Raman on carbon nanotubes using a tunable laser and comparison with photoluminescence

Stokes and anti-Stokes Resonance Raman experiments on carbon nanotubes using a tunable laser system are reported. Both arc discharge and HiPco samples are investigated with laser excitation energies varying continuously from 1.6eV up to 2.7eV. The usual features, i.e., the RBM, D-band and G-band are analyzed, as well as the intermediate frequency modes (IFMs) appearing between the RBM and the D/G band spectral region. The results are analyzed based on electron and phonon structure. Step-like dispersive behavior is observed for the IFMs and this anomalous result is analyzed based on Raman selection rules plus quantum confinement of electrons and phonons. An experimental determination of the van Hove singularity energies E(ii) is compared with photoluminescence results. The single versus double resonance nature of the G band SWNT spectra is briefly addressed.