Controlled Nucleation Sites with Large Coverage Area for Continuous Monolayered WS2Growth by Using the CVD Process

Two-dimensional (2D) atomically thin transition metal dichalcogenides exhibit remarkable interest due to their tunable band gap and novel electronic and optical properties for electronics and optoelectronic devices. Here, we demonstrate an effective approach to synthesize large-area monolayered tungsten disulfide (WS2) through the atmospheric pressure chemical vapor deposition technique on a SiO2/Si substrate. Initial parameters like amount of precursors, gas flow rate, and growth duration decide the crystal quality, crystallite size, and the coverage area. We observed that the amount of metal precursor (WO3) is responsible for creating nuleaction sites and excess presence of WO3–xleads to formation of multinucleation sites. Sulfur is experimentally found to govern the coverage area. A sulfur-rich environment leads to bilayer formation. The local concentration of the vapor phase of precursors is found to get affected by the gas flow rate and it governs the crystallite size and coverage area. Moderate gas flow rate (100 and 150 sccm) and optimum growth time (15 min) result in monolayer growth. Competing adsorption and desorption process in addition to the evaporation rate of the metal precursors decides the nucleation sites toward the formation of a monolayered structure. We have established the role of precursors, carrier gas flow rate, and growth duration toward the formation of a continuous monolayered WS2film. Such highly crystalline monolayered WS2films will be highly useful to achieve efficient electronic and optoelectronic devices.