Nucleation mechanism and morphology evolution of MoS₂ 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 MoO_3-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.