Unveiling Anisotropic in‐Plane Thermal Conductivity and Thermal Expansion Coefficient in Low‐Symmetry Few‐Layer ReS 2

Abstract

ABSTRACT ReS 2 belongs to a unique class of 2D materials with low structural symmetry and extremely weak interlayer bonding, which give rise to intriguing thermal properties, like in‐plane anisotropic thermal conductivity (ATC) and anisotropic thermal expansion coefficient (ATEC). This study provides experimental evidence of in‐plane ATC and ATEC in few‐layer ReS 2 , revealed through simultaneous two‐mode Raman thermometry. In a single optical configuration, ATC of ReS 2 has been determined by simultaneous analysis of two anisotropic Raman modes (III and V) corresponding to in‐plane crystallographic axes. For the five‐layer (5L) ReS 2 , the extracted thermal conductivities are ∼ 28.78 (‘ b ’‐axis) and ∼ 22.25 (‘ a ’‐axis) Wm −1 K −1 , respectively while the corresponding TECs are ∼ 6.1 × 10 −6 and ∼ 4.04 × 10 −6 K −1 . As the layer thickness varies from 5 to 8L, the ATC and ATEC ratios monotonically increase from 1.29 and 1.52 to 1.36 and 1.94, respectively. The ab initio calculations reveal that the directional in‐plane anisotropy in phonon dispersion, group velocity, and Grüneisen parameter gives rise to ATC and ATEC. This work paves the way for exploring in‐plane ATC and ATEC in other low‐symmetry 2D materials, with potential implications for device applications.