdamped with 1000. Therefore, once the gain is here set at 200, the circuit is optimized [4.22,23]. The frequency response of the feedback system can be derived from (4.17) by replacing s by !W.

A typical block diagram of a feedback circuit is presented in Fig.4.26. The tip voltage can be supplied by a battery or computer Digital-to-Analog Converter (DAC). The tunneling current can then be measured by a preamplifier with a gain of 10 6 -:- 10 9 V/ A. The preamplifier itself can either be a commercial unit or built around an operational amplifier (such as the AD529 or OPAI28). Special care must be taken in the design of the preamplifier, since the high source resistance makes the circuit particularly susceptible in electrostatic coupling. To limit interference, the current wire 94

should be kept short. A compact preamplifier can be mounted very close to the tunneling junction to minimize the capacitance. The signal output from the preamplifier can then be linearized by a logarithmic amplifier to improve the dynamic range since the tunneling current is exponentially dependent on the gap distance. However, the logarithmic amplifier is not necessary and a linear approximation of the exponential dependence can be used in the case of scanning a flat surface. The measured tunneling current is compared with the reference current, and the resultant error signal is fed into the main feedback amplifier, consisting of an analog integrator with a variable time constant and a proportional amplifier. The integrator has infinite gain at DC so there will be no net, long-term difference between the operating current and the demanded current. The feedback signal is then applied to the z-piezoelectric driver for adjusting the tunneling gap spacing. For the x-y piezos, separate high-voltage amplifiers controlled by DACs or function generators are utilized for controlling the scan area. A sample-and-hold amplifier, inserted in the circuit just ahead of the proportional and integral amplifiers, permits the measurement of the I-V characteristics and other spectroscopic data at a constant gap distance. Dur· ing the hold period, the error signal is set to zero so that the z-piezo voltage does not change, maintaining a fixed tunneling gap. The bias voltage can be varied at the same time, and the resulting current can be measured by an Analog-to-Digital Converter (ADC). Care needs to be taken to make sure that the system responds smoothly when the hold of the sample-and-hold

95

amplifier is turned off. The I-V relationship gives information about the electronic structure of the surface, as described in Chap.3. In practice, the overall gain is adjusted to be as high as possible without causing oscillation. Here again, the more rigid the microscope and the higher its resonant frequencies, the higher the gain can be set. This adjustment should be easily accessible and adjustable over a wide range since the optimum setting changes with the tip condition and an effective work function. Sometimes a small additional increase in gain and hence speed of response can be obtained by putting a low-pass filter with a cutoff frequency of above the unity-gain point but below the resonant frequency. In a typical STM using a current preamplifier with a gain of 10 MO, the proportional signal can have a gain adjustment that varies from 0.01 to 100 and a variable cutoff frequency that can be adjusted from 10 Hz to 10 kHz. In addition, since the feedback is a high-gain circuit, electrical noise and pick up can be a problem in some environments. Shielding signal and control wires is important. However, sometimes this may introduce undesirable capacitive effects. When the tip is scanned faster than the feedback can respond, the feedback-loop gain will be smaller than 1, and the signal applied to the z piezo cannot follow the topography of the surface. If the response of the preamplifier of the system is faster than the scanning speed, the measured tunneling current will reflect the surface topography. This is the constant-height scan mode that has been discussed in Chap. 1.

4.5.1 Hardware The equipment used for control, data acquisition, and image display of an STM differ from one installation to another. However, the basic hardware is similar in most cases. A basic approach to a computer-control system is exhibited in Fig.4.27. A personal computer is required for control, data acquisition and display. If STM images are directly stored on a hard disk. no special memory is required but, if they are stored in the random access memory in order to increase the data acquisition speed, a large extended memory is required. In order to optimize the performance and accuracy of all arithmetic computation for data processing, it is necessary to install a math co-processor. The Analog-to-Digital Converter (ADC) and the Digital-to-Analog Converter (DAC) cards used for data acquisition and microscope control must provide high precision and speed. The 12-bit DAC can resolve 5 mV differences in a ± 10 V range. Using a 16-bit ADC, each step is 0.305 mY. The dynamic range of inputs and outputs should be switch-selectable, for example from 0 to +10 volts, from -5 to +5 Volts, and from -10 to +10 volts. It is best to have a software-selectable ADC gain for data acquisition. If a digital feedback circuit is utilised, a special-purpose high-speed signal processor is required to ensure the fast response of the feedback loop. Basically, at least two analog inputs are required for determining the tunneling current I and the voltage V z applied to a z-piezoelectric element.

Control unit

Computer workstation ------, i

4.5 Computer Automation

I I

;1'\ ':W.;( ,,-n\VJ\4' ll.·~ n' OIl) I";·.·:'''''.....·<... ~'''"O.', 1)':<11 \)' II 'I \'IIl\V~

!~

1:

High-voltage power supplies

:1

~"

~...,:,,:.;, 'U"j':'r:.":.\"1.'.\"1 '~;.\«"-' ·u.'!/.\\~.'~' 'i'"i,' ~.'; .".:-'".::' tI :;~:,,·:.·:I:·~i

The early STM experiments were performed only with function generators for the x-y scan signals and a pen plotter to record data. The recorded images were either analyzed visually or, in a more sophisticated process, read by an optical scanner connected to a host system. However, for more precise image processing, this simple method could no longer satisfy increasing demands on the picture quality. Furthermore, the image data acquired are not in a computer-like format. Therefore, fast data handling, such as archiving, retrieving, and transfering between computers is too difficult to be realized with this system. In this section we deal with the development of computerized systems for STM control, data acquisition and real-time display and, moreover, the development of image processing techniques applicable to the restoration and analysis of STM images.

IUI!'. ,'..

x-y board

l r:.•,"':.V.-:......." l\./t.';\'J';("...~.•~"',I'J.~\\';.:.;,>:/; ;~':J~~

:>.....:"-'-',-O-;:."'+-+I------1I>-"1--tr

§

~

High-speed special- purpose digital signal processor

I

Hard drive

I

I

RAM

I

I

80386 microprocessor

z

~L.B

t.~,.

Bias

'.r,·'·.'·,.·.",·.. ,·,.',·".,"'.•'.'.'.·,····,"·,",".· ,.,.,..

!:~~ I

Base support

" .~ j .

Fig. 4.27. Block diagram of the STM control, data acquisition and display system indicating all of the equipment connections 96

97

For more efficient and precise instrumental control, the computer may need more analog inputs to measure some other parameters such as the tunneling bias Vb' the voltages V x ' V y applied to the x-, y-piezoelectric elements, and the offset voltages for selecting the scan window. In this way, data acquired are recorded automatically, and changes of each parameter can be monitored during scanning. It is important to note that connections to the ADCs have to be carefully made in order to avoid electric noise. As discussed above, the x- and y-raster voltages can be supplied by a function generator, but in many cases DACs are used instead. Therefore, at least two D/ A channels are required to generate x and y signals. Furthermore, in order to acquire the STS and work function information, other DACs for controlling the bias voltage Vb between the tip and the sample and a digital input are needed. The latter is used to trigger and turn off the feedback loop many times during the data acquisition process. Of course, offset voltages can also be controlled by DACs. In addition, if automaticapproach devices such as a louse or an Inchworm Motor are employed, it is possible for a computer to control the approach. Some kind of device is always needed to visualize the values of the zpiezoelectric voltage V z, while data are being collected. An analog x-y pIotter was traditionally employed by STM to graphically represent the modulation of Vz' These line scans separated by a fixed distance provide an illusion of three-dimensional surface topography. However, the slow motion of its pen sets a very restrictive limit on the scanning frequency and this kind of presentation provides limited image quality. To fully analyze the scanned-image data and resolve spatial information, gray-scale or top-view images should be generated. For these reasons, digital graphic display devices, such as color graphic controller cards and high-resolution graphic monitors are now widely used. Ideally, graphic display devices are expected to have the resolution of 1024 pixels and 256 levels of gray to enhance image processing.

(3) file management, and (4) some processing tools for postacquisition image processing. The following items are usually included in a parameter setting: an experimental identification name and descriptive text, the number of points per x scan (this number should be a power of two to facilitate the computation of the discrete Fourier transform), the maximum number of x scans (the actual number of scans may be smaller if the experiment ends prematurely), the size, color and position of the image to be displayed, the ranges of the DAC and ADC (if any), and the conversion factors of the digital counts to the real values of the voltages applied to the x-, y- and z-piezoelectric elements so that coordinate data can be given in Angstroms or nanometers. Default values of these parameters can be obtained from a program profile, which allows the investigators to avoid going through several menus. In addition, in order to save storage space and time, data have to be stored in binary format. However, the program should incorporate the possibility of converting STM image files from the binary format to character format. The following techniques can be utilized in the software for immediate data analysis: • Discrete Fourier Transform (DFT) of each scan is performed in order to find its frequency spectrum. This information is relevant in that it permits the microscopist to see whether there is any periodicity in the data, whether there exists any periodic noise that the system is getting, or whether the signal-to-noise ratio is too small. The DFT is computed with the Fast Fourier Transform (FFT) algorithm for a number of data points that is a power of 2. • Root-mean-square noise of the samples per scan is defined as I/NJE[Vz(X)-(V z)]2 ,

4.5.2 Software The computer software that performs instrumental control, data acquisition, and image display is usually written in a high-level language except for some subroutines written in an assembly language for critical tasks in order to get a fast and compact executable module. All the programs should be executed in a user-interactive way through menus. Generally, the program can be organized in four well-defined parts: (1) Setting of parameters that determine the operation of the microscope, (2) STM control, data acquisition and real-time display, 98

where Vz(x) represents the N samples of Vz collected in a scan, (V z) is the mean value of Vz' This value gives information about the reliability of the data. • Real-time display paramters are used to increase the dynamic range of the STM image displayed on the monitor. The image actually displayed in real time is [4.24] V;(x,y)

=

C[Vz(x,y) - (A+Bx)],

(4.18)

where C is a scaling factor, and the parameters A and B are obtained by a 99

least-squares fit of the values Vex, Yo) collected in a test scan. What should be taken into account is that the values v; (x, Yo) should range between a minimum (greater than 0) and a maximum (less than 255) selected by the microscopist to have a reasonably good probability that v; (x, y) does not fall out of the range of possible pixel values. • Real-time digital filtering can be performed within the program on each STM line scan. There are sources of acoustic and vibrational noise that can be filtered very efficiently by using a Finite Impulse Response (FIR) filtering relation [4.25] of the form: x(l) = ax(l) + bx(l-l)

+ cx(I-2) + dx(I-3) + ex(I-4)

,

(4.19)

where a, b, c, d and e represent the coefficients of a 5-tap FIR filter. The real-time filter windows over each data point I in the line scan range from 4 < I < N pixels per line. Software signal averaging is also utilized to improve the Signal-to-Noise Ratio (SNR) of the acquired images. • Since generally the surface normal is not parallel to the tip, the structures we are interested in are in an inclined plane. It is difficult to see details if we look at such pictures in a top-view presentation. In order to correct the microscope image that results due to this sample tilt, a plane must be fit and substracted from the image. An interactive subroutine uses a window (usually of ten previously acquired line scans) to compute a plane that can be written as Zplane

-

ax + {3y + 'Y

are difficult to visualize in the top-view image. A magnified image on the selected area produced by software zoom techniques with interpolation can help reveal detailed features on the surface. Additionally, some other tools which can be included in the software are listed below. a) Histogram Equalization

It is desirable for all surface features to be displayed within all of the available (256, for instance) gray levels. The plane subtraction algorithm described earlier can be used to approximate this condition during real-time image acquisition. During post-acquisition image analysis, the microscopist can expand the dynamic range of the image by performed histogram equalization. This requires the use for a new minimum and maximum pixel values in the range (0--:-- 255) to adjust the contrast of the image. The image pixels can then be adjusted in the following manner: All pixels < minimum are set All pixels

= 0 ;

> maximum are set = 255 ;

Remaining pixels =

(Old pixel value - minimum) x (maximum) (. ..) (4.21) maximum - mlOlmum

where maximum and minimum are the gray-scale limits specified by the user.

(4.20) b) Convolution Filter

where a, (3 and 'Yare computed coefficients. By using the computed plane, slope and offset values are computed to correct the current line scan from the sample tilt.

4.5.3 Image Processing Image processing can include algorithms for systematic data correction, such as a change of scale and a correction of the thermal drifts of the scan origin and of Vz' Two-dimentional fast Fourier transform is very useful for periodic images, and can be applied to a periodicity analysis of the image. Images can be displayed using the same color coding. However, a better matching of the dynamic range of the data to the set of colors can be achieved, in order to optimize the visual interpretation of the images, either based on the image histogram or done directly by the microscopist. Using the cross-sectional cut tool to display the height values of a topography in any direction is valuable for analyzing the detailed surface features which 100

STM images typically contain noise. Space-domain filters are available for smoothing (variable-size window averaging), sharpening (Laplacian filter), and edge detection (gradient filter using Sobel kernels). Although convolution filters do not provide an ideal frequency filter, they are often used as low-pass or high-pass filters. The following algorithm describes the 3 x 3 convolution of image pixels [4.26]:

Updated pixel valuex,y

x+l

y+l

L

L

W(i+x+2,j+y+2) Z(i,j)

i=x-l j=y-l 3 3

L L

(4.22) IWCi,j)1

i=1 j=1 where ZCi,j) are pixel values, and the W(i,j) are the window coefficients. 101

The values in the 3 x 3 matrix of window coefficients are integers, and can be set by the users to implement other filter types. The convolution algorithm requires nine multiplications, nine additions, and one division for each pixel in the image. This process is repeated for each pixel in the N x N image. The edges of the image are handled as special cases since there are no data beyond the edges with which to perform the convolution.

Fig. 4.28a,b. STM image of a Si (111)-7 X7 surface coated with Ag at low coverage. (a) Grey scale rendering of unprocessed data, (b) having been obtained from (a) by statistical differencing [4. 27J

c) Statistical Differencing Statistical differencing is a standard technique which attempts to make the corrugation uniform throughout the image by determining a local corrugation for the input data and empirically choosing a transformation. This method is suitable for boundary enhancement in cases where STM images simultaneously show two different structures. The transformation employed is

o

85 =

(D - A) a

+

A

17

+ "2 + 64

,

(4.23)

where 0 and Dare 8-bit output and input data for a given pixel, respectively. A is a local average, defined over an 8 x 8 array of surrounding pixels, and a is the variance of the data for the given pixel from the local average. The first term is designed to result in a uniform corrugation, the denominator contains an additional term to limit the enhancement in the regions where the corrugation is small. The second term preserves, but reduces, the effect of steps or average height changes. It can be eliminated to equalize the corrugation if desired. The last term simply sets the grey scale value to mid-range for a constant mid-range input image. It is important to note that the choices of the neighborhood size used in computing the local average is important, especially since the lateral extent of the features in the STM image are different for the two domains. The trial-and-error method is usually applied for choosing parameters. Figure 4.28 shows the Si (111) surface processed by means of this method by Wilson and Chiang [4.27]. d) Three-Dimensional Representation STM images can also be displayed as three-dimensional surfaces. Threedimensional perspective images offer additional clarification. especially when complex structures are present. The use of the contours, together with shading is also helpful for rendering images when image display formats have good resolution but few gray levels. The orientation of the surface can be chosen by the users, as well as the point from which the surface is illuminated and the relative proportions of direct and the isotropic diffuse light applied. Figure 4.29 exhibits a three-dimensional representation of the Si (111)-7 x7 surface. 102

Fig. 4.29. Three-dimensional representation of the Si (111)-7 X7 surface

103

50 Other Related Scanning Probe Microscopes

The successful achievement of G. Binnig, H. Rohrer and their colleagues at the IBM Zurich Laboratory initiated a surge of research and engineering activity. This brought about rapid advances in STM technology and led to the development of many other novel scanning probe microscopes, such as the Atomic Force Microscope (AFM), Lateral Force Microscope (LFM), Magnetic Force Microscope (MFM), Ballistic-Electron-Emission Microscope (BEEM), Scanning Ion-Conductance Microscope (SICM), Near-field Scanning Optical Microscope (NSOM), scanning thermal microscope, and Scanning Tunneling Potentiometry (STP). These microscopes take advantage of the remarkable ability to control the spatial position of the tip relative to the sample, and provide new information about the physical properties of surfaces on an atomic or nanometer scale. This chapter is devoted to brief descriptions of the operational principles and some applications of these new types of microscopes.

5.1 Atomic Force Microscope With regard to the operational principle of the STM described in ChapA it is important to note that the "tunneling" phenomenon utilized by STM requires that this instrument be used only for conductors or semiconductors. For a non-conducting material, its surface must be covered with a thin conducting layer. However, the existence of conducting film often lowers the resolution and thus limits the usefulness of the STM. It was chiefly this limitation that prompted the development of Atomic Force Microscopy (AFM) in 1986 [5.1-3], which can be applied to image conductors and non-conductors in air, liquids or vacuum. The operational principle of AFM is explained in Fig.5.!. The cantilever which is extremely sensitive to weak forces is fixed at one end; the other end has a sharp tip which gently contacts the surface of a sample. When the sample is being scanned in x-y direction, because of the ultrasmall repulsive force existing between the tip atoms and the surface atoms of the sample, the cantilever will move up and down in the direction vertical 105

(a)

5.1.1 The Force Sensor

B 1

E

u

1.

I

I

Block (Aluminium)

(b)

A: AFM sample B: AFM diamond tip C: STM tip (Au) D: Cantilever, STM sample E: Modulating piezo F: Viton

251lm1 Diamond tip

B

y

z

·.?S

~

Lever (Au-Foil)

k

TE

Mil}

c:;

6

1

Fig. 5.1. The principle of an AFM [5.1]

to the surface of the sample, corresponding to the contours of the interaction force between the tip and surface atoms of the sample. The topographic images can be obtained either by recording the deflection of the cantilever at each point (variable deflection mode) or by keeping the force constant. using an integral feedback loop and recording the z-movement of the sample (constant-force mode). While the STM yields images related to surface electronic energies near the Fenni level, AFM images are related to surface electronic energies up to the Fenni level. The AFM tip may be kept in direct contact with the surface (contact mode) or it may be vibrated above the surface (non-contact mode). For high-resolution imaging and most routine topographic profiling, the repulsive-force (10- 9 --:- 10- 8 Newton) or contact mode is usually used. In the non-contact mode, the van der Waals force, the magnetic force, or the electrostatic force is detected. The non-contact mode has been employed in other scanning force microscopes which will be reviewed in the next section. An application of STM and AFM of particular interest is the in-situ study in electrochemistry. As discussed in Sects.4.2.4 and 4.3.3, special requirements have to be satisfied in the design of an STM for in-situ electrochemical investigations because the electrolyte is highly conducting, which makes the detection of tunneling current difficult. With the advances in AFM, especially the non-conducting AFM tip (Sect.5.1.1) and the optical-beam-deflection technique (Sect. 5.1.2), the AFM has been used successfully to study the electrode surfaces under potential control in a fluid electrolyte. One example will be given in Sect.5.1.3.

106

An AFM can be designed to look very similar to the STM. The primary difference is that it is convenient to have the sample move rather than the delicate, and sometimes bulky, force sensor (composed of tip and cantilever). Much of the technology for implementing the AFM is now well developed. Techniques for vibration isolation, scanning, sample approach, feedback control and image processing are taken with little modification from the STM. Construction of the force sensing cantilever stylus and the measurements of the deflection of the cantilever still need careful consideration. The force sensor is a crucial component for the AFM, detennining its sensitivity and resolution. While the force sensor senses the forces across a sample, the role of the cantilever is to communicate this infonnation to the outside world. When the AFM is operated in the contact mode, in order to register a measurable deflection with small forces, the cantilever must flex with a relatively low force constant. The data acquisition rate in the AFM is limited by the mechanical resonant frequency of the cantilever. To achieve an imaging bandwidth comparable to that obtainable in the STM, AFM cantilevers should have resonant frequencies > 10 kHz. Fast imaging rates are not just a matter of convenience, because the effects of thermal drift are more pronounced with slow scanning speeds. If the scanning rate is too high or the cantilever resonance is too low, the inertia of the cantilever will cause the stylus tip to exert large forces on steeply sloped protrusions, and prevent the stylus tip from tracking steep downward slopes. The combined requirements of a low spring constant and a high resonant frequency can be met by reducing the mass of the cantilever stylus assembly. High lateral stiffness in the cantilever is desirable to reduce the effects of lateral forces in the AFM. Frictional forces can cause appreciable lateral bending of the cantilever, leading to asssociated image artifacts. Investigations have indicated that choosing a "V" or "X" shape of the lever can yield substantial lateral stiffness. When optical beam deflection is used to measure cantilever deflection, the sensitivity of the detector is inversely proportional to the length of the cantilever, since greater angular bending occurs for a given linear displacement of the end of the cantilever when the length of the cantilever is reduced. Reducing the length of the cantilever is an additional motivation for microfabricating the cantilever stylus. Provision must be made for the operation of the detector used to measure the cantilever deflection. If tunneling is utilized, a metal electrode should be fabricated on the back of the cantilever. If optical methods are applied, a reflective surface is needed. For optical beam deflection, the reflective surface should also be as flat as possible. The microscopic structure of the point of the tip is important for high-reso-

107

lution contact profiling, where the contact area between the tip and sample may involve only a few atoms. For high-resolution topographic imaging, the cantilever stylus used in the AFM should satisfy the following criteria: (1) A low force constant, (2) a high resonant frequency, (3) a high mechanical Q, (4) a high lateral stiffness, (5) short lever length, (6) incorporation of a mirror or electrode for deflection sensing, and (7) a sharp protruding tip. The force sensor used in the first AFM was a gold foil of 0.8 x 0 .25 x 0.025 mm 3 carrying a diamond stylus. The gold foil served as a lever and the diamond fragment attached to one end of the gold foil as the AFM tip. The observations of a ceramic (Al z 03) surface in air demonstrated a lateral resolution of 3 om and a vertical resolution less than 0.1 nm. Thereafter, force sensors have been constructed from thin metal foils (Au, W) or formed from fine metal wires (Au, W). Carbon and fused quartz fibers have also been used as the force sensor. Atomically resolved force maps of graphite were obtained utilizing AFMs equipped with these sensors. It has been shown, however, that microfabricated cantilevers are ideal since they have both low force constants and high mechanical resonance frequencies. The microfabrication technique developed by Albrecht et al. [5.4] (indicated in Fig. 5 .2) has been employed to produce cantilevers from thermally grown SiO z and Si 3 N4 by means of Low Pressure Chemical Vapor Deposition (LPCVD). The fabrication process begins with the deposition of the desired film on both surfaces of a (100)Si wafer. The thickness of the Free cantilever

------>...--

( c) (a)

Top '-----------' VIew

/

se~~i~n~""::':B~:?~4or (lll)plane in Si Fig. 5. 2a-d. Fabrication of thin-film microcantilevers. (a) A thin film of SiOZ or Si 3 N 4 is formed on the surface of a (100) Si wafer and patterned to define the shape of the cantilever and to create open ings on the top and bottom of the wafer. (b) The windows are al igned along (III) planes. (c) Anisotropic etching of the exposed Si with KOH undercuts the cantilever and self-terminates at the (III) planes as shown. (d) A small Si chip is cut from the wafer to serve as a pedestal for mounting the cantilever in the AFM [5.4] 108

Fig.5.3. SEM micrographs of SiOz microcantilevers. (a) Rectangular cantilevers. The shorter of the two has dimensions of 100 X20 XI. 514m3, with a force constant of I N/m and a resonant frequency of 120 kHz. The V-shaped cantilever shown in (b) has increased lateral rigidity which reduces its sensitivity to frictional forces [5.4)

film determines the thickness of the finished cantilever. The film is patterned photolithographically to form openings on the top and bottom of the wafer, with the cantilever protruding into the top opening (Fig.5.2a). The geometry and registry of the top and bottom patterns are chosen so that the edges of the openings lie approximately along common (111) planes in the Si lattice. The EDP (ethylenediamine/pyrocatechol/water) Si etchant selectively etches Si rapidly in the (100) direction, but is inhibited when the region etched away is bound by (Ill) planes. EDP does not attack SiO z appreciably. The bottom side of the cantilever and the surrounding Si is then coated with a thin film of metal for the deflection detector in the AFM. The finished structure is shown in Fig.5.2d, and SEM micrographs of finished SiO z cantilevers are depicted in Fig. 5.3. For imaging atomically flat samples one can simply use the end of a cantilever to act as an effective local tip. For imaging rougher samples, however, one desires a sharp, protruding tip of known shape so that the interaction between the sample and the cantilever can be characterized more precisely. A fabrication-process produced Si 3 N4 cantilevers with an integrated pyramidal tip is presef.ued in Fig.5.4. The first step is the formation of a pyramidal pit on the surface of a (100)Si wafer (Fig.5.4a). The wafer is coated with a suitable masking material, such as a thin film of thermal SiO z , and a small circular or square opening is etched in this film. The exposed Si is etched with an anisotropic etchant. After the pit has been formed, the masking material is removed from the wafer. The Si 3 N4 film from which the cantilevers will be made is then deposited on the wafer surface and conforms to the shape of the pyramidal pit (Fig.5.4b). Annealing in an oxidation furnace follows to prepare the surface of the nitride for the later step of anodic bonding. The Si 3 N4 film is patterned into the shape of a cantilever and aligned so that the end of each lever lies over a pit. All Si 3 N4 109

,,)

181

J,m:::::s<

C

Saw cut -+--

(e)

Cr

AJlastf , /

Si Masking material (Si0 2 )

(b)

,

(d)

Saw cut -:

Glass and Cr removed

l/f /' ,/ tr -- j ----: C'::)

Si 3 N4

c=

I

i

::J '~,;/

r-100!Jrn-1

I

~.

fueC:::

1P

Fig.5.4a-e. Fabrication of Si 3 N4 microcantilevers with integrated pyramidal tips. (a) to (e) illustrate the steps in the fabrication process, see text [5.4)

is removed from the bottom surface of the wafer so that all of the Si can later be etched away. A glass plate is prepared with a saw cut and a Cr-bondinhibiting region, and is then anodically bonded to the annealed nitride surface (Fig. SAc). After the bonding is complete, a second saw cut from the top of the glass surface is made, which intersects the previous cut. This releases the bond-inhibited part of the glass plate and exposes the cantilever (Fig.5Ad). All Si is etched away, leaving the Si 3 N 4 microcantilever attached to the edge of a glass block. The back side of the cantilever is coated with metal for the deflection detector. Complete cantilevers with integrated tips formed by the above process are shown in Fig.5.5. The tips are highly symmetric and have a tip radius of less than 30 nm. These Si 3 N4 cantilevers are mechanically more robust than Si0 2 ones, and have a well defined smooth tip shape although this fabrication process is relatively complicated and time consuming. Similar fabrication processes have been developed to produce Si02 cantilevers with integrated conical tips or tetrahedral tips [5 A]. The low mass of these cantilevers allows them to have high resonant frequencies (l0 -;- 100 kHz) with force constants (typical 0.0006 -;- 2N/m) small enough to detect forces of less than 10- 8 N. Images which show atomic periodicity of several types of crystalline samples, including graphite, MoS 2 , WTe 2 , TaSe 2 , and boron nitride (BN) were obtained applying the cantilevers described above. Wolter et al. [5.5] have pursued another method for the microfabrication of silicon force sensors. This method, which mainly involves a com110

/"

3!Jrn

I

Fig. 5. 5a-d. SEM micrographs of Si 3 N4 cantilevers with integrated pyramidal tips. (a) The Si 3 N4 film is attached to the surface of a glass block with dimensions of 2 X3 XO. 7 mm 3 . Four cantilevers protrude from the edge of the block. (b) Four pyramidal tips can be seen at the end of this V-shaped cantilever. (c) The pyramidal tips are hollow when viewed from the back side. (d) Each tip has very smooth sidewalls, and the tip appears to terminate virtually at a point, with less than 30 nm radius [5.4)

bination of wet and dry etching techniques, differs substantially from the fabrication processes described above and offers the following advantages: •

•

•

The tip of the sensor is pointing out of the silicon wafer surface rather than pointing into the silicon wafer as in the thin-film tips. Thus the use of silicon tips avoids the rather complicated process of bonding the sensor to a glass body, as described above. Tip, cantilever, and holder are made out of one piece of material. This avoids any strain due to thermal mismatch of bonded parts or stress associated with deposited thin films. All parts are single crystalline which yields exceptional mechanical properties.

The sensors made by this method have been applied in various microscopes in both attractive and repulsive modes. 111

I

5.1.2 Deflection Detection

The images in the AFM are generated by laterally scanning the sample under a fine tip while simultaneously measuring the displacement of the stylus induced by the interatomic forces. Seven methods have been used for measuring the deflection of a force-sensing cantilever with subnanometer sensitivity. They consist of two electronic ones, tunneling and capacitance, and five optical ones, homodyne, heterodyne, laser-diode feedback, polarization and deflection. In the orfginal AFM design, a tunneling junction was employed to detect the motion of a diamond stylus attached to an electrically conductive cantilever beam (Fig.5.l). Two feedback circuits were used to keep the force acting on the stylus constant. This method is convenient to set up and sensitive to the displacement of the cantilever. The disadvantage of this approach is that contamination on the sensing electrode or the back of the cantilever can prevent electron tunneling or drastically alter its distance dependence. Often erratic jumps in tunneling current are observed. Figure 5.6 shows a force microscope with a capacitively controlled lever displacement. Both the mechanical construction and the electronics are simple and lead to a very compact device. Force-induced lever displacements yield capacitance changes caused by changes in the interelectrode spacing of the control capacitor. The capacitor-control piezo is driven by feedback electronics to hold the capacitance constant. Thus, the driving signal of the piezo depends on lever deflection and is proportional to the acting force.

xyz Piezo

xy Control z Control

Fig. 5.6. Capacitive lever·displacement detection [5.6] 112

0

IL""'H'N"I-[]/

J

Beam splitter

II

Sample Calcite

-0-o/~+c-~~

Linear polarized light with 50% s, 50% P

-

--l I

I

I

'l'

Faraday Isolator

s:n::g" system-

I I I

__ J

~

t:::T;=J Wollaston 1\

I

\

I

\

,

2 Photodiodes

* *; J

A-B A+B

j

---r Electronics

Fig. 5.7. AFM with an optical differential displacement sensor [5.7)

The principle of a differential interferometer (polarization) method is presented in Fig. 5.7. The reference and sensing beams correspond here to two mutually orthogonal polarization states: the linearly polarized p-state with polarization in-plane (relative to the calcite crystal) and the s-state with perpendicular polarization, employing a polarized 7 mW HeNe laser. The beam passes through a Faraday isolator in order to protect the laser against spurious back-reflections from the optical parts. At the exit of the isolator the beam is linearly polarized at 45 0 relative to the s- and p-states, thus providing equal intensity in the reference and sensing beams. After passing through a beam expander, a beam splitter and a focusing lens, the beam is divided into reference (p-state) and sensing (s-state) parts by the birefringent prism, which in this case is a calcite crystal. Both beams are back-reflected from the cantilever. The sensing beam, however, is reflected close to the tip whereas the reference beam is reflected at the side of the shaft on which the cantilever is mounted. The deflections of the force-sensing cantilever are measured by means of the phase shift of two orthogonally polarized light beams, both reflected off the cantilever. This arrangement minimizes perturbations arising from fluctuations of the optical path length and the light source since the measured quantity is normalized versus the reflected intensity. The system is less sensitive to relative vibrations between 113

Light Photodiode

~~

I

<:±>

Lens

Fig. 5.8. The schematic diagram of optical-beam deflection AFM. (Reproduced from [5. 8J with permission)

I I I

.

• Cantilever

I

I--- 0.5 nm--..J

l-o. -i 5nrn

f--Inm

-j

Fig. 5.9. AFM images of (a) HOPG, (b) MoS Z and (c) boron nitride [5.9]

the optical parts (left-hand side of Fig.5.7) and the mechanical part (righthand part of Fig.5.7) of the AFM. In the optical-beam-deflection AFM (Fig. 5 .8) a surface feature deflects the cantilever through atomic repulsion by a distance oz. This deflection causes a laser beam being reflected off the cantilever to deflect by an angle of roughly 2oz1l, where l is the length of the cantilever, usually 100-7200 lLm. This deflection of the reflected beam is seen as a change in the difference between light striking the segments of a two-segment photodiode. Because of the optical lever, the deflection of the laser beam at the photodiode is roughly 1000 times the deflection of the cantilever. The optical detection system is very stable and reliable. The optical AFM has the disadvantage, however, that the cantilever must be large enough to reflect light without introducing too much diffraction. This is not necessary in a tunneling AFM.

5.1.3 Illustrating AFM Applications As the Atomic Force Microscope (AFM) can image conductors and nonconductors, and can image samples in air, liquids or vacuum, it is finding a wide range of applications. The first published atomic-periodicity images were of layered compounds: graphite, molybdenum disulfide, boron nitride (Fig.5.9) and mica [5.9-11]. Subsequently, gold atoms in an epitaxial film on mica were resolved in images obtained both in air and in water [5.12]. The AFM has also been used successfully to image ionic crystals of LiF in air [5.13] and NaCl in ultrahigh vacuum [5.14] with atomic periodicity. Figure 5.10 shows the AFM image of the amino acid D L-Ieucine in which the white dots represent topographic peaks of methyl groups at the end of individual DL-Ieucine molecules. The positions of the methyl groups in this 114

Fig.5.1O. AFM image of DL-Ieucine molecular crystal. (Reproduced from [5.15] with perm iss ion)

115

Fig. 5.12. (a) Crystalline mica covered with an acqueous solution. The image area is 2.6 X 2 2.6 nm . Polyalanine adsorbed on glass and (b) covered with water or (c) dry. Note that the chains appear to pack closer together without water. The image area is 3.4 nm by 3.4 nm. (Reproduced from [5.8] with permission) Fig. 5. I I. AFM image of a polymerized monolayer n-(2-aminoethyl)-IO, 12-tricosadiynamide reveals parallel rows of molecules with a side-by-side spacing of about 0.5 nm. (Reproduced from [5.17] with permission)

molecular crystal agree with the positions predicted from X-ray diffraction analysis of the sample, demonstrating that the surface is a simple termination of the bulk. The ATM also succeeded in resolving individual organic molecules at nitronyl nitroxide single-crystal surfaces [5.16] Besides these atomically flat surfaces, using an AFM, it has been possible to image organic molecules on substrates. For example, the AFM images of polymerized monolayers of AE-TDA [n-(2-aminoethyl)-IO, I2-tricosadiynamide] were observed and the parallel rows in Fig. 5 .11 represent a polymerized molecule with a side-by-side spacing of about 0.5 nm. This image was obtained with a tracking force of about 10- 8 N, which was just small enough to prevent the monolayer from being disrupted. Although the monolayer could be imaged repeatedly, it was eventually damaged after tens of images were recorded. Surfaces in air are typically covered with an adsorbed layer of water and unknown contaminants. When a tip covered with a layer of liquid contaminams comes near a sample that is also covered, there is a capillary force that pulls the tip towards the sample (with a typical value of 1O- 7 N). Operating the AFM within water can eliminate this undesirable and often destructive force and allows better control of the force between the tip and the sample. 116

Figure 5.12 shows AFM images of mica and polyalanine covered with water. The nearly hexagonal array of dark spots in the image of mica (Fig. 5 .I2a) corresponds to holes in the center of hexagonal rings of atoms in the flat plane. The holes in these rings are separated by approximately 0.52 nm. Figure 5.12b depicts the image of the amino acid polymer polyalanine dissolved in 85 % chloroform-I5 % trifluoracetic acid at a concemration of 40 ILg/ml. Operation within water allows not only better control of the applied force, but also physiologically more realistic environments. Drake et al. [5.8] reported a series of ten images of the polymerization of fibrin, the basic componem of blood clots, illustrating the potential of the AFM for revealing subtle details of biological processes as they occur in real time. In this experimem, a few drops of fibrinogen solution were first placed on a mica substrate. After a few minutes of scanning, fleeting images of fibrinogen mole'cules could be seen (Fig.5.I3a). They then removed a set of peptides from the central region of the fibrinogen molecule by adding a few drops of the clotting enzyme thrombin, thereby unmasking polymerization sites, which allows the molecule to polymerize spontaneously. Then follows a series of AFM images (Fig.5.I3b-i) recording the process of the fibrin monomers polymerizing imo larger aggregates, these aggregates growing larger and connecting to each other to form chains, and finally a fibrin net (Fig.5.I3j). For other biological applications, the AFM has also been used 117

40.0

Fig.5.B. Ten AFM images show cloning of lhe human blood prolein fibrinogen in real lime. Each image area is 450 nm by 450 nm. (Reproduced from [5.8] wilh perm ission)

Fig. 5.14. The AFM image of red bloodcells

30.0

20.0

10.0

o

10.0

20.0

30.0 ~m

to image red blood cells (Fig.5.14), crystallized lipid bilayers, DNA and purple membranes. Zeolites are crystalline aluminosilicates that have a regular pore or channel structure with a periodicity on the order of atomic dimensions. These materials, with the appropriate combination of molecularly sized holes and cation exchange ability, have found a great number of uses in the chemical industry. The AFM has been engaged to image the real time interactions of molecules or cations with a zeolite surface. The geometry of the 118

crystal c1inoptilolite is shown in Fig. 5.15a and its AFM image in water in Fig. S.1Sb. Images of the crystal in different liquids revealed that molecules could be bound to the surface in different ways; neutral molecules of tertbutanol formed an ordered array (Fig.5.1Sc), which suggests that the tertbutanol is adsorbed into the 8-membered-rings with its OH group pointing down into the pore, whereas tert-butyl ammonium ions formed clusters (Fig.S.lSd). These adsorbed molecules were not rearranged by the AFM tip when used in an imaging mode. However, when a sufficiently large force (greater than 10- 8 ) is applied while simultaneously oscillating the crystal up and down at a frequency of about 5 kHz, the tip of the AFM could rearrange the tert-butyl ammonium ions on the zeolite surface. Figure 5.15e depicts a cross "X" written in this way. This demonstration of molecule manipulation suggests new applications, including biosensors and lithography. The AFM has also been used successfully to image an Au (Ill) electrode surface showing atomic periodicity while the electrode was under potential control in a fluid electrolyte [5.19]. AFM images in electrolyte taken at +0.7 V prior to Cu deposition showed large areas exhibiting Au (111) structure. By sweeping the potential the Cu can be stripped down to an underpotential-deposited monolayer and finally returned to a bare Au (111) surface. The images revealed that the underpotential-deposited monolayer has different structures in different electrolytes. Specifically, for a perchloric acid electrolyte the Cu atoms are in a close-packed lattice with a spacing of 0.29 nm. For a sulfate electrolyte they are in a more open lattice with a spacing of 0.49 nm. As the deposited Cu layer grew thicker, the Cu atoms converged to a (111 )-oriented layer with a lattice spacing of 0.26 nm 119

demonstrates that the AFM is a valuable technique in studying other electrochemical processes in situ. Although the AFM has been utilized to image some biological samples, a major challenge for researchers is to develop new sample preparation techniques that will allow smaller details to be imaged. The sample must be rigid enough not to be damaged or distorted by the applied force. As mentioned earlier, imaging with the sample in liquid eliminates meniscus forces that can pull the stylus destructively into the sample, and thus makes it possible to use smaller forces, usually in the 10- 9 N range or less. Another possibility is to apply the AFM at very low temperatures to take advantage of the rigidity inherent in frozen biological structures. Additional technological applications for the AFM include imaging integrated circuit chips, optical and X-ray components, data storage media and other critical surfaces. An AFM can also be configured so that it measures the elastic and plastic behavior and hardness via nanoindentation, surface forces, and the adhesion of thin-film and bulk materials [5.20-22].

5.2 Other Scanning-Force Microscopies 5.2.1 Lateral Force Microscope

Fig. 5.15. (a) Tetrahedral framework of clinoptilolite (010). (b) AFM image of clinoptilolite (010) in water with inserted model (c) AFM image oftert-butanol molecules adsorbed on clinoptilolite (010) with an inserted model of one tert-butanol molecules (d) Clustering of tert-butyl ammonium cations. (e) Lithography on a layer of tert-butyl ammonium cations that was adsorbed on clinoptilolite. (Reproduced from [5.18] with permissiuon)

for both electrolytes. Images were also obtained of an atomically resolved Cu monolayer in one region and an atomically resolved Au substrate in another in which a 30 0 rotation of the Cu monolayer lattice from the Au lattice is clearly visible. The observation of a complete adsorption and desorption cycle for a metal onto another metal surface with such high resolution 120

In an AFM operating in the contact mode, the interaction force is manifested as a displacement of a cantilever (with or without a tip), where the magnitude of the force is the product of the displacement and the spring constant of the cantilever. The Lateral Force Microscope (LFM) is an extension of the AFM which allows the measurement of two forces simultaneously through the bending and twisting of the cantilever in the imaging process [5.23-25]. It measures topography just like the AFM but the additional sensor is used to measure frictional forces between the sample and the tip from the same scan. It is instructive to examine briefly the nature of the cantilever response to frictional and surface forces. In the latter case, the direction of the acting force is normal to the surface of the sample, and results in a vertical bending (z-axis direction) of the free end of the anchored cantilever. By contrast, in the lateral or frictional regime, where the cantilever is in presumed contact with the surface, upon scanning, the cantilever undergoes a torsion (twist) motion about its long axis (in the x-y plane). Both motions are orthogonal to each other. This orthogonality is what enables the simultaneous, yet independent, acquisition of topographic images and frictional data. 121

To measure the cantilever displacement, the direction of a laser beam reflected off the backside of the cantilever is monitored with a PositionSensitive Detector (PSD). In the case of surface topography, the bending of the cantilever is recorded with a segmented PSD, typically a bicell, which consists of two photoactive (e.g., Si) segments (anodes) that are separated by 10 Jlm and have a common cathode. Additionally, lateral forces induce a torsion of the cantilever which, in turn. causes the reflected laser beam to undergo a change in a direction perpendicular to that due to surface corrugation. Thus, with a simple combination of two orthogonal bicells, i.e., a quadrant PSD, one is able to measure the deflection of the cantilever independently yet simultaneously in two orthogonal directions [5.23]. This capability is unique to the optical beam deflection method which. due to its very nature, measures the orientation of the cantilever and not only its displacement. It should be noted that the torsion force constant depends critically on the tip length. Additionally, the tip should be centred at the free end of the cantilver. Both factors can be readily controlled with the use of microfabricated cantilevers and tips, as described in Sect.5.1.2. Figure 5.16 shows the AFM and LFM images of Langmuir-Blodgett film 2-(4-hexadecoxypheny1)-4,4,5 ,5-tetramethyl-4 ,5-dihydro-l H-imidazolyl-l-oxy-3-oxide on the surface of a glass substrate. These two images were obtained simultaneously in the same area by our group. The particularly larger grains in the topographic image of Fig.5.l6a reveal lower frictional forces in the LFM image of Fig. 5.16b obtained in the same area. Considering other experimental results we demonstrate that the large grains in Fig.5.16a are the defects on the glass substrate rather than the LB films. The result is in agreement with the well-known fact, namely the surface of a glass substrate should show less frictional force than organic LB films. The recent fast growing advances of nanotribology has largely benefited from the lateral force microscopy or FFM. With several fundamental findings, one can better understand the atomistic friction [5.26,27] and the frictions with solid lubricants [5.28.29]. It is further demonstrated that the friction behavior is closely related to the intrinsic ordering of the molecular layer [5.30.31] (Fig.5.17).

5.2.2 Force Microscope Operating in the Noncontact Mode As mentioned previously, the force microscope can be operated in either contact mode or non-contact mode. The AFM described in the last section and the LFM mentioned above are operated in the contact mode to measure the repulsive forces between the tip and the sample. A force microscope operated with relatively large tip-sample separation can be used to measure 122

Fig. 5.16. Three-dimensional represenlalions of Ihe AFM IOpography (a) and the lateral force properlies (b) of an organic LB film oblained simultaneously in the same scan area

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longer-range interactions between tip and sample across areas that are orders of magnitude larger than that of a single atom. Three basic forces can be mapped across a sample in the noncontact mode by detecting the deflection of the cantilever under the influence of the desired force. These consist of electrostatic, magnetic, and van der Waals forces, in the order of the complexity of the interaction. The first deals with monopoles, the second with dipoles, while the third requires a quantum mechanical treatment. These forces and their derivatives can be as small as 10- 12 Nand 10- 14 N/m, respectively, they are much smaller than those encountered with the AFM operating in the contact mode, requiring resonance enhancement techniques [5.32]. Much effort has been made to investigate magnetic force in force microscope. This kind of microscopy has developed into an independent field and is called magnetic force microscope (MFM) (Sect. 5 .2.4). The electrostatic force microscope for mapping electrostatic forces across the surface of a sample will be presented in Sect. 5 .2.5. For distances of a few Angstroms to a few humdred Angstroms. van der Waals forces are significant. They can be used to measure topography with a resolution of a few nanometers. For instance, Wickramasinghe [5.33] has developed a scanning-force microscope that works with attractive 124

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rather than repulsive forces and scans the tip 3 to 29 nm away from the surface. It allows one to measure attractive forces down to 10- 11 N that are one thousand times smaller than the forces typically encountered with the contact AFM. Figure 5.18 presents a sketch of the scanning-force microscope operating in the noncontact mode. The cantilever (tungsten or silicon) tip is attached onto a vibrating transducer which drives the tip at just above its mechanical resonance frequency (typically 50kHz). The attractive-force gradients encountered by the tip as it approaches the sample surface changes the spring constant of the cantilever and hence its resonance frequency. If the vibrating transducer is driven at a fixed frequency (50kHz) this results in a change in the vibration amplitude of the cantilever. This change is detected by a sensitive laser probe and is utilized in a feedback loop to stabilize the sample to average tip spacing, as the sample is raster scanned in x and y to record an image. The image recorded in this way represents contours of constant force gradient across the sample. In this way the force microscope can detect a surface relief of as little as five nanometers, and because it senses topography from a distance, it can inspect features inside the deep. narrow clefts. It promises to be valuable for mapping rough surfaces and for noncontact examination of finished microcircuits. The noncontact force microscope has also been used to monitor the quality of the silicon surfaces on which they are built. Figure 5.19 shows a typical scan of an anisotropically etched V-groove surface of silicon (l J.Lm period and 1J.Lm deep) clearly demonstrating the ability of the LFM to reproduce both the gross features and the fine terracing structure (down to 5 nm) caused by the etching process. 125

Fig. 5. 19. Scanning force microscope image of an isolropically etched V-grooves in silicon. Enlarged area: IXIJ.Lm 2 (5.33J

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The achievement of atomic resolution of AFM in the non-contact, especially on semiconductor surfaces under UHV condition, marked a significant progress of AFM probing capability, leading to minimized interactions and making it practical to direct comparison of STM and AFM results (Fig.5.20). Having the advantage of little electric field induced effects, it can be better used to study the point defects and their motions [5.35].

5.2.3 Force Microscope Operating in the Tapping Mode Applications of the non-contact mode operations are limited due to some fundamental drawbacks inherent to the technique. The tip must be vibrated close the sample surface with low energy because van der Waals forces are relatively weak. However, moving the vibrating tip closer to the surface increases the chances of getting the tip stuck in the water layer which covers the surface of all samples exposed to the atmosphere. Trapping the tip-tosample separation, which is typically between 5 and 10 nanometers, defines 126

Fig. 5.20. The nonCOnlact AFM image of Si (111)-7 X7. In [he upper section of the image, the tip was monatomic for about the width of a cell. The unit cell C indicates all 12 adatoms . The unit cells C and E show two adatoms that are not visible in A and B Unit cell A shows an atomic defect. The right of the two central adatoms is misplaced [5.34]

the lateral resolution of a force microscope operated in the non-contact mode. Due primarily to these limitations, the non-contact force microscope has found only limited applications. To overcome the limitations of the non-contact technique. Digital Instruments has developed a new technique for operating the force microscope, the so-called tapping mode. This technique vibrates the cantilever with a larger amplitude than the non-contact mode operation. In the tapping mode, high-aspect-ratio tips with small radii of curvature are used and the vibrating tip contacts the sample surface many times per data point. The cantilever oscillation is damped when the tip contacts the water layer and the sample surface, but the larger vibration amplitude gives the cantilever sufficient energy to overcome the surface tension of the adsorbed water layer. The force imparted onto the sample by the cantilever can be very small because small shifts in the vibration amplitude can be detected. The tapping mode allows delicate samples to be imaged with normal forces on the order of fractions of a nanoNewton and shear forces that are essentially zero. The applied force is significantly lower than the force applied by the contact AFM. The lateral resolution in the tapping mode compared favorably to the contact AFM because high vibration frequencies allow the tip to Contact the sample surface many times before it translates laterally by one tip diameter. Therefore, the tip shape defines the lateral resolution in the tapping mode just as it does with the contact AFM. In practice, this mode 127

can be used to get a topographic image of any sample regardless of its conductivity or mechanical composition. Tapping mode AFM has been widely used in studying surface properties of biological material and polymers, with great successes.

5.2.4 Magnetic Force Microscope The Magnetic Force Microscope (MFM) is essentially a kind of force microscope operating in the non-contact mode except that the tungsten or silicon tip is replaced by a nickel or iron tip which is magnetized along its length. Tips coated with ferromagnetic thin films were also investigated successfully by several groups and have became commercially available recently. Magnetic thin film tips have the substantial advantage of a significantly reduced tip stray field as compared to bulk-wire tips. Another advantage is that their magnetic properties can be controlled by choosing an appropriate coating material. Thus, it is possible to measure selected components of the sample field by coating tips with high coercivity films and suitably magnetizing them in an external field. The magnetic forces measured in the MFM are purely magnetostatic; they arise from the magnetic dipoles in the tip interacting with dipoles in the sample. When the cantilever which is oscillated at its resonant frequency is brought near, a magnetic sample and the tip encounter a magnetic-force gradient, the effective spring constant and, hence, the resonance frequency is shifted. By driving the cantilever above or below the resonant frequency, the oscillation amplitude varies as the resonance shifts. An image of magnetic field gradients is obtained by recording the oscillation amplitude as the tip is scanned over the sample. Much of the reason for this excitement is the fact that MFM is the only magnetic imaging technique that can provide high resolution (l0 --:- 100 nm) with essentially no special sample preparation. This microscope enables us to look at the structure of materials such as magnetic heads that determine the definition, uniformity and strength of the magnetic disks and other media, giving insight into both head performance and quality of the storage medium. Good-quality images can be taken even when the magnetic material is covered with a thin overcoat, an important feature when imaging many technological samples. Figure 5.21 exhibits a pair of images of a magneto-optical disk taken in two passes over each scan line. On the first pass, topographical information is recorded in the tapping mode where the oscillating cantilever lightly taps the surface, as described in the last section. On the second pass, the tip is lifted to a selected separation (typically 20 --:- 200 nm) between the tip and local surface topography. By using the stored topographical data instead of 128

Fig. 5.21. A pair of images of a magneto-<Jptical disk [5.36)

the standard feedback, the separation remains constant without sensing the surface. At this height, cantilever oscillation amplitudes are sensitive to relatively weak but long-range magnetic forces without being influenced by short-range surface interactions. Two-pass measurements are taken for every scan line, producing separate topographic and magnetic field images. The topographical image on the left of Fig. 5 .21 shows grooves that delineate the recording tracks. The right image shows magnetic force gradients as sensed in the lift mode. The gradient image gives a clear picture of the bits, in which magnetization is oriented perpendicular to the sample plane in a direction opposite to the background magnetization. In this experimental disk, the power of the laser used to write these bits was varied, creating magnetic bits of different sizes. The smallest bit shown in this figure is about 90 nm across. A resolution of 10 nm has been obtained on rapidly quenched FeNdB thin films by MFM [5.37]. By using MFM we have successfully imaged domains in very "soft" magnetic materials such as the organic LB films mentioned in Sect.5.2.I. Figure 5.22 depicts a pair of images of LB film 2-(4-hexadecoxyphenyl)4,4,5, 5,-tetramethyl-4, 5-dihydro-l H-imidazolyl-l-oxy-3-oxide. The topographical image on the left obtained in the tapping mode shows a rough surface of the films. The right image reveals that the LB films form quasi-onedimensional band structures composed of magnetic domains on a large scale up to 30 x 30 J.Lm 2 . The results demonstrate that the MFM is also a promising method for detecting weak magnetic signals originating from as thin as 10 molecular layers. In the short period since the first MFM image obtained by Y. Martin and H.K. Wickramasinghe in 1987, the field has grown rapidly. MFM is al129

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ready a powerful tool for magnetic imaging, with many scientific and industrial applications. Future advances in MFM will include improved tips, matching of tip-sample characteristics, and determination of field strengths and domain structure from force gradient images. Much effort has been given to improve the resolution of MFM [5.38, 39]. The ultimate goal is set to use the MFM technique to recognize species at the level of individual atoms [5.40].

