Probing Anisotropic Thermal Conductivity of Transition Metal Dichalcogenides MX2 (M = Mo, W and X = S, Se) using Time‐Domain Thermoreflectance

Transition metal dichalcogenides (TMDs) are a group of layered 2D semiconductors that have shown many intriguing electrical and optical properties. However, the thermal transport properties in TMDs are not well understood due to the challenges in characterizing anisotropic thermal conductivity. Here, a variable‐spot‐size time‐domain thermoreflectance approach is developed to simultaneously measure both the in‐plane and the through‐plane thermal conductivity of four kinds of layered TMDs (MoS2, WS2, MoSe2, and WSe2) over a wide temperature range, 80–300 K. Interestingly, it is found that both the through‐plane thermal conductivity and the Al/TMD interface conductance depend on the modulation frequency of the pump beam for all these four compounds. The frequency‐dependent thermal properties are attributed to the nonequilibrium thermal resistance between the different groups of phonons in the substrate. A two‐channel thermal model is used to analyze the nonequilibrium phonon transport and to derive the intrinsic thermal conductivity at the thermal equilibrium limit. The measurements of the thermal conductivities of bulk TMDs serve as an important benchmark for understanding the thermal conductivity of single‐ and few‐layer TMDs. Anisotropic thermal conductivities of layered 2D crystals (MoS2, MoSe2, WS2, and WSe2) are measured using a variable‐spot‐size time‐domain thermoreflectance approach at 80–300 K. Non‐equilibrium thermal transport between the high‐frequency and the low‐frequency phonons is observed in the measurement results, and a two‐temperature model is used to extract the intrinsic thermal‐conductivity values (at the equilibrium limit).