Insight into the Role of H₂ in WS₂ Growth by Chemical Vapor Deposition

Monolayer tungsten sulfide (WS2), a desirable transition metal chalcogenide semiconductor material for the next generation of promising electronics and optoelectronics applications, has attracted great attention. However, the lack of understanding of the underlying growth mechanism in the chemical vapor deposition process inhibits the precise dimension control of WS2 and its practical applications. Herein, we discovered that H2 played an important role in the growth of WS2 when tungsten trioxide (WO3) was used as W precursor. It was found that a prereduction of WO3 prior to the WS2 growth is necessary, and the time to introduce H2 into the growth system is crucial. The intermediate product of volatile W18O49 contributed to the formation of single-crystal WS2 only when the H2 was introduced after the growth temperature reached 850 °C. The Gibbs free energy of the involved reactions was examined, and an intermediate-driven growth mechanism was proposed. The photodetector fabricated from the monolayer WS2 exhibited a fast response speed and stable performance. A better understanding of the growth mechanism of WS2 and the photodetector application demonstrated in this work offer guidelines for the study of other 2D materials.