5.2.5 Electrostatic Force Microscope Similarly to the MFM, the Electrostatic Force Microscope (EFM) also uses a probe vibrating at its resonance frequency. However, in the EFM, the probe has an electric charge. Its vibration amplitude is affected by the electrostatic forces resulting from electrical charges in the sample. The force sensitivity of the EFM suggests that capacitive variations down to 10- 21 F could be measured in a 1 Hz bandwidth. The EFM has been used to map the electrical properties of the sample's surface. For example, the distribution and concentration of dopant atoms in doped silicon plays a critical role in chip performance. A voltage applied across the gap between the probe of the EFM and the sample will mobilize the conduction electrons or holes beneath the probe leaving a charged region that exerts an electrostatic force on the tip. The consequent movements of the tip provide a precise, finescale measure of the charge and hence of the number of mobilized electrons or holes, and the concentration of dopant atoms. 130

Fig.5.23. Images (2.5X2.5J1.m 2 ) of polished sapphire using repulsive contact forces (a) and attractive electrostatic forces (b) [5.4l]

The electrostatic images can also be obtained in another way. In studies of polished sapphire, charge is deposited on the sample by applying a bias of + 10 V to an electrode on the back surface of the sapphire sample for several hours, and allowing it to come to equilibrium. Images were taken by scanning the sample and using feedback (with the opposite polarity than that used for imaging with repulsive contact forces) to maintain a constant electrostatic force on the tip. Figure 5.23 exhibits two images taken over the same area of the sapphire surface with contact forces (Fig.5.23a) and electrostatic forces (Fig.5.23b), respectively. The electrostatic mode images reveal the same large-scale periodicity as with the repulsive mode, but with an increased amplitude. This result implies that the features are more than simple topography, and possibly correspond to regions of charge accumulation. The EFM has also been applied to measure voltages on circuits, to observe discrete steps in the force versus time curve corresponding to the discharge of single charge carriers during the decay process. For example, by monitoring the electrostatic force on the insulating Si 3 N4 films which had been deposited charge carriers [5.421. The recombination was attributed to the thermionic emission of electrons from the tip or sample, depending on the sign of the deposited charge. Therefore, the force microscope has demonstrated its ability to detect individual electrons or to measure currents of the order of 10- 19 A, which may have wide-ranging applications. Another application of the EFM is the observation of ferroelectric domain walls in the ferroelectric-ferroelastic material Gd 2 (Mo0 4 ). The features of the EFM image obtained by measuring electrostatic forces from the 131

5 .3 Ballistic-Electron -Emission Microscopy

Fig.5.24. The lopographic and the electric field images of a cross section of conducting nanowires [5.36)

polarization charge at the sample surface could be explained by modeling the wall as a step function in the electric potential. With a grounded conducting tip, the force microscope operating in the non-contact mode can measure electric field gradients by oscillating the tip near its resonant frequency, depending on which side of the resonance curve is chosen, the oscillation amplitude of the cantilever increases or decreases due to the shift in resonant frequency. By recording the amplitude of the cantilever, an image revealing the strength of the electric field gradient is obtained. Figure 5.24 shows a pair of AFM images depicting a cross section of conducting nanowires embedded in a non-conducting medium. The top surface of this sample exposes the ends of the nanowires. The other side of the sample was attached to a metal sample puck with conductive epoxy and was held at approximately 7 V relative to the cantilever tip. The image on the left is topography and the one on the right is the electric field gradient above the sample. The large spots roughly 200 nm in diameter in the left image are the metallic nanowires. The electric field gradient from these nanowires is shown as measured in the lift mode as described in the last section. Note that several of the nanowires that appear clearly in the topographic image are missing from the electric field image because electrical contact to these nanowires has failed.

The discovery and application of semiconductor materials have resulted in the necessity for a complete understanding of the fundamental characteristics of semiconductor surfaces and interfaces, such as the influence of thinfilm deposition on semiconductor surfaces, the transport properties of interfaces, superlattice carrier mobilities, quantum-well depths, etc. Subsurface interface electronic properties are not directly accessible to conventional surface analytical techniques. Although conventional Schottky-Barrier (SB) characterization methods, including photoemission, photoresponse, current-voltage, and other techniques, can be used for indirect investigation of the properties of interfaces, they are limited by their lack of spatial resolution for probing the variation of SB properties over the interface plane. Ballistic-Electric-Emission Microscopy (BE EM) is the first method for direct spectroscopic investigation and imaging of subsurface interface electronic properties with high spatial resolution.

5.3.1 The Principle of BEEM The BEEM developed by Kaiser and Bell [5.43] utilizes STM in a threeelectrode configuration (Fig.5.25). The sample for BEEM investigation consits of at least two layers separated by an interface of interest, in generaL is a metal/semiconductor SB heterojunction. The STM tunneling tip is positioned near the surface of the heterojunction to emit ballistic electrons into a metal/semiconductor structure via vacuum tunneling. These low-energy electrons have typical attenuation lengths of about 10 nm in the metal

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base, and some of the injected electrons may propagate through a surface layer to the subsurface metal/semiconductor SB interface before scattering. For a base-tip tunnel bias less than the base-collector barrier height Vb' there will be no ballistic-electron current into the collector since the injected ballistic electrons have insufficient energy to surmount the energy barrier. But, as the base-tip bias V is increased above Vb (Fig.5.25b), a dramatic increase in base-collector current Ie occurs. The spatial variation in Ie reveals the differences in electronic structure at the different areas of the interface. Moreover, the Ie-V spectrum provides a direct probe of interface electronic structure, including the important SB height, defect structure at the interface, quantum-mechanical reflection of electrons at the interface, and ballistic-electron transport properties of the base film. The collector current can be described by

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lected, rather than to reduce resolution. For example, for Au on Si or GaAs with mt/m = 0.067, this critical angle is less than 6°, so only electrons within 6° of normal incidence may be collected. For a tunneling voltage just above threshold (i.e., V-Vb is about 0.3V), a lateral resolution of the order of 1 nm is achieved for a 10 nm thick base layer. The resolution obtained by the BEEM experiment is in agreement with this treatment. This demonstrates that critical-angle reflection defines the spectrum shape and spatial resolution of the BEEM spectrom, and is also a dominant effect in interface carrier transport.

5.3.2 The BEEM Experiment There is not much difference between the apparatuses used for BEEM and STM experiments. In order to keep the surface clean and to isolate the vibration from outside it is necessary for some samples to perform the BEEM experiment in vacuum or in inert gas. The criteria to be satisfied in the design of the BEEM are the same as those for the UHV STM. However, in order to measure the collector current Ie' a high-sensitivity (gain :::,,) 011 VIA), low-impedance (l00) current amplifier should be added to the electronics. The typical collector current is less than 100 pA and is weaker than tunneling current by a factor of at least 10. This leads to the stricter requirements for the stability, reproducibility, and high Signal-to-Noise Ratio (SNR) of the BEEM system. The software for data acquisition should have the capability of acquiring the STM topographic and the corresponding BEEM images simultaneously. The Ie signals should be averaged many times at each surface location to improve the spectral SNR. The specific differential resistance Re = (dV/dJ)v=o for the Au/GaAs (l00) SB interface structure has been measured as a function of Vb' It demonstrates that a BEEM signal between 10 -:- 50 pA at room temperature is dominated by noise for a barrier of less than 0.75 V. At 77 K, however, the same signal can be measured for barriers down to 0.2 V. In addition, the current-amplifier noise decreases with increasing impedance of the basecollector junction. Since junction impedance depends on thermally activated processes, for low-energy barrier interface systems it is necessary to perform BEEM measurements at low temperature to obtain large impedance and low-noise spectra. Moreover, reduction in the BEEM system's operation temperature results in reduced smearing of the tunnel tip Fermi distribution and therefore improved spectral energy resolution. Thus the BEEM experiment performed at low temperature gains advantages over room temperature.

135

5.3.3 The Application of BEEM The complexity of Schottky-Barrier (SB) formation phenomena, including the role of interface-defect formation, electrode interdiffusion, and chemical reaction, is expected to induce inhomogeneity into interface structure and electronic properties. For example, the experimental results obtained by other techniques demonstrate that the properties of the Au/GaAs SB interface are strongly affected by interface-defect formation, pronounced interdiffusion and alloy-formation phenomena between the Au and GaAs electrodes. In contrast to Au/GaAs, the Au/Si SB shows simple, reproducible SB characteristics. The topography and the corresponding BEEM images of Au/Si (100) and Au/GaAs (lOa) SB heterojunctions have been obtained. The samples were prepared by evaporation of 1.6-mm diameter Au disk electrodes, 10 nm thick, on chemically etched n-type Si(lOO) and chemically etched n-type GaAs (lOa) wafers. The STM image shows smooth topography at the Au electrode surface for the Au/Si heterostructure. The BEEM image of an Au/Si SB interface also shows homogeneous electronic properties. In contrast to the Au/Si heterostructure, a large degree of interfacial heterogeneity was observed for the Au/GaAs system, with domains of high ballisticelectron transmittance about 2 to 20 om in size. BEEM images of the chemical etched Au/GaAs (lOa) interfaces, prepared whether with or without air exposure before Au deposition, show the same heterogeneous interfacedefect structure. This persistence of the interface heterogeneity indicates that the defect structure is not simply the result of substrate surface contamination. In order to compare SB structures prepared on MBE-grown GaAs with the melt-grown case, and to investigate the role of bulk defects on interface formation, 1-~m-thick GaAs buffer layers were grown on n-GaAs (loa) substrates and an oxide strip grown by chemical treatment prior to deposition of 10 nm-thick Au layers without air exposure. A typical BEEM collector current image of this interface and the corresponding STM topography of the Au surface are exhibited in Fig.5.26. As in the case of the interface systems described above, the BEEM images of this interface again show that only a fraction of the Au/GaAs interface supports ballistic electron transmission. These results indicate that the observed defects cannot be attributed simply to the greater bulk defect density of melt-grown GaAs. It has been known that the formation of the Au/GaAs interface involves dissociation of GaAs and diffusion of Ga and As into the Au electrode. The results of BEEM investigation for this interface indicate that this diffusion process dominates the interface-formation process. Interface heterogeneity observed by BEEM imaging and spectroscopy is attributed to diffusion-induced nonstoichiometry in the form of As-rich precipitates at the interface. 136

Fig. 5.26. STM topographic and BEEM images of a Au/GaAs (100) SB structure prepared by chemical etching of a MBE grown GaAs layer without air exposure before metal deposition. The STM (upper) and BEEM (lower) images were acquired simultaneously. Both images display a 51 X40 nm 2 area. The white calibration bar on the STM image indicates a height of 8 nm. The dark regions of the image are regions of zero detected collector current. Average collector current of the Iight areas is 5 pA [5.44]

The initial GaAs surface stoichiometry is also important in determining the characteristics of the final Au/GaAs interface. The chemical treatment which was used to prepare an oxide-free GaAs surface produces a surface which has been shown to be As-rich. In order to investigate the effects of different initial GaAs surface structures, the Au layers were deposited in UHV on a GaAs substrate grown by MBE without chemical treatment, yielding a Au/GaAs interface prepared completely in situ. Diodes fabricated by this method exhibited an 1-V behaviour dramatically different from samples prepared ex situ. The usual rectifying behavior seen for the Au/GaAs system has drastically been modified, producing an ohmic I-V curve [5.38]. This ohmic behavior persisted in I-V measurements performed at 77 K. These results are interpreted in terms of increased electrode interdiffusion at the interface. Such ohmic behavior was never observed for interfaces prepared on chemically treated GaAs substrates, either melt grown or MBE grown. This drastic difference in diode behavior emphasizes the important role of surface stoichiometry in moderating the diffusion process. In order to inhibit this diffusion, Kaiser et al. [5.44] have grown a diffusion barrier consisting of 2 monolayers (l unit cell) of epitaxial AlAs (loa) on the GaAs buffer layer grown by MBE prior to Au deposition. It is evident that the rectifying behavior has been completely restored in the I-V spectra for the resulting diode structure; the derived SB height is 0.88 eV . STM-topographic and BEEM images are shown in Fig.5.27. The BEEM im137

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age disp(ays homogeneous collector current in strong contrast to the heterogeneity of the interface image in Fig.5.26. In particular, there is no evidence of large areas of zero transmittance, which are prevalent for the chemically prepared samples. This result indicates a dramatic increase in the percentage of the total interface area which is transparent to the ballistic electron probe. Moreover, BEEM imaging demonstrates that the interposition of the 2-ML AlAs diffusion barrier does not significantly attenuate electron transmission across the interface, since the magnitude of the measured collector current is comparable to that through the highest-current regions of the chemically prepared samples. Spectroscopic results also indicate that the interface band structure is unaltered by the presence of the thin diffusion barrier. These results, together with the electrical and XPS measurements, demonstrate the effectiveness of the AlAs layer as a diffusion barrier while remaining electronically transparent. The defect structure identified by BEEM. and this method of removing the resulting interface heterogeneity, are of great importance to device technology, where the central requirements are the uniformity and reproducibility of SB interfaces. BEEM can also be used for the investigation of interface band structure. GaAs has a direct conduction-band minimum at the zone center and two higher indirect minima at the L and X points of the Brillouin zone, while Si has only a single conduction-band minimum along the [100] direction. For example, the Au/GaAs spectra display a threshold region which, in contrast to the Au/Si spectra, is apparently composed of multiple thresholds. This dramatic difference is seen by comparing typical spectra of Au/GaAs and Au/Si heterostructures. 138

~1O

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of ........ ~b Fig. 5.27. STM topographic and BEEM images of a Au/AIAs/GaAs SB structure. The STM (upper) and BEEM (lower) images were acquired simultaneously. Both images display a 51 X 40 nm 2 area. The white calibration bar on the STM image indicates a height of 8 nm (5.44]

20 ~:: 25

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0 0.5

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Fig.5.28a-d. Comparison of the minima of the GaAs and Si conduction bands at the Au/GaAs interface and Au/Si interface, respectively. (a) SEEM Ie -V spectrum (dots) for a Au/GaAs heterojunction. Curve a is a zerO-i:urrent reference, curves b, cand d represent calculated spectra treating one, two and three thresholds, respectively. Spectrum d yields thres· hold val ues of O. 89, I. 18 and 1.36 eV. (b) Derivalive dIe IdV vs V for the data (dots) and the calculated spectrum d shown in part (a). The three threshold values are indicated by arrows. (c) SEEM Ie -V spectrum (dots) for Au/Si structure. The solid I ine is a calculated spectrum for the theory given in (5. I), yielding a threshold of value of 0.82 eV. (d) dIe IdV vs V for the experimental SEEM spectrum and the calculated spectrum in part (c) (5.45]

The BEEM spectroscopy for the Au/GaAs system by fitting the theory according to (5.1) with three threshold values is indicated in Fig.5.28a. For comparison, theoretical spectra for one and two thresholds are also displayed. For the case of three thresholds, the agreement between theory and experiment is excellent, yielding theshold values of 0.89, 1.18 and 1.36 eV. The first threshold value is in agreement with the commonly accepted value for the Schottky-barrier energy for Au/GaAs, namely 0.9 eV. The differences between the upper thresholds and the lower one, 0.29 and 0.47 eV, agree well with the expected relative energies of the three lowest conduction-band minima in GaAs: 0.29 and 0.48 eV for the separation between the direct minimum and the satellite minima at the L and X points, respectiveIy. As a sensitive test of the agreement between experiment and theory, the experimental and theoretical derivative spectra, dIe/dV vs. V, are compared with Fig.5.28b. The thresholds, marked by arrows, appear as steps in the derivative spectra and the theory agrees well with the experimental BEEM spectrum. The changes in slope at the thresholds show relative magnitudes which are in agreement with the ordering of the different effective masses of the three minima. It is worth noticing that BEEM techniques can be used for observing not only the multiple thresholds in the conduction-band structure, but it can 139

Fig. 5.29. STM (a) and BEEM (b) images of Si-rich CoSi 2 /n-Si (100) showing the effect of atom ic modulation on the BEEM image [5.46]

Fig.5.30a,b. Comparison of BEEM electron and hole spectroscopies of sub· surface interface conduction and valence bands. The tunnel voltage V and the Schottky barrier eV b between the Fermi levels and the conduction minimum and valence·band maximum are shown. (a) Ballistic electron spectroscopy of a metal/semiconductor interface structure formed on a n-type substrate. (b) Ballistic hole spectroscopy of a metal/semi· conductor interface structure formed on a p-type substrate [5.48]

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also be applied to measure many lc -V spectra on different locations of the heterojunction, i.e., spatial variations in the relative energies of different thresholds can be detected, providing a direct measure of the local variation of the conduction-band structure. Variation in local interface band structure may result from variation in interfacial strain or diffusion-induced nonstoichiometry. For semiconductor/semiconductor strained-layer interfaces, the local variation in band structure plays an important role in determining the interface electronic properties. The ballistic-electron spectroscopy methods are applicable to investigation of such and many other systems. BEES studies at atomic resolution further revealed that the energy distribution of the tunneling electrons are affected by the atomic lattice [5.46] (Fig.5.29). BEEM has also been employed to study Au/Si (111), Au/CdTe, NiS 2 / Si and CoSi 2 /Si interface structures, providing new information about these interfaces. Furthermore, quantum-dot structures at an Au/GaAs interface were studied by BEEM [5.47].

5.3.4 Ballistic-Hole Spectroscopy of Interfaces BEEM methods enable direct spectroscopy of subsurface interface electronic structure on a nanometer spatial scale. The energy-resolved spectroscopy methods based on BEEM have been limited to investigation of electron transport. However, the properties of hole transport in semiconductors 140

I

Base

Collector

and metals, through interfaces, and through tunnel barriers, is central to the understanding and development of quantum-well, superlattice and other important systems. Hecht et al. [5.48] developed a ballistic-hole spectroscopy of interface transport and interface electronic structure based on BEEM. This new method has enabled direct measurement of subsurface valence-band SB height and valence-band structure. Comparison of BEEM ballistic electron and hole spectroscopies of subsurface interface conduction and valence bands are shown in Fig. 5.30. In the case of BEEM the ballistic-electron distribution created by tunneling is collected after transport through a multilayer structure (Fig. 5. 30a). In contrast, ballistic-hole spectroscopy (Fig. 5.30b), is based on tunneling of electrons from the interface structure to the tunneling tip with the tip bias positive with respect to the structure (tunneling bias. V, negative). For negative V, where tunneling probes occupied states in the base, the emission of an electron from the base electrode of the interface structure creates a hole in the conduction band of the base material. Since hole-attenuation lengths are as large as several hundred Angstroms, the hole may propagate ballistically through the base electrode and to the subsurface interface. Transmission through the interface is allowed if the hole energy (measured with respect to the base conduction-band minimum) is less than the threshold defined by the valence-band maximum (E F eV b)' Since the ballistic-hole energy is directly determined by the applied 141

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Tunnel vollage [VI Fig. 5. 31a, b. Experimental (circle symbols) and lheoretical (solid lines) ballislic carrier spectra for the Au/Si (100) SB interface. (a) Experimental ballislic hole (negative lunneling voltage) and ballistic electron (posilive lunneling voltage) spectra measured at 77 K for plype and n-type substrates, respectively. The Au film thicknesses for the p-type and n-type SB structures are 15 nm and 10 nm. respectively. The band gap, valence-band maximum. and conduction band minimum althe subsurface semiconductor interface are clearly seen as thresholds in the hole and electron spectra. The lheoretical spectra are in excellent agreement with the experimental results. (b) Comparison of the ballistic-hole spectrum (dots) with a theoretical fit to the data and evaluated without the requirement of transverse momentum conservation on hole transport at the interface. The drastic discrepancy demonstrates the primary influence of transverse momentum conservation on hole interface transmission for th is system (5.48]

bias, an accurate hole spectroscopy of interface transport and interface valence-band structure is possible. Since the incident hole energy is simply controlled by the bias voltage, direct spectroscopy of interface valence-band structure can be performed. Ballistic-hole spectroscopy can be performed by measuring the hole current transmitted through the interface and reaching the collector as a function of the tunnel bias V applied between the tip and base, Ballistic-hole and ballistic-electron spectra for the Au/Si(lOO) system are displayed in Fig.5.3!. The combined spectra show a region of zero observed collector current bounded by two abrupt thresholds in the current. The threshold for positive tunneling bias (tunneling tip negative with respect to the base) directly indicates the position of the conduction-band minimum. As expected, for negative tunneling bias, the observed collector current is opposite in sign to that observed for positive V. The current provides a spectroscopy of ballistic-hole interface transport. Further, the ballistic-hole spec142

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Tunnel voltage [V] Fig.5.32a,b. Experimental and theoretical derivative ballistic carrier spectra for the SB structures measured at 77 K. (a) An experimental derivative ball istic hole spectrum (dots) is compared to an experimental derivative ballistic electron spectrum (open circles) for the Au/Si (100) SB interface. The asymmetry between hole and electron spectroscopies is apparent in the experimental spectra and is accurately described by theory (sol id lines). (b) Experimental hole spectrum (dots) of the valence band of the Au/Si (100) SB interface. A theoretical spectrum for a single valence band (dashed line) is in disagreement with experiment. However, a theoretical spectrum for two valence bands (solid line) correctly reproduces the experimenlal result (5.48]

trum threshold directly yields the barrier height formed by the valence-band maximum at the subsurface Schottky barrier. The detailed characteristics of ballistic-hole creation and transport are compared to those for ballistic-electrons in the experimental derivative spectra (Fig. 5 .32a). The marked difference in spectral shape above threshold between the hole and electron derivative spectra reveals a fundamental asymmetry between the collected distributions for electron and hole injection. For ballistic-electron spectroscopy at a positive tunneling bias, the collected electrons originate from the top of the tunneling distribution where the distribution is maximum. The number of electrons created per unit energy remains nearly constant with the bias. However, for ballistic-hole spectroscopy at negative tunneling bias (Fig. 5.32b) the ballistic-hole distribution originates from the bottom of the tunneling distribution where the distribution is at a minimum. Therefore, the number of holes created per unit energy decreases with increasing bias. This fundamental asymmetry between BEEM electron and hole spectroscopy is directly revealed in the experimental derivative spectra for an Au/Si (100) SB interface (Fig.5.32a). The ballistic~hole derivative spectrum displays an abrupt maximum and a sharp 143

decay resulting from the decay in the ballistic-hole distribution with tune 1ing bias above threshold. In contrast, a smooth increase is observed in the ballistic-electron derivative spectrum. It is particularly significant that the accurate understanding of this contrasting behavior presents a sensitive and important test of interface transport spectroscopy and theory.

(a)

5.3.5 Interfacial Modification with BEEM Analogue to STM fabrication on surface, storing information at subsurface region covered by a protective metal film has its unique advantage in application, and also provides an approach to probe interfacial properties. Fernandez et al. [5.49-50] firstly reported their observation that modification in subsurface electronic properties can be induced by ballistic electrons in some specific Au/n-Si system when the energy of injected electrons is several eVs higher than the Schottky Barrier (SB). Their typical modification result was an area of a few hundred angstroms in diameter with decreased BEEM current, surrounded by a ring of enhanced contrast. In addition, protrusions were sometimes observed in the topographic image, indicating slight structural changes in Au film. Since the modification results were strongly dependent on the composition of interfacial impurity layer, the authors attributed the observed variations in BEEM current to be related to the changes at the Au/n-Si interface. As a plausible explanation, they proposed that the ballistic electron current at high bias enhances the interdiffusion of Au and Si at the interface. The created Au-Si intermix layer at the interface thus decreased the ballistic transmission. However, the later discovery of locally enhanced ballistic transmission after a voltage pulse in some cases [5.51-53] has implied that the previously assumed Au-Si alloyment mechanism seems not sufficient. BEES measurements performed inside and outside the modified region have further exhibited that [here exists a huge discrepancy in their natures [5.53]. On the other hand, the dynamic studies [5.52,53] of modification process have revealed a well defined linear relationship between the area of the modified interface region and the applied voltage (Fig.5.33), as well as the duration of the applied pulse, indicating the modification is very likely to be associated with the accumulation of injected electrons. Although the underlying mechanism is not thoroughly understood, yet it is more likely that certain chemical processes might be involved. Further investigations of the compositions of the modified regions should be crucial to understand the principles of the interfacial fabrications.

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duraticn of awIie::l \dta;]e PJlse (soc) Fig. 5.33. (a) The modificat ion outcomes at Au/n ·Si (Ill) interface by a series of the volt~ge pulse~ of -4.0Y and duration from 5 ~o 30 seconds, respectively. The image area is 1200 A X 1200A. The topographic range is 20 A and the BEEM current variation covers a range of2 "730 pA. (b) The modified interface regions increase monoronously with respect ro the duration of applied voltage pulses [5.53]

5.4 Scanning Ion-Conductance Microscope

The Scanning Ion-Conductance Microscope (SICM) is designed to image both the topography of nonconductors (such as membranes) that are immersed in electrolytes, and any ion flow through that sample. As shown in Fig.5.34, the probe of the SICM is an electrolyte-filled micropipette, and the insulating sample is located at the bottom of a reservoir of electrolyte. The probe is lowered toward the sample while the conductance between an 145

Fig. 5. 34. The SICM scans a micropipelle over the can LOurs of a surface by keeping the electrical conductance between an electrode inside the micropipette and an electrode in the reservoir constant by adjusting the vertical height of the probe [5.54]

------

electrode inside the micropipette, and an electrode in the reservoir is monitored. This current serves as the feedback signal for standard scanningprobe microscope electronics. As the tip of the micropipette approaches the surface, the ion conductance decreases because the space through which ions can flow is decreased. The micropipette is then scanned laterally over the sample surface. The SICM topographic images can be obtained by measuring the voltage applied to the z-axis of the piezo-tube translator while holding the conductance constant under a feedback control during a scan. For ion current images, the current flowing into the pipette at each point is monitored as the pipette is scanned over the surface at a constant height. It is also possible to follow the topography with the AC ion current from one electrode in the bath and measure the DC ion current from an electrode below the sample surface. In practice, the ion-conductance signal should be averaged many times at each surface location to improve the signal-to-noise ratio. Just as for other scanning probe microscopes, the sample approach mechanism and the feedback system used for the SICM can be the same as for STM. The critical point is to make a robust micropipette tip with a small inner diameter. The test results reveal that the resolution of the SICM is determined by the inner diameter of the micropipette. In other words, the SICM can resolve features as small as the inner diameter of the micropipette if the noise in the ion conductance signal can be reduced to less than 1 %. The most interesting application for the SICM is not, however, just imaging the topography of surfaces at submicrometer resolution. The SICM can image local ion currents coming out through pores in a surface. Comparison of topographic and ion current images can give a more detailed picture of the type of surface features that correlate with ion channels. Figure 5.35 shows the SICM topographic and ion current images of a Nuclepore membrane filter obtained by Hansma et al. [5.54]. In this model system, ion currents come through the holes in the sample surface. 146

Fig. 5.35. (a) A SICM LOpographic image of the 0.8 J-tm diameter pores in a Nuclepore membrane filter. (b) The same image presented in a plan view. (c) A SICM image of the ion currents emerging through the pores. The imaged area is 7.8 J-tm by 4.5 J-tm for all three images [5.54]

The performance of the SICM has been greatly improved by replacing fragile glass micropipettes by a silicon microfabricated probe [5.55]. The new microfabricated probes are end caps for a 1.5 mm glass capillary. A hollow tip is fabricated in the center of a silicon membrane. An aperture is formed at the apex using microfabrication techniques including photolithography and etching, giving typical minimum diameters of 250 nm. Unlike the glass micropipette, the new tips are compact and hence mechanically robust. The tips are mounted on a flexible membrane that allows the tip to deflect away from the surface in case of a collision with the sample surface. In addition, the microfabricated probes have been designed to have a high mechanical resonant frequency, allowing scan speeds up to 50 times faster than used with glass micropipette probes. The new probes have been used to image the surface topography of a plastic diffraction grating and the ion flow through porous polycarbonate membrane filters [5.55].

5.5 Scanning Thermal Microscope

The scanning thermal microscope can be used to make topographic measurements and to map temperature variations. The tip of a thermal microscope is designed as a tiny thermocouple, for example, tungsten wire with a specially configured tungsten-nickel junction, so that its voltage is propor147

Fig. 5.36. Schematic diagram of a scanning thermal microscope. The probe has a tungsten core, jacketed in nickel but insulated from it everywhere except at the 3D-nm-wide tip [5.56)

Piezoelectric control

Fig.5.37. Surface profile of fixed red blood cells on a glass substrate obtained by the scanning thermal microscope. Each cell is approximately 7 IJ-m in diameter [5.33)

Nickel

Insulator

tional to the temperature (Fig.5.36). If a steady current is passed through this thermocouple junction, it heats up and comes to an equilibrium temperature above the ambient value. When the heated tip approaches the sample surface, its rate of heat loss increases because the solid sample is a much better heat conductor than air. The resulting drop in the voltage across the thermocouple junction depends on the distance from the sample at any point and provides a measurement of the surface topography. The voltage can be used to control tip-sample spacing in much the same way as the tunneling current is used in a STM. In operation, instead of measuring the DC thermoelectric voltage, the tip is vibrated by a few nanometers in the vertical direction, and the ac change in the thermoelectric voltage is used as a monitor of the tip-sample spacing. This renders the system immune to ambient temperature variations caused by room temperature fluctuations and air currents in the vicinity of the probe tip. Figure 5.37 displays an image of the surface of fixed red blood cells obtained with the scanning thermal microscope. The thermocouple tip cannot be made much finer than about 30 nm, which limits the spatial resolution of surface profiles made with the scanning thermal microscope. The tunneling thermometer which can, in principle, have atomic resolution, is based on the fact that as a metal probe tip approaches within 0.5 nm or so from a different metal surface, a potential difference could be measured between the probe tip and the metal surface due to the equalization of the Fermi levels caused by two-way electron tunneling across the gap. In the situation where there is local thermodynamic equilibrium in the gap region, this potential is a measure of the local surface temperature. This technique has been applied to measure the optical absorption spectrum of the gold film [5.57] and to detect differences in the chemical potential signal between molybdenum and sulfur of a cleaved MoS2 sample [5.58]. 148

5.6 Scanning Tunneling Potentiometry and Scanning Noise Microscopy A variant of STM, Scanning Tunneling Potentiometry (STP), allows simultaneous measurement of the topography of surfaces and their electric potential distribution with microscopic resolution, giving an insight into conduction through granular structures, defects, and interfaces. STP requires a minor modification of the STM: two additional electrodes at the sample. A potential difference LlV = V2 - V I is applied across the sample surface (Fig.5.38), while an AC voltage (V TI = VT1 'sinwt) applied across the tunneling gap generates an AC tunneling current which can be used to maintain a constant tunneling gap. VTI is connected to both sample electrodes to generate nearly position-independent tunneling. The DC part of the tunneling current is regulated to zero by shifting the potential of the sample by the amount V R' VR gives the local potential at the scanning position. In operation, an independent control loop whose band (DC to 1kHz) is outside the band of the gap control loop is used to maintain zero DC tunneling current by continuously causing the voltage on the tip to track the voltage on the sample as tip is rastered across its surface. The tip voltage is then equal to the sample voltage at every point on the sample surface. Figure 5.38b shows a schematic diagram of the control system. The distance is regulated for a constant AC tunneling current by means of the lock-in amplifier (LI) in series with the usual logarithmic amplifier (LG) and the control circuity (PI). The STP has been employed to study a goldisland MIM structure [5.59]. The STP is useful for measuring nanometer scale potential variations on devices such as Schottky barriers, pnjunctions and heterostructures. The voltage resolution is typically of the order of a few millivolts. These tech-

149

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niques have been extended to the EFM which was described in Sect.5.2.5 for measuring potential distributions on insulating surfaces. Another modification of the STM is the scanning noise microscope which can be considered as a STM where no external bias voltage is applied to the tunnel junction [5.60]. Instead of the tunneling current, the meansquare noise voltage from the junction is used to control the tip-sample separation by a feedback loop in the same way as the STM. This technique can be applied to maintain a constant gap resistance when the tip is scanned across the sample because the mean-square noise voltage is proportional to the gap resistance. This technique could have important advantages for special applications such as those in electrochemistry where zero average current is desirable, or for observing delicate samples where extremely small AC currents are required.

5.7 Photon Scanning Tunneling Microscopy and Scanning Plasmon Near-Field Microscopy

Another variant of the STM, the Photon Scanning Tunneling Microscope (PSTM), enables us to probe directly the evanescent field outside the confined-propagating optical fields within the sample, thereby revealing variations in these fields due to topographic changes, the index-of-refraction inhomogeneities, or modal variations within the waveguide. PSTM is the exact analogue of STM. The difference between these two microscope types is that the PSTM works with photon tunneling rather than electron tunneling, and using an optical tip instead of a metal one. Figure 5.35 represents a schematic of the PSTM. In the PSTM the sample is the surface at which total internal reflection of photons occurs unless an optical-fiber probe tip is brought in close vicinity. The optical-fiber tip is drawn to a sharp point and is piezoelectrically positioned, in a manner similar to the tip of an STM, to be moved within the range of the evanescent field. Part of this field is coupled to the fiber tip (Fig. 5.39). The photons are transmitted in the optical fiber to a photomultiplier to produce an electrical signal. Just as in an STM, the tip can be scanned across an optically conducting sample in either of two modes. In the constant-height mode the intensity of the coupled light is detected, and in the constant-current mode the voltage for piezo controlling the height of the fiber tip is measured. By analyzing the signals from each of these

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modes, topographic corugations and dielectric variations can be separated with good spatial resolution. In comparison with other optical microscopes, PSTM offers 3-dimensional imaging with subwavelength resolution. It is possible to obtain lateral resolution of one tenth of the wavelength. The resolution normal to the sample is primarily limited by the electronics and is easily one nanometer or less. The PSTM has been employed to study several different waveguide samples, such as a silicon oxynitride planar waveguide, a titanium indiffused lithium niobate channel waveguide, an optical grating, and quartz. In addition, the measurement of photon scanning tunneling spectroscopies on PSTM samples using a split optical fiber may provide important information. Very recently, Specht et al. reported a new form of near-field microscopy-Scanning Plasmon Near-field Microscopy (SPNM) [5.62]. They have achieved lateral resolution of 3 nm (A/200) at optical wavelength. SPNM is based on the interaction of extended Surface Plasmons (SP), as introduced by Ritchie [5.63], with a sharp metal tip which is placed close to the surface of the object. Theoretical considerations indicate that this interaction can be understood mainly in terms of elastic plasmon scattering as well as radiationless energy transfer from the tip to the sample. These processes depend very strongly on the distance between tip and sample and are the key to a high lateral-resolution capability. By raster scanning the tip over the sample surface, nanometer-scale maps of interaction strength (related to the topography of the sample) can be recorded. Unlike most of other near-field optical techniques, SPNM is not based on an aperture (see below), but on metal tips. Taking an example here, Specht et al. utilized a simple, electrochemically etched, tungsten tip (radius of curvature about lOnm) as the near-field optical probe. SPNM uses an ingenious way of extracting an optical signal from the tip. A laser beam incident on the object (for example, a thin silver film evaporated on a glass prism) at a defined angle of total internal reflection excites resonant oscillations of conduction electrons in the silver film which gives a strong evanescent electromagnetic field at the air-silver interface. The plasmons can be recognized by a minimum in the reflected laser intensity which can be understood as destructive interference between light reflection from the silver-air and the silver-glass boundaries. A tungsten tip diving into the evanescent field of the surface piasmons at the air-silver interface reduces the scattering of surface plasmon which radiates in all directions. This also causes a local reappearance of the total internal reflection, which is recorded as a signal for the SPNM as the tip moves over the surface. This change in reflectivity is a measure of the complete tip-surface plasmons interaction, and not just the radiation emitted by the surface plas152

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mons into a small solid angle. That is why a relatively large signal is produced from the perturbation by a very small tip. This is an important ingredient in obtaining high lateral resolution. The origin of the surprisingly high resolution of SPNM is not clear. Specht et al. claimed that the elastic plasmon scattering and radiationless energy transfer from the tip to the sample are the main physical processes responsible for the high resolution. However, SPNM does demonstrate higher resolution than any other reported optical microscopy. Additionally, this new technique may permit the detection and spectroscopic identification of single adsorbed molecules. Successful applications of PSTM have revealed the propagation pattern of surface plasmons in silver films [5.64], and the optical modes of silver clusters [5.65]. Furthermore, this technique has provided direct evidence of the electromagnetic interaction between tip and substrate, using Au (110) as an example [5.66] (Fig.5AO).

5.8 Near-Field Scanning Optical Microscopy and Spectroscopy

Another optical microscope with subwavelength resolution is the Near-field Scanning Optical Microscopy (NSOM) [5.67-69]. Optical microscopy and spectroscopy have long been key techniques in medicine, biology, chemistry and materials science. There are a few advantages of optical microscopy and spectroscopy: 153

• Universality. All materials and samples attenuate light and have spectroscopic states. Optical microscopy can be used for observing a wide variety of biological and chemical samples. • Non-destructiveness. Optical microscopy can operate with any transparent fluid medium between the objective and the sample (e.g., air, water or oil) so the sample can be viewed in its native environment. When nonionizing, visible light is used, radiation damage is negligible. Most chemical reactions are not perturbed by long-wavelength light. • Convenience. In most cases, no sample preparation is needed to view specimens with optical microscopy; however, labeling, staining and sectioning are sometimes required. Optical microscopy is inexpensive and simple to operate. Also, optical microscopy is usually safe and precautions are mostly limited to protective eye-glasses. • Real-Time Observation. The speed of optical microscopy is limited only by signal-to-noise ratio considerations, and dynamic processes can be studied with optical microscopy. By using ultrafast light sources, the speed can be extended even into the femtosecond time domain. Thus, biological phenomena, chemical reactions, crystallization etc. can be observed under the microscope as they happen in-situ or in-vivo. • Contrast. There are various contrast mechanisms available in optical microscopy which permit clear imaging of features in a broad range of sampies. These mechanisms, in addition to absorption and scattering, include fluorescence microscopy [5.67-69], polarized-light microscopy, phase-contrast microscopy, and differential-interference microscopy [5.70-73). These are not easily accomplished by other techniques such as electron microscopy or X-ray crystallography. In summary, the conventional optical microscope has numerous advantages which have made it the most popular imaging system. Even with the proliferation of different types of microscopes today, there is no other microscope which can match all the advantages of optical microscopy. However, its primary disadvantage is a fundamental limit to the resolving ability of conventional optical microscopy. The spatial resolution is about A/2, the well-known diffraction limit [5.74], on the order of half a wavelength. This has greatly limited the application of optical microscopy. It was realized as early as 1928 [5.75] that light can be apertured down to much smaller sizes with no obvious theoretical limit. Thus, smaller light sources could be fabricated. The simplest example is the passage of light through a small hole. Whatever light that passes through is confined to the dimension of the aperture in the immediate vicinity outside this aperture, due to the rules governing the "near-field" regime of evanescent waves [5. 75-79]. This principle has been discovered and rediscovered several times, 1928 [5.75], 1956 [5.76], 1972 [5.77] and in the 1980s [5.78,79]. Actually, 154

the photon "scanning probe technique" has preceded all other scanning microscopy. However, only in the 1980s was the principle followed by the optical experiments [5.78,79). More recently came the idea of active subwavelength sized light-sources [5.80-82]. Very recently, these new light sources led to single molecule detection and imaging [5.83-85].

5.8.1 Principles of Near-Field Optics Conventional ("far-field") optical techniques are based on focusing elements such as a lens. This leads to the "diffraction limit" of about AI2 [5.58]. The realization of better resolution by smaller light sources has led to the concept of Near-Field Optics (NFO). The principle underlying this concept is schematically shown in Fig.5A1. The near-field apparatus consists of a near-field light source, sample and far-field detector. To form a subwavelength optical probe, light is directed to an opaque screen containing a small aperture. The radiation emanating through the aperture and into the region beyond the screen is first highly collimated, with dimension equal to the aperture size, which is independent of the wavelength of the light employed. This only occurs in the near-field regime. To generate a high resolution image, a sample has to be placed within the near-field region of the illuminated aperture. The aperture then acts as a subwavelength-size light probe which can be used as a scanning tip to generate an image. Therefore, this optical microscopy is called Near-field Scanning Optical Microscopy (NSOM) [5.67-69]. Unlike Scanning Tunneling Microscopies (STM) or Atomic Force Microscopies (AFM), imaging in NSOM is via the interaction of light with the surface by either a simple refraction/reflection contrast, or by absorption

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and fluorescence mechanisms. The advantages of NSOM are its non-invasive nature, its ability to look at non-conducting and soft surfaces, and the addition of a spectral dimension, the latter not existing in either STM (room temperature) or AFM. The potential for extracting spectroscopic information from a nanometer-sized area makes it particularly attractive for biomedical research and materials science.

5.8.2 Optical Probes for Near-Field Optics The light source which is the "heart" of the NFO technique has to be: (1) small, (2) intense, (3) durable, and (4) spatially controlled. As is well known, the tip's size determines the resolution, provided that the tip can be scanned close to the sample. In addition, to perform successful microscopy or nano-spectroscopy, the issue of contrast is of the highest importance. There are two major probes used in NFO: metal-coated glass micropipettes and nanofabricated optical-fiber tips [5.67-69]. Optical fiber tips and micropipettes are easily fabricated to sizes of approximately 50 nm, and the smallest nanofabricated optical fiber tip reported to date is about 20 nm [5.68]. The fabrication of miniaturized optical probes has keyed the development and application of NFO in a wide variety of fields. The first step in the probe nanofabrication process is the pulling of micropipette and fiber-optic tips of appropriate size and shape. The second step is the metal coating of such tips. This is followed by crystal (or polymer) growing if active optical or excitonic probes are desired. Here, we illustrate the nanofabrication of optical-fiber tips. Very similar techniques have been applied in the fabrication of micropipettes [5.67,86]. Optical-fiber tips have been used in many areas [5.68,81,82] and can be fabricated either by heating and stretching or by chemical etching. The apparatus for fiber-tip pulling usually consists of a micropipette puller and a CO 2 infrared laser. The CO 2 -laser beam replaces the electric filament in the puller to heat the optical fiber for the pulling process. The laser beam is reflected by a mirror and directed to heat the optical fiber which is fixed on the puller. The details of the pulling setup and procedures can be found in a series of references [5.68,81,82,86,87]. By using appropriate program parameters and laser power, optical fibers can be tapered to subwavelength diameters. After pulling, the optical-fiber tip is coated with aluminum by vapor deposition to form a small aperture. The procedure of vacuum deposition of metals is well known but far from trivial. A specially built highvacuum chamber is employed for coating these pulled fiber tips: only the fiber-tip sides are coated with aluminum, leaving the end face as a transmissive aperture. To make it into a light source, a visible or UV laser beam is coupled to the opposite end of the pulled tip. This probe delivers light very 156

efficiently since most of the radiation is bound to the core until a few micrometers away from the tip. A randomly chosen 0.1 J.Lm optical fiber probe gives 10 12 photons per second [5.86,88]. Using the same puller, glass micropipettes have been pulled with different subwavelength diameters. Both optical fiber tips and micropipettes have been used as optical nanoprobes. Near-field optical nanoprobes can be classified into three different kinds: passive optical probes, such as coated micropipettes or small holes on a screen [5.67,78,79], semi-active light sources, such as optical fiber tips [5.68,69,81], and active light sources, such as nanometer crystal light sources [5.80,82,88]. Compared to a hollow micropipette tip, a nanofabricated optical fiber tip is a "semi-active" photon tip which is orders of magnitude brighter, easily coupled to an optical source and at least as mechanically sturdy as a micropipette. It is interesting to notice that the top of a fiber tip is really very resistant to breakage. The photochemical stability for optical fiber tips is excellent and under very intense illumination it is the heat that damages the aluminum coating at the tip. Both probes have been made around 500 A in diameter without difficulties in applications as light sources.

5.8.3 NSOM Operation The two most important modes of traditional optical microscopy are transmission and luminescence. For NFO's chemical and biological applications these will probably continue to be the most important modes, especially if one includes nano-spectroscopy (see below). Also, various reflection and collection mode NSOM techniques have been devised [5.67-69]. Figure 5.42 gives several arrangements used in NSOM. There are several contrast methods: absorption, refractive index, reflection and fluorescence (luminescence), and not all of them are well understood. One can also count polarization [5.70,71] and spectroscopy as separate modes of contrast. Furthermore, there is a large number of quantum effects, such as energy transfer, energy down-conversion and energy quenching [5.69]. The simplest optical contrast mechanisms in the near-field regime, e.g. refractive index, are not yet well understood [5.93] and thus they are under intensive study. Actually, the microscopic quantum effects are better understood than the mesoscopic (near-field) optical interactions. The most important consideration for sample preparation is sample roughness. It is limited by the probe shape in the most obvious way (the same as for all scanning probes). The sample thickness is an important factor for all transmission (forward scattering) modes of operation, but not for the reflectance (back-scattering) and some "collection" modes. The near-field approach couples the optical resolution with the 157

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distance from the probe; the higher the desired resolution, the thinner the required sample. On the other hand, the contrast mode (absorption, refractive index) may limit the thinness of the sample. Even the fluorescence mode may be limited by the thinness of the sample i.e., the absorption cross section. However, this can be overcome by intensity, by auxiliary fluorophores or by quantum mechanisms (energy transfer). Thus, the various luminescence modes appear to be the most promising modes of forward-scattering near-field microscopy. The best resolution to date has been claimed [5.68] to be about 12 nm (with 514nm light). Presumably this was achieved with a 20 nm diameter aperture. A signal of 50 nanowatts has been claimed for an 80 nm aperture [5.68]. Also, NSOM has successfully been applied in single-molecule detection (see below) [5.83-85]. An example would be the study of the domain boundary and the web structures of phospholipid monolayers with SNOM [5.94] (Fig.5.43).

5.8.4 Near-Field Scanning Optical Spectroscopy Near-field Scanning Optical Spectroscopy (NSOS) [5.69,95] is based on NSOM. It basically adds one more dimension, spectroscopy, to NSOM and can be used to obtain spectra of various nanostructures, such as nanocry158

Fig. 5.43. FFM images of DPPC/O. 5 mol % Bodipy-PC monolayer sampled at different surface pressure. (a) 7mN/m; (b) IOmN/m; (c) 30mN/m. (d-f) are th NSOM images of these sample, respectively. The web structure in (e) and (f) reflect the presence of nanoscale crystals with hexagonally packed lipids [5.94)

stals and quantum wells. NSOS inherits all the advantages of NSOM and adds a spectral dimension to the near-field optics technique. The ability to obtain spectroscopic information with a naometer-sized resolution makes NSOS very promising for a wide variety of scientific researches. Examples include the detection of fluorescent labels on biological samples and isolating local nanometer-sized heterogeneities in microscopic samples. People have studied microscopic crystals in order to demonstrate that nanoscopic inhomogeneities can be detected in what might at first appears to be a homogeneous sample [5.95]. The eventual goal is to obtain spectroscopic information with single molecules. The NSOS apparatus is quite similar to that of NSOM [5.69]. In NSOS an optical probe with an emissive aperture that is submicrometer in size is positioned such that the sample is within the near-field region. With piezoelectric control of the fiber tip, the tip can be accurately positioned over a fluorescing region of the sample and a spectrum recorded. Excitation of the sample can be either external with detection through the fiber tip or with the fiber tip itself and subsequent detection of the emitting photons. This means that it is not necessary for the sample to be of any particular thickness or opacity, however it should be a relatively smooth surface. Optical 159

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probes used in NSOS are the same nanometer-size optical-fiber light sources for sensor preparation [5.68,69]. The experimental apparatus for measuring fluorescence spectra with high spatial resolution is sketched in Fig.5.44. Here a 422 nm line from a HeCd laser is coupled into an optical fiber with a high-precision coupler. The fiber tip is mounted in a hollow tube of piezoelectric material which is positioned by the usual STM control electronics. The sample (deposited on a glass slide) is mounted on the nearfield microscope such that it is perpendicular to the exciting tip and the entire apparatus rests on the base of an inverted frame microscope with a reflected light fluorescence attachment. Excitation of the sample via the fiber tip generates fluorescence which is collected by an objective, filtered (to remove laser light) and collimated before exiting the microscope. The fluorescence is then focused onto an Optical Multichannel Analyzer (OMA), and thus the data are collected and analyzed in a computer. Different samples have been studied. For example, films of a 1.0 wt. % mixture of tetracene in polymethylmethacrylate were prepared by spin-coating a dichloromethan solution on a glass slide [5.95]. Tetracene/PMMA films examined under the fluorescence microscope show microaggregates of tetracene with an average size of about 10 p.m embedded in the polymer. 160

The background fluorescence from the film appears greenish-yellow and is presumed to be from either isolated molecules or crystals which are of a size smaller than what can be resolved with the conventional microscope. What is surprising about the aggregates is that this fluorescence ranges in color from green to yellow to red. Thus, the macroscopic fluorescence spectrum obtained with conventional (far-field) light source excitation is very broad, containing contributions from the background and all colors of aggregates. With NSOS it is then easy to excite a specific aggregate and record individual clusters' fluorescence spectrum.

Micro-spectroscopy has often been utilized for chemical analysis and biological intracellular analysis. For instance, a reagent is introduced into the cell (e.g., with a micropipette under the microscope) and the color or spectrum of the cell changes and provides information about the pH or the calcium content of the cell. More recently, fiber optical chemical sensors have been introduced for such measurements [5.96]. However, their spatial resolution has been limited by the physical size of the optical fiber, typically 100 p.m. A spin-off from NSOM technology is the development of submicrometer, subwavelength Near-field Optical Chemical Sensors (NOCS) [5.82,88]. Their chemical preparation by photopolymerization is based on near-field optical excitation, which limits the size of the produced probe [5.80]. In addition, the sensing occurs in the near-field regime of the optical excitation, thus highly increasing the sensitivity per photon and per sensor molecule. This near-field operation has decreased the volume needed for non-destructive analysis to well below a femtoliters [5.88]. Such a subwavelength pH sensor is schematically shown in Fig.5.39. The first biological application of submicrometer NOCS [5.82,97] was demonstrated for lO-day and l2-day-old rat conceptuses. The NOCS consist of an aluminized fiber tip with a copolymer supertip containing the pH sensitive dye [5.88]. The analysis is based on ratios of fluorescence intensities at different wavelengths of the same spectrum, or on ratios of fluorescence intensities at different wavelengths of two different spectra obtained by two different excitations (ratios of ratios), providing for internal calibration [5.82]. The intra-embryo pH were 7.55 for 10-day rat conceptuses and 7.27 for l2-day rat conceptuses, respectively. These values are in good agreement with the reported results for "homogenized" rat conceptuses samples, where more than 1000 embryos had to be crushed. In contrast, only one single embryo was needed for the pH measurements via miniaturized NOCS. In addition, chemical dynamic alterations in pH of intact rat conceptuses, in response to several variations under their environmental condi161

Fig. 5.45. Schematic drawing of subwavelength optical fiber chern ical and biological sensor

tions, have been measured. This is the first time that such an experiment has been carried out on a single and live rat embryo. The ability of the sensors to measure pH changes, in real time, in the intact rat conceptus, demonstrates their potential application for dynamic analysis in small multicullular organisms and single cells. Compared to conventional devices [5.96], a thousand fold miniaturization of immobilized optical-fiber sensors, a million fold or more samplesize reduction and at least a hundredfold shorter response time have been achieved by combining nanofabricated optical-fiber tips with near-field photopolymerization. Also, the submicrometer sensors have improved the detection limits by a factor of a billion [5.88].

copy. It has been called Scanning Exciton Microscopy (SExM) [5.98]. It has also been called Molecular Exciton Microscopy (MEM) [5.99]. MEM is conceptually quite similar to STM. The excitons "tunnel" from the tip to the sample. However, there is no driving voltage or field. Rather it is the energy-transfer matrix element which controls the transfer efficiency. Its unusual matrix elements allow for the highest sensitivity to distance, higher than that of STM and comparable to that of AFM. In addition. the most striking result of this direct energy transfer is its ultrahigh sensitivity to isolated or single molecular chromophores. The quantumoptics energy transfer is highly efficient within the range of the Forster radius. Thus, a single excitation could be "absorbed" by the sample acceptor. In contrast, based on the Beer-Lambert law, about a billion photons are needed to excite a single acceptor in the absence of other acceptors. Furthermore, as the distance range is limited to about 10 nm for the direct energy transfer, MEM is as much a near-field technique as STM or AFM, i.e., very sensitive in the single digit nanometer range and much less sensitive beyond 10 nm. However, in combination with conventional NSOM, the range can be extended to about 200 nm. Thus MEM is a technique which is able to "zoom in" from macroscopic to nanoscopic distances. Obviously such a "zooming in" enhances the speed of operation. It also allows for a much more universal range of samples, from metal spheres and clusters to soft, in-vivo biological units. In addition, MEM can use fluorophores, metal-clusters, etc. to enhance contrast, sensitivity and resolution with the help of NSOM. It can also be used in conjunction with lateral force feedback, in the same way as NSOM.

5.8.7 Single-Molecule Detection by Near-Field Optics 5.8.6 Scanning Exciton Microscopy The concept of active light sources enables a totally new mode of NSOM, based not on the blocking or absorption of photons but rather on quenching directly the energy quanta that otherwise would have produced photons. For instance, a thin, localized gold film (or cluster) can quench an excitation (or exciton) that would have been the precursor of photons. Furthermore, a single atom or molecule on the sample could quench (i.e., by energy transfer) the excitations located at the tip of the light source. For simplicity, we assume that the active part of the light source is a single atom, molecule or crystalline site, serving as the "tip of the tip". This quenching energy transfer from the excitation source's active part (donor) to the sample's active part (acceptor) mayor may not qualify technically as an NSOM technique. However, it is the best hope, currently, for single-atom or molecule resolution and sensitivity. This technique basically is a quantum optics micros162

Emerging techniques aimed at Single-Molecule Detection (SMD) have potential applications across the physical and biomedical sciences. SMD represents the ultimate goal in chemical analysis and has been proposed as a tool for DNA sequence. Traditionally, molecular structure and dynamics were observed by averaging techniques, such as X-ray crystallography, electron diffraction, and various spectroscopies. On the other hand, electron microscopy and related methods do indeed image single molecules but at a heavy cost to their integrity - observing them in a vacuum and/or under highly perturbative conditions. Recent methods such as STM and AFM come closer to the ideal but there are still some difficulties. These problems are particularly acute for the soft organic/biological molecules. In addition, the observation cannot be performed in-situ or in-vivo, and rarely even invitro. Furthermore, it is impossible or nearly unfeasible to observe the molecule dynamics. Near-field optical microscopy and spectroscopy [5.51- 48] is 163

a new tool providing hopes for highly improved imaging at a single-molecule level [5.83-85]. In a recent study [5.84], individual carbocyanine dye molecules in a submonolayer spread have been imaged with NSOM. About two-dozen isolated dye molecules are imaged within seconds. The imaging resolution is about 50 om, and the molecular location is resolved within about 25 nm in the horizontal plane and 5 nm in the vertical direction. Furthermore, the much smaller molecular transition dipole is a point detector mapping out the electric-field distribution of the near-field light source. In addition to imaging individual dye molecules, we can also obtain information on the orientation of these molecules (via polarization and transition dipole fitting). Another avenue is to obtain spectra characteristic of a single-molecule or molecular aggregate. Also, the mechanism of light-matter interaction may be different in the far- and near-field regimes, leading to different spectral selection rules and, in particular, to an enhanced cross section of light absorption (and thus fluorescence) [5.68,86]. These phenomena are an extra bonus for near-field detection. Similar SMD work has been done on rhodamine-6G molecules. The photophysics and photochemistry of this molecule have been investigated on the single-molecule basis [5.85]. In short, the NSOM approach to SMD permits the determination and localization of single-dye molecules. The advances towards nanometer-resolved microscopy, spectroscopy and chemical sensor probes promise to push chemical analysis much closer to one of its ultimate goals - the non-invasive detection of a single molecule, radical or ion, the determination of its precise coordinates and the characterization of its structural conformation, as well as its internal dynamics and energetics, as a function of time and environmental perturbations.

