Pressure-controlled growth and mechanism of monolayer WSe2 via NaCl-assisted CVD

Precise control of monolayer WSe2 growth is critical for electronic and optoelectronic applications. Here, we systematically investigate the effect of growth pressure on chemical vapor deposition-synthesized WSe2 on SiO2/Si substrates. By tuning the pressure from atmospheric pressure to 2 × 103 Pa, the domain morphology evolves from multilayer triangular flakes with thick centers and edges to uniform monolayers at 5 × 103 Pa and finally to hollow structures at ultra-low pressure due to H2-assisted etching. Raman, photoluminescence, x-ray photoelectron spectroscopy, and x-ray diffraction analyses confirm high crystallinity, uniform thickness, and phase-pure 2H-WSe2 at optimal pressure. A microscopic growth mechanism is proposed: pressure controls the precursor mean free path, which governs collision frequency and flux, while higher edge surface energy and anisotropic armchair/zigzag attachment barriers direct preferential deposition. These findings establish pressure as a key parameter for reproducible, high-quality WSe2 monolayer growth.