Electronic Properties, Band Gap Renormalization, and Doping Effect in Epitaxial WSe$_{2}$ thin film

As a class of graphene-like two-dimensional materials, the layered metal dichalcogenides MX$_{2}$ (M $=$ Mo, W; X $=$ S, Se, Te) have gained significant interest due to the distinct properties in 2D limit. Examples include the indirect to direct band gap transition in monolayer, and giant spin-splitting of the valence band. These properties give MX$_{2}$ great application potentials in both optoelectronic and spintronics devices. For practically studying and applying the MX$_{2}$, the growth of high-quality MX$_{2}$ thin film with precise control of layers thickness is favorable. Here we report the molecular beam epitaxial growth of WSe$_{2}$ thin film, with controllable thickness from monolayer to 8 monolayer. By using \textit{in-situ} angle-resolved photoemission spectroscopy, we experimentally revealed the valence band evolution with film thickness. By applying the potassium doping on the surface, we observed the indirect to direct band gap transition in monolayer WSe$_{2}$, and the distorted band structure. Combining the \textit{ex-situ} photoluminescence and scanning tunneling spectroscopy, we further presented the giant band gap renormalization and excitonic effects. Our results will enrich the understanding of WSe$_{2}$, and bring it more application potential in practical devices.