Anisotropic thermal transport in van der Waals layered alloys WSe2(1-x)Te2x

Abstract

Transition metal dichalcogenide (TMD) alloys have attracted great interest in recent years due to their tunable electronic properties and the semiconductor-metal phase transition along with their potential applications in solid-state memories and thermoelectrics among others. However, the thermal conductivity of layered TMD alloys remains largely unexplored despite that it plays a critical role in the reliability and functionality of TMD-enabled devices. In this work, we study the composition- and temperature-dependent anisotropic thermal conductivity of the van der Waals layered TMD alloys WSe2(1-x)Te2x in both the in-plane direction (parallel to the basal planes) and the cross-plane direction (along the c-axis) using time-domain thermoreflectance measurements. In the WSe2(1-x)Te2x alloys, the cross-plane thermal conductivity is observed to be dependent on the heating frequency (modulation frequency of the pump laser) due to the non-equilibrium transport between different phonon modes. Using a two-channel heat conduction model, we extracted the anisotropic thermal conductivity at the equilibrium limit. A clear discontinuity in both the cross-plane and the in-plane thermal conductivity is observed as x increases from 0.4 to 0.6 due to the phase transition from the 2H to the Td phase in the layered alloys. The temperature dependence of thermal conductivity for the TMD alloys was found to become weaker compared with the pristine 2H WSe2 and Td WTe2 due to the atomic disorder. This work serves as an important starting point for exploring phonon transport in layered alloys.