Topotactic Conversion to High-Valent Metastable 2D Semimetallic W₂N₃ via Lateral Anion Exchange of Bilayer WS₂

Phase control in the vapor-phase growth of transition metal nitrides is typically restricted by substantial kinetic barriers during precursor dissociation and an inherent thermodynamic instability towards nitrogen loss. Consequently, extreme growth conditions such as high pressure or plasma environments are often required. Here, we circumvent these limitations to stabilize high-valent, metastable 2D semimetallic tungsten nitride (W₂N₃) by employing a tailored topotactic conversion of a bilayer WS₂ van der Waals template. In particular, we preserve the structural symmetry and stacking order of the W metal sublattice, while leveraging thermodynamic control to suppress vacancy aggregation, enabling homogeneous and synchronous nitrogen substitution across the layers. This uniform transformation facilitates "hyper-stoichiometric" nitrogen incorporation, inducing subtle structural distortions to trigger the electronic phase transition from a semiconductor to a semimetal. Beyond W₂N₃, our approach provides a universal route to synthesize nitrogen-rich, nonequilibrium 2D semimetallic nitrides from the broader family of transition metal dichalcogenides, advancing the development of functional 2D material engineering.