Understanding of monolayer WS₂ nucleation by in-situ monitoring CVD

Transition metal dichalcogenide (TMD), two-dimensional material, is expected to be promising candidates for novel transparent flexible devices because of their excellent electrical and optical properties. In terms of the industrial applications, there are still remained several issues to be solved in the growth stage of TMDs, which can be overcome by clarifying the synthesis mechanism of monolayer TMD. We have recently succeeded in developing the in-situ monitoring CVD, which can contribute to uncovering the detailed growth mechanism of TMD [1].

In this study, we improved the in-situ monitoring CVD and introduced auto-image analysis to quantitatively analyze the initial growth stage. Systematic investigations reveal that the growth of monolayer tungsten disulfide (WS₂) is mainly dominated by not vapor-solid (VS) transition but liquid-solid (LS) transition. The incubation time (?t_i), which is the time required for the nucleation of monolayer WS₂, was found to be strongly influenced by the substrate temperature. These results are in good agreement with the time-temperature-transformation (TTT) curve, which is known as a classical three-dimensional crystal growth model. Furthermore, detailed image analysis suggested that the nucleation of WS₂ follows two-step nucleation, which is known as a non-classical nucleation [2]. These findings can be useful to introduce plasma reaction in TMD synthesis for future high quality and single crystal synthesis of TMD in large scale.