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 1 – 4 . Although the epitaxial growth of wafer-scale graphene and hexagonal boron nitride on metal surfaces has been reported 5 – 8 , these techniques are not applicable for growing TMDs on insulating substrates because of substantial differences in growth kinetics. Thus, despite great efforts 9 – 20 , 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 2 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 2 interaction leads to two preferred antiparallel orientations of the WS 2 crystal, and sapphire step edge–WS 2 interaction breaks the symmetry of the antiparallel orientations. These two interactions result in the unidirectional alignment of nearly all the WS 2 islands. The unidirectional alignment and seamless stitching of WS 2 islands are illustrated via multiscale characterization techniques; the high quality of WS 2 monolayers is further evidenced by a photoluminescent circular helicity of ~55%, comparable to that of exfoliated WS 2 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.