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 MoO3-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.