Impact of Polar Edge Terminations of the Transition Metal Dichalcogenide Monolayers during Vapor Growth

The polar edges of two-dimensional monolayer transition metal dichalcogenides (TMD) and their alloys are examined by combined theoretical (density functional theory) and experimental approaches. For these polar edges, the growth reaction energies between different edge terminations are considered instead of the surface free energies. Due to different energy evolutions during growth on the zigzag edges between MoS2 and WS2, the S-ZZ edges in the WS2 monolayer flakes more easily decompose into sawtooth-like edges in M-ZZ type as compared to the MoS2 monolayer; thus, the hexagonal morphology can be seen more often in WS2. Moreover, the observed anisotropic short-range order in the MoS2/WS2 alloys is originated from the freezed edge configurations during growth, explainable by the growth kinetics and thermodynamics of the Mo-ZZ-edges. The determination of the growing edge terminations is of great importance for the controllable synthesis of the emergent two-dimensional TMD materials.