Distinguishing Optical and Acoustic Phonon Temperatures and Their Energy Coupling Factor under Photon Excitation in nm 2D Materials

Abstract

Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long-history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state-resolved Raman spectroscopy. ΔTOP −AP is measured to take more than 30% of the Raman-probed temperature rise. A breakthrough is made on measuring the intrinsic in-plane thermal conductivity of suspended nm MoS2 and MoSe2 by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman-based thermal conductivity measurement of 2D materials. GOP↔AP for MoS2, MoSe2, and graphene paper (GP) are characterized. For MoS2 and MoSe2, GOP↔AP is in the order of 1015 and 1014 W m−3 K−1 and GZO↔AP is much smaller than GLO/TO↔AP. Under ns laser excitation, GOP↔AP is significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP, GLO/TO↔AP is 0.549 × 1016 W m−3 K−1, agreeing well with the value of 0.41 × 1016 W m−3 K−1 by first-principles modeling.