Abstract: In recent years, single-layer crystals have attracted much research interest due to their potential application for ultrathin, transparent, and flexible optoelectronic devices. Besides graphene, the first and most prominent representative of single-layer crystals, transition metal dichalcogenides (TMDs), are coming into the focus of research attention.
Monolayer TMDs are especially interesting because of their transition from an indirect bandgap in the bulk crystal to a direct bandgap in the single layer. The bandgap is in the optical or near-infrared spectral region and the optical properties are dominated by bound electron-hole states, so-called excitons. In TMD monolayers, exciton binding energies are much larger than in bulk semiconductors owing to the spatial and dielectric confinement to two dimensions.
We have experimentally studied the coherent and incoherent dynamics of charged excitons in molybdenum diselenide monolayers. The dynamics can be explained by the existence of at least two types of defect states. I will give an overview of the current state of research in the field of coherent and incoherent spectroscopy of monolayer TMDs and will introduce two-dimensional coherent spectroscopy.