Tunable Thermal Anisotropy Triggered by Quasi-Ballistic Heat Transport in WS₂ Crystals

We investigate the influence of temperature and film thickness on the anisotropic thermal conductivity tensor of multilayer single-crystal WS₂ films of varying thickness (10 nm to 2.8 μm) across a wide temperature range (80-473 K). Experiments show that both in-plane (k_r) and out-of-plane (k_z) thermal conductivities increase with decreasing temperature, reaching, at 80 K in bulk WS₂, values up to k_r ∼ 1000 W m^-1 K^-1 and k_z ∼ 13 W m^-1 K^-1. The thermal anisotropy ratio η = k_r/k_z in bulk rises dramatically from 30 to 78 as the temperature decreases from 460 to 80 K, driven by the suppression of k_z due to phonon transport entering the quasi-ballistic regime. We further analyze the cumulative thermal conductivity as a function of phonon mean free path (MFP), showing that phonons with MFPs < 200 nm contribute to 70% of the total k_z. This work provides fundamental insight into the interplay between dimensionality, temperature, and anisotropic phonon transport in two-dimensional materials, where thermal anisotropy can be strategically leveraged for performance optimization.