Sodium salt-assisted edge structure control of atomically thin monolayers of transition metal dichalcogenides

The growth of large-sized, highly crystalline two-dimensional (2D) monocrystals with controllable edge structures is highly desired when exploring novel properties and optimize/control functionalities. The use of different precursors during chemical vapor deposition (CVD) facilitates the growth of 2D crystals by assisting the reaction or by modifying surface properties of the substrates. In this work, we used sodium halides (i.e., NaBr, NaCl) in CVD growth of MoS₂ monolayers when reacting MoO₂ and S. We successfully synthesized MoS₂ monolayer single crystals with lateral sizes up to 250 µm and controlled edge morphology (curved or straight). These different morphologies exhibited characteristic edge structures consisting of Mo- or S-terminated zigzag edges. These edge structures were studied by aberration corrected high-resolution scanning transmission electron microscopy (AC-HRSTEM). In this presentation, we will discuss the role of sodium halides in the presence of MoO₂ and S; as well as the role of substrates. This salt-assisted growth has shown to be an effective method for rationally controlling the size and morphology of 2D MoS₂ single crystals, and allows edge termination manipulation at the atomic level, which is crucial for tailoring the properties and applications.