6. STM Studies of Clean Surfaces

The first and major applications of STM have been in the area of surface physics. Metal and semiconductor surfaces are the seat of many interesting phenomena, both of a practical and fundamental nature. Almost invariably, understanding these requires a detailed knowledge of the atomic structure of the surface. A full array of surface analysis techniques have been used to probe metal and semiconductor surfaces and have shown that in many cases the surface adopts a structure different from a simple termination of the bulle Some surfaces are so complicated or subtle that these techniques have failed to completely determine the structure. The feature that sets Scanning Tunneling Microscopy (STM) apart from all the other structural techniques is the capability to probe not only the geometric but also the electronic structure with atomic-scale resolution in real space. Therefore, STM has already led to unprecedented new insights into the surface structure of metals and semiconductors. In this chapter scanning tunneling microscopy and spectroscopy on clean metal and semiconductor surfaces will be discussed, structures of adsorbate-covered surfaces will be dealt with in Chap. 7.

6.1 Metal Surfaces The metal surface is easily contaminated in air, therefore well controlled Ultra-High Vacuum (UHV) conditions are required for accurate and reliable studies of clean surfaces. When a surface is created by cleavage or annealing, the local energy of the system increases by the surface energy. The surface is either simply strained, or it adopts a completely reconstructed bonding configuration to minimize the surface energy. While large-density corrugations (ca. O.lnm) are often observed on semiconductor surfaces due to the presence of dangling bond, those on (1 x I) metal surfaces are small «O.Olnm), as measured by helium diffraction experiments, unless they are reconstructed or foreign atoms are chemisorbed on them [6.1]. For example, metal corrugations are 50 to 100 times smaller than those observed on the Si (111)-7 x 7 reconstructed surface. Atomic imaging of a 164

165

(1 x I) metal surface, therefore, requires high lateral and vertical resolution. In Sect. 4.3.4, we have discussed the role of the tunneling tip and its particular importance in metal studies.

6.1.1 Geometric Structures The surfaces of Pt, Ir and Au exhibit a variety of surface reconstructions of which the I x 2 structure of the (110) surface has attracted the most attention. Although various models have been proposed on the basis of LEED and scattering experiments, none of these models accurately fits all of the experimental data. One reason might be the nonlocal, averaging character of diffraction experiments. As mentioned previously, local information can be obtained by STM. Thus, this unique technique can be utilized to provide experimental evidence of the basic driving mechanism for the various reconstructions as well as the disorder. For example, STM studies [6.2] of a Au (110) surface have revealed various typical features which consist essentially of clearly separated parallel hills usually running several hundred Angstroms along the [110] direction. Most of the hills are separated by 0.8 nm, thus forming the I x 2 reconstructed ribbons which are further separated by steps and I x 3 channels. This results in considerably stronger disorder along the [00 I] than along the [110] direction. High-resolution images can also reveal an increased density of I x 3 channels and a transition from a I x 4 channel to two I x2 channels. Considering the deep and symmetric I x3 channel and such I x 3 channels together with a sequence of other channels and steps, the following basic driving mechanism for the various reconstructions has been proposed: the reconstructed surface consists of long ribbons of narrow (111) facets along the [110] direction with a maximum of three free rows. Two-row facets give rise to the I x 2 reconstruction of the missing-row type, while three-row facets produce the I x3 reconstruction. Combinations of two- and three-row facets can give other local reconstructions and are the cause of disorder. In other STM studies of the same surface [6.3,4], the topograph shows I x 2 reconstruction with alternate [110] missing rows, in agreement with other experimental results. The observed ordered domain size also agrees with the broadening of the fractional-order spots in LEED measurements. These investigations also provide evidence for significant atomic motion of gold atoms on the Au (110)-1 x 2 reconstructed surface at room temperature. Sequences of images reveal that structural changes are associated with kink sites on (100) microfacets, supporting earlier suggestions that they play an important role in mass transfer. Step edges are formed by closepacked atomic rows in general. In the vicinity of domain boundaries, how166

ever, steps exposing (331) microfacets or steps along the [l00] direction are pinned by domain boundaries. These observations provide local information of direct relevance to nucleation and growth processes of Au(110)-1 x 2 phases. Due to the delocalized character of metal valence electrons the atomic corrugation of metal surfaces observed in STM is found to be much smaller than in the case of semiconductor surfaces. In fact, only a few studies report on the resolution of the individual atoms on a metal surface. Au (111) surfaces have been imaged by STM with atomic resolution [6.5,6]. In the STM study of a Au (111) film prepared by epitaxial evaporte deposition, the atomically resolved images are obtained both in UHV and in air [6.6]. The high resolution and large corrugations are attributed to the existence of a surface state near the Fermi level. Individual atoms on the Au (100)-5 x 20 reconstruction have also been imaged by Kuk et al. [6.7], but they believed that the unusually high spatial resolution and corrugations are the result of foreign atoms picked up by the tip during scanning. Al (111) is another example of atomic resolution on a metal surface. The STM image taken in vacuum shows the individual atoms on a closepacked surface of the nearly free electron metal (Fig.6.1). Following extensive cycles of cleaning and annealing (to 800K), a large fraction of the surface is found to consist of extended, atomically flat terraces of several hundred Angstroms wide, which are separated mostly by atomic steps. Atomically resolved STM images of a bare Cu(1IO)-1 xl surface and a Ni (110) surface have also been investigated, and will be illustrated in the next chapter together with the discussion on gas-induced reconstruction. Although studies on metal surfaces with atomic resolution are scarce, the main characteristics of metal surfces, such as monatomic steps, terraces and flatness can be imaged relatively easily by STM at very high resolution, as in the cases of Ag, Pt, Cr films, Cu-Al, AI-Co-Cu and AI-Cu-Fe alloy

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systems, Au on the Ni (110), Ag and Ni on the Au (111) surface, AI, In, Pt, Au, Mo and Cu-AI alloy in oil or in insulating liquids.

6.1.2 Electronic Structures Most of the STM studies on metals have concentrated on topographic imaging of surfaces, only a few spectroscopic results have been reported so far. On semiconductor surfaces, mainly p-electron character (valence- or conduction-band electrons, and dangling-bond states) contribute to the electronic tunneling while, in metals, bands of s, p, d, f or hybrid character may be present depending on the governing electron states. Sometimes a surface state can dominate the spectroscopy, and some other times chemisorption-induced states (bonding or antibonding states) can result in a change of the spectroscopy. The STS studies on Au, Ag and Ir films have been performed, a near exponential dependence of the tunneling current as a function of the tunneling gap is shown. A series of d(1nI)/d(1nV)-V curves have been measured on well characterized Au(100)-5x20 [6.9]. The STS data for different gaps show resemblances, but the peak widths and heights vary slightly with the gap distance. It is also possible to obtain a spectroscopic image of a metal surface, such as has been demonstrated for semiconductor surfaces discussed in Chap.3. Electrons in the delocalized, Schockley-type, surface states of metals behave very much like a two-dimensional electron gas. These electrons are scattered by localized potentials such as those produced by surface steps, other defects and adsorbed species. These scattering phenomena have a profound influence on a number of properties of the solids, including its electrical resistivity, thermoelectric power, magnetic susceptibility, and electromigration. The electron-scattering processes are usually studied indirectly through their influence on the above material properties. Davis et al. have developed a simple theory of the effect of monatomic steps on the local density of states and on the conductance of a STM [6.10]. This theory is based upon reflection and transmission amplitudes for surface electron waves. The reflected electron waves can interfere with incident waves leading to an oscillatory Local Density Of States (LDOS). These LDOS oscillations were shown to be readily observed in spatial maps of dI/dV or its normalized form (dI/dV)(I/V). The theory for the conductance with the STM tip near a single step and in the center of a pit is shown to agree well with experiment. The formation of standing waves by the scattering of surface state electron at steps and defects on metal surfaces were first observed on the Cu (Ill) surface [6.11]. The constant-current STM images of the surface 168

obtained at low temperature shows three monatomic steps and about 50 point defects. The measured variation in dI/dV as a function of distance from a monatomic step reveals that the wavelength of oscillation in surface LDOS changes as a function of energy. The wavelength smoothly increases as the energy is lowered with respect to E F . At ::::: 0.45 eV below E F a transition occurs in which the LDOS sharply decreases in magnitude. Similar behaviour is also seen in the LDOS oscillations emanating from point defects on a surface. Complementary information regarding the LDOS of the Cu (Ill) surface can be obtained by measuring a full dI/dV spectrum at one point on the surface. The most dramatic feature in the spectrum measured over a clean section of terrace is the sharp drop in dI/dV at V ::::: -0.45 V. This corresponds to a sudden decrease in the surface LDOS at energies more than 0.45 eV below E F . The clean terrace dI/dV also displays an oscillatory component, as one might expect given the energy-dependent wavelength of the spatial LDOS oscillations. The spectrum measured over the step edge differs in that there is no sharp decrease at V ::::: -0.45 V. The existence of these surface-state standing waves were also observed on an Au (Ill) surface at room temperature by Hasegawa and Avouris [6.12]. The tunneling spectrum obtained over a terrace site of an Au(lll) surface shows a band characteristic of the surface state with an onset at about 0.45 eV below E F . In a tunneling spectrum obtained directly above the step, however, the surface-state feature is absent: a clear demonstration of the strong perturbation that the step exerts on the surface state. In addition to the topographic image of the Au(lll) surface, a spatial map of the quantity (dI/dV)/(I/V) for V = +0.15 V have also been obtained at 3 A intervals over the same area. It has been shown that the value of (dI/dV)/(I/V) provides a rough measure of the LDOS. Thus, the spectroscopic image corresponds to a map of the surface LDOS at an energy of 0.15 eV above the Fermi energy level. The spatial spectroscopic map shows a high LDOS right at the step, which may be an indication of the formation of a localized stepstate. Most importantly, the value of (dI/ dV)/(IIV) oscillates near steps with a period of about 16 A in the direction perpendicular to the step. This oscillatory structure is due to the standing waves formed by the scattering of surface state electrons at the steps. Figure 6.2 exhibits a series of (dI/dV)/(IIV) for different sample voltages as a function of the distance from the step. From these curves it is shown that the peak positions shift with voltage; they are drawn closer to the step, and period of oscillation becomes shorter with increasing voltage. Since the voltage at which (dI/dV)/(I/V) has been calculated corresponds to the energy of states measured from the Fermi level, these result indicate that LDOS at a higher energy has an oscillatory structure near steps with a shorter period. Such oscillations are expected due to the interference between the incident and reflected Bloch waves by the step. 169

~/

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Fig. 6.2. Plots of (dIldV)/(lIV) at different sample voltages as a function of the distance from (he step on Au(1II), showing shifts in peak positions and changes in the period of the oscillatory structure

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Ph. Avouris and co-workers at the IBM T.J. Watson Research Center have studied the Ag (111) surface. Figure 6.3a presents a 3-dimensional topograph (at +0.170V) of the surface showing two surface steps. Figure 6.3b is the corresponding LDOS map obtained at +0.43 V. This map displays clearly not only the oscillations associated with the steps, but also scattering by an impurity atom which is barely visible as a dark spot (see arrow) in the topographic image of Fig.6.3a. The (dIldV)/(I1V) curves measured in the direction normal to the Ag (111) step behave in a manner qualitatively similar to the Au (111) LDOS oscillations, but there are quantitative differences. Most important, the oscillations are weaker in Ag. The amplitude of the oscillations increases with increasing voltage, but for a high voltage it appears to decrease again. From the periods, peak positions, and intensities of the standing waves, the energy dispersion of the surface states, the scattering phase shifts as a function of energy and crystallographic direction, and the coherence length can be determined [6.12]. Figure 6.4 displays dispersion data for Au (111). The solid lines gives the dispersion determined by angleresolved photoemission [6.13]. One sees that the two measurements agree closely with the Fermi energy, but significant differences appear as one moves away from E F . The STS results lie close to the dotted line in Fig. 6.4, which represents the average (k II + k F )/2. Unlike photoemission, STS measures the dispersion in the perturbed region of the surface near the step. The step potential induces an oscillation, the so-called Friedel oscillation, in the total surface density of the form: cos(2k F x). The LDOS oscillations discussed here are part of the Friedel oscillation involving states in a narrow energy range. The intensity of the oscillations, in general, increases with increasing electron energy due to an increasing coherence length and a dec170

Fig. 6.3. Top: A three-dimensional STM topograph of a Ag (III) surface showing areas of three terraces (V S = +0.17 V). Bottom: Three-dimensional (dIldV)/(IIV) map at +D.44 V of the same area depicted in the topograph. The arrow points to an impurity atom. (Photograph courtesy of Ph. A vouris)

6

4

:> ~

2

>,

e.o

Fig. 6.4. Energy dispersion of the surface state of Au (III) measured by photoemission (solid line), obtained by STS (circles), and calculated from k = (k II + k F )/2 (dotted line); k F is the wave number at the Fermi level, and k II is the wave number measured by photoemission. (Courtesy of Ph. Avouris)

<)

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0.1

0.2

0.3

Wave number [A-I] 171

Au (Ill) 400 x 400 A2 o = Tip touch

Frame time = 8 min

Room temperature

Fig. 6.5. 400 X400 A2 topographic images of Au (111) surface taken every 8 minutes. The indentation was introduced at frame I by the tunneling tip [6. 14]

specific topographic features or composition, examine the initial state of the surface accurately, and then observe how the surface features change with time. The area can be as large as 300 x 300 nm 2 or more than enough to contain complicated structures such as depressions, hillocks, and very large multiple steps. Obvious extensions of this technique would make it even more useful. The ability to resolve individually diffusing atoms would allow a detailed study of their positions with time and provide a value for the local diffusion coefficient. Fast scanning rates are possible with the improved STM design and adjusting the temperature will allow individual atoms or groups of atoms to be slowed or frozen in place. Microscopic data would allow for a comprehensive theoretical description of diffusion on a macroscopic scale. Phase transitions of metal surfaces can be induced by changing the sample temperature and introducing adsorbates. Surface phase transitions have received increasing attention as equilibrium surface structures are better understood. Actually, structural, order-disorder and melting transitions on semiconductor surfaces have been studied by STM, which will be discussed in the section to follow and Chap. 7. STM has also been employed as a means for surface modifications, either by touching the tip to the surface or by the application of a voltage pulse to the tip. For further details the reader is referred to Chap.9. STM is also successful in observing diffusion of Ag clusters containing hundreds of atoms on Ag(lOO) surface [6.15], and the dynamics of the adsorbed benzene molecules on Cu(lll) [6.16]. With the addition of atomic tracking technique, STM can capture the dynamic process of Si dimers on Si(OOI) surface [6.17].

reasing oscillation period. At high electron energies, anomalous oscillation amplitudes indicate increased barrier permeability.

6.2 Elemental Semiconductor Surfaces 6.1.3 Surface Diffusion 6.2.1 The Si(lll) Surfaces STM is also a useful tool for studying microscopic details of the surface diffusion process. Taking surface self-diffusion on a metal as an example, Jaklevic and Elie [6.14] have obtained time-lapsed topography on a clean, annealed Au(lll) surface showing the effects of surface diffusion of Au atoms at room temperature (Fig.6.5). Within several minutes, features such as marks made by a gentle touch of the tunnel tip are seen to change as a result of the diffusion of Au atoms over the surface. The diffusion does not depend on the magnitude or sign of the tunneling current and the voltage parameters. These experiments demonstrate that the changing topography of a metal surface resulting from diffusion can be monitored with resolution sufficient to see surface features on the scale of single atomic steps. This method is clearly very useful because one can produce a surface with 172

a) Si(III)-7x7

A clean Si (III) surface has been known for over 30 years to reconstruct into a 7 x7 structure upon heating to approximately 1173 K. Electron diffraction reveals that a superlattice exists in the plane of the surface with a unit cell that is seven times larger laterally in each direction than a simply terminated surface would be. In spite of many hints from a plethora of surface techniques, the details of this structure remained unsolved for 25 years until the STM observation of this 7 x 7 structure in real space by Binnig et al. [6.18]. Since then, this surface has been considered the standard sample for checking the performance of the STM. 173

Similar STM images and LEED patterns can be obtained for either nor p-type Si (111) surfaces after annealing in vacuum at 1173 K. STM images of the Si(lII)-7x7 surface are exhibited in Fig.3.8 together with a Dimer-Adatom-Stacking-fault (DAS) model proposed by Takayanagi et al. [6.19J. The 7x7 unit cell has a rhombic configuration and contains 12 adatoms at the topmost layer and 4 vacancies at four corners which are usually referred to as corner holes. There are two triangular subunits separated by the shorter diagonal of the rhombus and the right one has a stacking fault. The STM topographic image (Fig.3.8b) at the feedback bias of +2 V closely follows the expected geometric contours of this surface, as calculated using atomic-charge superposition methods, which ignore electronic-structure contributions to the STM image. Within the cell are 12 distinct protrusions and, at each corner, a deep depression. The electronic structure of Si (Ill) -7 x 7, like the geometric structure, is also an intriguing topic. Photoemission and inverse photoemission spectroscopies have provided energy information of the occupied and unoccupied states, respectively, but unfortunatley it is the average of at least 10 atoms. The CITS (Current Imaging Tunneling Spectroscopy) methods mentioned in Chap. 3 allow real-space imaging of the spatial distribution of surface electronic states, thus one can identify these states with specific features of the structure of the 7 x 7 surface. Real-space images of these states can be subsequently obtained by taking the difference between current images just above and below the observed onsets in conductance. As shown in Fig.3.8c with the aid of a CITS image taken at bias voltages between 0 and -0.35 V, most of the tunneling current arises from the dangling-bond states on the twelve adatoms. More current arises from adatoms in the faulted half of the unit cell than in the un faulted half, and in both halves of the cell more current comes from the three adatoms adjacent to a corner hole than from the other three. The strong left-right asymmetry of the two triangular subunits in the filled-state image corresponds to the asymmetry due to a stacking fault in the subsurface layers. A nearly identical image is also obtained at the lowest positive bias of + 0.15 V. The similarity between images at the smallest positive and negative bias voltages suggests that they involve tunneling through the same metallic state, in agreement with UPS, IPS and energy-loss studies. Figure 3.8d is one of the CITS images taken at voltages between -0.6 and -1.0 V. Three regions of high current exist between the six adatoms in each half of the unit cell, resulting from the dangling bonds on the atoms in the layer beneath the adatoms. The CITS image can also be obtained with voltages between -2.0 V and -1.6 V [6.20J. The state is observed as regions of higher current density surrounding the adatoms where Si-Si back bonds are expected, and also as a diffuse circular patching at the corner hole where additional backbonds are exposed to the vacuum. This spatial distribution demonstrates that the 174

state observed at -1.75 eV in photoemission is a back-bond state instead of a dangling-bond state, as has been suggested. The closely related surface reconstructions of Si (111)-5 x 5 and 9 x 9 have also been examined by STM. Extending the interpretation of the 7 x 7 tunneling image to those of the 5 x5 and 9 x9, it is seen that the DAS model can be scaled down to the 5 x 5 and up to the 9 x 9 structure, accounting for the protrusions as adatoms on T 4 sites on the mesh halves, and the excess of filled-state signal between corner and middle protrusions as rest-atom dangling bonds. b) Si(111)-2xl

Apart from the Si (111)-7 x7, the 2 x 1 structure on the cleaved Si (Ill) surface has understandably received considerable attention. Mainly two classes of models have been proposed for the surface: the buckling model and the 1r-bonded chain model in which the (Ill) surface reconstructs to form zigzag chains of atoms directed in the [0 IIJ direction. The latter model has been confirmed by numerous subsequent experiments. The first STM study of the Si(111)-2x 1 surface [6.21J observed a dominant [211] corrugation with an amplitude of 0.054 nm. This amplitude is inconsistent with atomic charge densities expected for a buckling-type surface structure. Furthermore, the voltage dependence of the corrugation also demonstrates that the observed charge densities are not enhanced by possible charge transfer between dangling bonds in the buckled geometry. These results definitively exclude the buckling model as a possible structure for the Si (111)-2 x 1 surface, but are consistent with a 1r-bonded surface structure. Spectroscopic studies of this surface have also been performed over the entire energy range from -4 to +4 eV relative to the Fermi level. A flat spot with a width of 0.45 V exhibited in the I-V curve can be identified as arising from the 0.45 eV gap. This is in good agreement with the results obtained from optical absorption and that were predicted theoretically with the use of the 1r-bonded chain model of this Si (111)-2 x 1 reconstructed surface. As we know, the differential conductivity dlldV can provide a direct measure of the surface Density Of States (DOS). The obtained conductivity curves dIldV show the structure of the two 1r-bonding surface-state bands, together with an identified surface resonance at 2.3 eV arising from the critical point of the conduction band. The band width of the occupied band (0.8eV) is narrower than that of the unoccupied band (l.OeV), in agreement with theoretical predictions for the 1r-bonded chain. The magnitudes of the wave vectors of these surface states are obtained from the dependence of the tunneling current on the tip-sample separation (dIlds). States around the surface-band gap show an enhancement of their inverse decay length, indicating that they originate from the edge of the surface Brillouin 175

zone. By the observation of a phase reversal in the [011] direction it is found that the surface states on either side of the band gap have opposite polarities. This unique feature demonstrates that the STM images of this surface are dominated by the spatial dependence of the surface electronic structure, and not by the geometrical positions of the atoms. The occurrence of a band gap for the 2 x 1 surface states produces dramatic effects on the STM images. In particular, at voltages corresponding to energies just inside the band gap an enhancement of certain types of disorder-related features in the images is observed. These enhanced features are believed to arise from charge densities of some disorder-related state on the chains. States inside the gap tend to propagate along the chains, and can be associated with small tilts or translations of the chains in response to strain arising from nearby structural defects. Further investigation of this surface involves the determination of geometric structure from the observed STM images. The states constituting different dangling bonds, in general, lie at different energies and must therefore be imaged at different bias voltages. Figure 6.6 represents STM images of the cleaved Si (111)-2 x 1 surface at various bias voltages. Two different regions of the step edge are clearly visible in Fig.6.6a, and both regions have unit periodicity along the step edge (in the [011] direction). Figures 6.6b,c display the same region of the step, but imaged simultaneously at voltages of + 1.5 and -1.5 V. Tic marks on the images denote the step edge. We see that along the step edge the maxima in the images shift by half a unit cell (O.I92nm) in the [011] direction from Figs.6.6b-c, thus demonstrating the existence of two dangling bonds per unit cell along the step edge. One dangling bond is seen at a positive voltage, and the other at a negative voltage. The comparison between experimental and theoretical re-

(3)

\

suits leads to the conclusion that most observed steps are of the [2 II] type with a single periodicity along the step edge, and some regions of these steps are observed to reconstruct, forming nearest-neighbor dangling bonds along the step edge, that is, a 7r-bonded reconstruction of the step. This identification on the Si (111)-2 x 1 surface may lend some support to indications of related reconstructions on the Si(100), Si(112) and Si(113) surfaces. From the above discussions it is obvious that the form of the reconstruction of the cleaved Si (111) surface differs from that of the flash annealed surface, indicating that the properties of the semiconductor surface are sensitive to the surface preparation procedure. An STM device has been used to study the effect of cooling rate after annealing on the surface reconstruction of Si (111). The silicon annealed above 1140 K forms an apparent 1 x 1 structure. When the specimen is cooled slowly over several minutes, a sharp 7 x 7 LEED pattern is obtained, and the STM images show areas of well-ordered 7 x 7. In this slow-cooling regime, small areas of well-ordered 2 x 2 reconstruction have also been seen. As the silicon cooling rate is increased, the 7 x 7 LEED spots become less distinct. In contrast, the STM images reveal small area of well-ordered 2 x 1 reconstruction typically a few hundred Angstroms across, and very similar to the larger areas of 2 x 1 seen on cleaved Si (111). These results suggest that with rapid cooling (typically a few hundred degrees per minute), the high-temperature 1 x I surface is unable to form the energetically favorable 7 x 7 surface. The 2 x 1 structure is most easily formed, as is seen, on cleaved surfaces. As the cooling rate decreases, the surface is more likely to attain the 7 x 7 structure, but may be trapped in the 2 x 2 structure which forms the basic building blocks of the 7 x 7 reconstruction. A 1 x 1 structure can also be formed at room temperature by rapid cooling of the high-temperature surface, laser annealing, or stabilized by impurities. STM images exhibit very small areas of 2 x 2 and c 2 x4 reconstructions to be present on a laser-annealed surface.

6.2.2 The Si(OOl) Surfaces

(b

Fig. 6.6. STM images of Si (111)-2 XI surface at various bias voltage (3) +1.2 V. (b) +1.5 V. (c) -I. 5 V. Images in (b) and (c) are acquired simultaneously. The step edge is identified by tic marks at the border of the images [6.22) 176

The detailed atomic structure of a Si(OO 1) surface has been the subject of many experimental and theoretical studies because of its technological importance. (001) is the orientation of the most commonly used Si wafer in the semiconductor-device industry. Many models for the surface have been proposed, they can be divided roughly into three classes: the dimer model, the chain model, and the vacancy model. Previously, by other experiments and theoretical energy-minimization calculations, no consistent conclusion about the geometric structure of the Si (00 1) surface was ever reached. This is due to the presence of subsurface distortions extending as much as five 177

II

12

13

14

10

Fig. 6.7. STM image of the clean Si (001)-2 X 1 surface formed by annealing at 1325 K. White regions are surface protrusions and black regions are depresssions, with a total gray-scale range of 0.1 nm [6.23]

9

8 7

6

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atomic layers into the bulk, and the difficulty in preparing a well-ordered surface. STM resolved this problem by simply showing the dimerization in its images. a) Geometric Structure Figure 6.7 depicts an STM image of a clean Si (00 1) surface. The sample was outgassed for extended periods of time at 850 K and flashed to 1325 K for approximately 2 min. This treatment makes the sample routinely exhibit sharp 2 x 1 LEED patterns at room temperature. Several features of the STM image of this surface are immediately obvious, particularly the presence of rows of oblong protrusions, as in rows 6-;-. 10. Based on the distances between the oblong protrusions within a row and that between the rows, it can be seen that the local symmetry formed by these symmetric oblong protrusions is 2 Xl, and thus it can reasonably be proposed that these oblong protrusions are surface dimers formed by dangling bonds on each first-layer atom for lowering the high energy associated with these dangling bonds. Another striking feature in Fig.6.7 is the occasional presence of zigzag structures, as in rows 3 -;-.5 and 11. These structures are attributed to rows of buckled dimers in which the direction of buckling alternates from dimer to dimer along the row. A transition from nonbuckled to buckled dimers can be observed in row 5 of Fig.6.7; in the upper left the dimers are 178

nearly symmetric while in the lower right they are strongly buckled. Both buckled and nonbuckled dimers are present in roughly equal proportions suggesting that their energies are nearly degenerate. In many places, where the dimers are not buckled, the periodicity is 2 Xl, but in other places different symmetries, such as p2 x2 and c4 x2, are observed. A detailed analysis of the corrugation profiles for the experimental results and the theoretical models at various charge densities has been performed to evaluate proposed Si (00 1) structural models. The comparison indicate that the experimental results is in excellent agreement with a dimer-type model and in disagreement with chain and vacancy models. Another important feature is caused by local surface defects with relatively high densities. In the filled-state images, the bright dimer rows are interrupted by dark areas: the defects. These defects often consist of small clusters of missing dimers as in rows 3, 12 and 13, and sometimes they take the form of individual missing dimers as in rows 7 and 9 of Fig.6.7. In regions far from defects the surface apparently consists of symmetric dimers, whereas in defect areas buckled dimers predominate. Bamers et al. [6.23] suggested that these vacancy-type defects either induce or stabilize buckling in adjacent dimers. In order to investigate the effect of vacancy defects on dimer buckling, a Keating-type strain-energy minimization for missing dimer defects has been performed. The calculation predicts that the dimers in the same row adjacent to the defect are pulled slightly into the surface and point toward the defect. This effect can be observed in rows 12 and 13 of Fig.6.7. The direct observation of atomic steps on the surface is one of the unique advantages of STM, while further clues to the bonding and energies of the Si (00 1) surface can be obtained by examining the behavior near steps. STM images also show that various types of surface steps are formed by these dimers and vacancy-type defects mentioned above. For steps along the [110] direction, the row of dimers forming the upper step edge are strongly buckled; while for the steps along [110] where both the buckled and nonbuckled dimers are present, the dimers near the step edge are always buckled. Therefore, it may be concluded that buckled dimers may not be stable on the ideal defect-free surface at room temperature. The presence of defects or steps or even impurities, including some adsorbates, may induce the formation of the buckling dimerization. The STM experiments also indicate that the step density increases dramatically, eventually leading to microscopic faceting when Si (00 1) is heated in vacuum above approximately 1400 K. Real-space images of biatomic steps on a vicinal Si (00 1) have also been observed directly [6.24]. Technologically, the tendency to form double steps has important implications for heteroepitaxy on Si(OO 1). In the growth of a zincblende crystal, such as GaAs, on silicon's diamond lattice, 179

a monatomic step on the substrate causes disorder in the zincblende overlayer, whereas a biatomic step allows the growth of coherent Ga and As layers. Two structural models have been proposed for the biatomic steps. One is the 1I"-bonded chain model, the other involves a rebonded geometry with buckling. The STM observation reveals that the topography of the surface is dominated by straight double steps running along the [110] direction which are separated by terraces of relatively uniform width. The images are inconsistent with the 1I"-bonded chain reconstruction of the step. However, observation of the transition from a single-step edge to a double-step edge suggests a buckled rebonded geometry for the double step. b) Electronic Structure We now turn to the electronic structure of the surface. On Si (00 1)-2 x 1 there are two surface states within 2 V of the Fermi level: one state comprising the Si-Si dimer bond, and the other the two dangling bonds remaining on each dimer. Theoretical energy-minimization calculations predict that the buckling of the dimers may result in the change of the local electronic structure and particularly to split the two surface states, producing a filled dimer-bond state and an empty dangling-bond state. elTs measurements [6.25] of this surface found two electronic states separated by a surface-state bandgap of 0.5 V. Along the center of the dimer rows, where the tunneling probability is highest at -2 V bias voltage, the +2 V tunnelingcurrent image shows a minimum. Thus, Harners et al. [6.25) concluded that tunneling at negative sample bias takes place exclusively through the Si-Si dimer bond, while that at positive sample bias occurs into the empty dangling bonds. When the dimers are buckled, different electronic structures on each of the two atoms are observed based on the same method. Moreover, they also found that the positive-bias topography looks like the positive-bias current image, and vice versa. This shows that the same surface electronic structure can be obtained under either set of biasing conditions and also demonstrates that under both positive- and negative-bias conditions, the "topographic" image of Si (001) obtained in the constant-current mode arise primarily from the electronic structure of the surface rather than the geometric structure. STM and STS studies of the recombinative desorption of hydrogen from the Si (00 1)2 x 1 surface were reported [6.26,27]. Evidence is found the the existence of a pairing mechanism which drives the hydrogen atoms to occupy both sites on the Si dimers. Initially, at room temperature, hydrogen atoms singly occupy the Si dimer units while at higher temperature (about 60K) these atoms tend to pair up. However, rather than being due to an attractive interaction between adsorbed hydrogen atoms, this pairing phenomenon is the result of an intrinsic 1I"-bonding interaction on the 180

Si (001)-2 x 1 surface, which favors the remaining dangling bonds to pair up on the dimers. Desorption from the saturaterd surface also results in the formation of paired dangling bonds which are localized on the dimer units. In any case, these observations place severe constraints on any proposed desorption model and suggest that under some conditions the pairing step itself may become rate limiting. Such 1I"-bonded dangling bonds are characteristic of the clean Si (00 1)2 x 1 surface, and their presence following desorption suggests that the structure and bonding of the clean surface plays an important role in the desorption process. Indeed one might expect such pairing phenomena to be important whenever the clean surface exhibits a band gap, since the final state of the desorption process must reflect the properties of the clean surface. Figure 6.8 displays the individual 11" and 11"* molecular orbitals observed on the Si(OOI) surface. This observation can be explained as follows: on the nascent surface, every surface Si atom has a pz-type dangling bond. By saturating the Si (00 1) surface with hydrogen, all the Pz -type dangling bonds are capped with a hydrogen atom. By heating the hydrogen-saturated Si (001) surface carefully, a small fraction of the hydrogen atoms are desorbed. The pz-type dangling bonds on the Si atoms are paired to form bonding and antibonding orbitals. Figure 6.8a shows the filled 11" orbital

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Fig. 6. Sa-c. Individual 11" and 11"* molecular orbitals observed by STM on Si (001) surface. (a) The filled 11" orbital which has a symmetric structure. (b) The empty 11"* orbital exhibits a nodal plane between the dimer atoms. (c) Schematics of these orbitals [6.27] 181

which has a symmetric structure. This is a typical example of the chemical bond which binds atoms together to form molecules and solids. Figure 6.8b depicts the empty 1r* orbital which exhibits a nodal plane between the dimer atoms. Schematics of these orbitals are presented in Fig. 6.8c.

c) The 2 Xn Structure An interesting phenomenon occuring on a Si(OOI) surface is that the 2x 1 structure can be transformed into the "2 xn" one upon high-temperature annealing. Although it has been known that the structure is due to missing dimer defects on the basis of LEED and ABD studies, the periodicity "n" was reported to have a variety of values such as 8, 11 and 6 < n < 10. It has been found by AES measurements that the existence of Ni impurities and the quenching rate from high temperatures are the factors determining this structure: the "2 xn" phase is formed when the Ni concentration is above 0.35%, and that the value of "n" decreases from 10 to 6 as the Ni concentration increases from 0.35 to 1.0% [6.28]. The STM studies of the 5 % Ni containing the Si (00 1) surface shows a 2 X 8 structure after the sample being annealed at 1500 K. Similarly, the surface is also composed of the buckled and unbuckled dimers as well as the missing dimer defects. Not only is the defect density larger than that of the Si (00 1)-2 x 1 surface annealed at the same temperature, but also the vacancy-type defects appear to align into long arrays orthogonal to the dimer rows, forming ordered vacancy domains. In addition, a structure which Niehus et al. [6.29] refered to as "split-off dimers" where a single dimer is located between vacancy domains, is observed. They argued that the vacancy domains seem to be related to Ni impurities and to the ordering of the dimer defects that are probably caused by dimerization of the third-layer Si atoms. However, it remains a question as to whether or not the "2 x n" phase will form in aNi-free Si (00 1) surface. Sakurai and coworkers [6.30] have studied this surface by STM under conditions where Ni could not be detected with AES. In the STM images of the Si (001)- "2 xn" surface induced by repetitive annealings at 1473 K, individual dimers and missing dimer defects forming semi-periodic arrays of defect-cluster bands are clearly resolved. The two-dimensional Fourier transformation of one of the images is found to simulate the LEED result similar to that obtained by Niehus et al. for the Ni containing sample. It seems that as long as the energy for evaporation and migration is supplied to the surface atoms, the presence of Ni impurity in the Si may be unnecessary for the formation of the "2 x n" phase. They pointed out that the variety of detailed structures of the "2 x n" phase seem to be due to slight differences in the experimental conditions or in the nature of the sample employed.

182

6.2.3 Other Silicon-Surface Structures The main interest in the atomic structure of single-crystalline Si surfaces has been concentrated on Si(111) and Si(OOI), as described above. The lack of much experimental and theoretical effort on less common Si surfaces is partly related to the more complicated reconstructions and partly related to quite limited technical applicability. For example, although the Si (110) surface has been studied for more than 20 years, its structure is still poorly understood, as compared to those of the Si (111) and (00 I) surfaces. From STM studies, it has been found that native oxide removal at temperatures up to 1350 K leads to faceting on Si (110) and cleaning at temperatures in excess of 1450 K to atomically flat surfaces with step distributions which reflect the macroscopic misorientation of the sample. However, locally the step edges tend to follow directions determined by surface reconstructions. A number of different surface structures of Si (110) have been reported, including 2x 1, 5x 1, 5x2, 7x 1, 9x 1 4x5, 16x2 and 32x2. In early work all the observed structures were considered to be the consequence of phase transitions occurring at different temperatures on the clean surface. Later, some experiments revealed a strong correlation between the amount of Ni on the surface and the resulting reconstruction with only one exception: the 16 x2 structure. Several STM observations of this surface have been carried out in which 16 x2 and 2 x5 on the clean Si (110) surface,S x 1 on the 0.007 ML Ni contaminated surface, and 4 x 5 on the 0.25 ML Ni contaminated surface have been found, and these results are also supported by RHEED studies. STM images of 16 x2 structures show a period of 50 A perpendicular to the terrace edges and 13 A along the (1 i2) direction. The internal structure shows zig-zag chains of atoms which appear to be identical both on the upper and the lower terraces [6.31]. Based on these observations, it has been suggested that 5 x 1 and 4 x5 structures are due to the existence of Ni impurities. Neither the detailed structures of Si (112) and Si (223) surfaces are known, since well-developed two-dimensional long-range order of atomic structures in the corresponding macroscopic orientations has not been observed on these surfaces. These problems still need to be addressed in the future. Aside from all of the cases described above, STM has been used to investigate the behavior of metallic overlayers, including Ag, AI, As, Ni and In on a Si surface. Most of these overlayers show a metallSi (111)- (\13 x V3)R30° structure. Some studies determined the positions of metal atoms on the substrate, and described the nucleation and dynamics of the overlayers. As will be described in Chap.7 for metal surfaces, many important results have been achieved for gas-induced reconstructions, and for physical and chemical processes occurring on silicon surfaces. rather than the physi183

cal properties of the surfaces themselves. These achievements will be reviewed in Chap. 7 . In the previous sections we concentrated on STM studies in UHV on silicon surfaces prepared by vacuum cleaving and high-temperature annealing, which provided a great deal of information on the atomic arrangements of reconstructions. However, some air-exposed silicon surfaces have also been examined by STM in air. As we know, an air-exposed silicon surface always has a native oxide layer which, although not preventing STM observation, restricts the resolution of the images to a nanometer level. Such an oxide layer can be removed by irradiating in the UltraViolet (UV) region in an oxygen atmosphere first and then by chemical etching in Hydrofluoric acid (HF). Some results obtained by XPS, UPS, FT-IR and RHEED reveal that most of the dangling bonds are terminated by hydrogen and the (100) and (111) surfaces mainly form a 1 xl dihydride phase. However, STM observations of these air-exposed silicon surfaces still present many problems to be solved although atomic resolution has been achieved in a few percent of the whole body of samples. In addition, quasiperiodic nanoscale faceting has been revealed by STM [6.32].

6.2.4 The Ge Surfaces a) Ge(lll)

Three reconstructions of a Ge (111) surface and phase boundaries have been observed with STM [6.33]. The sample reported was a 50 nm thick germanium film grown at 823 K on a substrate by Molecular Beam Epitaxy (MBE). After sputter cleaning and then annealing at 773 K for 15 min, a very sharp 7 x 7 LEED pattern can be observed. No surface silicon is detected by Auger-Electron Spectroscopy (AES), and the STM image appears quite similar to that for the Si (111 )-7 x 7 structure. The surface periodicity consists of rhombohedral unit cells with deep depressions at the corners and twelve protrusions inside. It has been known that thermal processing (heating to 948K) of this sample causes the LEED pattern to change from 7 x 7 to a centered 2 x 8 structure. The STM studies reveal some important differences but also some striking similarities to the Si(lll) surface. Figure 6.9 exhibits a STM image which includes regions of c2 x 8, c4 x 2 and 2 x 2 local symmetry, along with two double-layer atomic steps running laterally across the image. The thin lines designate 2 x 2, c4 x 2 and c2 x 8 unit cells, while the thick lines indicate three twinned domains of c4 x 2. The uppermost terrace is almost entirely single domain c2 x 8. The most striking feature of this image is the tendency for Ge (111) to display multiple phases, in contrast to 184

Si(111)-7x7, despite the similarity in both appearance and apparent height of the protrusions dominating both surfaces. A comparison of tunneling images in both filled and empty states for the Ge (111) surfaces with similar images for Si (111)-5 x 5, 7 x 7 and 9 x 9 surfaces shows the surface structure to be inconsistent with the proposed dimer chain models. Comparison with filled- or empty-state images for the laser-stabilized Si (111) -2 x 2 surface indicates that the Ge (111) -c 2 x 8 is most consistent with a simple model of alternating rows of 2 x2 and c4 x2 adatoms on t4 sites on a 1 x 1 substrate. Therefore, Becker et al. [6.33,34] concluded that all reconstructions could be most readily explained as ordered arrangements of Ge adatoms atop a bulk-like (111) lattice. The basic structural unit consists of a bulk-like Ge(ll1) lattice with one Ge adatom and one three-fold coordinated rest atom. The larger 2 x2, c2 x8 and c4 x2 unit cells then arise from particular arrangements of this small unit. The tunneling images for these adatom reconstructed surfaces can principally be interpreted in terms of imaging occupied and unoccupied dangling bonds, and the apparent heights in the data are completely dominated by electronic rather than geometric sources of contrast. The equilibrium room-temperature structure of the Ge (111) is believed to be a centered 2 x 8 reconstruction, consisting of Ge adatoms bonded on top of a bulk-terminated (111) bilayer. At a temperature near 573 K, the surface is known to undergo a reversible phase transition in which the c2 x8 structure disorders, forming a structure characterized by an apparent I xl diffraction pattern with weak half-order spots. The formation of disordered regions on the Ge (111)-c2 x 8 surface has been observed by STM at a temperature in the range 423--;-623 K [6.35]. At a temperature of 423--;-493 K, the motion of individual surface adatoms and rows of adatoms can be clearly seen. As the temperature is increased, this activity accelerates, and the rapidly moving atoms form disordered regions on the surface. These disordered regions are found to form at domain boundaries or steps, and grow continuously in size as the temperature is increased. The transition temperature where the entire surface is disordered is about 580 K. This type of behavior is precisely what is expected for a two-dimensional phase transition which is the first order for an infinite-size domain, but is continuous near domain boundaries due to the occurrence of premelting at the boundaries. b) Ge(OOI)

As for the Ge (00 1) surface, it is an interesting example of a system that possesses both a strong short-range interaction as well as an energetically weaker, long-range ordering. Unlike the Ge(lll) surface whose c2x 8 resonstruction is markedly different from the 7 x 7 found on Si (111), the 185

clean Ge (00 1) surface is very similar to the Si (00 1) in that both display a 2 xl LEED pattern at room temperature. The basic 2 x 1 reconstruction is generally accepted to entail the formation of dimers, created through pairing of nearest-neighbor surface atoms. The driving force for pairing is bond formation with an interaction energy characteristic of chemical bonds, on the order of several electron volts. Associated with the dimer bond are two surface states roughly corresponding to the bonding and antibonding orbitals of the free diatomic molecule. The STM images of Ge (001) surface are consistent with the formation of dimers by pairing of adjacent rows of surface atoms along [110] crystallographic directions. These images have low concentrations of missing dimer-type point defects, de-emphasizing the importance of 7r-bond defects for the Ge (00 1) surface reconstruction. Terraces formed from the primary 2 x 1 surface reconstruction are found to be separated by steps of 0.14 nm height, with the orientation of the reconstruction rotating through 90 at each monatomic step. Regions of local p2 x 2 and c4 x 2 symmetry are also observed, and the atomic positions in these regions, as well as in 2 x 1 structure, are modeled with use of arrangements of asymmetric buckled dimers [6.36]. Furthermore, the STM study of vicinal Ge (00 1) surface [6.37] shows the terraces consisting of dimers and missing dimer defects, which are separated by monatomic or biatomic steps. The step height depends on the tilt of the sample cut. The surface exhibits a large fraction of biatomic steps if the tilt of the cut is aligned in the [110] direction; and is dominated by monatomic steps when it cut tilted about a line close to [010]. The terraces are predominantly 2 x 1 structure. In addition, the roughening and smoothing processes have been investigated with an in situ STM [6.38]. 0

6.2.5 The GeSi(ll1) Surface The LEED pattern of the Ge-Si alloy system shows a 5 x 5 reconstruction, which is quite similar to the 7x7 pattern observed on the clean Si(111) surface. Thus, one has reasons to suspect a close relationship between the two surfaces. As discussed in the last section, the normal c2 x 8 reconstruction is observed to bear little resemblance to the 7 x 7 case and it is not obvious what feature the intermediate alloy case might actually adopt. A series of studies on this subject by STM to compare the features of Si and Ge adatom surfaces included an investigation which provided new information on Si(111)-5 x5 [6.39]. The sample consists of a Si (111) wafer with several hundred Angstroms Ge grown by MBE. After the sample has been Ar-ion sputtered back to the original interface, an approximately equal concentration of Si and Ge atoms in the near-surface region is obtained due to the atomic mixing that 186

accompanied the sputtering process. This is followed by a brief anneal at 923 K which caused the 5 x 5 LEED pattern to develop. The most striking feature in the obtained STM image (Fig.6.1O) is an ordering of large depressions, each surrounded by a hexagonal array of protrusions. These depressions may be looked upon as defining the corners of a rhombohedral unit mesh and each unit mesh is seen to contain six large protrusions. Compared with the Si (111)-7 x 7 surface, both structures are characterized by deep depressions at the corner of a rhombohedral mesh. The cells in the 7 X 7 case have 3 nm edges and each cell contains 12 protrusions. The cells in the GeSi (111) -5 x 5 case, however, have 1.9 nm edges and each cell contains 6 protrusions. Thus the two structures are seen to be very closely related if the Si (111)-7 x 7 cell is shrunk to have the proper size for a 5 x 5 geometry by eliminating all noncorner protrusions. Based on the DAS model for Si (111)-7 x 7 structure a similar model for the GeSi (111)-5 x 5 phase has been proposed, which seems quite satisfactory except for the oscillation in ada tom height. It still remains a question as to the origin of the observed less modulation in the corresponding unoccupied state image.

6.3 Compound Semiconductors and Layered Compounds STM probes the electronic properties of a surface, with the images corresponding to contours of the constant state density. For metals, these contours usually just reflect the shape of the surface potential barrier, which closely follows the surface atomic positions. As far as a compound semiconductor is concerned, however, a given wave function is often preferentially localized on specific atoms or bonds due to their chemical nature. Thus, the energy and the spatial distribution of a surface state may depend sensitively on both the chemical identity and the positions of the surface atoms. While STM has enjoyed enormous success in elucidating several classical problems of geometric and electronic structure on homogeneous metallic and semiconducting surfaces, there remains a central problem with respect to a study of compound surfaces: to what extent can the chemical identity of individual atomic species be ascertained? Progress in this area will be illustrated in the following subsections.

187

6.3.1 GaAs Surfaces

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a) GaAs(l10) Both bulk and surface properties of gallium arsenide are most interesting to many researchers because of its potential applications in various fields. Investigations of the clean GaAs surfaces are mainly focused on the (110) surface since it is easy to cleave along this direction. Of course, studies on other surfaces have also been performed under practical conditions. The GaAs (110) surface has a chainlike structure similar to the Si (111) - 2 x 1 surface. The chain consists of alternating Ga and As atoms. The two chemical elements in the unit cell offer the possibility of chemically differentiating between the two atoms; an important technique which would be valuable to many inhomogeneous systems. Feenstra et al. [6.40] have obtained the first voltage-dependent STM images of the vacuum cleaved GaAs (110) surface with atomic resolution. Images show either only Ga atoms, or only As atoms, depending on the bias voltage. Not only did they identify the two kinds of atoms in real space, but also determined precisely the positions of As and Ga atoms on the surface. It is theoretically predicted that the STM images of the occupied states (at negative bias voltage) should reveal the positions of As atoms, and the images of the unoccupied states (at positive bias voltage) should reveal those of Ga atoms. Figures 3.9a,b exhibit two STM images, acquired simultaneously at the voltages of + 1.9 and -1.9 V, which correspond to states of Ga atoms and As atoms, respectively. A unit cell is positioned at identical positions on both images and the surface-atom positions are indicated in Fig. 3.9c. The difference in the lateral positions of state-density maxima between occupied and unoccupied states is 0.21 nm on average, which is larger than the geometric separation of about 0.14 nm. This large separa-

Fig. 6.9. STM image of the Ge (111) surface. The principal crystallographic directions are indicated, as are local regions of 2 X2, c4 X2 and c2 X8 reconstructions. Two atomic steps are seen to run across the image. which was acquired tunneling into unoccupied surface states [6.34) 188

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tion is a manifestation of the surface buckling appearing in the electronic structure of the surface. It turns out that the buckling of the surface atoms causes an increase in charge transfer between the Ga and As, along with a repulsion of the surface-state bands. The separation between the occupied and unoccupied states can then be used as a measure of the surface buckling when compared with theoretical calculations. Feenstra et al. thus concluded that the buckling angle [the amount of rotation out of the (110) plane] is larger than 23 0 , with a most probable value in the range 29 0 -:- 31 0 [6.35], although the overall uncertainlty would permit values between 23 0 and 35 0 . It was nicely demonstrated on Si doped GaAs (110) surface that the filled electronic state would display Friedel oscillatory pattern surrounding the defects [6.41] (Fig.6.11).

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Fig. 6. 11. (a) STM image of the (llO)-c1eaved Sidoped GaAs surface. (b) Cross section through the middle of the left dopant induced feature along Iine A. From th is figure the oscillation period can be obtained [6.41)

Distance 189

Fig. 6.12. STM image of empty states on a GaAs (100) surface, the basic structural unit consisting of three As dimers and one vacancy is marked [6.43J

b) GaAs(lOO) The other gallium-arsenide surface, arsenic-rich GaAs (100), is the most widely used gallium-arsenide face in MBE growth of device structures. It is therefore necessary to know the detailed structure of this surface for a better understanding of the growth mechanisms. Both RHEED and LEED results show disorder on the surface which could be explained by the presence of both 2 x 4 and c 2 x 8 domains where growth usually begins and ends. The difference betwen the 2 x 4 and c2 x 8 reconstructions arises from the way in which the basic 2 x 4 units are arranged relative to one another. It has proved difficult to make an absolute structure determination from diffraction. The STM images of the arsenic rich GaAs (100) grown by MBE obtained by Pashley and Haberern [6.42] show that the 2 x4 reconstruction arises from a regular array of missing dimers, with each unit cell containing three arsenic dimers and a missing dimer, as shown in Fig.6.12. Local areas of 2 x 4 reconstruction and c2 x 8 reconstruction arise from different arrangements of the basic 2 x 4 units. In addition, a change in the local ordering of the unit cells in some areas has been observed in STM images. This illustrates the small energy difference between these two reconstructions, and the ease to transform one into the other. The most striking features which may be important in the growth mechanism are small islands one step up and small holes one step down [6.43], typically only a few unit cells in size. The step height corresponds to the spacing between arsenic planes. The raised islands are largely made up from complete unit cells rather than individual atoms, as had been predicted, and can be as small as one 2 x 4 unit cell wide in the [110] direction. This shows the three dimers of a 2 x4 unit cell to be a very stable structure, either in the plane when it is bordered by missing dimers, or on a raised island when bordered by a step 190

edge. These localized features which can be obtained by an STM are of great importance in the mechanism of growth. The surface order can be much improved by using in-situ MBE. The STM image of the GaAs (100) surface prepared in such a manner shows arsenic dimerization and the formation of four-dimer units containing three As dimers and one missing dimer, with the 2 x4 and c2 x 8 unit cells formed by ordering of these structural units. A c4 x 4 arrangement, which arises from a four-dimer structural unit is also observed; each unit consisting of three dimers and a missing dimer; however, in this reconstruction the axis of each As-As dimer is rotated by 80 ° from its orientation in other reconstructions [6.44]. Lengel et al. [6.45] found the characteristics for As vacancies on GaAs (110), together with the theoretical analysis, and the migration process of the vacancies under the influence of STM tip bias.

c) GaAs(I11) and GaAs(Hf) The other two clean GaAs surfaces which have been studied by STM are GaAs(111) [6.39] and GaAs(111) [6.40,41]. The gallium-terminated GaAs (11l)-A surface exhibits a 2 x2 symmetry, as depicted in Fig. 6.13a. In analogy with the GaAs (110) surface, since Ga is electropositive and As electronegative, charge transfer between surface atoms leads to a high density of occupied states around the As atoms and a high density of unoccupied states around the Ga atoms. Figure 6.13a obtained while tunneling into the unoccupied states of this surface is thus interpreted as revealing the locations of the Ga atoms. Haberern and Pashley [6.46] concluded that the 2 x2 reconstruction rose from gallium vacancies in the outermost layer, as illustrated in Fig.6.13b. For the (111) surfaces a 2 x2 diffraction pattern is observed during and after growth in As-rich conditions. Subsequent annealing results in a transition to a (V19 XVI9)-R23.4° reconstruction. Both As-rich 2x2 and Garich v19 xv19 of the arsenic-terminated GaAs(111) surface have been studied by STM. STM images of the unoccupied and occupied states of the 2 x2 reconstruction look identical, indicating that the protrusions accurately reflect the atomic positions of the surface atoms. A model based on As trimers, with the As atoms in a trimer bonding amongst themselves and to As atoms in the underlying layer has been proposed by Riegelsen and co-workers [6.47,48]. These trimers then form a hexagonal array with 2x2 symmetry. Upon annealing to 500 C for about 10 minuts, the 2 x 2 surface converts to a v 19 xv 19 structure. On the basis of the STM images of this surface, they proposed a model consisting of hexagonal ring atoms, with six As atoms in the uppermost layer, and twelve Ga stoms in the second layer.

