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 $\mathrm{MoS}{\mathrm{e}}_2$ 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 Г 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.

• Received 10 May 2017

DOI:https://doi.org/10.1103/PhysRevB.97.075433