Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS 2 monolayer on vicinal a-plane sapphire

The growth of wafer-scale single-crystal two-dimensional transition metal dichalcogenides (TMDs) on insulating substrates is critically important for a variety of high-end applications . Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported , these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts , the direct growth of wafer-scale single-crystal TMDs on insulating substrates is yet to be realized. Here we report the successful epitaxial growth of two-inch single-crystal WS monolayer films on vicinal a-plane sapphire surfaces. In-depth characterizations and theoretical calculations reveal that the epitaxy is driven by a dual-coupling-guided mechanism, where the sapphire plane-WS interaction leads to two preferred antiparallel orientations of the WS crystal, and sapphire step edge-WS interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS islands. The unidirectional alignment and seamless stitching of WS islands are illustrated via multiscale characterization techniques; the high quality of WS monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS flakes. Our findings offer the opportunity to boost the production of wafer-scale single crystals of a broad range of two-dimensional materials on insulators, paving the way to applications in integrated devices.