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.