Phonon energy inversion in graphene during transient thermal transport

This work reports on the phonon energy inversion in graphene nanoribbons: after initial localized thermal excitation, the energy of initial cold phonons (flexural mode: FM) becomes higher than that of local hot phonons (longitudinal and transverse modes: LM/TM). Such energy inversion holds for about 50 picoseconds. Two physical factors combine together to give rise of this phenomenon: one is the much faster heat conduction by FM phonons than that by LM/TM phonons, and the other factor is the strongly temperature-dependent energy exchange rate between FM and LM/TM phonons: 3.7×1010 s−13.7×1010 s−1 at 84 K to 20.3×1010 s−120.3×1010 s−1 at around 510 K.

► Phonon energy inversion is observed in graphene during transient thermal transport. ► The lifetime of energy inversion is around 50 ps. ► Energy coupling between out-of-plane and in-plane modes phonons increases quickly with increasing temperature. ► Both dynamic and static heat sources can evoke the energy inversion in graphene. ► Energy inversion is weakened with the increasing layer number of graphene.