AbstractTwo-dimensional (2D) transition-metal dichalcogenides have attracted a considerable amount of attention because of their potential for post-silicon device applications, as well as for exploring fundamental physics in an ideal 2D system. We tested the chemical vapour deposition (CVD) of WS₂ using the gaseous precursors WF₆ and H₂S, augmented by the Na-assistance method. When Na was present during growth, the process created triangle-shaped WS₂ crystals that were 10 μm in size and exhibited semiconducting characteristics. By contrast, the Na-free growth of WS₂ resulted in a continuous film with metallic behaviour. These results clearly demonstrate that alkali-metal assistance is valid even in applications of gas-source CVD without oxygen-containing species, where intermediates comprising Na, W, and S can play an important role. We observed that the WS₂ crystals grown by gas-source CVD exhibited a narrow size distribution when compared with crystals grown by conventional solid-source CVD, indicating that the crystal nucleation occurred almost simultaneously across the substrate, and that uniform lateral growth was dominant afterwards. This phenomenon was attributed to the suppression of inhomogeneous nucleation through the fast and uniform diffusion of the gas-phase precursors, supported by the Na-assisted suppression of the fast reactions between WF₆ and H₂S.