Single wall carbon nanotubes (SWNTs) that are found as close-packed arrays in crystalline ropes have been studied by using Raman scattering techniques with laser excitation wavelengths in the range from 514.5 to 1320 nanometers. Numerous Raman peaks were observed and identified with vibrational modes of armchair symmetry (n, n) SWNTs. The Raman spectra are in good agreement with lattice dynamics calculations based on C-C force constants used to fit the two-dimensional, experimental phonon dispersion of a single graphene sheet. Calculated intensities from a nonresonant, bond polarizability model optimized for sp2 carbon are also in qualitative agreement with the Raman data, although a resonant Raman scattering process is also taking place. This resonance results from the one-dimensional quantum confinement of the electrons in the nanotube.

We report experimental and theoretical investigations on the pressure dependence of the Raman-active radial and tangential vibrational modes of single-wall carbon-nanotube bundles. Using 514.5-nm excitation, we find that the radial mode intensity disappears beyond 1.5 GPa, and the tangential mode intensity also drops considerably above this pressure. This observation is tentatively attributed to a loss in the electronic resonance in the Raman scattering cross section due to a hexagonal distortion in the cylindrical cross section of the tubes in the bundles under compression. Theoretical calculations were made as a function of pressure using a generalized tight-binding molecular dynamics scheme that included intertubule van der Waals coupling. Under certain model assumptions, the experimental pressure dependence of the radial mode is well described by the calculations, indicating that intertubule interactions strongly influence the ambient pressure frequency and the pressure behavior of the radial breathing mode.

We propose structural and electronic properties of recently synthesized SiC nanotubes. The nanotubes with a Si to C ratio of 1:1 exhibit rich morphologies and are shown to belong to two distinct categories that are close in energies but show significant differences in electronic and transport properties. Similarities and differences are invoked with the case of BN nanotubes to explain the observed surface reconstruction.