191

Fig. 6.13. (a) STM image of a GaAs (fii)-A surface, tunnel ing into unoccupied surface states. The superimposed lattice indicates the positions expected for Ga atoms on a bulk-terminated lattice. (b) Proposed model formed by Ga vacancies in the outermost surface layer [6.46)

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d) GaAs-AIGaAs Superlattices, consisting of layered arrangements of different III-V compounds, are generally grown in the (00 I) direction, and viewing of a (110) face then permits an examination of the structures in a cross section that exposes the interfaces between the two materials. From such studies, one can gain structural information concerning the superlattice, such as the extent of interdiffusion at the interface. STM images of MBE-grown GaAsAIGaAs layers and their interface show contrast between the GaAs and AIGaAs regions because of their different electronic structures. While the GaAs region appears very uniform, the AIGaAs layer has some apparent height modulations on the 1.5-:- 2.0 nm length scale which may arise from variations in the local composition. The AIGaAs region appears to be topographically slightly lower than the GaAs, probably owing to the larger band gap of the AIGaAs. The I-V curve taken on GaAs is typical of highly-doped p-GaAs, with a gap close to 1.43 eV, and the Fermi level located at the bottom of the gap. The curve representing measurements on the AIGaAs is found to be located 0.35 eV above the valence-band edge [6.49]. 192

Continued efforts have been seen that gave detailed information of the electron sub-bands together with interface roughening [6.50].

6.3.2 Layered Compounds Layered materials are characterized by the anisotropy of their physical properties, which stems from their structural anisotropy. According to their electrical conductivity they may be classified into three different categories: metal, semiconductor and insulator. Layered materials exhibit many properties which make them special with regard to STM studies. Graphite, for example, is a layered material which has been studied most widely by STM and AFM. Layered compounds, in general, and the transition-metal dichalcogenides, in particular, have played a prominent role in tunneling microscopy. The cleaved surface of layered compounds, such as MoS2 , SnS2 , TaS2 , TiSe 2 , WSe 2 and other transition-metal dichalcogenides can be imaged by STM in air at atomic resolution. This is not only because these materials do not have dangling bonds but are also inert due to the weak van der Waals forces acting between the surface and the subsurface. We use the molybdenum disulfide as an example to illustrate the STM studies of layered compound semiconductors. Some other transition-metal dichalagenides will be dealt with in the next subsection together with the discussions on ChargeDensity Waves (CDW). From the point of view of the (001) surface projection, the crystal structure of MoS 2 shows that the top layer is a hexagonal lattice of sulfur atoms with a lattice constant of 0.316 nm. Immediately below this plane is an identical hexagonal lattice of molybdenum atoms laterally displaced relative to the top layer. As shown in Fig. 6.14, the position of Mo atoms reduces the sixfold S planar rotational symmetry to threefold symmetry, producing a diamond-shaped surface unit cell with a Mo atom centered in one triangular half and a hollow located in the other. Figure 6.15a displays an atomic-resolution constant-height image of this surface. A centered hexagon of bright spots is evident in the figure, as are three distinct sites corresponding to the two constituent atom types and a surface hollow. An overlay of the surface unit cell is illustrated for comparison with Fig.6.14a. We see four bright spots at the corners, a secondary site in one half of the unit cell and a hollow in the other half, in agreement with the X-ray crystal structure. When we plot the variation in tunneling current relative to its mean value along the directions indicated in Fig.6.15a, we find a repeated pattern of three sites along the cell edge, as illustrated in Fig.6.15b. The same results have also been obtained in the conventional constant-current mode. 193

Fig. 6.14. The (DOl) surface structure of the MoS Z [010]

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6.3.3 Charge-Density Waves in Compound Semiconductors

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A more straightforward interpretation of the results directly identifies the two sites in this image with the two surface atomic species. Simple geometric considerations would then imply that the brightest points are due to surface sulfur atoms, which are closest to the tip, while the secondary peaks arise from molybdenum atoms in the second layer. However, numerous theoretical investigations of the band structure of this compound point to strongly covalent bonding with a substantial Mo 4d contribution at the top of the valence band and bottom of the conduction band. Thus, one cannot apriori ignore the possibility that Mo 4d levels, rather than S 3p levels, are primarily responsible for the tunneling current. This study demonstrates that it appears possible to clearly resolve two chemically and structurally distinct atomic sites in a layered semiconductor compound by STM. Since there are no intrinsic surface states on 2H-MoS z one should be able to directly probe the differences between tunneling into 194

the conduction band or out of the valence band. It will be interesting to see whether such experiments, combined with appropriate calculations, can uniquely establish the position of the transition-metal atom in layered semiconductors.

Another important application of STM to layered compounds is to study Charge-Density-Wave (CDW) phases. While the charge densities of many surfaces probed by STM resemble their atomic arrangements, this is not always true for materials exhibiting CDWs. The CDW phase was first predicted theoretically by R.E. Peierls in 1954; however, this phase was not observed experimentally until almost 20 years later. Thus far, CDWs have been recorded and studied in a number of low-dimensional inorganic and organic materials by various kinds of analysis techniques including electron, X-rays, atomic beam, and neutron diffraction, nuclear magntic resonance, nuclear quadrupole resonance and angle-resolved photoemission spectroscopies. Although significant advances have been made, key details of the structure and dynamics of the CDW phase remain uncertain in many systems. That STM has proven to be a particularly useful tool for investigating CDW phases lies in the fact that it can simultaneously image the atomic lattice and the charge-density modulation associated with these phases. On the one hand, STM study may judge the proposed models; on the other hand, STM may help establish a new model on the basis of this microstructural information to add to our understanding of the nature of the CDW phases. The amplitude of the charge-density modulations is dependent on the detailed geometry of the Fermi surface, which itself is a function of the type of material and even of the specific phase, e.g., IT, 2H or 4Hb for the transition-metal dichalcogenides. For example, TaS z and TaSe z can be grown fairly easily in these four phases. The IT phase exhibits octahedral coordination between the metal and chalcogen atoms and has one sandwich layer per unit cell. The 2H phase displays trigonal prismatic coordination between the metal and chalcogen atoms and has two sandwich layers per unit cell. The 4Hb phase has alternating sandwich layers of octahedral and trigonal prismatic coordination with a total of four sandwich layers per unit cell. The transition from the normal to the CDW state can occur in a wide temperature range. Since the initial report on the STM study of the CDW by Coleman et al. [6.52], amazing progress in this field has been made in the short time span of only a few years, from the objective observation to the study of the local structure and the key local properties of CDW phases. STM has been 195

Table 6.1. Comparison of the COW properties determined by STM measurements and predicted by the DC and NC models for IT-TaS z

Fig. 6. 16. (a) STM image of IT-TaSZ' Seven domains have been circles to highlight the symmetry of this phase. An apparent defect (D) is also marked. (b) Difference in the maximum COW ampIitude along the Iine marked in (a) [6. 53}

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used to investigate CDW amplitudes, the phase of the CDW relative to the lattice (i.e., the commensurability), the orientation and dynamics of the CDW phase transition. The materials which have been studied by STM include TaS z , TiS z , TaSe z ' NbSe z , VSe z , TiSe z , TaS 3 and NbSe 3 , among which the studies of TaS z are the most thorough and detailed. a) CDW Phases of 1T -TaS 2

Four distinct temperature-dependent CDW phases have been proposed for native IT-TaS z on the basis of evidence from diffraction and transport studies. These include a hexagonal InCommensurate (lC) phase (543 to 353K), the Nearly Commensurate (NC) phase (353 to l83K on cooling and 283 to 353K on warming), a triclinic incommensurate phase (223 to 283K on warming), and a low-temperature Commensurate (C) phase. The detailed structure of NC CDW, in the temperature range of 283 to 353 K, is controversial. Though NC and DC models have been proposed, STM studies support the DC model for the room-temperature CDW phase in 1T-TaS z , as may be inferred from Table 6.1. Large-scale STM images of this phase exhibit a domain-like pattern by a variation in the maximum CDW amplitude. As shown in Fig.6.16a, the circular domains, consisting of relatively highamplitude CDW maxima separated by lower amplitude regions (domain boundaries) are arranged in a regular hexagonal lattice (period 7.3nm) that is rotated relative to the CDWs. The modulation of the CDW amplitude be196

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tween the domains and boundaries is quantitatively displayed in a profile of surface corrugation (Fig.6.16b). From the analysis of atomic-resolution images it is determined that there is a well-defined phase shift between the CDWs in adjacent domains, and that within a domain the CDW superlattice is commensurate with the atomic lattice. These results provide unambiguous evidence for the hexagonal discommensurate CDW phase in 1T-TaS 2 and resolve the long-standing controversy concerning the structure of this CDW phase. As for the intralayer structure of the NC CDW phase existing between 230 and 350 K, a novel feature of the STM images is the periodic modulation in the CDW vertical corrugation that defines domains consisting of relatively high-amplitude CDW maxima. These maxima are separated by regions (domain walls) in which the CDW phase and amplitude change (Fig.6.17). The approximately circular domains of high-amplitude CDW maxima are arranged in a hexagonal superstructure with a period that depends strongly on temperature (see insets of Figs.6.17b, c). The variation in the amplitude of the CDW maxima within the domains (0.07 A), is significantly smaller than the 0.6 A decrease in the CDW amplitude that occurs at the domain walls. This decrease in amplitude at the domain-wall regions is independent of temperature (230 -;- 350K). As highlighted by lines through the CDWs in two adjacent domains in Fig.6.17b, evaluation of atomic-resolution images demonstrates that at all temperatures between 230 and 350 K the CDW undergoes a well-defined one-lattice-period phase shift across the 197

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Fig. 6.18. A plot of the NC-COW-phase domain period (e) and domain size (~) vs. temperature

Fig. 6.17. A series of STM images of IT-TaSz recorded at sample temperatures of (a) 242 K, (b) 298 K, (c) 349 K, and (d) 357 K. The images are of (a) 30 X30 nm z , (b) 17.5 X 17.5 nm z , and (c), (d) are 30 X 30 nm z areas. The insets in (b) and (c) are 30 X 30 nm z as in Fig. 6. 12a. The domain center positions for an ideal hexagonal domain structure are marked with white dots in (a) and are clearly different from the center positions of the two domains in the upper right corner. Lines drawn through the COW maxima of two adjacent domains in (b) hightlightthe one-lattice-period COW phase shift that occurs between domains [6. 54J

domain walls. This phase change represents an important feature that distinguishes this domainlike phase from a uniformly NC phase. Furthermore, an image of the IC phase (Fig.6.17d, T = 357K) exhibits the expected uniform triple CDW structure and shows no evidence for domainlike features, in contrast to the results for the NC phase. Both the period and domain size increase significantly as crystals are cooled over the temperature range between 230 and 350 K (Fig.6.18). Realspace and two-dimensional Fourier transform analysis of atomic-resolution images further demonstrate that within the domains the CDW is approximately commensurate with the atomic lattice, and that between domains the CDW amplitude decreases and the CDW phase changes. The growth (melting) of the hexagonal domain structure in IT-TaS z on cooling (warming) occurs over an order of magnitude large temperature range and in this res198

pect is quite remarkable. It has been predicted that the boundaries between domains should sharpen as the temperature is decreased. But the STM observations provide the contrary result that diffuse domain walls are 2 -;.- 3 CDW periods wide (ca. 2.4-;.- 3.6nm) and nearly independent of temperature. On the basis of these results. Wu and Lieber [6.54] suggested that continuous growth and melting of this domainlike CDW phase occurs via localized distortions or fluctuations of the hexagonal structure which results in a quasi-periodic packing of the commensurate domains. Further STM investigations lead to the observation that the T-phase could be induced from H-phase by the strong electric field at tip apex [6.55] (Fig.6.19).

Fig. 6.19. (a) STM image of a single crystal of 2H-TaSez. (b) Image of the surface region in (a) after a voltage modification pulse of - 1300 m V applied to the tip. The central hexagonal region corresponds to T -phase TaSez [6.55J

199

b) Charge-Density Wave Defects The fundamental question of how impurities affect the local properties of the CDW phase has also been addressed by STM. For example, STM investigation [6.56] provide the first local measurements of the effects of titanium impurities on the CDW phase in 1T-TaS 2 . The local CDW structure distorts significantly in response to the random-lattice potential associated with the distorted titanium sites, and the average CDW wavelength responds to the changes in the Fermi-surface geometry. STM studies [6.57] of the effects of impurities on the commensurate CDW phase in titanium-substituted IT-TaSe 2 , Ti x Ta 1_x Se2 demonstrate that the frequency of the CDW defects increases linearly with increasing titanium concentration in the commensurate phase. Analysis of this concentration dependence further reveals that defects in the CDW structure may be due to titanium clusters rather than isolated impurities. For x(Ti) < 0.04 the localized defects consist of a CDW amplitude distortion, while for x(Ti) = 0.07 defects consisting of a coupled-amplitude phase distortion are also observed. In addition, CDW twin domains that nucleate at these amplitude-phase defects have been imaged in real space, in which the adjacent superlattices are rotated 13.9° clockwise and counterclockwise relative to the lattice, and thus form a 28° angle with respect to each other. Other achievements in this area include the measurements of the CDW energy gap and some parameters of the CDW density of states in some dichalcogenides and trichalcogenides using STM [6.58] and angle-resolved tunneling spectroscopy [6.59], AFM measurements for distinguishing the commensurate and the incommensurate CDW structures [6.52] and the observation of a continuous long-range modulation of about 6 CDW wavelengths in TaS 2 [6.61]. The CDW structure observed in other phase and in the quasi-one-dimensional compounds has been reviewed in a number of recent books [6.62,63].

6.3.4 High-T c Oxides The recently discovered high-temperature oxide superconductors are the complex layered compound and have been the subject of many STM investigations. The traditional method of tunneling electrons through thin insulating films into superconductors is routinely used to determine important parameters of superconductors. However, this method takes place over a broad area and tends to average any spatial variations in the density of states. Firstly, STM can be used as a spectroscopic tool at low temperatures to determine the superconducting energy gap locally, which is advantageous, especially for inhomogeneous states in granular superconductors. Secondly, 200

Fig. 6.20. STM image of Bi-O plane of Bi 2 Sr2 CaCu2 08+0 single crystal obtained at 77 K

[6.66]

the atomic and electronic structure of high-T c superconductors can be mapped with atomic resolution from room temperature down to low temperatures. The surface structures of Bi- and TI-based copper oxides have been investigated by STM at room temperature. The main aspect of study for the ceramic materials is to examine the influence of the granular structure and twin boundaries on the superconductive properties. STM images of Bibased compounds obtained in ultrahigh vacuum reveal the atomic lattice and an incommensurate superstructure consisting of a sinusoidal modulation with a periodicity of nine to ten unit cells [6.64]. STM studies of T1-based copper oxides obtained in ultrahigh vacuum also show the atomic lattice and a weak one-dimensional superlattice with a period of about 1 nm [6.65]. In the case of Bi-based compounds, only one species, either Bi or the atoms are mapped according to the measured distance between the atomic scale protrusions. In contrast, both the TI and atoms of the top surface plane can be imaged by STM indicating that TI and make similar contributions to the density of states near the Fermi lever. We have imaged the Bi-O plane of Bi2Sr2CaCu208+0 (Bi-2212) single crystal with home-made STM at 77 K [6.66,67]. Figure 6.20 depicts a 5 x5 nm STM image with lattice resolution. The measured average distance between the atomic scale protrusions is 3.7 A. The most remarkable STM images of the Bi-O plane that we have observed are shown in Fig.6.21 with the scanning area of 10 x 10 nm 2 . This image reveals a two-dimensional superstructure with an average period of about 2 nm and the orientation angle between the 2D superstructure and Bi or lattice is about 23 ° .

°

°°

°

201

Fig. 6.21. STM image of Bi-O plane of Bi 2 Sr2 CaCu2 08+0 single crystal obtained at 77 K. The two-dimensional superstructure is clearly visible [6. 67J

Scanning-tunneling-spectroscopic studies have been performed on high-T c oxides in both the normal and superconducting state. One example is that conductance measurements on Bi-based compounds have revealed a very small electronic density of states at the Fermi level. This implies that the surface BiO layedr is not metallic and would indicate that the metallic conduction, and thus the superconductivity, is mainly located in the Cu02 layers (Sect.3.4.3) [6.68]. There is clear evidence in the STM observation of YBa 2 CU 3 07 -single crystals for a periodic 1.3 nm modulation, suggesting a charge density wave transition in CuO chain [6.69] (Fig.6.22). The central question in low-temperature tunneling spectroscopy is the magnitude of the energy gap. Kirtley [6.61] gave an extensive survey of the tunneling data that are aimed at determination of the energy gap in high-T c materials. STM and STS have also been utilized to study classical superconductors such as layered compounds Nb 3 Sn and NbSe 2 . These investigations include the determinations of surface structures and magnetic flux lines called vortices. A review of this topic can be found in [6.70,71] and will not be further discussed here.

202

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Fig. 6.22. STM image of CuO chain in the layer ofYBa2 CU3 7 , (a) Y bias = +240 mY; (b) Y bias = -240 mY; (c) the average of (a) and (b): (d) the difference of (a) and (b). (e) A cross section along the CuO chain marked by the end points of the arrows showing the periodity [6.69J

203

7. Surface Adsorbates and Surface Chemistry

As demonstrated in Chap. 6, STM has had a substantial impact on studies of the atomic and electronic structure of metal and semiconductor surfaces. This technique has also been employed to investigate physical and chemical processes occurring at surfaces, rather than the physical properties of the surfaces themselves. A theoretical model has been developed for the study of tunneling of electrons from the probe to the surface where molecular species had been adsorbed, and with atomic and molecular species intervening between the probe and the surface [7.1]. This theory relies on assessing the changes in the tunneling current caused by variations in the size and structure of surface adsorbates. Some results have been achieved in studies on such issues as the nature and the activities of atoms, molecules and clusters in adsorbate/substrate systems, nucleation and growth of metal films, surface diffusion, as well as the atomic details of surface chemistry. The present chapter deals with the application of STM in this area.

7.1 Adsorption on Metal Surfaces The study of adsorbate-covered metal surfaces is one of the important subjects of surface science. Determination of the atomic positions on surfaces and measurement of the coverage of surface species are crucial in modeling surface structure. A variety of techniques has been used to study adsorbate bonding, surface diffusion, ordering processes and reactions with the surface or other adsorbates. Except for a few methods, a common disadvantage for those conventional techniques is that the structural information is a result that is statistically averaged over large areas, comparing with the dimensions of typical features of the surface structure rather than local properties. STM, on the other hand, has led to unprecedented new structural insight into local features of the surfaces such as the chemisorption of adsorbates on metal surfaces and especially the restructuring of metal surfaces. Its high-resolution capability opens new possibilities for the investigation of a large number of adsorbate-substrate interactions, even for those involving non-periodic structures. Not only are static surface structures rev205

ealed atom by atom but, perhaps even more important, the dynamics of the chemisorption process, i.e. the nucleation and growth of the reconstructed phases, can be studied in real space and time at a rate of several images per second.

7.1. 1 Cu (11 0)-0 The chemisorption of gases on transition-metal surfaces is of particular interest for its role in heterogeneous catalysis. For adsorbates which interact strongly with the substrate, the chemisorption process is often accompanied by the breaking of several nearest-neighbor bonds within the substrate lattice, resulting in a reconstructed surface phase with a substantially altered atomic density in the topmost layer. Oxygen chemisorption on Cu surfaces is a prototype of this category of reconstruction which has provoked considerable experimental and theoretical interest. It is known that molecular oxygen chemisorbs dissociatively on Cu (110), and that LEED patterns show a 2x 1 structure at an oxygen coverage of 0.5 ML (MonoLayer). The halfand integer-order spots of the LEED patterns are of comparable intensity, indicating a surface reconstruction. However, the detailed atomic structure of this Cu (110)-2 x 1 surface is still open for vigorous dispute [7.2]. It appears that such information is of utmost importance also in the understanding of the static surface structure. The two most common structural models proposed in the past are the "missing row" model, where every second [001] row on the surface is absent, and the "buckled row" model, where every second [001] row is shifted outward. By means of STM it is possible to study the microscopic mechanism for nucleation and growth of such adsorbate-induced reconstructions. Figure 7.1 illustrates a series of STM images of a surface for increasing oxygen exposure. Atomic resolution of the bare Cu (110)-1 x I structure is clearly observed in Fig.7.1a. The formation of "added" rows of atoms (interpreted as O-Cu chains) along the [001] direction is initiated when the surface is exposed to oxygen at 373 K for exposures ranging from 0.1 to 1 L (1 L = 10- 6 torr' s). The shortest O-Cu chains appear to be about 6 x 0.36 nm 2 indicating a critical minimum length. At higher exposures (ca. 1--:-2L) resulting in an 0 coverage of 0.1--:-0.2 ML, these "added" rows are found in islands developing a unit mesh, with a periodicity that is doubled in the [110] direction (Fig.7.1b). Typical dimensions for these islands are 10--:-20 om in the [001] direction and 1.5 --:- 2.0 nm in the [110] direction, corresponding to preferential growth in the [001] direction. This structure is consistent with the observation of streaky 2 x 1 LEED pattern for low oxygen exposures, indicating a lack of order in the [l10] direction.

206

Fig. 7.1. (a) Atomically resolved STM topographic image of a 2 X2 nm 2 region of a bare Cu (110)-1 XI surface recorded with V = -0. 35 v and I = 2. 5 nA. (b) STM image (7 X7nm 2 ) showing the formation of "added rows" after exposure with ::::::1 L of oxygen. (c) STM image of the Cu (110)-2 XI-O reconstructed phase at an oxygen exposure of about 10 L recorded with V ::::::-0.8 V and I ::::::0.8 nA. The [00 I] direction coincides with the y-axis in all cases [7.2]

Exposures up to about 10 L lead to an 0 coverage of ::::::0.5 ML where most of the surface is covered with the reconstructed phase. However, several types of defects or irregularities may be observed, as shown in Fig. 7.lc. There is a reconstructed terrace (A) one atomic layer below the top layer, a single chain of atoms (B) between two reconstructed anti-phase domains shifted away from the nearest neighbor chain by an extra [l 10] 2 x 1 lattice parameter, a point defect (C) developed from a vacancy, and a region (D) between in-phase reconstructed areas which shows a very weak corrugation with a periodicity consistent with a c6 x 2 reconstruction. This Cu (11O)-c6 x2 0 structure will be discussed later.

207

7.1.2 Cu (100)-0 In the case of the oxygen-induced reconstruction of a Cu (I 00) surface, the STM images show a phase different from the Cu (110) reconstruction. Prior to oxygen exposure, the single Cu atoms on the Cu(lOO)-1 x 1 surface can be seen in Fig.7.2a. After the crystal is dosed with 1000 L oxygen at 573 K and at a pressure of 2.5 .10- 6 mbar, and followed by an anneal at 573 K for 5 min, the sharp LEED pattern obtained reveals the (2v2xv2)R45° 0 structure. Figure 7.2b depicts an STM image of this surface over an area of 1.5x1.3 nrn 2 . The [001] and [010] directions are equivalent on the Cu(lOO)

surface, and thus the reconstruction is twined into two different domain orientations which appear to be randomly distributed. In this image the chains appear to be grouped in pairs of two, with separations from A to B and from B to Care 0.29 nm and 0.43 nm, respectively. This implies that the structure in the [010] direction repeats itself for every 0.72 nm. The chains of bright spots along the [001] direction can be interpreted as Cu-OCu chains/bonds, equivalent to the observations for the Cu (110)-2 X 1 0 structure. The [010] periodicity of 0.72 nm can be explained by a removal of every fourth row of the Cu atoms.

7.1.3 Dynamics

Fig. 7 _2. Atomically resolved STM image of a 1. 5 X 1. 5 nm 2 region of (a) a bare Cu (100)-1 X 1 surface, and (b) a fully developed Cu (100)(2V2X V2)R45° 0 structure. (c) Atomistic model of the Cu (100)- (2V 2 XV2) R45 ° 0 reconstructed phase. The small and large open circles represent the 0 atoms and the remaining Cu surface atoms, respectively, whereas the gray and black circles represent Cu atoms in the 2nd and 3rd layer. The arrows indicate the missing row of Cu atoms. In (b) and (c) a unit cell is indicated [7.3]

208

While the examples described above provide information on atomic arrangement of surface structure in equilibrium, some studies have been performed on the dynamics or the driving force responsible for structural transformation, which include the nucleation and growth of oxygen-induced Cu(l10) and Cu (100) reconstructions, instabilities inherent in the Si (I 11)-pseudo 5 x 5 Cu reconstructed surface and the formation of disordered regions on the Ge (l11)-c2 x 8 surface. Here, we discuss the Cu-O system, the case for Si and Ge will be considered later. As mentioned in the last chapter, if the STM has a high mechanical resonance frequency, images can be recorded sequentially in about 1 second, and one can follow snapshots of dynamical processes. Apart from being of utmost interest in itself, the dynamic information gained from such STM "movies" is very decisive also for the understanding of the static surface structures. It has been shown that in some cases, although there are strong similarities in the final gas-induced reconstructions, the growth modes may be dramatically different, and that direction information on the number of atoms in the unit cell of a certain reconstruction can be derived from the change in area of particular domains. As described above, when the surface of Cu (II 0) is exposed to oxygen at about 373 K with exposures ranging from 0.1 to 1 L, the 2 x 1 structure is initiated and the reconstruction appears in the form of rows of atoms (interpreted as O-Cu chains) along the [001] direction. In order to investigate the detailed mechanism underlying the formation of the O-Cu chains, the dynamical growth of the reconstructed phase has been studied by imaging a region of several terraces separated by steps. While the crystal is exposed at room temperature to oxygen at a pressure of 1· 10- 8 mbar, it is seen from the sequential STM images that Cu atoms are removed exclusively from edges of the terrace, and that the rate of removal varies at different points along the terrace edge. Simultaneous growth of O-Cu chains, which later agglomerate to reconstructed islands on the terraces, preferentially along 209

the [001] direction, is observed. From the observed great mobility of both single and groups of O-Cu chains, Besenbacher et al. [7.4] concluded that the reconstructed phase of "added row" grows on top of the terraces by nucleation of Cu atoms (coming from step edges) and 0 atoms diffusing onto the surface. The "added-row" model is identical to the previously adopted "missing-row" model at the saturation coverage of 0.5 ML, but the two models differ significantly in terms of mass transport. For the added-row model, the Cu atoms are supplied from step edges, as discussed above, whereas the missing-row model would lead to a mass transport from terraces to step edges. A distinction between the added-row and the missingrow reconstruction types can hardly be obtained by any other technique. The nucleation and growth of the Cu(100)-(2V2XV2)R45° 0 reconstruction phase has also been studied by continuous imaging of two terraces separated by a monoatomic step during oxygen exposure at room temperature at a pressure of 1.5,10- 5 mbar. The dramatic change depicted in the STM topographs reflects the nucleation and growth of small islands on both terraces, whereas the step is essentially intact. This is in contrast to the growth mode for the Cu (110)-2 x 1 phase. The islands, which have a height of 0.18 nm [the interlayer distance for Cu(lOO)], grow preferentially along the [010] and [001] directions. At saturation they cover 25 % of the surface area, as determined from a plot of the height distribution. These observations give the first direct proof that the oxygen-induced reconstruction of the Cu(100) surface is of the missing-row type, with one quarter of the Cu (001) rows in the surface layer "squeezed out", and that these extra atoms nucleate and grow epitaxially in small islands on top of the Cu surface. At oxygen coverages above 0.5 ML a c6x2 phase coexists with the 2x 1 phase on the Cu(110) surface. A dynamical study shows that patches with one layer of Cu missing appear on the surface in the late stages of the formation of the 2 x 1 structure. These patches serve as Cu reservoirs for the added rows in the absence of steps. Further oxygen exposure leads to a build up of the c 6 x 2 structure both on the terrace and within the patches, starting at the edges of the patches. The c6 x2 structure consists of two 0Cu chains for each three [110] 1 x 1 lattice spacings [7.5], as compared to the 2 x 1 structure where there is only one O-Cu chain per two [110] 1 x 1 lattice spacings. The O-Cu chains are connected by Cu atoms, coordinated to every second 0 atom along the chain. These Cu atoms, which are gliding on top of the structure, constitute a c6 x2 "superstructure" with respect to the underlying bare 1 xl Cu surface. The O-Cu chains have been seen as pairs of rows in between the protrusions only in very highly resolved images. The STM movie shows that the c 6 x 2 phase grows isotropically (in contrast to the growth mode for the 2 x 1 phase) and that the protrusions forming the c6 x 2 structure have a high mobility along the [001] direction as single units. During the build up of c6 x2 there is an increase in the sizes 210

of the patches. Since the 2 x 1 structure at this point is already fully developed, this additional Cu supply from the patches indicates an increased density of Cu atoms in the c 6 x 2 phase compared to the 2 x 1 phase. The result of a thorough analysis of the height distributions reveal that the c6x2 unit cell contains 10 Cu atoms corresponding to 5/6 ML. As discussed above, with a high-stability STM one can record STM images sequentially in about one second and thereby can visualize in real time space-dynamical processes on metal and semiconductor surfaces. Such information is very decisive for a full understanding of both the growth mode of a reconstructed phase and the resulting static structure. Furthermore, by analyzing a large number of pictures (with atomic resolution) concerning dynamical processes on surfaces, it is possible to study fundamental atomic quantities like diffusion constants and interaction energies by STM measurements.

7.1.4 Ag (110)-0 Silver is used in the chemical industry as a catalyst for partial oxidation of ethylene. Thus, extensive investigations have been carried out to examine the adsorption characteristics of oxygen on Ag surfaces. However, little is understood of the role of molecular oxygen on the surface, even though it is believed that the epoxidation of ethylene involves molecular oxygen. Hashizume et al. [7.6] have applied STM to investigate the reaction on such a surface and proposed an "added-row" model for the atomic oxygen adsorption on the Ag(110) surface, similar to the case of oxygen adsorption on the Cu (110) surface (Sect. 7 .1.1). After the Ag (110)-1 x 1 surface had been exposed to 5.10- 9 torr oxygen, a series of STM images reveal that the monoatomic steps of the clean Ag (110) surface are not stable at room temperature and diffuse much faster than the STM scanning speed, which results in the zigzag-step-line shape. At an oxygen exposure of 14 L, bright lines ("added rows ") can readily be observed in the direction of [001], which is perpendicular to the Ag atomic row on the Ag (110) surface. The average spacing between "added rows" is 13ao (ao = 2.98;\') at this oxygen exposure. After about 60 L of oxygen exposure the movement of the steps is reduced. A zoomed-in image exhibits the "added row" formation with an average separation of 5.2ao ' With increasing oxygen exposure, the average separation between these "added rows" is reduced (Figs. 7 .3a, b) and after the surface is exposed to a large amount of oxygen, the well-ordered 3 x 1 (at around 1600 L), and eventually (at, or more than, 4000 L) the terminal 2 x 1 surface, consisting of the "added rows", are obtained (Fig.7.3c), in agreement with LEED observations. Note the presence of various separations at the same time (4ao , 211

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3ao and 2ao for the case of Fig7.3a, and 3ao and 2ao for the case of Fig. 7.3b). Based on these STM and LEED observations together with other experimental results, a model has been proposed for the oxygen behaviour on this silver surface, as shown in Fig.7.4. The bright lines observed in the STM images of Fig.7.3 are Ag-O-Ag linear chains running perpendicular to the Ag rows (Fig. 7.4); "added rows" similar to the case of the Cu (110) surface. The "added-row" model has been confirmed by comparing the STM/ STS results with a first-principle theoretical calculation ofy M. Tsukada's group on the Ag (110)-2 x 1-0 surface having "added-row" atomic configuration. Figures 7.5a,b display the bias dependence of simulated images for the 2 x 1 phase "added-row" configuration. In the empty-state image (V := + 1.5V) Ag atoms are imaged and in the filled-state image (V := -1.5V) oxygen atoms are imaged. Figures 7.5c,d show the empty- and filled-state STM image, respectively, observed from the same surface area. The experimental values for the corrugation height (Figs. 7.5c, d and Figs. 7.5e, f, respectively) reveal a good qualitative agreement with the theoretical calculation. A critical terrace width of 100 A was found on the vicinal Ag (110) surface [7.7]. The oxygen causes spontaneous faceting with little nucleation barrier (it should be noted that nuclei growth is key to faceting).

212

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7.1.5 Ni(llO)-H and Ni(111)-H The chemisorption of hydrogen on transition-metal surfaces is of particular interest due to its role in catalysis, and as a simple case for the general chemisorption phenomenon. Extensive studies of hydrogen adsorption on Ni (110) have produced conflicting results on the structure of the surface. Six different reconstructions of the Ni (1lO)-H system have been observed by low-energy electron diffraction and helium diffraction: including 2 Xl, 2 x6, c2 x4, c2 x6 and 1 x2 reconstructions below 220 K, and "streaky" 1 x 2 reconstruction at room temperature. However, a STM study [7.8] at room temperature produces the convincing result that more than 80 % of the surface is usually disordered, despite the appearance of the typical streaky 1 x 2 LEED pattern before the STM measurement. Patches of 2 x 1 or 1 x 2 structure are sometimes observed in the disordered areas; whereas 5 x 2 structure, formed by a combination of row pairing and missing [001] rows, are most frequently found in the ordered area. The STM images also show the 5 x 2-ordered domain with an average size of 100 nm along the [110] and 10 nm along the [001] directions. The small domain size along [001], 213

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7.1.6 Sulfur Adsorption

('}I~41 Fig.7.5. Theoretical simulation of the (a) empty and (b) filled state STM images of the Ag (110)-2 X 1-0 "added row" configuration and experimental results of (c) empty and (d) filled state STM images (100 XIOOA2). (e) and (f) are the cross section plots of (c) and (d), respectively

and frequent antiphase boundaries and dislocation explain the observed streaky I x2 LEED pattern. In the 5 x2 structure, all the top-layer Ni atoms are paired along the [001] direction by a 0.05 nm lateral displacement to form tetramers, and every fifth [001] row is missing. Although the position of hydrogen is not determined in this experiment, the asymmetric measured heights of the Ni atoms suggest that H atoms may occupy some of the lowsymmetry bridge sites. In the 5 x 2 reconstruction of Ni (110) by adsorbed H at room temperature, all the first-layer Ni atoms are laterally displaced by 0.05 nm along the dimerizing direction, forming tetramers. In the Ni (111)-H system, however, micrographs of the (111) plane show a hexagonal pattern with a dimension of twice the unit structure, and a corrugation of 0.1 nm which increases towards a step. This suggests a p2 x 2-2H monolayer, induced by the high partial pressure of hydrogen [7.9]. Figure 7.6 represents a hardsphere model of part of the surface drawn according to the STM images.

214

It is known that Sulfur (S) forms ordered overlayers on many metal surfaces, often producing several distinct LEED patterns with increasing sulfur coverage. More than 100 LEED patterns have been reported for S chemisorbed on various crystal faces of at least 16 metals. The S-chemisorption system is of technical importance since sulfur is a common ingredient of lubricants. Investigating the structure of sulfied surfaces may help explain the fundamental mechanisms of friction and lubrication. Moreover, even more importantly, the interaction between sulfur and metal surfaces is also a classical problem in the field of heterogeneous catalysis, mainly because it plays an important role in poisoning metallic catalysts. Catalytic activities of most transition metals can drastically be reduced in the presence of S, which is a common impurity in many industrial processes. The reason is that the chemisorption bonding between S and the metal surfaces modifies the electronic or structural properties of the neighboring metallic atoms which are responsible for the adsorption of reactants. If the interaction between S and a metal surface is relatively weak, the structure of the substrate remains unchanged; however, it results in a perturbation all around the adsorption sites which, in turn, deactivates the surface. If the interaction is strong enough to significantly modify the metal-metal bonding, surface reconstruction may occur and produce new superficial structures which are inactive in chemical-reaction processes. In an attempt to understand the poisoning mechanism of metallic catalysts deactivated by S adsorption, extensive studies have been done essentially on copper, nickel, palladium, rhodium and platinum. The S adsorption, on the other hand, is able to modify the selectivity in some catalytic processes; i.e., a S-adsorbed metal catalyst is preferentially active for a particular reaction process. In this respect, a reaction via a Sadsorbed catalyst can mainly produce one pre-selected compound with fewer by-products, and subsequently leads to a higher commercial interest 215

since it enhances the efficiency in related industrial processes. Though a large number of efforts have been undertaken, the mechanism of selectivity modification caused by sulfur adsorption is still far from being understood. There are different models to explain modifications of catalytic selectivity, emphasizing geometry, ligand effect or restructuring. In order to ascertain the fundamental behavior of these S-induced properties, it is essential to characterize the structure and bonding between sulfur and the substrates. Generally speaking, surface structures produced by adsorption are classified into two categories. As mentioned above, when the interaction between adsorbates and a substrate is relatively weak a simple adsorbate layer is produced on the unreconstructed substrate. However, when adsorbates such as hydrogen, carbon, oxygen, sulfur, potassium chemisorb on a metal surface, the surface undergoes reconstructing due to a strong interaction between the adsorbate and the substrate. As a consequence, the atomic structure of the substrate is strongly modified to adopt to the adsorbate-induced chemical environment. Such a rearrangement of atomic positions can result in a slight distortion or displacement of the substrate without invoking significant mass transport (weak reconstruction), or the substrate structure can dramatically change, involving a long-range or local mass transport (strong reconstruction). In the latter case, some energy is needed to break the metal-metal bonding and migrate the metal atoms on the surfaces. This energy requirement can ultimately be overcompensated by the formation of a new adsorbate-metal bonding, and the net lowering of surface energy is the driving force of surface reconstructing. In fact, thermal energy is needed to activate surface reconstructions which are kinetically limited at room temperature; hence those reconstructions occur only at elevated temperature. Often an adsorbate-induced reconstructed structure can exhibit features significantly different from the clean surface, due to a severe change of the substrate.

7.1.7 Cu(lll)-S Different models have been proposed for the adsorption of S on the Cu(111) surface. Among them only the eV7XV7)RI9° structure attracted much interest and was studied by LEED, Surface-Extended X-ray Adsorption Fine Structure (SEXAFS), and X-ray standing-waves technique. In all those models the S coverage was referred from a radioactive-trace measurement, but the Cu coverage was subjectively assumed to fit with the individual proposals under consideration. Moreover, the source of Cu for the formation of the adsorbate-metal layer remained unknown, since only information about the static structure was provided by these techniques.

216

(e)

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A

Fig. 7.7. (a) STM image of the zigZ4g struClure (75 X75 z), lhe unil cell is ou11 ined in a zoomed image (b) in a scale of 30 ><30 A z, lhis slruclure can be idenlified by imposing a I Xl grid in (c) (7.10]

When a structure with a large unit cell exists in several equivalent domain orientations, a complicated LEED pattern can appear. This complexity produces considerable difficulty in analyzing the diffraction data. Nevertheless, such a problem can be tackled by the local imaging probe of the STM; indeed, it is fairly easy to establish the unit mesh of the surface structures on atomically-resolved images, thus providing direct identification of the complicated structures. More importantly, the STM movie technique offers an advantage in observing in-situ the dynamic process of adsorption, thus providing further understanding of the static structures. Ruan et al. studied this system with STM [7.10]. The complicated LEED pattern as reported by early researchers, prior to the (V7xv7)RI9° structure appears at a low exposure of HzS (ca.6L). The STM results yield that the pattern corresponds to a well-ordered phase with a zigzag fashion (Fig.7.7a). A total of six equivalently coexisting domains revealed by the STM measurements is consistent with the six-fold symmetric LEED patterns. The unit cell (a = 9.3, b = 11.7A and (3 = llr) sketched in Fig. 7.7b is 17 times bigger than that of the (1 X 1) Cu. There are two apparent protrusions (3 A in diameter) in each unit cell, one in the corner and the others located about 1 A away from the centre along the a axis. In Fig. 7. 7c a mesh derived from the clean surface is drawn in order to determine this previously unidentified structure, here the mesh and the unit cell of the zigzag structure have been kept in their original orientations. The (V7 xV7) R 19° structure appears after 20 L exposure of Hz Sand competes on the surface at an exposure of 600 L. An STM image, shown in Fig.7.8, exhibits a hexagonal structure with a spacing of 7.7 A [V7 times the (l X I) lattice parameter]. In its unit cell there are two protrusions, one is distinct, the other seems faint. The large sizes of the brightest protrusions (3A), like that in the zigzag structure, are not due to poor resolution, but in217

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Fig.7.8. STM image of the C'/7 XV7) R19° structure [7.10]

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7.1.8 Ni (lll)-S In previous studies four different models [7.12-15] have been proposed for

dicate that they correspond to a Cu-S complex rather than individual atoms. Compared with a height of 0.3 A contributed from individual S atoms, according to a theoretical predication by Lang [7.11], the apparent protrusions of 0.8 A in the STM images corresponds to a complex, indicating the formation of a reconstructed structure rather than a simple S overlayer. Dynamic observation via an STM movie technique [7.10] demonstrates that the steps gradually retract with increasing H2 S exposure, like in the case of 0 adsorption on Cu (llO) (Sect. 7 .1.1) revealing a step-etching mechanism. The observed long-range mass transport gives strong evidence of surface reconstruction, i.e., the Cu atoms are removed from the steps and diffuse on the terraces to form a Cu-S mixture when encountering S adatoms. Consuming Cu atoms from the steps is not surprising from an energetic point of view, because the reconstruction process requires a relatively low energy to remove Cu atoms from the step sites in comparison with that on the terraces due to the lower coordination number. Owing to being a spontaneous process, this energy requirement for relocating Cu atoms must eventually be overcompensated by forming the new chemical bonding between Cu and S in the mixture layer. By combining the STM results with an X-ray diffraction experiment, a reconstruction model for the (v7xV7)RI9° structure has been proposed in which there are three Cu atoms and three S atoms in one unit cell, as shown in Fig.7.9.

218

the 5v3 x2-S structure which can be expressed in matrix notation

2 0 5 lO

[7.16], including two simple S-overlayer structures [7.12J, a reconstructed, sulphide-like surface layer [7.13], and a structure in which the Ni surface layer has been transformed into a pseudo-(lOO) phase, and the S atoms are adsorbed in the four-fold hollow sites in a c2 x2 structure [7.14]. STM studies of this system [7.17] indicate that the surface undergoes a reconstruction to the 5v3 x 2-S phase which is therefore not a simple overlayer structure. The two models [7.12,13] for the 5v3 x 2-S phase which are based on simple overlayers can thus be eliminated. Figure 7.1 Oa exhibits an STM image of the 5¥ 3 x 2-S phase recorded with atomic resolution. The original six-fold symmetry of the surface has been replaced with a "row and trough" structure along a [l01] surface direction. The rows of protrusions appear in sets of three separated by troughs. With a careful calibration of the microscope, which is achieved by imaging a clean Ni (111) surface with atomic resolution, it was found that the troughs have a separation (measured along a [121]-type direction perpendicular to the troughs) of 10.8 A and that the distance between the rows adjacent to the troughs is 6.0 A. In the [l01] direction along the troughs the structure repeats itself every 5.0 A. Small protrusions along the troughs were observed with a spacing of 5 A. These protrusions are also aligned in the [121] direction. Taking the symmetry of the substrate into account, a unit cell of 21.6 x 5.0 A2 is drawn in Fig. 7.lOa which confirms the 5v3 x2 assignment to the phase. It is, however, evident from the symmetry in the observed STM image that the two previously proposed models for the 5v3 x 2 phase which involve a reconstruction of the substrate [7.14, 15] are incompatible with the STM images. 219

7.1.9 Cu(llO)-S

(b)

Fig. 7.10. (a) STM image (40 x40A 2) of the sv' 3 X2-S Slfucture with atomic resolution, obtained after 30 L exposure to H 2 S. The troughs are formed along a [10 1] surface direction which is vertical in the image. The unit cell is indicated. (b) The missing-row model for the SV3X2-S structure on Ni(lll). The vertical direction coincides with a [101] surface axis. The large circles are Ni atoms, and the small circles S atoms

It is natural to arrive at the missing-row model which is shown in Fig. 7 .lOb for the 5v3 x2-S phase on Ni (111). Note that the S atoms in two adjacent troughs have a different coordination number, thereby doubling the repeat distance in the [121] direction. The STM studies have provided the first experimental evidence for a missing-row reconstruction of a fcc(111) surface. A novel driving force is proposed in which the S adsorbates induce a compressive surface stress which drives the restructuring of the surface. The strong coupling between the S adsorption and the changes in metal-metal binding has interesting implications for many catalytic reactions. Indeed, it represents the basis for an understanding of trends in the hydrodesulfurization activity of the transition-metal sulfides. 220

The interaction between Sand Cu (110) was studied by LEED [7.18] and EXAFS [7.19]. To interpret the p5x2 and c8x2 LEED patterns, Domange and Oudar [7.18] proposed the formation of a sulfide compound in which the constitues are compressed in the close-packed direction. Nevertheless, computer-simulation studies [7.20,21] rather preferred simple overlayer models wherein all S atoms locate in the two-fold hollow positions formed by the unreconstructed substrate and many antiphase boundaries and vacancies exist, corresponding to the observed patterns. Figure 7 .11a displays a STM image of the c2 x 2 structure [7.22]. The apparent protrusions should correspond to adsorbed S atoms, according to a theoretical prediction by Lang [7.11]. Unlike the Ni (1lO)-S system wherein the S (protrusion) was clearly imaged in the two-fold hollow sites (Sect. 7 .2), it is impossible to resolve both the c2x2 and bare 1 x 1 Cu structure simultaneously, due to a high mobility of S on this surface at room temperature. However, comparative adsorption sites in the current system could be expected since both Ni (110) and Cu (110) have similar structural and electronic properties. EXAFS study [7.19] first confirmed experimentally the two-fold hollow positions, implying that each S atom is bonded with five Cu atoms, four in the substrate and one nearest in the next Cu layer. The "p 2 X 5" structure produced at a further exposure, shown in Fig. 7 .11 b, which exhibits two features running along the [011] direction, with a periodicity of 5ao (the ao is the Cu fcc lattice parameter of 3.16A). The background of the row structure still exhibits a c2 x 2 feature. It would be expected that the interaction among the rows is relatively weak due to a

Fig. 7. 11. (a) STM image. of the c2 X2 Slfucture (20 X20 "p2 XS" structure (71 X7SA2)

A2), and (b) a STM image of the 221

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Fig. 7.13. (a) STM image of the c8 X2 structure (70 x70A 2), and (b) a simple S-overlayer model

large separation. Indeed, the STM observation shows that segments of the rows are mobile and the protrusions in some rows are sometimes shifted by 2.S A along the row direction with respect to those in the other rows. The STM dynamic study [7.17] reveals an unusual reversible step-etching mechanism, i.e., the steps retract at the initial exposure of H2 S but the tendency reverses under a further dose, and the step growth ends up by returning to the original positions. The observed mass transport suggests a surface reconstruction process in which the Cu atoms are removed from the step sites to build the row structure. With increasing H2 S coverage to a certain stage, a lifting of the reconstruction takes place by releasing the Cu atoms from the rows back to the step sites, generating S overlayers. The pS x2 structure (Fig. 7 .12a) produced by a further exposure of H2 S to the "p2xS" state is accounted for the "splitting" LEED spots in the [OIl] direction. The appearance of the "splitting" in both (001) and [OIl] direction is therefore ascribed to the coexistence of the p2 x Sand pS x 2 structures. Development of the latter structure is at the cost of consuming the former structure. As a consequence, the Cu atoms in the rows eventually have to be released and diffuse to the step sites, i.e., the observed reversible step-etching (step-growth) process. The fact that the steps return to their original positions indicates that the pS x2 is a simple S overlayer rather than a reconstructed structure because the net mass transport is null. The S coverage of 0.60 ML reported in the early studies suggests that each of the six protrusions in one unit cell (10 times the primary one) corresponds to an individually adsorbed S atom. Based on that, one can easily determine the position of S adatoms from the atomic-resolved image. AS overlayer model is illustrated in Fig. 7.12b. Here, it is reasonable to assume that most S atoms adsorb in (or close to) the two-fold hollow sites in order to lower the surface energy.

Figure 7.13a exhibits a STM image of the c8x2 structure. For the same reason, stated above, the c8 x2 structure is a simple S overlayer other than a reconstructed structure. Individual S atoms contribute to each of ten protrusions in one unit cell outlined in this STM image. That gives rise to an ideal S coverage of 0.625 ML, consistent with early studies [7.18]. In the model displayed in Fig. 7.13b, most S atoms are assumed to be located in the two-fold hollow sites, as indicated by the mesh drawn in the STM image. Note that the protrusions in the hollow sites are brighter than those on the bridge sites for both the pS x2 and c8 x2. At first sight this seems to be contradictory to their geometric positions. Further discussion will be found in Sect.7.1.10 where similar observations are found in the Ni(IIO)-S system. One may ask why the S-induced reconstruction occurs on the Cu (111) (Sect. 7.1. 7) rather than on the Cu (110) (the superstructure is not a final state), since the energy requirement is higher for breaking the metal-metal bonds in the most closely-packed (111) surface. In order to understand this issue we should probably focus on the coordination number of Cu atoms bonded with a S adatom, and suppose that a larger coordination number corresponds to a lower surface energy. The S atom would be bonded by a maximum of three Cu atoms if a simple overlayer formed on the Cu (111) surface. This number can increase to five in the reconstructed V7 xv7 structure which is the S-saturated structure, according to X-ray diffraction. Through a surface reconstruction the substrate can be rearranged to allow the S atom to adsorb in a higher coordinated site. In contrast to the Cu (111), the formation of the S overlayer on Cu (110) results in a coordination number of five for the c2 x 2 structure, and most S atoms have the same number for the p5 x 2 and c8 x 2 structures. It does not require a surface reconstruction on the Cu (110) to increase the coordination number.

222

223

From the STM studies of S adsorption on the Cu(110) surface, it is clearly seen that the superstructure, appearing after the c2 x 2 state is a reconstructed structure characterised by rows along the (001) direction, which contain both Cu and S. The lifting of the surface reconstruction occurs under further Hz S exposure, producing simple S overlayers, the p5 x2 and c8 x2, with most S atoms adsorbing in the two-fold hollow positions.

7.1.10 Ni(llO)-S For the Ni (1lO)-S system a sequence of LEED patterns appear with increasing S coverage at room temperature [7.18], including a c2 x2, p5 x2, c8 x 2 and p3 x 2. A high S-covered p4 x 1 was only obtained by exposing HzS above 473 K. In contrast to a theoretical study [7.23], experimental data suggested that the S atoms in the c 2 x 2 structure were located in the two-fold hollow sites rather than the long-bridge sites. By simulating the LEED patterns, it has been proposed that each structure except the p4 x 1 corresponds to a simple adsorbate layer consisting of many anti-phase c2 x 2 domains and extensive vacancies [7.21,24]. Besides, the p4 x 1 structure was supposed to be a reconstructed structure [7.13,25]. In the Cu(1lO)-S system, as discussed in Sect. 7. 1.9, the nx2 structures have been found to be simple S overlayers. Thus, it is very interesting to compare the Ni (110)-S with the Cu (1lO)-S system, since the two substrates have similar electronic and structural properties. The STM studies [7.26] show that the n x 2 structures in the current system are found to be simple S overlayers as well. Besides, experimental evidence that the substrate surface undergoes reconstruction corresponding to the p4 x 1 structure is provided. Two new reconstructed structures, formed by S adsorption at elevated temperature p7 x 1 and p 11 x 1 structure were also reported. The STM dynamic study indicates that no reconstruction occurs, therefore all adsorbed S atoms should be in the outermost layer. In the STM images (Fig.7.14a-c), there are 6, 10 and 4 apparent protrusions in each unit cell, respectively. The corresponding S coverage of 0.60,0.65 and 0.67 ML [7.18] suggest that all the adsorbed S atoms contribute to the apparent protrusions, though showing different levels of brightness. Note in Fig. 7 .14b the c2 x2 structure existing in the upper-right shares of an ideal phase boundary with the p5x2 and c8x2 structure, respectively. The STM data indicate that most S atoms, corresponding to the brightest protrusions, are located in the two-fold sites. Three simple S overlayer models for p5 x 2, c8x2 and p3x2 structures are given in Fig.7.14d,e and f, respectively. For Hz S adsorption at room temperature the p3 x 2 structure, upon annealing the p 3 x 2 state above 200 0 C a conversion into both c 8 x 2 and 224

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(b)

Cu (11O)-S system, an intermediate structure appeared just after formation of the c2 x2 structure. It is supposed to be a reconstructed structure, based on the mass transport involved in the S adsorption process. With increasing Hz S dose at room temperature the intermediate structure transforms into a simple S overlayer. Unlike the Cu(110)-S case, the reconstructed p4x I structure in the Ni (11 O)-S system is found to be a stable structure.

