The G* (2450 cm$^{-1})$ Double Resonance Raman Peak in single-, few-layer graphene and DWNTs

The dispersion and skewness of the 2450 cm$^{-1}$ peak in the raman spectra of carbon structures was analyzed. The dispersion of this peak for graphene is smaller in magnitude and of opposite sign than that for the G' ($\sim $2700cm$^{-1})$. This dispersion is independent on number of layers. The peak shows asymmetry (skewness) which increases with E$_{laser}$. The observations can be explained by viewing this double resonance process arising by the scattering with both an iTO and a iLA phonon. The peak becomes more symmetric in DWNTs and it shows a stronger curvature dependence than the G' peak.