We performed high pressure Raman experiments on purified open-ended single wall carbon nanotubes using different pressure transmitting media (paraffin oil, argon and 4:1 methanol ethanol mixture) and two excitating wavelength (514.5 nm and 632.8 nm). We state that the behavior of the Raman spectrum under pressure is significantly dependent on the pressure transmitting medium. This result points out the relevance of the interactions between the medium and the nanotubes, at the origin of the disagreement between the phase transition sequences reported in literature. The comparaison of tangential modes profiles clearly shows that 4:1 methanol ethanol induces few strain on nanotubes in the opposite of what is observed for argon and paraffin oil pressure transmitting media. These observations are discussed in terms of intercalation and stress due to non hydrostatic conditions.

We performed high pressure resonant Raman experiments on well characterized purified single-wall carbon nanotubes up to 40 GPa using argon as pressure transmitting medium. We used two different excitating wavelengths, at 632.8 nm and 514.5 nm. In contrast with other studies no clear sign of phase transformation is observed up to the highest studied pressure of 40 GPa. Our results suggest that the progressive disappearance of the radial breathing modes observed while increasing pressure should not be interpreted as the sign of a structural phase transition. Moreover, a progressive change of profile of the tangential modes is observed. For pressures higher than 20 GPa the profile of those modes is the same for both laser excitations. We conclude that a progressive loss of resonance of single-wall carbon nanotubes under pressure might occur. In addition, after high pressure cycle we observed a decrease of intensity of the radial breathing and tangential modes and a strong increase of the D band.

Infrared intramolecular vibrations and lattice modes in the crystalline phase of 2,2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. For the first time, the far- and mid-infrared responses have been calculated from the Gamma-point modes and the Born effective charge tensors of the 2T crystalline phase. The relative good agreement between the calculated and experimental infrared spectra allows us to assign the origin of the main features of the experimental spectra, which is of particular interest in the far-infrared domain. These assignments are useful for understanding all the properties of the 2T crystalline phase in which phonon-phonon and electronphonon interactions play an important role.

Phonons in the α-quaterthiophene (4T) and α-sexithiophene (6T) polymorph phases are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. The simulation of inelastic neutron scattering spectra (INS) on the LT and HT polymorph phases of 4T and 6T enable the corresponding spectral signatures of these materials to be identified. In particular, there are two fingerprints: (i) the low-frequency vibrational modes (frequencies lower than 200 cm(-1)) and (ii) the vibrational modes in the 600-900 cm(-1) frequency range. The good agreement with the INS experimental data allows us to assign unambiguously the origin of all features (first-order and high-order processes) of these spectra and to predict that the LT phase is the phase measured experimentally both on the 4T and 6T materials. Moreover, the broad background in the 600-1400 cm(-1) frequency range and the well-defined features which appear around 940 cm(-1) in the calculated INS spectra of 4T/HT and 6T/HT are assigned to multiphonon contributions. This multiphonon contribution at 940 cm(-1), which is absent in the 4T/LT and 6T/LT INS spectra, also constitutes a fingerprint of the HT phases. Finally, the calculated dispersion curves of the two polymorph phases of 4T and 6T are given.

The nonresonant Raman spectra of infinite peapod were calculated in the framework of bond-polarization theory by using either direct diagonalisation of dynamical matrix or the spectral moments method. The effect Of C-60-tube interaction on the mode frequencies both of the peas and the pod has been calculated using the Lennard-Jones, potential showing that the lowest frequency mode region is more affected than the higher one. The tube chirality's and diameter effects on Raman spectra were studied. Also, we show that filling factors Of C-60 molecules inside some single-walled carbon nanotubes (SWCNT) may induce a splitting of the radial breathing like mode (RBLM) into two components. Comparison with experimental results is reported. (c) 2005 Elsevier B.V. All rights reserved.