Steplike dispersion of the intermediate-frequency Raman modes in semiconducting and metallic carbon nanotubes

Resonance Raman spectra of semiconducting and metallic single-wall carbon nanotube bundles have been investigated in the spectral range between 600 and 1100 cm-1, which is associated with intermediate frequency modes. A large range of nanotube diameters corresponding to HiPco and electric-arc samples (0.7-1.8 nm), and laser excitation energies between 1.62 and 2.71 eV have been used in the measurements. A rich set of peaks is observed, some of them independent of the excitation laser energy and some exhibiting a steplike dispersive behavior. The nondispersive peaks are assigned. The model proposed by Fantini [Phys. Rev. Lett. 93, 087401 (2004)] to explain the steplike dispersive behavior of the intermediate frequency modes is then extended to different SWNT diameters and different Eii transitions, including semiconducting and metallic SWNTs.