Raman spectroscopy of nondispersive intermediate frequency modes and their overtones in carbon nanotubes

We performed resonance Raman spectroscopy on HiPCO-produced single-walled carbon nanotubes in the spectral region of 846 and 1692 cm(-1), using laser excitation energies between 1.58 and 2.73 eV. We observe a characteristic peak around 846 cm-1 and its overtones around 1692 cm-1. Especially the latter was controversially discussed in the past and has been assigned to a double-resonant Raman process involving two oTO phonons. We demonstrate that both peaks slightly depend on the excitation laser energy by means of their shape, intensity, and peak position. Following the model proposed by Dobardzic et al., we relate both peaks to oTO Gamma-point phonons. As we analyze both, the first-order band and its overtone simultaneously, we are able to provide a unified explanation of these low-intensity Raman bands in carbon nanotubes. [GRAPHICS] (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim