Systematic comparison of the Raman spectra of metallic and semiconducting SWNTs

Raman spectra of individual SWNTs were systematically investigated by comparing the behavior of 21 semiconducting and 32 metallic SWNTs. It is found that, compared to semiconducting SWNTs, the RBM of metallic ones is softened, which exhibits chirality dependence. There are significant differences in the line shape of the G-band between semiconducting and metallic SWNTs, which was attributed to electron-phonon coupling previously. However, we found that the differences cannot be explained only by the electron-phonon coupling via the Kohn anomaly mechanism. Curvature effect and other unknown reasons appear to also contribute to the dissimilarities. As for the D-band, the frequency of metallic SWNTs does not show a softening effect when compared with that of semiconducting SWNTs, which is not consistent with theoretical predictions.