Fullerenes, nanotubes, onions and related carbon structures

Materials Science and Engineering, RI5 95 209-262

Fullerenes, nanotubes, onions and related carbon structures C.N.R. Rao *, Ram Seshadri, A. Govindaraj, Rahul Sen Solid State and Structural Chemistry Unit, CSIR Centre of Excellence in Chemistry and Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India Received 6 March 1995

Abstract Fullerenes, containing five- and six-membered carbon rings, of which CeO and CT0 are the prominent members, exhibit phase transitions associated with orientational ordering. When CsO is suitably doped with electrons, it shows novel superconducting and magnetic properties. We review these and other properties of fullerenes in bulk or in film form along with the preparative and structural aspects. Carbon nanotubes and onions (hyperfullerenes) are the other forms of carbon whose material properties have aroused considerable interest. Besides discussing these new forms of carbon, we briefly introduce other possible forms, such as those involving five-, six- and seven-membered rings and hybrids between diamond and graphite. Keywords: Fullerenes;

Nanotubes;

Hyperfullerenes;

Superconductivity;

Magnetism

1. Introduction 1.1. Background

and history

Diamond and graphite are the traditional forms of crystalline carbon familiar to us. Diamond has four-coordinate sp3 carbon atoms forming an extended three-dimensional network whose motif is the chair conformation of cyclohexane, a puckered six-membered ring molecule. Graphite, on the other hand, has three-coordinate sp2 carbons forming planar sheets whose motif is the flat six-membered ring (Fig. 1) , The new carbon allotropes, the fullerenes, are closed-cage carbon molecules with three-coordinate carbon atoms tiling spherical or nearly-spherical surfaces. The best known of these molecules is structure Buckminsterfullerene, CeO, which has sixty carbon atoms forming a truncated-icosahedral with twelve pentagonal rings and twenty hexagonal rings, as shown in Fig. 2. The structure is essentially that of a soccer ball. The coordination at every carbon atom is not planar but rather slightly pyramidalised at every carbon atom. In other words, some sp3 character is present in the essentially sp2 carbons of fullerenes. While regular hexagons can tile a plane, pentagons can tile a sphere. The simplest example of pentagons tiling a sphere is a pentagonal dodecahedron with twelve pentagons. The structure of Cc0 can be visualised as being obtained by spacing apart the pentagons of the pentagonal dodecahedron with hexagons (Fig. 3). The key feature of the fullerenes is the presence of five-membered rings which provide the curvature necessary for forming a closed-cage molecule. Such structural motifs are not new to chemistry - particularly in the chemistry of elemental boron, C&,-like motifs are ubiquitous [ I]. Paquette and coworkers [2] had earlier synthesised dodecahedrane (C2Hr2), a molecule with the same symmetry as C&. The prediction and discovery of the closed carbon cage is an interesting story. In 1966, Jones [ 31, proposed large hollow cage molecules constructed from sheetlike materials in order to bridge the large discontinuity between the densities of gases and of the condensed phases. He suggested that such * Corresponding

author.

0927-796X/95/$29.00 0 1995 Elsevier SSDIO927-796X(95)00181-6

Science S.A. All rights reserved

210

C.N.R. Rao et al. / Fullerenes. nanotubes. onions and related carbon structures

(a)

(b)

Fig. 1. The structures of (a) diamond and (b) graphite. The respective structural motifs, the chair form of cyclohexane the benzene ring are shown with thick lines.

and

molecules could be fashioned out of graphite, for instance. In his later writings Jones [ 41 pointed out that from Eulers phase rule for regular polyhedra exactly twelve pentagons are required to close a cage comprising only pentagons and hexagons. In making this suggestion, he was inspired by Thompson’s

Fig. 2. The structure of the Cso molecule.

211

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 3. The truncated icosahedral structure of C,,, obtained by spacing apart the five-membered six-membered rings.

rings of a dodecahedron

with

seminal book on forms in biology [ 51. In 1970, Osawa [ 61 conjectured the soccer ball-shaped CeO molecule. Soon after, Bochvar and Gal’pem [7] published the results of their Huckel calculations suggesting that CeO would be stable with a closed shell electronic configuration. In 1985, Kroto, Smalley and others [ 81 reported the detection of a sixty-carbon cluster in the mass spectrum of laserevaporated graphite; a seventy-carbon cluster was responsible for the next most abundant peak in their spectrum. The unusual stability of C&, as evidenced by its high relative abundance, led them to propose the closed cage truncated-icosahedral structure. They named the molecule Buckminsterfullerene in honor of the American architect R. Buckminster Fuller who introduced geodesic structures in architecture [ 93. The experiments of Kroto and Walton, who were not aware of the earlier theoretical work of Osawa and others, were partly motivated by experiments to determine the forms that molecular carbon might take under extra-galactic conditions, and particularly by the knowledge that unusual carbon molecules such as HC7N had been detected by microwave observations [ lo]. Based on mass spectrometric studies on laser-ablated carbon plumes by Rohlfling et al. [ 111 had earlier pointed out the preferential stability of even-atom clusters. These authors reported a mass spectrum with a markedly high concentration of a sixty carbon species but did not take any special note of this observation. Between 1985 and 1990, the work on fullerenes was confined to mass spectrometric and related vapour-phase studies [ 121 and on theoretical calculations on the structure and stability of the systems [ 131. In 1990, Kratschmer et al. [ 141 found that the soot produced by arcing graphite electrodes in a bell jar produced a simple IR spectrum and the sort of absorption spectrum expected for CcO. Soon after, they found that the material responsible for the visible absorption spectrum of the soot could be extracted from the soot into benzene and could thus be obtained in reasonably large quantities [ 151. Various experiments to determine the structure of the new carbon molecule strongly supported the soccer ball hypothesis. It was this ability to generate fullerenes in gramme quantities in the laboratory using a relatively simple apparatus, combined with the fascinating structure and properties that has given rise to the burst of research activity on these molecules (generically known as the fullerenes) and has caused a veritable renaissance in the study of carbon in its various forms. As if the excitement of closed cage molecules was not enough, Iijima in 1991 [ 161 observed helical nanotubules of graphite deposited on the negative electrode during the DC arcing of graphite for the preparation of fullerenes. These nanotubes are concentric graphitic cylinders closed at either end due to the presence of five-membered rings. A single nanotube could be visualised by cutting CT0 along the centre and spacing apart the corranulene end-caps by a cylinder of graphite of the same diameter. Concentric spherical shells of graphite forming onion-like structures were observed by Ugarte in 1992, when samples of carbon nanotubes (plus small graphitic particles) were simultaneously heated and imaged by the electron beam in a microscope [ 171. These structures are similar to hyperfullerenes or the Russian doll structures proposed earlier [ 181. Kroto et al, [ 191 have reviewed

212

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Laser

1 OAtm He Pulse

Rotating graphite Disc

Fig. 4. Schematic diagram of the apparatus used for the laser-ablation

of graphite in the original experiment

of Kroto et al.

1151. some of the early work. A survey of the discovery of the fullerenes is available in a special issue of the Philosophical Transactions of the Royal Society [ 201. Various reviews on fullerenes and related carbon materials have appeared in the recent literature, including special issues of the Accounts of Chemical Research [ 211, Indian Journal of Chemistry [22], Journal of Physics and Chemistry of Solids [23,24], Carbon [25], and Solid State Physics [ 261. Reviews by Hebard [ 271 and by Ebbessen [ 281 have appeared in the Annual Reuiew of Materials Science. Some of the important early papers on fullerenes and nanotubes have been compiled in the form of a book by Stephens [ 291. The November 1994 issue of the MRS BuZZetin [ 301 provides a general review of the subject for nonspecialists. 1.2. Scope of the article The present article is devoted to the structure and solid- state properties of fullerenes, nanotubes and related carbon structures, with brief reference to materials applications where possible. We shall not cover the chemical reactivity of the fullerenes or the large number of compounds formed by them by reaction in solution. In the following section (Section 2) we deal with the preparation and characterisation of fullerenes followed by an extensive discussion of the structure, phase transitions, superconducting and ferromagnetic properties of doped fullerenes, thin-films of fullerenes and possible materials applications in Section 3. Section 3 also refers to solid materials derived from C6,, and organic solvents/compounds. Section 4 deals with carbon nanotubes and Section 5 with carbon onions. In Section 6 we examine fullerenes and nanotubes within the context of the various carbon materials and present some speculative structures. In the final section (Section 7), we present some concluding remarks along with a tabulation of typical patent literature.

2. Fullerenes 2.1. Preparation 2.1 .l. Laser ablation The first observation of a 720 amu peak due to Cho in the mass spectrum was from analysis of the carbon plume produced by laser ablation of a rotating graphite disc using a Nd:YAG laser operating (after frequency doubling) at 532 nm. The plume of carbon was expanded in a burst of He gas (at 10 atm) before the mass analyses in a time-of-flight spectrometer [ 81. More recently, it has been found that heating the graphite disc to ca. 1000 “C results in high yields of CeO upon similar laser ablation

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

[ 32,331. The schematic diagram of the Smalley cluster apparatus used in the original paper is shown in Fig. 4. The laser ablation technique, despite yielding only small amounts of fullerenes, is still important because if one includes the later modification of being able to heat the graphite disc, it allows for considerable control of fullerene distribution and the production of speciality fullerenes. Thus boron can be doped into the CsO cage and the resulting species titrated in the vapour phase with ammonia [ 331. Castleman and coworkers [34] have made extensive use of the laser ablation combined with mass spectrometry to study a series of metal-carbon cages, dubbed metcars for metallo- carbohedranes. Chen and Lieber [ 351 have used laser ablation to produce 13C& efficiently. Lieber and Chen [ 361 point out that systematic studies exploring the various parameters involved in the production of the fullerenes, such as pressure and nature of the buffer gas, laser power, plasma characteristics, etc., are yet to be carried out. 2. I .2. Arc-evaporation This is undoubtedly the best established technique for the production of carbon soot and carbon coatings in the laboratory, being particularly useful for the production of carbon-coated copper grids for transmission electron microscopy. For the production of fullerenes by this method, an arc is struck between two graphite electrodes separated by l-10 mm in 100-200 Torr of helium. Typically, a current of 100-200 A across a potential drop of lo-20 V results in copious quantities of soot being produced. This soot might contain as much as lo%-15% of soluble fullerenes; in order to optimise this production, efficient cooling of the soot is required. For this purpose, the arc may be surrounded by a water-cooled copper jacket on which the soot is deposited. Alternately, the vacuum chamber itself may be cooled. The soluble portions in the soot are extracted continuously in toluene (preferred over benzene because of its lower toxicity) using a Soxhlet apparatus. The material obtained upon removing the toluene solvent is ca. 80% C& and 15% Co, the rest being higher fullerenes such as C& C,*, etc. Purification of this material is performed by chromatography on alumina columns using light petroleum as the mobile phase [ 37-391. Kratschmer et al. [ 151 utilised the technique of directly subliming fullerenes from this solid material on to glass or other substrates. However, this does not result in pure fullerenes. Fullerene soot has also been made by RF sputtering [ 401 and by inductive heating of graphite in an inert atmosphere [ 4 11. Patents for the production of fullerenes by carrying carbon dust suspended in argon through a hot plasma [ 421 or by heating organics (e.g. CS2) in a hot plasma [ 431 have been filed. Fullerenes are also known to form in sooting flames, albeit in very small amounts. While the arc-evaporation procedure seems to be the method of choice for the production of soot, the chromatographic separation of the fullerenes from the toluene extract has many variants. Apart from chromatography on alumina mentioned earlier, fullerenes can be separated on charcoal-silica mixtures using toluene as the mobile phase [ 44,451. This is extremely quick and convenient, particularly for producing large quantities of C6,,. Gel-permeation chromatography [ 461 and fractional crystallisation [47] have also been employed for C,, separation. Cb,, easily forms a complex with AlCl, and this can be used to separate it from higher fullerenes [ 481. Likewise, calixarenes have been employed to preferentially remove CeO from solution 149,501. Fullerenes have been separated by chromatography over powdered graphite [ 5 11. Special chromatographic HPLC columns for ChOand its derivatives have also been developed [ 521. For obtaining higher fullerenes CT6, C,* etc., it is necessary to carry out High Performance Liquid Chromatography using Silica columns with nonpolar solvents [ 531, as well as reverse phase columns (hydrocarbons bound to silica) with polar solvents [54,55]. Many chemical companies list CsO in their stock, including Aldrich, Strem, and MER Corporation, Tuscan, Arizona. Methods for the preparation and extraction of fullerenes have been exhaustively reviewed by Lamb and Huffman [ 561. Reference [ 541 provides useful information of

213

214

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

ARCING GRAPHITE RODS IN He

EXTRACTION OF SOLUBLE MATERIAL FROM SOOT INTO TOLUENE

CHROMATOGRAPHY TOLUENE EXTRACT CHARCOAL-SILICA

OF THE OVER

Fig. 5. Schematic diagram of the steps involved in the preparation of fullerene soot, extraction of the fullerenes from the soot, and separation over a charcoal-silica column.

the separation of higher fullerenes. Fig. 5 shows a schematic of the steps involved in the laboratory production of fullerenes. 2.2. Characterisation offillerenes The initial assignment of the structure of CeOwas speculative, being based on the stability of the molecule as inferred from its relative abundance in the vapour phase. The cage structure received considerable support from gas-phase experiments where metal atoms encapsulated within the cage could be retained even after the ejection of successive Cz units in the mass spectrometer [ 57,581. Such experiments are termed shrink-wrapping. Thus M-&, gave M-C 58, etc. where the -refers to the metal atom being within the cage. The most important experimental results on vapour-phase properties of CeObetween 1985 and 1990 included the UV-photoelectron spectrum of CsO- showing the low-energy LUMO consistent with a species with a closed electronic shell [59] and the determination of the ionisation energy of ChOby Zimmerman and coworkers [ 60,611 using the technique of charge-transfer bracketing. While experiments were restricted to the gas phase, considerable theoretical progress was made in this period and, as we shall see shortly, this paved the way for the easy characterisation after the Kratschmer-Huffman discovery. This included calculations of the vibrational spectra [ 62,631 for IR and Raman modes. Electronic structure calculations had shown that C6,, would be expected to have an intense absorption in the UV [ 641. Thus, Kratschmer et al. could easily infer that the optical as well as vibrational properties of their soot was actually due to ChO. Some important early work includes that of Taylor et al. [65] and by Ajie et al. [38], who obtained pure CsO and CT0 by means of column chromatography. As would be expected for the icosahedral structure, the NMR spectrum of C& showed only a single line for Ceo and five lines for

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 6. Structures of CeO and C70. The C,O structure can be obtained by cutting the CW structure across the middle and adding five six-membered rings. The distinct atoms in the CT0 structure are indicated a, b, c, d, e, with respective abundances of 10, 10,20,20, 10.

Co. The single NMR line in CeO is a testament to its very high symmetry. Even when aligned in a nematic liquid crystalline host, a highly anisotropic medium, this single line does not split whereas the signal due to the protons of tetramethylsilane does [ 391 (see Fig. 6 for distinct atoms in ChOand C,,,) . CT0 has the shape of a rugby ball in that it has a major and minor axis. Two-dimensional NMR has been used to obtain the bond lengths of CeO [ 661. Raman and IR spectra of both Ceo and CT0 were also in agreement with the predictions based on the cage structures [ 671. Perhaps the most satisfying of the early characterisation techniques was the STM observation of the spherical molecules deposited on a gold substrate which showed up small ball-shaped molecules [ 68-701. Attempts to directly determine the structure of the CeOmolecule from single-crystal X-ray diffraction data were unsuccessful because of dynamic orientational disorder associated with the rapid motions of the individual molecules in the solid state at room temperature. This problem was overcome by Hawkins et al. [ 711 who broke the spherical symmetry of the Ceo molecule by osmylation and obtained the crystal structure of C,,0s04 (t-butylpyridine) 2. The molecular structure of this complex is shown in Fig. 7. This was soon followed by the determination of the structure of a platinum complex of ChOby Fagan et al. [ 721. Gas phaseelectron diffraction has also been used to obtain very accurate bond lengths for ChO [ 731.

Fig. 7. Molecular structure (ORTEP diagram)

of the C,-0~0,

(t-butylpyridine)

complex.

215

216

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Table 1 Structures of Cm and CT0

Symmetry

C 60

C 70

1,

D 5h

FCC a= 14.17 A (300 K)

FCC a= 15.1 A (440 K)

Pentagons

12

12

Hexagons

20

25

Bond lengths

1.44 A (6-5) 1.40 A (6-6)

eight kinds of bonds in the range 1.37-l .46 A

High temperature

structure

The molecular structure of CT0 has been confirmed by NMR [ 371 and electron diffraction [ 741. The problem of molecular reorientation has plagued attempts to obtain accurate crystal structure information on Co. Balch et al. [ 751 have obtained the crystal structure data of a iridium derivative of Go, which does not suffer from the problem of orientational disorder. Table 1 summarises the structural features of CeOand C70.

3. Solid state properties of fullerenes 3. I. Structure and phase transitions of C,, 3. I. 1. Results of experiments Symmetrical molecules tend to show some degree of orientational disorder in the solid state [ 761, and the fullerenes CeO and CT0 are no exception. The intermolecular forces in solid CeOare essentially van der Waals in nature and, being spherical, the rotational energy barriers are small. These factors contribute to the molecules being able to rapidly revolve around their centres of mass at room temperature. Another interesting feature in the organisation of C& molecules in the solid lattice is that the molecular point group ( Ih) is incompatible with any space group. The structure and phase transitions of Cso associated with orientational ordering have been reviewed adequately in the literature [ 77-821. The first attempt to characterise the structure of solid ChOwas by Kratschmer et al. [ 151, who carried out powder X-ray diffraction and electron diffraction studies on solid &,. They showed that the structure could be explained as being comprised of 10 A spheres packed in a hcp lattice, which displayed the sort of disorder that one sees in elemental cobalt, i.e. a preponderance of fee-like stacking faults, which they ascribed to residual solvent and possible contamination from higher fullerenes. These authors made no attempt to resolve the atomic details in their relatively low-resolution study. In their X-ray study of sublimed single crystals, Fleming et al. [ 831 sought to overcome the problem of frustration of the Ih point group by assuming that the molecules formed merohedral twins, wherein pairs of CeO molecules are related to each other via a mirror symmetry (shown in Fig.8). This assumption implies that the average density probed by X-ray diffraction would be a CrZOmolecule at every fee lattice site, with half-occupancy at every atom. Their structural model required large thermal parameters in order to obtain reasonable fits to the data. They obtained a 14.2 A fee unit cell for &,. Heiney et al. [ 841 took the alternate view that the molecules were dynamically disordered at room temperature, and that the density probed by X-rays was in fact that of a shell of 3.5A radius, whose structure factor could be modelled by a zeroth-order Bessel function. In this, they were encouraged by the NMR studies of Johnson et al. [ 851 and of Tycko et al. [ 861 who showed that the room temperature powder NMR spectrum of solid CGOwas a sharp single peak.

217

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 8. Two neighbouring two molecules.

CeOmolecules showing merohedral twinning. The view is down the axis joining the centres of the

Fig. 9 shows the NMR spectrum of Cc0 as a function of temperature. NMR spectra of solids normally show the effects of dipolar broadening. For a sample such as solid (I&, the concentration of 13C is too small for dipolar broadening to be significant and instead, one would expect the dominant broadening to be due to chemical shift anisotropy. Whatever be the mechanism for broadening, it is usually necessary to rapidly spin the NMR sample tubes at the magic angle in order to obtain high resolution data on solids. The observation of a sharp resonance in the powder pattern (without spinning the sample) implied molecular rotation at GHz timescales was taking place. With this assumption,

i

A

193K

153k.J

-.P-E p---q, 0

200 wm

Fig. 9. NMR spectra of solid C, at different temperatures showing orientational motion at near-ambient structure due to chemical shift anisotropy (CSA) at low temperatures (adapted from Tycko et al. [ 861)

temperatures

and

218

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

14.16

(a)

t

0

8 w- 0.25

0

%

L

240

250 260 Temperature(K)

270

Fig. 10. (a) CT,,,lattice parameter and (b) the fee fraction as a function of temperature transition (from Heiney [ 771) .

across the orientational

ordering

Heiney et al. [ 841 fitted the room temperature X-ray data to the Fm3m space group with a unit cell of 14.2 A. Differential scanning calorimetry showed that cooling the sample below 250 resulted in a sharp phase transition, which they assigned as being due to orientational ordering. Fig. 10 shows the lattice parameter of ChOand the DSC trace across the phase transition. The structure of C,, below this transition is simple cubic, fitting into the Pa3 space group [ 871. It must be noted that the molecules retain fee packing even below the ordering transition. The simple cubic space group comes about because orientational ordering results in the four molecules in the fee unit cell becoming nonequivalent. An accurate neutron structure determination was performed by David et al. [ 88,891 who confirmed the simple cubic space group at low temperatures. At 5 K when the orientational motion is frozen, the molecules pack very compactly. There are two ways of viewing this compact packing. One is that the inter-C,, separation is ca. 2.95 A at room temperature, considerably shorter than typical nonbonded carbon+arbon distances. To accommodate this short distance, the electron-rich 6-6 double bonds of one C& molecule occur adjacent to the relatively electron-deficient five-membered rings of the neighbour. The other view is that this packing is chosen because of the differences in electron density at these sites. The views are actually not in conflict and the construction of intermolecular potentials seems to require a charge-transfer term. Fig. 11 shows this packing geometry between two adjacent CsO molecules. The structure of solid CeOhas been studied exhaustively by single-crystal X-ray diffraction. Liu et al. [ 901 obtained the crystal structure of a twin at 140 K. This structure was further refined by the group of Burgi [ 9 11. Chow et al. [ 921 performed a synchrotron X-ray study on a single crystal of Ccc, at room temperature. Their aim was to find the effect of the reduced symmetry of the fee solid lattice on the electron density. They found that the charge density was not totally spherical even at room temperature. They obtained a barrier to rotation of ca. 600 K but pointed out that the height of this barrier was different in different directions.

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 11. Diagram showing the packing arrangement between adjacent Cc0 molecules at low temperatures. The 6-6 double bonds of one C,, molecule (light lines) sit adjacent to the five membered rings of the neighbouring C,, molecule (thick lines).

