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. 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 MoS₂ and MoSe₂ by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman-based thermal conductivity measurement of 2D materials. G _OP↔AP for MoS₂, MoSe₂, and graphene paper (GP) are characterized. For MoS₂ and MoSe₂, G _OP↔AP is in the order of 10¹⁵ and 10¹⁴ 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 × 10¹⁶ W m^-3 K^-1, agreeing well with the value of 0.41 × 10¹⁶ W m^-3 K^-1 by first-principles modeling.

Keywords: 2D materials; acoustic phonon temperature; energy coupling; energy transport state‐resolved Raman; optical phonon temperature.