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. Δ T 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.

Nanostructured and Heterostructured 2D Materials for Thermoelectrics

“…[55] These accurate measurements of the bulk crystals provide a baseline for understanding thermal transport in 2D TMDCs. The thermal conductivities of single-and few-layer TMDCs have been reported by a few Raman-based [57][58][59][60][61][62][63] and micro-bridge-based [64][65][66] measurements, but the limited data scattered in a large range (between ～15 and ～100 W/m•K at RT for monolayer MoS2). Meanwhile, careful MD simulations, [68], [50] first-principles [67] and BTE [69] calculations agreed well on the in-plane thermal conductivities of 2D MoS2.…”

Energy and Charge Transport in 2D Atomic Layer Materials: Raman-Based Characterization

“…Thus, neglect of nonequilibrium between ZA phonons and optical phonons can induce significant underestimation of thermal conductivity. Wang et al designed and employed a nanosecond ET-Raman technique to explore the temperature nonequilibrium among different phonon branches [ 35 ].…”

“…The length, width, and thickness of the sample are around 17 mm, 0.28 mm, and 28.6 μm, respectively [ 36 ]. Then, based on the coupling factors, the temperature difference between electrons and AP is calculated to be around 2.8 × 10 −8 K, which is negligible compared with a measured temperature rise of 2 K [ 35 , 37 ].…”

“…( c ) The determination of thermal conductivity and energy coupling coefficient between optical phonons and acoustic phonons. Figure reproduced from [ 35 ], with permission from John Wiley and Sons.…”

“…( c ) Distinct temperatures of different phonon branches. Figure reproduced from [ 35 ], with permission from John Wiley and Sons.…”