Large area chemical vapor deposition and spectroscopic properties of bilayer WSe2

•The controllable synthesis of bilayer WSe2 films by chemical vapor deposition is achieved.•As-grown bilayer WSe2 exhibits good optical characteristics comparable to that of the exfoliated bilayers.•An asynchronous growth mode for bilayer WSe2 films is proposed. Large area preparation of WSe2 in different atomic layers are essential for practical applications in integrated electronic/photoelectric devices. Compared with the monolayer WSe2, the bilayer WSe2 has shown higher carrier mobility and better stability at room temperature, but scalable and rapid growth of the bilayer WSe2 with proper optical performance is a fundamental synthetic challenge. Herein we report a chemical vapor deposition (CVD) approach for growth of millimeter-sized bilayer WSe2 on SiO2/Si substrates. By finely controlling the hydrogen flow at low flow rates and the growth temperature at 780 °C, WSe2 can undergo a second layer growth shortly after the start of the monolayer growth and eventually form a complete bilayer structure. The Raman and photoluminescence characterizations demonstrate that the resulting CVD WSe2 bilayers have similar spectroscopic characteristics to that of the exfoliated bilayers. Our study provides a simple pathway for growth of bilayer WSe2 and deepens the understanding of CVD growth mechanisms of TMD materials.