7.1.11 Mo(OOI)-S and Re(OOOI)-S

Fig. 7.15. (a) STM imate of the P(4' I) structure showing a row feature running along the (001) direction (38 X38Az). and (b) a reconstructed model, in which the black dots mean S atoms and the grey ones are Ni atoms

p4x I structures is observed [7.26]. The fact suggests that the p3X2 structure has a surface energy higher than the other two structures, but the conversion into the other two structures is kinetically limited at room temperature. The p4 x 1 structure is usually obtained by exposing the crystal to Hz S at 473 K. It has been presumed to be a reconstructed structure with a S coverage of 0.75 ML [7.18,25]. A typical STM image is shown in Fig.7.15a, in which the protrusions are separated by 3.5 and 10 A along the [001] and [0 I 1] directions, respectively. A reconstruction model may be designed as follows: One third of the total 0.75 ML S adsorbs in a two-fold hollow site formed by 0.5 ML Ni underneath and the remaining S adsorbs in two-fold sites formed by the second Ni layer. p7 x I and p II x I structures are also observed by STM [7.26]. These two new structures are produced in coexistence with the p4 x 1 structure. Studies also demonstrated that direct heating of the crystal at the p3 x 2 state produces both p4 x 1 and c8 x2 structures. The early discussions brought to light that the S coverage of the p3 x2 structure is interposed between that of the other two structures. To maintain a constant amount of S on the surface, some S atoms must be removed from the areas appearing as the c8 x 2, and move to other regions to form the higher S-contained p4 x I structure. Chemisorption on the Ni (110) and Cu (110) surface produces comparative structures because of their similar electronic and structural properties. The c2 x 2, p5 x 2, c8 x 2 and p3 x 2 structures have been observed in the Cu (110)-S system. All of them have been identified as simple adorbate overlayers, with S coverage identical to that of the corresponding Ni (110)-S structures. Although no stable reconstructed structure has been found in the 226

STM has also been employed to study the atomic structures of sulfur chemisorbed on molybdenum Mo(OOI) [7.27] and rhenium Re(OOOI) [7.28]. At a saturation coverage of one monolayer, the sulfur overlayer on a Mo (00 1) surface shows an ordered LEED pattern, as described by a p2 x I superlattice. Although some speculations have been advanced that sulfur atoms occupy both fourfold hollow and twofold bridge sites, no real structural data on this system are available. In addition, an important point about this surface is that it appears to be completely inert towards the adsorption of the most common atmospheric reactants: CO, Hz 0, Oz, etc. A Mo (00 1) crystal is first cleaned and annealed in UHV, followed by dosing it with sulfur, a sharp LEED pattern is obtained that reveals a two-domain structure with 2 x I and I x 2 symmetries. The surface so prepared is exposed to air for a period of 14 h and subsequently re-examined by LEED and Auger spectroscopy after pumped down to pressures below 10- 6 Torr. The LEED pattern is as sharp as the initial one, while the Auger spectrum reveals insignificant amounts of carbon and oxygen, and no loss of sulfur after this exposure to air. The STM images obtained at atmospheric pressure reveal that the pseudohexagonal arrangement of bright spots has the I x 2 symmetry of the sulfur overlayer which was observed by LEED at low pressures. The shortest atom-atom distance corresponds to the sulfur-sulfur distance of 0.31 nm in the [00 I] direction. By analysing the measured corrugation amplitudes it can be concluded that the [120] direction would contain sulfur atoms in alternating bridge and fourfold hollow sites. This result demonstrates for the first time that atomic-resolution imaging of surfaces with a chemisorbed layer is achievable in an atmospheric-pressure environment. This opens the way for detailed in-situ studies of the structure of catalytically important surfaces, obtained under reaction conditions. For the sulfur overlayer chemisorbed on Re (000 I), the structure of the (2v3 X2v3)R30 ° surface is observed. The ordered sulfur overlayer is prepared in ultrahigh vacuum and then transported through air into an STM operating at a vacuum of 10- 7 torr. Similarly, the (2v3 x2v3)R30° sulfur overlayer also passivates the rhenium substrate in air. STM images show the atomic structure of the overlayer unit cell to be a hexagonal ring of six 227

sulfur atoms adsorbed in threefold hollow sites. The S overlayer has a variety of imperfections, including distorted and broken unit cells. The disordered regions usually consist of small aggregates of S atoms separated by troughs similar to those between hexagons - neither isolated S atoms nor large S islands are observed.

7.1.12 Other Non-metal Adsorbates on Metals Iodine adsorbates also passivate the Pt (111) surface. The adsorbate structure reveals (V7xV7)RI9.lo symmetry at a coverage of 3/7 and 3x3 structure at 4/9 ML coverage [7.29]. Other adsorbates on metal surfaces which have been observed by STM include Cu (11, I, I)-S [Cu (II, I, I) surface is vicinal surface of the Cu(lOO)], Cu(ltO)-K, Ni (ltO)-Xe, Ni(lOO)-O, Ni (110)-0, AI(lII)-O, Ru(OOOI)-O, Pt(lll)-I, Pt(110)-CO, Pt(lOO)-CO, Pt (100)-NO and Cr(1tO)-Cr2 0 3 . A review of most of these systems can be found in [7.30]. STM imaging of larger organic molecules such as benzene on Rh (III), Cu phthalocyanine on Cu (100) and C 60 on various surfaces will be discussed in Sect. 7.3. The interaction between adsorbates with metal surfaces and surface chemistry are dealt with in Sect.7.6.

7.1.13 Metallic Adsorbates Adsorption of metallic ad layers and subsequent growth of thin metal films have widely been studied by STM. These results yielded not only structural information of overlayer films, but also the dynamics of growth when the overlayer diffusion is sufficiently slow, as described previously. On the basis of STM studies on the structure of surfaces, nucleation and growth processes can be studied and understood to a large extent from systematic STM observations following different procedures for metal deposition and SUbsequent annealing. The formation of a 2-D island can be described in terms of a nucleation and growth process. Following common terminology, the formation of islands in the middle of flat terraces can be described as homogeneous nucleation, whereas condensation at step edges or nucleation at surface heterogeneities correspond to heterogeneous nucleation. For example, since the lattice mismatch between Au and Ag is very small (0.24%), Ag overlayer growth on the Au (III) is nearly homo-epitaxial. Although the Au (III) substrate surface exhibits a 22-23 xV3 reconstruction, the Ag overlayer does not show this reconstruction [7.31]. As an example for heterogeneous nucleation, in contrast, the early stage of Ni growth on an Au (11l) surface exhi228

bits unique nucleation sites determined by the Au substrate reconstruction [7.32]. Since the Au reconstruction is caused by a change of stacking sequence, the Ni overlayer grows preferentially at the areas where two stacking sequences meet to form a surface dislocation. An ordered array of Ni islands is formed with lattice spacings of 73 A and 140 A, corresponding to the unit cell of the reconstruction superlattice. Similarly, STM studies of Au overlayer on the Ni (ltO) surface reveal a 7 x 4 structure with a c2 x 4 subunit structure [7.33]. A short-range interaction yields the local registry of Au on the Ni (ltO) that results in the c2 x4 unit cell, while the 7 x4 longrange structure originates from a lattice mismatch between Au and Ni. At similar deposition conditions Au islands on Ru (000 I) showed a high preference to nucleate at points where vertical screw dislocations emerge at the surface [7.34]. Similar nucleation characteristics have also been observed with Fe on Au(lll) by STM [7.35].

7.2 Adsorption on Semiconductor Surfaces The geometry of metals and their electronic properties on semiconductors have been investigated, particulary motivated by the technological importance of metal-semiconductor interfaces [7.36]. Fundamental interest arises because a wide variety of coverage-dependent structural arrangements occur, including ordered metal overlayers on reconstructed semiconductor surfaces, the formation of surface alloys, and the alteration of semiconductor reconstructions by trace amounts of metallic adsorbates. These interfacial rearrangements pose challenging problems for surface science, requiring the application of many experimental and theoretical techniques to determine the atomic positions, the nature of metal-semiconductor bonding, and the electronic structure of the interfaces [7.37]. Defects and impurities at such interfaces play particularly important roles by providing new electronic states lying within the semiconductor band gap which pin the Fermi level and also act as recombination centers. By combining tunneling microscopy with local tunneling spectroscopy, the electronic properties of single-atom defects in several metal/semiconductor systems have been studied. Silicon anti-site defects occurring in Al overlayers on Si (III) are shown to give rise to new electronic states within the surface-state band gap, which are important in pinning the Fermi level [7.38]. The electronic structures of Al and Si adatoms are sufficiently different so that they can be separately imaged. The AI adatom has a pronounced unoccupied surface state about 1.1 e V above Ep , while the Si atom has an occupied surface state just below E p ; as a consequence, imaging at a 229

positive sample bias reveals the Al adatoms, while imaging at negative bias reveals the Si adatoms. The STM image of Si (1 1I)-V3 xV3 Al was displayed already in Fig. 3.11. Band-gap states are also identified on small islands of Sb deposited at room temperature on GaAs (110) [7.39]. At higher Sb coverage it is shown that the Fermi-level pinning is caused by bands of filled and empty states which are localized near the edges of Sb islands. For BilGaAs (110), the STM spectrum, taken on a Bi terrace away from the edges, shows a zero conductivity region (0.7eV wide) which represents the band gap of the Bi overlayer. Its position essentially lies centered in the GaAs band gap. Thus the Bi/GaAs (110) ordered monolayer is semiconductor-like as is its Sb counterpart, but with only one half of its band gap [7.40].

7.2.1 Ag/Si(lll) Ag/Si (111) has intensively been investiagted, but its geometry, even its surface stoichiometry, remain controversial. For example, Ag/Si (111)-V3 x

V3 structures are observed by LEED and STM after Ag deposition onto the Si substrate. The unit cell contains two maxima, with the maxima arranged in a honeycomb structure. On spectroscopic grounds van Loenen et al. argued that these have to be Si atoms [7.41], but Wilson and Chiang insisted that the bumps are Ag atoms [7.42]. Theoretical investigations of this surface using first-principles total-energy calculations [7.43] indicate that the lowest-energy configuration consists of a top layer of Ag atoms arranged as honeycombed-chained trimers lying above a distorted "missing-top-layer" Si (111) substrate. The honeycomb structure observed in STM images arises not from the top-layer atomic positions but rather from the wave-function behaviour of empty surface electronic states above the Fermi level. The maxima of the electronic distribution for the empty states occur at the center of the Ag trimers, and are situated above the fourth Si layer, in agreement with the registry determined by Wilson and Chiang [7.44]. This example indicates that there is no sure and simple way to understand the results obtained by STM (Sect. 3.4.4). In addition to Ag/Si(lIl)-V3 XV3, the initial stage of Ag film growth on a Si (111)-7 x7 surface has also been studied at substrate temperatures of 363 and 403 K. At the first step of Ag condensation, many of the atomic subunits appearing as ringlike structures on the inner adatoms of both halves of the 7x7 unit cell are observed (Fig.7.16a), which show a triangular distortion and indicate bonding of the Ag atoms to the dangling bonds of the second atomic layer. The first step of Ag condensation therefore consists of saturation of these dangling bonds, which means that the ringlike structures would contain at least six Ag atoms. These subunits represent the 230

(a)

(b)

0J Fig. 7.16. Constant-current topography of 1/3 ML Ag on Si (Ill)-7 X7 obtained at (a) a bias voltage of 2 V, tunneling current of 3 nA and substrate temperature of 363 K, and (b) a bias voltage of 2 V, tunneling current of 2 nA and substrate temperature of 400 K. The faulted and unfaulted halves are labeled f and u, respectively. The area of (b) is about II XI8 nm 2

[7.45]

critical nuclei for Ag condensation, which continues by the addition of further Ag atoms until the 7x7 unit-<:ell half is covered completely. Several, not yet completed triangular Ag islands are visible in Fig.7.l6a. At a substrate temperature of 403 K, the Ag deposit mostly condenses in the form of such triangularly shaped, flat two-dimensional islands of equal size, which are preferentially located on faulted halves of the 7 x7 unit cells and are explained by a close-packed arrangement of Ag atoms (Fig. 7.16b). On the uncovered parts of the surface is the usual 7 x 7 reconstruction of clean Si (Ill). The Ag islands still reflect details of the 7 x 7 reconstruction, which means that the 7 x 7 structure is preserved on the entire surface. The shape and the size of the Ag islands indicate a very weak or even repulsive interaction of Ag atoms with those substrate sites where, according to the DAS structure, the dimers of the second atomic Si layer are located. On the Ag islands a reduction of the local density of states both above and below the Fermi level E F compared with the ada toms of clean Si (111)-7 x7 is observed, which may be understood by the nonmetallic character of the local electronic structure of the Ag islands. The shape of the Ag islands in the submonolayer range is essentially determined by the underlying 7 x7 reconstructed Si (111), which is not changed by deposition of 1/3 ML Ag at substrate temperatures less than 407 K.

7.2.2 Au/Si(lll) Many metals induce different reconstructions of the Si (111) surface at different metal coverages. In the case of Au,S xl, V3 XV3, and 6 x6 phases are seen with increasing coverage up to 2 ML. Normally, each metal-induced reconstruction has a well-defined structure charcterized by a specific density of metal atoms, and a change in the average metal density on the 231

surface is accommodated by changes in the relative abundance of each phase. In this manner, the surface makes transitions between the different phases with increasing metal coverage. The transition between the Au/Si(l11)-V3x v3 and 6x 6 phases shows anomalous behavior. A previous LEED study proposed that the v3 x v3 structure itself is not a perfectly ordered phase, but reflects the local order of a structural subunit of the 6 x 6 reconstruction. The transition between the v3 x v3 and the 6 x 6 structures was viewed as a gradual ordering of v3 xV3 subunits into a larger 6 x6 unit cell. An ion-scattering study supports the picture of the v3 xV3 as being a partially complete 6 x6 structure. STM images of the v3 xv3 phase support the basic premise of the earlier work. The transition between the v3 xv3 and the 6x6 reconstruction of the Au/Si (111) surface is observed, which is unusual in that as the Au coverage increases, there is a continuous evolution of the surface structure with Au coverage, rather than a simple change in the relative abundances of two distinct, well-ordered phases. STM images show that the v3 x v 3 structure is broken up into sub-1O nm domains that decrease in size with increasing Au coverage. The 6x6 phase can be described as a periodic arrangement of small v3 xV3 domains [7.46]. This progression terminates with the establishment of the 6 x6 phase, which can be described in terms of a network of domain walls separating subunits of local v3 x v3 order.

7.2.3 Cu/Si(lll) Furthermore, in studies of metal adsorbate/semiconductor systems, STM has successfully been used to follow the instabilities of Cu/Si(111) surface structure in real time and space. In a system with multiple, nearly degenerate "ground states", a configuration cannot be found in which all competing interactions are feasable, it is said to be frustrated. For frustrated physical systems like spin glasses (as opposed to the computational problems of optimization) the possibility exists of spontaneous (physical) changes from one virtual ground state to another. and the frustration may be physically observed. Mortensen [7.47] reported on the first direct atomic-scale observations of frustration in a surface structure via STM. Unusual dynamics of a distinct structure in the tunneling topographs of the Si (111) "pseudo 5 x5" Cu structure are seen, and evidence is presented that the instabilities are due to an intrinsic frustration in the surface structure. The constant-current topographs acquired for different bias voltages show that the density of the bright features in the images is independent of the copper coverage as well as of the annealing temperature. They always appear asymmetrically in the pseudo 5 x 5 unit cell, and the direction of the displacements (ca. 0.5 nm) 232

away from that center of the cell is random. When comparing sequentially recorded images of exactly the same location, it is most vividly visualized that the bright features exhibit remarkable changes over time: they are observed moving within a pseudo 5 x 5 unit cell, to "jump" between neighboring cells, and to appear and disappear in time at room temperature; and all changes are reversible. The changes of the bright features are apparently uncorrelated, and while the rate of change is observed to span from features changing every 5 --:- 10 s to features never changing on the time scale of the image acquisition (3 --:- 5s). Based on the experimental evidence Mortensen hence reached the conclusion that the bright features are purely electronic and not associated with distinct geometric features on the surface, i.e., they are intrinsic to the pseudo 5 x 5 structure. Furthermore, he argued that the observed reversible changes reflect an intrinsic instability (a frustration) in the surface structure. These observations demonstrate the feasibility of STM to follow dynamical processes as, e.g., instabilities in real time and space.

7.2.4 Group-III Metals on Si(lll) Determinations of the atomic structure of semiconductor surfaces have indicated that the total surface energy can often be substantially lowered through a reduction in the number of "dangling bonds". On the Si (111) surface, this is achieved by a remarkable combination of a stacking fault, adatoms, and dimers in the 7x 7 reconstruction. Here, each Si adatom replaces three dangling bonds with a single dangling bond. If metals from group III of the Periodic Table (AI, In, Ga) are deposited onto Si(111), the number of dangling bonds can be completely eliminated by placing the metal atoms in threefold adatom geometries on top the Si (111) surface in a v3 xV3 superstructure. The topographic structure and density of occupied and unoccupied states of the AIISi(111)-v3xv3 surface were studied by STM and CITS [7.38,48], The image is depicted in Fig.3.1!. The results confirm a threefold adatom geometry for AIISi (111)-v3 x v3 and prefer assignment to the T 4 site over the H3 . Similarly, STM images of the indium-induced v3 xV3 reconstructions of the Si (111) surface [7.49] are also consistent with 1/3 ML of In adatoms resting in threefold sites. At elevated temperatures, as more metal is deposited, the surface order changes from v3xv3 to v31xv31 and then finally to 4x1 at about 1 ML. The higher-coverage 4 x 1 surface consists of large reconstructed terraces often bounded by abrupt, stepped edges. Growth of flat metal islands is also seen around 1 ML. Like the Al and In systems previous, Ga exhibits a v3 xV3 symmetry when the surface coverage is about 0.3 ML. The Si (111)-Ga surface appeared as a simple adatom phase on a 1 x 1 substrate and the predominantly Ga adatoms are located on the T 4 site [7.50]. 233

7.2.5 B/Si(lll)

7.2.6 CIISi(lll)

In the case of chemisorption of AI, Ga and In on Si (111), as we discussed above, it was found that the group-III atom occupies an adatom position at a T 4 site of the Si(111) surface. Should B occupy an adatom T 4 site? AI, Ga and In are all larger than Si, while B is significantly smaller (Si covalent radius: 0.111nm, B: 0.082nm). As a result of boron's small size, the shon B-Si bonds will induce considerable tensile strain and will bring the B atom very close to the second-layer Si atom below the T 4 site, leading to strong overlap repulsion. Using a molecular species, DecaBorane (nido-B IO H 14 , DB) as the source of boron, Avouris et al. [7.51] determined the details of the interaction of B with Si (111) surfaces and its effects on surface electronic structure, reconstruction and chemistry. They have been able to image the B-containing molecules on the surface and follow the structural and electronic modifications of the surface as a function of the annealing temperature. At room temperature the DB molecules adsorb preferentially on defects and on center-adatoms of the Si (111)-7 X 7 surface and appear as round disks when viewed from above. On heating the surface to temperatures above the hydrogen desorption temperature (800K), the hydrogen desorbs and B is deposited on the surface. A surface produced by exposing the 7 X 7 surface at 300 K to 0.4 L of DB and briefly annealing to =1000 K has a v3 xV3 structure. On the stable B/Si(111)-v3 xV3 surface, B occupies a nova I configuration where it substitutes for a Si atom in the 3rd atomic layer directly below a Si adatom. Because of a Si-to-B charge transfer, the top Si adatom layer has no occupied dangling-bond states and is insulating. As a result, the chemical properties of Si adatoms on the B/Si (111)-v3 xV3 surface are very different from those of the adatoms on the Si (111)-7 X 7 surface (Sect. 6.2.1). As to the electrical characteristics of localized minority sites on the v3 x v3 surface, it is found that I-V curves over such sites may show regions of negative differential resistance and that this behavior is localized in areas of atomic dimensions (ca. 1nm). The essential requirement for negative resistance to develop is the existence of narrow peaks at appropriate energies in the density of states spectra of both sample and tip. The above observations have important implications on discussions regarding the ultimate size-limit of electronic devices. A detailed analysis on Si (00 1) suggests that the boron atoms sit at the substitutional sites in the first bulk-like Si layer, and covered with Si dimers and dimer vacancies [7.52].

In the case of an interaction of CI with Si surfaces, CI forms well-ordered overlayers on Si (111) and has become a model system for studying chemisorption on semiconductor surfaces. CI adsorption plays a key role in many technologically important processes such as reactive ion etching and chemical-vapor deposition [7.53]. At low coverages, reacted and unreacted sites are distinguishable in both current-voltage curves and STM topographs, and the interaction involves the adatom dangling-bond states. At higher coverages CI atoms penetrate the adatom structure and insert themselves in the bonds between the adatom and rest-atom layers [7.54]. As a result there is extensive mass transport on the surface in which the CI serves to stabilize various surface structures. In particular, after saturation coverage followed by an anneal, most of the Si-adatom layer is stripped from large areas of the surface and accumulated in pyramidal Si structures, revealing the complete underlying Si rest-atom CI-stabilized 7 x 7 domains, but further annealing converts it slowly to the more favorable CI-stabilized bulk-terminated 1 xl structure. The transition between these structures is sufficiently slow that isolated intermediate structures between the 7 x7 and 1 x 1 are observed corresponding to the stepwise progressive motion of the dimer-row domain walls. Boland and Villarrubia [7.54] pointed out that under appropriate annealing conditions it is possible to quantitatively convert the surface to the bulk-terminated structure. The adsorption is attributed to precursor mediated chemisorption of Cl. the formation of Si-F is directly related to the translation energy of F2 molecules [7.55].

234

7.2.7 Bi on Si(lOO) and Si(lll) Surfaces The motivation for the investigation of Bi on silicon is that the adsorption of group-III and -V elements can passivate Si surface eliminating all surface dangling bonds and to restore bulk terminated structure. In the case of Si (111), the bulk terminated (Ill) plane consists of threefold bonded atoms with one dangling bond on each surface atom (surface passivation). For group-III metals the individual adatom bonds to three Si atoms and all of the surface dangling bonds are saturated, whereas group-V atoms can also saturate surface dangling bonds by being theefold coordinated and creating a pair of nonbonding electrons. In the case of Si(100), each surface atom in bulk geometry is twofold bonded and has two broken bonds. The group-V adsorbate can saturate all dangling bonds by forming dimers between groupV atoms and bonding to two Si atoms.

235

On the Si (l00) surface, the adsorption of some group- V metals such as As, Sb, are known to remove Si (l 00)-2 x 1 reconstruction and to passivate the surface. On the Si (l00) surface, the adsorption of group-III, -IV , -V metals as well as noble metals induces most frequently a ("v3 XV3)R30° reconstruction. A lager number of experimental and theoretical studies has been performed to determine the atomic structure of this reconstruction. The adsorption of group-III metals is of great interest since these metals are known to fully passivate the surface dangling bonds. It is known that bismuth also forms a v3xv3 structure on the Si(lll) surface. However, the geometrical structure and the saturation coverage for the 11'3 xV3-Bi surface are still controversial despite several models for the 11'3 xv3 structure having been proposed corresponding to different coverages. TheSi(l00)-2xn-Bi and Si(lII)-V3xV3-Bi surface have been studied with STM and LEED [7.56]. The STM images show a series of well-ordered 2 Xn superlattice, with n ranging from 5 to up to 12. The 2 xn-Bi structure is not the result of the mixture of the randomly-spaced missing rows, but results from the periodic distribution of the missing Bi-dimer rows in the 2 xl Bi overlayer. The periodicity of n in the 2 x n structure depends on the Bi coverage with the relationship = I-lin. The formation of successive 2 x n phases as a function of coverage and annealing temperature reveals the strain relief processes of the compressed Bi-dimer rows by creating missing rows and line dislocations as well as rectangular vacancies. The remarkable ordering of 2 x n phases suggests the presence of strong longrange repulsive elastic interactions among missing Bi-dimer rows. However, for the phases with n less than 5, the ordering of missing rows is found to be suppressed due to the strong repulsive interaction and the noncomitant formation of rectangular and linear-type defects. On Si (111) surfaces STM images of different Bi coverages revealed three distinct structures by different type of reconstructions for the well known 11'3 xv3 LEED pattern for the Bi!Si (111) system. At low coverages, Bi atoms are found to occupy the T 4 site and form a monomer phase. At the saturation coverage of 1 ML, the clusters of Bi atoms in the trimer symmetry are found. Between these two coverages, STM image showed the honeycomb type 11'3 x 11'3 reconstruction coexisting with the trimer phases.

e

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7.2.8 Na/Si(lll) Owing to the relatively simple electronic properties of the alkali metals, their adsorption on the metal and semiconductor surfaces has been one of the most interesting topics, from both an experimental and a theoretical point of view. Some of the well-known phenomena accompanying alkali 236

Fig. 7.17. STM il11age of Na/Si (111)-3 X 1 surface showing individual Na atoms. The area shown is 140 Xl90 A2. and a sample bias of -1.6 V is used

adsorption are the promotion of chemical reactions or the reduction of the work function. For instance, it is known that the oxygen uptake rate of the Si (111)-7 x 7 surface reduced by as much as 40 % upon structural transformation of the clean 7 x7 to aNa-induced 3 x 1 surface. STM studies of this surface provide detailed information regarding the structural transformation with atomic resolution. From the STM image obtained after depositing Na which showed a good 3 x 1 LEED pattern, it is readily seen that the 7 x7 structure is completely replaced by domains of the 3 x 1 structure formed by a single layer of Na atoms. These domains are rotated by 120 0 from each other. A 3 x 1 surface image with atomic resolution is depicted in Fig.7.17. Each 3 x 1 unit cell contains two Na atoms so that the saturation coverage is 2/3, which agrees with the saturation coverage deduced from the RHEED study on the alkali-metal-induced 3 x 1 structure on the Si (111) surface. To understand the origin of the island formation and the mechanism of the 7 x 7 to the 3 x 1 structural conversion, the images at various stages of Na deposition have been obtained [7.57]. At an early stage of the conversion, the 3 x 1 regions are found on the lower terraces adjacent to the steps. The 7 x 7 periodicity in the uncovered regions could be observed until the entire surface converted to the 3 x 1 structure, indicating the importance of the dimer wall breaking up for the conversion to the 3 x 1 structure. Further Na deposition resulted in a reversal of the contrast of the 7 x 7 and 3 x 1 LEED pattern intensities until finally the 7 x 7 pattern disappeared completely. The 7 x7 structure, in addition to the 3 x 1 structure, is the only periodic structure observed during Na deposition. This is an important difference from the surface obtained after annealing the Na-saturated 3 xl sur237

face, in which case the Na layer could be gradually peeled off and various intermediate structures (nxn, n = 2,5,7,9) are observed in the regions where the Na layer is removed. Based on the STM results, a general picture of the 7 x7 to 3 x 1 conversion can be conjectured as follows: the Si adatoms back bonds break up upon Na deposition and annealing. The adatoms are confined to the unit cells because of the dimer walls. Near the steps, however, the unit cell structure is imperfect, and break-up of the dimer walls and adatom diffusion occur rather easily. Therefore, the 3 x 1 conversion starts at the steps, Si adatoms diffuse into the corner holes of the 7 x 7 structure or to the steps, concomitant with the restacking of the rest-atom layer, also under stress. A new substrate structgure, i.e., the unreconstructed Si (111) is then stabilized by the 3 xl adlayer. Once the 3 x 1 structure is formed, it expands by converting the adjacent 7 x 7 unit cells. If the annealing temperature during Na deposition is lower or higher than the optimum temperature range, a complete conversion to the 3 x 1 surface does not take place. Tunneling I-V curves measured from the Na adsorbed 3 x 1 regions show that the Na monolayer-covered surface is insulating but the tunneling gap closed when a Na double layer is formed. When oxygen is adsorbed onto the Na monolayer-covered surface, the surface morphology does not change very much. However, a small amount of oxygen adsorption on the Na double-layer-covered surface caused the dark patches in the STM image, although the LEED pattern remained unchanged. Concomitantly, the work function decreased further, suggesting that this morphology change is an electronic effect. This result is of significance in understanding the metallization of the alkali metal overlayer and the mechanism of negative electron affinity.

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7.2.9 Na/GaAs(llO) and Cs/GaAs(llO) The geometry and the coverage-dependent growth structures formed by Na on other semiconductore, e.g. a GaAs (110) surface, have been investigated by STM at room temperature [7.58]. When the coverage is low, Na adatoms reside on the bridge site encompassing one Ga and two As surface atoms, as illustrated in Fig.7.18. It is interesting that the Na adatoms appear to form low-density linear chains with the nearest Na-Na distance of 8 A along the [110] direction and exhibit no long-range order in the [001] direction. The Na chains are separated by more than three As rows and are short in length (Fig. 7 .19). The formation of the ordered one-dimensional chain at low coverage suggests that Na atoms can move easily on the GaAs(1IO) surface at room temperature, and that repulsive interactions along the [001] direction can be significant at low coverages. The fact that the short chains form 238

Fig. 7.19. Occupied state imatges of Na on the GaAs (110) surface with a 2 (ca. scanning area of 130 X 180 0.07ML) recorded with sample bias of -2.5 V and tunneling current of 0.1 nA

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Fig. 1.20. Schematic drawing of the c4 X4 Cs overlayer observed on GaAs (110) with STM, ipdicating the c4 X4 unit cell and the five-atom Cs polygons, with average nn distance of 4. 9 A. The Cs density in this ideal overlayer is 5/16 ML. Only the As lattice is shown for clarity, Segments of a Cs (110) lattice are superimposed over the two polygons on the right [7.59]

even assuming that the Na adatoms could also be arranged in the same highdensity 2D structure c4x4 as in the case ofCs/GaAs(llO), the average NaNa nearest neighbor distance (5.3A) is still much larger than that in bulk Na (3.66A). The formation of such 2D structures, therefore, could not gain the necessary cohesive energy to overcome the free energy barrier. This simple argument holds with the STM results of Sm, Ce, Yb and Mg on the GaAs (110) surface [7.60,61], which do not form any ordered high-density 2D overlayer. In all cases the adatom nearest neighbor distance (5.3A) in the c4 x 4 high-density 2D structure is much larger than the bulk atomic spacing of these elements: 3.6, 3.6, 3.5 and 3.2 A, respectively. We speculate, based on this, that the element whose nearest-neighbor distance in the bulk is close to 5.0 A, such as Rb (5.1A), may form an ordered high-density 2D structure on the GaAs (110) surface. It should be pointed out here that this speculation rests on the basis that the adsorbate-adsorbate interaction is stronger than the adsorbate-substrate interaction in the present 2D nucleation process. The overlayer can grow quite differently even for the same atom if the adsorbate-substrate interaction is dominant over the adsorbateadsorbate interaction.

7.2.10 Alkali MetalsonSi(100)-2xl With increased Na coverages, the surface is found to be dominated by clusters at about 0.11 ML, and completely covered by bigger clusters at about 0.13 ML. The saturation coverage of Na is thus estimated to be roughly 0.1 ML. The I-V curves recorded over the various Na-covered surfaces show no evidence of metallic characteristics. It is interesting to compare the Na/GaAs (110) with the Cs/GaAs (110), in which Cs atoms form a zigzag chain at low coverages and, at high coverage, a high density c4 x 4 overlayer consisting of a five-atom polygon [7.59], as illustrated in Fig. 7.20. In contrast, neither the zigzag chain nor a high-density two-dimensional ordered structure is observed from Na/GaAs (110). As mentioned above, the linear chains are almost always separated by three units, even at the coverage close to the saturation. It is not clear why Cs form ordered 2D structure at high coverage when Na do not. As for the stability of the 2D nucleation, any 2D ordered structure must be energetically favorable to be formed on the surface. There are many important factors to be considered, such as changes in the electronic e~ergy through the change of bond lengths and angles, and the dipole-dipole interaction through the charge transfer. However, it is significant that the average Cs nearest-neighbor distance of the c4x4 structure formed on the GaAs (110) surface is 4.9 A [7.59] which is 7 %smaller than that of the bulk Cs (5.3A). Hence, the formation of the Cs 2D ordered structure may gain the cohesive energy. In the case of Na adatoms on GaAs(l10), however, 240

The adsorption of Li, Na, K and Cs on the Si(l00)-2x 1 surface has been systematically studied by STM [7.62,63]. All of these alkali atoms are brightly imaged in the filled states, suggesting that these atoms on the Si (l00)-2x 1 surface are not completely ionized. The evidence from STM results and theoretical arguments led to the following conclusion: • Individiual Li atoms sit on top of the dimerized Si atoms. Na and K atoms reside over two neighboring dimerized Si atoms. Cs atoms take up a position similar to that of K with further inclination towards the valleybridge site. All of these alkali atoms adsorbe alternatively in the dimer-row direction to form zigzag chains. • Alkali atoms (Li, Na, K, Cs) do not preferentially adsorb on defect sites (dimer defects of terrace edges). • Li, Na and K adsorption appears to stabilize the buckled dimerization upon adsorption, while no such effect is observed with Cs adsorption. The spatial range of buckled dimer stabilization due to alkali metal adsorption reaches approximately 20 units (xO.384nm) in the dimer-row direction and a few units (XO.768nm) perpendicular to the dimer-row direction. • Some clusters, which are composed of 2 or 3 alkali atoms, are formed. When the 2 x 1 surface is exposed to higher doses of Li, Na, K and Cs atoms (ca. 0.05ML), it is found that the alkali metals form linear chains 241

perpendicular to the Si dimer rows. At higher coverages, the formation of the linear chains is less noticeable. At saturation coverage, i.e. at 0.5 ML, adsorbates will occupy all dimer pairs, and end up with linear chains formed in both directions. The influence of temperature on the Li/Si (100)-2 x 1 system has also been studied. By annealing the sample at 773 K for 1 minute after 0.5 to 1 ML deposition of Li, the surface tuned into a 8 x I structure. The filled states of the Na saturated surface exhibit more pronounced bifurcation and is very much like the empty state image of the clean surface [5.62].

7.3 Molecules, and Molecular Adsorbates 7.3.1 Molecular Crystals The adsorption of small molecules on clean surfaces has been a field of study in surface science because of its applications to catalysis, corrosion, and etching. After STM had demonstrated atomic resolution on semiconductor and metal surfaces, both with and without atomic adsorbates, it soon became a challenge to image molecules at surfaces and interfaces as well. Early results for molecular adsorbates on metal surfaces have been somewhat less encouraging. For example, chemisorbed carbon monoxide molecules have not been resolved on Pt (100) surfaces, even though the CO-induced restructuring of the metal surface was observed. Images of Cu phthalocyanine on Ag surfaces showed low symmetry and resolution, which was interpreted in terms of molecular motion induced by the electric-field gradients near the tip. The difficulties in the imaging of molecular adsorbates have been thought to be due either to rapid surface diffusion, possibly augmented by electric fields, or to the absence of molecular orbitals near the Fermi level, which are necessary for STM. Recently, some STM studies of well-ordered surfaces have shown that molecules can be resolved and that their STM images provide information about molecular properties and molecule-surface interactions. In the first of these studies the surface of the conducting organic molecular crystal TTF-TCNQ (tetrathiafulvalene tetracyanoquinodimethane) has been examined with STM in air [7.64]. The STM images show a periodic surface corrugation with a 0.4 x 1.2 nm 2 unit cell in good agreement with the intermolecular spacings in the bulk crystal. More importantly, detailed structure within a unit cell arising from individual molecules is seen in the TTF-TCNQ images (Fig.7.2I). The image can be simulated assuming that it reflects the electron density of the Highest Occupied Molecular Orbital 242

Fig. 7.21. (a) An STM image of the TTF-TCNQ surface. Arrows indicate a row of triplets of dots are assigned to individual TCNQ molecules. (b) A simulated STM image by calculating surfaces of constant probability density. Arrows indicate a row of TCNQ molecules along the b direction (7.64]

(HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) of TTF and TCNQ, respectively. The triplets of raised balls in the STM image are attributed to the TCNQ molecules and reflect the nodal structure of the highest occupied TCNQ molecular orbital. The principal features of the experimental images are reproduced in simulations that are based on molecular orbital calculations, demonstrating that the STM provides a picture of the electron probability density of specific orbitals of surface molecules. Following this pioneering work, various other organic conductors, including the BEDT-TTF family [7.65,66] and conducting polymers, have been imaged by STM at molecular resolution. Individual organic molecules at nitroyl nitroxide single crystal surface [7.67] and some biomolecular crystals have also been observed by AFM (Chap.5).

7.3.2 Chemisorbed Aromatic Molecules in Ultrahigh Vacuum A second example of molecular imaging is chemisorbed aromatic molecules in ultrahigh vacuum. After some promising experiments on phthalocyanines on silver, it was an important breakthrough when high-resolution images of arrays of benzene co-adsorbed with carbon monoxide (CO) on a Rh (Ill) surface were obtained [7.68]. Previous work had shown that benzene and CO form a stable ordered 3 x 3 superlattice on Rh (Ill). The STM images reveal an ordered array of protrusions that are attributed to benzene molecules. For certain values of the tip bias and tunneling current, the individual benzene molecules appear as rings with a possible threefold distorted symmetry (Fig.7.22). Defects within the adsorbate layer at atomic-scale resolu243

Fig. 7.22. A three-dimensional view of the 3 X 3 superlallice of benzene and carbon monoxide coadsorbed on the Rh (Ill) surface [7.68]

tion [7.69] and movements of individual molecules can be resolved with the STM. One pleasing result of this work is that one can image certain chemisorbed molecules by examining mixed metal-adsorbate states near E F , thereby avoiding the use of high bias voltages which often damage the surface and the tip. Although there is no simple rule for predicting the magnitude of the corrugation to be expected for a particular molecule on a given metal surface, it is likely that large molecules which have many closely spaced electronic states split by strong molecule-surface interactions, will often lead to states near E F and result in a useful STM footprint. Following the benzene work, the same group further studied naphtalene chemisorbed on Pt(111) [7.70]. STM images indicate that the planar molecules pack in a herring-bone structure, as has been proposed from LEED measurements on this system. However, the herring-bone unit cells are formed only within very small domains of typically less than 5 nm diameter. At the domain boundaries a molecular shift by an additional Pt lattice constant occurs. Other aromatic molecules which have been studied by STM include Cu-phthalocyanine on Cu(100) [7.71] and GaAs(1lO) [7.72], which allowed the internal structure of an isolated molecule to be imaged. Unusual molecular binding sites at step edges were observed for the first time. These results are an important step towards real-space imaging of surface-molecule interactions, and further suggest that co-adsorption techniques may be helpful in reducing surface diffusion and for moving E F closer to molecular states. STM studied on carefully chosen metal-adsorbate surfaces appear promising for observations of surface chemical processes, such as molecular diffusion, nucleation phenomena, step or defect-related reactivity.

244

7.3.3 Physisorbed Molecules in Ultrahigh Vacuum As has been mentioned in Sects. 3.4.4 and 7.2.1, the STM is not going to tell us whether the bumps are Ag or Si in the images of the Ag/Si (111) -v3 x v3 surface. While we interpret the images of adsorbed molecules, a note of warning is that the appearance of adsorbates in STM images can vary according to the binding site. Weiss and Eigler [7.73] found three distinct types of STM images for benzene adsorbed on the Pt (111) surface, depending on the benzene adsorption site (Fig. 7.23). The type of image seen in Fig. 7.23a matches the appearance of benzene coadsorbed with CO on Rh (Ill). While Ohtani et al. [7.68] noted only one orientation of the three lobes on the surface, Weiss and Eigler observed two possible orientations, rotated 60 0 with respect to one another. These two orientations are assigned to both hcp-type and fcc-type adsorption sites. Theoretical calculations indicate that these sites have nearly identical chemisorption energies, are stable in that no lower energy sites exist on the perfect crystalline surface, and give STM images with three symmetric lobes [7.74]. The ring-like image type shown in Fig. 7.23b is only observed near other adsorbates or defects on the surface, and is thus assigned to the atop sites. This indicates

Fig. 7 _2)a-c. STM images of three different 15 X 15 A 2 regions of p[ (III) each showing a single adsorbed benzene molecule. The images were recorded with (a) V = -0.050 V. I = 100 pA; (b)V =-O.OIOV. I = InA; and(c)V =0.010 v. I = 100 pA. respectively. The minimum to maximum .heigh[ difft';rences in [he images are (a)0.58 A. (b)O.72 A, and (c)0.91 A, respectively. The observed images of individual molecules did nOl change qualitatively for a wide range of tunneling parameters [7.73]

245

that the chemical environment, i.e., the nearby adsorbates and defects, may playa role in stabilizing adsorption at these sites. Finally, the single-bump structure visible in Fig.7.23c is assigned to be bridge-bonded benzene. This site dependence is in agreement with theoretical calculations [7.74]. This study demonstrates that the characteristic image types are due to adsorption of benzene at different sites on the surface, providing useful information on how electronic structure varies with chemical environment. Another example of molecular imaging is the STM study of the adsorption and reactions of ethylene adsorbed in UHV on Pt (Ill) as a function of temperature [7.75]. Adsorbed ethylene has often been used as a model for hydrogenation-dehydrogenation reactions in catalysis studies. In particular, its interaction with the close-packed Pt (111) surface has been investigated by a variety of surface-science techniques [7.76]. The STM images taken at 160 K show an ordered structure of adsorbed ethylene which molecularly bonds to the Pt surface. Annealing to 300 K produced ethylidyne (C-CH 3 ) irreversibly, as has been demonstrated by a wide variety of surface-science techniques, but the ethy lidyne on Pt (Ill) has not been detected by the STM at room temperature. However, by cooling the sample to 180 K, the ethylidyne ordered structure can be observed. Annealing in the range of 430--:700 K causes further dehydrogenation and decomposition of ethylidyne until only carbon remains on the surface. The decomposition products appear as small clusters which are localized and uniformly distributed over the surface. Further annealing to temperatures >800 K results in the growth of graphite islands on the Pt (111) surface. The annealed graphite islands exhibit several superstructures with lattice parameters of up to 2.2 nm, which can be explained nicely by the higher-<>rder commensurability of the graphite and platinum substrate at different relative rotations. STM has been used to distinguish a series of related molecules on Pt (111), including naphthalene, its isomer azulene, and l-methylazulene, 2-methylazulene, 6-methylazulene, 4,8-dimethylazhulene and 4,6,8-trimethylazulene. Typical lower-resolution images are used to measure the relative sticking coefficients and relative diffusion rates of the molecules. The orientations and binding sites of several of the molecules on the surface are also assigned. Tip dependent higher resolution images can show internal structural details on molecular species which have low diffusion rates [7.77].

7.3.4 Physisorbed Long-Chain Molecules As we know, STM allows high-resolution imaging not only in UHV but also at the internal interface between two condensed media, one being a conducting solid and the other a gas, a liquid or a soft solid. Under these con~ 246

ditions various molecules physisorbed to inert substrates have been investigated. The first unambiguous report on high-resolution molecular imaging at the solid-fluid interface was on two liquid crystals, 4-n-octyl-4'-cyanobiphenyl and 4-(trans-4n-pentylcyclohexyl) benzonitrile on HOPG [7.78]. The STM images reveal that the molecular order at the interface is increased over the bulk, and that the adsorbate lattice is oriented relatively to the substrate. Subsequently other liquid crystallHOPG interfaces [7.79] as well as a homologue series of cyanobiphenyls [7.80,81] have been imaged. The latter were prepared as thin films, whose exact thickness is not relevant to STM imaging, since the tip advances in any case all the way through the insulating organic layer. Different packings of molecules on different substrates [7.82,83], and a bias-dependent rearrangement within a molecular layer have also been observed [7.84]. Interesting is the fact that the first monolayer at the interface with graphite could be imaged even if the film forming material was in a crystalline phase. Besides liquid crystalline phases, long-chain alkyl derivatives can also form highly ordered monolayer at the interface between organic solutions and HOPG substrate. It should be emphasized that the interaction energy between a small molecule and a chemically inert substrate is usually too small to immobilize an individual molecule sufficiently at room temperature, therefore, alkyl-derivatization turned out to be a method to immobilize small molecules within a monomolecular layer on graphite and image them by STM. The first image of such a layer was obtained on dotriacontane (C 32 H66 ) [7.85]. Subsequently, similar two-dimensional molecular patterns were obtained on many other alkanes with chain lengths ranging from nonadecane (C I9 H 40 ) through pentacontane (C 50 H I02 ); alcohols such as octadecanol (C 18 H 37 OH), tetracosanol (C24 H 49 OH), and triacontanol (C 30 . H61 XOH); fatty acids including stearic acid (C n H 35 COOH), arachidic acid (C 19 H 39 COOH), and tetracosanoic acid (C 23 H47 COOH) and didodecylbenzene [H 25 C l2 (C 6 H 4 )C 12 H 25 ] [7.86,87]. In-situ STM studies reveal that all of these molecules organize in lamellae with the extended alkyl chains oriented parallel to a lattice axis within the basal plane of graphite. The planes of the carbon skeletons, however, can be oriented either predominantly perpendicularly to or predominantly parallel to the substrate surface, causing the lamella lattice to be either in or near registry with the substrate (alkanes and alcohols) or not in registry (fatty acids and dialkylbenzenes). In Fig. 7.24 the structures of alkanes, alcohols, fatty acids and a dialkylbenzene are compared. Note the superstructure along the lamellae of the flatly lying fatty acid molecules, which does not exist in the alkane lamellae. Since the images contain information on both adsorbate and substrate lattices they are attributed to an incommensurate and commensurate adsorbate, respectively, settling a scientific issue of long standing. In the case of the alcohols and the dialkylbenzene the molecular axes are tilted by +30 or _30 with res0

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pect to an axis normal to the lamella boundaries, giving rise to molecularly well-defined domain boundaries. It was found by in situ AFM studies that the adsorption of anions of the organic acid onto cationic surface is jointly affected by electro-neutrality and steric interactions, and that the adsorption could be reversible [7.88]. STM images of organic molecules are usually interpreted as time averages over their molecular dynamics, which typically occur on the picosecond time scale. However, there are also slower molecularly defined processes occurring on a time scale longer than milliseconds, which can be directly followed by STM. Fast STM image recording allowed the spontaneous switch between the two tilt angles to be observed in the alcohol mono layers on a time scale of a few milliseconds. Similarly, the movement of a domain boundary has also been observed in the didodecylbenzene monolayers, which can be explained by the quasi-simultaneous rotation of at least 248

about a dozen didodecylbenzene molecules around their long axis by 180 0 , together with a lateral shift by half a molecular length [7.89]. The imaging mechanism for adsorbed organic molecules has been a very interesting topic, as it not only involves the sample properties but also the possible interactions between tip and sample, sample-substrate. So far, several proposed mechanisms gave reasonable accounts for the data from different systems. The polarizability of the adsorbed Liquid Crystal (LC) molecules is seen likely to modify the local tunneling barrier. The tunneling current will thus not only reflect the LDOS distributions, but also the properties of the adsorbed dipole layer [7.90]. An attractive interpretation of STM images of LC molecules would be the correlation of tunneling image to the orbitals of molecules. It has been demonstrated for nCB molecules that the electric field under the tip apex is sufficient to induce significant shift of energy levels [7.91]. Furthermore, the STM images may involve several adjacent orbitals close to the Fermi level (Figs. 7.25,26). It could be perceived that validity of the models depends on whether there are orbitals available for tunneling electrons or not. If there exist orbitals involved in the resonant tunneling then the later model applies, otherwise the polarizability associated barrier modification should be the dominant machanism.

7.3.5 Chemisorption of Long-Chain Molecules An important category of chemisorbed molecules are self-assembled species on the surface of noble metals. Self-assembled molecules have been widely perceived as a venue to obtain novel material properties at nanometer scale. The application of STM 249

Fig. 7.26 Molecular mechanics modeling and molecular orbitals in superposition, along with a STM image of MDW74. Molecular model of MDW74 on graphite, with lowest four unoccupied orbitals

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250

251

in this fast growing field has generated considerable interests. The scope of the research spans from the basic mechanisms for self-assembly (Figs. 7.7,28), conductivity, elasticity, molecular ordering, the relevant interfacial properties [7.94-96]. These molecular systems provide an ideal testing ground for STM technique. STM has been remarkable in studying many chemisorbed species, especially the related dynamic processes. By using variable temperature STM, a series of experiments have revealed the unique nature of the diffusion of chemisorbed CO molecules [7.97,98], Si dimers and allowed artificially control of the precursor state by application of an electric voltage pulse [7.99] (Fig.7.29). It was shown that the Fermi energies of the tip and sample could be shifted by the applied bias. In case of the aligned molecular energy level would lead to resonnant tunneling [7.100]. Openning up possibilities for differenting structurally similar molecules based on their redox properties (Fig.7.30).

IIA

Fig. 7.30. STM images of Fe-Protoporphyrin IX molecule embedded in anordered array of Protoporphyrin IX molecules when the substrate was held at (A)-0.15 (B) -0.30 (C) 0.42 (D) -0.55 and (E) -0.65V, respectively. (F)-(J) are the correponding plots of the cross sect ion along the Iine indicated in

(A) [7.100]

7.3.6 Fullereoes After breakthroughs in the synthesis of CliO and related fullerenes, monolayer and multilayer structure of C 60 , a high-temperture van der Waals solid, has been successfully imaged with STM [7.10 1-113]. Since the spherical C 60 molecule is bound by van der Waals interaction and desorbs in ultrahigh vacuum at about 823 K, C 60 monolayers, multilayers, and solids offer unique opportunities to explore crystal growth driven by a simple interaction potential. The substrates have been used for STM studies including GaAs (110), Si(lOO) and (111), HOPG(OOOI) and MoS 2 (0001), Au(lII), (110) and (100), Cu (Ill) and Ag (Ill) surfaces. Some fundamental questions about the nature of the interaction at the substrate-C60 interfaces have been addressed, such as: do substrate atoms interact in concert with C 60 molecules as a whole or do they form covalent bonds with just a few of the nearest carbon atoms within each C 60 molecule. In either case, how can we best describe the interaction force and its strenth?

a) C 60

00

GaAs (110)

On the GaAs (110) , STM studies [7.10 1, 102] indicate that C 60 has fairly high mobility on the surface due to its weak interaction with the substrate. The STM results reveal that ordered C 60 structures can be commensurate or incommensurate with the GaAs (110) substrate. Structures grown on GaAs (110) at 300 K and at elevated temperatures show significantly different 252

(C)

Fig. 7.31. (A) STM image of a C 60 layer on two adjacent terrace of a Fe (100)-(2 X I) surface with sparse brighter C 70 . (B) C 60 molecules reside in the trough between the dimer rows of the uncovered surface. (C) STM image of a partially ordered C 60 overlayer on the Si (100)-(2 XI) surface [7. 105]

253

morphologies because of balances between thermodynamics and kinetics. Multilayer growth at 300 K produces growth structures with point defects, dislocations, domain boundaries, and surface faceting. Multilayer C60 growth at 470 K yields structures that are almost perfectly ordered. Condensation onto stepped surfaces demonstrates preferred bonding and nucleation at step edges. Detailed studies of potassium incorporation in crystalline C 60 indicate highly-ordered structures in the K 3 C 60 metallic state but disordered non-metallic structures for high potassium concentrations. These are the first direct images of multilayer structures of a van der Waals solid, and they demonstrate complex order and disorder due to kinetic constraints.

b) C 60 on Si(IOO) and Ga(OOI) It is known that the C 60 molecules are rotating in the bulk fcc crystal at the rate of 10 0 turns per second at room temperature. On Si(100), however, the STM studies [7.104] show that C 60 remains stable on the surface without rotation at room temperature because of the strong bonding with Si dangling bonds. The sticking probability for C 60 is much higher although island formation begins after the first disordered layer is completed, the bulk-like ordered fcc islands are only found above the third layer. At the monolayer coverages, the local c4 x4 and c4 x 3 orderings are observed. The formation of incommensurate superlattice of C 60 on Si (100) and Ge (100) surfaces [7.105] provides evidence that attractive and repulsive intermolecular forces are jointly responsible to stabilize and organize the C 60 on surfaces (Fig.7.31). c) C 60 on Si (III) On Si(lll) surface, fcc crystalline C60 islands are oriented primarily in two distinct directions independent of their height or size. STM images reveal that these two preferred orientations are clear manifestations of the formation of a double domain C 60 solid on the Si(111)-7X7 surface. The process leading to such a unique structure involves a disorder-to-order interface transition in which interplanar molecular interactions play an indispensable role [7.106]. The force binding the C 60 molecules to the semiconductor substrates is comparable to the van der Waal's forces between the molecules of the C 60 solid in general, but it is extremely sensitive to the detailed local atomic and molecular configurations.

d) C 60 on MoS 2 (000 I) The cleaved (0001) surface of MoS 2 has large atomically flat areas with small defects, as compared to Si surfaces, but no dangling bonds. STM images of submonolayer-to-monolayer coverages of C 60 on the 2H-MoS 2 surface reveal unique adsorption and initial growth patterns that are not 254

observed in similar coverages of C60 on metal and other semiconductor surfaces. Intramolecular contrast of the adsorbed C 60 reveal both the local density states of the molecule and the underlying crystalline lattice features. STM results [7.107] indicate that (1) the adsorbate-substrate interaction is van de Waals like and (2) the electronic structure of the adsorbates is therefore only weakly perturbed by the adsorption process; the electronic structure of an isolated C 60 is, to a large extent, preserved on the MoS 2 surface in contrast to adsorption on metal surfaces.

e) C 60 on Cu (Ill) In the case of C 60 adsorption on the Cu (111) surface, C60 molecules are mobile on the terrace and segregate to step edges at the initial stage. After filling up the adsorption position at the step edges. they form 2D islands grown from step edges to the upper terrace and then form the 4 x4 domains at the monolayer coverage. A large amount of charge transfer from Cu substrate to C 60 molecules is observed [7.108]. Because of the interaction between Cu surface atoms and C 60 molecules and possibly because of the interaction among adjacent C 60 molecules, including the interaction associated with the orientational ordering of the C 60 molecules, the rotation of the C 60 molecules is reduced or terminated. The bias-dependent intramolecular structures with threefold symmetry are observed and interpreted as 3D mappings of the charge density of states near the Fermi level. f) C 60 on Au(lll) and Ag (Ill) The Au (Ill) surface undergoes an unusual reconstruction that acts as an ordered array of nucleation sites for the growth of metal films. The behaviour of C 60 on Au (11l) and Ag (111) has been studied and compared because Ag (Ill) has the same lattice constant and nearly the same electronic structure as Au (111) but does not reconstruct. On both Au (111) and Ag (111), C 60 are found to grow in a layer-by-layer manner forming close-packed adsorbate phases with a spacing of 1.0 nm [7.109], the same as fcc (111) C 60 . On Au (111) two preferred adsorbate phases are observed: an "in-phase" structure with roughly 38 x 38 periodicity in which the crystallographic directions of the overlayer match those of the substrate and the lattice matching is poor, and a (2v3 x 2V3)R30° structure with nearly perfect lattice matching (less than 0.4 % mismatch) in which all the molecules are in equivalent surface sites. The former is attributed to the kinetics of the growth process and the latter is expected to be thermodynamically favored. On Ag (111) only the (2V3 X2V3)R30° structure is observed indicating a higher driving force for adsorption into specific sites on the more reactive surface [7.110]. Adsorbed C 60 molecules are found to rotate faster in Au (111) than on Ag (Ill) also indicating a stronger interaction between C 60 and Ag. 255

It was nicely demonstrated that the underlying Au atoms could be affected to maximize the bonding between C 60 the substrate Au atoms, leading to a formation of (1 x5) interfacial reconstruction pattern [7.111]. On both surfaces, spatially-resolved tunneling spectroscopy and barrier-height measurements indicated that bonding occurs by charge transfer from the metal to C 60 . The internal structure of adsorbed C 60 observed by STM was found to be a strong function of the adsorption site, the molecular orientation and the tunneling bias. Occupied state images were found to reflect the adsorption site symmetry while unoccupied state images reflected the molecular symmetry. In domains where all the molecules are in equivalent adsorption sites, the observed internal structure of adsorbed C 60 has been assigned to an on-top site with the five-membered ring on the surface. The 23 x v3 Au (111) reconstruction has been found to influence the nucleation and growth of C 60 films. The fcc sites at steps are found to act as nucleation sites for C 60 resulting in periodic arrays of three and four molecule chains at hcp sites at low coverages. Conversely, on the uniform Ag (111) surface, the steps tended to saturate before growth onto the terraces. STM studies show that at equilibrium the Au atoms at the C 60 interface occupy fcc or bulk terminated sites but that the large scale rearrangement of Au atoms required to lift the reconstruction causes a kinetic constraint that can trap the Au surface in nonequilibrium configurations. These examples indicate that the strength of the interfacial interaction varies appreciably for different substrate surfaces. Besides the results mentioned above, C 60 LB films on Au (100) surface [7.112] and a film prepared by sublimating C60 solid at 360 C in vacuum on a mica-supported Au (111) film substrate have also been studied [7.113]. 0

7.3.7 Langmuir-Blodgett Films Some organic Langmuir-Blodgett (LB) films, such as diyonic, arachidic, w-tricosenoic acids and 4-octyl-4'-(5-carboxy-pentamethylene oxy)-azobenzene (ABD) have also been imaged by STM. A review of this subject can be found in [7.114]. Although STM is adequate for investigating films in real space and under ambient conditions, in some cases it still seems difficult to define experimental parameters providing images with the individuaIly resolved molecules of LB films. Besides these organic molecules, many biomolecules adsorbed on a variety of substrates have also been studied by STM. For biological applications of STM, see Chap.8.

