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To address this issue, researchers have used the metal–organic (MO) CVD method, which uses MO compounds with a vapour pressure higher than those of conventional metal oxides and elemental sulphur. This high melting point makes it difficult to control the precursor supply and achieve uniform, large-scale growth of TMDs 30, meaning that solid-source CVD is not suitable for industrial TMD production. WO 3, which is commonly used for growing WS 2) have low vapour pressures at typical growth temperatures because of their high melting points (e.g. One persistent problem encountered during the conventional CVD method is the low vapour pressure of the solid-source precursors, such as sulphur and metal oxides.

Another hypothesis is that a catalytic effect at the edges of the forming TMDs may be promoting 2D crystal growth 22, 29. For example, the volatile metal oxychloride species and/or eutectic intermediate compounds generated during the process, such as Na 2MoO 4, could be affecting the chemical reaction route and enhancing the diffusion of the metal precursors 23– 28. Although the effect of alkali metals has not been completely clarified in terms of the chemical processes involved, several models have been proposed to explain the phenomenon. Because this effect is valid not only for group-VI TMDs, but also for other TMDs, such as NbS 2 and TiS 2 25, gaining insight into the underlying mechanisms of this phenomenon is important for expanding the range of future applications of TMDs. When alkali-metal compounds, such as NaCl and KBr, are added into a solid-source CVD system, TMD grains are able to grow much larger than they otherwise would 18– 24. One topic of recent importance for the CVD growth of TMDs is the ‘alkali-metal assistance’ method. TMD monolayers with grain sizes larger than 10 μm have been prepared using this approach 15– 17. Typical CVD growth methods for TMDs use a metal oxide, such as MoO 3 or WO 3, and elemental sulphur as precursors, by heating the solid precursors along with a substrate under an inert gas atmosphere at a temperature above 500 ☌. The CVD method, in particular, is extensively used to fabricate large-scale monolayer TMDs with better reproducibility than that afforded by other methods. Monolayer TMDs can be prepared using a range of techniques, such as mechanical or chemical exfoliation of bulk single crystals, chemical vapour deposition (CVD) and molecular beam epitaxy 10– 14. Since monolayer group-VI TMDs, such as MoS 2 and WS 2, are flexible direct-gap semiconductors 3, 4, they are considered promising candidates for use in next-generation semiconductor devices, including extremely thin-body transistors, flexible electronic devices, photodetectors and light-emitting devices 5– 9. Two-dimensional (2D) materials, such as graphene, hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs), have gained considerable attention in recent few years, due to their electronic and optoelectronic properties 1– 3. 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 WF 6 and H 2S. We observed that the WS 2 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. 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.

By contrast, the Na-free growth of WS 2 resulted in a continuous film with metallic behaviour. When Na was present during growth, the process created triangle-shaped WS 2 crystals that were 10 μm in size and exhibited semiconducting characteristics. We tested the chemical vapour deposition (CVD) of WS 2 using the gaseous precursors WF 6 and H 2S, augmented by the Na-assistance method. Two-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.

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