Synthesis of monolayer tungsten nitride: Rapid optical visualization and electrical impact of grain boundaries

Two-dimensional (2D) transition metal nitrides (TMNs) have garnered significant attention in fields such as energy storage and nanoelectronics due to their unique electrical properties, high chemical stability, and excellent mechanical strength. In polycrystalline 2D TMNs films, grain boundaries (GBs) are inevitable structural defects that could play a crucial role in determining the material's properties. Developing rapid optical visualization methods is essential for obtaining large-scale information on the distribution of GBs. However, the rapid visualization of GBs in 2D TMNs, as well as the impact of GBs on the material's electrical properties, has never been previously reported. In this study, we demonstrate the growth of monolayer tungsten nitride crystals on SiO₂/Si substrates by chemical vapor deposition (CVD). High-resolution transmission electron microscopy reveals the presence of GBs at the junctions of twisted grains. A wet-etch process utilizing buffered oxide etchant (BOE) enables rapid and effective visualization of these GBs with optical microscopy. By analyzing grains with different twist angles, we find that GBs at specific angles demonstrate increased stability during etching. Electrical measurements revealed that tilted GBs hinder electrical transport, with GBs of a 62° twist angle showing sheet conductance nearly half that within the monolayer grain. This work not only provides insights into GBs in monolayer tungsten nitride but also lays the groundwork for exploring GBs-related properties in other 2D TMNs.