256

7.4 Observation of Clusters

The understanding of metallic cluster compounds has a specific significance in the study of catalyst structures and catalyitic mechanisms. Metallic cluster compounds are not only used as homogeneous catalysists in some organic synthetic reactions, but also as ideal models for the study of heterogeneous catalytic reactions. Previous experimental and theoretical analyses indicated that one cluster composed of 2 to 100 atoms may have some unique bulk and surface properties which any other solid cannot have. For example, when the number of atoms increases from one to several hundred in a cluster, the energy levels of atoms and molecules will gradually change into an energy band. The energy-level diagram of clusters of middle size is the subject of many theoretical and experimental studies. For determining the relationship between adsorption, catalytic activity and steps, as well as relevant sites, two kinds of measuring methods are employed: one is the measurement of the activity difference of clusters; the other is measurement of reactions and adsorptions on the surfaces of single-crystal steps by means of surface spectra.

7.4.1 Metal Clusters One class of systems having interesting catalytic properties is the metal carbonyl clusters such as those based on rhenium. When supported on aluminum and activated by heat treatment, these complexes readily catalyze the formation of methane from carbon monoxide in a hydrogen ambient. One possible mechanism for this behavior is that the rhenium-carbonyl clusters group into aggregates upon heat treatment and behave in concert to promote the observed catalytic reactions. STM studies have been initiated to test this hypothesis. Both individual and aggregates of clusters are observed and changes brought about by heat treatment as well as using different solvents are investigated. In the case of small Ag and Au clusters, the electronic structure and chemical reactivity have been treated extensively, but the morphology has remained open to debate. For systems containing only a few atoms, the local environment of each atom may be unique, and it is thus desirable to study these systems on an atomic scale, since the morphology forms the basis for predictions of the behavior of these systems. Ag, Au, AI and Pd monomers, groups of monomers, dimers, trimers and smaIl, two-dimensional islands on graphite have been observed [7.115-118]. Figure 7.32a represents an image of a portion of a silver island, while Fig.7.32b is the corresponding computer model. A portion of a gold island is shown in Fig. 257

substrate lattice. The fact that the long axis of the Au lattice is observed to smoothly with time also suggested that the Au-graphite coupling is weak, and that there is a shallow minimum in the potential energy as the long axis is varied. One can hope to resolve these issues only with detailed model calculations. STM has been applied to examine ultra fine Pt metal particles (a few nanometers in diameter) dispersed on the surface of a vacuum-deposited carbon film that simulates an industrial Pt/C supported metal catalyst. Figure 7.34a illustrates the center region of STM images of a Pt/C surface. Close examination shows that there exist several flat faces on the particle surface, as marked by dotted circles in the figure. It indicates that these ultrafine Pt particles are bounded by relatively flat, specific crystallographic faces. It is reasonable to assign thermodynamically stable, lowindex planes to those faces. There are various ways to construct a particle with low-index planes. Five possible shapes for face-centered cubic (fcc) metals are examined for fitting the three particles depicted in Fig.7.34a. It is found that a cubo-octahedreon structure represents most closely the shape of the three particles. In Fig.7.34b, the three ultrafine Pt particles found in (a) are reconstructed using three cubo-octahedron halves. It is noted that the orientation of those cubo-octahedra are not the same for the three particles, d~crease

Fig. 7.32. (a) 3.5 X3. 5 nm 2 current image of part of a monolayer of Ag island on graphite. The graphite honeycomb lattice is visible at the lower right. (b) Computer model showing the positions of the adatoms (filled cirles) on the graphite honeycomb lattice (small dots at (3 sites). Lines have been drawn to guide the eye [7.115]

y

Fig. 7.33. (a) 3.5 X3. 5 nm 2 image of a monolayer Au island on graphite. The graphite is imaged as dots at the top. (b) Computer model showing a rectangular lattice on the left and a honeycomb lattice on the right [7.115] II !lm

7.33. With the low coverage (roughly 0.1 % of a monolayer), it is possible to image both thB edge of an island and the neighboring graphite lattice simultaneously. The islands observed contain ordered regions of roughly 50 atoms in rectangular lattices, incommensurate with the substrate lattice, which are not closely packed as in the bulk fcc structure. In one series of images, the shorter lattice spacing (short axis of the rectangular lattice structures) remained constant at 0.23 nm, whereas the longer decreased from 0.40 to 0.35 nm over a period of roughly 10 min. These results demonstrate that the interaction between ada toms and the substrate is small since the island structures are neither commensurate nor aligned with the 258

Fig. 7.34. (a) The center region of an STM image. Dotted circles indicate relatively flat surfaces on the Pt particles. (b) Representation of Pt particles shown in (a) with halves of a cubo-octahedron [7.119]

259

indicating that there is no epitaxy in the growth of these Pt particles. This may be a reflection of the noncrystallinity of the carbon substrate employed.

7.4.2 Semiconductor Clusters Apart from metal clusters, STM has also been employed to image semiconductor clusters. Semiconductor clusters embedded in dielectric solids or liquids exhibit third-order nonlinear susceptibilities which can be utilized for optical signal processing applications. An important issue in understanding the optical response of such a system concerns the size and composition of these semiconductor clusters. Composed of a few to several hundred atoms, semiconductor clusters also exhibit features not seen in their bulk analogs. In small silicon clusters, for example, band-structure calculations indicate that silicon atoms bind with a coordination much higher than the four-fold tetrahedral arrangement found in the bulk material. To achieve an understanding of the size-dependent materials properties, one would like to experimentally determine the structure of clusters containing different numbers of atoms. STM studies of Bil 3 clusters [7.120], typically composed of ca. 10 -:- 1000 atoms, demonstrate that microcrystallites can be probed with an STM to yield atomically resolved images of small clusters. The image depicts ten atoms arranged as two fused six-membered rings which have a distance of 0.65 nm between centers.

7.5 Nucleation and Growth Epitaxial growth is of wide technological interest, particularly in integratedcircuit fabrication. Theoretical models for nucleation and epitaxial growth have been verified by direct imaging techniques such as SEM. TEM and REM. However, many of these methods do not have sufficient resolution to analyze growth on an atomic scale. Moreover, SEM cannot study autoepitaxy. Because of its atomic resolution, STM offers the possibility to study the microscopic mechanisms of crystal growth, initial stages of epitaxy, monolayer-thick metal films on semiconductors and surface order processes at a detail heretofore unachievable.

260

7.5.1 Epitaxial Growth of Metal Films The studies of the properties of metal films with thicknesses in the range 1-:10 monolayers (IML = 8.85'10 14 atoms/cm 2 ) are important for an understanding of both the evolution of the film structure up to the bulk phase, and the concomitant development of the electronic properties of the film. Rows of close-packed metal atoms on the surfaces of Cu(111), Ag(lll), Au (111) and Au (110) thin films with the thickness of a few hundred Angstroms have been resolved in air using STM. An average corrugation amplitude of about 0.03 nm is found for these surfaces. In the case of Sb films at 10 ML, a layered structure is observed and very low conductivity is found at the Fermi level, indicating semimetallic behaviour. For Sn films, a layered structure with semimetallic nature has been observed below 2 ML. It was found that the first layer growth always forms a double-layer structure, which consists of a 1 xl Sn and an adatom Sn layer. This adatom layer appears to play an important role for layer-by-layer growth of the Sn overlayer. At Sn coverages above 3 ML, a structural phase transition is observed, accompanied by an electronic transition into a metallic state. Several stages of autoepitaxy on Au (111) from submonolayer up to twenty monolayer coverage at room temperature have been investigated by STM in ultrahigh vacuum [7.121]. The substrate Au (111) epitaxially grown on mica, exhibits several hundred Angstrom wide atomically flat terraces separated by monatomic steps. At submonolayer coverages, the gold nucleates into single-layer clusters arranged preferentially in rows along [112] directions. As the metal coverage increases, cluster coalescence by growth is observed. Cluster-size distributions and spatial correlation functions have been extracted from the STM data. Higher layers start forming before the lower ones are completely filled. The number of incomplete layers increases with deposition rate and total thickness of the film. Room-temperature diffusion smooths the terrace structure over a period of several hours. This process is observed to accelerate with a moderate anneal. Normally, metal-on-metal growth is believed to follow one of the following three well-known growth modes: layer-by-Iayer growth, layer-bylayer followed by 3-Dimensional (3D) growth, and 3D growth, depending on the magnitude and sign of the sum of the interface energies and the surface-free energies for the substrate and the adsorbate, respectively. However, it has been realized from STM studies of the growth of Au on Cu (100) [7.122] and Ag (110) [7.123], Ag on Pt (111) [7.124] as well as for Au on Ni (110) [7.108] that intermixing may be much more important than previously anticipated. In all these cases, however, the two metals involved are miscible. For a system such as Au on Ni (110), one would anticipate that Au grows on top of the Ni (110) crystal since Au is inmiscible in Ni, and no 261

ordered or disordered alloys exist. However, STM studies [7.125] have shown that this is not the case, and an unexpected growth mode is revealed in which a surface Au-Ni alloy is formed. When even very small amounts of Au (ca. 0.05ML) are evaporated on the Ni(llO)-1 xl surface, Au atoms are substituted into the first Ni layer. Both the density of substituted Au atoms and the surface area covered by Ni islands increase linearly with the amount of Au deposited up to Au coverage of about 50 %, and the area covered by the Ni island is equal to the integrated area of the substituted Au atoms. A model has been proposed to explain this surface alloying [7.126]: diffusing Au adatoms on the surface replace Ni atoms in the close-packed rows through a concerted cross-channel diffusion mechanism. The Ni atoms thereby squeezed out agglomerate into Ni islands which grow epitaxially on the Ni(llO) surface. The Au atoms are about 16% larger than the Ni atoms, .which causes a relaxation of the surrounding Ni lattice, as observed in the STM images which gives the appearance that the Ni atoms next to the substituted Au atoms are imaged higher than the remainder of the Ni atoms in the surface layer. The key factors affecting the nucleation and the growth have thouroughly been investigated by STM. Examples are the silver dimers which are stable nuclei for T < 110 K on Pt (111) [7.126]. A typical diffusion barrier has been estimated to be at about 0.2 eV (Ag on Pt(lII), Au on Au (100» [7.127]. It has been shown that lattice strain is important for film growth [7.128]. Furthermore, both thermodynamic and kinetic considerations should be taken into account [7.129]. Another interesting aspect is the surfactant effect which may display substantial impact on the overlayer-growth behavior by affecting the mobility of the adatoms. An example is Ag (111) in the presence of Sb [7.130]. This phenomenon can be observed in a wide variety of surface processes.

7.5.2 Growth of Si on Si (00 1) It has been known that growth is a nonequilibrium phenomenon, involving a variety of kinetic processes whose rates are generally thermally activated. Surface structural disorder, defined as any disruption of perfect order in the surface, influences a wide variety of surface kinetics and thermodynamic processes, surface electronic properties and surface chemistry. STM studies of surface disorder are valuable both in assessing models of surface processes determined from other techniques and in formulating them where other techniques fail. Lagally et al. [7.131] first studied the ordering process of Si on Si(OOI) from a viewpoint of two-dimensional ordering. The schematic diagram is sketched in Fig.7.35.

262

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All atoms are assumed to occupy lattice sites given by the substrates. The existence of a superlattice structure in the overlayer leads to translational and rotational antiphase domains that are energetically equally like (ground-state degeneracy) and leads to domain boundaries, as shown in Fig. 7.35c,d. Domain boundaries have little importance at low coverage because ordering is assumed to proceed by monomer desorption-diffusion condensation rather than by boundary motion. Figure 7.36 displays a series of STM images of a Si (00 1) surface on which 0.2 ML of Si has been deposited. The initial deposition is at room temperature. Subsequently, the sample is heated to three higher temperatures. Si(OOl) reconstructs to form dimer rows, which are orthogonal on alternate terraces separated by a single atomic-height step, as is evident in the images. All the above images show two terraces of the Si (00 1) substrate separated by a monatomic step. The lines running orthogonally on the two terraces are rows of dimerized pairs of Si atoms corresponding to the 2 x 1 and I x2 reconstructions of the surface. Several things are immediately obvious from these images: the deposited Si atoms form small islands implying a net attractive interaction; these islands grow at higher temperatures and have anisotropic shapes with the long axis orthogonal to the dimer rows of the terraces underneath. 263

7.6 Chemical Reactions on Metals

Fig. 7.36. STM images of the growth and coarsening of Si islands deposited on Si (001). The dose is about 0.2 ML. The size of each panel is 36 X36 nm 2 . Top left: deposition at room temperature. Top right: after annealing for 4 min at 520 K. HOllom left: after annealing for an additional 2 min at 583 K. Hollom right: after annealing for an additional 2 min. at 623 K [7.I3l)

Several possibilities have been advanced for these anisotropic island shapes during the growth of Si on Si (00 1): (i) they are an equilibrium crystal shape caused by adatom interactions that differ in different directions; (ii) they are caused by anisotropic surface diffusion; and (iii) they are a nonequilibrium structure caused by aspects of the growth dynamics. By appropriate measurements at submonolayer coverage, and - during the initial stages of growth - further STM study demonstrated that the shapes are not equilibrium structures and are not a consequence of anisotropic diffusion [7.132]. Rather, an anisotropic lateral accommodation coefficient seems responsible for most of the growth shape anisotropy. At a growth temperature of T ::::: 500 K the two-dimensional accommodation coefficient for atoms arriving at the edge of an island differs by an order of magnitude at the ends and at the sides of dimer rows in bulk crystal growth. It is likely that other observed anisotropic island shapes are also growth structures and that this result can be generalized to many more structures evolving in a highly nonequilibrium manner.

A study of chemical reactions occurring on surfaces, i.e., surface reactions, is of great importance because of fundamental interest as well as a large number of important technological applications in catalysis and corrosion. To understand the reaction mechanisms it is first necessary to obtain a characterization of the structures and bonding of adsorbed species on the surfaces. Surface-specific sites, such as defects and steps, are often believed to be active in surface processes due to a possibly low-energy configuration compared to the perfect substrate regions. It would therefore be expected that many surface reactions preferentially take place on those active sites. Nevertheless, the surface reactions can be affected by other facts, e.g., surface diffusion and activation energy. It is of critical interest to observe what really happens in a particular reaction system and how the change of surface structures when the reaction proceeds, issues important in understanding the reaction mechanisms. Conventional techniques encounter great difficulty in answering these types of questions because of an inherent inability to probe local information in investigating surfaces. The power of STM is to explore surfaces in real space and real time at atomic scale. While a chemisorbed system reacts with a gas to form a new structure, this process is reflected by a structural transformation on the surface. With the STM, it is feasible to study the dynamic processes and the reactions mechanisms, if the adsorbates (reactants and products) stay on the surface for a considerable period (on the order of a second), they can thus be imaged. Such an ability also offers an advantage for studying the surface reactions in the presence of a gas environment. Previous investigations revealed that the formation of the O-induced 2x 1 reconstruction on Ni(1lO) [7.133,134] and Cu(110) (Sect.7.1.1) proceeds via a novel mechanism, an added-row growth mode, in which metal atoms are released from, for example, step edges, diffuse across terraces, and combine with chemisorbed 0 atoms into low-coordinated-metalo added rows running along the [001] direction. The added rows nucleate homogeneously on the terrace and grow anisotropically in 2 x I-reconstructed islands, with a long/short coherence length in the [001]/[110] directions, respectively.

7.6.1 Reaction on Ni(llO) The dynamics of a reaction between H2 Sand preadsorbed 0 on Ni (110) was analysed by exposing the well-ordered (2x1)-O surface (Fig.7.37a) to a 264

265

Fig. 7 _37a-f. A series of STM images (V :::: 10m V and I :::: 1--:-2nA) recorded during the reaction, after progressively higher exposure of HZ S (a) 0 L (b)? L, (c) 8 L, (d) 20 L, (e) 25 L, (f) 35 L. All the images,are recorded on an area of 85 X91 AZ except the last one (f), for which the area is 59 x63 AZ only [7.135]

HzS gas at Room Temperature (RT). STM images were recorded sequentially on the same area. Figures 7.37b-f are a series of STM images of the surface structure during the reaction after progressively higher Hz S exposures [7.135]. Hz S reacts with preadsorbed 0 on Ni (110), resulting in the formation of water which desorbs at RT, leaving chernisorbed S on the surface, i.e., HzS(g)+Oad ~ HzO(g)f+S ad . The initial phases of the HzS-O reaction are visualized through the formation of the troughs and islands next to them. When 0 atoms are removed from the (2 x 1)-0 structure, the resulting low-coordinated "cl~a~" Ni rows are ",ery unstable. The Ni atom~ 266

in the rows accumulate in small Ni-1 x 1 islands, leaving Ni-1 x 1 troughs behind. Sulphur chemisorbs on these Ni-1 x 1 areas, and c2 x 2-S troughs and islands result. For increased Hz S exposure, the small c2 x 2 islands suddenly become unstable. They dissolve and, simultaneously, from the islands. One observes (Fig.7.37e) the growth of low-coordinated rows along the [001] direction; the periodicity along the rows is one. During the continuous transformation of the islands to the rows, the areas of the c2 x 2 troughs gradually decrease and, as more rows are formed, they develop into ordered regions with a periodicity of four in the [l10] direction. Finally, after a total Hz S exposure to 50 L, all the islands and troughs have disappeared, and the surface ends up locally with a perfectly-ordered 4 x 1 reconstructed phase, as seen from Fig. 7 .37f. One may ask why the 4x l-S structure develops at RT from the rough c2 x 2-S structure with all the islands and troughs, and not from the flat Ni (110) sulfphur-exposed surface. Apparently the detailed surface morphology plays a crucial role in the structural transformation from the c2 x 2-S overlayer structure to the reconstructed 4 x l-S structure. The rough c2 x2-S surface with many small islands and troughs can be considered as a metastable, intermediate surface state which has a higher surface energy than the flat, homogeneous c 2 x 2-S surface. Thus, the energy barrier for S atoms to react with and/or to penetrate into the surface and initiate the surface restructuring is reduced. During the transformation into a thermodynamically stable 4 x l-S state, diffusing Ni atoms removed from the islands react with S atoms and form the 4 x l-S structure. The energy requirement to remove a Ni atom at the periphery of a Ni island is significantly smaller than to pull out a Ni atom from a flat terrace. The results suggest how 0 chemisorption is able to "activate" a surface at the atomic level and thereby open reaction pathways otherwise not accessible. The decomposition of NH 3 on a Ni (110) surface with preadsorbed oxygen has also been studied by STM [7.136] with the aim of revealing which microscopic mechanism is active when NH 3 decomposition is promoted by preadsorbed oxygen, as well as of understanding the deactivating behaviour of excessive oxygen on the surface. STM results provided an atomic-scale interpretation of why the oxygen at low coverage promotes the decomposition but at high coverage deactivates the surface. The high reactivity observed at low 0 coverage is ascribed to a direct interaction between the NH 3 molecules and 0 atoms, which terminate the short, mobile -Ni-Oadded rows present on the surface under these conditions. The reaction model may also explain other observations in which preadsorbed oxygen has been found to act as a promoter for dissociation of H-containing species, such as NH 3 on eu (110) and Hz 0 on Ni (110).

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Fig. 7.38. A series images from an STM movie showing the structural change of a reaction between H 2 S and the 0 preadsorbed Cu (l1O)-p2 Xl recorded at H 2 S expsoure 01'3 L (a) 5 L. (b)7 L (c) and 9 L (d) [7. 137]

7.6.2 Reaction on Cu (110) Figure 7.38a-d represent a series of STM images recorded during the surface reaction between H2 S and the 0 preadsorbed Cu (110) [7.137]. A c2 X 2 structure is initially formed on fringes of the gaps (defects) between the p2 X 1-0 islands (Fig. 7.38a). Dynamic observations (Fig. 7 .38b-d) show that the reaction gradually spread to the surroundings with increasing H2 S exposure. The fact indicates that the reaction occurs preferentially on the edges of the p2 X 1-0 domains. As a result of the reaction, small islands appear and develop in size rather than quantity. As mentioned above, the p2 X 1-0 structure is stabilized by forming -0Cu- bonds in the rows along the [001] direction, and the rows are separated by a double lattice parameter along the [101] direction. When H2 S species access the adsorbed 0, the two react to produce water and atomically adsorbed S. While water desorbs from the surface at RT, the metal atoms (0.5ML) can be released from the row structure due to a loss of 0, then diffuse and accumulate to form 1 X l-Cu, on which the S adatoms form the c2 X 2-S structure. Unlike in the Ni (110)-0-S case, whereas a local diffusion 268

occurs to form many small islands and identical patches with the c2 X 2-S structure, the reaction CU(110) produces the c2x2-S islands and patches in large sizes. This different behavior could be ascribed to the considerably large difference of self-diffusion rate between the Cu(110) and the Ni (110). It has been also demonstrated that formaldehyde could be synthesised from methnol on oxygen treated CU(110) surface upon desorption [7.138]. On Cu (110), the high mobility of the Cu atoms at RT is responsible for the appearance of large islands but less in quantity. To the end of the reaction a simple S overlayer forms, and the surface morphology is characterized by the formation of large islands [7.22]. Surface reactions produce not only chemical replacements, but also structural transformation. Direct observations of the resultant structural transformation resulting provide an understanding of the reaction mechanisms from the structural aspect. At atomic scale, the S replaces the adsorbed o and results in c2 X 2-S structures. At a relative macroscale for surface morphology, the Cu atoms released from the p2 X 1 structure accumulate into islands due to the high Cu diffusion rate on the surface. The Ni atoms can form small islands stabilized by adsorbed S atoms at room temperature owing to a low mobility as well as the adsorbed S atoms. Activated by 0 adsorption, the Ni atoms bonded with adsorbed S atoms to form a metastable structure of small islands ascribed to the formation of the reconstructed p4 x l-S structure which otherwise would be obtained by exposing a clean Ni(llO) at elevated temperature. The results suggest how it is possible at the atomic level to activate a surface and thereby open reaction pathways. In this way, the results may have important consequences for the field of surface reactivity in general, and heterogeneous catalysis in particular.

7.6.3 Chemical Identity with STM Although STM. as mentioned in Chaps. 1 and 3, is able to reveal new unique and unprecedented information, extraction of topographic information from STM images is not always straightforward and simple, and it is not a priori possible with the STM to discriminate between the different chemical species on the surface. By reversibly manipulating the chemical identity of the apex of the tunneling tip, Ruan and Besenbacher et al. [7.139] were able to discriminate between the 0 and Ni atoms in low-coordinatred -Ni-Orows on Ni(llO) and, equivalently, between 0 and Cu atoms in -Cu-Orows on Cu (110). Figure 7.39 shows a 2 x 1-0 island on aNi (110)-1 x 1 surface imaged with a clean W tip. The protrusions of the (2 x 1)-0 phase are in line with the [110] rows of the clean surface (Fig.3.32a). Thus with a "clean W" tip, the 0 atoms are imaged. By letting tiny amounts of oxygen 269

7.7 Chemical Reaction on Semiconductors 7.7.1 Reaction of NH 3 with Si (111 )-7 x7 Surfaces

Fig. 7. 39a, b. STM topographs (37 X47 A2) of the added -Ni-O- [OOIJ rows on Ni obtained with a clean W tip (a) and after chern isorption of all 0 atom at the apex of the tip (b). For both images V = IO mV and I = InA. The grids indicate the positions of the Ni atoms in the I XI Ni layer underlying the added -Ni-O- rows. In (a) the protrusions of the added rows are in line with the close-packed rows of the underlying Ni-I XI surface. whereas in (b). they are out of "registry" [7. 139J

into the chamber (0.05 L) or by just scanning the 2 X I-a surface for a while, a sudden uncontrolled tip change may occur, and correlated with the tip change, the protrusions of the 2 X I structure change to be out of registry with the close-packed Ni [110] rows (Fig. 7.39b). R uan and Besenbacher et al. [7.139] suggested that the tip change is associated with the binding of an a atom at the apex of the tip. Thus, with an "0 tip", the Ni atoms are imaged as protrusions. By reversibly manipulating the apex of the tunneling tip to have either a "w atom" or an "0 atom" at the apex, the a and the Ni atoms, respectively, are imaged in the -Ni-O- rows, i.e., chemically different elements such as a and Ni can be discriminated with an STM. Identical conclusions are reached for the Cu (ll 0)-2 XI-a surface. A possible tentative explanation for these observations is to assume that tip-sample interactions may cause a "w tip" to form a stronger chemical bond with the a atoms along the rows than with the metal (Ni, Cu) atoms, whereas an "0 tip" may form a stgronger chemical bond to the Ni or Cu atoms than to the a atoms. By manipulating the chemical identity of the tip apex, the formation of a chemical bond between tip and sample may increase or decrease the density of states at the Fermi level E F since antibonding states, resulting from tip-surface atom interactions, are shifted up or pushed away from E F [7.126].

270

The usefulness of an STM in the study of chemical reactivity on semiconductor surfaces has been demonstrated in dissociative chemisorption of NH 3 on Si(lII)-7X7 [7.140] and on Si(001)-2x I surfaces [7.141]. This was achieved primarily by comparing STM images of clean, welkharacterized surfaces taken before and after dosing with reactive gas. Atomic-scale variations in surface chemical reactivity are observed in the interaction of NH 3 with Si (l11)-7 X 7. Pronounced differences have been found in the chemical reactivity at various sites within the ideal Si (Ill )-7 X7 unit cell. STM topographs allowed the course of the chemical reaction to be mapped on an atom-by-atom basis, and simultaneous tunneling spectroscopy measurements established the relation between local electronic structure and chemical reactivity. Figure 3. Ila exhibit an STM image of the unoccupied states of the clean Si (lll )-7 x7 surface in which the 12 adatoms in the 7 x7 surface unit cell are clearly seen. As we described in the last chapter, STM images of a clean Si(l1l)-7X7 surface clearly show two inequivalent types of surface Si adatoms, called "corner" adatoms and "center" adatoms. Upon exposure to about 1 L of NH 3 , reaction takes place and about half of the adatoms disappear from the images as their dangling bonds are passivated by reaction with NH 2 or H producing surface Si-H and Si-NH 2 groups (Fig.3.11b). Even from a simple inspection of the STM images, important chemical information can be obtained. For example, there are roughly 4 times as many unreacted corner adatoms than center adatoms in Fig.3.llb. The fact that center adatoms are more reactive than corner adatoms would be difficult to determine by conventional surface-science techniques. At a sample bias of +3 V, there is a finite contribution to the local density of states by reaction product states. As a result, product sites can be imaged. The preservation of the 7 X7 reconstruction suggests that the reaction with NH 3 has primarily saturated existing dangling bonds with limited, if any, Si-Si bond breaking. In order to obtain insight into the reasons for the above behavior and to answer questions such as the chemical identity of the products, tunneling' spectroscopy has been used to study the electronic spectra of the clean and NH 3 -exposed Si (l11)-7 X 7 surfaces. The atom-resolved tunneling spectra shown in Fig.3.ll were recorded for the positions indicated by the arrows. The curves A, Band C in Fig. 3 .lla give the spectra over rest-atom, corneradatom and center-adatom sites on the clean surface, respectively. Negative energies correspond to occupied states and positive energies to unoccupied states. The rest-atom spectrum (A) shows a strong occupied states peak at about 0.8 eV below E F . This peak is characteristic of the rest-atom's dangl271

ing bond. The corresponding dangling-bond states of the adatoms appear near 0.5 eV above E F (B and C). However, the center-adatom spectrum (curve C) reveals important differences from that of the corner adatom (curve B). The intensity of the occupied dangling-bond state has decreased while, correspondingly, the intensity of the unoccupied state has increased. The filling of the rest-atom's dangling-bond state and the small occupation of the adatom state suggests an adatom to rest-atom charge-transfer process. In addition, the spectra (curves Band C) indicate that most of this charge is contributed by the center adatoms. This is because center adatoms have two rest-atom neighbors while corner adatoms have only one. The above differences in the occupation of the dangling bond states could be the cause of the observed differences in the reactivity of center and corner adatoms. Figure 3. 11 b (bottom) depicts spectra obtained on a partially reacted surface. The 0.8 eV characteristic state has been eliminated in the spectrum A for a rest-atom site. Adatom spectra (curve B, dashed line) show that the corresponding surface states are also eliminated upon reaction. The elimination of the surface states is the reason for the "disappearance" of the reacted adatoms in Fig.3.11b. More systematic studies which involved spectral maps of large areas of the partially reacted surface demonstrate that rest atoms are more reactive than adatoms, reacting faster than one would predict on the basis of their relative numbers on the 7x7 surface. For example, upon NH 3 exposure, rest atoms react first, and under conditions such as those of Fig.3.11b where about half of the adatoms are still unreacted, but no unreacted rest atoms remain. The spectra B (solid line) and C of unreacted corner and center adatoms on the partially reacted surface (Fig. 3 .11 b) illustrate the effects of the reaction on the electronic structure of still unreacted sites. These two curves reveal that, after the neighboring rest atoms have reacted, the electronic spectra of unreacted corner and center adatoms become virtually indistinguishable. The differences between the two sets of adatomic spectra can be explained in terms of the effects of the reaction on the charge-transfer interactions present on the clean 7 x 7 surface. On the clean surface the center-adatom dangling-bond state is nearly empty (Fig.3.11a, curve C), but on the surface where the rest atoms have reacted (Fig.3.11b, curve C) it shows a much higher occupation with a simultaneous decrease in the intensity of the unoccupied part at +0.5 eV. The above results suggest that during reaction a rest atom to adatom reverse charge transfer takes place which allows the rest atom to transfer the extra charge and react with NH 3 . STM can be utilized to investigate the effect of local structure on the chemical reactivity of the various dangling-bond sites of the 7 x 7 surface. In the DAS model, rest-atom sites involve triply coordinated surface Si atoms. Theoretical calculations find a normal dangling-bond character at these sites and a dihedral angle between bonds close to tetrahedral. Reac. 272

tion at these sites should not produce surface strain. The situation is different at adatom sites which involve considerable strain. Adatoms in this model are members of three four-membered Si rings. These ring structures bring the adatoms close to the Si atoms directly below them in the third atomic layer. This proximity leads to repulsion, and distortion of the structure with adatom dihedral bond angles close to 90°. Another important difference between the two kinds of sites is the occupancy of the respective dangling-bond states. The spectra in Fig. 3.11 a indicate that the rest-atom dangling bonds are fully occupied while the adatom dangling bonds are less than half occupied. The reduced density brought about by the delocalization of the dangling-bond charge at adatom sites and the large deviation from tetrahedral geometry are responsible for the reduced reactivity of adatoms as compared to rest atoms.

7.7.2 Reaction of NH 3 with B/Si (111)-V3 xv3 Surface The other example which demonstrates the chemistry of the Si surface and depends very strongly on the local structure and strain considerations at the reactive site, is the effect of boron on surface chemistry. As we saw earlier, the equilibrium configuration of the B/Si (111)-V3 xV3 surface involves a Si-adatom top layer, with the B dopants below the Si adatoms in substitutional sites. Because of the Si-to-B charge transfer, the Si top layer of the B/Si (l11)-V3 xv3 system has no occupied dangling-bond levels. This drastic change in the dangling-bond level occupancy should be reflected in the chemical properties of the surface. Indeed, it is experimentally found that the top Si layer of this B/Si (111)-V3 xv3 surface has chemical properties very different to those of clean Si surfaces. For example, in contrast to the behavior described above for the reaction of a clean Si (111)-7 x 7 surface with NH 3 , exposures of the B/Si(111)-V3XV3 surface to even a few hundred Langmuirs of NH 3 at room temperature leads to very little reaction [7.51]. This indicates that boron incorporation has a drastic influence on the reactivity of Si adatoms. However, not only is the reaction rate affected by B-doping but the nature of the reaction itself is different: NH 3 adsorbs reversibly on the B-modified v3 x V3 surface by donating N lone-pair electrons to the empty Si dangling-bond state of the adatoms, that is, by a Lewis acid-base reaction. This is in contrast to the dissociative adsorption of NH 3 observed at the adatom sites of the clean 7 x 7 surface. It is a novel shortrange doping effect on chemical reactivity which involves direct chargetransfer interaction between the dopant atom and the surface active (i.e., dangling-bond) site.

273

7.7.3 Reaction of NH 3 with Clean Si (00 1) Surface The chemisorption-induced changes in surface chemical bonding have also been observed, using the interaction of NH 3 with Si(OOI) as a prototypical gas-surface reaction system. The dissociative adsorption of NH 3 on Si(OOI) produces hydrogen atoms which change the local bonding of the Si (001) dimers. These changes allow reacted and unreacted Si (00 I) dimers to be distinguished (Fig. 7 .40), and tunneling spectroscopy is utilized to elucidate the detailed nature of the bonding before and after the reaction. Since the atomic positions for Si remain virtually unchanged by H adsorption, these changes can be directly attributed to the different spatial distributions of SiSi and Si-H bonding states. The STM results on the reacted surface are interpreted in terms of tunneling through localized Si-H bonding orbitals of a Si (00 1)-2 x I monohydride.

A

A

,

7.7.4 Si(I11)-7x7 Oxidation The nature of the initial stages of the oxidation of silicon is a long-standing problem. A large number of studies on this issue has been carried out utilizing a great variety of different techniques. Several configurations have been proposed for the oxygen-containing sites in the early stages of the reaction. They involve oxygen atoms saturating the dangling bonds of top-layer Si atoms, oxygen atoms inserted in back bonds but leaving the dangling bonds intact, or molecular forms of oxygen attached to surface atoms or bridging two surface Si atoms. Avouris et al. [7.142] found that two different oxygen-containing structures are formed in the early stages of Si (111)-7 x 7 oxidation. These structures appear as dark and bright sites, respectively, in STM topographs of the unoccupied states of the sample. At very low exposures «0.2L), the numbers of dark and bright sites are comparable but as the exposure to 02 or N2 increases, the number of dark sites increases steadily, leading eventually to rough surfaces with no obvious order. The number of bright sites does not increase nearly as fast, suggesting that the dark sites are more representative of the main oxidation process. The dark sites remain dark for both positive and negative sample bias voltage, indicating that the adatom dangling bond is saturated by a group that does not have any low-lying unoccupied or occupied states. The different surface site distributions and different growth rates observed with increasing 02 -exposure show them to be two distinct early products. By correlating the spectroscopic results and the results of theoretical calculations one is able to identify the dark sites as adatom sites whose dangling bond has been saturated by an oxygen atom while another oxygen atom has been inserted on one of its back bonds. The bright sites represent adatoms with an intact but emp-

°

274

Fig.7.40a-d. STM topographic images probing the (a) occupied (-2V bias) and (b) unoccupied (+ 1.2 V bias) surface states of clean Si (001) and the (c), (d) occupied (-2 V sample bias) states ofNH 3 -dosed Si (001). In each figure, the arrows point in the (110) direction and point directly at the center of the dimer rows. Each box out I ines a 2 XI unit cell with a dimer centered in the box (a) contains a single "missing-dimer defect". In (d), the position of a single atomic step is indicated by the solid line, and the upper and lower terraces are identified by "U" and "L" [7.141)

275

tied dangling-bond state which is shifted to higher energy and contains an oxygen atom inserted in one of the adatom back bonds. In general, new dark sites tend to nucleate at old dark sites, indicating an electronic-structure perturbation by the oxidation products. The preference of these products for the faulted half of the 7 x 7 unit cell and for corner-adatom sites can be explained in terms of a site-dependent sticking coefficient involving a process analogous to the gas-phase "harpooning" processes. It should be pointed out that the majority of the resulting molecular precursors involve 02 interacting with a single dangling-bond site. The adsorption of 02 on Si (Ill) produces many molecular sites which are highly reactive [7.143].

Hydrogen serves to relax the strained surface bonds primarily by allowing the formation of isolated trihydride species and adatom islands. While reaction with the bulk is thermodynamically feasible, the barrier is too large for the reaction to proceed appreciably under the present conditions. The manner in which this relaxation occurs determines both the nature and the population of different hydrides present on the surface. Hydrogen desorption involves the restructuring and ultimately the breakup of these island and leads to be re-establishment of a 7 x 7 adatom surface. Boland suggested models that fail to account for the presence of these strained bonds due to an inadequate description of the surface chemistry [7.146].

7.7.5 Si(l00)-2x1 Oxidation

7.7.7 Reaction of Sb4 with Si (100)

In contrast to the Si (Ill) -7 x 7 surface which has a metallic Density Of States (DOS), the dangling bonds of the Si dimers on the 2 X 1 surface are paired, leading to the formation of a surface gap and a vanishing DOS near E F . A vouris and Cahill [7.144] studied the initial stages of oxidation of the Si (100)-2 x 1 surface by STM, and observed a reduced DOS near E F . While on the Si(100)-7X7 surface the most reactive sites are the top-layer Si adatom sites, on the Si (111)-2 x 1 surface the majority of the dimer sites are not very reactive compared to defect sites, particularly the C-defect sites. Defects on the Si (100)-2 x 1 surface which have a metallic DOS dominate the reactivity towards 02 in the early stages of the reaction. Among the new sites generated by the exposure to 02 are 1.4 A high bumps on top of the surface. Upon annealing of the 02 -exposed surface or upon 02 exposure at an elevated temperature these bumps form elongated islands. Evidence is presented suggesting that the bumps and islands are probably due to silicon ejected to the surface by the oxidation reaction. The nucleation of oxide clusters are essential in forming multilevel Si islands [7.145].

7.7.6 Reaction of H with Si (111)-7 X7 Little is known about the factors which control the reactivity of hydrogen on the Si(l11)-7x7 surface although the chemistry of hydrogen on this surface has been extensively studied over the years. A combined STM and STS study [7.146] has shown that the chemistry of H on the Si (111)-7 x7 surface is driven by the relaxation of the strained bonds formed by the reconstruction. Such bonds have a reduced activation barrier, and reaction continues until the supply of these weak bonds is exhausted. The limited number of such bonds explains the apparent saturation of hydrogen on this surface. 276

The reaction of Sb 4 with Si(lOO) has been studied with STM. Five distinct types of Sb clusters are observed on Si (l00), each containing four Sb atoms. The final-state cluster consists of two dimers with the dimer bonds perpendicular to the Si dimer bonds in the substrate, consistent with past studies of the equilibrium structure of the Sb layer on Si (100) formed at elevated temperatures. The remaining four types are precursors that form the pathway for the Sb 4 molecules to reach the final state of dissociative chemisorption. These precursor clusters can be converted to the final state of chemisorption either by thermal annealing or through an STM-tip-induced conversion process. Using a new STM method of image-anneal-image cycles, i.e., imaging the same Sb clusters before and after a cycle of thermal annealing at a certain temperature, Mo [7.147] determined the reaction path of the dissociative chemisorption of Sb 4 on Si(lOO). Combined with the measurements of the average population distribution of Sb clusters as a function of thermal treatment, the energy barriers and the prefactors for conversions between different states are obtained. From the conversion rate of precursors to the final state vs temperature, an effective activation energy of 0.5 ±0.1 eV and an effective prefactor of about 10 3 Hz were deduced [7.148,149]. These precursors are found to have no thermal mobility before dissociation, contrary to the popular notion about precursor states. Such results indicate that the reaction of molecules with solid surfaces can be extremely complex. Studying the reaction path of molecules on surfaces with the STM has unique advantages. First, it allows direct identification of different types of coexisting precursors. Second, direct measurements of the conversion rates are less model dependent than the conventional molecular-beam method. The new STM method of image-anneal-image cycles also allows direct observation of the diffusion process while avoiding potential STM tip effects. The method has been used to study the anisotropic diffusion of Sb 277

dimers on Si(100) [7.147]. The energy barrier and the prefactor for the faster diffusion across the substrate-dimer rows are measured. This method offers the same advantages as the FIM method, yet it allows studies of surface diffusion on a much broader range of substrate materials. It is obvious from the above examples that the demonstrated capability of STM to probe the topography and electronic structure of surfaces and adsorbate layers with atomic resolution makes it a powerful tool in the study of surface chemistry, providing unique insight into the mechanisms of surface reactions at the atomic scale.

8. Biological Applications

Shortly after the STM was invented by G. Binnig and H. Rohrer, the application of this new technique to biomolecules was initiated and followed by rapid growth. Many meaningful results have been achieved in the structural investigations of nucleic acids, proteins, biological membranes and supermolecular biosystems. This has demonstrated the great potential of the newly developed technique in the studies of surface structures of biological materials.

8.1 Advantages and Problems As mentioned in Chap. 1, very high resolution can be obtained with STM. Although higher resolution can be achieved by TEM and FIM in the lateral directions, they are limited by the fact that a coated conductive layer on the surface of the sample is required in SEM imaging [8.1]. TEM is only suitable to the study of bulk and interface structures of very thin samples [8.2]. Only the two-dimensional structure of the sample atoms on tips with a diameter of less than 100 nm can be detected by FIM. Additionally, these three kinds of microscopes must be operated in a vacuum environment. Atomically resolved structural information can also be obtained with several other diffraction techniques such as X-ray diffraction, He diffraction and low-energy electron diffraction, but these techniques require crystalline samples, and only provide averaged information rather than local structural information in real space [8.3]. Unlike electron microscopy and diffraction techniques, requiring a vacuum environment and crystal samples, respectively, STM images of the surface structure of crystalline or non-crystalline samples with resolutions ranging from Angstroms to nanometers, can be operated not only in the vacuum, but also in air, at low temperature, at ambient pressure and temperature, and even in solution. Biological samples die in such sterile environments and their structures may be far different from that in the active state. Therefore, to observe directly the structure of biological samples under natural or nearly natural conditions (at ambient pressure and tempera278

279

ture, or in acqueous solutions) remains a dream for biologists. Such a possibility generated the wide interest of biologists in STM. Although these unique advantages imply hopeful prospects for the application of STM to life science, some problems still exist which can be listed as follows:

pies to substrates with strongly adsorbent groups. The second is to reduce the interaction between the sample and the tip, including choosing a smaller reference current to increase the gap between the sample and the tip, or using a hopping technique (which will be described in Sect. 8.2). Salmeron et al. [8.9] argued that the main contribution of coating the sample with a conductive film for STM imaging comes from the fixation rather than the conductivity.

8.1.1 Substrates Highly-Oriented Pyrolitic Graphite (HOPG) had been a convenient substrate commonly used in STM studies on biological molecules before 1991: DNA, for instance. However, some reports [8.4] indicate that features similar to that of DNA and other biomaterials are present in STM images of freshly cleaved HOPG surface onto which no biological samples were deposited. It has been suggested that gold and platinum are also not suitable substrates for use with these materials, no matter how the substrate surface is prepared [8.5]. Unfortunately, choices among other materials which can be employed as substrates are very limited except for HOPG and gold. To improve the applicability of STM to biological materials, one must either find suitable substrates or refine the available methods to distinguish the artifacts of the substrates from the biological molecules themselves. Crystalline gold, which is prepared easily by evaporating gold onto mica heated to a temperature between 600 and 800 K [8.6], is becoming increasingly popular. For AFM study, mica is a convenient substrate commonly used. STM is based on the flow of an electrical current and thus cannot be used to directly image insulating material. It has been speculated theoretically [8.7] and confirmed experimentally [8.8], however, that a very thin film of water (about one monolayer) adsorbed to a surface exhibits a surprisingly high conductivity that is sufficient to allow STM imaging hydrophilic insulators. Biological specimens, such as DNA on mica, have been imaged in humid air by an STM with high resolution recently [8.8].

8.1.2 Fixation of Samples onto Substrates Usually, biological samples must be dispersed on flat and conductive substrates before imaging by STM. Adsorption may be so weak that the interaction between the tip and the sample at such a small distance can cause the movement of the sample on the surface of the substrate with the tip scanning across the surface, thus stable and high-resolution images of adsorbent cannot be easily achieved. Two methods might be used to address this problem. The first is to enhance the adsorption between samples and substrates, including coating samples with conductive films or covalently linking sam280

8.1.3 Flexibility of Biological Samples Most biological samples are not rigid structures and are flexible to various extents. The long chains of peptides, lipids and carbohydrates can be turned or folded. Even DNA, with a structure which is usually considered stable, can form various conformations under different conditions and undergoes structural variances such as twisting and unwinding. During tip scanning across the sample, the tip-sample interaction and the thermal vibration of the sample may lead to changes in the sample surfaces, resulting in a distorted image of the natural structure of samples on the one hand, and low resolution on the other. This problem might be solved by reducing the tipsample interaction. Although placing samples at low temperatures can reduce thermal vibrations; a low temperature is far from the native condition and has some effect on the structures of samples and, moreover, increases the difficulties associated with manipulation.

8.1.4 Identification and Interpretation of STM Images No systematic theories or a simple way for identifying and interpreting the images obtained have been established. It has proved to be a more difficult task to identify and interpret the images of more complicated and irregular biosystems than the images of simple crystal samples. Generally, biological samples have a low conductivity. Although low conductivity of biological materials is not an inpenetrable barrier for STM imaging, the contrast mechanisms for imaging samples with low conductivity are still not clearly understood. This limits image identification and interpretation.

281

8.2 Preparation 8.2.1 Dispersion of Samples on Substrates The simplest method of sample preparation is to deposit a droplet of dilute sample solution on the substrate so that the sample is dispersed and adsorbed naturally after being dried on the substrate. What is to be noted in using this method is that: firstly, the concentration of the sample solution cannot be too high, otherwise the thick deposition of the sample will prevent the STM from providing good images; meanwhile the concentration of salts in the solution cannot be too high, so as to avoid the effect of crystallization of salts from the solution. Usually, the concentration of the solution should be just as high as, or slightly lower than, what is needed to form a monolayer. Secondly, the dispersing abilities of the molecules under investigation should be taken into consideration. In order to promote homogeneous dispersion of sample molecules, a small amount of detergents (such as SDS) or other solvents (such as ethanol) which can reduce surface tension, are added into the solution. Tailored monolayers which either create favorable charge patterns at the interface or offer ligands interacting specifically with molecules in the subphase can serve as intermediate high-affinity substrates. Alternatively, electrochemical methods can be utilized for deposition by applying appropriate potentials between solution and substrate. Biomolecules usually remain attached to the substrate even if the potential is changed within certain limits. With electrochemical STM and AFM the process of adsorption may be monitored in situ (Chaps. 4 and 5). A FIM technique for sample preparation has been applied to depositing samples onto a gold ball. The device employed is shown in Fig. 8.1. A gold ball on which the sample is deposited is held in a threaded pin that is then inserted into the plexiglass cover. This pin is so positioned that the gold ball is inside a small tungsten coil. Deposition of the sample is started by placing the deposition device into the sample solution. Due to surface tension,

sample droplets are pulled up into the coil and in this way the gold sphere is immersed in the solution. After a while the sample will be adsorbed onto the gold sphere [8.10). The adsorbate concentration can be controlled by adjusting the time of adsorption and the concentration of the sample solution. The sample prepared by this method can be observed by STM immediately, or after being rinsed with alcohol or distilled water to remove the undesired salts, or even covered with a layer of conductive film.

8.2.2 Fixation of Samples a) Sample Coatings After the biological sample is dispersed on the substrate it can be coated with conductive films such as metal and carbon. This kind of treatment can provide not only good conductivity but also fixation and rigidness which favor stable STM imaging, but the resolution is limited because of the limitation of particle size of the coating films. For the purpose of reducing the particle size, low-temperature evaporation techniques can be employed to improve the resolution. The results which have been obtained, indicate that the sample-coating method is most suitable to topographical studies of biological samples on a large scale. The topograph of a recA-DNA complex and the whole cell sheath of bacteria, etc. have thus been obtained. To make sure in advance that the preparation of samples fulfills certain quality criteria such as good and homogeneous coverage of the support, imaging metal-coated samples alternatively by STM and TEM or SEM is a good way to examine the quality of the preparation. Such control experiments also help to unambiguously identify the structure under scrutiny. b) Covalently Binding Samples with Strongly Absorbent Groups p-toluenesulfonyl, for example, can serve as an absorbent group on the HOPG surface to link the sample to the substrate. While tris( l-aziridinyl) phosphine oxide (T APO) can be used to anchor DNA to a gold surface, because T APO contains an ethylene imine group which can readily be reacted with a pentose group of DNA on the one hand, it also has a phosphorus oxide group to provide linkage to gold on the other. Therefore, T APO can not only provide a linkage to the a gold surface, but also a conductive pathway. Some highly-resolved images of DNA have been obtained by means of this method (Sect.8.3.1).

Tungsten coil

Fig. 8.1. Devices employed in the FIM sample absorption method [8.10) 282

283

c) Binding Samples to the Substrate Covalently The Langmuir-Blodgett technique has been employed by STM researchers to link samples to substrates (Fig.8.2a). Heckl et al. [8.11] once fixed lipid molecules on the graphite surface for STM observation. By using this technique, individual lipid molecules can be observed. On the basis of the Langmuir-Blodgett technique, Lindsay et al. developed a new technique of anchoring DNA on the graphite surface [8.12]. Graphite is oxidized and then joined with -SH groups which can react with Hg, thus DNA modified by Hg can be linked to the graphite substrate covalently, as shown in Fig.8.2b.

(a)

~

~

SOCl'I -C-CI ~

-C-OH--

I,IO-Decanediol

•

HOCH ,(CH,J,CH,OH

1 ~

KMnO,

-C-0(CH2)9CH20H---

Pyridine

1

2

3

Oxidized graphite surface

o II

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0

II

-C-0(CHz)9CO~H

1

DMAP

•

0

0

II

II

I

II

I I

CHOH

8.2.3 STM Imaging in Acqueous Solutions

I

5

Z 6

Z

-C-0(CHZ)9C-0-OH

CH zOC(CH2)6 CH 3

4

CH OC(CH ) CH

3

0

II

CHrO-P-OCH2CHzN+Me3

I 0

0

II

CH z -0-P-OCH 2CH zN+ Me 3

I

Lindsay et al. first obtained STM images of DNA in acqueous solutions [8.13]. With regard to the special problems arising when STM is applied under water, they managed to use the following techniques: a tiny cell of ca. 50 ~I was employed to overcome the vaporization of the solution; the scanning probe was covered with glass, leaving only the very end of the tip naked so that the Faradaic current was reduced to about 0.1 nA and the detection of the tunneling current became possible. A tiny gold sphere was utilized as the substrate, and a reference electrode was employed to induce adsorption of DNA molecules to the substrate when a voltage was applied between the reference electrode and the gold sphere.

O' (b)

Step 1:

~

Cr03 -

+

ill ~ U-

C - OH

Step 2:

nC~ U-

OH

+

SOCI 2 - - -

n~ U-

C - C1

Step 3:

n U- ~

C C1 -

.: ~

+

~

:: -C-~-(CHzCH2S-)z-NHz .' H

(NH zCH 2CH 2SH)2 -

Step 4:

8.2.4 Hopping Technique

.:~

~

:: -C-N-(CHzCHzS-)z-NH z + 2-Mercaptoethanol -

Jericho et al. [8.14] developed a new kind of STM technique which was named the hopping technique after its operating principles: when the tip is scanning laterally, it is drawn back from and then reapproaches the sample in the z direction periodically just like "hopping". The height of each "hop" can be kept constant. If the tip "hops" high enough, the lateral force caused by the scanning tip will be reduced and the damage to the sample surface will also be reduced. From the operating principle of the hopping technique shown in Fig. 8.3, it is apparent that "hopping" is achieved by adding a digital modulator into the feedback system of a STM instrument.

-: ~

~

I

H

~

::

C

H

:. C

Ste~5:

II :: -C-~-CHz-CH2-SH+Hg-DNA

~:::: -C-N-CHz-CHz-SH ~

::

-

I

H

0

II

:: -C-NHCH 2CH 2S-Hg-DNA

.

