Unveiling Anisotropic in-Plane Thermal Conductivity and Thermal Expansion Coefficient in Low-Symmetry Few-Layer ReS2.

ReS2 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 ReS2, revealed through simultaneous two-mode Raman thermometry. In a single optical configuration, ATC of ReS2 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) ReS2, the extracted thermal conductivities are ∼ 28.78 ('b'-axis) and ∼ 22.25 ('a'-axis) Wm-1K-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.