Anharmonic phonon effects in Raman spectra of unsupported vertical graphene sheets

Temperature-dependent Raman scattering is performed on unsupported vertical graphene sheets, which are approximate to free graphene without supporting the substrate. Here the observed G peak line shift with temperature is completely consistent with the theoretical prediction based on the first-principles calculation on free graphene, and our result is helpful to understand intrinsic anharmonic phonon characteristics of free graphene and the divergence on the G peak line shift with temperature. However, the observed linewidth variation is different from the prediction. To reveal the origins, a simplified Klemens model is used, and the dominating anharmonic phonon scattering mechanism is explored. In addition, line shift and linewidth variations of D and 2D peaks of the graphene sheets with temperature are revealed, and the possible mechanisms dominating the results are discussed.