Ultrahigh thermal anisotropy in 2D van der Waals films with interlayer rotation*

The miniaturization and densification of modern-day electronics call for the concomitant advancement of thermal management strategies, including the discovery of novel, extreme materials that can serve as heat spreaders or insulators. Thermally anisotropic materials can perform both functions simultaneously; however, the ratios between heat transport along different axes in existing engineered anisotropic materials are usually around 20 or less. Here, we introduce layer-by-layer stacked transition metal dichalcogenide films with interlayer rotation, which boast a record-breaking thermal conductivity anisotropy ratio of nearly 900. Interlayer rotation maintains the crystallinity within each constituent monolayer and breaks the through-plane crystal symmetry. Notably, we achieve an ultralow through-plane thermal conductivity that is only twice the value of air. We interrogate the effect of interlayer rotation on thermal transport via molecular dynamics simulations, which reveal a 1D glass-like transport. Our films can spread heat from current carrying gold nanoelectrodes while insulating their surfaces. We show that interlayer rotation can be a new degree of freedom in manipulating thermal transport in crystalline, layered solids toward the realization of 2D phonon transport.