Topotactic Conversion of Two-Dimensional WSe₂ into Atomically Thin Nonlayered Metal Nitrides

To date, much of the synthesis efforts pertaining to atomically thin materials have been directed toward van der Waals layered structures. When nonlayered materials are thinned down to the atomic scale, they exhibit markedly different emergent properties compared with their bulk counterparts. However, the lack of a scalable synthesis technique for atomically thin nonlayered materials possessing single crystallinity poses a substantial barrier to exploring their intrinsic physical properties and potential applications. Here, we present a topotactic synthesis approach for producing atomically thin nonlayered tungsten dinitride (WN₂) single crystals, utilizing van der Waals layered tungsten diselenide (WSe₂) as precursor materials. Our investigations reveal the conversion of even bilayer WSe₂ into WN₂ with high degrees of single crystallinity and nitrogen-rich elemental composition. Employing an h-BN mask-assisted spatially controlled topotactic conversion strategy, we fabricate lateral WN₂-WSe₂ heterojunctions, leading to a notable enhancement in the on-off ratio compared to conventional Pt/WSe₂ planar contacts. Furthermore, local hydrogen evolution reaction (HER) measurements highlight the improved electrocatalytic activity of WN₂ compared to WSe₂. Our study provides insights into scalable synthesis methods for atomically thin nonlayered materials and offers a promising platform for developing transitional metal nitride (TMN)-based electronics and advanced catalysts.