Raman spectroscopy of HiPCO single-walled carbon nanotubes at 300 and 5 K

Abstract

Single-walled carbon nanotubes (SWNTs) produced by the high pressure CO disproportionation (HiPCO method) and purified by controlled thermal oxidation in air have been studied by Raman spectroscopy at 300 and 5 K. Raman spectra have been observed at λ exc=632.8 and 441.6 nm laser excitation in the range of 160–1800 cm −1. In the low-frequency part of the spectra (the radial breathing mode range) eleven narrow lines can be detected at low temperatures, enabling an estimation of nanotube diameters (0.8–1.3 nm) and chirality. The width at half-maximum intensity of these spectral lines is about 3–4 cm −1 at 5 K. The Stokes and anti-Stokes spectra are measured at λ exc=632.8 nm at room temperature. The most intense lines in these spectra are caused with the resonant Raman-scattering process. With increasing temperature from 5 to 300 K the shift (3–4 cm −1) of the most intense high-frequency component of the tangential mode (G mode) to lower frequency is observed. Based on the analysis of the Stokes/anti-Stokes spectra and the G band shape, the corresponding lines were identified with metallic or semiconducting type of nanotubes.