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The growth of thin C 60 films on GaSe(0001) has been studied by infrared High Resolution Electron Energy Loss Spectroscopy (HREELS) in specular reflection geometry. In contrast to previously studied disordered films on Si(100), it was found that deposition on a GaSe substrate heated to 120°C leads to well-ordered, epitaxial C 60 films. The structural order, manifested by a sharp LEED pattern, was confirmed by the clear detection in the specular beam of all four T lu dipole-active vibrations of the C 60 molecule. Several other infrared inactive modes, which were prominent in the C 60 /Si HREELS spectrum, are still seen in C 60 /GaSe but with reduced intensities. Application of the dielectric theory of reflection EELS allows a quantitative determination of the contribution of the dipole-active modes to the total spectrum. Although the relative oscillator strengths of the four infrared modes are in good agreement with optical absorption measurements, their absolute values, as determined by theoretical simulation, are found to be three times larger than the optical results.

The evolution with hydrostatic pressure (0 < P < 8.1 kbar) of the interchain ordering in trans-polyacetylene was measured using 4-circle synchrotron x-ray diffractometry and a Merrill-Bassett diamond anvil cell. The P-dependence of (HK0) reflections signify a lowering of the two-dimensional space group symmetry from pgg to p2 with a threshold P < 0.5 kbar, with an angle γ between the a and b axes of about 92° throughout the entire P range. The interchain compressibility is large and highly anisotropic. Below 6 kbar, Δa/a and Δb/b are 8.3 × 10−3/kbar and 3.8 × 10−3/kbar respectively. Both a and b stiffen above 6 kbar as the system approaches true close-packing.

We present new x-ray diffraction data on trans-(CH)x which shows that the blond alteration is out of phase on adjacent chains, and therefore the correct space group is P2 1 n . Previous attempts to resolve this question by analyzing only the (00L) intensities have yielded ambiguous results; all experiments show some (001) intensity which is not allowed P2 1 n . By analyzing indidual off-axis peak intensities, we show unambiguously that the bulk 3-D structure is inconsistent with P2 1 a (in-phase bond alternation), and therefore the weak, sample-dependent (001) intensity probably results from uncorrelated local defect regions with in-phase bond alternation on adjacent chains.

X-ray and neutron-diffraction measurements have been used to study the evolution of the lattice parameter of solid C{sub 60} through the orientational-ordering transition. The lattice parameter jumps by +0.044{plus minus}0.004 A on heating, indicating a strongly first-order transition. The average isobaric volume thermal-expansion coefficient both above and below the transition is 6.2{plus minus}0.2{times}10{sup {minus}5} K{sup {minus}1}. We observe phase coexistence over a 5-K range, but little if any hysteresis.

Translating the unique characteristics of individual single-walled carbon nanotubes into macroscopic materials such as fibres and sheets has been hindered by ineffective assembly. Fluid-phase assembly is particularly attractive, but the ability to dissolve nanotubes in solvents has eluded researchers for over a decade. Here, we show that single-walled nanotubes form true thermodynamic solutions in superacids, and report the full phase diagram, allowing the rational design of fluid-phase assembly processes. Single-walled nanotubes dissolve spontaneously in chlorosulphonic acid at weight concentrations of up to 0.5 wt%, 1,000 times higher than previously reported in other acids. At higher concentrations, they form liquid-crystal phases that can be readily processed into fibres and sheets of controlled morphology. These results lay the foundation for bottom-up assembly of nanotubes and nanorods into functional materials.

The high-temperature structure of solvent-free C(70) has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions chi in Rb(chi)C(60) is reported. In addition to the face-centered cubic (fcc) (chi = 3, superconducting), body-centered tetragonal (chi = 4), and body-centered cubic (bcc) (chi = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c/= 1, and for a substoichiometric bcc phase, chi approximately 5, is presented. In contrast, chi = 3 and chi = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram.

Single-crystal diamond-anvil x-ray diffraction at 300 K reveals a reversible stage-1--to--stage-2 transition in ${\mathrm{Ag}}_{0.35}$${\mathrm{TiS}}_{2}$, with an onset pressure 1 kbar and coexisting stages up to 25 kbar. In contrast, no stages higher than 1 are observed in ${\mathrm{Li}}_{0.5}$${\mathrm{TiS}}_{2}$ up to 55 kbar. We argue that the different behavior of Li and Ag intercalates cannot be understood in terms of a continuum elastic model but depends instead on the different interatomic distances and atomic radii.

