Improved Grain Growth in Large-Area Monolayer WX 2 (X: S, Se) Films through Controlled Reduction of Tungstate Precursors

Successful integration of two-dimensional (2D) semiconductors into next-generation hardware will require methods that can reliably produce highly crystalline monolayer films over large areas. While the synthesis of semiconducting 2D transition-metal dichalcogenides (TMDs) through chemical vapor deposition (CVD) has shown promise, CVD crystal growth is hampered by inconsistent nucleation events and irregular lateral growth rates. Here, we investigate the role that carrier gas plays in regulating the nucleation and growth of WSe 2 films synthesized via salt-assisted low-pressure CVD. We achieve monolayer films covering areas in excess of 5.5 mm 2, limited only by the size of the reaction vessel, by controlling the onset of crystal nucleation through an initial thermal ramp conducted in the absence of hydrogen gas. Films grown without hydrogen during this initial stage have larger, more uniform grains when compared to those grown with hydrogen carrier gas throughout the entire reaction. Photoluminescence (PL) mapping of these more uniform films reveals uniformly intense and narrow emission peaks comparable to those seen in exfoliated WSe 2 flakes. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction studies reveal that exposure to hydrogen gas during this initial phase promotes over-reduction of intermediate Na x WO 3 species, resulting in a significantly more disordered film. A careful study of intermediate deposits has yielded insights into the details of TMD growth chemistry. Finally, we show that 2D WS 2 growth in the absence of hydrogen during the initial phase confers similar benefits vis-à-vis structural and optical quality. Our study highlights that even subtle changes to carrier gas regimens in 2D TMD crystal growth protocols can have a substantial influence on the morphology and performance of large-area films.