Effective shape-controlled growth of monolayer MoS₂ flakes by powder-based chemical vapor deposition

In the recent years, transition-metal dichalcogenides such as MoS₂ have attracted considerable attention owing to their unique structure and electronic properties. Chemical vapor deposition (CVD) is a popular method for producing MoS₂ flakes with different shapes. Here, we report an effective method for achieving a broad range of shape evolution in CVD-grown monolayer MoS₂ flakes. By controlling the S and MoO₃ temperatures, the shape of the monolayer MoS₂ flakes was varied from hexagonal to triangular via intermediate shapes such as truncated and multi-apex triangles. The shape evolution of the MoS₂ flakes can be explained by introducing the term "nominal Mo: S ratio", which refers to the amount of loaded MoO₃ and evaporated S powders. By using the nominal Mo: S ratio, we predicted the potential reaction atmosphere and effectively controlled the actual proportion of MoO_3−x with respect to S in the growth region, along with the growth temperature. From the systematic investigation of the behavior of the shape evolution, we developed a shape-evolution diagram, which can be used as a practical guide for producing CVD-grown MoS₂ flakes with desired shapes.