| dc.description.abstract | Recently, the large area WS_2 is prepared by utilizing CVD growth. However, the growth result is mostly polycrystalline, which leads to the formation of grain boundary, and the grain boundaries block electronic transmission. In CVD, the growth substrate is usually the primary parameter that affects the grain orientation, so it is important to understand the role of the substrate in CVD in order to achieve optimizing the fabrication. Thus, we utilized SEM to analyze the growth result for CVD growth WS_2 under different growth substrate. Then, we found that the nucleation mechanism of WS_2 on HOPG and exfoliated-h-BN can be divided into 1. Nucleation in the defect region due to the surface defected state reduces the formation energy. 2. Nucleation in the wrinkle or bubble region because the surface morphology leads to a higher local precursor concentration and accumulates into a larger cluster of precursor. In addition, we used NaCl to participate in the CVD growth of WS_2, and succeeded in switching the nucleation mechanism of WS_2 between above two modes when CVD growth graphene was used as the substrate, and as observed in the subsequent TEM measurements that WS_2 nucleated at the wrinkle and bubble had a tendency to grow in the same crystal orientation as the growth substrate. Furthermore, the degree of surface undulation determines the size of the precursor cluster that dominates the nucleation, making the angle between WS_2 and graphene are 0° or 19°. | en_US |