Band engineering in epitaxial monolayer transition metal dichalcogenides alloy MoxW1−xSe2 thin films

The direct bandgap transition and spin–orbit-coupling-induced spin-splitting in monolayer transition metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te) show great application potential in high-efficient optoelectronic devices and valleytronics and, thus, have attracted enormous research interest in recent years. Various MX2 monolayers usually show a distinct bandgap and spin-splitting size. Here, we realized the molecular beam epitaxial growth of monolayer MoxW1−xSe2 alloys with a controllable stoichiometric ratio x. Combining with the in situ angle-resolved photoemission spectroscopic and x-ray photoemission spectroscopic measurements, we determined the evolution of the valence band dispersion and the spin-splitting size with the change in the Mo ratio x. We found that the energy difference of both the valence band between the Γ and K points and the spin-splitting size at the K point reduce monotonically with the increasing Mo ratio x. The growth of MoxW1−xSe2 monolayer alloys and the method to control the stoichiometric ratio of Mo/W atoms provide an effective way to engineer the band structures in the two-dimensional MX2 materials.