Methane-Mediated Vapor Transport Growth of Monolayer WSe₂ Crystals

The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe₂ has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe₂ hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe₂, but the difficulty of precisely controlling solid-state TMD precursors (WO₃, MoO₃, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe₂ by utilizing methane (CH₄) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe₂ without a gas-phase promoter, the catalytic growth of the monolayered WSe₂ with a CH₄ promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe₂ up to 100 μm. The significant improvement in the optical properties of the resulting WSe₂ indicates that CH₄ is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.