The phase transition in CeO has been investigated by a variety of techniques. Independent of the report by Heiney et al. [ 841, Dworkin et al. [ 931 and Tse et al. [ 941 have reported the orientational ordering phase transition by differential scanning calorimetry (DSC) . The large enthalpy change of 6.98 kJ mol- ’ suggested the transition to be first order. Heiney et al. [ 951 and David et al. [ 891 have found that the unit cell volume changes discontinuously across the phase transition, signifying a firstorder phase transition. The symmetries between the phases above and below the phase transition are also not related continuously, as would be necessary according to the Landau criteria for a secondorder phase transition. The phase transition has also been studied using electron microscopy by van Tendeloo et al. [ 961 who find evidence for a 2% supercell at low temperatures. Raman measurements show that below the phase transition, the phonon frequencies harden [ 97,981. A striking hysteresis in the Raman studies of the orientational ordering phase transition in CeO films has been found by Akers et al. [99], which has been ascribed to photochemical changes induced by laser irradiation. IR spectroscopy [ 100-l 021 provides similar information. Intermolecular phonons in the Raman spectrum are not easily observed in the fullerene solids, and have not been unambiguously assigned. Recent far IR measurements have provided information on these modes [ 103,104]. Sound velocity measurements [ 1051 and dilatometry [ 1061 have also been used to characterise the phase transition. It should be noted that most of these techniques have a characteristic timescale so that the data can be used to complement each other. NMR spectroscopy permits a study of the dynamics of the transition of CeO. Thus orientational correlation times can be obtained from the spin-spin relaxation time T,. Assuming thermal activation for the orientational correlation time 7, one can plot ln( T) as a function of the reciprocal temperature. Such plots show biexponential behaviour corresponding to two activation energies of ca. 300 and 3000 K above and below the orientational ordering transition at 260 K, respectively [ 85,86,107-1091 (as shown in Fig. 12). Similar behaviour is also seen in the zero-field muon spin resonance spectra [ 1 lo]. The motion below the orientational ordering transition is activated, as mentioned, with the molecules jumping between preferred orientations. This has been referred to as ratchetting. Molecular motion on NMR timescales freezes only at ca. 100 K in C6,, [ 1111.

219

220

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

lOOO/T(K-‘) Fig. 12. Temperature dependence of the orientational correlation time of solid CC0from NMR studies showing biexponential behaviour (from Heiney and Fischer [ 791) .

Since neutrons can probe the structure (via diffraction) as well as the dynamics (via diffuse and inelastic scattering), they provide the most powerful probes for studies on systems such as CsO. The technique of quasi-elastic neutron scattering allows the vibrational energetics of solids to be resolved in a spatial manner that is unmatched by other techniques. The other advantage of neutrons is that there is no selection rule for scattering by phonons so that the entire phonon structure can be mapped. Neutrons have been used to determine the amount of hydrogen impurities in ChO samples, using a technique called prompt y-ray neutron activation analysis; studies that are crucial, since the structures of solid fullerenes are known to be very sensitive to impurities. David et al. [ 88,891 have performed a complete crystal structure study on solid CeO. At 5 K the structure refines into a simple cubic Pa3 space group as mentioned earlier. There is a rotational angle between nearest-neighbour Cc0 molecules of ca. 22”and 22” + 60”. The molecules are distributed between these two orientations with a probability p of 0.835 in favour of the 22 orientation. This probability is independent of temperature till ca. 90 K, which is the glass transition temperature. The value of p decreases above this temperature reaching p= 0.61 at the orientational ordering transition temperature of 260 K. Whereas the orientational ordering transition shows up in X-ray as well as neutron studies as a jump in the lattice parameter, the glass transition, below which the molecules are frozen into their respective orientations, is seen in high resolution neutron studies as a gentle change of slope of the lattice parameter and a concomitant decrease in the coefficient of thermal expansion as one traverses the transition from above. The nature of this transition is of interest and we return to it later. Hu et al. [ 1121, confirmed the earlier results albeit with a different approach of fitting the neutron data to orientations and structure in real space. Neutrons have been used to directly probe the temperature-dependent dynamics of ChO [ 1131 151. In order to probe inelastic scattering due to the orientational motion, data at high Q (approaching 6 nm- ’ ) are required. Above 260 K the high Q inelastic scattering data shows a single broad Lorentzian centred at zero energy transfer, characteristic of orientational diffusion. Below this temperature, peaks corresponding to non-zero energy transfer develop. At 115 K these peaks are centred at 2.5 meV and correspond to librational energies of the low temperature phase (Fig. 13). It is interesting that as the temperature is raised to 260 K, the librational amplitudes become approximately 7% - roughly a third of the near-neighbour interatomic angles. This could be construed as some sort of a Lindemann criterion for orientational melting. The phonon structure of solid ChOhas also been extensively probed using neutrons. Cappelletti et al. [ 1161 made the first measurements of energy transfer between 25 and 215 meV (1 meV= 8 cm-‘, so these correspond to mostly intramolecular phonon modes, these being stronger than intermolecular modes by almost an order of magnitude). They obtained phonon spectra which corresponded well with the known Raman, IR and HREELS data. In subsequent measurements using a spallation source, Coulombeau et al. [ 1171 and Prassides et al. [ 1181 have confirmed

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

T=260K

T=115K

l&A&l

- 5.0

2.5

0.0 2.5 Energy (meV)

5.0

Fig. 13. Inelastic neutron scattering about the Q = 5.65 A- ’ Bragg peak above and below the orientational ordering transition showing the development of librational modes of the Cm molecules as the temperature is lowered (adapted from Neumann et al. [Sl]).

the earlier results and obtained improved resolution. The intermolecular modes are considerably more difficult to obtain, but this has been managed on a 6 mm3 single crystal by Pintschovius et al. [ 1191 who obtain a phonon structure for the room-temperature fee phase which is reminiscent of that of xenon. Solid Cso is thus like a pseudo noble gas. Below the orientational ordering transition, the structure is much more complicated [ 1191. In an interesting gas-phase experiment, mass spectra of (C,,), clusters have been studied by Martin et al. [ 1201. They find the magic numbers corresponding to Mackay icosahedra to predominate the mass spectra (y1= 13, 55, etc.).

3.1.2. Modelling and simulation Early studies on the compressibility of solid C,, [ 121,122] indicated that bulk C,, had a compressibility similar to that of graphite along the c-axis. This is to be expected since ChOhas a closed electronic shell and the interactions between molecules is largely van der Waals in nature. The van der Waals nature of the interaction receives further support from the fact that the photoionisation spectrum of C& in the gas phase as well as of thin films are almost identical [ 1231. Wang et al. [ 1241 modelled CeO as shells interacting with their nearest neighbours via an intermolecular Morse potential and obtained phonon frequencies that were in excess of the measured values by ca. 30%. Treating C,, as a sphere has the disadvantage that orientational details are lost. Girifalco [ 1251 showed that using an atom based Lennard-Jones potential could lead to better models at the price of considerably greater computational expense. Thus each ChOmolecule is treated as a rigid 60 atom unit and the interaction between the atoms of one C& molecule and the atoms on the rest are of the 12-6 form. Cheng and Klein [ 1261 found that molecular dynamics (MD) simulations of such C& molecules does result in an orientational ordering transition but the predicted low- temperature structures [ 127,128] have lower symmetry than the experimental structures, ie. the Pa, space group is not reproduced in these simulations. The predicted frequencies of the librational modes are also usually underestimated by simple LJ potentials. The acoustic modes due to molecular translations are usually in the right range.

221

222

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

The temperature at which the orientational ordering transition occurs as well as the correct space group of the low-temperature phase can be obtained from Molecular Dynamics simulation of LennardJones CsO provided extra Coulombic terms are included. The stratagem used by Sprik et al. [ 1291 in their MD simulations is to place a charge of - q on the double bonds and q/2 on the atoms (q = 0.35 e) . In their mean field approach Lu et al. [ 1301 place a charge of q on the centres of the single bond and - 2 q on the centres (q = 0.27 e) of the double bonds. The placing of charges results in the correct low-temperature orientation as obtained from neutron data of the electron-rich 6-6 bond being wedged into the relatively electron-poor five-membered ring. Neither of these approaches gives a sufficiently stiff librational mode. Recently, split partial charges have been proposed for the electron-rich double bond in order to explain low-temperature neutron scattering results [ 13 1] . 3.1.3. Orientationally glassy state of C,, Molecular reorientation persists in the simple-cubic phase below 260 K. The orientational correlation time however, gradually decreases until some temperature Tg is reached, below which, on the time scale of the particular experiment, the orientations are completely frozen. In the neutron diffraction study of David et al. [ 891, this transition shows up as a change in slope at 85 K in the plot of lattice parameter vs. temperature. Such freezing is also seen in NMR, dilatometry and ultrasound attenuation measurements. Monte-Carlo studies show that quenching fee-ChO to below 80 K results in a transition [ 1321. Michel [ 1331 has pointed out that the glass transition may actually be a sequence of orientational ordering transitions with the successive orientational modes freezing out one after the other. Structurally, the glass transition is characterised by the orientational disorder, the fraction of major and minor orientations being temperature independent below Tg. The height of the barrier separating the ground state orientation and the defect orientation fits the Arrhenius form r= T,, exp(BIT) over a range of time scales. Most glass formers display a Vogel-Fulcher temperature dependence of the form T= r0 exp[ Bl ( T- To) ] where T is the relaxation time and B and To are constants [ 1341, rather than an Arrhenius form. This suggests that C,, fits into the classification of strong glass formers. 3.2. Phase transitions of CT0 CT0 is an elongated molecule with a major and minor axis. Based on our knowledge of the phase diagram of C6,,, we would expect two phase transitions corresponding to rotations around the longer and shorter axes being restricted one after the other as the temperature is lowered. Extensive Molecular Dynamics simulations by Sprik et al. [ 1351 using a combined LJ and partial charge potential do suggest such a view. The predicted high temperature structure is fee. Cooling results in structures of lower symmetry, going through a rhombohedral structure to a monoclinic ground state. Unfortunately, the experimental situation is rather complicated, partly due to the difficulty in producing good quality samples. At ambient temperatures and pressures, there seem to be at least two phases of nearly equal energies. This results in crystals being twinned and faulted. Also, solid C& like solid C&,, has a great propensity to absorb solvent molecules and these have a significant effect on the structure. The first powder diffraction study of solid CT0 was by Vaughan et al. [ 1361, who determined the 440 K structure to be fee. This is possible for elongated molecules only if they are completely orientationally disordered, so that the average molecular density corresponds to a sphere. The molecules do however, spin around the long and short axes at different rates. These authors also reported two phase transitions by DSC at 276 and 337 K, but could not establish the low-temperature ordered structures. Verheijen et al. [ 1371 attempted to grow single crystals for an X-ray study but found that on subliming solid C,, one obtains crystals with both fee and hcp morphologies. The fee crystals they obtained were extremely faulted and unsuitable for diffraction. They tentatively propose a rather

C.N.R. Rao et al. / Fullerenes, nanotubes. onions and related carbon structures

Temperature

.

1568

(K)

__a

1566 1564

0

1

1 100

I

I 200

300

400

Temperature(K) Fig. 14. Selected (a) IR and (b) Raman

modes of CT0 across the orientationalordering phase transitions.

complex phase diagram involving both shearing and displacive transitions. The phase transition in C,,, thin films has been studied using IR [ 1381 and Raman [ 139,140] spectroscopies. The phase transitions can be followed by monitoring the intramolecular phonon modes. There are changes in slope of the linewidth and intensity of the peaks which are suggestive of phase transitions (Fig. 14). The phonon frequency hardens perceptibly only below the transition around 280 K. In fact, from their neutron diffraction study, Christides et al. [ 1411 suggest that the only first-order phase transition accompanied by a change in cell volume is the one at around 280 K. DSC studies show that there may be as many as three phase transitions in CT,, [ 142,143]. This is supported by the resistance studies under pressure by Ramasesha et al. [ 1441, who find that pressure delineates these phase transitions. The zero pressure intercepts of the phase transition temperatures are respectively at 280, 330 and 340 K. The thermal history of the sample is known to influence the phase transitions rather dramatically, and in fact thermal cycling can lead to fracture of crystals [ 1451. NMR studies [ 1091 indicate that around 340 K, the rotation becomes restricted to the five-fold long axis. Orientational freezing takes place on NMR time scales only at 130 K. Orientational dynamics in solid CT0 have also been followed using zero-field muon spin resonance [ 1461. Other techniques that have been used include electron diffraction [ 1471 and thermal expansion measurements [ 1481. 3.3. Pressure efsects

on C,, and C,*

Pressure has considerable effect on the structural phase diagram of Ceo and CT,, as expected of these relatively soft solids. DSC measurements of Samara et al. [ 1491 on Ceo under pressure showed that the transition temperature had a pressure dependence of about 10 K kbar- ‘. Thus even at pressures as low as 10 kbar, the room-temperature structure is orientationally ordered and the solid crystallises

223

224

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

1400

1500 Raman shif

1600 t(ct-2)

Fig. 15. Raman spectrum (pentagonal

pinch mode) of C, under pressure showing the formation of a glassy state.

in a simple cubic space group. The measurements of Ramasesha et al. [ 1441 show that even in the case of C&, pressure results in two rather than a single phase transition. This is partly supported by the DSC measurements of Samara et al. [ 1491 who find that at higher pressures, the DSC trace develops a shoulder. Raman investigations on Cho single crystals under pressure show that the pentagonal pinch mode undergoes considerable softening around 3.5 kbar [ 1501. At higher pressures the linewidth increases until at ca. it merges into the background, indicating the formation of an orientational glass possibly related to that found at lower temperatures (Fig. 15). Under pressure, CT0 clearly shows the occurrence of three phase transitions as seen from electrical resistivity measurements of Ramasesha et al. [ 1441 (Fig. 16). It appears that the application of pressure delineates like phases of imilar energies in both Ceo and C70, giving rise to two and three orientational phase transitions, respectively. X-Ray diffraction studies [ 1411 under pressure have been carried out at room-temperature showing the transition to a rhombohedral phase. According to the phase diagram of Ramasesha et al. [ 1441 which has the advantage of probing lower pressure regions than the X-ray studies, at room temperature only one of the phase transitions would be traversed along the pressure axis. Kawamura et al. [ 15 1,152] have studied CT0 as a function of both temperature and pressure, the lowest pressure studied being 0.5 GPa. Their studies show that &, exists in the rhombohedral form at low temperatures and high pressures. At high temperatures, the fee structure is preferred. 3.4. Amorphization

and conversion to diamond

Early X-ray diffraction studies showed that Cso transforms to a lower symmetry structure at pressures of around 20 GPa under nonhydrostatic compression [ 1211. Raman [ 153,154] and other

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

260’

I 0.2

0

I I I 0.4 0.6 0.8 Pressure (GPa)

I 1.0

225

12

Fig. 16 The p-T phase diagram of C,, from resistance measurements. be unambiguously determined.

The structures within the phase boundaries

are yet to

studies show that CeOforms amorphous phases at pressures higher than 22 GPa. The amorphous phases show evidence for sp3 carbons and are considered to result from chemical reactions of the Diels-Alder type between the ChOmolecules. Above a pressure of about 25 GPa, the changes in the ChOsolid are irreversible. Fig. 17 shows the results of a Raman study of CsO under pressure. C,,, is interesting in that Raman studies show that at around 12 GPa, only a single broad peak due to sp2 carbons is seen [ 1551. This is a signature of an amorphous phase. What is noteworthy is that decreasing the pressure from 30 GPa to atmospheric pressure results in the complete recovery of the original spectrum, suggesting reversible amorphization (Fig. 18). One possible reason for the distinct behaviour of C,O under pressure could be that the molecule is elongated and more easily pinched around the central waist. The exact nature of the distortion requires detailed structural as well as simulation studies. It has been suggested [ 1561 from the Murnaghan equation of state for ChO and Co, that at the same pressures, the C6,, molecules approach each other much closer than do Co. The Diels-Alder transition

(4

1400’

0

I 5

I 10

lrreversi Reversible t

Pressure

I 15

I 20

(GPa)

ble --c

I 25

J

30

500

900 Raman

1300 Shift

170(

(cm’)

Fig. 17. (a) Raman phonon frequencies of C,, as a function of pressure showing the regions where the changes are reversible or irreversible. (b) Raman spectrum of a sample recovered from 27 GPa (from Yoo and Nellis [ 1541) .

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

226

15.3

:.

N<.._

..‘-..A

7

. . .; :y :”

8.8

10x

.

.I. . .?:L?‘)_ L.

::

*

.:’ j-> ::.. .. . :. a.

d

1”

i

.:

.:

.

.. .

.

.

+;_;y

.,.

. 9..

.I

..

0.1

.

.y:t:._.:!:. ... 2

FL,, ,, 0 (Recovered

......

1700

19001500 Raman

shift

‘...

( , P(GPa, ,,,,,,

1700

1900

(cm-‘)

Fig. 18. Raman modes of a CT0single crystal under (a) increasingand (b) decreasingpressure showing the formation of a reversible amorphousphase. state has a carbon<arbon separation of ca. 2.1 A, and any approach which is smaller than this is likely to result in bonding which is irreversible. We have examined the effect of pressure on lattices of rigid CeO and C,O molecules interacting via a Lennard-Jones potential using NPT and NVT Monte-Carlo techniques. We find, in support of the compressibility data that even at pressures as high as 30 GPa, the shortest interatomic distances in CYoare of the order of 2 A, so a Diels-Alder reaction is not favoured. Such interatomic distances are reached at lower pressures for CsO. Thus the fact that the amorphization of CeObeing irreversible while the amorphization of C,O is reversible is clearly due to better packing of the latter. The reason for the amorphization itself can only be speculated at this stage. Interestingly, the centre-of-mass-centre-ofmass-pair-distribution functions remain crystalline throughout for both C6,, and C,,,. More work is required, involving possibly the simulation of nonrigid molecules, before the nature of the phase changes are understood. There is considerable evidence that &,, under conditions of nonhydrostatic pressure can be converted to diamond [ 157,158]. The conversion to graphite has also been studied. The topological relation of C6,, to diamond and graphite has been discussed in Ref. [ 1591. Recently, Roy and coworkers [ 1601 have reported the conversion of graphitic carbon into diamond under conditions of low pressures and temperatures , The use of fullerenes as a carbon source in solid-state conversions by low-pressure, low-temperature routes to diamond is being explored in this laboratory. The fullerenes have been used as nucleation centres for the growth of diamond films. This will be covered at a later point in this review.

3.5. Pressure induced closing of the PL bandgap in Cd0 High pressure studies [ 1611 on single-crystals of CeO show that with increasing pressure, the photoluminiscence band, initially centred around 1.6 eV, is gradually red-shifted till, at a pressure of

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

11.5

12.5 Wavenumber

13.5

14.5

(103cm-‘)

Fig. 19. PL band of a C,,, single crystal under pressure. 3.2 GPa, the band merges into the background (Fig. 19). The crystal is observed between the diamond anvils to turn from red to black at around the same pressure. Since at such low pressures, the C& molecule shows very little structural distortion, the closing of the PL gap can be interpreted as arising from the broadening and overlap of the HOMO and LUMO of the molecules. Such broadening would be expected because the decreased interball distance would increase the interball hopping integral. Electrical resistivity measurements have shown that the activation energy for conduction decreases markedly with increase in pressure, becoming negligible at high pressures. These studies have definite implications for the strength of electron-phonon coupling in these systems and therefore, for superconductivity and other low-temperature ordering phenomena in doped fullerene phases.

3.6. Superconductivity

in thefullerides

There are principally two factors which are responsible for electronic structure of the fullerenes being different from that of graphite. The first is the curvature, which serves to increase the 7r orbital density on the convex side of the molecule. If a band is derived from this level, the curvature would result in raising its level vis-a-vis the equivalent graphitic (zero-curvature) level. The other factor is the presence of five-membered rings in the fullerene cage which destroys the HOMO-LUMO symmetry. Thus, unlike polyacetylene or graphite, solid fullerene can only be doped by electron donors and not by electron acceptors. The five-membered rings on the cage behave like cyclopentadiene units, resulting in the fullerenes having high electron affinity (see Fig. 20 for a hand waving explanation of the high electron affinity). In the molecule, the highest occupied molecular orbital is the five-fold degenerate h, level. The lowest unoccupied levels are respectively the ti, and T,, levels. In the solid state these HOMO and LUMO give rise to the valence and conduction bands separated by a 1.7 eV band gap (Fig. 21) . The fee structure of ChOhas large octahedral and tetrahedral voids that can easily accommodate dopant atoms. Added to this is the soft, van der Waals nature of the C60-C6,, cohesion, making solid ChOa good host for the intercalation of alkali-metal atoms. In fact C6,, could be described

227

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon swuctures

228

2e-

Fig. 20. CeO molecule emphasising dienyl anions.

a pyracylene unit, showing the stabilisation

of electrons by the formation of cyclopenta-

tNJ flu

Fig. 21. The molecular electronic structure of ChOshowing the Cm HOMO and LUMO (one-electron bands formed thereof in the solid state.

picture) and the energy

as a zero-dimensional host because atoms and small clusters can be intercalated without destroying the fee structure. The maintenance of the original structure/packing allows for referring to the doping as intercalation. Superconductivity has been reviewed in Refs. [ 162-1641 and [ 1831. 3.6.1. Alkali-metal&Derides Early studies on films showed K&,, to be metallic [ 1651. Soon the discovery of superconductivity in this system followed [ 1661. Haddon [ 131 has pointed out there exist few, if any, rules for the design of new superconductors, and rather the stratagem is to avoid other kinds of ordering phenomena (e.g. magnetic) at low temperatures. This is easier for materials which are not low-dimensional. Thus CsO which is cubic in the solid state is an ideal system to work with. The other advantage is that phonons due to carbon atom vibrations are quite strong owing to carbon being a light atom. If electron-phonon coupling were possible in doped CsO phases, it is conceivable that the ground state would be superconducting. have been prepared by a variety of means including reacting Bulk samples of K& purified ChO samples with K vapour [ 1661, refluxing toluene solutions of CeO with potassium metal [ 1671 and using azides as the alkali-metal source [ 1681. Other methods include the use of alkali-

Preparation.

