▎ 摘　　要

Although graphene holds great promise in thermal applications owing to its superior thermal conductivity, an intriguing question remains as to which polarizations and frequencies are dominant in its heat conduction. In this work, by incorporating the direction-dependent phonon-boundary scattering and the special selection rule for three-phonon scattering into the linearized phonon Boltzmann transport equation, we systematically investigate the relative contributions from longitudinal-acoustic, transverse-acoustic, and out-of-plane acoustic (ZA) branches to the thermal conductivity of graphene ribbons, focusing on the effects of their size and temperature. We find that the relative contribution from ZA branch to heat conduction increases with decreasing the size, specularity parameter, and temperature of graphene ribbons. Our analysis reveals that this change arises from the huge difference in the phonon dispersion and in the phonon mean free path of Umklapp process between in-plane and out-of-plane branches. (C) 2014 AIP Publishing LLC.