Controllable selenization transformation from MoO${}_2$ to MoSe${}_2$ by gas pressure-mediated chemical vapor deposition

During the chemical vapor deposition (CVD) of MoSe${}_2$, controlling its selenization reaction and understanding its reaction mechanisms are of great significance to obtain high-quality 2D transition metal selenide semiconductors. Herein, a variable-pressure CVD (VPCVD) method is reported to achieve the controllable transformation from MoO${}_2$ to MoSe${}_2$ monolayer based on gas pressure-mediated selenization. At the gas pressure lower than 20 KPa, high-temperature decomposition of MoO${}_3$ in the Ar/H${}_2$ mixture only produces MoO${}_2$ nanosheets without any selenization. When the gas pressure is between 20 and 60 KPa, quadrilateral MoO${}_2$ nanosheets are partially selenized. By further increasing the gas pressure from 60 to 100 KPa, they are completely selenized. At the downstream margins of as-selenized films, 2D MoSe${}_2$ monolayers demonstrate a morphological evolution from triangle to hexagon and then to a continuous film. In addition, their Se vacancy concentrations and nucleation sizes depend directly on gas pressure. Therefore, the gas pressure-mediated selenization provides a feasible way for the in situ synthetic control of chemical composition and vacancy doping of 2D transition metal selenides/oxides. (© 2022 Wiley‐VCH GmbH)