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.