Measuring the absolute Raman cross section of nanographites as a function of laser energy and crystallite size

In this paper, the dependence of the differential Raman cross section of the D, G, D^', and G^' bands of nanographites on the excitation laser energy and also on the crystallite size is reported. We show that ₆ is proportional to the fourth power of the excitation laser energy (E₋), as predicted by the Raman scattering theory. For the bands which arise from the double-resonance mechanism (D, D^', and G^'), the differential cross section does not depend on E₋, explaining the strong dependence of the ratio I₃∕I₆ on the excitation laser energy E₋ used in the Raman experiment. The L₀ dependence of D and D^' band differential cross sections is measured, confirming that the proportionality I₃∕I₆L₀^-1 originates from the strong dependence of ₃ on the inverse of the crystallite size. In the G^' band case, the data show that its differential cross section increases with the increasing crystallite size L₀, following an opposite behavior when compared with the disorder induced D and D^' bands. An analysis on the dependence of the full width at half maximum () of the D, G, D^', and G^' bands on the crystallite size L₀ of nanographites is performed, showing that the phonon lifetime is proportional to the crystallite size.