Direct synthesis of moiré superlattice through chemical vapor deposition growth of monolayer WS2 on plasma-treated HOPG

Vertical van der Waals (vdW) heterostructures composed of two-dimensional (2D) layered materials have recently attracted substantial interests due to their unique properties. However, the direct synthesis of moiré superlattice remains a great challenge due to the difficulties in heterogeneous nucleation on smooth vdW surfaces. Here, we report a controllable chemical vapor deposition growth of complete monolayer WS2 on highly ordered pyrolytic graphite (HOPG) substrates through the plasma pretreatment. The results show that the morphologies of the grown WS2 have a strong dependence on the plasma parameters, including gas composition, source power, and treatment time. It is found that the surface C-C bonds are broken in the plasma pretreated HOPG, and the formed small clusters can act as the nucleation sites for the subsequent growth of WS2. Moreover, the height of clusters dominates the growth mode of WS2 islands. A transition from a 2D mode to three-dimensional (3D) growth mode occurs when the height is higher than the interlayer spacing of the heterostructure. Besides, diverse moiré superlattices with different twist angles for WS2/HOPG heterostructures are observed, and the formation mechanism is further analyzed by first-principles calculations.