Atomic insight into the effects of precursor clusters on monolayer WSe.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been attracting much attention due to their rich physical and chemical properties. At the end of the chemical vapor deposition growth of 2D TMDCs, the adsorption of excess precursor clusters onto the sample is unavoidable, which will have significant effects on the properties of TMDCs. This is a concern to the academic community. However, the structures of the supported precursor clusters and their effects on the properties of the prepared 2D TMDCs are still poorly understood. Herein, taking monolayer WSe as the prototype, we investigated the structure and electronic properties of Se, W ( = 1-8), and WSe ( = 1-7) clusters adsorbed on monolayer WSe to gain atomic insight into the precursor cluster adsorption. In contrast to W clusters that tightly bind to the WSe surface, Se clusters except for Se and Se are weakly adsorbed onto WSe. The interaction between WSe ( = 1-7) clusters and the WSe monolayer decreases with the increase in the Se/W ratio and eventually becomes van der Waals interaction for WSe. According to the phase diagram, increasing the Se/W ratio by changing the experimental conditions will increase the ratio of Se and WSe clusters in the precursor, which can be removed by proper annealing after growth. W clusters induce lots of defect energy levels in the band gap region, while the adsorption of WSe and Se clusters ( = 3-6, 8) promotes the spatial separation of photo generated carriers at the interface, which is important for optoelectronic applications. Our results indicate that by controlling the Se/W ratio, the interaction between the precursor clusters and WSe as well as the electronic properties of the prepared WSe monolayer can be effectively tuned, which is significant for the high-quality growth and applications of WSe.