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

metal mercury and thallium alloys [ 169,170] . For example, Rosseinsky et al. [ 1711 have used NaH and NaSHg, to prepare a range of Na$& and NaxACeO solids (A= Cs, Rb). This helps to achieve better control of stoichiometry and the use of lower temperatures. Another strategy is to make the fully doped samples A&& and then react these with the weighed quantities of C&. This is probably the best method for attaining samples of controlled purity and stoichiometry [ 1721. Poirier [ 1731 has described a distillation technique for removing excess alkali metal. He used electron spectroscopy to follow the stoichiometry and could construct a phase diagram. Electron doping studies. Photoemission studies, both normal and inverse, allow the nature of the electron doping to be followed [ 1741. Exposing CeOfilms in the vacuum chamber of a photoelectron spectrometer results in the filling of electrons donated by K into the ChOtl, LUMO. This results in Fermi-level pinning and a shift in the CeOmanifold to higher binding energies (Fig. 22). Between the compositions x = 0 and x = 3 in AXCsO,the material is actually a simple mixture of CsO and A&-,. Likewise, between x = 3 and x = 4, the material exists as a mixture of the A#& and A&,,. The phases A&,, A&, and A6& seem to be line phases. Since the tr, level is threefold degenerate, half-filling correspond to K3&, and this stoichiometry is metallic. A,&,,, however, corresponds to a fully filled tr, LUMO and is insulating. Magnetic measurements on KXCsOsystems shows that the maximum Meissner fraction in the superconducting state corresponds to the composition K,C, [ 1741. Raman spectroscopy [ 1761 has also proved to be a powerful tool in following electron doping across the series A&,,. Thus Raman H, modes are shifted to lower frequency and broadened on electron doping. The extent of electron-phonon coupling can be followed from such shifts in phonon frequency. K3&, has a superconducting transition temperature of 19 K. Rb& has a higher T, of 29 K [ 1771. Doping has also been followed by NMR [ 1781, which confirms that A#& is a line phase and between CsO

x=3

x=6

EF NORMAL PES hv

INVERSE PES

e-

\/

=-\

,“‘

PES PROBES Fig. 22. Schematic diagram of the photoemission DOS of CeOas a function of alkali metal doping. The unoccupied levels are normally probed by inverse photoemission. The experimental geometries are indicated at the bottom of this figure.

229

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

230

40r

0’ 13.8

I’

I

I

I

14.0 14.2 14.4 14.6 Lattice constant a (A)

Fig. 23. Field of T, vs. a-lattice parameter for an entire series of A$& well as alloying experiments.

phases including data from both variable pressure as

and A3C60, no other stable phases exist, at least for A = Rb and K. NMR studies also show that dynamic orientational disorder of the Cm molecules persists at room temperature even in doped phases. Kochanski et al. [ 1791 have followed alkali metal doping by studying the electrical resistance of samples as a function of alkali metal content. They found a dip in the resistance only at x= 3. The p-x curve is not linear but reflects effects due to granularity and percolation thresholds. Alloying at the A site with other alkali metal can give rise to a plethora of phases and a whole family of alkali-metal-doped CGo compositions are now known; most of the fee phases are superconducting. As a general rule, the larger the cation the higher the superconducting transition temperature. In fact, T, seems to depend solely on interball separation. Thus, sodium-doped CeOphases which are otherwise nonsuperconducting, can be made superconducting by further doping with NH, [ 1801. The NH3 molecules solvate the Na+ cations and result in larger effective radii of the cations thereby increasing the C6,,--Cso interball separation. Fig. 23 shows the field of T, vs.a-lattice for the A3Ce0 phases. Structure. Structures of the alkali metal fulleride phases have been studied, mostly by Rietveld refinement of powder X-ray diffraction data (both laboratory and synchrotron) . An A&, phase for A = Na was reported by Rosseinsky et al. [ 1811, with Na in the tetrahedral voids. The A3C60 phases are fee, with ions occupying both the octahedral and tetrahedral sites [ 182,183] . In species such as NazRbCeo, the larger ion occupies the octahedral site. NMR [ 1841 studies have been used to follow the states of nuclei such as Rb. From NMR, it is known that there is a possible structural distortion in Rb3Ceo which makes the tetrahedral sites inequivalent. 13C NMR [ 1861 has also shown that there is considerable orientational disorder in the A3Cso species. The phases A,Cso [ 1871 and A6&, [ 1881 are also known, crystallising respectively in a body-centred tetragonal structure or a body-centred cubic structure. While A6Ce0 is a band insulator with a filled ti, derived band, A4Ce0 is an insulator because the structural distortion (to a tetragonal structure) results in the opening of a gap. Larger amounts of K, Cs or Rb (than x = 6)‘cannot be doped into these phases. Sodium is small enough that as many as 11 Na atoms can be intercalated per C& molecule [ 1891. The sodium atoms form cubic clusters inside the structure. This structure has been studied by both NMR and by X-ray techniques. AC60 is more complicated. These are for the most part, insulating. Ace0 [ 190,191] (eg. RbC&) is an interesting new system with a metal insulator transition at low temperatures, with concomitant structural distortion. This system also displays an unusual phase transition associated with covalent bond formation [ 1911. The structures of some of the A$,, phases is shown in Fig. 24.

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 24. Structures of AXC& showed as slices along the c-axis. The structures can be visualised by stacking the layers. From left to right, the layers are z = 0, l/4, l/2, 3 /4 and 1. Electrical

transport. In situ electrical resistivity studies have been used to obtain tentative phase diagrams and study doping of electrons in the fullerenes in general, as mentioned in the previous sections. The availability of crystalline films and single crystals has allowed more careful measurements of the electrical transport to be performed. Early reports of the resistivity of K$& suggested a value at room temperature of ca. 5-10 rnfl cm [ 179,192]. Xiang et al. [ 1931 have obtained high quality single crystals of K$& and Rb&, by first growing C 6,, crystals from the vapour phase and then doping them with the alkali metal vapour. They found clear metallic behaviour in their samples with little evidence for granularity. The residual resistivity p( 280 K) lp( 20 K) was ca. 2. Crespi et al. [ 1941 have analysed the data of Ref. [ 1931 and fit it to the quadratic form A+BT2. They find that phonon scattering by phonons in the 400 cm-’ range seem to explain most of the transport data. Coupling to higher energy phonons as required by certain mechanisms for superconductivity (see the sections that follow) do not provide as good fits. Xiang et al. [ 1951 find that the excess conductivity just above the T, in K& and Rb& due to fluctuating superconducting domains fits the expression of Aslamazov and Larkin [ 1961, as modified by Maki and Thompson [ 1971 to include effects due to breaking of the superconducting pairs. The data indicate that these are 3-D superconductors. This samples in fact are the first systems to fit the expression for 3-D systems since the right combinations of small N(E,) and high T, allow the fluctuations to be large enough to be measured. Most recently, the same Berkeley group have found that if the resistivity of A$& samples are measured at constant volume, (by suitable scaling) the p-T behaviour is linear.

Magnetism

and tunnelling. The lower and upper critical fields of K+& were first measured by Holczer et al. [ 1991, who carried out measurements to 5 T and extrapolated to obtain a 0 K value of 50 T for the upper critical field. This results in a Ginsberg-Landau coherence length of 25 A. This is rather short. The lower critical field was determined to be 0.013 T, and from these values they obtained a penetration length of 2400 A. From this the picture of an extreme dirty limit superconductor emerges. There is no universal agreement on the precise values of the coherence length and of the penetration depth. However there is general agreement on these superconductors being in the dirty limit. For example, the muon spin rotation data of Uemura et al. [ 2001 agree with this finding. There is an overall

231

232

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Table 2 Properties of AxCeOsuperconductors

a

Composition

L@ce (A)

Na2W&so 4%

14.138(3) 14.132(2) 14.176(9) 14.253( 3) 14.436(2) 14.493(2)

NazCsC,O K3Go

Rb&o Rb,CsC,O

parameter

T, (K)

Meissner fraction (%)

8 10.5 14.0 19.3 29.4 31.3

3 8 9 30 35 48

a Taken from Ref. [ 1831.

consensus that orientational disorder in the samples of the doped fullerenes has a role to play in the superconducting properties [ 2011. Tunnelling studies have played a crucial role in the past in unravelling mechanisms of superconductivity, particularly for the traditional BCS superconductors. Tunnelling results on the doped CeO phases of Lieber and coworkers [ 202,203], who probed the conductance gap from measurements of the I-V data above and below the superconducting transitions, show a gap value 26/k,T, of 5.3 for K& and 5.2 for Rb$&. These are in the strong-coupling limit and are considerably larger than the usual BCS value of 3.5. IR [ 204,205] and NMR [ 206,207] measurements, however, suggest a smaller value for the gap, closer to the usual BCS estimates. Some of the properties of alkali fulleride superconductors are summarised in Table. 2 (data from Ref. [ 1831) . 3.6.2. Alkaline-earthfillerides The alkaline-earth metal fullerides Ca&60 [ 2081, Sr&, and surprisingly, Ba&, [ 2091 are superconducting. In these systems, not only the ti, LIMO but also the ti, are involved. In Ca&,, and BaJ&,, there is multiple occupancy of the fee tetrahedral site of CsOby alkali metal atoms. Particularly in B+ChO, charge transfer from the metal to the fullerene cannot be complete. Calculations show that there is considerable hybridisation between alkaline-earth metal d levels and the ChO n states [ 2101. Again, photoemission studies have played a major role in following the nature of electron doping [ 2121, as have studies of in situ resistivity [ 2131. In the photoemission studies, it is observed that first the threefold degenerate ti, level gets filled and Ca,C,,, shows no density at EF. Further exposure to Ca vapour in the spectrometer results in the ti, level getting filled. Fig. 25 shows a schematic diagram of the photoemission DOS with alkaline-earth metal doping. Ba,CeO adopts the Al5 structure of the traditional workhorses of the superconductor field, viz. NbsSn, NbsGe, etc. This phase is an insulator. These systems are more difficult to prepare than the alkali-metal systems because of the lower vapour pressures of the metals. The azide route [ 1681 seems to be an effective method for their preparation. 3.6.3. Mechanism of superconductivity in fuller-ides The finding that the superconducting transition temperature seems to depend critically on the inter-& separation seems to suggest that the alkali-metal atoms do not play a significant role, apart from doping electrons and stabilising the structure. Also, isotope effects are not seen in *‘Rb doped samples [ 2131. There is also other evidence that seems to point to intramolecular phonons as playing the dominating role in mediating electron pairing. Both Raman [ 1761 and neutron scattering [ 2141 measurements show that the tangential H, modes are strongly coupled to the electrons. 13C isotope effect measurements [ 2151 seem to indicate this. Varma et al. [ 2161 have presented the argument that superconductivity in the doped ChOphases arises as a result of electron-phonon coupling of the BCS

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

EF Fig. 25. Schematic diagram of the photoemission

DOS of Cso as a function of doping by alkaline earth metals.

type, except that the phonons are intramolecular and are of relatively high energy (of the order of 1000 K) . Some authors, however, are of the view that in the fullerenes the value of the Hubbard U parameter, as determined from combinations of normal and inverse photoemission and Auger electron spectroscopy [ 2171, is too large to be ignored in mechanisms for the superconductivity in these systems. This would mean that correlation effects are important. Accurate measures of certain critical parameters such as the magnitude of the superconducting gap and the density of states are still to be obtained. It has also yet to be established that the alkaline earth metal doped phases have similar mechanisms for superconductivity as the alkali-metal doped phases. The details of the electronic structure and the nature of the superconducting phase (e.g. the magnetic field dependence of 7’,) have been studied in detail. Orientational disorder is suggested as an important scattering mechanism. 3.7. Ferromagnetism

in C,,-TDAE

and other systems

When a benzene solution of CeO is mixed with tetrakis-dimethylamino ethylene, which is a very strong electron donor, a black precipitate is obtained [ 2181. Cooling this black powder to below 16 K results in a transition to a state which is ferromagnetic. C,,-TDAE is a soft ferromagnet, which means that there is no remanence in the M-H curve. It presently holds the record for the highest Curie temperature amongst purely organic ferromagnets. Electron spin resonance studies confirm that one electron is doped from TDAE to CeO [ 219,220]. The ESR linewidth narrows with decreasing temperature, suggesting that these samples are metallic. Conductivities of pressed pellets of C&,-TDAE also suggest this [ 2181. Recent microwave studies, however, suggest that the samples might be slightly semiconducting. Below the transition at 16 K, the ESR g-value shifts to a higher value owing to internal fields. Raman studies [ 2211 show that the phonon frequencies, when compared with the phonon frequencies of alkali-metal-doped fullerene phases, correspond quite well to what one would expect for single-electron doping (Fig. 26).

233

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

234

-5oc 0

2

Doped

QlQctrons

4

6 per

c

IlJO~QCU~Q

Fig. 26. Titrating the Raman phonons of C,,-TDAE A,C, for x = 3 and 6.

against the number of electrons (x) on the Cm anion using the data for

A key property of C,,-TDAE is that the nonspherical structure of TDAE forces C,,-TDAE into adopting a low-symmetry monoclinic structure [ 2221 (Fig. 27). This figure also shows a space-filling view of the TDAE molecule. The structure can be visualised as independent stacks of CeO and TDAE running along the c-direction, reminiscent of organic metals. This quasi-l-D nature of CeO-TDAE possibly favours the formation of a magnetically ordered ground state. Configuration-interaction pictures for the stabilisation of a triplet ground state between two CeO species have been presented in

z=o -TDAE

2=1/2

l

z=l a

CGO t

b

Fig. 27. Schematic structure of (a) Cm-TDAE and (b) of the TDAE molecule (from a molecular mechanics calculation).

C.N.R. Rao et al. / Fullerenes. nanotubes, onions and related carbon structures

the literature, in keeping with the usual McConell model for organic ferromagnetism [ 223,220]. One of the important factors required by the McConell model is a degenerate frontier orbital, which is satisfied by the tI, LUMO of ChO,and not by higher fullerenes. There are however, some recent studies that suggest that the ChOdianion is a ground state singlet [ 2241. Also, the temperature dependence of the microwave conductivity of C&, suggests that it is not a metal, but a small-gap semiconductor [ 2253. Li et al. [ 2261 have recently reported the novel magnetic properties of a complex formed between a mixture of brominated CeOspecies and tetrathia fulvalene (‘ITF&Br,) . This system is ferromagnetic with a reported Curie temperature (by extrapolation since the system decomposes) of 334 K. Below 60 K the system appears to show a magnetic anomaly. Systematic studies underway in this laboratory have shown the complex nature of this system. Elucidation of the properties of the TTF adduct with a single brominated species (rather than a mixture) would be of interest. 3.8. FullereneJilms:

Structure, electronic and optical properties

The low vapour pressure of CsO and CYOallows them to be easily deposited onto suitable substrates by physical vapour deposition. At ea. 10-5-10-6 Tot-r, deposition is appreciable at temperatures around 600 K for Ch,, and 800 K for CTO. STM of thin films of ChO on gold [68] showed round molecules but the internal structure could not be seen because of rapid molecular reorientation. The molecules were seen to be ca. 10 A apart. From photoemission studies [ 226-2281, it is known that band-bending takes place when CeOis deposited on a substrate, aligning the Fermi surface of the metal and the deposited fullerenes. Santra et al. [ 2271 have studied the development of both valence and core-level photoemission spectra as a function of Ni deposition on thin CeOfilms on gold. Some degree of charge transfer from the metal to Cm was observed in these studies - such effects were not seen for the metal clusters on graphite. This was taken to imply that substrate+verlayer interactions are important even in these systems which have closed shell electronic structures. On Si( 111) the C&,substrate interaction is sufficient that for low CeO coverages, the molecular rotation is frozen and intramolecular features can be imaged in STM [ 2301. CeOforms epitaxial films on GaAs ( 110) surface and this is a system that has been extensively studied by STM [ 23 l-2331. STM studies of fullerene films on various metal surfaces has been reviewed by Wang et al. [ 2341. High resolution electron energy loss spectroscopy [ 2352361 has been employed to study epitaxy and overlayer formation of ChO on GaSe(OOO1) and Si( 100). One very interesting feature of CbO growth on Si is that C,, can preserve the Si ( 111) 7 X 7 reconstruction [ 2371. The Si( 111) 2 X 1 reconstruction is also preserved. This might turn out to be an important application of C&,, as a surface capping system in nanoscale devices where surface reconstruction needs to be preserved. Heating C60Si( 111) 7 X 7 results in all but a single monolayer being desorbed. The molecules in this monolayer open up at 1020 K with concomitant formation of Si-carbon bonds [ 2381. Xu et al. [ 2391 have shown from STM studies that ChOon Si( 111) 7 X 7 forms a double domain structure with some of the C&, molecules being pinned to the surface defects. The threshold height for the movement of CeO on Si( 111) 7 X 7 when the molecule is moved with STM tips has been determined by Maruno et al. [ 2401. They find that direct contact or close proximity between the STM tip and the molecule is required to move the molecule. C@,molecules can be dragged across the Si( 100) and Si ( 111) surfaces, suggesting possible uses in nanofabrication of surfaces using STM/AFM. On CaF2( 111) CeO forms epitaxial films when grown by molecular beam deposition [ 2411. Reflection high energy electron diffraction (RHEED) shows that the growth involves incommensurately stacked hexagonal layers. When the substrate temperature is below 443 K, the growth is unidirectional. Above 433 K, two equivalent domain orientations are seen. This helps reduce the mismatch. On highly ordered/oriented pyrolytic graphite (HOPG), C& films grow in a spiral manner with triangular terraces as seen by

235

236

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 28. STM lineplots of (a) C, and (b) CT0 on a HOPG substrate.

Fig. 29. STM picturesof (a) C,,onAg(llO); of disorder is seen to vary.

(b) C,,onAg(

111); (c) C,,-C,mixtures

(80:20) onAg( 1 ll).Thedegree

AFM. The steps in the terraces are separated by 10 A distances, corresponding to distances between the 111 planes of fee CeO [ 2421. Aiyer et al. have studied CT0 on HOPG and Ag substrates by a combination of STM and scanning tunnelling spectroscopy [ 2431. They found evidence for orientational disorder at room temperatures. They were also able to obtain intramolecular features. Fig. 28 shows STM lineplots of (a) C6,, and (b) (&, on HOPG substrates. The lattice is better formed in the latter. Fig. 29 is a collection of STM images of CT0 and Cs0-C7,, mixtures on silver substrates. AFM and RHEED show that CeOalso grows well on MoSz [ 2441. On metal surfaces the &,-metal interaction is quite strong. Hashizume et al. [ 2451 have studied ChOon Cu( 111) 1 X 1 by STM and by using local density approximation calculations for the charge

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

density. They find that CeO forms a 4 X 4 superlattice. There is charge transfer from Cu to CsO and the CeO molecules ratchet in the three-fold hollow sites. Using varying biases of - 2, 0.1 and 2 V these authors were able to observe different C& frontier orbitals, seen as lobed structures in STM. On Au( 001) , Cc0 forms stressed overlayers [ 2461. This has been observed by STM and photoelectron spectroscopy. The CeOmolecules are commensurate and uniaxially stressed along the ( 110) directions. Both UV photoemission spectroscopy (UVPES) and scanning tunnelling spectroscopy (STS) show charge transfer from metal to C,. Interestingly, C6,, can cause cooperative self-assembly on the Au( 110) surface. ChOinduces Au ( 110) 1 X 2 to rearrange itself to form a 1 X 5 reconstruction. The CeOitself forms a 6 X 5 ordered structure [ 2471 .The charge states of CsOhave been probed by HREELS and EELS by titrating vibrational frequencies on various Au surfaces and K-doped Au surfaces [ 2481. The measured electron transfer from the substrate to C,, were respectively 11, 3 1 and 61 for Cm on Au( 110)) K-Au( 110) and bulk K. Silver is known to diffuse into C,, films at temperatures slightly above room temperature [ 2491. Silver atoms form an impurity band with an activation energy for conductance of 0.26 eV. This is important in the forming of ohmic contacts between Ag and C6,, in possible devices. The use of CeO films for forming surfaces of uniform potential has been suggested [ 2501. In their Kelvin probe scans of Cc0 films, Camp and Schwarz [ 2501, used a phosphor bronze/ Ge( lnm) /&, architechture. The thin Ge film helps clamp down CsOto the metal surface and prevents the formation of C6,, islands. The same authors found Ge to help clamp down Au films in earlier work. Tribology is an area where fullerenes could make a possible impact. Against steel, tribological studies suggest that C6,, on Si( 111) is as friction-free as MoS, [ 2511. Ion-beam-modified fullerene films have also been examined for their frictional properties [ 2521. The ion beam irradiation results in mixtures of crystalline and amorphous areas as seen by Raman and transmission electron microscopy studies. The amorphous films in this study were found to have lower coefficients of friction. Microhardness studies of CeO films [ 2531 show effects of the orientational ordering phase transition around 260 K (seen as a discontinuity in the hardness). Interestingly, the hardness goes through a maximum at 370 K and then starts to decrease. The authors implicate dissolved O2 in their CsO samples for this anomalous behaviour. AFM studies have been used to characterise C& on Si and mica [ 2541, following both morphology and frictional characteristics. Nanocrystals 40-60 nm were observed with small area scans, showing the molecules to be disordered. Frictional measurements carried out by monitoring the buckling of the AFM cantilevers showed that the &, covered areas had a higher coefficient of friction. At high forces, lines could be etched by the AFM tip. Charged CeOions have been impacted on epitaxial CeO films on mica by Lill et al. [ 2551, who found that both fusion and rainbow scattering take place. C6,, films can be polymerised by exposing them to light [ 256-2581, or electronically by using a 3 V bias in STM [ 2591. In their STM study of CGOon GaAs, Zhao et al. [ 2591 found that CsO-(& addition can take place. A 1.5 kV electron beam achieved the same effect. Low energy electron diffraction (LEED) and inverse photoemission spectroscopy (IPES) confirm the bonding. The polymerisation is reversible and annealing results in the original C,, being recovered. Menon and Subbaswamy [ 2601 have calculated the stabilities of the possible (C& 2 dimers formed by such irradiation. Because of their rich r-electron structure, fullerenes display complex electro-optical properties. These constitute important applications for fullerenes and fullerene-based materials. These are largely molecular properties, however, and are not really within the scope of this review. Tutt and Kost [ 2611 have shown that &,-doped polymers have useful optical limiting properties, i.e. the amount of light transmitted by CeO samples initially increases with the intensity of the source and then saturates. Some of these properties, including that of reverse saturable absorption (RSA) , have been reviewed by Tutt and Bogess [ 2621. Fig. 30 shows the typical characteristics of an optical limiter. ChOthin films, despite the molecule as well as the crystal being centrosymmetric, show a large second harmonic response [263], with ax (2) about twice that of quartz using 1064 nm radiation from a Nd:YAG laser. There is

237

238

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

I 0

I

1

5

10

I

I

20

25

Input Fluence Fig. 30. Schematic plot of incident and transmitted fluence for an optical limiter.

