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. ΔT (OP -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 MoS(2) and MoSe(2) by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman-based thermal conductivity measurement of 2D materials. G (OP↔AP) for MoS(2), MoSe(2), and graphene paper (GP) are characterized. For MoS(2) and MoSe(2), G (OP↔AP) is in the order of 10(15) and 10(14) W m(-3) K(-1) and G (ZO↔AP) is much smaller than G (LO/TO↔AP). Under ns laser excitation, G (OP↔AP) is significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP, G (LO/TO↔AP) is 0.549 × 10(16) W m(-3) K(-1), agreeing well with the value of 0.41 × 10(16) W m(-3) K(-1) by first-principles modeling.