P. C. Eklund

Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering

Single-walled carbon nanotubes (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls. Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br2) frequencies. Little change is seen for I2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements, suggests that doping does increase the carrier concentration of the SWNT bundles.

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.

Chemical Attachment of Organic Functional Groups to Single-walled Carbon Nanotube Material

We have subjected single-walled carbon nanotube materials (SWNTMs) to a variety of organic functionalization reactions. These reactions include radioactive photolabeling studies using diradical and nitrene sources, and treatment with dichlorocarbene and Birch reduction conditions. All of the reactions provide evidence for chemical attachment to the SWNTMs, but because of the impure nature of the staring materials, we are unable to ascertain the site of reaction. In the case of dichlorocarbene we are able to show the presence of chlorine in the SWNT bundles, but as a result of the large amount of amorphous carbon that is attached to the tube walls, we cannot distinguish between attachment of dichlorocarbene to the walls of the SWNTs and reaction with the amorphous carbon.

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.

Reaction mechanism for the photopolymerization of solid fullerene C60

Abstract A photochemical “2+2 cycloaddition” reaction is proposed to drive the photopolymerization of pristine solid C 60 . This mechanism proceeds via the triplet T 1 state and exhibits a temperature threshold near 250 K, close to the orientational ordering transition of solid C 60 . Below this threshold, the topochemical requirement of reactive double bond alignment on adjacent C 60 molecules becomes improbable, thereby suppressing the reaction rate.

Nanocrystalline α-Fe, Fe3C, and Fe7C3 produced by CO2 laser pyrolysis

Nanocrystalline α–Fe, Fe 3 C, and Fe 7 C 3 , particles with narrow size distributions were produced by CO 2 laser pyrolysis of vapor mixtures of Fe(CO) 5 and C 2 H 4 . Details of the synthesis procedure are discussed. Mossbauer spectroscopy and x-ray diffraction were used to identify the structural phases and the former was used also to study the magnetism of the nanoparticles. All the nanoparticles were observed to be ferromagnetic in this size range. If excess C 2 H 4 appears in the reactant gas mixture, several monolayers of pyrolytic carbon were observed to form on the particle surface, as deduced from transmission electron microscopy and Raman scattering studies. Results of thermo-gravimetric analysis/mass spectroscopy studies of this carbon coating indicate it is gasified in hydrogen at temperatures T ∼ 250 °C.

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.

Photo-assisted structural transition and oxygen diffusion in solid C60 films

Using Raman scattering we have observed a phototransformation of C60 films on Si(100) from a face‐centered‐cubic (fcc) phase to a second solid phase leaving the C60 molecules intact. Furthermore, photoassisted oxygen diffusion into the deep bulk is detected using alpha resonance scattering. The Raman spectrum for oxygen‐doped C60 is found to be similar to the result for oxygen‐free fcc C60, and oxygen is found to harden the film against phototransformation.

Raman Scattering in Fullerenes

Since the discovery in 1990 of a relatively simple arc method to synthesize gram quantities of carbon cage molecules (fullerenes), considerable research effort has been expended to understand the molecular and solid-state properties of fullerenes and fullerene-derived materials. Raman scattering has played an important role in this effort which has focused on the fullerene C60, and to a lesser extent C70. From Raman spectra and their comparison with model andab initiocalculations, fundamental questions regarding the intramolecular bonds, the structure and properties of crystalline, fullerene-derived solids, including the superconducting K3C60 and Rb3C60 compounds, have been addressed and largely understood. Raman scattering has also been used to probe pressure- and temperature-driven phase transitions and the photopolymerization of fullerenes in the solid state, in addition to the bonding of fullerene molecules to metal substrates. In this review the highlights of this Raman scattering research are discussed. Brief introductory remarks concerning the discovery and structure of fullerenes are also given.