a strong temperature dependence of xc2) and at 413 K the value is ten times larger than at room temperature. These authors suggest that the large value of xc2) could be due to either impurities reducing the molecular symmetry or to the interaction of the magnetic dipole/electric quadrupole of the incident radiation with the molecule, rather than the electric dipole. The dispersion of the thirdorder optical susceptibility of C,,, thin films has been measures by Kajzar et al. [ 2641. These authors point out that the absence of hydrogen in these materials ensures that there is no C-H stretch in the IR region, thereby increasing the spectral window for communication in the IR. Wang [ 2651 has doped CeO in polyvinylcarbazole to increase its photoconductivity and has suggested possible uses of this system in xerography. Various studies of the optical properties of Ceo, both static and dynamic, have been performed. These include the femtosecond dynamics of Ceo and A3Ce0 by Fleischer et al. [ 2661. Rb&, and K& relax from the photoexcitation at 625 nm within the very short time of 0.02 ps owing to their band-like nature. Pristine Ceo on the other hand takes as long as 30-40 ps and the relaxation is nonexponential. Fluorescence from X traps in C 6. single crystals has been studied [ 2671. In this study, the dominant species were Frenkel excitons, as would be expected for a molecular solid. The fluorescence intensity shoots up below 80 K owing to the glass transition in solid CeO. Singlet oxygen is produced in quantity by C6,, and its highly structured pholotoluminiscence has been measured [ 2681. CeO is normally transparent in the near-IR in the region 900-2000 nm. Electrochemical insertion of Li+ from a solution of LiC104 in butyrolactone results in the formation of Chop which is black and absorbs in the near-IR region [ 2691. This has been suggested as a novel display device. The dielectric function of solid C&, films has been measured [ 2701 as has the persistent photoconductivity [ 27 11. Harigaya et al. [ 2721 have used the Su-Schrieffer-Heeger model to understand the optical absorption properties of C6,, and CT0 solids with particular reference to the effect of Coulomb interactions, lattice fluctuations and anisotropy. Some of the exciting applications of the fullerenes involve their properties as dopants and charge acceptors in polymeric systems. Thus CeO is an effective charge sensitiser of the photo-refractive polymer polymethylmethacrylate-pnitoaniline-diethylaminobenzaldehyde-diphenylhydrazone (PMMA-PNA-DEH) . As little as 0.2 wt% of C6,, considerably improves the efficiency of the optical writing [ 2731. A nearly resonant two-photon absorption at 3.76 eV (633 nm) has been observed by Han et al. [ 2741. They obtained a value for /3 of 4.4 cm W - ‘. Dark and photoconductive behaviour of CeOfilms sandwiched between metal electrodes has been studied by Yonehara and Pat [ 2751. They used a A1/A1O,/CeO sandwich to obtain a photon quantum yield as high as 54% They also studied the

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

temperature dependence of the photoconductivity and found that it decreases with temperature. Orientational ordering does not seem to affect carrier mobility. Polysilane films containing ChOhave been studied in terms of photoinduced charge transfer and charge carrier generation [ 2761. CeO doped poly(methylphenylsilane) films showed an 8% quantum yield for the carrier generation. The hole transport in this system was however, unaffected. Saricifti et al. [ 2771 have studied diodes, photodiodes and photovoltaic cells constructed from semiconductor-polymer-C6, heterojunctions. The polymer was poly (paraphenylene vinylene) mixed with CeO and the charge transfer from 5 (2’methylhexyloxyl,4-phenylene vinylene) (MEH-PPV) to the C&,-doped polymer was studied. The rectification ration exceeded 104. In an important step, Eom et al. [278] have made free-standing ChO films using deposition in high vacuum on to Si/Si,N, substrates followed by suitable lithography. Such free-standing films could be used in various separation technologies. They also serve to indicate the mechanical stability of these materials, which is an important issue in the fabrication of molecule-based devices. The use of &, deposited on Si as a substrate for diamond growth has been described by Meilunas et al. [ 279,280]. C&, did not serve as well. Gruen et al. [ 2811 have described the use of of fullerenes as precursors for diamond growth (i.e. as the carbon feedstock) in a microwave plasma. The substrates were diamond-polished Si ( 111) . Interestingly, H and 0 additions were not required for this synthesis. Optical measurements implicate the involvement of C2 units. Chen et al. [ 2821 have studied the sorption of He by CeOcrystals and thin films, and suggest that their experiments open up the possibility of studying 3-D quantum fluids. 3.9. Other solids obtained by the intercalation

of C,, and CT0

While the alkali and alkaline earth metal derivatives of CsO are interesting because they are superconducting, there exist many other CeO and CT0 based solids that are perhaps best described as cocrystals of C&, with organic or inorganic molecules. More often than not, there is little or no evidence for charge transfer from these molecules to CeO. Early evidence for the possible existence of such phases came from the fact that it is very difficult to crystallise CeOfrom solvents without some degree of solvent incorporation in the crystal. In this section we present examples of some of the wellcharacterised samples of such cocrystals. Fleming et al. [ 2831 found that crystals of CeO or CT0 crystallised from n-pentane as elongated ten-sided columns. X-Ray diffraction studies suggested that the remarkable crystallographically forbidden ten-fold symmetry was actually a result of twinned unit cell. Geiser et al. [ 2841 have reported the structure of C60-C6H6CHZ12. Crystals of C&&H6 have been obtained by slow evaporation of &,-benzene solutions [ 285,286]. The benzene as well as the CsO molecules stack down the c-axis in independent stacks. There is not much evidence for specific benzene-&, interactions. Balch et al. [ 2861 have determined that the CeO are oriented in two ways, with relative populations of 55:45. As is obvious from this discussion, the benzene molecules freeze the orientational motion of the C6,,. Crane et al. [ 2871 have similarly cocrystallised CeO with ferrocene in the stoichiometry 1:2. In this structure, too, the CsO pack independently along the c-axis. The ferrocene molecules stack along the c-axis with the five-fold axis in the a-b plane. In this structure, there is some evidence for chargetransfer from ferrocene to CeO. However, the ferrocene structure is fully retained and the Fe atoms remain in the + 2 state. Bis-ethylenedithio tetrathiafulvalene (BEDT-ITF) forms a 2: 1 crystal with CeO [ 2881 with nr* charge transfer from sulphur to &,. Unfortunately, there is no evidence for metallic or superconducting behaviour in this system, which combines fullerenes with traditional organic donors. Even though the C& electronic levels are not symmetric about the HOMO-LUMO gap, early attempts were

239

240

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

made to dope CeOwith electron acceptors. C6,, was found to intercalate I2 from the vapour phase [ 2891. The formula of this phase is C60(12)2. There is no evidence for charge transfer from or to CeO in this structure. The packing of the C6,, and the IZ resemble that in the C,,-ferrocene phase. Douthwaite et al. [ 2901 have intercalated P4 units into ChOby treating a CeOtoluene solution with an excess of white phosphorous in CS2. The P4 tetrahedra occupy the trigonal prismatic sites in the hexagonal packed ChO lattice. Both 31P and 13C NMR support the view that there is orientational disorder of both species at room temperature. Ermer [ 2911 has reported a molecular complex of C& with hydroquinone in a 3: 1 ratio. The hydroquinone formed a host lattice with rather tightly enclathrated ChOmolecules. The ChO molecules are, however, orientationally disordered. Ermer and Robke [292] have also reported a 1:4.5:1 complex of C,,, hydroquinone and benzene. Wang et al. [ 2931 have reported attempts to intercalate CeO derivatives with alkali metals. Gugel et al. [ 2941 have shown that CeO can be incorporated into molecular sieves from the vapour phase. Specifically, they were able to demonstrate ChO uptake into the pores of AlPOe- and the zeolite VPI-5. NMR studies indicated that rotation of the ChO molecules was restricted. Anderson et al. [295] further showed that the samples of VPI-5 which incorporated CeO did not convert to A1P04-8 on heating, unlike pristine VPI-5, showing that the CeO serves to stabilise the large cavities of this zeolite. Some workers have reported spectroscopic and calorimetric studies of C6,, and CT0 solids which are either solvated or cocrystals. The heat capacity and orientational phase transitions of C,, samples prepared from different solvents has been reported [ 2961. The IR and Raman spectra of Cso n-pentane clathrate crystals has been reported by Kamaras et al. [297]. Bowmar et al. 12981 have made an extensive Raman and IR study of single crystal CsO and its derivatives, including the BEDT-TTF cocrystal and a CS,-solvated solid phase. CeOsolids with sulphur and chlorobenzene have been reported along with their IR and UV/Vis spectra [ 2991. Massively cyclohexane-solvated ChOhas been studied [ 3001, as have toluene containing solid Co phases [ 3011. Some progress has also been made in the study of functionalised Cc0 phases. Thus the structures and phase transitions of C6,0 [ 3021 and C&H2 [ 3031 have been studied. Functionalising ChOresults in the orientational ordering transition temperature being raised; in this particular series, the transition temperatures increase as CeO< &,O < C&HZ. 3.10. The higherfillerenes

CT6, CT8, C,, and C,,

In Section 2.2 we made a brief reference to the higher fullerenes CT6, CT*, C8*, Cs4, etc. These fullerenes are formed under the same arcing conditions used for preparing ChOand CT0 except in much smaller quantities. In fact, the limited quantities of these higher fullerenes has been the main barrier to their being studied as extensively as the lower ones, particularly with reference to their solid state properties. Much of the separation strategies and the material for study have come from the laboratory of Y. Achiba of the Tokyo Metropolitan University. CT6 adopts only a single electronically favoured chiral structure with the Dz point group [ 3041. Isomer counting, in order to assign the lines in the NMR spectra of the higher fullerenes, has been simplified by use of an algorithm devised by Manolopoulos and Fowler [ 3051. The kinetic resolution of the optical isomers of CT6 has been achieved by Hawkins and Meyer [ 3061, who osmylated CT6 using an 0~0, derivative with a chiral alkaloid ligand. C& has interesting electronic properties in that unlike ChO and Co, it has a low first oxidation potential in solution of 1 eV [ 3071. C,* is the next higher fullerene that has been isolated and characterised. It is the smallest fullerene that can undergo a certain unusual pyracylene rearrangement that is now termed Stone-Wales isomerism [ 3081. This isomerism relates two of the naturally occurring CT8 isomers, D,, and CZv. The other isomer is C2+ All these have been characterised by NMR as have the CZ, CZv and C3” forms of Cs2 and the D2 and DZd forms of Cg4 [ 3091. K doping into the LUMO’s of Cg4 have been studied by combinations of UV

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

photoemission studies and resistance measurements. The material remains semiconducting throughout [ 3101. Weaver et al. have studied Cg4 and alkaline metal-C,, salts by STM [ 3111. The chiral D2 isomer of Cs, has been resolved using the osmylation procedure by Hawkins et al. [ 3 121.

4. Carbon nanotubes The discovery by Iijima [ 161 in 1991 that the carbon deposit formed on the cathode during the process of generating fullerene soot in a DC arc contains tube-like graphitic structures has given added impetus to the fullerene field. These nanotubes comprise concentric sheets of graphite with the ends capped by hemispherical domes of fullerene-like structures, as shown in Figs. 3 1 (a) and 3 1 (b) . During the curling of graphite sheets, helicity can be introduced. This is shown in Fig. 3 1 (c) . Carbon nanotubes are the only forms of carbon with extended bonding and yet no dangling bonds. Since the structures of these nanotubes are derived from those of the fullerenes, some authors refer to them as buckytubes. 4.1. Preparation and structure The arcing process can be optimised such that the major portion of the anode deposits on the cathode as nanotubes and other graphitic nanoparticles [ 3 131. Nanotubes can be prepared by striking an arc between graphite electrodes in 1 atm helium - considerably larger than the pressures of helium used for the production of fullerene soot. A current of 60-100 A across a potential drop of ca. 25 V results in high yields of carbon nanotubes. The structure of carbon nanotubes has been explored in

Fig. 3 1. Molecular structure The difference is the in the nanotube with finite helicity. turn. (adapted from Saito et

of a single carbon nanotube in (a) the sawtooth configuration and (b) the ziPTan ____ -~__Dconfi~rration __~___D-_~.__~.. disposition of the axes of the benzene ring. (c) Folding a graphene s heet in order to obtain a This is obtained by coinciding the dotted lines. A-A’ would then corre_,_.._ a single helical ~nond to al. [ 3531).

241

242

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 32. Transmission

electron micrograph of a carbon nanotube and some associated carbon nanocapsules.

considerable detail by Iijima and others using high resolution transmission electron microscopy (HRTEM) and allied techniques such as electron diffraction [ 16,3 14-3 171. The nanotubes as formed in the usual arcing process are revealed in the HRTEM studies to be multilayered, concentric cylinders of single graphitic (sometimes referred to as graphene) sheets. The diameter of the inner tubes are of the order of a few nanometres. The outermost tubes could be as large as 20-30 nm. Fig. 32 shows a transmission electron micrograph of a carbon nanotube and associated capsule-like nanoparticles. MER Corporation (Tuscan, Arizona) in a flier dated July 1993 lists carbon nanotubes with typical outer diameters of 19.60 f 3 nm and a length of 843 ) 185 nm. Aspect ratios (ratio of the length to the diameter) are ca. 50. Electron diffraction studies by Iijima [ 16,317] indicate that these tubes possess helicity and this suggests that the growth of nanotubes would be as in typical spiral growths of crystals. The separation between the concentric sheets is ca. 3.5 A, corresponding to the 0002 planes of graphite. These are the lowest energy surfaces of graphite and contain no dangling bonds, so these nanotubes are in fact the expected structures. Typically, one observes the nanotubes along their lengths, with the electron beam falling perpendicular to the axis of the nanotube. In very high resolution pictures, it is possible to see spots due to the lattice planes running along th,e length of the beam and Iijima [ 3 171 has shown one such picture for the 1120 planes, separated by 2 A. The selected area electron diffraction patterns (ED) show spots due to the 0001 planes. Ring-like patterns can be indexed to each tube comprising cylindrical graphitic sheets which are independently oriented (there is no registry between the sheets) with helical symmetry for the arrangement of the hexagons. Graphitic cylinders by themselves would have dangling bonds at the tips. The carbon nanotubes are capped by dome shaped hemifullerene units. These capping units have pentagons to provide the curvature. Ajayan et al. [ 3141 have studied the distribution of pentagons at the caps of carbon nanotubes and found that these caps need not be perfectly conical or hemispherical, but can form skewed structures. The simplest possible singlelayer carbon nanotube can be visualised by cutting the CT0 structure across the middle and adding a large number of hexagonal rings such that the tube is elongated. Such a nanotube would not be helical. Rather, it would have the symmetry of CT0 viz. D5,,.

C.N.R. Rao et al. / Fullerenes, nanorubes,onions and related carbon strucm-es

Tubular or capsule-like structures are not common in chemistry. Tenne et al. [ 3181 and Margulis et al. [ 3191 have recently reported a capsule-like structure of small WS2 and MoS2 particles. These materials have structures similar to that of graphite in the sense that they form sheets with only van der Waals interactions between them. Ghadiri et al. [ 3201 have recently reported a nanotube based on a cyclic peptide architechture. The structures of carbon nanotubes are closely related to the well known structures of carbon fibres, usually produced by the pyrolysis of polymers [ 321-3231. The difference is really one of degree - the carbon fibers are not so crystalline (graphitised) as nanotubes. More importantly, there is no evidence of either helicity or of closing fullerene-like caps in the pyrolytic carbon fibers. Carbon nanotubes have also been produced by using plasma arc jets [ 3241 and in large quantities by optimising the quenching in an experiment where the arc is struck between a graphite anode and a cooled copper electrode [ 3251. Jose Yacaman et al. [ 3261 have prepared carbon nanotubes by the catalytic decomposition of acetylene at 1023 K over iron particles supported on graphite. Ivanov et al. [ 3271 have prepared nanotubes by the catalytic decomposition of acetylene on well dispersed Fe, Co, Ni and Cu particles on graphite or silica supports. Ge and Sattler 13281 have reported from STM studies that the deposition of carbon vapour on cooled HOPG substrates results in tube-like structures. 4.2. Characterisation

by other techniques

Scanning electron microscopy (SEM) has been extensively used to study carbon nanotubes and it represents a good technique for checking bulk yields [ 329-33 11. Nanotubes seem to form in bundles on the cathode, held together by van der Waals interactions. The alignment of the tubes in these bundles seems to depend on the stability of the arc [ 331,332]. Fig. 33 shows typical SEM pictures of nanotubes that have been cleaned of extraneous graphitic material. Wang et al. [ 3321 have observed the formation of tightly packed buckybundles in the cross-sectional SEM of the cathodic deposit. These bundles are formed when the arc is maintained for very small separations between the electrodes. AFM has also been used to study the as-formed nanotube bundles [ 3291. Ebbesen et al. [ 3291 found that the bundles

500

Fig. 33. Scanning electron micrographs

nm

of carbon nanotubes which have been cleaned by oxidative cleaning.

243

244

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

> 200t u-l i5 !s

200-

TUBES

h

-40

Ii

100 I-JLL20 I. 20

I

I

I

40

I

60 2THETA,Cu

I

I

J

80

Koc

Fig. 34. XRD patterns of polycrystalline graphite and carbon nanotubes. The absence of general hkil reflections in the latter, as well as the sawtooth peak shape of the hki0 reflections are indicative of turbostratic disorder.

are self-similar in the sense that large cylindrical bundles are comprised of smaller ones and these smaller ones are made up of nanotubes, and so on. STM has been extensively used to probe the structure of carbon nanotubes on various substrates [ 328,333-3361. These studies have provided some insight into the electronic properties of these materials. They have also been used to probe sp3 defect structures and the closure of the tips [ 3361. X-Ray diffraction studies on carbon nanotubes have been performed using laboratory sources [ 33 1 ] as well as synchrotron radiation [ 3371. XRD patterns show that for nanotubes, only hki0 and 0001 reflections but no general hkil reflections. This is known for turbostratically modified graphites [ 338,339] and Warren [ 3381 has suggested special methods for the analysis of the hki0 reflections. These studies support the electron microscopy data in showing that structural correlations exist along the axis perpendicular to the carbon nanotube as well as within each individual tube, but not in any combination of these. The correlation lengths obtained from the XRD analysis are in the same regimes as those seen in microscopy. Fig. 34 shows typical powder XRD patterns from graphite and from carbon nanotubes. Raman spectroscopy has provided some important insights into these structures. Jishi et al. [ 3401 have predicted the Raman-active phonon modes that one would obtain on folding graphitic sheets. Hiura et al. [ 3411 found, quite surprisingly, that the linewidth of carbon nanotubes in the Raman spectrum is quite narrow, of the order of 20 cm-‘. This is not expected since nanotubes are quite diverse in their diameters. These authors use this as evidence for the crystallinity of these new forms of carbon. The Raman phonon frequency of nanotubes is seen by Hiura et al. [ 3411 to be softer than those of HOPG. They ascribe this to the curvature of the nanotubes. Holden et al. [ 3421 have performed Raman investigations of cobalt-catalyzed single layer carbon nanotubes and have compared them with the predictions in Ref. [ 3401. 4.3. Mechanism of formation Oberlin et al. [ 3211, from their extensive TEM studies of graphitisation and the formation of filamentous carbon fibers through benzene decomposition, propose a two-step model for the formation of such fibres. We present this here because of the relation of these fibres to nanotubes. The initial core regions, made of long straight layers, are formed by a catalytic process. Subsequent pyrolysis leads to a secondary thickening, with graphitic particles cladding the inner layers. Heating leads to a larger degree of graphitisation and improved alignment of the sheets. Endo and Kroto [ 3421, based on the observation of CZ ejection from C,, observed by mass spectrometry experiments, suggest that tube formation processes are a natural consequence of the formation of fullerenes, viz. that the process is

C.N.R. Rao et al. /Fullerenes,

nanotubes, onions and related carbon structures

by C2 addition. Smalley [ 3441 has raised the objection to such a picture that only the growth of outer layers of multilayered tubes would be permitted within such a mechanism. Iijima et al. [ 3151 present evidence from electron microscopy for the open-end growth of carbon nanotubes. They suggest that the termination of incomplete layers of carbon seen on the tube surface suggest that the extension and thickening of the tubes occurs by the growth of graphite islands on the surfaces of existing tubes. In this sense, the growth mechanism is similar to that for the traditional carbon fibres. They point out that the nucleation of pentagons and heptagons on the open tube ends results in a change in the growth direction of the growing tube. In their paper, they also observe some novel morphologies including an example where the tube turns around 180” during the growth. The authors suggest that the growth is self-similar and fractal-like with the inner tubules telescoping out of the larger ones with logarithmic scaling of the size. The very elegant isotope scrambling experiments of Ebbesen et al. [ 3451 have shown that in the conditions of the plasma used for fullerene formation, complete scrambling of i2C and 13C isotopes are observed. This implies that the plasma has vaporised atoms of carbon. Ebbesen et al. [ 3291 also observe C2 units in considerable abundance in the plasma spectrum. From the tube morphologies, they suggest a mechanism similar to the one discussed by Saito et al. [ 3461, wherein the carbonaceous material reaching the cathode anneals into polyhedral particles. Given the right conditions, the tip might remain open and continue to grow. Saito et al. [ 3461 suggest that such growth takes place from the outside inwards. In both Refs. [ 3291 and [ 3461 the authors point out the need for electric fields to align the tube bundles. Smalley [ 3441 has pointed out that plasmas employed for the preparation of nanotubes are rather dense and have high dielectric constants. This results in high field strengths, of the order of V A-’ near the tube tips due to efficient screening. These fields serve to keep the growing tube open. This has been contested by Maiti et al. [347] who from their Car-Parinello Molecular Dynamics simulation of nanotube growth found that the electric fields in typical growth conditions are too weak to keep the tube tips open. They suggest that the reason for the tips remaining open is actually the manner in which defects rearrange themselves on the growing tip. Ebbesen et al. [ 3291 make a passing remark as to the possibility of tubes forming directly from the closing of a large graphene sheet. Such a suggestion gains credence from the simulations of Robertson et al. [ 3481 who have studied the curling and closure of small graphitic ribbons. They find that the formation of cylinders is favoured by both entropy and enthalpy. It is possible that this could serve to nucleate the growth of multilayers by cladding, as in the mechanism of Iijima et al. [ 3151. Recently, Colbert et al. [ 3251 have tried to optimise tube yields by improving the cooling of the cathode. They suggest that tube yield and length are arrested by the sintering of tubes to one another and that to obtain more, longer tubes, one needs to prevent such sintering. 4.4. Properties Nanotubes, in terms of their relations to the structure of graphite and graphitic fibres, pose, as a natural consequence, questions regarding their structure and electronic properties. As with the fullerenes, the curvature of the graphitic sheets might be expected to influence the electronic structure. Calculations show that nanotubes may be as good conductors as copper, while combinations of the degree of helicity and the number of six-membered rings per turn around the tube can serve to tune the electronic properties from those of a metal to those a semiconductor [ 348-3561. Using local density functional theory calculations, Mintmire et al. [ 3491 showed that the simplest D5h 7 A nanotube could have as many free carriers as copper. Their mean-field estimates suggest that at room temperature these systems are well above a Peierls transition. Tanaka et al. [ 3551 have pointed out that the reasons for the existence of a gap is largely topological and that isolated (in terms of conjugation) cis polyacetylene skeletons running along the length of the nanotube assure the existence of conductance

