Engineering ultrahigh thermal anisotropy in 2D van der Waals films*

Thermally anisotropic materials, whose thermal conductivity differs depending on the direction of heat conduction, are technologically important for heat management and fundamentally intriguing in terms of their mechanism of heat transport. Previous studies have achieved thermal anisotropy through anisotropic bonding, fabricating heterostructures, and introducing low dimensional defects, giving rise to anisotropic ratios of 1-2 orders of magnitude. Herein we report an ultrahigh thermal anisotropy (~1000 at room temperature) in large-area thin films with tunable thicknesses made by stacking transition metal dichalcogenide (MoS₂ or WS₂) monolayers. We measure an ultralow cross-plane thermal conductivity comparable to that of air, which can be attributed to interlayer rotation and the lack of lattice order. In the in-plane direction, a high thermal conductivity close to that of the single crystal counterpart is maintained due to the long-range lattice order and grain connectivity in the polycrystalline monolayers. The overall thermal anisotropy ratio in our films is higher than that of any man-made or natural material, demonstrating interlayer structure as a new degree of freedom for engineering thermal anisotropy in matter.