Pressure behaviour of single wall carbon nanotube bundles and fullerenes: a Raman study

Abstract

We shall discuss our recent high pressure Raman studies on single wall carbon nanotube (SWNT) bundles and compare them with pressure-induced amorphous and polymeric phases of crystalline C₆₀ and C₇₀. Our high pressure Raman studies on SWNT bundles carried out upto 25.9 GPa show that the intensities of both the radial modes (ω≈170 cm⁻¹) as well as tangential modes (around 1590 cm⁻¹) decrease significantly with pressure, so much that the radial modes cannot be observed beyond 2.6 GPa. Most interestingly, the frequency of the dominant tangential mode increases upto 11 GPa, then decreases till 16 GPa and again increases. Raman spectra were resolved into four modes upto 10 GPa beyond which only one or two modes could be fitted to the recorded spectra. The pressure-softening of the mode between 11 and 16 GPa as well as other features of the Raman spectra are reversible in the decreasing pressure cycle. These results, though not understood at present, can be associated with the distortion of the circular cross-section of the tubes in the bundle, eventually leading to a possible transition of the SWNT bundle to graphite like carbon at 11 GPa which is completed at 16 GPa. This transition is reversible on decompression.