245

246

C.N.R. Rao et al. / Fullerenes, nanotubes, onions ana’ related carbon structures

pathways and hence metallic behaviour. Measurements of tunnelling conductivity using a scanning tunnelling microscope suggest that these materials are semiconducting [333-3351, with an inverse diameter dependence of the energy gap [ 334,335] ubiquitous in many systems showing the effect of finite size [336]. Bulk electrical transport studies, however, suggest that the transport behaviour is very similar to that of graphite [ 335,3583601, i.e. of a semimetal. In their study of electrical transport of nanotube bundles Song et al. [ 3581 observe that at low temperatures there is a change in the sign of the magnetoresistance, which they ascribe to weak localisation. Dravid et al. [ 3161 have studied the electronic structure of these material using electron energy loss spectroscopy in the transmission mode (in an electron microscope) and have mapped the u- and T-like states. No direct measurements on the strength of these materials have been possible so far, even though calculations suggest that nanotubes would be exceptionally stiff and strong along their length [ 3613631. The aspect ratios of nanotubes as prepared by the usual methods is ca. 50-lOO;, too small for use as reinforcers in composite materials. However, newer techniques are being developed that may serve to increase the aspect ratio [ 324,325]. Tersoff and Ruoff [ 3641 have examined the stability of crystals of nanotubes from force-field calculations and have determined that while the narrower tubes would be cylindrical in a crystal (a close-packed bundle), larger tubes are expected to be hexagonal in order to maximise van der Waals contact between the tubes. Recently, Ajayan et al. [ 3651 have aligned nanotube arrays in a polymer resin-nanotube composite by cutting the composite. They point to the implications of such aligning in the design of nanostructured materials. The combustion of carbon nanotubes has been studied using thermogravimetry in air [ 3661 and in O2 [ 3311. Since carbon nanotubes have no dangling bonds, their attack by oxygen at elevated temperatures is kinetically less favourable that the attack of oxygen on systems such as graphite and diamond. However, C,, in spite of having no dangling bonds, bums at lower temperatures due to the strain associated with the small cage. Thus carbon nanotubes can be cleaned of extraneous material, usually small graphitic particles, by burning these away in an oxidising atmosphere [ 367,33 11. Kosaka et al. [ 3681 have monitored the quantity of dangling bonds present in the arc-produced cathodic material (which includes the nanotubes) using ESR spectroscopy, as a function of the weight loss in a thermogravimetry experiment. As expected, the initial combustion results in the quantity of dangling bonds decreasing owing to their greater reactivity. Further reaction results in the signal due to dangling bonds once again increasing. This is suggested to be due to the opening by oxidation of the closed tubes. Such tube opening was first studied by Tsang et al. [ 3691 and by Ajayan et al. [ 3701 who found that the heating of carbon nanotubes in an oxidising atmosphere results in the attack of the fivemembered rings in the fullerene caps. This permitted them to also suggest the mechanism for the insertion of metals into carbon nanotubes by heating them with Pb304 [ 3711 (see next section). The magnetic properties of carbon nanotubes have been studied by Zhou et al. [ 3721, Ramirez et al. [ 3731 and Heremans et al. [ 3741. They found the susceptibility of nanotubes to be enhanced with respect to graphite, and showing a greater temperature dependence. The enhanced diamagnetism of nanotubes is ascribed to ring currents around the body of the nanotubes. The electron spin resonance spectra of nanotubes show that nanotubes have a factor of ten fewer spins than graphite. In Ref. [ 3721 the defects in these materials is also followed by K and Rb doping. If the nanotube were totally closed structures, it would not be possible to perform such doping. Stephan et al. [ 3751 found that nanotubes can be doped with BN using graphite-amorphous boron electrodes and carrying out the arcing in a dinitrogen atmosphere. 4.5. Catalysis and stuffing Heating carbon nanotubes with Pb30, results in the opening of the tube tip and the stuffing of the tubes with what was originally thought to be lead metal [ 37 1] but is now believed to be either the

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

carbonate or oxide of lead [ 3 171. The process of stuffing has been ascribed to nanocapillarity and this was predicted by model calculations of Pederson and Broughton [ 3761, who showed from local density functional calculations that a HF molecule would be drawn into a small nanotube. Attempts have been made to introduce other metals into carbon nanotubes. One of the reasons for the interest in such structures is that the nature of these metals in terms of their structure and electrical transport properties, including superconductivity, might be vastly different from that of the bulk metal. For example, a leadfilled carbon nanotube might be an ideal one-dimensional nanowire. Ruoff et al. [ 3771 obtained single crystal particles of alpha-La& encapsulated in carbon nanoparticles (but not in nanotubes) on arcing a graphite anode filled with La,O,. Ajayan et al. [ 3781 found that one of the few metals that can be incorporated into nanotube-like structures during the arcing procedure is manganese. They confirm the encapsulation of Mn metal by constructing an image of the transmission EELS line scans. Tantalum and cerium carbides (TaC and CeC2) have been encapsulated in carbon nanotubes by Murakami et al. [ 3371 using graphite-Ta and graphite-CeO, composite rods. These authors have studied the antiferromagnetic transition in the encapsulated CeC2 and the superconducting transition in the encapsulated TaC. The latter shows no change in the transition temperature. Recently, Tsang et al. [ 3791 have employed a simple chemical technique to fill nanotubes with metals. They oxidised the tube tips in a concentrated metal nitrate/nitric acid solution. The tips opened and the nitrate solution filled the tube. Reduction resulted in the metal-filled tube. When carbon nanotubes are prepared in the presence of transition metals such as Co or Fe, not only does one observe a small metal encapsulated carbon particle (discussed in the section on carbon onions) but there are also some catalytic effects. The most dramatic of these is the formation of singlelayer carbon nanotubes [ 380,381]. These are in the 0.7-2.5 nm diameter range and are exciting because they allow measurements to be made which can help support or disprove theoretical predictions made for single layer tubes. Catalysis can also be achieved using mixed metals and the yield of singlelayer tubes is said to improve [ 3821 on using mixtures of Co and Pt, for example. Ajayan et al. [ 3831 have studied the formation and growth morphologies of transition-metal catalysed nanotubes as a function of the quantity of the catalyst used and the He pressure in the chamber. The same group [ 3841 also suggest the use of carbon nanotubes as supports for heterogeneous catalysts (specifically Ru) in the catalytic hydrogenation of cinnamaldehyde. Wang and Zhou have studied single-layer nanotubes by electron microscopy [ 3851.

5. Carbon onions and nanocapsules Along with carbon nanotubes, one also sees small polyhedral graphitic particles in the cathodic stub formed during the DC arcing of carbon (Fig. 35) [ 313,346,386]. These particles are also referred to as capsules because they are hollow in the centre. Ugarte [ 17,386] (see also Ref. [ 3881) found that exposing these polyhedral graphitic particles to intense electron beam irradiation (lo-20 times what is used normally for high resolution microscopy, which is 10 A cm-*, 200-300 kV) results in their curling and closure, forming remarkably spherical concentric graphitic shells. Sometimes, the innermost shell is of the order of 7 A in diameter and is thought to be CeO. These structures can thus be considered to be concentric, nested hyperfullerenes of carbon, of which CsO is the smallest, and have been christened carbon onions. Carbon onions have also been made by heat treating soot at temperatures around 2500 K [ 3891. Recently, it has been found that the thermal annealing of dispersed nanodiamonds also results in the formation of carbon onions [ 39 11. These structures have no dangling bonds. In small particles, the ratio of surface to volume is large and the surface can make a considerable contribution to the total stability. Thus small particles adopt structures that eliminate dangling bonds.

247

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

248

It is interesting to speculate that at least in the nanometer regime, carbon onions are the most stable carbon structures. Calculations on the stability of these materials have been performed [392-3951. Maiti et al. [ 394,395] have used the Tersoff-Brenner potential in combination with a Lennard-Jones interaction between sheets to find the number n for which C, becomes a two-shell onion rather than remaining as a single shell fullerene. Large fullerenes have less strain because the pyramidalisation angle at every carbon is less and all the carbons are nearly sp*. Multishelled structures are favoured by the van der Waals interactions between the shells. The same group has also asked the question of why these structures should be so spherical. They suggest that the main reason for the spherical (rather than polyhedral) structure are the defects that are quenched in from the high temperatures of formation. These defects might include seven-membered rings. Carbon onions can be filled and emptied with metals using the electron beam to incise the onion and melt the metal so that it is sucked in. This has been performed by Ugarte [ 3861 with gold and

2.0

z? E 2

1.8 1.6 1.4

1.0

0

I

I

80

120

I

40

I

I

160 200

240

280

T,K

-763

0 Bias,mV

763 -763

0

763

Bias,mV

Fig. 35. (I) Temperature dependent resistance of a pellet of carbon nanotubes and of the cathodic stub containing nanotubes. The resistance of a graphite pellet is also shown for comparison. (II) Tunnelling conductance and density of states (left and right panels) of individual carbon nanotubes are shown for nanotubes of diameters (a) 1.3 nm, (b) 3 nm, and (c) for graphite.

C.N.R. Rao et al. / Fullerenes. nanotubes, onions and related carbon structures

(a)

43nm -

Fig. 36. (a) and (b) Transmission electron micrograph of carbon wrapped iron nanoparticles. These particles are stabilised from oxidation by the carbon sheaths. The contrast due to iron is darker than that due to carbon.

LaC, particles. Ruoff et al. [ 3771 and Tomita et al. [ 3971 have reported the formation of small multishelled carbon capsuled filled with single-crystalline La& particles. Saito et al. [ 3961 have synthesised Y&-filled carbon nanocapsules and studied their electron-beam incision. Transition metals and transition metal carbides easily fill carbon nanocapsules [ 399-4011 when the graphite anode is filled with metal or metal oxide particles. What is exciting about such metal particles (e.g. small Fe particles) is that despite being small enough (5 nm) that they are superparamagnetic; the carbon sheathing provides them with a great deal of stability and these materials can be stored for months on end under ambient conditions without any oxidation taking place. Fig. 36 shows the HREM micrograph of small iron particles wrapped in graphitic carbon.

6. Other carbon forms The discovery of fullerenes and nanotubes has not only revolutionised research in carbon but has also provided a new paradigm, namely structures derived from curved graphitic surfaces. While pentagons provide positive curvature in a graphitic sheet and help the sheet fold up, heptagons provide negative curvature, and can undo the folding provided by pentagons. More interestingly, a combination of pentagons, hexagons and heptagons can be made to tile any curved surface, the heptagons being found wherever there is a saddle point. The most striking example of such a structure is when hexagons and other polygons (each three-coordinate as in graphite) are used to tile a periodic minimal surface of the type described by the mathematician Schwarz. This speculative structure resembles the structure of many zeolites and was first proposed by Mackay and Terrones [ 4021. Lenosky et al. [ 4031 have proposed the name schwarzites for such materials and have investigated their stability. Townsend et al. [404] have considered the tiling of a random surface with n-membered rings comprising threecoordinate graphite. They do this by constructing a random surface from the domain boundaries of a 3-D Ising model and then tiling this surface using certain rules regarding local coordination, angles and distances. A comparison of the RDFs obtained from their model and from the electron diffraction of amorphous carbon films suggests that random schwarzite might provide a structural model for amorphous carbon in the graphitic limit. Periodic minimal surfaces tiled by graphite related networks have also been explored by Vanderbilt and Tersoff [ 4051. Negatively curved graphitic structures have been observed amongst carbon nanotube related material by Iijima and coworkers. The structures of the schwarzites are too crystalline for one to expect that they could form by chance, and systematic synthetic methodologies such as those suggested by Diederich and others [ 4061 might be required for carbon schwarzites to become reality. Fig. 37 shows the unit cell of a typical (speculative) schwarzite based on the P-surface (from Ref. [ 4021) . Fig. 38 shows a picture of a more complex stellated structure

249

250

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Fig. 37. Structure of a typical schwarzite, based on the P surface (from Ref. [ 4021) .

that has been recently proposed. Such structures are constructed by tiling complex surfaces using certain valence rules. Zeger and Kaxiras [ 4071 suggest a CloO molecule (composed of a CzOinside a ChOmolecule with 20 tetrahedral carbon atoms connecting the inner cage to the outer one) as a plausible model for the collapsed fuller&e phase obtained on subjecting Ceo to high pressures. Other interesting suggestions for novel materials include a solid formed from the intercalation of C,, into graphite [ 4081 and various toroidal forms of carbon containing pentagons and hexagons [409]. Tsang et al. [410] make a case for the careful examination of the soot which is generated in standard fullerene arcs, suggesting it to

Fig. 38. A stellated carbon structure with five-, six- and seven-membered

rings (from Ref. [416] ).

C.N.R. Rao et al. / Fullerenes, nanotubes. onions and related carbon structures

Fig. 39. A typical diamond-graphite hybrid structure. This structure is mostly graphitic. The dangling bonds are capped with hydrogen atoms and the net stoichiometry is CS4HM.

be a novel microporous carbon. Coiled carbon fibres have been prepared by Motojima et al. [ 4111 and Zhang et al. [ 4121 by the pyrolysis of acetylene over Ni catalysts in the presence of a phosphorusor sulphur-containing impurity. Further microscopic characterisation is awaited. In the light of the new carbon structures, it is of interest to look at some of the carbon structures that have been proposed in the literature. As early as 1983 Hoffman and coworkers [ 4131 proposed a space-filling sp2 carbon based structure related to the structure of ThSi,. In this structure, the carbon atoms run along polyene chains that are perpendicular to each other and connected. Huckel calculations show that the structure would be metallic, largely due to the polyene chains being separated by a short nonbonded contact of ca. 2.5 A. More recently, Karfunkel and Dressler [ 4141 have proposed extended networks of triptycene molecules interconnected by benzene rings. Such a structure would combine

Expanded

I

I

I

10

20

30

I

I

I

40 50 60 28tCuKti) Fig. 40. Powder X-ray diffraction pattern of carbon spherulites extracted by the leaching of cast irons in hydrochloric acid. The appearance of the general hkil reflections and the squared Lorentzian nature of the hki0 peaks suggests that the graphite is flat (cf. the XRD of carbon nanotubes) .

251

252

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Table 3 Representative

patents

(I) Synthesis (a) T. Irie, K. Murata, M. Matsumoto and N. Hatsuda (Jpn. Kokai Tokkyo Koho JP 0632609 awarded 8 Feb. 1992) Manufacture of fullerenes. Carbon-containing compounds, e.g. CS2, are heated in a hot plasma (b) T. Yoshida, K. Eguchi and K. Yoshie (Eur. Pat. Appl. EP 527035 10 February 1993) Process for the preparation of the fullerenes. Carbon dust suspended in argon is carried into a hot plasma in the presence of other compounds that catalyze the formation of fullerenes (c) A.B. Smith III, J.P. McCauley Jr. and D.R. Jones (PCT Intl. Appl. WO 931004118 Nov. 1994) C,,O from the chromatographic separation of the products formed on irradiating &,-benzene solutions with UV light (d) K. Shigematsu (Jpn. Kokai Tokkyo Koho, JP 05924604, awarded 9 Nov. 1993) Selective conversion of high molecular weight fullerenes to Cm. The higher fullerenes are hydrogenated with hydrogenating catalysts. Dehydrogenation yields Cso (2) Superconductivity (a) CM. Lieber, (US 5196396 awarded23 March 1993) Method of making a superconducting fullerene composition with an alloy containing alkali metal. Cm + binary or ternary alloy to obtain Rb,Kr -&a or Cs&,, high T, superconductor, bulk single phase (b) N. Okuda, Y. Ueha, K. Ookura and J. Hisagai, (Jpn. Kokai Tokkyo Koho JP 05213610 awarded 24 Aug. 1993) Manufacture of carbon clusters for electrical conductors and superconductors. Carbon or graphite vapourised in dopant containing atmosphere to obtain fullerene + dopant (c) T. Sakugi (Jpn. Kokai Tokkyo Koho JP 05258621 Appl. 8 Oct. 1993) Alkali metal doped fullerenes ( C6,,) superconductors. Alkali doped Cm (H, lower hydrocarbons adsorbed) disordered regions (nonsuperconducting clusters). Good room temperature stability and high T,

to give

(d) Y. Ueha, N. Okuda, K. Ookura, J. Hisagai and K. Tada (Jpn. Kokai Tokkyo Koho JP 05238885 awarded 17 Sept. 1993) Manufacture of carbon cluster electrical conducting and superconducting film. Rb doped C,, on ZnSe( 100) T, = 30 K (3) Light limiters (a) K. Shigematsu (Jpn. Kokai Tokkyo Koho JP 0625461 awarded 1 Feb. 1994) Fullerene compositions for light limiters and shields. Transparent film + toluene + polystyrene + fullerene. (4) Photoresist (a) N. Aoki (Ger. Offen. DE 43 11547 awarded 13 Jan. 1994) Photosensitive material. C,,, + n-propylamine + methacryloyl chloride (5) NLO material (a) Y. Wang (PCT Intl. Appl. WO 9302012 Appl. 15 July 1991) Charge transfer complexes. Charge transfer involving fullerene + donor in a transparent film (6) Magnetooptical recording (a) T. Yoshikawa (Jpn. Kokai Tokkyo Koho JP 0602106 awarded 24 Jan. 1992) Magneto-optical recording material using fullerene Kerr-effect enhancement layer. Magnetic layer with a thin coating of fullerene (7) Photocharge generation (a) T. Suzuki, T. Matsui, H. Kimura, K. Tsuda, K. Koe and T. Ishi (Jpn. Kokai Tokkyo Koho, JP 05254815 awarded 5 Oct. 1993) Electrical conductivity of doped fullerene. Remarks: C,(CGM), on Si wafer; CGM = charge generating material (8) Room temperature ferromagnetism (a) H. Watanabe, M. Ata and H. Machida (Jpn. Kokai Tokkyo Koho, JP 05159921 Appl. 25 June 1993) Fullerene magnetic material and its manufacture. Cm + azoisobutyronitrile ( AiBN) (9) Magnetic material (a) A. Matsufumi, H. Machida and H. Watanabe (Jpn. Kokai Tokkyo Koho JP 05 129120) Fullerene magnetic material. C, (n = 60,70,76, 84, etc.) doped with halogen from alkali halide or I. Also organic polymer containing C, + MX (M = Li, Na, K, Cs, Rb; X = F, Cl, Br, I) (continued)

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

Table 3 (continued) (b) T. Yamada, M. Ata, H. Machida and H. Watanabe (Jpn. Kokai Tokkyo Koho JP 05159915) Magnetic substance. Dispersion of anisotropic aggregates containing a paramagnetic material from fullerenes in an organic polymer or a nonelectrolytic conducting liquid or a solid ferromagnet at room temperature (10) Magnetic resonance spectroscopy and imaging (a) W.L. Neumann and W.P. Cacheris (PCT lntl. Appl. WO 9303771 Appl. 4 March 1993) Fullerene compositions for magnetic resonance spectroscopy and imaging. C&F, used for 19F NMR imaging and spectroscopy (I I) Diamond preparation (a) D.M. Gruen (US 5209916) Conversion of fullerenes to diamond. Ionising fullerene molecules in the vapour and accelerating beam and impinge on the surface

> 250 eV to form an ion

(12) Lubricants (a) M. Taniguchi, Y. Tomioka, M. Kunegawa and M. Ishibashi (Jpn. Kokai Tokkyo Koho JP 05179296 awarded 20 July 1991) Lubricants. C,F,, (m = 6-54), C&F,,, (m = 664) adsorbed on materials containing metal complexes, peroxides and polymerisable functional groups. (Lubricants for mechanical parts) (b) A.O. Pati, G.W. Schriver and R.D. Lundberg (US 5292444 awarded

sp2 and sp3 carbon atoms. Solid state MNDO calculations suggest that such a structure would be as stable as diamond. Balaban et al. [ 4151 have studied conduction pathways in diamond-graphite hybrids. A variety of diamond-graphite hybrids have been studied by Sen at al. [416] using molecular mechanics minimisation. These studies help clarify some of the issues in the study of amorphous carbon phases. These could be either in the sp3 rich regime (called ta for tetrahedrally amorphous), or in the graphiterich regime (as in soots, or glassy carbon). An example of such a structure is given in Fig. 39. There has been some speculation concerning the nature of the spherical graphitic precipitates found in certain cast irons [417]. These carbon spherulites are known to be graphitic, but in light of their curvature it is intriguing that these structures might be similar to carbon onions. Microscopy studies from this lab suggest, however, that the graphite is mostly flat. The X-ray diffraction pattern of carbon spherulites obtained by the acid leaching of cast iron is shown in Fig. 40. The appearance of general hkil reflections and the absence of the sawtooth lineshape for the hki0 reflections suggest that the graphite is not turbostratically disordered, contrary to what one would expect for any sort of curved graphite.

7. Concluding

remarks

In the foregoing presentation we have examined the synthesis, structure and solid-state properties of fullerenes, carbon nanotubes and carbon onions. We have also commented on the nature of other types of carbon structures of interest. Clearly, carbon research today presents interesting challenges, not only in terms of discovering, preparing or designing new types of structures but also in exploring possible materials applications. Carbon structures with combinations of five-, six- and seven-membered rings as well as diamond-graphite hybrids typify such possibilities. Materials applications of fullerenes and related structures have already attracted considerable attention and we have indicated some of these in the text. In order to provide some flavour of the patent literature on this subject, we have listed typical patents in Table 3.

