Dependence of Raman spectra G′ band intensity on metallicity of single-wall carbon nanotubes

We report the peculiar behavior of the G(') band Raman intensity, which is dependent on the metallicity of single-wall carbon nanotubes (SWCNTs). In the metallic SWCNTs, the G(') band intensity was enhanced relative to the G band intensity, while the G(') band intensity was suppressed in the semiconducting SWCNTs. Resonance Raman spectroscopy (using laser energies of E-laser=2.41, 1.96, 1.58, and 1.165 eV) showed these features on the metal-enriched and semiconducting-enriched SWCNT samples that had been selectively separated by the nitronium ions. The metallicity dependence was explained theoretically by calculating the resonance Raman intensity within the extended tight-binding calculations. The calculated results confirm that the G(') band intensity of the metallic SWCNTs is stronger than that for the semiconducting SWCNTs because the electron-phonon matrix elements for the TO phonon at the K point is larger for metallic SWCNTs and the resonance window for E-33(S) is larger than that for E-11(M).