Double-resonant Raman scattering with optical and acoustic phonons in carbon nanotubes

Abstract

We analyze and assign two low-intensity, double-resonant Raman bands at around 1950 ${\mathrm{cm}}^{\ensuremath{-}1}$ and 2450 ${\mathrm{cm}}^{\ensuremath{-}1}$ in carbon nanotube ensembles. We present Raman spectra of a high-pressure carbon monoxide sample for laser energies in the visible range. By using density functional theory plus zone folding, we simulate both combination modes for various excitation energies with respect to the chiral-index distribution of carbon nanotubes in the sample. For a comparison, double-resonant Raman spectra of single carbon nanotubes are also modeled within a symmetry-based nonorthogonal tight-binding approach, resulting in a different assignment of the 1950-${\mathrm{cm}}^{\ensuremath{-}1}$ band. Apparent discrepancies in the assignments are discussed in the article.