253

254

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

References J. Donohue, The Structure of the Elements, Interscience, New York, 1974. L.A. Paquette, R.J. Temansky, D.W. Balogh and G.S. Kentgen, J. Am. Chem. Sot.. IO5 (1983) 5446. D.E.H. Jones, New Scienrisr, 35 (1966) 245. D.E.H. Jones, The Invenrions of Daedalus, W.H. Freeman, Oxford, 1982, p. 118. D.W. Thompson, in J.T. Bonner (ed.), On Growth and Form, Cambridge University Press, Cambridge, 1961. E. Osawa, Kagaku, 25 (1970) 854 (in Japanese). D.A. Bochvar and E.G. Gal’pem, Dokl. Acad. Nauk. SSSR, 209 (1973) 610. H.W. Kroto, J.R. Heath, S. O’Brien, R.F. Curl and R.E. Smalley, Nature, 318 (1985), 162. For a discussion of some of Fuller’s ideas, see R. Mark and R. Buckminster Fuller, The Dymarion World of Euckminsrer Fuller, Anchor Books, Garden City, 1973. Most of Fuller’s geodesic domes had five- or six-coordinate vertices and triangular faces and are thus duals of the fullerenes. The fullerenes can be obtained by truncating geodesic spheres at their vertices. [lo] For disci sion of the earlier work and the astrophysical motivation/implications, see for example, H.W. Kroto and D.R.M. Walton, Philos. Trans. R. Sot. London, A343 ( 1993) 103. [ 1 l] E.A. Rohl’hng, D.M. Cox and A. Kaldor, J. Chem. Phys. 81 (1984) 3322. [ 121 See S.W. McElvaney, M.M. Ross and J.H. Callahan, Act. Chem. Res., 25 ( 1992) 162 for a review of the work both before and after the bulk synthesis. [ 131 R.C. Haddon, Act. Chem. Res., 21 ( 1988) 243; 25 (1992) 127 reviews some of the theoretical work. [ 141 W. Kratschmer, K. Fostiropoulos and D.R. Huffman, Chem. Phys. I&t., 170 (1990) 167. [ 151 W. Kratschmer, L.D. Lamb, K.Fostiropoulos and D.R. Huffman, Nature, 347 ( 1990) 354. [ 161 S. Iijima, Nature, 354 (1991) 56. [ 171 D. Ugarte, Nature, 359 (1992) 707. [ 181 R.F. Curl and R.E. Smalley, Scientific American, October 1991, p. 54. [ 191 H.W. Kroto, A.W. Allaf and S.P. Balm, Chem. Rev., 91 (1991) 1213. [20] Phil. Trans. R. Sot. London A., 343 (1994) pp. 1-154. [21] Act. Chem. Res., 25 (1992) pp. 98-174. [22] Indian J. Chem., 3ZA,B (1992) pp. Fl-Flll. [23] J. Phys. Chem. Solids, 53 (1992) pp. 1321-1485. [24] J. Phys. Chem. Solids, 54 (1993) pp. 1635-1877. [25] Carbon, 30 (1992) No. 8. [26] H. Ehrenreich and F. Spaepen (eds.), Solid Stare Physics, 48, Academic Press, San Diego, 1994, pp. l-434. [27] A.F. Hebard, Annu. Rev. Mater. Sci., 23 (1993) 159. [28] T.W. Ebbessen, Annu. Rev. Mater. Sci., 24 (1994) [29] P.W. Stephens (ed.), Physics and Chemistry of Fullerenes, Vol. 1.. World Scientific, Singapore, 1993. [ 301 MRS Bulletin. XIX, November 1994. [31] R.E. Smalley, Act. Chem. Res., 25 ( 1992) 98. [32] Y. Chai, T. Guo, C. Jin, R.E. Haufler, L.P.F. Chibante, J. Fure, L. Wang, M.J. Alford and R.E. Smalley, J. Phys. Chem., 95 (1991) 7564. [33] T. Guo, C. Jin and R.E. Smalley, J. Phys. Chem., 95 ( 1991) 4948. [ 341 B.C. Guo, S. Wei, J. Pumell, S. Buzza and A.W. Castleman Jr., Science, 256 ( 1991) 515. [35] C.-C. Chen and CM. Lieber, J. Am. Chem. Sot., 114 (1992) 3141. [36] CM. Lieber and C.-C. Chen, in H. Ehrenreich and F. Spaepen (eds.), Solid State Physics, 48 (1994) 109. [37] J.P.Hare,H.W.KrotoandR.Taylor,Chem. Phys.Lerr., 177(1991) 314. [38] H. Ajie, M.M. Alvarez, S.J. Anz, R.D. Beck, F. Diederich, K. Fostiropoulos, D.R. Huffman, W. Kratschmer, Y. Rubin, K.E. S&river. D. Sensharma and R.L. Whetten, J. Phys. Chem., 94 (1990) 8630. [39] C.N.R. Rao, T. Pradeep, R. Seshadri, R. Nagarajan, V. Narasimha Murthy, G.N. Subbanna, F. D’Souza. V. Krishnan, G.A. Nagannagowda, N.R. Suryaprakash, C.L. Khetrapal and S.V. Bhat, Indian J. Chem., 3ZA,B (1992) F5. [40] R.F. Bunshah, S. Jou, S. Prakash, H.J. Doerr, L. Isaacs, A. Wehrsig, C. Yeretzian. H. Cyn and F. Diederich, J. Phys. Chem.. 96 ( 1992) 6866. [41] G. Peters and M. Jansen, Angew. Chem. Inr. Edn. Engl.. 104 (1992) 240. [42] T. Yoshida, K. Eguchi and K. Yoshie, Eur. Pat. Appl. EP 527035 ( 10 Feb. 1993). [43] T. Irie, K. Murata, M. Matsumuto and N. Hatsuta. Jpn. Kokai Tokkyo Koho, JP 0632609 ( 15 July 1992). [44] W.A. Scrivens, P.V. Bedworth and J.M. Tour, J. Am. Chem. Sot., II4 (1992) 7917. [45] A. Govindaraj and C.N.R. Rao, Fullerene Sci. Technol., 1 ( 1993) 557. [46] A. Gugel, Angew. Chem. Intl. Edn. Engl., 31 ( 1992) 57. [47] N. Coustel, P. Bemier, R. Aznar, A. Zahab, J.M. Lambert and P.J. Lyard, J. Chem. Sot. Chem. Commun., (1992) 1402. [48] I. Bucsi, R. Aniszfeld, T. Shamma, G.K. Suryaprakash and G.A. Olah, Proc. Natl. Acad. Sci. (USA), 91 (1994) 13. [49] J.L. Atwood, G.A. Koutsantonis and C.L. Raston, Nature, 368 (1994) 229. [50] T. Suzuki, K. Nakashima and S. Shinkai, Chem. Left., (1994) 699. [ 1] [2] [3] [4] [5] [6] [7] 181 [ 91

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

[5 1] A.M. Vassalo, A.J. Palmisano, L.S.K. Pang and M.A. Wilson, J. Chem. Sot. Chem. Commun., ( 1992) 60. [52] W.H. Pirkle and C.J. Welch, J. Chromatogr., 609 (1992) 89. [53] K. Kikuchi, N. Nakahara, M. Honda, S. Suzuki, K. Saito. H. Shiromam, Y. Miyake, K. Yamauchi, I. Ikemoto, T. Kuramochi, S. Hino and Y. Achiba, Chem. Left., (1991) 1607. [54] F. Diederich and R.L. Whetten, Ace. Chem. Rex, 25 (1992) 119. [55] F. Diederich, R. Ettl, Y. Rubin, R.L. Whetten, R. Beck, M. Alvarez, S. Anz, D. Sensharma, F. Wudl, C.K. Khemani and A. Koch, Science, 252 (1991) 548. [56] L.D. Lamb and D.R. Huffman, J. Phys. Chem. Solids, 54 (1993) 1635. [57] F.D. Weiss, J.L. Elkind, SC. O’Brien, R.F. Curl and R.E. Smalley, J. Am. Chem. Sot., 110 (1988) 4464. [58] SC. O’Brien, J.R. Heath, R.F. Curl and R.E. Smalley, J. Chem. Phys., 88 (1988) 220. [59] S.H. Yang, CL. Pettiette, J. Coceicao, 0. Cheshnovsky and R.E. Smalley, Chem. Phys. Left.. 139 (1987) 233. [60] J.A. Zimmerman, J.R. Eyler, S.B.H. Bach, SW. McElvaney, J. Chem. Phys., 94 ( 1994) 3556. [61] SW. McElvaney, Inrl. J. Mass Specrrom. Ion Process, 102 (1990) 81. [62] R. Haddon, L.E. Brus and K. Raghavachari, Chem. Phys. Let?., 125 (1986) 459. [63] F. Negri, G. Orlandi and F. Zerbetto, Chem. Phys. Left., 144 (1988) 31. [64] S. Larsson, A. Volosov and A. Rosen, Chem. Phys. Lerr., I37 (1987) 501. [65] R. Taylor, J.P. Hare, A.K. Abdus-Sada and H.W. Kroto, J. Chem. Sot. Chem. Commun., ( 1990) 1423. [66] R.D. Johnson, G. Meijer, J.R. Salem and D.S. Bethune, J. Am. Chem. Sot., 113 (1991) 3619. [67] D.S. Bethune, G. Meijer, W.C. Tang and H.J. Rosen, Chem. Phys. Lerr., I74 (1990) 219. [68] R.J. Wilson, G. Meijer, D.S. Bethune, R.D. Johnson, D.D. Chambliss, MS. deVties, H.E. Hunziker and H.R. Wendt, Nurure, 348 (1990) 621. [69] J.L. Wragg, J.E. Chamberlain, H.W. White, W. Kratschmer and D.R. Huffman, Narure, 348 (1993) 623. [ 701 T. Chen, S. Howells, N. Gallagher, L. Yi, D. Sarid, D.L. Lichtenberger, K.W. Nebesny and C.D. Ray, in R.S. Averbach, D.L. Nelson and J. Bemholc (eds.), Proc. Symp. G on Clusters and Cluster-Assembled Marerials, Marer. Rex Sot. Proc., MRS Publications, New York, 1991. [71] J.M. Hawkins, T.A. Lewis, S.D. Loren, A. Meyer, J.R. Heath, Y. Shibato and R.J. Saykally, J. Org. Chem., 55 (1990) 6250. [72] P.J. Fagan, J.C. Calabrese and B. Malone, Science, 252 (1991) 1160. [73] K. Hedberg, L. Hedberg, D.S. Bethune, C.A. Brown, H.C. Dom, R.D. Johnson and M.S. devries, Science, 254 (1992) 410. 1741 D.R. McKenzie, CA. Davis, D.J.H. Cockayne, D.A. Muller and A.M. Vassalo, Nature, 355 (1991) 622. [75] A.J. Balch, V. Catalano, J.W. Lee, M.M. Olmstead and S.R. Parkin, J. Am. Chem. Sot., 113 (1991) 8953. [76] J.N. Sherwood (ed.), The Plasrically Crystalline Srare, Wiley, Chichester, 1979. [77] P.A. Heiney, J. Phys. Chem. Solids, 53 (1992) 1333. [78] J.R.D. Copley, D.A. Neumann, R.L. Cappelletti and W.A. Kamitakahara, J. Phys. Chem. Solids, 53 (1992) 1333. [79] J.E. Fischer and P.A. Heiney, J. Phys. Chem. Solids, 54 (1993) 1725. [SO] D.A. Neumann, J.R.D. Copley, D. Reznik, W.A. Kamitakahara, J.J. Rush, R.L. Paul and R.L. Lindstrom. J. Phys. Chem. Solids, 54 (1993) 1699. [ 811 J.D. Axe, SC. Moss and D.A. Neumann, in H. Ehrenreich and F. Spaepen (eds.), Solid Sure Physics, Vol. 48, 1994, p. 149. [82] J.E. Fischer, P.A. Heiney and A.B. Smith III, Act. Chem. Res., 25 (1992) 112. [83] R.M. Fleming, T. Siegrist, P.M. March, D. Hessen, A.R. Kortan, D.W. Murphy, R.C. Haddon, R. Tycko, G. Dabbagh, A.M. Mujsce, M.L. Kaplan and S.M. Zahurak, Marerials Research Society Symp. Proc., Materials Research Society, Pittsburgh, 1991, Vol. 206, p. 691. [84] P.A. Heiney, J.E. Fischer, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr. and A.B. Smith III, Phys. Rev. Lerr., 66 (1991) 2911. [85] C.S. Yannoni, R.D. Johnson, G. Meijer, D.S. Bethune and J.R. Salem, J. Phys. Chem., 95 (1991) 9. [86] R. Tycko, R.C. Haddon, G. Dabbagh, S.H. Glarum, D.C. Douglass and A.M. Mujsce, J. Phys. Chem., 95 (1991) 518. [87] R. Sachidanandam and A.B. Harris, Phys. Rev. Len., 67 (1991) 1467. [88] W.I.F. David, R.M. Ibberson, J.C. Matthewman, K. Prassides, T.J.S. Dennis, J.P. Hare, H.W. Kroto, R. Taylor and D.R.M. Walton, Nature, 353 (1991) 147. [89] W.I.F. David, R.M. Ibberson, T.J.S. Dennis, J.P. Hare and K. Prassides, Europhys. L&r., 18 (1992) 219. [90] S. Liu, Y.J. Lu, M.M. Kappes and J.A. lbers, Science, 245 (1991) 410. 1911 H.-B. Burgi, E. Blanc, D. Schwarzenbach, S. Liu, Y. Lu, M.M. Kappes and J.A. Ibers, Angew. Chem. Inr. Ed. Engl., 31 (1992) 640. [92] P.C. Chow, X. Jiang, G. Reiter, P. Wochner, S.C. Moss, J.D. Axe, J.C. Hanson, R.K. McMullan, R.L. Meng and C.W. Chu, Phys. Rev. Lerr.. 69 (1992) 2943. [93] A. Dworkin. H. Szwarc, S. Leach, J.P. Hare, T.J.S. Dennis, H.W. Kroto, R. Taylor and D.R.M. Walton, C.R. Acad. Sci. (Paris), 312 U(1991) 979. 1941 J.S. Tse, D.D. Klug, D.A. Wilkinson and Y.P. Handa, Chem. Phys. L&r., 183 (1992) 387. [95] P.A. Heiney, G.B.M. Vaughan, J.E. Fischer, N. Coustel, D.E. Cox, J.R.D. Copley, D.A. Neumann, W.A. Kamitakahara, K.M. Creegan, D.M. Cox, J.P. McCauley Jr. and A.B. Smith III, Phys. Rev., 845 ( 1992) 4544. 1961 G. van Tendeloo, S. Amelinckx, M.A. Verheijen, P.H.M. van Loosdrecht and G. Meijer, Phys. Rev. L-err.,69 (1992) 1065. 1971 P.H.M. van Loosdrecht, P.J.M. van Bentum and G. Meijer, Phys. Rev. L&r., 68 (1992) 1176. 1981 M. Matus, T. Pichler, M. Haluska and H. Kuzmany, SpringerSeries in Solid Srare Science, 113 ( 1993) 446.

255

256

C.N.R. Rao et al. / Fullerenes,

[99] [ 1001 [ 1011 [ 1021 [ 1031 [ 1041 [ 1051 [ 1061

[ 1071 [108] [ 1091 [ 1101

[ 1111 [ 1121 [ 1131

[ 1141 [ 1151 [ 1161 [ 1171 [ 1181

[ 1191 [ 1201

[ 1211 [ 1221 [ 1231 [ 1241 [ 1251 [ 1261 [ 1271 [ 1281 [ 1291 [ 1301 [ 13 11 [ 1321 [ 1331 [ 1341 [ 1351 [ 1361 [ 1371 [ 1381 [ 1391 [ 1401 [ 1411 [ 1421 [ 1431 [ 1441 [ 1451

[ 1461

nanotubes,

onions and related carbon structures

K. Akers, K. Fu, P. Zhang and M. Moskovits, Science, 259 ( 1992) 1152. B. Chase, N. Herron and E. Holler, J. Phys. Chem., 96 (1992) 4262. L.R. Narasimhan, D.N. Stoneback, A.F. Hebard, R.C. Haddon and C.K.N. Patel, Phys. Rev.. 846 (1992) 11346. V.S. Babu and MS. Seehra, Chem. Phys. L&r.. 196 (1992) 569. S. Huant, J.B. Robert, G. Choteau, P. Bemier, C. Fabre and A. Rassat, Phys. Rev. Left., 69 (1992) 2666. S.A. Fitzgerald and A.J. Sievers, Phys. Rev. Lea.. 70 (1993) 3175; H. Bonadeo, E. Halac and E. Burgos, ibid., 3176; S. Huant, J.B. Robert and G. Choteau, ibid., 3177. X.D. Shi, A.R. Kortan, J.M. Williams, A.M. Kini, B.M. Savall and P.M. Chaikin, Phys. Rev. Len., 68 (1992) 2708. F. Gugenberger, R. Heid, C. Meingast, P. Adelmann, M. Braun, H. Wuhl, M. Haluska and H. Kuzmany, Phys. Rev. Lea.. 69 (1992) 3774. R.D. Johnson, D.S. Bethune and C.S. Yannoni, Act. Chem. Res., 25 ( 1992) 169. R. Tycko, G. Dabbagh, R.M. Fleming, R.C. Haddon, A.V. Makhijaand S.M. Zahurak, Phys. Rev. Lerr., 67 (1991) 1886. K. Mizoguchi, J. Phys. Chem. Solids, 54 ( 1993) 1693. R.F. Kiefl, J.W. Schneider, A. Macfarlane, K. Chow, T.L. Duty, T.L. Estle, B.H. Jitti, R.L. Lichtl, E.J. Ansaldo, C. Schwab, P.W. Percival, G. Wei, S. Wlodek, K. Kojima, W.J. Romanow, J.P. McCauley Jr., N. Coustel, J.E. Fischer and A.B. Smith III, Phys. Rev. Len., 68 (1992) 2708. Y. Jin, J. Chen, M. Varma-Nair, G. Liang, Y. Fu, B. Wunderlich, X.-D. Xiang, R. Mostovoy and A.K. Zettl, J. Phys. Chem., 96 (1992) 5751. R. Hu, T. Egami, F. Li and J.S. Lannin. Phys. Rev., 845 (1992) 9517. J.R.D. Copley, D.A. Neumann, R.L. Cappelletti, W.A. Kamitakahara, E. Prince, N. Coustel, J.P. McCauley Jr., N.C. Maliszewskyj, J.E. Fischer, A.B. Smith III, K.M. Creegan and D.M. Cox, Physica, B180/281 (1992) 706. D.A. Neumann, J.R.D. Copley, R.L. Cappelletti, W.A. Kamitakahara, R.M. Lindstrom, K.M. Creegan, D.M. Cox, W.J. Romanow, N. Coustel, J.P. McCauley Jr., NC. Maliszewskyj, J.E. Fischer and A.B. Smith III, Phys. Rev. Len., 67 (1991) 3808. D.A. Neumann, J.R.D. Copley, W.A. Kamitakahara, J.J. Rush, R.L. Cappelletti, N. Coustel, J.E. Fischer, J.P. McCauley Jr., A.B. Smith III, K.M. Creegan and D.M. Cox, J. Chem. Phys., 96 ( 1992) 8631. R.L. Cappelletti, J.R.D. Copley, W.A. Kamitakahara, F. Li. J.S. Lannin and D. Ramage, Phys. Rev. L&r., 66 (1991) 3261. C. Coulombeau, H. Jobic, P. Bemier, C. Fabre, D. Schutz and A. Rassat, J. Phys. Chem., 96 (1992) 22. K. Prassides, T.J.S. Dennis, J.P. Hare, J. Tompkinson, H.W. Kroto, R. Taylor and D.R.M. Walton, Chem. Phys. Lerr., 187 (1991) 455. L. Pintschovius, B. Renker, F. Gompf, R. Heid, S.L. Chaplot, M. Haluska and H. Kuzmany, Phys. Rev. Mr., 69 (1992) 2662. T.P. Martin, U. Naher, H. Schaber and U. Zimmerman, Phys. Rev. Lerr., 70 (1993) 3079. S.J. Duclos, K. Brister, R.C. Haddon, A.R. Kortan and F.A. Thiel, Narure, 351 (1991) 380. J.E. Fischer, P.A. Heiney, A.R. McGhie, W.J. Romanow, A.M. Denenstein, J.P. McCauley Jr. and A.B. Smith III, Science, 252 (1991) 1288. D.L. Lichtenberger, K.W. Nebesney, CD. Ray, D.R. Huffman and L.D. Lamb, Chem. Phys. Lerr., I76 (1991) 203. Y. Wang, D. Tomanek and G.F. Bertsch, Phys. Rev., B44 (1991) 6562. L.A. Girifalco, J. Phys. Chem.. 98 (1992) 858. A. Cheng and M.L. Klein, J. Phys. Chem., 95 (1991) 6750. Y. Guo, N. Karasawa and W.A. Goddard III, Nature. 351 (1991) 464. A. Cheng and M.L.Klein, Phys. Rev., B45 (1992) 1889. M. Sprik, A. Cheng and M.L. Klein, J. Phys. Chem., 96 (1992) 2027. J.P. Lu, X.-P. Li and R.M. Martin, Phys. Rev. L.&r., 68 (1992) 1551. S.L. Chaplot (personal communication). A. Chakrabarti, S. Yashonath and C.N.R. Rao, Chem. Phys. Lerr.. 215 (1993) 519. K.H. Michel, Chem. Phys. L.&r., 193 (1992) 478. R. Zallen, The Physics of Amorphous Solids, Wiley-Interscience, New York (1983). M. Sprik, A. Cheng and M.L. Klein, Phys. Rev. Lea, 69 (1992) 1660. G.B.M. Vaughan, P.A. Heiney, J.E. Fischer, D.E. Luzzi, D.A. Ricketts-Foot, A.R. McGhie, Y.-W. Hui, A.L. Smith, D.E. Cox, W.J. Romanow, B.H. Allen, N.Coustel, J. P.McCauley Jr. and A.B. Smith III, Science, 254 (1991) 1350. M.A. Verheijen, H. Meekes, G. Meijer, P. Bennema, J.L. deBoer, S. van Smaalen, G. van Tendeloo, S. Amelinckx, S. Muto and J. Landuyt, Chem. Phys.. 166 (1992) 287. V. Varma, R. Seshadri, A. Govindaraj, A.K. Sood and C.N.R. Rao, Chem. Phys. Len.. 203 (1993) 545. N. Chandrabhas, K. Jayaraman, D.V.S. Muthu, A.K. Sood, R. Seshadri and C.N.R. Rao, Phys. Rev. B47 (1993) 10963. P.H.M. van Loosdrecht, M.A. Verheijen, H. Meekes, P.J.M. van Bentum and G. Meijer, Phys. Rev., 847 (1993) 7610. C. Christides, I.M. Thomas, T.J.S. Dennis and K. Prassides, Europhys. L-err.,22 (1993) 545. E. Grivei, B. Nysten, M. Cassart, J.-P. Issi, C. Fabre and A. Rassat, Phys. Rev., 847 (1993) 1705. J. Sworakowski, K. Palewska and M. Bertault, Chem. Phys. L-err.,220 (1994) 197. S.K. Ramasesha, A.K. Singh, R. Seshadri, A.K. Sood and C.N.R. Rao, Chem. Phys. Len.. 220 (1994) 203. A.R. McGhie, J.E. Fischer, P.A. Heiney, P.W. Stephens. R.L. Cappelletti, D.A. Neumann, W.H. Mueller, H. Moln and H.- U. ter Meer, Phys. Rev., B49 (1994) 12614. K. Prassides, T.J.S. Dennis, C. Christides, E. Roduner, H.W. Kroto, R. Taylor and D.R.M. Walton, J. Phys. Chem., 96 ( 1992) 10600.

