Evolution of the broadband optical transition in large-area MoSe2

The dependence of the optical and electrical properties of two-dimensional transition metal dichalcogenides on the number of layers has garnered significant interest. In particular, the indirect-to-direct band gap transition and the resulting changes, such as improved quantum yield, have been widely studied. However, an experimental investigation of the dependence of the optical transition for a wide range of photon energies is still lacking. Here, we report the broadband optical response of large-area MoSe2 grown from monolayer to pentalayer thicknesses by molecular beam epitaxy, for photon energies in the 0.9-5.5 eV range. We observed a dramatic evolution of the absorption spectrum that depends on the number of layers. Using the density functional theory, we show that this feature is related to a change in the energy and geometric shape of the band structure at the. point in the Brillouin zone. The dependence of these optical properties on the number of layers yields insights into the underlying physics and is promising for photonic and optoelectronic applications.