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Polyelectrolyte solutions of Na-doped single-wall carbon nanotube (SWNT) salts are studied by Raman spectroscopy. Their Raman signature is first compared to undoped SWNT suspensions and dry alkali-doped SWNT powders, and the results indicate that the nanotube solutions consist of heavily doped (charged) SWNT. Raman signature of doping is then used to monitor in situ the oxidation reaction of the nanotube salt solutions upon exposure to air and to an acceptor molecule (benzoquinone). The results indicate a direct charge-transfer reaction from the acceptor molecule to the SWNT, leading to their gradual charge neutralization and eventual precipitation in solution. The results are consistent with a simple redox titration process occurring at the thermodynamical equilibrium.

We have performed electron spin resonance (ESR) spectroscopy and x-ray diffraction experiments at low temperature on KC60 single crystals. ESR data reveal the occurrence of a metal-insulator phase transition at about 50 K. In the same temperature range, we observe the stabilization of a superstructure which doubles the volume of the unit cell. We suggest that displacements of the K atoms and a modulation of the C60 charge may be involved in the mechanism of this phase transition. These results shed new light on the subtle interplay of structure, dimensionality, and electronic properties in the AC60 fullerides.

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We have investigated, by means of temperature-dependent vibrational spectroscopy, the properties of neutral single crystals of C 60 intercalated with biphenyl. Raman scattering results from a biphenyl-C 60 single crystal in the temperature range 75-300 K are in support of the two reported phase transitions in the material, as determined by X-ray diffraction (at 147 and 212 K). Following the variations of the Raman features of the single components, biphenyl and C 60 , it is possible to infer the behavior of the two molecules at such temperatures. In particular, at least three of the main biphenyl mode energies and/or intensities plus the C-H Raman band show modifications correlating with the reported phase transition temperatures. From the variation of the relative intensity of two of the modes, a partial tentative qualitative picture of the biphenyl planarity as a function of temperature can be drawn. The behavior of most of the C 60 mode energies, widths, and intensities correlate with either of the transitions, but not necessarily both. Especially abrupt H g mode intensity modifications at ∼147 K seem to result from the influence of the corresponding phase transformation on C 60 electronic transitions, by changing the C 60 site symmetry.