C.N.R. Rao et al. / Fullerenes,

nanotubes,

onions and related carbon structures

[ 1471 V.P. Dravid, X. Lin, H. Zhang, S. Liu and M.M. Kappes, J. Muter. Rex, 7 ( 1992) 2440. [ 1481 C. Meingast, F. Gugenberger, M. Haluska, H. Kuzmay and G. Roth, Appl. Phys., A56 (1993) 227. 11491 G.A. Samara, J.E. Schirber, B. Morosin, L.V. Hansen, D. Loy and A.P. Sylwester, Phys. Rev. Lerr., 67 (1991) 3136. 11501 N. Chandrabhas, M.N. Shashikala, D.V.S. Muthu, A.K. Sood and C.N.R. Rao, Chem. Phys. Lerr., 197 (1992) 319. 115 11 H. Kawamura, M. Kobayashi, Y. Akahama, H. Shinohara, H. Sato and Y. Saito, Solid Stare Commun., 83 (1992) 563. [152] H. Kawamura, Y. Akahama, M. Kobayashi, H. Shinohara, H. Sato, Y. Saito, T. Kiegawa, 0. Shimomura and K. Aoki, J. Phys. Chem. Solids, 54 ( 1993) 1675. [ 1531 F. Moshary, N.H. Chen, I.H. Silvera, C.A. Brown, H.C. Dom, M.S. deVries and D.S. Bethune, Phys. Rev. Len., 69 (1992) 466. [ 1541 C.S. Yoo and W.J. Nellis, Chem. Phys. Left., 198 (1992) 379. [ 1551 N. Chandrabhas, A.K. Sood, D.V.S. Muthu, C.S. Sundar, A. Bharathi, Y. Hariharan and C.N.R. Rao, Phys. Rev. Lerr., 73 (1994) 3411. [ 1561 A.K. Sood, N. Chandrabhas, D.V.S. Muthu, Y. Hariharan, A. Bharathi and C.S. Sundar, Philos. Mug., 870 (1994) 347. [ 1571 M. Nunez-Reguero, P. Monceau, A. Rassat, P. Bemier and A. Zahab, Nature, 354 (1991) 289. [ 1581 Y. Ma, G. Zou, H. Yang and J. Meng, Appl. Phys. Z&f., 65 ( 1994) 822. [ 1591 E. Sandre and F. Cyrot-Lackmann, Solid Stare Commun., 90 ( 1994) 431. [ 1601 R. Roy, H.S. Dewan and P. Ravindranathan, Mater. Res. Bull., 28 (1993) 861. [ 1611 A.K. Sood, N. Chandrabhas, A. Jayaraman, N. Kumar, D.V.S. Muthu, H.R. Krishnamurthy, T. Pradeep and C.N.R. Rao, Solid Stare Commun., 81 (1992) 89. [ 1621 J.H. Weaver and D.M. Poirier, in H. Ehrenreich and F. Spaepen (eds.), SolidSrare Physics, Vol. 48, 1994, p. 1. [ 1631 W.E. Pickett, in H. Ehrenreich and F. Spaepen (eds.), SolidSrate Physics, Vol. 48, 1994, p. 226. [ 1641 C.M. Lieber and Z. Zhang, in H. Ehrenreich and F. Spaepen (eds.), SolidSrate Physics, Vol. 48, 1994, p. 349. [ 1651 R.C. Haddon, A.F. Hebard, M.J. Rosseinsky, D.W. Murphy, S.J. Duclos, K.B. Lyons, B. Miller, J.M. Rosamilia, R.M. Fleming, A.R. Kortan, S.H. Glarum, A.V. Makhija, A.J. Muller, R.H. Eick, S.M. Zahurak, R. Tycko, G. Dabbagh and F.A. Thiel, Nature, 350 (1991) 320. [ 1661 A.F. Hebard, M.J. Rosseinsky, R.C. Haddon, D.W. Murphy, S.H. Glarum, T.T.M. Palstra, A.P. Ramirez and A.R. Kortan, Nature, 350 (1991) 600. [ 1671 H.H. Wang, A.M. Kini, B.M. Savall, K.D. Carlson, J.M. Williams, K.R. Lykke, D.H. Parker, M.J. Pellin, D.M. Gruen, U. Welp, W.K. Kwok, S. Fleshter and G.W. Crabtree, Inorg. Chem., 30 (1991) 2838. [ 1681 M. Tokumoto, Y. Tanaka, N. Kinoshita, T. Kinoshita, S. Ishibashi and S. Ihara, J. Phys. Chem. Solids, 54 ( 1993) 1667. [ 1691 S.P. Kelty, C.-C. Chen and C.M. Lieber, Nurure, 352 (1991) 223. [ 1701 C-C. Chen, S.P. Kelty and CM. Lieber, Science, 253 (1991) 886. [ 1711 M.J. Rosseinsky, D.W. Murphy, R.M. Fleming, R. Tycko, A.P. Ramirez, T. Siegrist, G. Dabbagh and S.E. Barret, Narure, 356 (1992) 416. [ 1721 J.P. McCauley Jr., Q. Zhu, N. Coustel, 0. Zhou, G.B.M. Vaughan, S.H. Idziak, J.E. Fischer, SW. Tozer, D.M. Groski, N. Bykovetz, CL. Liu, A.R. McGhie, B.H. Allen, W.J. Romanow, A. Denenstein and A.B. Smith III, J. Am. Chem. Sot., I13 ( 1991) 8537. [ 1731 D.M. Poirier, Appl. Phys. Left., 64 (1994) 1356. [ 1741 J.H. Weaver, J. Phys. Chem. Solids, 53 (1992) 1433. [ 1751 K. Holczer, 0. Klein, S.M. Huang, R.B. Kaner, K.-J. Fu, R.L. Whetten and F. Diederich, Science, 252 (1991) 1154. [ 1761 S.J. Duclos, R.C. Haddon, S.H. Glarum, A.F. Hebard and K.B. Lyons, Science, 254 (1991) 1625. 11771 M.J. Rosseinsky, A.P. Ramirez, S.H. Glarum, D.W. Murphy, R.C. Haddon, A.F. Hebard, T.T.M. Palstra, A.R. Kortan, SM. Zahurak and A.V. Makhija, Phys. Rev. Let?., 66 (1991) 2830. [ 1781 R. Tycko, G. Dabbagh, M.J. Rosseinsky, D.W. Murphy, R.M. Fleming, A.P. Ramirez and J.C. Tully, Science, 253 (1991) 884. [ 1791 G.P. Kochanski, A.F. Hebard, R.C. Haddon and A.T. Fiory, Science, 255 ( 1992) 184. [ 1801 0. Zhou, R.M. Fleming, D.W. Murphy, M.J. Rosseinsky, A.P. Ramirez, R.B. van Dover and R.C. Haddon, Nature, 362 ( 1993) 433. [ 1811 M.J. Rosseinsky, D.W. Murphy, R.M. Fleming, R. Tycko, A.P. Ramirez, T. Siegrist, G. Dabbagh and S.E. Barrett, Nature, 356 (1992) 416. [ 1821 P.W. Stephens, L. Mihaly, P. Lee, R.L. Whetten, S.-M. Huang, R.B. Kaner, F. Diederich and K. Holczer, Nature, 352 (1991) 632. [ 1831 D.W. Murphy, M.J. Rosseinsky, R.M. Fleming, R. Tycko, A.P. Ramirez, R.C. Haddon, T. Siegrist, G. Dabbagh, J.C. Tully and R.E. Walstedt, J. Phys. Chem. Solids, 53 ( 1992) 1321. [ 1841 Y. Maniwa, K. Mizoguchi, K. Kume, K. Tanigaki, T.W. Ebbesen, S. Saito, J. Mizuki, J.S. Tsai and Y. Kubo, Solid Srare Commun., 82 (1992) 783. [ 1851 R.E. Walstedt, D.W. Murphy and M.J. Rosseinsky, Nature, 362 (1993) 611. [ 1861 S.E. Barrett and R. Tycko, Phys. Rev. Let?., 69 (1992) 3754. [ 1871 R.M. Fleming, M.J. Rosseinsky, A.P. Ramirez, D.W. Murphy, J.C. Tully, R.C. Haddon, T. Siegrist, R. Tycko, S.H. Glarum, P. Marsh, G. Dabbagh, SM. Zahurak, A.V. Makhija and C. Hampton, Nature, 352 (1991) 701. [ 1881 0. Zhou, J.E. Fischer, N. Coustel, S. Kycia, Q. Zhu. A.R. McGhie, W.J. Romanow, J.P. McCauley Jr., A.B. Smith III and D.E. Cox, Nature, 351 (1991) 462. 11891 T. Yildirim, 0. Zhou, J.E. Fischer, N. Bykovetz, R.M. Strongin, M.A. Cichy, A.B. Smith III, C.L. Lin and R. Jelinek, Nature, 360 (1992) 568. 11901 0. Chauvet, G. Oszlanyi, L. Forro, P.W. Stephens, M. Tegze, G. Faigel and A. Janossy, Phys. Rev. L&r., 72 (1994) 2721. [ 191 I S. Pekker, A. Janossy, L. Mihaly, 0. Chauvet, M. Carrard and L. Forro. Science, 265 ( 1994) 1077.

251

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

258

[ 1921 [ 1931 [ 1941 [ 1951 [ 1961 [ 1971 [ 1981 [ 1991 [200]

Y. Maruyama, T. Inabe, H. Ogata, Y. Achiba, S. Suzuki, K. Kikuchi and I. Ikemoto, Chem. L&t., ( 1991) 1849. X.-D. Xiang, J.G. Hou, G. Briceno, W.A. Vareka, R. Mostovoy, A. Zettl. V.H. Crespi and M.L. Cohen, Science 256 (1992) 1190. V.H. Crespi, J.G. Hou, X.-D. Xiang, M.L. Cohen and A. Zettl, Phys. Rev., B46 (1992) 12064. X.-D. Xiang, J.G. Hou, V.H. Crespi, A. Zettl and M.L. Cohen, Nature, 361 (1993) 54. L.G. Aslamasov and A.I. Larkin, Phys. Let?., A26 ( 1968) 238. K. Maki and R.S. Thompson, Phys. Rev., B39 (1989) 2767. W.A. Vareka and A. Zettl, Phys. Rev. L.&t., 72 (1994) 4121. K. Holczer, 0. Klein, G. Gruner, J.D. Thompson, F. Diederich and R.L. Whetten, Phys. Rev. Lett., 67 ( 1991) 271. Y.J. Uemura, A. Keren, L.P. Le, G.M. Luke, B.J. Stemleib, W.D. Wu, J.H. Brewer, R.L. Whetten, S.M. Huang, S. Lin, R.B. Kaner, F. Diederich, S. Donovan, G. Gruner and K. Holczer, Nature, 352 (1991) 605. [201] S.G. Louie and E.L. Shirley, J. Phys. Chem. Solids, 54 (1993) 1767. [202] Z. Zhang, C.-C. Chen, S.P. Kelty, H. Dai and CM. Lieber, Nature, 353 (1991) 333. 12031 Z. Zhang, C-C. Chen and CM. Lieber, Science, 254 (1991) 1619. [204] L.D. Rotter, Z. Schlesinger, J.P. McCauley Jr., N. Coustel, J.E. Fischer and A.B. Smith III, Nature 355 (1992) 532. [205] L. Deglorgi, P. Wachter, G. Gruner, S.-M. Huang, J. Wiley and R.B. Kaner, Phys. Rev. Len., 69 (1992) 2987. [206] R. Tycko, G. Dabbagh, M.J. Rosseinsky, D.W. Murphy, A.P. Ramirez and R.M. Fleming, Phys. Rev. Lett., 68 ( 1992) 1912. 12071 R. Tycko, J. Phys. Chem. Soli&.. 54 (1993) 1713. [208] A.R. Kortan, N. Kopylov, S. Glamm, E.M. Gyorgy, A.P. Ramirez, R.M. Fleming, F.A. Thiel and R.C. Haddon, Nature, 355 ( 1992) 529. [209] A.R. Kortan, N. Kopylov, S. Glarum, E.M. Gyorgy, A.P. Ramirez, R.M. Fleming, 0. Zhou, F.A. Thiel, P.L. Trevor and R.C. Haddon, Nature, 360 (1992) 566. [210] S. Saito and A. Oshiyama, J. Phys. Chem. Solids, 54 (1993) 1759. [21 l] G.K. Wertheim, D.N.E. Buchanan and J.E. Rowe, Science, 258 (1992) 1638. [212] R.C. Haddon, G.P. Kochanski, A.F. Hebard, A.T. Fiory and R.C. Morris, Science, 258 (1992) 1636. [213] T.W. Ebbesen, J.-S. Tsai, K. Tanigaki, H. Hiura, Y. Shimakawa, Y. Kubo, T. Hirosawa and J. Mizuki, Physica, C203 (1992) 163. [214] K. Prassides, J. Tomkinson, C. Christides, M.J. Rosseinsky, D.W. Murphy and R.C. Haddon, Nature, 354 (1991) 462. [215] C-C. Chen and C.M. Lieber, Science, 259 (1993) 655. [216] CM. Varma, J. Zaanen and K. Raghavachari, Science, 254 (19910 989. [217] R.W. Lof, M.A. van Veenendal, B. Koopmans, H.T. Jot&man and G.A. Sawatzky, Phys. Rev. Lett.. 68 (1992) 3924. [218] P.M. Allemand, K.C. Khemani, A. Koch, F. Wudl, K. Holczer, S. Donovan, G. Gmner and I.D. Thompson, Science, 253 (1991) 301. [219] K. Tanaka, A.A. Zakhidov, K. Yoshizawa, K. Okahara, T. Yamabe, K. Yakashi, K. Kikuchi, S. Suzuki, I. Ikemoto and Y. Achiba, Phys. Let?., Al64 (1992) 221. [220] R. Seshadri, A. Rastogi, S.V. Bhat, S. Ramasesha and C.N.R. Rao, Solid State Commun., 85 (1993) 971. 12211 D.V.S. Muthu, M.N. Shashikala, A.K. Sood, R. Seshadri and C.N.R. Rao, Chem. Phys. Lett., 217 ( 1994) 146. [222] P.W. Stephens, D.E. Cox, J.M. Lauher, L. Mihaly, J.B. Wiley, P.-M. Allemand, A. Hirsch, K. Holczer, Q. Li, J.D. Thompson and F. Wudl, Nature, 355 ( 1992) 33 1. [223] F. Wudl and J.D. Thompson, J. Phys. Chem. Solids, 53 (1992) 1449. [224] K. Yamaguchi, S. Hayashi, M. Okumura, M. Nakano and W. Mori, Chem. Phys. Let?.. 226 (1994) 372. [225] A. Schilder, H. Klos, I. Rystan, W. Schutz and B. Gotschy, Phys. Rev. L&t., 73 (1994) 1299. [226] Y. Li, D. Zhang, F. Bai, D. Zhu, B. Yin. J. Li and Z. Zhao, Solid State Cormnun., 86 ( 1993) 475. [227] A.K. Santra, R. Seshadti, A. Govindaraj, V. Vijayakrishnan and C.N.R. Rao, Solid State Commun., 85 (1993) 77. [228] R. Seshadri, V. Vijayakrishnan, A.K. Santra, A. Govindaraj and C.N.R. Rao, Fullerene Sci. Technol., 1 (1993) 75. [229] T.R. Ohno, Y. Chen, S.E. Harvey, G.H. Kroll, J.H. Weaver, R.E. Haufler and R.E. Smalley, Phys. Rev., 844 (1991) 13747. [230] Y.Z. Li, M. Chander, J.C. Patrin, J.H. Weaver, L.P.F. Chibante and R.E. Smalley, Phys. Rev.. 845 (1992) 13837. [231] Y.Z. Li, J.C. Pa&in, M. Chander, J.H. Weaver, L.P.F. Chibante and R.E.Smalley, Science, 252 (1991) 547. [232] Y.Z. Li, M. Chander, J.C. Patrin, J.H. Weaver, L.P.F. Chibante and R.E. Smalley, Science, 253 (1992) 429. [233] J.H. Weaver, Act. Chem. Res., 25 (1992) 143. [234] X.D. Wang, T. Hashizume and T. Sakurai, Mod. Phys. Lett., 8 (1994) 1397. [235] G. Gensterblum, L.-M. Yu, J.J. Pireaux, P.A. Thiry, R. Caudano, Ph. Lambin, A.A. Lucas, W. Kratschmer and J.E. Fisher, J. Phys. Chem. Solids, 53 (1992) 1427. [236] A.A. Lucas, J. Phys. Chem. Solids, 53 (1992) 1415. [237] H. Hong, W.E. McMabon, P. Zschack, D.S. Liu, R.D. Aburano, H. Chen and T.C. Chiang, Appl. Phys. Lett., 61 (1992) 3127. [238] M. Balooch and A.V. Hamza, Appl. Phys. Z&t., 63 (1993) 150. [239] H. Xu, D.M. Chen and W.N. Creager, Phys. Rev. L&t., 70 ( 1993) 1850. 12401 S. Maruno, K. Inanaga and T. Tsu, Appl. Phys. Lett.. 63 (1993) 1339. [241] S. Folsch, T. Maruno. A. Yamashita and T. Hayashi. Appl. Phys. Left., 62 (1993) 2643. [242] T. Fujita, S. Kuroshima, T. Satoh, J.S. Tsai, T.W. Ebbesen and K. Tanigaki, Appl. Phys. Lett.. 63 (1993) 1008. [243] H.N. Aiyer, A. Govindaraj and C.N.R. Rao, Bull. Mater. Sci., 17 (1994) 563; Bull. Mater. Sci., to be. published. [244] K. Tanigaki, S. Kuroshima, J.-I. Fukuda and T.W. Ebbesen, Appl. Phys. Let?., 63 (1993) 2351.

