Direct Visualization of Grain Boundaries in 2D Monolayer WS₂ via Induced Growth of CdS Nanoparticle Chains

To date, wafer-scale synthesis of two-dimensional (2D) materials are well achieved by chemical vapor deposition, but the obtained monolayers typically have multidomains with electrical and optoelectronic properties affected by grain boundaries and domain sizes. When these 2D materials are used as the growth templates, these boundaries would also provide unknown influences to the successive heterostructure formation for extended applications. Here, for the first time, direct visualization of grain boundaries in monolayer WS₂ film can be realized by the growth of CdS nanoparticles. Specifically, CdS is found to first preferentially nucleate and form as nanoparticle chains along WS₂ grain boundaries in a random manner, independent of the grain boundary characteristics. Due to electron scattering and type II band alignment at the WS₂–CdS heterojunction, WS₂ reduces in its mobility while becoming enhanced in its electron concentration. Notably, the WS₂–CdS heterostructure also yields improved carrier separation and collection for the photodetection performance enhancement. All these results can facilitate the detailed evaluation of crystalline grains-related information of 2D materials and provide thorough understanding on the effect of these overgrown CdS on underlying WS₂ monolayers, being extremely important to further optimize and enable their functionalities for advanced device applications.