J.M. Holden

Vibrational modes of carbon nanotubes; Spectroscopy and theory

Abstract Experimental and theoretical studies of the vibrational modes of carbon nanotubes are reviewed. The closing of a 2D graphene sheet into a tubule is found to lead to several new infrared (IR)- and Ramanactive modes. The number of these modes is found to depend on the tubule symmetry and not on the diameter. Their diameter-dependent frequencies are calculated using a zone-folding model. Results of Raman scattering studies on arc-derived carbons containing nested or single-wall nanotubes are discussed. They are compared to theory and to that observed for other sp 2 carbons also present in the sample.

Photochemical transformation of C60 and C70 films

Thin solid films of C60 and C70 have been found to be sensitive to UV-visible light. In the absence of oxygen, which acts as a triplet state quencher, C60 and C70 have been observed to phototransform from a toluene-soluble to a toluene-insoluble state. This phototransformation has been studied via Raman and FTIR spectroscopies, UV-visible transmission spectroscopy and laser desorption mass spectroscopy. The results of these experiments have been interpreted as evidence for a phototransformation from a van der Waals solid to one in which the fullerenes are linked by covalent bonds. For C60, it is proposed that a transformation to a polymeric solid has occurred, whereas a similar flux of UV-visible light applied to C70 is proposed to lead to a random dimerization of the lattice and a much smaller population of higher oligomers. For both phototransformed C60 and C70, the covalent bonds between fullerenes can be broken thermally and the phototransformed material returns to the pristine, toluene-soluble state. UV-visible light can also be used to photochemically assist the diffusion of dioxygen into the interstitial voids in the solid C60 and C70 lattices. For C60, a photochemical enhancement of the O2 diffusion rate by a factor of ~ 10 is observed by alpha particle backscattering, leading to a stoichiometry of ~ C60O2. Similar to C60-polyfullerene, C60(O2)x is also toluene insoluble. As a result, these C60-based films might find photolithographic applications.

Thermal decomposition of polymeric C60

Abstract Raman scattering is used to study the thermal decomposition of photochemically produced polymeric C60 films. Under the experimental conditions, a steady state monomer population is maintained through a competition between: (a) the photoattachment of monomers to other monomers and higher oligomers (e.g. dimers, trimers) driven by the Raman laser and (b) the thermal scission of the intramolecular CC bonds. The rate for thermal dissociation of these bonds is found to follow an Arrhenius-type behaviour with an activation energy Ea=1.25 eV.

Raman scattering from nanoscale carbons generated in a cobalt-catalyzed carbon plasma

Abstract Carbonaceous material including nanoscale soot, carbon-coated nanoscale Co particles and nanotubes have been generated from a dc arc discharge between carbon electrodes. Sharp first- and second-order lines are observed in the Raman scattering spectra of the arc-derived carbons only when Co metal is present in the core of the anode. The sharp lines in the Raman spectrum of the Co-catalyzed, arc-derived carbons have not been observed previously in carbonaceous materials and are tentatively assigned to carbon nanotubes on the basis of a zone-folded model for the vibrational spectra of armchair tubules.

Optical properties of C60- and C70-based solid films

Abstract We review results obtained from our laboratory regarding the optical properties of C 60 - and C 70 -based thin solid films, including alkali metal (M) doped compounds. The films were deposited on various substrates (e.g. KBr, Si(100), quartz, Cu) by vacuum sublimation. First- and higher-order IR and Raman spectroscopies were used to determine values for most of the 46 intra-molecular vibrational modes of the pristine solid, and the results are compared with theory. Doping (M)-induced shifts and splittings of these mode frequencies are also discussed. Finally, results of UV-visible-IR reflection-transmission spectroscopies and ellipsometry are presented which reveal structure in the dielectric function due to electronic absorption. For pristine C 60 , the Clausius-Mossotti relation is used to extract the frequency-dependent molecular polarizability. Results from these studies of the electronic contribution to the optical dielectric function are discussed in connection with recent calculations.

Optical studies of fullerene-based solids

Results of optical studies on oxygen-doped, alkali-metal(M)-doped and photopolymerized solid C60 films are reported. Results of Raman scattering studies of MxC60 films are presented, but discussion is focused on recent Raman scattering studies of superconducting Rb3C60 films (Tc ≈ 28 K) from T = 300 to 9 K. The effects of the exposure of C60 to light and/or oxygen are also presented. If pristine C60 is exposed at T = 300 K to visible or UV light, a photoinduced polymerization of the structure is obtained, which is consistent with results from laser desorption mass spectroscopy and optical spectroscopy. If oxygen is also present, the photoinduced polymerization is suppressed, and the oxygen is observed to diffuse about 2000 A in about 1 h, eventually leading to oxidation of the structure.

Electronic and vibrational properties of Rb-intercalated MoS2 nanoparticles

Abstract Nanoplatelets (diameter about 8 nm) synthesized by laser pyrolysis and bulk powder of MoS2 have been intercalated with Rb to establish the metallic state. The properties of these powders are investigated by Raman scattering, X-ray diffraction, transmission electron microscopy and electron spin resonance (ESR). With increasing time of Rb intercalation the Raman-active interlayer A1g mode is observed to red shift in the nanoplatelet sample and blue shift in the micron-size bulk powder sample. In both samples the Rb doping eventually induces a metallic state and the Raman signal is lost. A broad ESR line was detected for the Rb-saturated MoS2 bulk powder sample, also indicating the metallic character. Owing to the small particle size of the nanoplatelets, no broad line ESR signal was observed for the intercalated nanoplatelets. A low field (ESR) signal was used to search for superconductivity and a Tc = 6.4 K was observed in the bulk, in good agreement with the literature. However, no superconductivity down to 3.5 K was observed in the nanoplatelet sample.

Photochemical Transformations and Polymerization in Solid C60

In this paper we review theory and experiment on the photopolymerization of solid C60 and related C60 polymers. In addition, we summarize experimental results on a second interesting phototransformation in solid C60, viz., the photo-enhanced doping, or diffusion, of molecular oxygen into the interstitial voids in the host lattice. Both of these phototransformations render the transformed solids insoluble in organic solvents, and this property has been patented for applications as photoresists[l]. Using vibrational spectroscopy as a tool, we discuss recent infrared and Raman studies of the photopolymer and other C60 polymers produced at high pressure and high temperature (HPHT). Finally, recent results on the identification of charge transfer excitons in solid C60 will be given. These optical excitations have been proposed as the “trigger mechanism” for the photopolymerization process[2].

VIBRATIONAL MODES OF CARBON NANOTUBES; SPECTROSCOPY AND THEORY

Experimental and theoretical studies of the vibrational modes of carbon nanotubes are reviewed. The closing of a 2D graphene sheet into a tubule is found to lead to several new infrared (IR)- and Raman-active modes. The number of these modes is found to depend on the tubule symmetry and not on the diameter. Their diameter-dependent frequencies are calculated using a zone-folding model. Results of Raman scattering studies on arc-derived carbons containing nested or single-wall nanotubes are discussed. They are compared to theory and to that observed for other sp2 carbons also present in the sample.