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

[245] T. Hashizume,

K. Motai, X.D. Wang, H. Shinohara, Y. Saito, Y. Maruyama, K. Ohno, Y. Kawazoe, Y. Nishina, H.W. Pickering, Y. Kuk and T. Sakurai, Phys. Rev. Let?., 71 (1993) 2959. [246] Y. Kuk, D.K. Kim, Y.D. Suh, K.H. Park, H.P. Noh, S. Joh and SK. Kim, Phys. Rev. L&r., 70 ( 1993) 1948. [247] J.K. Gimzewski, S. Modesti and R.R. Schittler, Phys. Rev. Left.. 72 (1994) 1036. [248] S. Modesti, S. Cerasari and P. Rudolf, Phys. Rev. L&r., 71 (1993) 2469. [249] D. Sarkar and N.J. Halas, Appl. Phys. L&r., 63 (1993) 2438. [250] J.B. Camp and R. Schwarz, Appl. Phys. Lat., 63 (1994) 455. [251] B. Bhushan, B.K. Gupta, G.W. van Cleef, C. Capp and J.V. Coe, Appl. Phys. Z.&f., 62 (1993) 3253. [252] R.S. Bhattacharya, A.K. Rai, J.S. Zahinski and N.T. McDevitt, J. Mater. Res., 9 (1994) 1615. [253] M. Tachibana, M. Michiyama, K. Kikuchi, Y. Achiba and K. Kojima, Phys. Rev., 849 (1994) 14945. [254] T. Thundat, R.J. Warmak, D. Ding and R.N. Compton, Appl. Phys Serf. ,63 (1993) 891. 12551 T. Lill, F. Lather, H.-G. Busman and IV. Hertel, Phys. Rev. Left.. 71 (1993) 3283. [256] A.M. Rao,P.Zhou, K.-A. Wang, G.T. Hager, J.M. Holden,Y. Wang, W.-T.Lee, X.-X.Bi,P.C.Ecklund,D.S.Comett,M.A.Duncan and I.J. Amster, Science, 259 (1993) 955. [257] P. Zhou, A.M. Rao, K.-A. Wang, J.D. Robertson, C. Eloi, M.S. Meier, S.L. Rin, X.-X. Bi, P.C. Ecklund and M.S. Dresselhaus, Appl. Phys. Lerr., 60 (1992) 2871. [258] W.S. Bacsa and J.S. Lannin, Phys. Rev., 849 (1994) 14750. [259] Y.B. Zhao, D.M. Poitier, R.J. Pechman and J.H. Weaver,Appl. Phys. Left., 64 (1994) 577. [260] M. Menon and K.R. Subbaswamy, Phys. Rev., B49 (1994) 13966. [261] L.W. Tutt and A. Kost, Narure, 346 (1992) 225. [262] L.W. Tun and T.F. Bogess, Progr. Quanr. Electr., 17 (1993) 299. [263] X.K. Wang, T.G. Zhang, W.P. Lin, S.J. Liu, G.K. Wong, M.M. Kappes, R.P.H. Chang and J.B. Ketterson, Appl. Phys. Left.. 60 (1992) 8110. [264] F. Kazjar, C. Taliani, R. Danielli, S. Rossini and R. Zamboni, Phys. Rev. I.&r, 73 (1992) 1617. [265] Y. Wang, A’arure,356 ( 1992) 585. [266] S.B. Fleischer, E.P. Ippen, G. Dresselhaus, M.S. Dresselhaus, A.M. Rao, P. Zhou and P.C. Ecklund, Appl. Phys. Len.. 62 (1993) 3241. [ 2671 W. Guss, J. Fedman, E.O. Gobel, C. Taliani, H. Mohn, W. Muller, P. Haussler and H.-U. ter Meer, Phys. Rev. L&r., 72 ( 1994) 2644. [268] M.K. Nissen, S.M. Wilson and M.L.W. Thewalt, Phys. Rev. Lerr., 69 (1992) 2423. [269] J.D. Kein, A. Yen, R.D. Rauh and S.L. Clauson, Appl. Phys. La., 63 (1993) 599. [270] S.L. Ren, K.-A. Wang, P. Zhou, Y. Wang, A.M. Rao, M.S. Meier, J.P. Selegue and P.C. Ecklund,Appl. Phy.s. Left., 61 (1992) 124. [271] A. Hamed, H. Rasmussen and P.H. Hor,Appl. Phys. Len., 64 (1994) 526. [272] K. Harigaya and S. Abe, Phys. Rev., 849 ( 1994) 16746. [273] S.M. Sileace, C.A. Walsh, J.C. Scott and W.E. Moerner, Appl. Phys. Let?., 61 (2992) 2967. [274] Y.N. Han, W.J. Zhang, X.M. Gao, Y.B. Cui, Y.X. Xia, Cl. Gu, WC. Zhang, P. Yang, X.W. Du and D. Feng, Appl. Phys. Lat., 63 (1993) 447. [275] H. Yonehara and C. Pat, Appl. Phys. Lerr., 61 (1992) 575. [276] J. Mart, M. Machonkin, R. Ziolo, D.R. Huffman and M.I. Fergusen, Appl. Phys. Len., 60 (1993) 1735. 12771 N.S. Sari&i, D. Braun, C. Zhang, V.I. Srdanov, A.J. Heeger, G. Stucky and F. Wudl, Appl. Phys. Lat., 62 (1993) 585. [278] C.B. Eom, A.F. Hebard, L.E. Trimble, G.K. Celler and R.C. Haddon, Science, 259 (1993) 1887. [279] R. Meilunas, R.P.H. Chang, S. Liu and M.M. Kappes, Narure, 354 (1991) 271. [280] R. Meilunas and R.P.H. Chang, J. Mater. Rex, 9 (1994) 61. [281] D.M. Gruen, S. Liu, A.R. Krauss, J. Luo and X. Pan, Appl. Phys. L&r., 64 (1994) 1504. I2821 C.P. Chen, S. Mehta, L.P. Fu, A. Petrov, F.M. Gasparini and A. Hebard, Phys. Rev. Left., 71 (1993) 739. [283] R.M. Fleming, A.R. Kottan, B. Hessen, T. Siegrist, F.A. Thiel, P. Marsh, R.C. Haddon, R. Tycko, G. Dabbagh, M.L. Kaplan and A. Mujsce, Phys. Rev, E44 (1991) 888. [284] U. Geiser, S.K. Kumar, B.M. Savall, S.S. Harried, K.D. Carlson, P.R. Mobley, H.H. Wang, J.M. Williams, R.E. Botto and M.H. Whangbo, Chem. Mater. (4 (1992) 1077. [285] M.F. Meidine, P.B. Hitchcock, H.W. Kroto, R. Taylor and D.R.M. Walton, J. Chem. Sot. Chem. Commun., (1992) 1534. [286] A.L. Balch, J.W. Lee, B.C. No11 and M.M. Olmstead, J. Chem. Sot. Chem. Commun., (1993) 56. [287] J.D. Crane, P.B. Hitchcock, H.W. Kroto, R. Taylor and D.R.M. Walton, J. Chem. Sot. Chem. Commun., (1992) 1764. [288] A. Izuoka, T. Tachkawa, T. Sugawara, Y. Suzuki, M. Konno, Y. Saito and H. Shinohara, J. Chem. Sot. Chem. Commun., ( 1992). [289] Q. Zhu, D.E. Cox, J.E. Fischer, K. Kniaz, A.R. McGhie and 0. Zhou, Nature. 355 (1992) 712. [2901 R.E. Douthwaite, M.L.H. Green, S.J. Hayer, M.J. Rosseinsky and J.F.C. Turner, J. Chem. Sot. Chem. Commun., (1994) 1367. 12911 0. Ermer, Helv. Chim. Acra., 71 (1994) 1339. [292] 0. Errner and C. Robke, J. Am. Chem. SIX., I15 (1993) 22. [2931 H.H. Wang, J.A. Schlueter, A.C. Cooper, J.L. Smart, M.E. Whetten, U. Geiser, K.D. Carlson, J.M. Williams, U. Welp, J.D. Dudek and M.A. Caleca, J. Phys. Chem. Solids, 54 (1993) 1655. [2941 A. Gugel, K. Mullen, H. Reichert, W. Schmidt, G. Scott, F. Schuth, J. Spickerman, J. Titman and K. Unger, Angew. Chem. tnr. Ed. Engl., 32 (1993) 556.

259

260

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

[2951 M.W. Anderson, J. Shi, D.A. Leigh, A.E. Moody, F.A. Wade, B. Hamilton and S.W. Carr, J. Chem. Sot. Chem. Commun., ( 1993) 533. [2961 T. Atake, T. Tanaka, H. Kawaji, K. Kikuchi, K. Saito, S. Sujuki, Y. Achiba and I. Ikemoto, Chem. Phys. Lerr., 196 (1992) 321. [2971 K. Kamams, V.G. Hadjiev, C. Thomsen, S. Pekker, K. Foder-Csorba, Cl. Faigel and M. Tegze, Chem. Phys. Lerr., 203 (1993) 325. 12981 P. Bowmar, M. Kurmoo, M.A. Green, F.L. Pratt, W. Hayes, P. Day and K. Kikuchi, J. Phys. Condens. Mart., 5 ( 1993) 2739. [299] N.D. Kusch, T. Majchrzak, W. Ciesielski and A. Graja, Chem. Phys. Letr., 215 (1993) 137. [300] Y. Nagano, T. Tamura and T. Kiyobayashi, Chem. Phys. L&t., 228 ( 1994) 125. [301] A. Agafanov, R. Ceolin, D. Andre, J. deBruijn, A. Gonthier-Vassal. H. Szwarc, N. Rodier, J. Dugne, S. Toscani, P.-Y. Sizaret, C. Fabre, V. Gregny and A. Rassat, Chem. Phys. Len., 208 (1993) 68. 13021 G.B.M. Vaughan, P.A. Heiney, D.E. Cox, A.R. McGhie, D.R. Jones, R.M. Strongin, M.A. Cichy and A.B. Smith III, Chem. Phys., 168 (1992) 185. [303] A.N. Lommen, P.A. Heiney, G.B.M. Vaughan, P.W. Stephens, D. Liu, A.L. Smith, A.R. McGhie, R.M. Strongin, L. Brard and A.B. Smith III, Phys. Rev., 849 (1994) 12572. [304] R. Ettl, I. Chao, F. Diederich and R. L. Whetten, Nature. 353 (1991) 149. [305] D.E. Manolopoulos and P.W. Fowler, Chem. Phys. Letf., I87 (1991) 1. [306] J.M. Hawkins and A. Meyer, Science, 260 (1993) 1918. [307] Q. Li, F. Wudl, C. Thilgen, R.L. Whetten and F. Diederich, J. Am. Chem. Sot., 114 ( 1992) 3994. [308] A.J. Stoneand D.J. Wales, Chem. Phys. Left., 128 (1986) 501. I3091 K. Kikuchi, N. Nakahara. T. Wakabayashi, S. Suzuki, H. Shiromaru, Y. Miyake, K. Saito, I. Ikemoto, M. Kainoshi and Y. Achiba, Nature, 357 ( 1992). 13101 S. Hino, K. Matsumoto, S. Hasegawa, K. Kamiya, H. Inokuchi, T. Morikawa, T. Takahashi, K. Seki, K. Kikuchi, S. Suzuki, I. Ikemoto and Y. Achiba, Chem. Phys. Len., 190 (1992) 169. 1311 I Y.Z. Li, J.C. Patrin, M. Chander, J.H. Weaver, K. Kikuchi and Y. Achiba, Phys. Rev., B47 (1993) 10867. [312] J.M. Hawkins, M. Nambu and A. Meyer, J. Am. Chem. Sot., I16 ( 1994) 7642. [313] T.W. Ebbesen and P.M. Ajayan, Nature, 358 (1992) 220. [314] P.M. Ajayan, T. Ichihashi and S. Iijima, Chem. Phys. Len., 202 (1993) 384. [315] S. Iijima, P.M. Ajayan and T. Iichihashi, Phys. Rev. L-err., 69 (1992) 3100. [316] V.P. Dravid, X. Lin, Y. Wang, X.K. Wang, A. Yee, J.B. Ketterson and R.P.H. Chang, Science, 259 (1993) 1601. [317] S. Iijima, MRS Bulk&, XIX, November 1994.43. [ 3181 R. Tenne, L. Margulis, M. Genut and G. Hodes, Nature, 360 ( 1992) 444. [ 3191 L. Margulis, G. Salitra, R. Tenne and M. Tallanker, Nature, 365 (1993) 113. [320] M.R. Ghadiri, J.R. Granja, R.A. Milligan, D.E. McRee and N. Khazanovich, Nature, 366 (1994) 324. [321] A. Oberlin, M. Endo and T. Koyama, J. Cryst. Growth, 35 (1976) 335. 13221 A. Oberlin, in P.A. Thrower (ed.), Chemistry und Physics of Carbon, Marcel Dekker, New York, 1988, Vol. 22, pp. 1-143. [3231 M. Endo, K. Takeuchi, S. Igarashi, K. Kobori, M. Shiiaishi and H.W. Kroto, J. Phys. Chem. Solids, 54 (1993) 1841. [324] N. Hatta and K. Murata, Chem. Phys. Left., 217 (1994) 398. [325] D.T. Colbert, J. Zhang, S.M. McClure, P. Nikolaev, Z. Chen, J.H. Hafner, D.W. Owens, P.G. Kotula, C.B. Carter, J.H. Weaver and R.E. Smalley, Science, 266 (1994) 1218. [326] M. Jose-Yacaman, M. Miki-Yoshida, L. Rendon and T.G. Santiesteban, Appl. Phys. Lett., 62 (1993) 202. [3271 V. Ivanov, J.B. Nagy, Ph. Lambin, A.A. Lucas, X.B. Zhang, X.F. Zhang, B. Bemaerts, G. van Tendeloo, S. Amelinckx and J. van Landuyt, Chem. Phys. Len., 223 (1994) 3129. 13281 M. Ge and K. Sattler, Science, 260 (1993) 515. [329] T.W. Ebbesen, H. Hiura, J. Fujita, Y. Ochiai, S. Matsui and K. Tanigaki, Chem. Phys. Left, 209 (1993) 83. [330] Y. Ando, Jpn. J. Appl. Phys. Left., 32 (1993) L1342. 1331 I R. Seshadri, A. Govindaraj, H.N. Ayer, R. Sen, G.N. Subbanna, A.R. Raju and C.N.R. Rao, Curr. Sci. (India), 66 ( 1994) 839. [332] X.K. Wang, X.W. Lin, V.P. Dravid and J.B. Ketterson, Appl.Phys. Len. 62 (1993) 1881. [333] Z. Zhang and CM. Lieber, Appl. Phys. Left, 62 (1993) 2792. [334] C.H. Olk and J.P. Hereman, J. Mater. Res., 9 ( 1994) 259. [335] R. Seshadri, H.N. Aiyer, A. Govindaraj and C.N.R. Rao, Solid Stare Commun., 91 (1994) 195. [336] H. Hiura, T.W. Ebbesen, J. Fujita, K. Tanigaki and T. Takada, Nafure, 367 (1994) 148. [337] Y. Murakami, T. Shibata, K. Okuyama, T. Arai, H. Suematsu and Y. Yoshida, J. Phys. Chem. Solids, 54 (1993) 1861. [338] B.E. Warren, Phys. Rev., 59 (1941) 693. [339] W. Ruland, in P.L. Walker (ed.), Chemistry and Physics of Carbon, Marcel-Dekker, New York, 1968, Vol. 4, pp. l-84. [340] R.A. Jishi, L. Venkataraman, MS. Dresselhaus and G. Dresselhaus, Chem. Phys. Len., 209 ( 1993) 77. [341] H. Hiura, T.W. Ebbesen, K. Tanigaki and H. Takahashi, Chem. Phys. Let?., 202 (1993) 509. [342] J.M. Holden, P. Zhou, X.-X. Bi, P.C. Ecklund, S. Bandow, R.A. Jishi, K.D. Chowdhury, G. Dresselhaus and M.S. Dresselhaus, Chem. Phys. L.&t., 220 (1994) 186. [343] M. Endo and H.W. Kroto, J. Phys. Chem., 96 (1992) 6941. [ 3441 R.E. Smalley, Mater. Sci. Eng., B19 ( 1993) 1. [345] T.W. Ebbesen, J. Tabuchi and K. Tanigaki, Chem. Phys. Lett., 291 (1992) 336. [346] Y. Saito, T. Yoshikawa, M. Inagaki, M. Tomita and T. Hayashi, Chem. Phys. Len., 204 (1994) 277.

C.N.R. Rao et al. / Fullerenes, nanotubes, onions and related carbon structures

[347] [348] [349] [350] [351] [352] [353] [354] [355] [356] [357] [358] [359] [360] [361] [362] [363] [364] [365] [366] [367] [368] [369] [370] [371] [372] [ 3731 [374] [375] [376] [377] [378] [379] [380] [381 I [382] [383] [384] 13851 [386] [387] [388] [389] [390] [391 I [392] [393] [394] [395] [396] [397] [3981 [399]

A. Maiti, C.J. Brabec and J. Bernholc (preprint). D.H. Robertson, D.W. Brenner and C.T. White, J. Phys. Chem., 96 (1992) 6133. J.W. Mintmire, B.I. Dunlap and C.T. White, Phys. Rev. Left, 68 (1992) 631. N. Hamada, S. Sawada and A. Oshiyama, Phys. Rev. Len., 68 (1992) 1579. C.T. White, D.H. Robertson and J.W. Mintmire, Phys. Rev., B47 (1993) 5485. R. Saito, M. Fujita, G. Dresselhaus and M.S. Dresselhaus, Appl. Phys. Lett., 60 (1992) 2204. R. Saito, M. Fujita, G. Dresselhaus and M.S. Dresselhaus, Phys. Rev., B46 (1992) 1804. K. Tanaka, K. Okahara, M. Okada and T. Yamabe, Chem. Phys. Len., 191 (1992) 469. K. Tanaka, K. Okahara, M. Okada and T. Yamabe, Fullerene Sci. Technol., I (1993) 137. D.J. Klein, W.A. Seitz and T.G. Smalz, J. Phys. Chem., 97 (1993) 1231. W.P. Halperin, Rev. Mod. Phys., 58 (1986) 533. S.N. Song, X.K. Wang, R.P.H. Chang and J.B. Ketterson, Phys. Rev. Lert., 72 (1994) 697. L. Lange& L. Stockman, J.P. Heremans, V. Bayot, C.H. Olk, C. van Haesendonck, Y. Bruinseraed and J.P. Issi, J. Muter. Res.. 9 (1994) 927. R. Heyd, A. Charlier, J.F. Mareche, E. McRae and O.V. Zharikov, Solid&are Commun.. 89 (1994) 989. D.H. Robertson, D.W. Brennerand J.W. Mintmire, Phys. Rev. 845 (1992) 12592. G. Ovemey, W. Zhong and D. Tomanek, Zeit. Phys.. 027 (1993) 93. J.-C. Charlier and J.-P. Michenaud, Phys. Rev. Lert., 70 (1993) 1858. J. Tersoff and R.S. Ruoff, Phys. Rev. L&r., 73 (1994) 676. P.M. Ajayan, 0. Stephan, C. Colliex and D. Trauth, Science, 265 (1992) 1212. L.S.K. Pang, J.D. Saxby and S.P. Chatfield, J. Phys. Chem., 97 (1993) 6941. T.W. Ebbesen, P.M. Ajayan, H. Hiura and K. Tanigaki, Nature, 367 (1994) 519. M. Kosaku, T.W. Ebbesen, H. Hiura and K. Tanigaki, Chem. Phys. L.-a, 225 (1994) 161. S.C. Tsang, P.J.F. Harris and M.L.H. Green, Nature, 362 (1992) 520. P.M. Ajayan, T.W. Ebbesen, T. Iichihashi, S. Iijima, K. Tanigaki and H. Hiura, Nature, 362 (1992) 522. P.M. Ajayan and S. Iijima, Nature, 361 (1993) 333. 0. Zhou, R.M. Fleming, D.W. Murphy, C.H. Chen, R.C. Haddon, A.P. Ramirez, S.H. Glarum, Science, 263 (1994) 1744. A.P. Ramirez, R.C. Haddon, 0. Zhou, R.M. Fleming, J. Zhang, S.M. McClure and R.E. Smalley, Science, 265 (1994) 84. J. Heremans, C. Olk and D.T. Morelli, Phys. Rev. B49 (1993) 15122. 0. Stephan, P.M. Ajayan, C. Colliex, Ph. Redlichs, J.M. Lambert, P. Bemier and P. Lefin, Science, 266 ( 1994) 1683. M.R. Pederson and J.Q. Broughton, Phys. Rev. Lerr., 69 (1992) 269. R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra and S. Subramoney, Science, 259 (1993) 346. P.M. Ajayan, C. Colliex, J.M. Lambert, P. Bemier, L. Barbedette, M. Fence and 0. Stephan, Phys. Rev. Lert., 72 ( 1994) 1722. S.C. Tsang, Y.K. Chen, P.J.F. Harris and M.L.H. Green, Nature, 372 (1994) 159. S. Iijima and T. lchihashi, Nature, 363 (1993) 603. D.S. Bethune, C.H. Kiang, M.S. deVries, G. German, R. Savoy, J. Vazquez and R. Beyers, Nature, 363 ( 1993) 605. J.M. Lambert, P.M. Ajayan, P. Bernier, J.M. Planeix, V. Brotons, B. Coq and J. Castaing, Chem. Phys. I.&., 226 ( 1994) 364. P.M. Ajayan, J.M. Lambert, P. Bernier, L. Barbedette, C. Colliex and J.M. Planeix, Chem. Phys. L&t., 215 (1993) 509. J.M. Planeix, N. Coustel, B. Coq, V. Brotons, P.S. Kumbhar, R. Dutarte, P. Geneste, P. Bemier and P.M. Ajayan, J. Am. Chem. Sac., 116 (1994) 7935. S. Wang and D. Zhou, Chem. Phys. Z&r., 225 (1994) 165. D. Ugarte, Chem. Phys. Lert., 198 (1992) 596. D. Ugarte, Chem. Phys. I_&., 207 (1993) 473. D. Ugarte, MRS Bullerin, XIX, November 1994, 39. W.A. deHeer and D. Ugarte, Chem. Phys. Let., 207 (1993) 480. V.L. Kuznetsov, A.L. Chuvilin, Y.V. Butenko, I.Y. Mal’kov and V.M. Titov, Chem. Phys. Len.,222 (1994) 343. K.G. McKay, H.W. Kroto and D.J. Wales, J. Chem. Sot. Faraday Trans., 88 ( 1992) 2815. M. Yoshida and E. Osawa, Fullerene Sci. Technol.. 1 (1993) 55. D. York, J.P. Lu and W. Wang, Mod. Phys. Letr., 849 (1994) 8526. A. Maiti, C.J. Brabec and J. Bernholc, Phys. Rev. Lert., 70 (1993) 3023. A. Maiti, C.J. Brabec and J. Bernholc, Mod. Phys. Len., B47 (1993) 1883. D. Ugarte, Chem. Phys. Len., 209 (1993) 99. M. Tomita, Y. Saito and T. Hayashi, Jpn. J. Appl. Phys., 32 (1993) L280. Y. Saito, T. Yoshikawa, M. Okuda, M. Ohkohchi, Y. Ando. A. Kasuya and Y. Nishina, Chem. Phys. Lett., 209 (1993) 72. Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, S. Yamamura, K. Wakoh, K. Sumiyama, K. Suzuki, A. Kasuya and Y. Nishina, Chem. Phys. Letr., 212 (1993) 379.

[400] T. Hihara, H. Onodera, K. Sumiyama, K. Suzuki, A. Kasuya, Y. Nisbina, Y. Saito, T. Yoshikawa and M. Okuda, Jpn. J. Appl. phys. Letf., 33 (1994) L24. I4011 R. Sesbadri, R. Sen, K.R. Kannan, G.N. Subbanna and C.N.R. Rao, Chem. Phys. Lert., 232 (1994) 308. [4021 A.L. Mackay and H. Terrones, Nature, 352 ( 1991) 762. [4031 T. Lenosky, X. Gonze, M. Teter and V. Elser, Nature, 355 (1992) 333.

261

C.N.R.

262

[404] [405] [406] [407 ] [408] [409] [410] [41 l] [412] [413] [414] [415] [416] [417]

Rao et al. / Fullerenes. nanotubes, onions and related carbon structures

S.J. Townsend, T.J. Lenosky, D.A. Muller, C.S. Nichols and V. Elser, Phys. Rev. Len., 69 (1992) 921. D. Vanderbilt and J. Tersoff, Phys. Rev. Len., 68 (1992) 511. F. Diederich and Y. Rubin, Angew. Chem. Int. Ed. Engb, 31 (1992) 1101. L. Zeger and E. Kaxiras, Phys. Rev. La. , 70 ( 1993) 2920. S. Saito and A. Oshiyama, Phys. Rev., B49 (1994) 17413. S. Itoh and S. Ihara, Phys. Rev., B49 (1994) 13970. S.C. Tsang, P.J.F. Harris, J.B. Claridge and M.L.H. Green, J. Chem. Sot. Chem. Commun., (1993) 1519. S. Motojima, I. Hasegawa, S. Kagiya, M. Mimoyama, M. Kawaguchi and H. Iwanaga,Appl. Phys. Letr., 62 (1993) 2322. X.B. Zhang, X.F. Zhang, D. Bemaerts, G. van Tendeloo, S. Amelinckx, J. van Landuyt, V.I. Ivanov, J.B. Nagy, Ph. Lambin and A.A. Lucas, Europhys. Len., 27 (1994) 141. R. Hoffman, T. Hughbanks, M. Kertesz and P.H. Bird, J. Am. Chem. Sot., IO5 (1983) 4831. H.R. Karfunkel and T. Dressier, J. Am. C&m. Sot., I14 (1992) 2285. A.T. Balaban, D.J. Klein and CA. Folden, Chem. Phys. L&t., 217 (1994) 266. R. Sen, R. Sumathy and C.N.R. Rao, J. Muter. Res., in press. H.W. Kroto, J.P. Hare, A. Sarkar, K. Hsu, M. Terrones and J.R. Abeysinghe, MRS Bulletin, XIX, November 1994.51.