High-pressure optical study of small-diameter chirality-enriched single-wall carbon nanotubes

We have investigated the mechanical stability of small-diameter single-wall carbon nanotube (SWCNT) films via optical absorption spectroscopy under high pressure. The studied sample was enriched in (6,5) SWCNTs, in order to observe sharp optical transitions even at high pressures. We observed two well-defined absorption bands in the studied frequency range, both of which red-shifted with increasing pressure. The rate of the pressure-induced red shift of one absorption band was found to change dramatically at 8GPa, which we interpret as a structural phase transition of the nanotubes' cross-section from circular to oval. By comparing the data with that for a film of mixed-chirality SWCNTs with an average diameter of 1.4nm, we conclude that smaller-diameter SWCNTs have higher mechanical stability, which is consistent with theoretical expectations. No collapse of (6,5) SWCNTs was observed up to 22GPa.