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 (kr) and out-of-plane (kz) thermal conductivities increase with decreasing temperature, reaching, at 80 K in bulk WS2, values up to kr ∼ 1000 W m-1 K-1 and kz ∼ 13 W m-1 K-1. The thermal anisotropy ratio η = kr/kz in bulk rises dramatically from 30 to 78 as the temperature decreases from 460 to 80 K, driven by the suppression of kz 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 kz. 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.