Nucleation mechanism and morphology evolution of MoS_2 flakes grown by chemical vapor deposition

We study the nucleation mechanism and morphology evolution of MoS2 flakes grown by chemical vapor deposition （CVD） on SiO2/Si substrates with using S and MoO3 powders. The MoS2 flake is of monolayer with triangular nucleation, which might arise from the initial MoO3-x that is deposited on the substrate, and then bonded with S to form MoS2 flake. The ratio of Mo and S is higher than 1：2 at the beginning with Mo terminated triangular nucleation formed. After that, the morphology of MoS2 flake evolves from triangle to similar hexagon, then to truncated triangle which is determined by the faster growth speed of Mo termination than that of S termination under the S rich environment. The nucleation density does not increase linearly with the increase of reactant concentration, which could be explained by the two-dimensional nucleation theory.