Softening of the radial breathing mode in metallic carbon nanotubes

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 (εF) dependence of the RBM Raman peak. In situ Raman spectra were obtained from several individual m-SWNTs while varying εF electrochemically. The RBM frequency of an intrinsic m-SWNT is shown to be down-shifted relative to highly doped tubes by ∼2cm-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 εF dependence of their frequencies and linewidths.