Tunable Thermal Anisotropy Triggered by Quasi-Ballistic Heat Transport in WS<sub>2</sub> Crystals.
basic_science · Level V
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- Record sourced from PubMed, PMID 41125540.
- Also identified by DOI 10.1021/acs.nanolett.5c04514 and PMC identifier 12593381.
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Abstract
We investigate the influence of temperature and film thickness on the anisotropic thermal conductivity tensor of multilayer single-crystal WS<sub>2</sub> films of varying thickness (10 nm to 2.8 μm) across a wide temperature range (80-473 K). Experiments show that both in-plane (<i>k</i><sub>r</sub>) and out-of-plane (<i>k</i><sub><i>z</i></sub>) thermal conductivities increase with decreasing temperature, reaching, at 80 K in bulk WS<sub>2</sub>, values up to <i>k</i><sub>r</sub> ∼ 1000 W m<sup>-1</sup> K<sup>-1</sup> and <i>k</i><sub><i>z</i></sub> ∼ 13 W m<sup>-1</sup> K<sup>-1</sup>. The thermal anisotropy ratio η = <i>k</i><sub>r</sub>/<i>k</i><sub><i>z</i></sub> in bulk rises dramatically from 30 to 78 as the temperature decreases from 460 to 80 K, driven by the suppression of <i>k</i><sub><i>z</i></sub> 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 <i>k</i><sub><i>z</i></sub>. 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.