Synchrotron-x-ray powder-diffraction and differential-scanning-calorimetry measurements on solid ${\mathrm{C}}_{60}$ reveal a first-order phase transition from a low-temperature simple-cubic structure with a four-molecule basis to a face-centered-cubic structure at 249 K. The free-energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature.

Ni/Nb and Nb/Ni bilayers on Si(100), Ni/NbSi2/Si(100) and Nb/Ni/Si/SiO2/Si(100) were produced by rf sputtering and annealed by rapid thermal processing. Thereafter, phase sequence and morphology were deduced by Auger electron spectroscopy and x‐ray diffraction. The first three cases lead to phase separation of the binary compounds NbSi2 and NiSi2. Conversely when SiO2 is used as a buffer to limit the consumption of Si, the ternary phase Nb4Ni4Si7 or Nb4Ni4Si7+NbSi2 can be produced depending on the amount of Ni available. These observations are consistent with the Nb‐Ni‐Si phase diagram, in which phase separation occurs in a Si‐rich environment whereas the ternary phase is stable in a Si‐poor environment.

The formation of NbSi2 from Nb/Si(100) is studied using rf sputtering and rapid thermal processing. For a long sputter‐etching time (10 min) of the Si substrate, NbSi2 is first formed at 720 °C. The lowest resistivity attained is 38 μΩ cm, one of the best for refractory metal silicides. The variation of the sheet resistance versus annealing temperature is correlated to grain growth. The grain size is deduced from the x‐ray coherence length of the NbSi2(111) peak which sharpens and increases in intensity for T≳720 °C. For shorter sputter etching time (5 min), NbSi2 only forms above 895 °C. This is attributed to the native oxide at the Si interface. An estimated ternary phase diagram of Nb‐Si‐O shows that Nb and SiO2 coexist at this temperature. Unlike other refractory metal silicides, oxygen in the Nb film is expelled as the silicide is formed.

Nanowires have great potential as building blocks for nanoscale electrical and optoelectronic devices. The difficulty in achieving functional and hierarchical nanowire structures poses an obstacle to realization of practical applications. While post-growth techniques such as fluidic alignment might be one solution, self-assembled structures during growth such as branches are promising for functional nanowire junction formation. In this study, we report vapor-liquid-solid (VLS) self-branching of GaN nanowires during AuPd-catalyzed chemical vapor deposition (CVD). This is distinct from branches grown by sequential catalyst seeding or vapor-solid (VS) mode. We present evidence for a VLS growth mechanism of GaN nanowires different from the well-established VLS growth of elemental wires. Here, Ga solubility in AuPd catalyst is limitless as suggested by a hypothetical pseudo-binary phase diagram, and the direct reaction between NH3 vapor and Ga in the liquid catalyst induce the nucleation and growth. The self-branching can be explained in the context of the proposed VLS scheme and migration of Ga-enriched AuPd liquid on Ga-stabilized polar surface of mother nanowires. This work is supported by DOE Grant No. DE-FG02-98ER45701.

The electrical resistance of single-wall carbon nanotubes (SWNT) produced by condensation of a laser- vaporized graphite=Ni=Co mixture at 1200C was studied under quasihydrostatic pressures up to 90 kbar. The resistance exhibits a positive temperature coefficient, characteristic of a metal, up to 10 kbar, whereas the absolute value decreases abruptly by a factor 10. From 10- 30 kbar R increases with pressure and the TCR becomes negative. At still higher pres- sure, up to 90 kbar, R decreases gradually with pressure, similar to the case of graphite. Raman scattering and electron microscopy performed after 25-kbar pressurizations indicate that the SWNT and its lattice are preserved. We propose that the sequential behavior of R.P/ reveals in turn the processes of compaction, defect formation by kinking, and finally the van der Waals compression of the inter-tube spacing in the triangular lattice.

X-ray diffraction and resistivity measurements on bulk single-wall carbon nanotube material are described. The tubes self-organize during growth into a triangular lattice. The temperature dependence of the resistivity is consistent with a metallic system at high temperatures.

