▎ 摘　　要

Graphene is promising for thermal management applications due to its superior thermal conductivity, but the inherent thermal transport mechanism is not fully understood. In this work, we directly extract the mode contributions to the thermal conductivity of free-standing and supported graphene nanoribbons using nonequilibrium molecular dynamics simulations. With the aid of the mode analyses, it is uncovered that the acoustic contribution increases with respect to the characteristic length in suspended graphene, and such a size-dependency is compromised in supported graphene. In addition, the contribution of optical modes could be more than 25% for heat conduction in narrow graphene nanoribbons because of strong boundary scatterings. Furthermore, the LA and TA modes rather than the ZA modes are dominant heat carriers in suspended and supported graphene owing to their large group velocities and long mean free path. Our results are instructive for understanding the mode thermal transport in free and supported graphene.