Direct observation of the indirect to direct band gap transition in epitaxial monolayer MoSe$_{2}$ 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 indirect to direct band gap transition in monolayer. Because of this direct band gap, monolayer MX$_{2}$ is favorable for optoelectronic applications. Here we report the direct observation such band gap transition by using angle-resolved photoemission spectroscopy on high-quality thin films of MoSe$_{2}$, with variable thickness from monolayer to 8 monolayer, grown by molecular beam epitaxy. The experimental band structure indicates a stronger tendency of monolayer MoSe$_{2}$ towards direct band gap, and with larger gap size, than theoretical prediction. Moreover, we observed a significant band splitting of $\sim$ 180 meV at valence band maximum of a monolayer MoSe$_{2}$, which was theoretically predicted to be 100{\%} spin-polarized. This spin signature gives the layered MoSe$_{2}$ great application potential in spintronic devices, as well as a new playground to investigate spin-obit physics beyond the topological insulators.