Gallium nitride (GaN) nanowires (NW) and various nanostructures were grown by thermal reaction of gallium oxide and ammonia. The interplay between Ga/N reactant ratio and characteristic lengths of polar surfaces explained morphology variation. Field effect transistors were patterned on 40~185 nm diameter NWs by Ga + focused ion beam (FIB) direct Pt deposition. The devices exhibited no gate responses with liner I-V for "small" diameter NWs. Linear I-V was unexpected since Pt forms Schottky barriers on n-GaN. I-V-T characteristics of the FIB-Pt contacts evolved from ohmic to rectifying with increasing NW diameter with strongly nonmetallic T-dependence. For small diameters, two-dimensional variable range hopping explained the contact conduction, the disorder being associated with ion-beam-induced sputtering and amorphization in the GaN under the FIB-Pt, as corroborated by transmission electron microscopy (TEM). For large diameters, back-to-back Schottky barriers explained the nonlinear I-V. High carrier concentration was confirmed explaining the absence of gate responses. Finally, Young's modulus E and quality factor Q of GaN NW were measured using in-situ TEM electromechanical resonance analysis. For large diameters, E was ~300 GPa but decreased for smaller diameters. Q was greater than was obtained from micromachined Si resonators with comparable surface-to-volume ratio, implying significant advantages of GaN NW for nanoelectromechanical applications.

The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires.

Percolation conductivity of a stick network depends on alignment as well as concentration. We show that both dependences exhibit critical (power-law) behavior, and study the alignment threshold in detail. The highest conductivity occurs for slightly aligned, rather than isotropic, sticks. Experiments on single wall carbon nanotube composites are supported by Monte Carlo simulations. These results should be broadly applicable to percolating networks of anisotropic conductors.

Liquid anhydrous sulfuric acid forms a partly ordered structure in the presence of single-walled carbon nanotubes SWNTs. X-ray scattering from aligned fibers immersed in acid shows the formation of molecular shells wrapped around SWNTs. Differential scanning calorimetry of SWNT-acid suspensions exhibits concentration-dependent supercooling/melting behavior, confirming that the partly ordered molecules are a new phase. We propose that charge transfer between nanotube electrons and highly oxidizing superacid is responsible for the unique partly ordered structure.

The electronic properties of $p$-doped single-walled carbon nanotube (SWNT) bulk samples were studied by temperature-dependent resistivity and thermopower, optical reflectivity, and Raman spectroscopy. These all give consistent results for the Fermi level downshift $(\ensuremath{\Delta}{E}_{F})$ induced by doping. We find $\ensuremath{\Delta}{E}_{F}\ensuremath{\approx}0.35\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and $0.50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for concentrated nitric and sulfuric acid doping respectively. With these values, the evolution of Raman spectra can be explained by variations in the resonance condition as ${E}_{F}$ moves down into the valence band. Furthermore, we find no evidence for diameter-selective doping, nor any distinction between doping responses of metallic and semiconducting tubes.

We find strong evidence for a metal-insulator sMId transition in macroscopic single wall carbon nanotube sSWNTd conductors. This is revealed by systematic measurements of resistivity and transverse magnetoresistance sMRd in the ranges 1.9‐300 K and 0‐9 Tesla, as a function of p-type redox doping. Strongly H2SO4-doped samples exhibit small negative MR, and the resistivity is low and only weakly temperaturedependent. Stepwise dedoping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with rsTd obeying a power-law temperature dependence, rsTd ~ T ˛b . In the insulating regime shigh annealing temperaturesd, the rsTd behavior ranges from Mott-like three-dimensional s3Dd variable-range hopping sVRHd, rsTd ~ expfsT0 / Td ˛1/4 g, to Coulomb-gap sCGVRHd behavior, rsTd ~ expfs˛T0 / Td ˛1/2 g. Concurrently, MRsBd becomes positive for large B, exhibiting a minimum at magnetic field Bmin. The temperature dependence of Bmin can be characterized by BminsTd = Bcs 1˛ T/ Tcd for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature Tc, MRsBd is positive for all B. The observed changes in transport properties are explained by the effect of doping on semiconducting SWNTs and tube-tube coupling.

High-pressure reaction of Li metal and graphite yields LiC 2 , three times as dense in Li as the usual LiC 6 . LiC 6 decomposes rapidly but incompletely into LiC 6 + Li upon releasing pressure. A dramatic enhancement is observed in the metastability of the high Li density when the graphite contains 0.5 at.% substitutional boron. Average compositions from X-ray (OOL) intensities, determined several months after releasing the pressure, are LiC 2.2 and LiC 3.8 with and without boron respectively. The Li “layer” is split along c by 0.5 A suggesting a puckering which reduces the repulsive interactions.

