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.

SUBMITTER: Wang R 

PROVIDER: S-EPMC7341092 | biostudies-literature | 2020 Jul

REPOSITORIES: biostudies-literature