First Brillouin zone and out-of-plane phonon modes. The phonon dispersion relation of the ZA and ZO modes as a function of the in-plane reciprocal vector . The vertical axis is the phonon frequency, while the horizontal axes are momentum space on the graphene lattice. The dispersion relation is obtained using the second-nearest-neighbor model for graphene [11]. The gray surface corresponds to the ZO (optical) mode, whereas the pink surface shows the ZA (acoustic) mode. Also shown are the corresponding , , and points of the Brillouin zone.