The high-temperature f.c.c. phase of KC 60 is unstable upon cooling and transforms into the orthorhombic polymer (OP). We show in this paper that the polymeric phase is also unstable above room temperature and separates into the thermodynamically stable K 3 C 60 + α-C 60 phases. The analysis of the lifetime of both f.c.c. and polymeric phases, probed by ESR as a function of temperature, allows the determination of activation energies of both f.c.c. → polymeric and polymeric → K 3 C 60 + α-C 60 phase transformations, as being 0.17 and 1.1 eV, respectively.

We have measured the bulk modulus K for C61D2 up to 1 GPa in the temperature range 175–343 K. For face-centered cubic C61D2 above 290 K, we find an anomalously low value for K below about 0.15 GPa, ...

The objectives of this project were (1) establish proof of principle by demonstrating enhanced polymer properties by the addition of single walled nanotubes (SWNTs), (2) provide guidelines for materials selection that give improved mechanical, electrical and/or thermal properties, and (3) define processing methods most appropriate for the materials identified. Our study of SWNT-polymer composites focuses on thermoplastics, because these materials can be readily drawn into fibers. As we have shown previously, the extensional flow exerted on the composite during fiber spinning aligns the SWNT to an unprecedented level. The anisotropic orientation of SWNT in these composite fibers provides an advantage when bolting for changes in macroscopic properties, particularly tensile strength. Composite processing and composite characterization received comparable effort in this project. Processing efforts include our coagulation method as well as in situ polymerization of nylon in the presence of well-dispersed SWNT. In addition to mechanical testing of the SWNT-thermoplastic fibers, we established methods for testing the electrical and thermal conductivity of these highly anisotropic materials. We used scattering methods for quantifying the extent of dispersion within the composites, a widely appreciated and long standing question in nanocomposites.

Structural, electrical and thermal methods are applied to characterize single-walled carbon nanotube (SWNT) fibers with post-extrusion stretching as the independent variable. HiPco SWNTs are dispersed in water using sodium dodecyl sulfate (SDS), and then co-extruded with polyvinyl alcohol (PVA)/water through a long syringe into a rotating water/PVA coagulation bath. Partial axial alignment is thereby achieved, and further enhanced by applying tension to the flexible green fibers in the coagulation bath. Our findings include: (1) X-ray diffraction shows that the full width at half maximum (FWHM) of the Bragg peaks decreases from 55 (as-extruded) to less than 30 degrees by 80% elongation. That is, SWNT alignment increases linearly with stretch (up to 80%). (2) In resistivity at room temperature vs. stretch ratio, result shows an initial rapid decrease followed by saturation; essentially all the improvement in electronic transport is obtained once alignment reached 40° FWHM. (3) Annealing in hydrogen at 1000°C is performed to drive out PVA, to improve inter-tube and inter-bundle contacts, and to heal damage on the tube walls. Such annealing is found to increase conductivity by at least 4 orders of magnitude. (4) Below 25 K, resistivity vs. temperature of the annealed fiber is well-represented by Coulomb gap variable range hopping (CG-VRH). It is rationalized that the Coulomb interactions in disordered systems open a gap at the Fermi energy. Above 25 K, the thermal energy is greater than the Coulomb gap, so thermal activation is more probable than correlated electron hops. (5) Finally, a measurable thermal conductivity is observed as stretch alignment increases.

The electrical transport properties of field effect transistor (FET) devices made of silicon nanowires (SiNWs) synthesized by pulsed laser vaporization (PLV) were studied. From as-grown PLV-SiNW FET, we found p-channel FET behavior with low conductance. To improve conductance, spin on glass (SOG) and vapor doping were used to dope phosphorus and indium into SiNW, respectively. From doping after synthesis, we could successfully make both n- and p-channel FET devices.

A spherically averaged potential was used to calculate solid state properties for C{sub 70} in the disordered fcc phase. Each molecule was treated as a spherical shell of 70 carbons with radius of 3.8 A, determined by fitting the equation of state to the experimental X-ray powder diffraction data. The anharmonic properties were treated by using the extended Mie-Gruneisen theory. This approach enabled us to compute compressibility and thermal expansion as well as the phase transition point between the solid and the vapor phases of C{sub 70}. The calculated enthalpy of sublimation, 193 kJ/mol, lies within the range of measured heats, while the predicted sublimation point of 1023 K is higher than the experimentally determined one. 13 refs., 3 figs., 1 tab.

