Anisotropic in-plane thermal transport in monolayer ReSe ₂ and its modulation through layer control and selenium vacancies: experiment <i>vs</i> . theory

Understanding phonon transport and thermal anisotropy in two-dimensional (2D) materials is essential for their integration into electronic and thermal nanoscale devices. In this work, we achieved the growth of large-area, contamination-free monolayer rhenium diselenide (ReSe₂) <i>via</i> chemical vapor deposition, confirmed by atomic force microscopy and HAADF-STEM imaging. To probe its anisotropic thermal properties, we employed non-contact low-temperature Raman spectroscopy with unpolarized laser excitation to measure its thermal conductivity (<i>κ</i>). We report an exceptionally low value of in-plane thermal conductivity <i>κ</i> ∼ 25.2 Wm^-1 K^-1 for the pristine monolayer, the lowest among the TMDs. Critically, we found that the introduction of more selenium vacancies further decreases the thermal conductivity to <i>κ</i> ∼ 20.7 Wm^-1 K^-1. Polarization-dependent Raman analysis reveals a layer-dependent change in the anisotropy ratio, with the ratio decreasing from 6.23 (pristine monolayer) to 4.42 (monolayer with vacancies) and further to 3.82 (trilayer), highlighting the distinct effects of both interlayer interactions and point defects on phonon transport. The decrease of <i>κ</i> with increasing thickness suggests enhanced phonon scattering from structural distortions and weak van der Waals coupling. These findings provide critical insights into how both layer thickness and intrinsic defects such as Se vacancies can be used to modulate anisotropic transport in low-symmetry 2D materials. In addition, we employed density functional theory and Boltzmann transport theory to elucidate the lattice phonon dynamics and thermal transport behaviour of monolayer ReSe₂. The computed lattice thermal conductivity (<i>κ</i>_l) exhibits excellent agreement (<i>κ</i>xl ≈ 21.5 Wm^-1 K^-1 and <i>κ</i>yl ≈ 23.8 Wm^-1 K^-1) with the experimental data, thereby providing strong validation for our experimental approach. This work establishes ReSe₂ as a strong candidate for thermoelectric and nanoelectronic applications where tunable thermal properties are paramount.