Vibrational properties of single walled carbon nanotubes under pressure from Raman scattering experiments and molecular dynamics simulations

We have investigated theoretically and experimentally the pressure dependent structural and vibrational properties of single-walled carbon nanotubes (SWNT). Bundles of SWNTs are studied inside a diamond anvil high pressure cell with micro-Raman scattering. The tube diameter and size distribution are determined from the radial breathing modes (RBM) of the tubes. The pressure coefficient and linewidth of one of the high frequency C-C bond stretching modes was found to change suddenly at similar to 3 GPa which is well below the structural transformation pressure (P-c) for the tubes in our sample. Molecular dynamics simulations were used to calculate the vibrational density of states of SWNTs as a function of pressure. Our simulations suggest that the experimental results can be explained by a softening of a low-frequency optical mode of the SWNTs - the "squashing" mode - and the dynamical effect of this softening on other phonon modes. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.