We report the production and characterization of polymer nanocomposites with single-walled carbon nanotubes having improved mechanical properties and exceptional nanotube alignment. High-pressure carbon monoxide nanotubes (HiPco) were efficiently distributed in polystyrene (PS) and polyethylene (PE) with a twin-screw compounder. Nanotube concentrations were 1, 5, 10, and 20 wt% in PE composites and 0.7 wt% in PS composites. PE composites were melt-spun into fibers to achieve highly aligned nanotubes. Polarized Raman spectroscopy shows that the degree of alignment increases with decreasing fiber diameter and decreases with increasing nanotube loading. The orientation distribution function of a 1 wt% HiPco/PE composite had a full width at half-maximum of approximately 5 degrees. The elastic modulus increases up to 450% relative to PE fibers for 20 wt% nanotube loading at an intermediate fiber diameter of 100 microns.

We measured thermoelectric power S of bulk single-wall carbon nanotube materials p doped with acids. In contrast to oxygen-exposed or degassed samples, S is very small at the lowest temperatures, increases superlinearly above a characteristic and sample-dependent T, and then levels off. We attribute this unusual behavior to 1D phonon drag, in which the depression of the Fermi energy cuts off electron-phonon scattering at temperatures below a characteristic T0. This idea is supported by a model calculation in which the low temperature behavior of phonon drag is specifically related to the one-dimensional character of the electronic spectrum.

Powder profiles have been obtained from unoriented films of (CH) x and (CD) x solution-doped with lithium by using x-ray and neutron diffraction respectively. Detailed intensity analyses show improved fits with a lower symmetry R3 model for the two-dimensional interchain packing, as compared to the previously proposed high-symmetry R3C space group. The R3-based structure is derived from the R3C one by a local distortion of the interchain packing motif, in a very similar way to the symmetry breaking observed in the tetragonal phases for larger alkali metals.

Electrochemistry has proven to be very useful for the study of guest-host systems, particularly, carbon intercalation compounds. Not only does electrochemistry provide essential information about the thermodynamics and kinetics of these systems, but it also offers accurate control of guest stoichiometry which is difficult to achieve by other doping methods. Therefore, electrochemical doping has been used extensively to study the properties of carbon guest-host systems. In situ X-ray diffraction and electrochemical doping were used to study the phase diagram of Li xC6 graphite, 1 phase transitions in Li-doped polyacetylene 2 and the structure of Li-doped solid C 60. 3 In situ resistivity measurements were used to study the electronic transport properties of K- and Na-doped polyacetylene. 4,5 In this work, electrochemistry was used to study a new carbon guest-host system: Li/carbon nanotubes. Two types of carbon nanotubes can be distinguished according to their structural properties: multiwall (MWNT) and single wall (SWNT). 6 MWNT consist of graphitic sheets rolled into closed concentric cylinders, with a structure similar to that of Russian dolls. The concentric tubes are separated by Van der Waals gaps of ,3.4 A, a typical interlayer spacing in turbostratically disordered graphite. External diameters can be as large as 50 nm, and lengths are of micrometer scale. SWNT can be envisioned as a single graphene sheet rolled into a cylinder, with diameters in the range 1-2 nm and lengths of several micrometer. SWNT of nearly uniform diameters self-organize into long crystalline “ropes” in which parallel nanotubes are bound by Van der Waals forces. 7 The diameter of a rope is typically 10-50 nm corresponding to 30-600 tubes per rope. Ropes containing as few as 2-3 tubes or as many as several thousand are occasionally found. Figure 1 presents a high resolution transmission electron microscope (HRTEM) image of purified and annealed SWNT, in which several entangled ropes with different diameters can be observed. The parallel fringes within each rope are due to the constructive scattering from the parallel planes of SWNT. The fact that the fringe spacings differ among ropes does not arise from a wide distribution in nanotube diameters, but rather from the different orientation of each rope zone axis with respect to the electron beam. Figure 2 shows an X-ray profile from purified and annealed SWNT. The well

