Probing the single-wall carbon nanotube bundle: Raman scattering under high pressure

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.