Fig. 8.2a, b. Two methods of binding samples to the substrate covalently [8.11]. (a) Langmuir-Blodgett technique for fixing lipid molecules. (b) DNA fixation technique developed on the base of Langmuir-Blodgett technique [8. 12J

284 285

Pre amp

Tunnel current monitor

8.3 Nucleic Acids

(-I08Y/A)

1,_

CrlflF

IC 4 to STM

z axis

SIQ

I

ChlflF~

Digitalcontrol~-15Y input

R h IkQ

100Q

Fig. 8.3. The principle of the hopping technique [8. 14]

A

Icm

t------i

Fig. 8.4. STM designed to be used in combination with an optical microscope [8.15]. (A: approach screw, S: sample, T: tip, P: piezoelectric tube, and 0: optical microscope)

8.2.5 STM Directed by an Optical Microscope If enough room is left between the tip and the sample, it is possible to use an optical microscope with high magnification to bring the tip to the sample as closely as possible so that the position of the tip can be controlled and the location of the scanned area can be chosen purposely. Figure 8.4 presents a combined instrument consisting of an STM and an optical microscope.

286

Nucleic acids are classified into two major groups: DeoxyriboNucleic Acid (DNA) and RiboNucleic Acid (RNA), of which DNA is the main genetic substance in the life process, with the only exception being viruses where RNA is the main genetic substance. In the field of life science, the structure and functions of DNA are of central interests. Up to now, a lot of results about the structure of nucleic acids have been accumulated mainly from the sources of X-ray diffraction, NMR, rotatory dispersion, circular dichroism analysis and the analysis of the primary structure of nucleic acids, i.e. the sequence. The interest of many researchers is focused on the three-dimensional structures of nucleic acids at the native and active states, and the structural variances which nucleic acids undergo in the process of life, a key to the explanation of the nature of many life phenomena. The emergence of STM and AFM provides scientists with a possibility to observe directly DNA and RNA under natural or almost natural conditions. Another major goal of this research is to assist in sequencing the DNA of the human genome in order to bring people relief from some of the physical and mental disease that is of genetic origin.

8.3.1 DNA in Air and in Vacuum Right-handed DNA with pitches ranging between 2.5 and 3.5 nm and a mean width of 2.0 nm has been observed by STM [8.16] when DNA is fixed on a gold surface with T APO. Furthermore, in the image taken at greater magnification, the details of one helical pitch are shown. The two sections of approximately rectangular shape (2.0 x 1.5nm2 ) form an angle of 40° with molecular axes and clearly represent the shallow minor grooves of the helix, separated by an imperfectly imaged region which corresponds to the deep major groove. The poor imaging of the major groove is probably due to the tunneling current originating from both sides as the tip is trying to enter the deep groove and drawing current from its sides, thus blurring the images. More detailed structure can be resolved in sections of the minor groove. The periodicity of the helix is approximately 3.5 nm, and the width of the minor groove is 1.2-:--1.5 nm. Besides B-DNA as discussed above, other kinds of nucleic acids, such as Z-DNA [8.17], A-DNA [8.18] and single stranded DNA [8.19], have also been imaged with STM on graphite. For example, Fig.8.5a shows the STM image of a double-stranded A-DNA fragment of about 500 bp sampled from B cells in region V of a mouse, and the corresponding model is presented in Fig.5.3b. Four periods of the helix are imaged together with clear 287

Table 8.1. A comparison of X-ray diffraction results with STM images of A-DNA

Helix pitch [run) Minor groove width [nm] Major grrove width [run] Molecular width [run] Phosphate backbone width [nm] Axial nucleotide rise [run] Base-pair angle [0] Helix symmetry

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minor and major grooves. In wider and shallower minor grooves, base pairs at an angle of + 150 a with the helical axis can be discerned. The dimensions of the image are consistent with X-ray diffraction results (Table 8.1). The surface atoms of the base pairs in the minor grooves can be distinguished through cross-sectional line analysis at the positions indicated in the image. This is in good agreement with the standard cross-sectional lines of the A-DNA model, suggesting a possibility to observe the atomic structure on nucleic acid surfaces. Plasmid DNA on mica has been imaged by STM in humid air [8.8] with an apparent width of 3.5 nm, which is close to its true diameter of about 2.5 om. This STM mode is useful for imaging biomolecules on insulating substrates, such as glass and mica surfaces on which no defects similar to those of DNA and other biomolecules are found.

of:

'"

a:l

2.4

Lateral displacement [nm]

Fig.8.5a-d. An STM image of A-DNA in vacuum [8.18). (a) Unsmoothed, unfiltered plane-substracted STM image of A-DNA (8 XI2nm 2). (b) A-DNA model base on X-ray diffraction results. (c) Solid line: interpolated cross section following line marked A on Fig. 8.5a; dashed line: corresponding data from a simultaneously acquired gap-modulated barrier-height image. (d) Solid line: interpolated cross section following line marked B on Fig. 8. Sa; dashed line: corresponding data from a simultaneously acquired barrier-height image

288

STM

8.3.2 DNA Studies under Water with an Electrolyte As mentioned in Sect.8.l.l, some kinds of defects on a HOPG surface similar to that of DNA and other biomolecules. STM images of DNA on graphite are controversial. An altogether different approach uses electrochemical methods to bind molecules onto a gold electrode which is then imaged under a covering layer of electrolyte [8.13,20]. Reliability has been improved by maintaining potential control of the electrode during imaging [8.21]. The deposition of molecules can be monitored with this process. Thus, in addition to imaging in an aqueous environment, the substrate artifacts that have plagued some other work can be ruled out. With the technique described in Sect.8.2, an effort has been made to directly observe the structure and electrochemical behavior of DNA in acqueous solutions.

289

STM images of aggregate and individual DNA molecules have been obtained with sonicated calf-thymus DNA taken as the sample. Regular arrangements of texture-like DNA molecules similar to liquid crystals are found in the STM image of aggregate DNA, the corrugation perpendicular to the long axis is always 20 A; while intact DNA strands are shown as trenches 20 A wide and several Angstroms deep in the image of individual DNA molecules [8.13]. Different adsorption geometries of DNA molecules on the substrate surface reflecting the bioelectrochemical processes of DNA were also observed by changing the potential between the reference electrode (AgCl) and the substrate electrode (a gold film grown epitaxially on the freshly cleaved mica surface). At a voltage of -2.3 V, regular and repetitive changes of DNA images are found with the changing of the scanning direction of the tip, corresponding to the reaction of the bases at negative electrodes. At -1.3 V the aggregate bundles of DNA molecules appear to be all side-on bound. At -1.0 V DNA molecules are observed to be isolated and adsorbed individually on the substrate surface. At -0.5 V the substrate surface appears to be uncovered in many regions. ling et al. [8.22] used STM to image several synthetic single- and double-stranded oligomers adsorbed onto a positively charged Au (Ill) electrode. The molecules were deposited and imaged in an aqueous electrolyte under potential control. Experiments were carried out with two types of single-stranded molecules (11 and 20 bases long) and three types of doublestranded molecules (20 and 61 base pairs and 31 bases with 25 bases pairs and 6-base "sticky" ends). The molecules lie along symmetry directions on the reconstructured 23 xv3 gold surface, and length measurements indicate that they adopt simple base-stacked structures. The base stacking distances are equal to the 0.33 nm measured for polymeric aggregates of stacked purines by direct imaging in identical conditions. The single helices appear to be more tightly twisted. Features in the images correspond qualitatively to turns of double- and single-stranded helices. A simple tunneling model for contrast produces good agreement with the images if the helices are B-like with a rise-per-base of 0.33 nm and a twist of 40° (single strands) and 36° twist (duplexes) which is consistent with a major-groove periodicity for double helices. Since Atomic Force Microscopy (AFM) can image nonconducting surfaces, it shows great promise for imaging nonconducting biological macromolecules and cells. AFM of uncoated DNA has also great potential for contributing to a detailed knowledge of the substructure of individual DNA molecules and the understanding of processes involving DNA. Reports on the AFM of double-stranded plasmid DNA are appearing at a rapid rate. The images obtained correlate well with what has been seen in the electron microscope. Imaging under propanol shows. in addition, reproducible struc290

ture along the DNA strands as small as 3 to 5 nm, while imaging in air yields mainly structure at the level of super-coiling. 4> X 174 single-stranded DNA in formaldehyde on mica can be imaged in an AFM under propanol or butanol or in air [8.23]. Measured lengths of most molecules are on the order of 1 p.m, although occasionally more extended molecules with lengths of 1.7 to 1.9 p.m are seen. Single-stranded DNA in the AFM generally appears lumpier than double-stranded DNA, even when extended. Double-stranded DNA usually appears more uniform along the strand than single-strand DNA. Images of double-stranded lambda DNA in the AFM show more sharp kinks and bends than are typically observed in the electron microscope (Fig.8.6a). The AFM image (Fig. 8.6b, c) exhibits a fairly regular series of lumps along the strand, 6 to 8 nm apart, and the strand has an apparent width of 7 to 9 nm. The apparent widths depend strongly on the width of the AFM tip, which is a source of variability that will be reduced when it becomes possible to make uniformly sharp tips. The

Fig. 8.6a-c. AFM images of lambda DNA under 2-propanol. Note the sharp bends in the strands, especially in a, and the uniform appearance of stands, with substructure evident. particularly in c. Image c is a small scan in the region of b indicated by the arrow. The image sizes are 2000 X2000 nm 2 (a), 1000 X 1000 nm 2 (b), and 200 X200 nm 2 (c) [8.23] 291

minimum spacing of lumps that are imaged along the strand is also limited by the tip width, which may at present be preventing us from routinely imaging features as small as the turns of the double helix or the individual nucleotides. Dense aggregate of double-stranded bluescript plasmids DNA was also observed by AFM. The molecular aggregate shows evidence of super-coiling. This aggregate can partially or completely be dissociated by rinsing the sample with several drops of hot water and drying it with compressed air. The aggregated fields of double-stranded DNA can thus be converted to well-spread fields.

8.3.3 DNA-Protein Complex As for the recA-DNA complex, STM images have been obtained on Pt-C coated recA-DNA clusters and metal-coated and uncoated single recA-DNA fragments. From the image of the Pt-C coated recA-DNA clusters, a periodical structure of 10 nm can be discerned, implying a parallel alignment of recADNA chains. Additionally, finer structures are also found which are believed to be recA monomers. In the image of the Pt-C coated and uncoated single-stranded recADNA complex [8.24], typical right-handed helical structure can be visualized in the recA-DNA chains. Three or four separate parts, each of which is a recA promoter, can be determined in each period on the side of the fragment facing us. With the addition of the other three parts in the invisible half of each period, there are approximately six recA promoters in each period. Some other characteristics, such as fusion of the two adjacent pitches and splitting of one pitch into two parts can be found in both images, indicating that these characteristics are due to the nature of recADNA complex. It is evident from the comparison of both images that the uncoated recA-DNA is better resolved than the coated one.

8.3.4 DNA Bases Motivated by the possibility of sequencing DNA with STM, guanine and adenine have been imaged by STM on graphite and on MoS 2 in air. In these studies, drops of guanine or other base solutions were placed onto graphite or MoS 2 substrates. Tao et al. studied all the four DNA bases on Au(lll) with STM in NaCI0 4 [8.25]. Different from the studies on graphite, in which the molecules condensed into ordered structures were observed, they found that adenine and guanine formed polymeric aggregates in which the bases stacked with a repeat distance of about 0.34 nm (similar to that in 292

Fig. 8. 7a-c. Potential-induced changes in the electronic states of monolayer guanine. (a-c) are STM images of monolayer guanine obtained at 80, 280 and 480 m V, respectively. Higher magnification STM and AFM images at the corresponding potentials are shown as insets at the upper-right and lower-right corners of (a-c) [8.26)

double helical DNA). The STM and AFM studies of guanine and adenine on graphite in NaCI solution [8.26] indicate that guanine and adenine condense into monolayer films on graphite spontaneously. The films dissolve at low substrate potentials which start either from step edges or by developing pits in the films, and they grow back at high potentials. Figure 8.7 shows three STM images of monolayer guanine obtained at surface potentials of 80, 280 and 480 mY, respectively. Both the superperiodic structure and the ordered monolayer guanine lattice (blobs) are clearly seen in the figure. As the surface potential changes, the STM image changes dramatically. For example, blobs in every two or three columns along the superperiodic strip at 80 mY (Fig. 8.7a) are turned into lines at 289 mY (Fig.8.7b), which transform back to blobs at 480 mY (Fig.8.7c). Higher magnification STM images (insets at the upper-right corners of Fig.8.7a-c) reveal that the transformation of blobs to lines is due to splitting of one blob into two. A unit cell is sketched in each STM image in the high-resolution STM images. Since the tunneling current is proportional to the local electron density of states of the molecules to a first approximation, the brightness variation in the images as a function of the surface potential provides information about the electronic properties of the molecules at various surface potentials. However, AFM images over a broad potential range show no obvious changes in the packing structure, as explained by the insets at the lower-right corners of Fig.8.7a-c. Similar to monolayer guanine, STM images of monolayer adenine were also found to be sensitive to the substrate potential, while no visible changes in the AFM images were detected. This observation demonstrates that the electronic states of biomolecular films at solidliquid interfaces can be modified and controlled by the surface potential. It also indicates that, in addition to tunneling spectroscopy (difficult to apply in solution), surface-potential-induced changes in STM images can be used to identify molecules. 293

8.3.5 DNA Sequencing by Scanning-Probe Microscopes Two goals of the human and other Genome Projects are (i) to map the relative positions of all known genes in the genome and (ii) to determine the sequence of bases in the genome. The four bases, adenine, guanine, cytosine and thymine, are arranged in different sequences in the human genome, which is 3 billion bases long. Gel-based DNA-sequencing requires small DNA fragments 0.3 to 0.5 kilobases (kb) long and can typically generate up to 15 kb of sequence per person per week [8.27], although automated sequencing rates of 36 kb per day have also been reported. If the pieces of DNA to be sequenced have been generated randomly, one typically needs to sequence about 5 to 7 times as many kb of DNA as the actual length of DNA in order to overlap all sequences along the length of the DNA [8.27]. Scanning-probe microscopes, such as STM and AFM etc., can image some surfaces at atomic resolution, even in air or water. It can also produce high-resolution images of DNA. Sequencing by imaging might provide a rapid alternative to present methods. From the viewpoint of an investigator asking for support from the Human Genome Initiative, one might argue that, since the STM and AFM tip can form images while scanning at several hundred Angstroms per second, and since it might be possible to attach a few micrometers of extended DNA to a substrate, then sequencing rates of kilobases per minute could be achieved. From the viewpoint of a critic familiar with the present state of the art, one would respond by noting that an image that resolves individual bases is probably limited to no more than 100 of them across the field of view, each scan would take 1 minute or more, there is no clear evidence that such data could be interpreted. Preparation and location of the samples would limit the measurement rate to a few samples (each of no more than a hundred bases) per day. The truth may lie between these extremes, and the matter deserves further investigation [8.28,29]. It has been shown that STM and AFM appear to be able to resolve individual bases [8.22,29,30]. To be useful as a sequencing device, the STM would have to be able to scan long portions of single-stranded material in which the bases have been labelled unambiguously. It is unlikely that images of a single-stranded DNA of arbitrary composition will easily be interpreted in terms of its sequence. We have attempted to circumvent this problem by making large electronic modifications to selected bases. A mercury atom has been attached to the adenosine nucletodes by using thiolated reagents in a Polymerase Chain Reaction (PCR) synthesis. A final sulfur-mercury reaction completes the labelling. Such polymers provided a few high-resolution images showing remarkable contrast [8.29]. It appears possible that STM could indeed form images of the sequence of a DNA, if a reliable way could 294

be found to bind monolayers of single-stranded material to a substrate. A procedure for reversible adsorption of DNA onto a substrate maintained under potential control has been developed [8.31]. This electrochemical method gives reproducible and uniform images of DNA; it may even prove valuable in the quest for a reliable deposition of extended single strands. The problem of distinguishing the path of individual strands becomes much more difficult with the increasing length of the fragment. However, it is clear that the STM will have a role as a probe of the shape and flexibility of genes. The utility of the STM as a sequencing tool is far from clear at this point. It may, however, have other applications to genetic analysis that are just as important. An AFM disturbs the biopolymer much less than the STM, so that many more options for pinning the molecules down can be explored. Although the AFM lacks the electronic sensitivity of the STM, it may be sensitive enough to distinguish the bases by shape alone. At present the AFM has the potential to map DNA at a resolution of less than 0.3 kb [8.28], based on AFM images of DNA-protein complexes. For example, a 120-kD complex of conjugated steptavidin has been visualized on 0.3-kb pieces of DNA [8.32]. Also streptavidin on 5 nm gold particles and RNA polymerase complexes have both been imaged on longer pieces of DNA [8.33,34]. Thus, an AFM can easily resolve proteins and other particles bound to DNA. Scanning-probe microscopes also offer the possibility to do a lot of other exciting DNA research in areas such as sequence-dependent binding of regulatory proteins, sequence-dependent kinking, and B- to Z-DNA transitions. In summary, at present, both STM and AFM show great potential for high-resolution mapping of DNA but they are not capable of sequencing DNA without further improvements. It seems reasonable to predict, however, that rapid single-molecule sequencing will eventually be possible with a scanning probe, based on examples from both nature and technology. In nature, DNA is read by the scanning probes called DNA and RNA polymerases at speeds up to 1 kb per second [8.35]. Regardless of the time scale needed to develop a rapid scanning probe sequencer for single DNA molecules, the benefit of such an instrument will be great [8.28].

295

8.4 Proteins Proteins, as one of the most important components of life structures and the main executor of life functions, are composed of twenty kinds of basic amino acids. Peptide chains made of animo acids can be coiled and folded to form various proteins with diverse structures. Proteins are conventionally divided into two basic categories: structural and functional proteins. STM and AFM studies of artificial peptides, structures and functional proteins have already begun. For the studies of peptides, it is apparent from the STM images of Poly(r-Benzyl-L-Glutamate) (PBLG) [8.36] that PBLG can have different conformations when dissolved in different solvents: helical conformation in DiMethylFormamide (DMF), chloroform and benzene; and random coil in DiChloroacetic Acid (DCA). The image of PBLG dissolved in DCA exhibits amorphous structure, in agreement with the effect of DCA. PBLG in chloroform appears to have a linear periodic structure, and the structure of the same sample dissolved in DMF even shows liquid crystal features of high orientation.

8.4.1 Animo Acids and Peptides Besides the AFM results on the lattice structures of DL-leucine described in Sect.S.1.3 (Fig.S.IO), the AFM has been used to image six amino acid crystal surfaces: glycine, L-aspartic acid, L-valine, L-isoleucine, L-leucine, and L-phenylalanine [8.37]. The ordered lattice of each amino acid could be imaged on the crystal sheets. The quality of images at the molecular scale (Fig.8.8) varied considerably, with L-ile and L-asp being the best and least resolved, respectively. Two-dimensional Fourier transforms of the realspace images aided in identifying periodicities (Fig.8.8). The measured lattice resolution results, averaged over the number of trials, are summarized in Table 8.2. Gly, L-asp, L-val, L-ile, and L-Ieu agree well with expected dimensions. The L-phe structure was reproducible, though it does not agree with the expected structure of either the a or the (3 form. This is assumed to be due to a combination of bulk hydration and surface hydration or reconstruction. The AFM images revealed periodicities corresponding to the bulk terminations in most cases, although frequently with reproducible extra spots which probably correspond to molecular structure within the unit cell. Step-motion kinetics were also imaged in situ during dissolution of L-Ieucine in flowing propanol. These results indicate that motion of steps oriented along (010) is limited primarily by reaction kinetics, whereas motion of orthogonal bends may be additionally controlled by solvent diffusion. 296

Fig. 8.8a-f. Molecular resolution AFM images of six amino acids. Image size is 15 nm. Insets show Fast Fourier Transforms (FFTs) of real space images. FFT spots corresponding to the expected surface structure (Table 8.2) are circles, and other spots that repeat from tip to tip are indicated by an arrow. Samples are as follows: (a) glycine, (b) L-aspanic acid, (c) L-valine, (d) L-isoleucine, (e) L-Ieucine, and (f) L-phenylalanine [8.37).

297

Table 8.2. Expected and measured surface lattice structures of the amino acids (dimensions in nm. drawings to scale) [8.37] Amino Acid

Expected Lattice 0511

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~.~

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Measured Lat1ice

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0.55

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056

-----

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\dr 110·

8.4.2 Structural Proteins The success of STM in the field of structural proteins is mainly with cytoskeletal proteins [8.38], collagens [8.39] and HPI-Iayer [8.39]. The images of network structures and single filaments of metal-coated type-IV collagen have first been obtained. Terminal globular domains 4-:- 5 nm high and associated with filaments were resolved. Collagen is an important protein component in animal tissue structure and in manufactured materials. The different properties of tissues with large amount of collagen (e.g., bone, tendon, skin) depend, in part, on the different forms and fibrillar organization of the collagen in them. Three types of collagen form large fibrillar structures: type I (skin, tendon, bone), to type II (cartilage) and type III (skin). All collagens contain three proteins arranged in a triple helix with nonhelical ends. AFM images of fibrillar and monometric type-I collagen display well-defined D banding of the fibrils, with a period of 70 nm (Fig.8.9). Beneath the fibrils finer fibrillar material was seen, probably oligometric collagen [8.40]. Samples prepared at acidic pH showed a meshwork of monometric collagen with an occasional structures of oligometric size. In STM images of bare type-I collagen, regular protrusions at a basic interval of 9 nm are found on individual collagen chains, which are believed to demonstrate the periodicity of monomer chains of collagen; while the 298

Fig. 8.9. AFM image of a group of type-! collagen showing characteristic D-banded fibrils. The dark bands (arrows) are indentations corresponding to the dark-staining gap regions in negatively stained TEM preparations of fibrillar collagen. Scanning range is 2500 X2500 nm 2 [8.40]

periodic structure of 3 nm reflects the helical triple strands. The width of filments in the image is about 1.5 nm, consistent with the known value. Cytoskeletal proteins such as microtubules and intermediate filaments have also been imaged. In STM images of microtubules [8.38] five or six protofilaments constructed by peanut-like structures (8 x 4nm 2 ) are discerned, which are suspected to be tubulin subunits. In the image of intermediate filaments, subfilaments coiled together are visible. The HPI layer which is found on the surface of bacterium (Deinococcus radiodurans) is a natural two-dimensional protein crystal with a period of 18 nm. STM images of an uncoated sample are obtained in the humidity range between 30 % and 45 % using carefully selected tips at a very low tunneling current of 0.3 pA and a sample bias of -8 V and +4.7 V, respectively. The images exhibit the hexagonal packing of the 2D crystal with a lattice constant of 18 nm. In the best cases the resolution is about 3 nm, sufficient to resolve the hole in the middle of each protein.

8.4.3 Functional Proteins The hopping technique was adopted to obtain STM images of porcine pepsin. Both bare and carbon-coated samples in the single and cluster state have been imaged. In images of bare single pepsin molecules, the lateral dimension and the surface topography of molecules are consistent with X299

8.5 Biological Membranes

Fig. 8. 10. (a) The plasma membrane of a dry cardiac myocyte has been stripped off to reveal a typical intricate array of filaments (23 X23fLm 2). (b) A live glial cell imaged in a physiological buffer is seen to move over a long period of time (65 X65 fLm 2 ) [8.4 J]

ray diffraction results, while the height of the molecules is only 4/5 of the expected result. In images of the coated sample, a larger lateral dimension and the lower height, only l/4 of that of the bare sample is shown, which are related to the coating process. Complicated structures, the explanation of which needs further research, are found on the surface of the coated sample. AFM has also been used to study proteins, such as immunoglobulins, fibrin, phosphorylase and actin [8.41]. Proteins embedded in membranes, such as ion channels and receptors, have the advantage that they are supported by a membrane. Bacteriorhodopsin in purple membranes [8.42] has been imaged at high resolution and the 2-D surface layer (S-Iayer) from Bacillus coagulans E38-66 and Bacillus sphaericus CCM2177 has been investigated at molecular resolution under physiological conditions [8.43]. AFM images of a drycardiac myocyte and a live glial cell in a physiological buffer (Fig.8.IOa,b) reported by Ho and Hansma [8.41] are remarkably similar to those from negative-strain electron microscopy [8.44], a significant improvement over previous results with this structure. The activity of the protein lysozyme was captured by an AFM working in tapping mode [8.45], where the conformational variation of lysozyme could be detected by the observed height variations.

300

Biological membranes are basic components of life which not only divide organisms into small cells but also control the outer and inner environment of cells to preserve normal life activities. Membranous systems of a cell, to which many enzymes combine, are also fundamental to biochemical reactions. The understanding of membranes forms an important aspect in the understanding of life phenomena. Although the progress in this area is not as striking as that in the areas of DNA and protein research, some preliminary results have been achieved which have aroused wide interest among STM researchers and biologists. Replacing the fluid, nonconductive DiMyristoyl PhosphatidlyCholine (DMPC) bilayers with rigid and highly conductive freeze-fracture replicas which can provide stable STM images, the ripple phase structures of DMPC with a periodicity of 13 nm and amplitude of 4.5 nm are observed [8.46]. The ripples shown in the STM image are asymmetric, rising more steeply to the left than to the right. In addition, some fine structures which are difficult to interpret are also found. The Langmuire-Blodgett technique has been employed to link Iyso-sn1-0ctanoyl-glycero-PhosphoryICholine (LOPC) to HOPG covalently before STM observation. In the images obtained, single phospholipid molecules of 2 -;- 2.5 nm in length and 1 nm in height are resolved. The failure to achieve atomic resolution may result from the flexibility and thermal vibration of the molecular chains. Small "islands" of 21 nm in diameter separated by a distance of 4 nm are observed on purified egg-PC by STM. Every island is argued to consist of about 170 lipid heads assembled together. In some regions, ordered arrangements of lipid heads are found, perhaps due to the ordered alignments of lipid molecules before drying. The ripple phase of the lipid with a periodicity of 13 nm is also detected below phase transition temperatures [8.47]. Filament structures are observed in the image of TE671 cell membranes. In images of oocyte cell membranes, not only are filaments of 12 nm similar to those of TE671 cell membrane found, but also sub-filaments of 3 nm are resolved. In addition, some hilly structures of variable size (5nm in average diameter) are detected in the same sample. Although the identity of either structure cannot be determined, it is supposed that the filamentous and hilly structures should be the cytoskeletal structures and complexes of lipids and membraneous proteins, respectively. Lipids and fatty acids have also been imaged with AFM in a variety of artificial membranes (LB films); both the head groups and tail groups of individual molecules have been resolved. The hydrophobic tail groups in

301

highly crystalline membranes can be imaged in air, while polar head groups require an aqueous environment [8.48]. One of the significant powers of STM and AFM is its ability to follow processes in real time. Currently, images are usually acquired in 10 to 100 s, which should allow visualization of processes as fast as translation or as slow as cell division. A dramatic demonstration of this capability is the AFM visualization of thrombin-dependent fibrin polymerization (Fig. 5.13, Sect. 5.1.3).

Fig. 8. 11. STM images of bacteriophage T7 [8.49]. (a) Two adjacent T7 phages and (b) lysed T7 phages

8.6 Imaging Cells and Other Applications

The preceding discussion in this chapter is based on direct STM and AFM observations on the molecular level. A Scaning Probe Microscope (SPM) can not only observe biological samples on this leveL but it can also detect surface topography over a larger area. Therefore, a SPM can be a powerful tool in observing surface structures of biological samples on various scales. Using an STM with a large scanning range, images of the complete cell sheath of Methanospirillum Hungateie (MH) have been obtained by applying a drop of dilute MH solution to the freshly cleaved graphite surface, and then the remaining solution is removed with a dropper or absorbed away with filter paper. The prepared sample can either be observed directly by STM after being dried or over-coated with a metal film. In order to reduce the damage of the tip to the sample surface, a slow linear scanning speed « 100 run/s) and the small tunneling current (ca. 0.1 nA) are required. STM images of coated and bare samples indicate that the sheath thickness is 8 nm. In the images of uncoated samples, the top of the sheaths show parallel rows, or corrugations, having a dominant lateral period of 6 nm. Bacteriophages, 1'7 and fd phages, have been studied by STM. Phages are absorbed on gold spheres and coated with Pt-C membrane before observation. In the STM image of the whole 1'7, as shown in Fig.8.11a, two complete 1'7 heads of approximately 70 nm in diameter and 50 run in height are observed. However, the 17 nm tail of the phage cannot be seen, which is similar to results using TEM. The STM image of lysed 1'7 in triply distilled water is depicted in Fig. 8.llb. Two adjacent phageheads are on the upper left corner of the image, with strands extending outward from the phages to the lower right. The phage head is easily identifiable in the lower phage. Four of the six sides of the head can be seen and the angles between adjacent faces are all near 302

120 0 . Additionally, individual proteins from the lysed 1'7 phages (average diameter of 5.5nm) are also observed. As described in the last section, AFM has been used to image a variety of fixed and dried cells including erythrocytes and bacteria dried onto a glass coverslip, white blood cells, neurones (Fig.8.lOb), cardiac myocytes (Fig.8.lOa). AFM images of whole chromosomes and chromosomes labelled by in-situ hybridization open a variety of exciting possibilities for investigating genomic organization and structure. There is, of course, significant interest in developing imaging conditions for live cells, and some successes have been reported by Haberle et al. [8.50]. Living monkeykidney cultured cells were imaged with AFM under normal growth conditions and showed reproducible features on the 10 nm scale. Upon adding a suspension of pox viruses, characteristic changes of the cell membrane were repeatedly observed in different experiments on different cells. They noted the exocytosis of proteins related to viral reproduction and the exocytosis of the progeny viruses themselves. While working with live cells is' of more basic interest, the ability to image surfaces of dried and fixed cells easily and quickly at high resolution may be valuable for the rapid detection and diagnosis of pathological condi(;ons in cells and tissues. Features as small as 10 nm have been described on the surface of whole cells, although none of these have been identified with certainty. Initial results from studies that used an antibody against the cell surface protein CD3 conjugated to colloidal gold have shown that the gold can be de303

tected on the surface of cells that were fixed and dried after labelling. This is a promising step towards labelling with naked antibodies in aqueous environments, which has so far proven difficult. With a force as small as 0.1 nN, AFM detected two conformations of extracellular porin[8.51]. Imaging is the most obvious use for STM and AFM. However, there are several other applications that may become important, particularly micromanipulation. The application of STM in nanofabrication will be dealt with in Chap. 9. The precise positional control and small size of the STM and AFM tips make it useful for micrmanipulation of molecules and structures on the nanometer scale. Several groups have reported moving protein molecules around on a mica surface, and nanometer-size holes have been made in artifical lipid membranes [8.52]. For isolated gap junctions, which are composed of two apposed bilayers [8.44]. The AFM tip has been used to dissect away one bilayer leaving the extracellular region of the remaining bilayer accessible for experimental manipulation (Fig. 8.12). DNA plasm ids

Fig. 8.13. (a) AFM image of the (111) plane of a cubic crystal of STMV (Satellite Tobacco Mosaic Virus). (b-f) Series of AFM images showing the STMV cryslal growth. (a) t = 0, 7.5p.m X7.5p.m, (b-e) t = 1500, 1590, 1670, 1840 [s], 25p.m X25p.m, (f) t = 3520 s, 23 p.m X23 p.m [8.55]

Fig. 8.12. 800 X800 nm 2 AFM images of an isolated gap-junction membrane. This double bilayer structure is stable when imaged at low force (A), but as the force applied to the tip is increased, the top bilayer is removed (B and C), exposing the extracellular surface of the bottom bilayer (D) for further examination [8.53] 304

have been cut into specific sizes apparently independently of sequence, which could lead to new approaches in molecular cloning and DNA sequencing (Sect.8.3.5). Attaching active molecules such as antibodies or proteases to the tip will extend it beyond a tool for mechanical manipulation to use in local chemistry and biochemistry. AFM has provided direct observation of the crystalization of proteins with high precisions on various stages of the growth [8.54,55]. These results are quite illuminating for the application of AFM to this important field (Fig.8.13). 305

(a)

Although STM studies on biological samples are only in the preliminary stage, the progress made has strengthened the vitality of scanning probe microscopy as a powerful tool to build up knowledge of microscopic life structures in their natural and subnatural states, thus providing a possibility to reveal changes in life structures at active states.

(b)

3·

8.7 Force Spectrum Analysis of Biological Materials There have been fast growing interest in using AFM to probe the characteristic interactions between biological species, as well as a large variety of organic molecules and polymers, etc .. The mechanism of performing force spectroscopic analysis using AFM is straightforward, and the revealed information is far-reaching. As an example, the exploration on the binding strength of cell adhesion proteoglycans from marine sponge produced a value of 400 pico-Newtons, leading to the believe that a single pair of molecules could held a weight as high as 1600 cells [8.56]. Using the measured force and binding energies, the effective rupture length of avidin-biotin could be estimated at 9.5 ± 1 A [8.57]. The technique has also been applied to investigate the bonding strength of the bases of nucleic acids (Fig. 8.14) and ligand-receptor pairs (Fig. 8.15). Force measurements have also been used to study the local elastic properties of polystyrene chains in different solvent conditions [8.60,61]. As the importance of force spectrum being gradually realized, attention has also been given to the principal factors involved in the measurements. The interaction is definitely a summation of forces from various origins, such as Van der Waals, electrical, chemical, and so on. On the other hand, adhesive force is also a vital part in the measurement. The later one could be quite sensitive to the environmental conditions, humidity, ionic strength, temperature, electrostatic potentials. Understanding the roles of these forces ha.s lead to the concepts of new microscopic analytical methods, and opened up possibilities for novel sensory systems [8.62-64] (Fig.8.16). These force could be differentiated by their dependence on tip-sample separations, even through this is normally a challenging task and are demonstrated in several cases. Since the deformations of substrate and the cantilever add considerable complexity to the observed behavior.

306

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Fig. 8.14. Schematics of (A) the interchain interaction of (ACGT)5 and (CAGT)5, (B)the intrachain interaction of C 20 -poly(I). The forces versus surface displacement measured between (C) (ACTG)5- and (CAGT)5-. (D) A C20-poly(I) surface and C 20-functionalized probe [8. 58J

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In the previous chapters we have described the applications of STM to surface topographies and electronic structures. STM, being a surface-analysis technique to study the surface properties, can also be used to modify or etch many surfaces on a nanometer scale. This ability is an important application aspect of STM and will be discussed in detail in this chapter.

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In the STM device, the tip is so close to the target as to make the electron current highly spatially confined - it is the key to the extremely high spatial resolution of STM images. The key element in surface modifications is also the STM tip. To affect these modifications one utilizes a variety of tipsample interactions, including attractive and repulsive forces, electric fields, and the effect of highly spatially confined electron currents. The small distance between tip and sample, which is about one nanometer, causes electrons to tunnel to (or from) a region on the sample that is approximately one nanometer in diameter, with an even smaller major distribution area. Thus, the surface fabrication produced by STM must be performed on the nanometer scale, i.e., STM can nanofabricate. Later, we will see that it is also possible to manipulate a single atom or molecule adsorbed on the surface with STM. Since the invention of STM in 1981, as a nanofabrication tool it has been used in direct surface identation, electron-beam-induced deposition, etching, single-atom manipulation, and so on. All of these techniques have a wide-spread application potential. First, it is possible to reduce the linewidth of large-scale integrated circuits from the micrometer scale to the nanometer scale by lithography, beam-induced deposition and etching, which is one of the goals of high technology. In most cases the resulting feature with dimensions on the order of hundred nm [9.1], but features with dimensions of a few nm have also been achieved [9.2-4]. An exciting possibility will be to use the tip to "operate" on biomolecules such as DNA and proteins. The electronic properties of devices may be dominated by quan309

tum-size effects when their size is reduced to the nanometer or atomic scale. By STM and other techniques, it is possible to discover new phenomena, design new devices and fabricate them. Next, STM can be utilized to repair masks and integrated circuits. The surface topographies can be imaged in situ during the surface fabrication process by STM, which makes it possible to discover defects in masks and circuits, to repair them by surface deposition and etching, and then to examine the final results by STM. Lastly, using the STM as a tool, the essential research on the growth, migration and diffusion of clusters on surfaces, and the interactions between small particles or between substrates and particles can be performed in order to manipulate clusters or atoms on purpose. Lens-focused electron beams, ion beams and X-rays can also be employed in nanofabrication. Although the STM seems unlikely to become competitive in some areas of nanofabrication such as wafer-scale resist patterning, it has its own characteristics. First, an STM can work in either the tunneling mode or the field-emission mode. When working in the latter, a low applied voltage (higher than a few volts) can produce a strong enough electric field to make electrons emit from the tip over the barrier, because the distance between tip and sample is very small. These emitting electrons with a certain current and energy, will not diverge greatly because of the small separation which results in a nanometer beam diamter on the substrate surface. Unlike conventional high-energy electron lithography, the lowenergy STM beam reduces the problems associated with electron backscattering and the generation of secondary electrons. A resolution of about 10 nm, and exposure rates comparable to those of conventional electron lithography have been achieved. Secondly, by moving the tip to contact the sample, the STM tip can also produce local contact forces and electrostatic forces in a small region on the sample surface to create indentations directly. Lastly, at present, STM is the only instrument that can provide a nmsized beam of very low energy electrons (0 -;- 20 eV). The importance of electrons with low energy is obvious when it is considered that many of the processes such as migration, bond breaking, chemical reactions that would be interesting to control, have activation energies less than 10 eV per atom which require a low-energy beam.

shortening the tip-sample distance, or applying a voltage pulse across the tunnel junction at selected points on a surface, there will be holes or hillocks formed on the surface which are the nanostructures most commonly created with a STM. In this application aspect, it is not required to coat the substrate with resist films or to modify the substrate under liquid or gaseous environments; the process may be regarded as the direct indentation with the tunnel tip on the sample surface. The indentation process will not degrade the sharpness of the tip, which makes it possible to image the surface after indentation in order to check its results in situ. The mechanism is different for different indentation methods, not all of them are well understood at present. By quantitative analysis of the tip-sample approaching process, it was found that the alternating elastic and yielding stages, and the Young's modulus and the yield stress could also be estimated accordingly [9.5]. For decreasing the tip-sample distance, there are two ways to move the tip. One is to control the tunneling resistance by the choice of reference current or tip-sample bias when constant-current feedback is used. The larger the reference current or the smaller the bias, the closer the tip moves toward the surface, which results in a lower tunnel resistance. The minimum resistance of a single-atom point contact has been shown theoretically and experimentally to be larger than, or equal to, 10 kQ, but sometimes the resistance will drop to 200 Q in making indentations on surfaces. In another method, STM is first brought into the tunneling range, then the feedback signal is fixed and the tip is moved toward the surface by applying an additional voltage to the Z piezo. This way allows more linear control of the change of the tunnel gap width than when the feedback loop is involved, and is thus a better method. By moving the tip towards the surface, direct indentation has been performed on metal surfaces such as Au, Ag and semiconductor surfaces such as Si and Ge in many experiments. Most of the tips are made of W, other metals such as Pt and Ir or their alloys are also suitable. From experiments it has been found that the tip conditions may influence the indentation results.

9.2.1 Modification of Metal Surfaces

9.2 Direct Indentation with the Tunneling Tip The key element in surface modification is, of course, the STM tip. There are several different ways the tip can accomplish such surface modifcations. It can act as a "scalpel" to dig trenches or produce indentations. By 310

The indentation experiments on an Ag surface in UHV, using the second technique mentioned above to move the tip to sample, have been performed by Gimzewski and Moller [9.6] with a W tip. They found that after contact with the sample, the dirty tips (i.e., after several hours of use) left depressions (holes) and the clean tips left protrusions (hillocks). The features created for both tip conditions were about 10 nm in diameter and 2 nm in the z direction when the tip was driven about 2.5 nm into the surface. It was con311

cluded that the clean tips adhered to the surface when contact was made which left hillocks. At this time, in order to return the current to its pretouch level, the tip had to be retracted further than it had been driven in, which supported the existence of a hillock due to the adhesion between the clean tip and the surface. Tip contamination prevented adhesion to the surface, so that retracting the dirty tips after contact did not pull the surface back up, but left it indented. Each indentation consisted of a hole with a surrounding wall of atoms pushed out from the center. Working at high current and small tip-to-sample distances, one can deposit material from the tip onto the sample, resulting in the formation of a hillock typically 20 nm across [9.7]. By applying 600 ns voltage pulses of about +4 V between an Au tip and an Au surface, a mound of about 10 nm in diameter and 1-:-2 nm in height was formed [9.8] due to the emission of atoms from the tip. The location of the mound can be precisely controlled. By programing the positions of the mounds, a gold map of the Western Hemisphere was constructed (Fig.9.1). The diameter of the map is about 1 j.Lm, giving the map a scale of 10 trillion to 1. It was pointed out that if the tip had previously touched the sample several times which usually happens for the experiment being performed in this mode, a lot of material may be first transferred from the sample to the tip, and then could be redeposited in small fractions afterwards during the lithographic process. In general, local heating should be important for adsorbate atoms, which are easily excited by electrons. Perturbations applied locally by the STM tip may lead to nonlocal, large-scale changes owing to the fact that

Fig.9.1. A map of the Western Hemisphere constructed by applying a voltage pulse between an Au tip and an Au surface. A mound of 10-:-20 nm in diameter and J -:-2 nm in height is formed [9.8] 312

surface atoms are coupled to each other by elastic interaction. This was demonstrated by the modification the reconstructed gold surface. This modification was accomplished by transferring a number of surface atoms to the STM tip to generate a surface multivacancy (hole) on Au (111)-22 xV3 surface which modifies the stress distribution at the surface. Hasegawa and Avouris [9.9] removed atoms from the Au surface by first bringing the tip close to the sample to establish an interaction between tip and sample, and then they applied a voltage pulse. The barrier for atom transfer between the tip and sample is reduced at close distances, and the high electric field generated by the voltage pulse further reduces the barrier, induces directionality, and makes the transfer of the Au atoms possible. By this method, they could make holes at specific locations, and can redeposit the removed atoms at other locations. The size of a hole can be controlled by adjusting the pulse height, pulse width, and the distance between the tip and sample. It is clear that high field strength and current intensity are two important aspects in the fabrication process. The mechanisms of field evaporation and current induced heating are well adapted to account for the observed variations of the fabrication results [9.10], as demonstrated by the threshold phenomenon shown in Fig.9.2. Further attention has been given to the detailed characteristics of the fabrication results. Using graphite as an example, a relationship was illustrated between the characteristics of the cross sectional profile with the numerical calculations involving mainly the anisotropy of the local transport coefficients (Fig. 9.3). The structural changes that follow the tip-induced surface perturbation were imaged in a time-resolved manner. Figure 9 Aa is an STM image taken before a hole was made: pairs of partial dislocation lines (their separation is about 4A om) are seen with a periodicity of 6.3 nm, that is, 22 times the lattice spacing of the Au(111) surface. After the generation of the hole (Fig. 9.4b), the two segments of the dislocation line on which the hole was formed change their relative positions slightly due to stress relief by the hole. Figure 9Ac shows that the original two dislocation lines have split. As time progresses, the size of the hole decreases as diffusing Au atoms enter the hole. In Fig.9Ad, the hole is now very clearly faceted (inset) with steps. Finally, in Fig.9Ae, within 6 min after Fig.9.4d, the hole has disappeared and a new arrangement of the dislocation lines has emerged. This forked structure is relatively stable, as can be seen in the image (Fig.9Af) taken 1 hour later. Most importantly, the STM results show that, on this surface, structural changes occur not only by single-atom motion but also by the concerted motion of large numbers of atoms through the motion of dislocation patterns. Hasegawa and Avouris [9.9] concluded that the structural modification is the result of both short-range interactions, which lead to local atomic relaxation, and long-range elastic interactions, which produce large-scale rearrangements. 313

Fig.9.3. The cross sectional profile of a STM fabricated crater on HOPG fitted with the numerical results derived from a low energy electron diffusion model (9. 15]

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A subsequent relief of the stress which had been induced by the formation of the indentation was also observed on the surface of Au(210) by touching the surface with a tungsten tip [9.7]. For the modification of a Au(lll) surface by loading to a soft-tip crash, Jaklevic and Elie [9.13] have also found that the indentation annealed out within a few hours by surface diffusion. The rate of atoms diffusing into the hole was calculated to be between 6 to 9 atoms per minute. These experiments demonstrate that the structures formed on metals like Au and Ag were unstable at room temperature due to diffusion and became significantly distorted within several hours. Scanning the tip transversely while it contacts the surface can cause scratches or grooves on the surface; the mechanism is the same as that of hole or hillock formation. However, it is impossible to create structures on elastic surfaces only by decreasing the tip-sample distance. For example, moving a tip into a graphite surface as much as 100 nm usually causes no

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surface damage, due to the elasticity of graphite. In this case, by applying a bias pulse or increasing the tunneling current, nanostructures may be formed. The experiments have been performed on the surfaces of metallic glasses, graphite, the semiconductors (Si and Ge) and some metals, despite the fact that on some of them nanostructures can be formed by contacting the tips to the surfaces. There are different nanostructure-formation mechanisms for the different substrate materials. A glassy Rh 25 Zr 75 sample was prepared by the splat cooling technique under high-vacuum conditions. Structures were written directly on its surface by the method mentioned in the preceeding paragraph. When working in the constant-current mode, the tip was moved to the selected point and then the sample-to-tip bias voltage was increased to 2 V by raising the tun-

314 315

Fig. 9.4. STM images of the Au (Ill) surface obtained: (a) before making a hole, (b) after the hole was formed, (c) 0 to 6 min after (b), (d) after 6 to 12 min, (e) after 12 to 18 min, and (f) after 50 min (9.12)

the bias voltage from -0.1 to -1.4 V while scanning of the tip was continued, small hillooks 1.0 to 1.5 nrn in diameter were observed all over the surface [9.14]. Other glassy sample that has been fabricated by STM is Hg1_xCd xTe [9.16]. A platinum tip was brought closer to the surface by increasing the set current a thousand times. The area to be "standed" was then scanned at a very fast rate, such that the feedback loop could not maintain a constant distance between the sample and the tip. After having made a crater by impacting a tip into the sample, "emptying" of the crater by post-impact scans can be subsequently observed. It is assumed that during these scans material which was broken off during the impact is moved out of the crater [9.15]. The tunneling electrons from a sharp tip apex have been demonstrated very helpful in fabricating nanometer scale metal-oxide wires [9.17]. This would provide a unique approach to study and control local electrical properties.

9.2.2 Modification of Semiconductor Surfaces

neling current up to 300 nA, and a hillock with a diameter of 30 nm and a height of 15 nm was created at that point [9.14]. The dimension of the hillocks increased with increasing bias voltage. The hillocks are formed because the high current density causes the substrate material around the selected point to melt, at this point the liquid is pulled up into a Taylor cone by the negative tip bias voltage. Reducing the current to 1 nA, the cone can be cooled while the electrostatic force is maintained, thus a hillock forms on the surface. It is not possible to form hillocks on all surfaces by applying this method. The melting temperature of the substrate determines how much heating is required to melt the substrate. For a pencil-shaped electron beam, the heating of the substrate depends not only on the current density but also on the mean-free path of the electrons in the substrate and the thermal conductivity of the substrate. By taking into account the small meanfree path of electrons in this disordered metallic glass substrate, and its lower melting temperature of 1340 K, it is easy to melt it by the incident electron beam and then to form hillocks on its surface. For some other substrates, such as Ir, with a higher melting temperature of 2683 K, the hillock formation cannot be observed under the same experimental conditions. But the localized heating and subsequent Taylor cone were supposed to be the origin of bias-voltage-induced modification on a gold surface. By increasing 316

As mentioned in the last subsection, the structures formed on the metals are unstable at room temperature due to diffusion. However, it is possible to form stable structures on semiconductor surfaces such as Si(110), Si(lOO) and Si(111) surfaces with STM tips. By moving the tip to the samples, holes and arrays of holes have been created on Si (11 0) and Si (100) surfaces in UHV. W tips were not cleaned after being put into the UHV chamber, which indicated a lack of adhesion between the tips and the sample during contact so that depressions were formed. Figure 9.5 shows STM images of the Si (110) surface before and after a hexagonal array of indentations were created by indenting the surface at 10 nm intervals. Each indentation consisted of a hole with a surrounding wall of atoms, pushed out from the

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center. The area around an indentation remained undisturbed. The total height difference between the top of the wall and the bottom of the hole was 0.7 nm when driving the tip into the surface about 2 nm. All written structures were highly stable and fully unchanged after a long time in UHV at room temperature. The mechanism of creating the structures on the Si (110) and (100) surfaces by shortening the tip-sample distance is due to a mechanical displacement of surface atoms produced by the tip-sample contact force. The sharpness of the tip is apparently not degraded by the intimate contact which makes it possible to image the surface with the same tip. As mentioned above, in some cases the deposition of tip materials or contaminations may also exist. Recently, by increasing the tunneling current from 1.0 nA to about 30 nA while keeping the bias voltage unchanged (+2.0V), Ma et al. [9.18] produced lines consisting of missing atoms on the Si(1l1)-7x7 surface. Figure 9.6 depicts three parallel grooves formed by this method. After the grooves were formed, if the bias voltage was reduced to less than 0.5 V while the high tunneling current of about 30 nA remained unchanged, the material from the STM tip could be redeposited onto the Si surface. forming protrusions (hillocks). Atomic-scale modification using STM was first reported by Becker et al. [9.19], who observed that by increasing the tip bias voltage from -1.0 to

Fig. 9.6. Three parallel grooves were created on a Si (111)-7 X7 surface by an STM tip in UHV. The tunneling current of about 30 nA, bias voltage of2.0 V and a tip movement speed of about 400 nm/s were appl ied in the experiment 318

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-4.0 V, atomic-size bumps (0.8nm in diameter) were deposited on the C-2x 8 reconstructed surface of the Ge (111). The topographic images before and after indentation are shown in Fig. 9.7. They suggested that these bumps have been germanium atoms that were picked up earlier by the tip. Later, Lyo and Avouris [9.20] observed that atoms strongly bound to a silicon surface could be removed or induced to diffuse by scanning the surface with the STM tip at an increased bias voltage. It was proposed that the removal mechanism involved a combination of chemical tip-sample interactions and electric field effects ("field-desorption "). Modification as a result of electronic excitation of the sample has also been successfully demonstrated. Electrons emitted by the tip were utilized by Becker et al. [9.21] to excite Si-H bonds to repulsive antibonding states, leading to the desorption of the hydrogen from the silicon surface. Similarly, Dujardin et al. [9.17] utilized electron-induced excitation to dissociate individual boron-containing molecules on silicon surfaces. As an initial step toward the development of nanostructure devices, a tunnel-diode effect has been discovered in studies of boron-rich Si(lll) surfaces [9.22,23]. In contrast to the conventional Esaki tunnel diode, a Negative Differential Resistance (NDR) in this STM experiment arose as a result of tunneling between localized quasiatomic states. The existence of such localized states gives rise to allowed and suppressed energies for tunneling, leading to NDR. These NDR-active sites are of atomic dimensions (ca. 1nm). A lot has been written on graphite surfaces, which is the most frequently imaged surface. The writing process requires no intentional surface coating or contamination. When working in the constant-current mode, applying voltage pulses (3 -;- 8 V, 10-;- 100 j1s) to the tunneling junction in air cre319

Fig.9.8. The letters CAS (an abbreviation of Chinese Academy of Sciences) were written on a graphite surface with a line width of about IOnm [9.25)

ates many holes. The dimensions of the holes are different for different tips, with a 2 nm minimum diameter [9.24]. Figure 9.8 displays the letters CAS (an abbreviation of Chinese Academy of Sciences) written on graphite. It was shown that the formation of holes is due to the removal of one or more layers of graphite over a small area directly below the tip, because the structures can also be visible after tipping one or more layers of graphite off the surface. The pulse amplitude necessary to write patterns on the HOPG surface varied from tip to tip, ranging from 3 to 6 V. They indicate that the tip may play an important role in the modification [9.25]. The same writing process can also be done on the surface of MoS 2 and epitaxial metals. It has been found that the above-mentioned writing process on the graphite surface was effective in air or water vapor, but in UHV the hole formation could seldom be observed, even with pulse heights up to 10 V. This suggests that the mechanism is a chemical process; but there is no full understanding of the mechanism. Applying smaller pulse voltages may sometimes cause hillocks on the graphite surface [9.26]. In summary, the experimental process of applying a pulse voltage on a tunneling junction or increasing the tunneling current in order to increase the local field strength and the current density is very simple. The mechanism of structural formations, however, has not been understood well. Some proposals, such as the chemical reaction induced by the current, the evaporation, melting or recrystallization of the atoms in the local region of the surfaces induced by a high current density, have been suggested. Some structural formation may be due to the deposition of the tip material or contamination. Besides the modification of metal and inorganic semiconductor surfaces, STM has also been employed in nanofabrication of organic materials. Examples may be considered the tests with the 1: 1 mixture organic 320

complexes of m-nitrobenzal malononitrile(m-NBMN) and diamine benzene (DAB) [9.27]. The capability of controlling individual domains at a scale of 500 nm in ferroelectric crystal of tri-glycing sulphate has also been shown [9.28]. More detailed information can be found in the review article by Nyffenegger and Penner [9.29]. It is also possible to create microstructures using STM on surfaces coated with resist films or under solution, or in gaseous environments. The experiments can be divided into two categories. One is electron-beam-assisted chemical etching and the other is electron-assisted chemical vapor deposition. We will discuss these in the following two sections.