The thermal conductivity, k(T), of bulk single-wall carbon nanotubes (SWNT's) displays a linear temperature dependence at low T that has been attributed to 1D quantization of phonons. To explore this issue further, we have measured the k(T) of samples with varying average tube diameters. We observe linear k(T) up to higher temperatures in samples with smaller diameters, in agreement with a quantization picture. In addition, we have examined the effect of annealing on k(T). We observe an enhancement in k(T) for annealed samples which we attribute to healing of defects and removal of impurities. These measurements demonstrate how the thermal properties of an SWNT material can be controlled by manipulating its intrinsic nanoscale properties., Comment: Proc. of the XV. Int. Winterschool on Electronic Properties of Novel Materials, Kirchberg/Tirol, Austria, 2001

A simple model explains both the Bragg peaks and the diffuse scattering observed in neutron diffraction measurements on Solid C 60 at 295 and 14 K. Neutron inelastic scattering measurements reveal dynamic orientational disorder above the order-disorder transition temperature. An analysis of the diffraction results shows that significant disorder remains at 14 K.

X-ray diffraction and diamond anvil techniques were used to measure the isothermal compressibility of K 3 C 60 and Rb 3 C 60 , the superconducting, binary alkali-metal intercalation compounds of solid buckminsterfullerene. These results, combined with the pressure dependence of the superconducting onset temperature T c measured by other groups, establish a universal first-order relation between T c and the lattice parameter a over a broad range, between 13.9 and 14.5 angstroms. A small secondorder intercalate-specific effect was observed that appears to rule out the participation of intercalate-fullerene optic modes in the pairing interaction.

METALLIC and superconducting donor-type intercalation compounds of C60 are now well established. These involve electron transfer from a sublattice of alkali-metal dopants to a sublattice of fullerene molecules. Three stoichiometric phases have been identified1–3, and a binary phase diagram describing composition regions of two-phase coexistence has been proposed2. Oxidative intercalation by electron acceptors, meanwhile, has not previously been reported. Ohno et al.4 recently presented photoemission data suggesting that solid C60 will take up only minor amounts of iodine, resulting in a non-metallic product which is not a stoichiometric compound4. Here we report that, by using different reaction conditions, we have prepared a highly crystalline C60: iodine compound of ideal stoichiometry C60I4, with an alternating guest–host layer structure closely resembling that of classic intercalation compounds. The 300-K resistivity exceeds 109 Ωcm and we observe no superconductivity down to 4 K, despite the fact that the inter-fullerene separation lies in the range of that in the superconducting M3 C60 phases (where M is K, Rb, Cs). C60I4 is apparently the first example of a fullerite intercalation compound in which there is no electron transfer between C60 and the intercalate.

AN understanding of the superconductivity and other electronic properties of the alkali-metal-doped fullerenes1 will require measurements of their solid-state physical properties. In the normal state one would like to know the electron effective mass, the Fermi energy and conduction bandwidth, and the mean free path for electron transport; the superconducting state, meanwhile, can be characterized at the most basic level by length and energy scales such as the coherence length, penetration depth and superconducting energy gap. Here we describe the use of far-infrared reflectivity measurements to probe the low-energy response of Rb3C60. We derive a value for the characteristic energy scale associated with reflectivity, which allows us to estimate a value for the gap energy 2δ of 3–5 kTC, in reasonable agreement with tunnelling measurements2. Combining this result with data obtained in previous studies, we estimate other length and energy scales, such as the bandwidth and penetration depth.

The authors succeeded in intercalating fullerite electrodes with lithium using all-solid-state electrochemical cells and polymer electrolytes. This method offers several advantages over previous studies of fullerenes in solution. The first redox cycle for pure C{sub 60} is shown as a linear sweep voltammogram. Three well defined peaks: A, B, and C, are apparent at 2.3, 1.9, and 1.5 V respectively vs Li. At each potential within the C peak, a slow relaxation of the current was observed. The absence of trailing current after the A, B, and C peaks proves that all the available sites have been occupied in the C{sub 60} solid at each of these reduction steps. Reduction beyond the fourth wave implies extensive reorganization of the C{sub 60} solid, as corroborated by the markedly different shape of the first oxidation following the deep reduction to 0.2 V. Only three oxidation waves were observed, even on extending the sweep to 3.5 V, and no peaks were observed on the second reducing potential sweep. It is interesting to note that the roughly constant 0.5 V separation between waves A and B, B and C, and C and D corresponds approximately to the electron-by-electron reduction in solution. This could bemore » interpreted as evidence for weakly perturbed molecular orbitals in the solid state which continue to dominate electroactivity.« less

