Synthesis of large-area monolayer and few-layer MoSe continuous films by chemical vapor deposition without hydrogen assistance and formation mechanism.

Two dimensional (2D) MoSe2 with a layered structure has attracted extensive research due to its excellent electronic and optical properties. The controlled synthesis of large-scale and high-quality MoSe2 is highly desirable but still remains challenging. Ambient pressure chemical vapor deposition (APCVD) is an excellent method for the synthesis of 2D materials but the inevitable use of hydrogen during the growth and the easy formation of cracks in the ultrathin films still need to be solved. In the present work, we reported the synthesis of large-area continuous MoSe2 films with different layers by the APCVD method without the assistance of hydrogen on SiO2/Si substrates just by raising the reaction temperature of Se. The synthesized continuous MoSe2 films can reach several centimeters, which can be seen clearly by naked eyes, and, more importantly, the size of the monolayer film can reach up to 3 mm. The morphology, structural characteristics, and optical properties of the synthesized MoSe2 films have been investigated, demonstrating good performance and high crystallinity of the films. Raman spectra give the empirical expression of the frequency difference between E2g1 and A1g dependence of the layer number (N = 1-10 L) for CVD grown MoSe2, which is useful in layer number identification. Further, the formation mechanism of the MoSe2 continuous film is of interest as a fundamental scientific problem and needs to be studied. We proposed the wing model, boundary layer theory, and diffusion theory to account quantitatively for the formation behavior of the MoSe2 film. The presented facile growth method and theoretical model are useful to synthesize other ultrathin transition metal dichalcogenide films and understand the formation behaviors of the systems.