9.3 Nanolithography on Resist Films Direct writing of nanometer-scale features on organic materials opens the door to using film as a resist for the etching mask and the possibility of etching in air [9.30]. The resists can directly be exposed with conventionally focused electron beams to create various structures. The e-beam resist materials frequently utilized in conventional lithography are PolyMethylMethAcrylate (PMMA) and polydiacetylene with urethane substitutuents (P4BCMU). They are typically sensitive to low-energy «20eV) electrons. The primary beam of a conventional e-beam lithography system has a considerably higher energy, so exposure of the resist occurs through interactions with secondary and backscattered electrons produced by the primary beam; a resist is consequently exposed over an area which is significantly larger than the primary beam spot size. The STM, even working in the field-emission mode, can supply focused electron beams with low energy which can interact with resists directly. Because the tip can be held within a few nanometers from the sample, which leads to an effective beam spot size on the sample of the order of the tip-sample separation, and the tip can transversely scan over the surface controlled precisely by a computer, STM can easily be employed in lithography for writing directly on the resist surface. The degradation in resolution because of interactions between the resist material and secondary electrons in conventional lithography can be overcome in STM lithography, which makes it possible to obtain more precise structures. Using STM it is also possible to make a thorough investigation of the exposure mechanism by controlling the bias voltage precisely (i.e., the energy of electrons with which the resists are exposed) in a certain time interval under constant current. In addition, STM generally ought to work in the fieldemission mode because the electrons must have enough energy to induce a chemical reaction in resists (i.e., to expose resists). In this mode, a linear 321

dependence of the tip-sample separation on the bias is expected in the absence of geometric effects, which make the widths of the features increase with the bias voltage. In order to be successfully exposed with an STM, the resist film coated on conducting substrates such as Si, GaAs, Au, graphite and so on must be extremely thin, on the order of a few tens of nanometers, for two reasons. First, the low-energy electrons must be able to completely penetrate the film in order to properly expose it and to prevent excessive charging of the surface. Second, if the film thickness is greater than the gap between the tip and the conducting substrate, the tip will penetrate and damage the resist film. Experimentally, the thickest film that can be used is Y nm, where Y is the bias voltage in volts. There are different kinds of materials which can be used as resist films in STM lithography, including polymers such as PMMA and P4BCMU which were often employed in conventional e-beam lithography, metal halides such as GaF2 and AlF 3 , etc. The resist can be applied to the substrates by evaporation or deposited from a Langmuir-Blodgett (LB) film balance and spin coating. P4BCMU, acting as a negative resist, is soluble in chloroform. Under exposure of the electron beam, cross-linked bonds will be formed to make it insoluble in chloroform. After coating it on a conducting substrate such as Si, STM lithography can be performed on the surface in UHY condition (l0-6pa) [9.31]. The experimental results have shown that the resist is thin enough to be imaged with an STM. For each film and each set tunneling current a tip-sample bias voltage could usually be found at which a repeatable image could be obtained. The thicker the film, the greater the bias voltage. Exposure on the resist had been achieved by making the tip-sample voltage more negative. During the exposure process the STM was also working in the constant-current mode, which made it possible to control the incident electron energy by changing the bias voltage. In most cases, the exposure resulted in a raised feature formed on the resist surface; the feature was stable and can be imaged by the STM. At voltages less negative than-8 Y or so, no features were detected, which showed that there exists an exposure-energy threshold and only when the exposure energy is greater than that, can the cross-linked bonds be formed in the resist. The minimum feature size was 20 run for a 76 nm thick resist film, which increased with the writing voltage, this is because the tip-sample separation which is the same as the minimum feature size should be proportional to the tip-sample voltage in the field-emission mode. The minimum feature size of 20 run is less than a third of the minimum-feature size observed following exposure with a 10 nm 50 kY electron beam. However, lithography with a vacuum STM and a 10 nm 50 kY e-beam has shown that a state-of-the-art high-resolution negative resist (SAL-601) is inherently capable of sub 25 nm resolution [9.32]. 322

To move the electron beam across the sample without exposing the resist, a technique for beam blanking is required. In a conventional electron-beam lithography system, the beam can be blanked by simply applying a suitable electric field to prevent the beam from passing down the electronoptical column. In an STM, the beam can be blanked in two ways. One is to simply retract the tip away from the surface - no current will then flow to the sample. The other is to decrease the tip-sample voltage below the exposure threshold when the sample is to be imaged. In the first method the tip can be moved quickly, resulting in a high exposure efficiency. It is also possible to expose PMMA with an STM [9.33]. Depending on the exposure and development, PMMA can act either as a positive or as a negative resist. At electron energies larger than 25 eY and doses larger than 10-2 C/cm 2 • a PMMA film coating on a Si substrate acts as a negative resist with the cross-linking occurring under exposure, development in acetone removes the unexposed film. Lower doses or lower energies followed by development in a solvent made of 3:7 Cellosolve-methanol results in a positive resist action, where the PMMA molecules undergo chain scission rendering them soluble in the developer. In experiments, below 25 eY exposure energy even with very large doses does not result in negative exposure. In X-ray lithography, it has also been found that at large exposures, PMMA acts as a negative resist with cross-linked bond formation, and at low exposures, PMMA acts as a positive resist with the chain scission. The resist patterns can be transferred onto a metal film using liftoff. Liftoff is a high-resolution technique for transferring resist patterns onto a metal film. First the resist is patterned, then the metal film is deposited, and finally the resist is dissolved away, removing the metal wherever it is deposited on top of the resist, which leaves a metal film pattern. After coating PMMA on Si, a thin-film resistor of gold-palladium 2 /Lm in length and 120 nm in width, being 13.5 nm thick, has been fabricated with the aid of liftoff. The room-temperature resistance of the device was 2.5 kU [9.28]. Lithography with STM leaves some theoretical and technological questions unsolved. First the need to use very thin resist films with a thickness of nanometers will make it difficult to fabricate structures with high aspect ratios, i.e., the metal film cannot be thicker. Another more serious question is the limited writing speed, which confines its use to very small, individual devices and structures for research applications. Significant progress has been made in the fabrication of nanometer-scale tip arrays. The use of such arrays may enhance the lithographic exposure rates. Complicated nanofeatures have also been fabricated sucessfully on inorganic resist (Ag-Se film of 10 to 100nm thickness) on Si, Si0 2 and AI substrates [9.34] by applying a positive DC voltage to the tip at a constantcurrent mode when writing, implying the potential possibility of transcription of nanopaterns on a wide variety of substrates. 323

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Electron-beam-induced etching and deposition is a way of writing patterns on substrates in solution and in gaseous environments. The basic idea for etching and deposition is very simple. The focused beam is used to supply energy to decompose chemicals in a localized region. The decomposition products can include a metallic species to be deposited on a surface, or a corrosive species intended to participate in an etching reaction resulting in locally etched structures on a surface. The substrates used include Si, GaAs, graphite and metals. The STM system for operation in solution should supply the necessary solution. To minimize unwanted Faradic leakage current, the tip must be treated using some special methods such as coating wax up to the extreme end of the tip (ChapA). Deposition and etching can also be induced with the STM tip as an electrochemical electrode to drive a localized Faradic current of ions. A gaseous environment can be utilized by introducing organometallic gas with a pressure of several Pa into a vacuum chamber with a base pressure of 10- 5 -:- 10- 6 Pa. For different deposition metals, the introduced gas is different. They include DMCd, W(CO)6' WF 6 , and organometallic chemical of Au. Three possible mechanisms have been assumed to account for the dissociation of organometallic molecules [9.35J. (i) Electrons tunneling inelastically between the tip and sample break apart gas molecules adsorbed on the surface of the substrate. (ii) Current traveling between the tip and sample can locally heat the surface of the substrate enough to cause pyrolytic dissociation of adsorbed gas molecules. (iii) High fields between the tip and sample break down the gas creating a microscopic plasma between the tip and sample which then deposits the metal atoms on the surface. All of these will break the chemical bonds from the energy of electrons traveling between the tip and sample, thus the STM ought to be operated in the field-emission mode.

Fig.9.9. Plot which shows the extent of etching on a Si (100) surface as a function of the areal scan rate. In this particular case, the tunneling current was 1 nA, the scan rate was 10 Hz and the scan length was kept constant (1000 nm) [9.36]

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of etching proportional to the areal scan rate (Fig. 9.9). The extent of etching also depends on the tunneling current and the tip; increasing the tunneling current would also increase the extent of the etching. The minimal etching linewidth is as small as 20 nm. The etching mechanism may be due to the intense electric field between the tip and surface causing oxide formation. The oxide was henceforth etched chemically by the dilute HF solution to form features. Organic molecules and metal ions in solution can also be deposited onto a substrate by applying a voltage pulse to the STM tip immersed in solution. Nanostructures of Ag, Cu, and Pt have electrochemically been deposited on a gold surfaces [9.37]. Kolb et al. demonstrated a scheme that uses mechanical contact of tip and surface to deposite nanoclusters, and the tip apex can automatically be recovered in the electrolyte at proper electric potential. These studies demonstrate that molecules' self-assembly provides a method for the protective, and chemically selective, encapsulation of reactive nanometer-scale metal structures.

9.4.2 Nanofabrication in Gaseous Environments 9.4.1 Nanofabrication in Solution Direct etching can be performed under solution. Placing dilute (0.05%) HF solution on the surface of Si or GaAs, allows imaging and etching to be done using an STM apparatus with 1 nA tunneling current and 1.4 V tipsample bias (under these conditions a leakage current of less than 50 pA is required) by varying the scan rate [9.36]. It was found that a critical threshold of the scan rate exists, with a scan rate faster than that the surface can only be imaged and slower than that the surface can be etched to the extent 324

In an organometallic gaseous environment, by applying a high bias voltage between the STM tip and the substrate, the gas adsorbed on the substrate under the tip is dissociated, leaving the metal from the organometallic gas deposited on the substrate, a complex pattern can be deposited by tracing out that pattern with the tip. An Auger Electron Spectroscopy (AES) analysis indicated that there is metal and a large amount of carbon in the deposits. The composition of the deposits depends on the deposition conditions and the gases involved. The amount of metal in the structure depends on the tip voltage and the emission current. In experiments, it is bet325

ter to apply a voltage pulse with a duration of less than the time constant of the feedback system in order to ensure that the writing pulse will not cause the tip to retract significantly during the pulse time, thus the resolution of the writing pattern remains high, so that the pulse bias only creates a pulsed tunneling current. If the bias is slowly increased, the feedback system retracts the tip to ensure constant current, and the increased tip-sample distance broadens the linewidth. Metallic Cd was deposited on Si or metal surfaces by operating the STM under DiMethylCadmium (DMCd) with this method [9.38]. The experiments showed that if the tip pulse was less than 3 Y, no visible surface modifications occurred. As the pulse voltage was increased beyond 5 Y, the size of the deposit increased rapidly; the size and shape of the deposition did not seem to depend upon the pulse frequency. The results of AES analysis determined that the deposits from DMCd contained a large amount of carbon, as well as a much smaller amount of Cd. By changing the composition of the gas, Al and W deposits could also be produced. Additionally, WF 6 was used to etch silicon. In investigating STM-induced deposition of W from W(CO)6' it was also found that for biases of -10 to -40 Y (field emission regime), conducting deposits resulted if the current exceeds a threshold value (e.g., ::::::10nA at -20Y). The conducting deposit, analyzed by AES, contained a certain amount of carbon as well as W. Resistivity was measured to be 0.01 Q'cm, three to four orders of magnitude higher than the resistivity of bulk W. High-aspect-ratio columns were deposited at 16 mTorr W(CO)6 pressure, with a tip bias of 25 Y and a current of 20 nA. The columns, which could be deposited as closely spaced as 80 nm, were 25 -:- 30 nm in diameter, and as high as 280 nm. This technique may overcome the disadvantage of STM lithography, namely that it is difficult to fabricate structures with high aspect ratios. It has also been found that deposits can be formed without allowing any organometallic gases into the vacuum chamber [9.33]. This is because of the existence of a layer of organic contamination on the substrate surface, which is polymerized or broken down by the writing process leaving a carbon deposit. The pattern generated by contamination resist can be very useful in forming either lines in surface structures or resist masks. For example, an Au line with a 0.1 !Lm width was obtained by forming a contamination deposit on a Si surface coating with an Au film of 100 nm thickness. The Au film below the resist was protected from sputtering by ions, thus Au lines could be formed by sputtering the unprotected Au film. Allgair et al. [9.39] have studied nanocsale STM patterning of silicon dioxide thin films by catalyzed HF vapor etching. Using ambient hydrocarbons as the carbon source, careful selection of voltage and current settings allowed the controllable production of oxide trenches of widths of 10 to 75 nm following HF vapor etching.

326

9.5 Atomic-Scale Manipulation In addition to its uses in direct writing, electron-beam-induced deposition and etching, an STM may be applied to affect reconstructions or to move adsorbates such as metal particles, clusters, molecules or single atoms from one place to another, i.e., manipulating these small particles. This capability would be useful for building devices out of small particles, perhaps from particles of different materials. It could also be utilized to study interactions between particles, and between particles and substrates. Eventually it may become possible to build or dismantle molecules atom by atom.

9.5.1 Manipulation of Atoms a) Xenon Atoms The simplest adsorbate on a surface is a single atom. Such an atom can be moved by means of an STM according to our imagination. It is known that the tip of the STM always exerts a finite force on an adsorbate atom. This force contains both van der Waals and electrostatic contributions. By adjusting the position and the voltage of the tip, the magnitude and direction of this force may be chosen. This, taken together with the fact that it generally requires less force to move an atom along a surface than to pull it away from the surface, makes it possible to track an atom across the surface while the atom remains bound to the surface. Moving atoms across the surface will result in a patterned array of atoms according to our own design. This wonderful idea has first been realized by Eigler and Schweizer [9.40]. Their decision to study xenon on Ni (110) was dictated by the requirement that the corrugations in the surface potential must be sufficiently large for the xenon atoms to be imaged without inadvertently moving them, yet sufficiently small that, when desired, enough lateral force could be exerted to move xenon atoms across the surface. The experiments were performed using an STM put into a UHY system and cooled to 4 K in order to reduce the contamination rate of the sample surface through adsorption of residual gases so that no measurable contamination occurred over weeks. After cooling to 4 K and depositing xenon to a desired coverage, some xenon atoms were adsorbed on the Ni(110) surface. Figure 9.10a represents an STM image of the surface taken under constant-current conditions, the randomly located xenon atoms on the surface can be recognized clearly. This image was obtained with a tip bias voltage of 0.01 Y and a tunnel current of 10 nA. The tip was made of W wire. At this gap impedance 327

the interaction of the xenon with the tip was sufficiently weak to leave the xenon essentially unperturbed during the imaging process. To move an atom the interaction between atom and tip has to be strengthened. Operating the STM in the non-perturbating imaging mode just described in order to find an atom to be moved, the stopping, scanning and placing the tip directly above the atom, then changing the tunneling current to a higher value, because of the control of the feedback system, the tip moves towards the atom. This results in an increase of the tip-atom interaction. Thereafter, moving the tip again under the new closed-loop condition across the surface, the xenon atom below the tip is dragged with it and moves across the surface with it to another position. At this new position, the tip motion is stopped and retracted by reducing the tunnelling current to the value used for imaging, the attraction between the xenon atom and the tip is effectively terminated, leaving the xenon atom bound to the surface at the desired position. After that the tip can be moved away to manipulate other xenon atoms. Thus, it is possible to move the xenon atoms adsorbed on the Ni(lIO) surface at will. Using this method Eigler and Schweizer [9.40] successfully moved xenon atoms adsorbed on a Ni(lIO) surface by changing the tunneling current to 1007600 nA and shifting the tip at a rate of 0.4 nm per second. On the basis of this, they constructed an ordered array spelling out the letters I, Band M (Fig.9.10a-f). By adjusting the experimental conditions, one can adjust the interactions of the STM tip with one adsorbed Xe atom. Eigler and Schweizer successfully repeatedly transferred a Xe atom back and forth between the tip and the substrate surface. The electrical conductance between the tip and the substrate depends on the position of the Xe atom, which results in a switching device with a low-conductance state when the Xe atom is on the substrate and a high-conductance state when the Xe atom is on the tip. This atomic switch is a bistable element; components such as this are vital in developing microcircuits. Although the chemical identity and the structure of the outermost atoms of the tip were not known, it was found that for any given tip and bias voltage, there was a threshold height. Only when the tip-sample distance was lower than the threshold, i.e. the tunnel current was high enough, were the adsorbed xenon atoms moved. Simple investigations showed that neither the magnitude nor the sign of the applied voltage had significant effect on the threshold tip-sample distance. This suggested that the dominant force between the tip and the xenon atom is due to the van der Waals interaction. As described in Chap.2, the generally accepted theory of Tersojj and Hamann (9.41 J demonstrated that for small bias and constant current the STM image corresponds to a map of constant local-state density at the Fermi level. The extent to which an adsorbate will be "visible" to the STM 328

Fig. 9. lOa-f. A sequence of STM images taken during the buill-up of a pallerned array of the lellers I, Band M constructed of xenon atoms on the Ni (110) surface. The atomic Structure of the nickel surface is not resolved. (a) The surface after xenon dosing. (b)-(f) Various stages during the construction. Each leller is 5 nm from lOp to bOllom [9. 40J

depends on how it locally changes this state density. Lang [9.42, 43J has shown that for single-atom adsorbates on metal surfaces, the crucial parameters in determining the apparent height of the atom in a low-bias STM image are the s-state and p-state densities due to the adsorbate at the Fermi level. Now Xe, like the other rare-gas atoms, when adsorbed on a metal surface makes virtually no contributions to the state density at the Fermi level. The STM studies indicated that the Xe atom appears as a nearly cylindrically symmetric 1.53 ±0.02 A high protrusion from the Ni(lIO) surface. Similar images for Xe adsorbed on the Pt (Ill) surface at 4 K have also been obtained by Weiss and Eigler (9.44]. It was therefore somewhat surprising to find that Xe is readily visible in the STM. To understand this phenomenon, the atom-on-jellium model has been used to calculate for the apparent height ofaXe atom as imaged with the STM (9.45]. The result was found to be in good agreement with experiment. The Xe 6s resonance, although lying close to the essentially unfilled vacuum level, is the origin of the Fermi-level local-state density which renders Xe "visible" in the STM. To perform atomic-scale modifications, one relies on the fact that the STM tip and the sample can interact through a variety of different mechan329

isms. For examples, ionization followed by field evaporation has been suggested by Lyo and Avouris [9.46] for the reversible transfer of Si atoms between the tip and an Si surface of the STM. Mamin et al. [9.8] argued that negative-ion formation and subsequent field evaporation can explain the transfer of gold atoms from a negatively-biased gold tip to a substrate. Haberland et al. [9.47] have presented evidence for the existence of a negative xenon ion with a lifetime longer than 1.10- 4 seconds. However, this proposed mechanism is not consistent with all the observed phenomena in the case of moving Xe atoms [9.48]. Ralls et al. [9.49] have studied the electromigration of impurities in metal nanobridges. They found that the dominant contribution to the impurity comes from heating the impurity above the lattice temperature by inelastic electron scattering at the impurity. Electron migration in the applied field can often results in motion of the impurity in the direction of electron flow. Walkup et al. [9.50] presented a simple model for the multiple vibrational excitation by inelastic tunneling. Calculations for Xe atoms adsorbed on metals by using this model indicate that there should be substantial vibrational heating for tunneling current > 100 pA. The observed sideways motion of the xenon atom at larger tip-sample separation is consistent with the xenon atom being vibrationally excited. The absence of sideways motion at smaller tip-sample separation may be due to the increased van der Waals attraction to the tip as the tip is brought closer to the surface. Heating-assisted electron migration is the only mechanism that is consistent with all the observed phenomena [9.48]. b) Iron Atom

Crommie et al. [9.51] made use of the hard-sphere nature of adatoms by building enclosures on their Cu (111) surface out of small numbers of Fe adatoms. They positioned the ada toms with an STM at 4 K. Figure 9.11 shows a circular enclosure of 48 Fe adatoms that are spaced at about 3.7 times the 2.55 A nearest-neighbour spacing of the Cu atoms. The Fe-Fe repulsion prevents much closer spacing, while a wider spacing would be "leakier". Also, this particular spacing allows formation of an almost perfect circle on the hexagonal grid of allowed sites. As can be inferred from the image of Fig. 9.11, dramatic patterns appear inside such "quantum corrals". In the topograph scans, the trajectory of the STM tip just above the surface corresponds to a contour on which the local ~ensity of electron staIes is constant, and the density of states depends on the square of the amplitude of the wave functions available to the electrons on the surface. The corral is about 14.3 nm in diameter, and the ripples closely match what is expected for a particle in a circular box. STM can also induce movements of other metal atoms such as Ag, Cu, etc. [9.52]. These practices have stimulated a series of theoretical investiga330

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tions which suggest that the controlled movement of single atoms are jointly affected by the field strength, the adsorption nature, and the electronic structure of the electrodes [9.53-55]. c) Silicon Atom Another manipulation scheme that can exhibit atomic resolution involves field desorption. Field desorption/evaporation has been studied using a field-emitter tip for a long time. The STM's capability to image a surface and to place the tip over a particular atomic site of interest opens up the possibility of atom-selective field-desorption. The high electric fields (ca. 1V/ A) needed can be generated, for example, by applying a voltage pulse when the tip is over the selected site. Figure 9.12 exhibits an example of a nanometer-scale modification of Si (111) at room temperature [9.46J. Panel A depict the modification of the Si (Ill ) surface when a W-tip is brought to 3 Afrom the surface and a 3 V pulse is applied to the sample. This leads to a modification with a characteristic morphology composed of a hillock surrounded by a depression. In a second step, the cluster of atoms forming the hillock is transferred to the tip by placing the tip over the hillock and applying a second 3 V pulse. The Si cluster is then redeposited to the left of the hole by applying a negative voltage (- 3 V) pulse. To remove a particular surface atom, its bonding to the substrate has to be preferentially weakened by the interaction with the tip, and its transfer completed by the application of a near-threshold voltage pulse. The STM tip must not only be sharp, but also perfectly clean. Tips with oxides or other surface impurities may be adequate for imaging purposes, but not for manipulation. Shown in Fig. 9.13 is a series of atomic-scale modifications (the 331

Fig. 9.12a, b. Nanoscale modification of Si (III). (al A +3 V pulse applied at about 3 A away from the electronic contact has led to the formation of a structure involving a central hill surrounded by a depression. (b) A second +3 V pulse applied over the hill leads to the transfer of the hill to the tip. The tip is then moved to the left of the hole and the cluster is redeposited by applying a -3 V pulse [9.46]

voltage pulse is 1 V and the tip-sample distance is about 1A). Figure 9.l3a depicts a section of a Si(l1l)-7x7 surface containing a defect in the lower right (dark site) which is used as a marker. In Fig. 9.13b the voltage pulse was applied while the tip was centered over the site indicated by the arrow. Three Si atoms were removed, leaving a fourth under the apex of the tip. This fourth Si atom was left in an unstable configuration, and it migrated to the left to occupy a center-adatom site of the 7x7lattice (Fig.9.l3c). This atom was then removed with a second pulse (Fig.9.l3d), Finally, with another pulse a corner adatom was removed (Fig.9.l3e). Avouris et al. [9.56] found that for Si in general and specially for clusters, the STM can not put back a removed atom or cluster of atoms to reform the original structure. Although they can place the cluster over the original site, it does not incorporate itself. This is due to rebonding that takes place at both the substrate and the cluster after the removal of a number of atoms. The redeposited atoms must overcome a sizeable activation barrier to occupy their original site. However, like in Fig.9.l3f, in the case of single-atom removal, incorporation occasionally can be achieved by bringing the tip over the vacancy site and applying a negative pulse to the sample. Although the manipulation of both Si and Xe involves tip-sample interactions and a voltage pulse, the mechanisms by which Xe and Si atoms are transferred appear to be different: Xe atoms move in the same direction as the tunneling electrons, while Si moves in the opposite direction. As discussed previously, in general, local heating should be important for adsorbate atoms, which are easily excited by electrons.

d) Sulfer Atoms

Fig. 9. 13a-f. A series of atomic scale manipulations on Si (III) at room t~mperature. (a) The tip is placed at about I A away from the electronic contact over the site indicated by the arrow. (b) A I V pulse removes 3 atoms leaving the fourth under the tip. (c) The first attempt to remove this atom leads to its migrating to the left (see arrow). (d) A second pulse removes this fourth atom. (e) A new corner-adatom is eliminated, and in (f) it is placed back to its original position [9.56)

It is also possible to manipulate single atoms using other methods. Japanese scientists have successfully written a "peace 91" pattern on a MoS 2 surface by applying voltage pulses. The height of the written letters is just 1.5 nm. The tip, which was 0.3 A away from the surface, was positioned above a S atom and applying a voltage pulse, which made the S atom ionize. The ionized S atom disappears on the surface and leaves a position vacant. These vacant positions can produce patterned structures.

9.5.2 Manipulation of Molecules and Clusters a) Carbon Monoxide Using the same method, it is also possible to move a molecule across a surface. After moving Xe atoms adsorbed on a Ni surface, Eigler and Schweizer have successfully moved CO molecules adsorbed on a Pt surface and created a CO molecule in the shape of a "man". It was found that the CO mole333

332

cule stood on the surface with the oxygen atom above the carbon atom. The distance between the molecules was approximately 0.5 nm, so that the "CO man" was just 5 nanometers from head to toe. The movement of the CO molecules was also due to the increase of the tunneling current, which weakened the bond between the molecule and surface, and strengthened the bond between the molecule and the tip. Thus, when the STM tip was moved, the molecule moved with it, all the while remaining bonded to the surface. However, not every adsorbed atom and molecule can be positioned by this method. For example, it is difficult to move oxygen atoms adsorbed on Ni. Lateral manipulation of CO molecules on a Cu (211) substrate has been performed with STM at temperatures above 4 K [9.57]. The STM picture (Fig. 9 .14) shows the two letters FU formed by CO molecules adsorbed on Cu (211) at 30-K substrate temperature. The CO molecules are here imaged as dark spots, but in other cases the molecules can appear as hemispherical protrusions. Meyer et ai. [9.57] assumed that in both cases the CO molecules on Cu (211) are imaged with STM tips of different local chemical composition. Whereas the STM pictures of Fig.9.14 were recorded with a tip having adsorbed CO or one of its constituents near or on the tip's apex, the molecules appearing as protrusions in the STM picture were obtained with a clean metallic tip.

Fig. 9.15a-d. The STM tip-activated conversions of three types (8, C, and D) of Sb pre4 cursors to the final stage. All four panels are observation scans taken with a tip bias of +1.0 V, which does not cause any structural changes. The conversion scans are taken between observations scans at +3.0 V. The tunneling current is 0.4 nA. The time sequence of the scans is from (a) to (d) [9.59]

b) Antimony Molecules It is of particular interest to induce reversible transitions between different states of individual molecules without fragmentation or other damage because a reversible process could be used to make logic or memory devices with atomic or molecular dimensions. Dissociation and reorientation of antimony molecules on the (00 I) surface of silicon (probably an electric field effect) have been studied by Mo [9.58]. The STM tip induced a reversible rotation between two orthogonal

Fig. 9.14. The letters FU formed with CO molecules on a Cu (211) substrate by controlled lateral manipulation of the molecules with an STM tip [9.57] 334

orientations of individual antimony dimers on the surface. Upon deposition of a small fraction (ca. 2 %) of a monolayer of Sb4 on Si (00 I), five distinct types of Sb clusters (type A, B, C, D and E) were observed to coexist. At higher bias voltage (ca. 3V, both polarities), all the types of precursor clusters could be induced to convert to the final state, sometimes going through one or two of the other states. Figure 9.15 presents the STM tip-induced conversions of three types (B, C, and D) of Sb4 precursors to the final state. From both the above and the thermal annealing experiments, Mo concluded that the type E cluster is the final state for the dissociative chemisorption of Sb4 on Si (00 I) and that the order of relative stabilities of the five cluster types, from the least to the most stable. is as follows: ball (A), dumbbell (C) or rotated dumbbell (D), rotated dimer (B), and the final state (E). Between any two states there is an energy barrier that prevents a conversion at room temperature in the absence of the STM tip. Interestingly, a dimer can be converted back and forth between the rotated state (B) and the final state (E), even though all other conversions are irreversibly toward the more stable states. The conversion of a dimer from the final state (E) to a rotated state (B) and then back to E again was also observed. It was more difficult to convert a final state dimer to a rotated dimer than vice versa. He estimate that, at 3.0-V bias, the "reverse" 335

conversion from E to B is less frequent by a factor of 1 in 5 than the "forward" conversion from B to E. It typically takes one high bias scan to convert a rotated dimer to the final state, but it takes an average of five scans with the same parameters to reverse it. This simple rotation can be explained by an atomic-scale torque exerted on the antimony dimers by the STM tip. The reversibility of this process could provide a basis for making atomic-scale memory cells.

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c) Decaborane and Other Organic Molecules 1

With the STM, one can select a particular adsorbed molecule, probe its electronic structure, dissociate the molecule, and then examine the dissociation products. These capabilities are demonstrated for decaborane (B 10 H 14) molecules adsorbed on a Si(l1l)-7x7 surface [9.22]. For low exposures of 8 10 H 14 (ca. 0.01 L), isolated molecules generally adsorbed near defects on the surface. Preferential adsorption near defects indicates that B IO H 14 is initially in a mobile precursor state. After selecting a particular adsorbed molecule, by operating the STM in the field emission mode (higher than 4 V), several effects have been observed including displacement of adsorbed molecules, dissociation and fragmentation, and cluster formation [9.22). Foster et al. [9.59) pinned single di(2-ethylhexyl)phthalate molecule on a graphite surface with STM by applying a 100 ns pulse of 3.7 V to the tip during normal scanning. For these experiments, the graphite substrate was first covered with a drop of the solution containing the molecules, and the tip was then immersed in the drop. The pinned molecules could later be removed or cleaved by another voltage pulse. Bernhardt et al. [9.60) were also able to produce similar structures on graphite without the presence of the organic molecules but with otherwise unchanged experimental conditions. They concluded that the production of the features is associated with the loss of tip material. However, pinning of a molecule cannot be ruled out in the case of the organic molecule present. Manipulation of molecules such as Cu-T8P-porphyrins [9.61) and C 60 gives an example for moving relatively large single molecules in combined steps (Fig.9.16).

d) H2 0 Molecules In other case, a Si (Ill )-7 x 7 surface was exposed to H2 0 which then dissociates on it, with the Hand OH fragments tying up surface dangling bonds. The objective is to remove the adsorbates, regenerate the dangling bonds and thus restore local reactivity. This can be achieved by scanning the area at a high positive sample bias voltage (> 3 V) while keeping the current constant. After a single high-voltage scan the bias is lowered and the surface is

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imaged again. Avouris [9.62] found that this procedure regenerated six dangling bonds, all at their proper positions in the 7x7 unit cell, and, in addition, it stimulated the diffusion of the adsorbate. Schemes such as this can have nanolithographic applications. For example, one could envision passivating a semiconductor surface, then fielddesorption adsorbates to form a pattern of dangling bonds. A new chemical can then be introduced to react with the dangling bonds and "develop" the pattern.

9.6 Quantization of Conductance in Nano-Contacts Produced by STM Along with the achievements in atomic manipulation and nanofabrications, considerable interests, both technologically and theoretically, have been seen in the quantized conductance behavior in the nanowires or nanocontacts produced by STM. The experimental results have provided direct evidence of the dimensional restriction on the electron density of states. A zero or one dimensional contact is defined as two electrode connected by a single atom or a linear chain of atoms, respectively. A O-d contact can be readily formed by the attachment of an individual atom to the tip [9.63-65], or the sample surface, where a 1-d contact could be formed by mechanical deformation of the STM tip from its contact state [9.66,67] (Fig. 9.17). For the responsible electrons at Fermi level, at quantized energy unit of 2£2/h is expected. This value would be dramatically reduced if the involved electrons come from energy levels below the Fermi level (Fig. 9.18). The closely related studies of the deformation of the nanocontact [9.68,69] and electron scattering [9.70,71] have provided much help to clarify the observed conductance quantizations. 338

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It should be noted that other types of quantizations in tunneling conductance are also very interesting topics. The investigation of Coulomb blockade behavior is certainly one of the interesting ones, together with the internal molecular orbitals HOMO, LUMOs [9.72].

9.7 Fabrication with Other Scanning-Probe Microscopes 9.7.1 Machining Thin Films In addition to the STM, the Atomic Force Microscope (AFM) has been used to modify materials. The concept of surface modification based on an increase of the tip-sample interaction strength is generally applicable to AFM as well. For instance, by increasing the strength of the force interac339

tion between tip and sample in a force microscope, submicrometer-scale structure can directly be written onto the substrate under ambient conditions [9.73]. The surfaces have been modified by the AFM include metal surfaces (Au, Cu), semiconductor surfaces (Si, GaAs), Langmuir-Schaeffer films, polycarbonate films, high-T c superconducting and metallic films [9.74]. In addition, AFM-tip-induced wear of transition-metal dichalcogenide materials has been reported on a ~50 nm scale [9.75]. The advantage of the AFM is that it can be applied to nonconducting materials. Several groups have shown that direct-contact (repulsive mode) imaging of soft organic layers under sufficiently high loads can lead to orientational ordering or removal or both of this organic layer from the area scanned by the AFM tip. The length scale or resolution of these modifications typically has been ~ 100 nm. In the past, most AFM-induced modifications produced relatively large-scale features. The situation is improving, and recently Kim and Lieber [9.76] were able to "nanomachine" thin layers (ca. 1.5nm) of Mo0 3 grown on the surface of MoS 2 with a resolution of about 10 nm. A typical image of a Mo0 crystallite formed after thermal oxidation of MoS 2 at 3 4800C is displayed in Fig.9.19a and an image of the same area of the surface acquired after machining a line in the Mo0 3 thin film by applying a force ~ 5· 10- 8 N is shown in Fig. 9 .19b. The line has a resolution of about 10 nm at the Mo0 surface and 5 nm at its bottom and is approximately one 3 unit cell deep. These features are shown clearly in a three-dimensional line scan image and cross-sectional view (Fig.9.19c). Continued scanning does not lead to features deeper than the thickness of the Mo0 3 thin layer. The MoS substrate functions as a self-limiting stop in this modification pro2 cess. Kim and Lieber concluded that the mechanism by which this structure is created is simply tip-induced wear of the Mo0 3 surface. They have nanomachined a series of lines to pattern "HU" in the Mo0 3 (Fig. 9 .19d- f). The resulting HU structure is stable during continuous imaging with loads ~ 10-8 N. These results suggest that by employing appropriate substrate material and methods, this technique might be used to fabricate nanometerscale diffraction gratings, high-resolution lithography masks, and possibly the assembly of nanostructures with novel properties.

9.7.2 Charge Storage An AFM tip has been used to store information density by locally trapping and de-trapping charge in the silicon nitride-silicon dioxide-silicon system. When a high-voltage tip-substrate programming pulse is applied to the top of the silicon nitride, charges tunnel from the substrate silicon, through the oxide, and are trapped in the silicon nitride. These charges can remain trap-

0 V")

0

-~A

E c:

50

100

Fig. 9. 19a-f. A series of 500 nm by 500 nm AFM images were recorded with an applied load of 1· 10- 8 N. (a) Mo0 3 grown on the surface of a MoS 2 single crystal. The irregularly shaped object occupying the central portion of this image is the Mo0 3 crystallite, and the surrounding regions correspond to the MoS 2 substrate. The insets shown in the lower and upper corners of this figure are atomic resolution images (3.5X3.5nm 2 ) recorded on the MoS 2 and Mo03 areas, respectively. (b) A line machined in the Mo0 3 crystal with an applied load of 5· 10- 8 N. (c) Zoomed view of this structure rendered as a three-dimensional Iine scan. A single scan across the line is also shown in the upper portion of this image. (d to f) A series of images illustrating sequential machining of the Mo0 3 crystallite to define the HU pattern. The white bar in (a) represents 50 nm and also defines the length scale for (b, d, e, and f) [9.77]

ped for many years. The presence or absence of these charges may be detected by their effect on the capacitance-voltage characteristic of the tipsubstrate capacitor. The charge can be removed with a suitable programming pulse of the opposite polarity. Using an instrument which combines a scanning capacitance microscope [9.77] and an AFM the storage of 256 kilobits of information over a (1201Lm2) area has been achieved with an error rate of less than 0.03% [9.78,79]. Figure 9.20 shows two complex patterns written with a -40 V pulse lasting 20 1Ls. Figure 9.20a was written in about 8 min and contains 24189 written charge bits. Figure 9.20b shows a zoomin near the center of the image, revealing more of the detail of the image. Figure 9.14c was written similarly and contains 49194 written charge bits from a 512x 512 nrn 2 grid. The reading and writing rates used for these images was about 550 bits/s, but rates of 10 6 bits/s are to be reasonably expected. 341

340

Fig.9.20. Two images of pallerns of stored charge. (a) 120 p.m across image with a pallern consisting of 24189 wrillen charge bits. (b) A zoom-in of (a). (c) A second pall ern consiting of 49194 wrillen charge bits. This image is 90 p.m across. Each of these patterns represents 256 kilobits of information 19.801

9.7.3 Magnetic Structures and Writing into an Interface Hosaka et al. [9.62] applied current heating to make nanometer-scale magnetic domain structures in magneto-optical material by means of STM. Using a current-imaging tunneling-spectroscopy technique, small magnetic domains with around 100 nm in diameter were made in a PtCo multilayer at the voltage of less than 2 V for 1 ms. It is considered that the temperature in the local area rose up to above the Curie point of about 400 0 C. Similarly, magnetic structures can be created by increasing the interaction strength between a magnetic tip and a magnetic sample, for example, in a Magnetic Force Microscope (MFM) [9.81] or in magnetic-sensitive STM. By using an in-situ prepared ferromagnetic probe tip (Fe) and a magnetite (00 I) substrate, a clear contrast between Fe2 + and Fe3+ on the octahedrally coordinated B-sites of the magnetite has been obtained. Local modifications in the surface spin configuration have been observed, which have been determined to a scale of 1 nm [9.82]. It is most likely that such localized changes are induced by the exchange interaction between tip and sample at close tip-surface distance. On the other hand, hot electrons injected by a STM tip with a few volts of tunneling bias scatter and modify a metal film not only at the top surface of the film, but throughout the film and at the inner or interfacial surface. As described in Chap.5, the Ballistic Electron-Emission Microscopy (BE EM) technique is a powerful method with which one can probe such modifications. BEEM has been used to write 80-nm tall letters into a Au/Si interface without modifying surface topography. The lithographic image can only be read by the BEEM [9.83].

9.8 The Future

For other substrates modified by a scanning probe microscope and applying a pulsed voltage to the gold-thin-film-covered AFM tip has made nanometer-scale gold dots of around 10 nm in diameter on a Si02 / Si with negative tip bias. On the other hand, positive tip bias has made small balls to evaporate and remove the silicon dioxide [9.80l The former is assumed due to field evaporation of gold atoms on the silicon dioxide and the latter due to current-induced heating of the surface. The silicon appears in the bottom of the ball after the voltage application. This means that the current heating 0 makes temperature in the local area risen up to higher than 1000 C.

As the previous sections show, the feasibility of a number of modifications involving small numbers of atoms or even single atoms or molecules has been demonstrated. These manipulations include: sliding and positioning of atoms or molecules on a surface, transferring atoms from a sample to the tip and redepositing them at another location on the sample, dissociating individual molecules, desorbing and depositing atoms or clusters of atoms, and inducing local heating and phase transitions. In other cases, reactions which are specifically induced by low-energy electrons, such as dissociative electron attachment, can be used to create locally free radicals and ions. Moreover, tip-induced modifications can be performed under a wide range of conditions, i.e. in air, in ultra-high vacuum, at solid-liquid interface, and 343

342

even at a solid-solid interface. At the same time our knowledge of the microscopic interaction mechanisms between tip and sample has improved significantly. The control and reproducibility of such experiments will undoubtedly improve. In the near future most applications are likely to appear in basic science. Examples include assembling precisely controlled atomic or molecular arrangements with novel physical, chemical or biological properties, studying interatomic/molecular interactions in such assemblies using scanning probe microscope, and extending electrical transport studies from the mesoscopic to the nanoscale regime.

References

Chapter 1 1.1 1.2

1.3

I. 4

1.5 I. 6 I. 7 I. 8 1.9 I. 10

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Subj eet Index

Actin 300 A-DNA 287 Added-row model210, 211, 212 Advantages of STM 1-3 AE-TOA 116 AFM modification 340 - application 114-121 Ag (110)-0 211 - (111) surface 110 - -O-Ag chains 212 Ag/Si(111) 230 AI(111) surface 161 Alkali metals on Si (100) surfaces 241 Amplitude transfers 66 Analog-to-digital converter (AOC) 95,97 Animo acid 296 Antibody 303 Antimony molecules 334 Apertue 158 Atom-selective field desorption 331 Atomic-force microscope (AFM) 77,105 Atomic step 179 Atomic tracking technique 113 AU(110) surface 166 Au(111) electrode 119 - - surface 168,169,313 Au/GaAs(100) interface 136-139 Au/Si interface 140,142,143 Au/Si(lll) 231 Average work function 5 B/Si (111) 234 Ballistic carrier spectra 142, 143 - electron emission microscopy (BEEM) 133,343 - hole spectroscopy 140-144 Barrier heights 5,44 BEOT-tlF 243 Benzene on Rh (111) 243 - on Pt(111) 244

Bi/Si (100) 235 Bi/Si(1ll) 235 Bilayers 30 1,304 Bimorph 70 Biological membranes 30 I BiSrCaCuO 58,20 I Boron nitride 114 Buckling angles 52 Buffer layer 137 C60 /Ag(111) 255 C60 /Au(111) 255 C60 /Cu (111) 255 C60 /GaAs (110) 252 C 60 IMoS z (0001) 254 C60 /Si (100) 254 C60 /Si(1ll) 254 C6 H6 31,32 Cantilever 107 Capacitance detection 112 Cardiac myocyte 300 COW defects 200 Cells 302 Charge bits 341 Charge storage 340 Charge-density wave (COW) 193,202 Chemical identity 269 - reactions 265 - sensory 161 Chemisorption 277 CIISi (111) 235 Classical spectroscopies 49 Clinoptilolite 120 Coarse-approach system 74 Cohesive energy 240 Collagen 298 Collector current 134 Commensurate phase 196 Computer automation 96 Conductance quantization 338 Constant-current mode 6,46

365

- - topography 46 - -height mode 6 Convolution filter 10 I Critical angle 134 Cs/GaAs (I 10) 238 Cu(lOO)-O 208 - (110)-0 206,209 - (I IO)-S 221 - (III) 169 - (l11)-S 216 Cu/Au 119 Cu/Si(lll) 232 Current imaging tunneling spectroscopy (ClTS) 46, 174 - -separation characteristics 44 - -voltage characteristics 43 Cutoff time 82 Damping system 64 Dangling bond 56, 182 Deflection sensor 112 Density of States (DOS) 40 Differential interferometer 113 Diffusiou barrier 262 Digital filtering 100 DigitaHo-analog converter (DAC) 97,98 Dimer-adatom-stacking (DAS) fault model 52, 173 Dimethylcadmium (DMCd) 324 Discrete Fourier transform (OFT) 99 DL leucine 115 DMPC 301 DNA bases 292 - sequencing 294 - -protein complex 292 Double-tip imaging 80 Effect of tip geometry 84 Egg PC 301 Electric-field gradient 132 Electrochemical etching 81 - polarization 82 Electron litography 301 - microscope (EM) 1,3,4 Electronic states 37 Electronics 92 Electrostatic-force microscope (EFM) 130 Electrostatic imaging 131 Empty state 52 Energy gap 59 366

Energy resolution 47 Epitax.ial growth 261 Fatty acid 246 Fibrin polymerizing 117 Feedback control 93 FeNdS thin film 127 Field - emission microscope (FEM) I - evaporation 328,313 - ion microscope 1,4 Fixation of samples 280,283 Flex.ibility of biological samples 281 Force 107 - sensor 107 - spectrum 306 Free-electron model 18,19 Friedel oscillatory pattern 189 Fulleres 252 Functional proteins 298 GaAs (I 00) 188 GaAs(l 10) 52,188 GaAs(l 10)-052 GaAs (III) 191 GaAs-AIGaAs 192 Ge(OOI) 185 Ge(lll) 184,318 GeSi(lll) 186 Glass-insulated tip 88 Glial cell 300 Glycine 296 Gold foil 108 Graphite 35 see also HOPG Guanine 292 Group-III metals on Si (III) 233 H2 S 216,222 Hgl_xCd xTe 317 High-T c oxide 200 Highest occupied molecular orbital (HOMO) 31,243 Histogram equalization 10 I HOPG 114,280,319 Hopping technique 284 HPI layer 298 Imaging processing 100 Inchworm motor 74 Inclined plane 100

Incommensurate (C) phase 196 Indentation 310 Inelastic electron tunneling spectroscopy (lETS) 38 Integrated pyramidal tips 110 Interfacial modification 144 Inverse photoemission spectroscopy (lPES) 57 L-aspartic acid 296 L-isoleucine 296 L-phenylalanine 296 L-valine 296 Langmuir-Blodgett film 122,129 Lateral force microscope (LFM) 121 - stiffness 107 Layered compounds 193 LFM image 123 Liftoff 323 Liquid crystal/HOPG interfaces 247 Local density of states (LDOS) 37, 168 LDOS oscillations 169 Lock-in amplifier 44 Long-chain molecules 246 LOPC 301 Louse 74 Low-energy electron diffraction (LEED) 1,177,206,216,227 Machining thin films 339 Magnetic forces 127 Magnetic-force microscope (MFM) 124,127,343 Magneto-optical disk 129 Manipulation - CO molecules 333 - Fe atoms 330 - H2 0 molecules 336 - S atoms 333 - Sb molecules 334 - Si atoms 331 - Xe atoms 327 Mass transport 216 Mechanical designs 67 Mechanically cut tips 85 Meniscue 82 Metal clusters 257 - films 261 - surfaces 165 - -vacuum-metal tunneling 40 Metallic adsorbates 228

Methanospirillum hungateie 302 MFM image 127 Mica 116 MIM structure 12,38 Mo(OOI)-S 227 Modification - of metal surfaces 311 - of semicoductor surfaces 317 Molecular exciton microscopy (MEM) 162 Monkey-kidney cultured cells 303 MoO) 340 MoS 2 1115,193 Na tip 41 Na/GaAs (110) 238 Na/Si(lll) 236 Nanofabrication 309,324 - in gaseous environments 324 - in solution 324 Nanolithography 321 Near-field - scanning optical spectroscopy (NSOS) 158 - optical chemical sensors (NOCS) 161 - optics (NFO) 155 - scanning optical microscopy (NSOM) 153 Nearly commensurate (NC) phase 196 Nearest-neighbor distance 241 Negative feedback loop 93 Ni(lIO)-H 213 Ni(lIO)-S 224 Ni(lII)-H213 Ni (III )-S 219 Nanocontact mode 122 Nanoscale faceting 184 Nanotube tip 89 Non-contact AFM 127 Nucleic acids 287 Nuclepore membrane 147

o tip 270 O-Cu chains 209 Optical - beam deflection 114 - displacement sensor 114 - fiber tip 151, 155 - probe 156 - microscope 3, - microscopy 153 367

P4BCMU 321 PBLG 296 Peptides 296 Photoemission spectroscopy (PES) 57 Photon scanning tunneling microscopy (PSTM) lSI Piezo-electric ceramics 68-71 PMMA 321, 322 Polyalanine 117 Polymerization of fibrin 117 Porcine pepsin 299 Porphyrin 336 Position sensitive detector (PSD) 122 Preparation of cantilevers 108-111 Proteins 296 Pt-Ir tips 84 PZT materials 68 PZT tube 69, 73 Quantum corrals 330 Quantum mechanics 10 Reaction - of H with Si (111) 276 - of NH) with Si (00 I) 274 - of NH) with Si (1 I I) 56,271 - of NH) with B/Si (Ill) 273 - of SB4 with Si (100) 277 - on Cu (l 10) 268 - on Ni (110) 265 Re (000 I)-S 228 Real-time display 99 recA-DNA complex 292 Rectangular barrier 12 Red blood ceIl 119,149 Resist films 321 Resistence of oxide films 13 Rh 25 Zr75 315 Sample - coating 283 - positioning 74 - preparation 282-286 - wave function 22 Sapphire surface 131 saturation coverage 240 Scanners 71 Scanning electron microscope (SEM) 1,3 - exciton microscopy (SexM) 162 - force microscope 121 368

- ion conductance microscope (SICM) 145 - noise microscopy 149 - plasmon near-field microscopy (SPNM) 151 - probe microscopes 105 - thermal microscope 147 - tunneling microscope (STM) 5 - tunneing potentiometry (STP) 149 - tunneling spectroscopy (STS) 37-39 Schottky barrier (SA) 133 Semiconductor clusters 260 - surfaces 173,317 Separation-voltage characteristics 44 Si(OOI) 177-182 Si(001)-2xn structure 182 Si(lIO) 183,317 Si (11/ )-CV3 XY3)Ag 60 Si (111 )-(Y3 xY3)Al 56 Si(III)-2xl 52,175 Si(111)-7x7 50-54,173,318,333 Si 3 N4 cantilever 108 Si (111) oxidation 274 Si (100) oxidation 276 Silicon nitride 340 Single-molecule detection (SMD) 163 Si02 cantiliver 108 Solid-state barrier 38 spectroscopy 37 SrTiO) (STO) 340 Statistically differencing 102 STM - conferences 8 - movie 209 - resolution 2 - limitations 8 Strain coefficient 69-71 Structuaral proteins 298 Substrates 280 Sulfur adsorption 215 Superconductivity 57 surface diffusion 172 - modification 309 - plasmon 153 - states 50,55

Thermal occupation probability 21 Three-dimensional representation 102 Thresholds 139 Tip 80-92 - characterization 90-92 - influence 48,60,85 - insulation 89 - orbital 31 - preparation 80 - wave functions 22 Tip-surface interaction 28 Transmission electron microscope (TEM) 1,4 TRAPSQR barrier 15 ttF-TCNQ 242 Tube scanner 73 Tungston tips 80 Tunnel-diode effect 319 Tunneling 11 - conductance 24,29 - current 5, 17,25 - effect II - gap 5

T7 phages 302 Tapping mode 126 TaS 2 196 TE671 cell membrane 301 Ten-butanol 120 Druck: Verarbeitung:

Strauss Offsetdruck, Miirlenbach Schaffer, Griinstadt

- geometry 23 - probability 14 - spectroscopy 37 Type-I collagen 298 UHV-STM 68,76 Vibration amplitUde 66 - damping system 64 - isolation 63-67 Vicinal surface 212 Viton spacers 65 W 10 cluser 30 Wave function 22 Work function 45 X-ray diffraction I X-ray photoemission spectroscopy (XPS) 2 Zeolites 118

SPRINGER SERIES IN SURFACE SCIENCES Editors: G. Ertl, R. Gomer and D.L. Mills Physisorption Kinetics

Founding Editor: H.K.V. Lotsch 21

Surface Phonons

22

Chemistry and Physics of Solid Surfaces VIII Editors: R. Vanselow, R. Howe

23

Surface Analysis Methods in Materials Science

24

The Structure of Surfaces III

Editors: W. Kress, F. W. de Welle

By H. }. Kreuler, Z. W. Gortel The Structure of Surfaces

Editors; M. A. Van Hove, S. Y. Tong Dynamical Phenomena at Surfaces, Interfaces and Superlattices

Editors; F. Nillolj, K.·H. Rieder, R. F. WiUis

Edilors; D. J. O'Connor, B. A. Sexton, R. 51. C. Smart

Desorption Induced by Electronic Transitions, DIET II

Editors: W. Brenig, D. Menzel Chemistry and Physics of Solid Surfaces VI Editors: R. Vanselow, R. Howe

Editors: S. Y. Tong, M. A. Van Hove, K. Takayanagi, X. D. Xie 25

NEXAFS Spectroscopy By J. Stohr

26

Semiconductor Surfaces and Interfaces

By W. Monch 2nd Edition

Low-Energy Electron Diffraction

Experiment, Theory and Surface Slruclure Determination By M. A. Van Hove, W. H. Weinberg, C.-M. Chan Electronic Phenomena in Adsorption and Catalysis By V. F. Kiselev, O. V. Krylov

27

Helium Atom Scattering from Surfaces Editor: E. Hulpke

28

Scanning Tunneling Microscopy II

Further Applications

Kinetics of Interface Reactions Editors; M. Grunze. H. J. Kreuzer

and Related Scanning Techniques

Editors: R. Wie,endanger, H.. J. Giintherodt 2nd Edition

Adsorption and Catalysis on Transition Metals

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Chemistry and Physics of Solid Surfaces VII &Hlors: R. Vanselow, R. Howe

11

The Structure of Surfaces II

Editors: J. F. van dec Veeo, M. A. Van Hove J2

Diffusion at Interfaces: Microscopic Concepts

Editors; M. Grunze, H. J. Kreuzer, J.I. Weimer I)

14 15

Desorption Induced by Electronic Transitions, DIET III Editors; R. H. Stulen, M. L. Knotek

19

Scanning Tunneling Microscopy III Theory of STM and Related Scanning Probe Methods Editors; R. Wiesendanger, H..J. GUntherodt 2nd Edition

)0

Concepts in Surface Physics

By M. C. De'jonqueres, D. Spanjaard·) 31

Desorption induced by Electronic Transitions, DIET V Editors: A. R. Burns, E. B. Stechel, D. R. Jenni,on

32

Scanning Tunneling Microscopy and its Applications

Solvay Conference on Surface Science

By C. Bai

Editor; F. W. de Welle

2nd Edition

Surfaces and Interfaces of Solids

33

Adsorption on Ordered Surfaces oflonic Solids and Thin Films Editors; H.• J. Freund, E. Umbach

34

Surface Reactions

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Applications of Synchrotron Radiation

By H. Uilh") 16

Atomic and Electronic Structure of Surfaces Theoretical Foundatjons

By M. Lannoo, P. Friedel 17

Adhesion and Friction Editors; M. Grunze, H.'. Kreuzer

18

Auger Spectroscopy and Electronic Structure

Editor: R. J. Madix High-Resolulion Studies of Molecules and Molecular Adsorbates on Surfaces

Editor: W. Eberhardt

Editors; G. Cubiolli, G. Mondio, K. Wandelt 19

20

Desorption Induced by Electronic Transitions, DIET IV Editors: G. Betl, P. Varga Scanning Tunneling Microscopy I

General Principles and Applications to Clean and Adsorbate-Covered Surfaces

Editors; H.-I. GUntherodt, R. Wiesendanger 2nd Edjtion

") Available as a textbook

)6

Kinetics of Metal-Gas Interactions at Low Temperatures: Hydriding, Oxidation, Poisoning

)7

Magnetic MuJtilayers and Giant Magnetoresistance Fwtdamentals and Industrjal Applications Editor: U. Hartmann')

By E. Fromm