Atomic insight into the effects of precursor clusters on monolayer WSe₂

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 WSe₂ as the prototype, we investigated the structure and electronic properties of Se, W (N = 1-8), and W_8-Se (N = 1-7) clusters adsorbed on monolayer WSe₂ to gain atomic insight into the precursor cluster adsorption. In contrast to W clusters that tightly bind to the WSe₂ surface, Se clusters except for Se₁ and Se₂ are weakly adsorbed onto WSe₂. The interaction between W_8-Se (N = 1-7) clusters and the WSe₂ monolayer decreases with the increase in the Se/W ratio and eventually becomes van der Waals interaction for W₁Se₇. According to the phase diagram, increasing the Se/W ratio by changing the experimental conditions will increase the ratio of Se and W₁Se₇ 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 W₁Se₇ and Se clusters (N = 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 WSe₂ as well as the electronic properties of the prepared WSe₂ monolayer can be effectively tuned, which is significant for the high-quality growth and applications of WSe₂.