Intermediate-frequency Raman modes for the lower optical transitions of semiconducting single-walled carbon nanotubes

A new class of intermediate-frequency modes (IFMs) associated with the E22S and E11S optical transitions of bundled HiPco single-walled carbon nanotubes (SWNTs) have been investigated via tunable laser (700- 985 nm) resonance Raman spectroscopy. "Steplike" dispersive behavior was observed for these IFMs, along with associated clusters of radial breathing mode (RBM) overtones at higher frequencies. While the excitation profiles of both RBM and RBM overtones follow a classical behavior predicted by resonance Raman theory, significant differences are observed for the IFM excitation profiles. The observed IFM maxima were found to obey a resonance behavior based on a combination of the E22S and E11S transition energies, scaled by the inverse diameter of the respective nanotube. Only IFMs for the mod(n-m,3)=2 nanotubes are visible, with intensities found to obey the family-based chiral angle dependence similar to previously reported electron-phonon interaction patterns observed for the RBM.