This is the final report of the fourth of four three year grants of the same title. The program evolved from an earlier DOE grant on graphite intercalation compounds. Since its inception eight years ago, the focus evolved continuously from conjugated polymers to fullerenes, disordered carbons for Li-ion battery applications, and most recently carbon nanotubes, with side excursion back to GIC's to exploit a recent advance in synthesis of a potentially exciting new phase. The unifying themes are the versatility of carbon in forming novel solids, and the flexibility of intercalation chemistry to provide new materials with potentially useful properties.

The vibrational density of states of single-wall carbon nanotubes (SWNT) was obtained from inelastic neutron scattering data from 0 to 225 meV. The spectrum is similar to that of graphite above 40 meV, while intratube features are clearly observed at 22 and 36 meV. An unusual energy dependence below 10 meV is assigned to contributions from intertube modes in the 2D triangular lattice of SWNT bundles, and from intertube coupling to intratube excitations. Good agreement between experiment and a calculated density of states for the SWNT lattice is found over the entire energy range.

In this chapter we discuss the structures and properties of alkaline-earth, rare-earth and lanthanide metal intercalated C60 compounds. A brief review is also given to the metal intercalated C70. Some recent works on intercalated carbon nanotubes are summarized.

This chapter is devoted to the alkali metal intercalated C60 fullenrides. The emphasis is on superconductivity and the correlation between the key materials parameters such as crystal structure, molecular orientation, valence state, and the superconducting transition temperature. The structures of other alkali metal fullende phases are also reviewed.

In three chapters we review the intercalation compounds of various new carbon allotropes: C60, C70, and the carbon nanotubes. This chapter reviews (1) the structure of C60 solid; (2) the common materials synthesis and characterization techniques that have been used to investigate the fullerene compounds; (3) the fullerenes that have been intercalated with neutral species. Chapter 3 is devoted to alkali and alkaline-earth metals intercalated fullendes. The emphasis is placed on structure and superconductivity. In particular, the relation between superconductivity and various materials parameters are discussed. Chapter 7 summarizes the recent works on (1) rare-earth and lanthanide intercalated fullendes; (2) intercalated C70; (3) carbon nanotube intercalation compounds.

IT has been shown recently that MxC60 (M = alkali metal) is metallic at 300 K for some M and x (ref. 1) and superconducts below 18 K for M = potassium2. These observations give further impetus to studies of the molecular3 and solid-state4 properties of fullerenes. Here we report on X-ray diffraction studies of the structure and bonding in alkali-metal-doped solid C60 (fullerite). Powder diffraction data obtained from equilibrium compositions of C60 doped to saturation with K or Cs show that the face-centred cubic lattice of pure C60 (refs 5–7) transforms to body-centred cubic at these doping levels, with a saturation composition close to M6C60. The rotational disorder of the fullerene molecules in pure C60 at 300 K is absent in the fully doped compounds.

Room-temperature powder x-ray diffraction profiles have been obtained at hydrostatic pressures P = 0 and 1.2 gigapascals on the solid phase of cubic C(60) ("fullerite"). Within experimental error, the linear compressibility d(ln a)/dP is the same as the interlayer compressibility d(ln c)/dP of hexagonal graphite, consistent with van der Waals intermolecular bonding. The volume compressibility -d(ln V)/dP is 7.0 +/- 1 x 10(-12) square centimeter per dyne, 3 and 40 times the values for graphite and diamond, respectively.

Neutron and gamma diffraction (00L) rocking curves of stage-1 AsF5-graphite intercalation compounds have been performed. The intrinsic c-axis mosaic FWHM is 0.8°, significantly less than the value typical of donor compounds. This suggests that intercalation with large molecules may provide new options for the development of long-wavelength neutron monochromators and filters.

The conduction electron spin resonance (CESR) of K3C60 single crystal and powder samples was studied from temperatures well below the superconducting transition (Tc=19 K) to 800 K. Several ESR frequencies were used with corresponding magnetic fields up to 8 T. We observed an anomalous change of the g factor with temperature in the superconducting state which may arise from an anisotropy of the superconducting energy gap. We proved that the change of the g factor is not caused by diamagnetic screening.