Layer-dependent electronic structure of an atomically heavy two-dimensional dichalcogenide

The first direct measurements are reported of the evolution of the thickness-dependent electronic band structure of monolayers of a material of much current interest. Angle-resolved photoemission is performed on the few-layer single-crystal dichalcogenide WSe${}_{2}$, which is in a class of graphene-like semiconductors with a desirable band gap in the visible frequency range. These results strongly support the presence of a predicted transition from an indirect (bulk or bilayer) to direct (one monolayer) band gap. Values are obtained for the effective mass and spin-orbit splitting of few-layer WSe${}_{2}$, parameters which are very important for transport studies and device applications.