G– and G+ in the Raman spectrum of isolated nanotube: a study on resonance conditions and lineshape

We analyze the high-energy Raman modes, G+ and G - , in a pair of one metallic and one semiconducting nanotube grown across a 100 μm wide slit. By combining Raman resonance profiles of the radial breathing mode and the high-energy modes, we assign the broad G- peak to a metallic and the G+ peak to a semiconducting nanotube. Considering theoretical predictions we show that both peaks, G- and G + , originate from the LO phonon. The G- peak is the longitudinal mode of the metallic tube; it is broadened and downshifted due to strong electron-phonon coupling in the metallic nanotube. The G+ peak is due to the longitudinal mode in the semiconducting tube. An asymmetric lineshape of the G- peak agrees with observations of the asymmetry to be an intrinsic feature of metallic nanotubes.