Raman study of individually dispersed single-walled carbon nanotubes under pressure

We report the resonant Raman spectra of individualized =debundled single-walled carbon nanotubes SWNTs having diameters d 0.8–1.3 nm subject to compression up to 10.5 GPa. Both SWNTs in watersurfactant dispersions as well as SWNTs deposited onto glass microfibers and compressed with methanolethanol were studied. In the low pressure regime 1 GPa , linear and reversible up-shifts of the Raman bands associated with the radial breathing vibrational mode RBM and tangential G mode were observed. The pressure derivative of the RBM frequency increases with increasing d and, unexpectedly, is larger for metallic than for semiconducting tubes. Above 1–2 GPa, RBM bands of additional SWNT species appear. This is due to pressure-induced shifts into resonance and broadening of the corresponding optical transitions. The latter could be quantified by photoluminescence PL measurements of the corresponding semiconducting tubes that are also reported here. Disappearance of the RBM and G bands contributed by nanotubes with d 0.8–0.9 and 1.2–1.3 nm at 10 and 4 GPa, respectively, is tentatively assigned to extensive radial deformation of the nanotubes, in accordance with theoretical predictions. After the application of several GPa pressure, a significant loss of Raman signals and a relative increase of the defect-induced D band are found, in particular for metallic SWNTs. We attribute these and other irreversible effects in the Raman spectra to defects generated in nanotubes under compression most likely via chemical processes . Comparable structural deterioration of semiconducting nanotubes is evidenced by strong irreversible changes in their PL spectra.