In , it is stressed the role of curvature. "The Raman-allowed tangential mode in graphite", shifted at 1582 cm -1 , is the G mode (see figure in Jorio et al.). "Unlike graphite, the tangential G mode in SWNTs gives rise to a multi-peak feature, also named the G band, where up to six Raman peaks can be observed in a first-order Raman process. However, a simple analysis can be carried out considering the two most intense peaks, that basically originate from the symmetry breaking of the tangential vibration when the graphene sheet is rolled to make a cylindrically shaped tube" . The two main components of the G band are G + , for mode displacements along the axis of the nanotube, and G -, for mode displacement "along the circumferential direction". "The lowering of the frequency for the G -mode is caused by the curvature of the nanotube which softens the tangential vibration in the circumferential direction" . Moreover, in metallic and semiconducting SWNTs, the G band is different in the shape of the G -feature. In metallic SWNTs the G -line is broader than that observed in semiconducting nanotubes; "this broadening is related to the presence of free electrons in nanotubes with metallic character" (Jorio et al., mentioning . "This broadened G -feature is usually fit using a BWF line that accounts for the coupling of a discrete phonon with a continuum related to conduction electrons" (Jorio et al., mentioning Brown et al., . Also in , we find stressed that the "BWF line is observed in many graphite-like materials with metallic character, such as n-doped graphite intercalation compounds (GIC), n-doped fullerenes, as well as metallic SWNTs".