Radial and tangential vibrational modes of HiPCO-derived carbon nanotubes under pressure

We investigate the differences in the pressure dependence of the Raman-active phonons in carbon nanotubes of different diameter and conducting nature. The pressure dependence of the tangential bands of the metallic tubes is found to be about the same as that of the semiconducting tubes. However, for the radial bands, the larger diameter semiconducting tubes are seen to exhibit a larger pressure coefficient than smaller diameter metallic tubes. We assign this behavior to the diameter dependence and not the conductivity of the tubes. We find a strong inverse diameter dependence for the normalized pressure derivative of the radial band frequency (omega(R)) in contrast to the recent predictions for isolated tubes. This suggests that faceting of the tubes in the bundles may be the dominant contribution to the pressure dependence of omega(R).