Softening of the Radial Breathing Mode in Metallic Carbon Nanotubes

Abstract

The softening of the radial breathing mode (RBM) of metallic single walled carbon nanotubes (m-SWNTs) due to electron-phonon coupling has been studied by observing the Fermi level (ϵ[subscript F]) dependence of the RBM Raman peak. In situ Raman spectra were obtained from several individual m-SWNTs while varying ϵ[subscript F] electrochemically. The RBM frequency of an intrinsic m-SWNT is shown to be down-shifted relative to highly doped tubes by ~2 cm[subscript -1]. The down-shift is greatest for small diameter and small chiral angle SWNTs. Most tubes show no change in RBM linewidth. A comparison is drawn between the RBM and the G band (ALO phonon) with respect to the ϵ[subscript F] dependence of their frequencies and linewidths.