Excitons in charge-tunable monolayers of transition metal dichalcogenides and their heterostructures

The two-dimensional nature of monolayer transition metal dichalcogenides (TMDs) results in a strongly enhanced Coulomb interaction of excited electron holes pairs, called excitons. The large exciton binding energies, paired with strong spin-orbit coupling of the electronic states at the direct band gap, give new perspectives for both, excitonic devices and fundamental studies on excitons. In particular, TMDs are promising platforms for the investigation of correlated phases such as exciton superfluids. Moreover, excitons in monolayer TMDs possess an additional degree of freedom, which is related to the momentum of the charge carriers. This valley pseudo-spin is accessible by optical selection rules, making it promising for potential applications in information technology. The intriguing phenomena and functionalities found in monolayer TMDs are even expanded by the possibilities of artificially stacked heterostructures. Heterobilayers, for example, consisting of two different TMD monolayers, can host interlayer excitons with drastically prolonged lifetimes. In the prospect to establish TMD structures as comprehensive platforms for exciton physics, we apply optical spectroscopy to study excitons in monolayer TMDs and their heterostructures. In a first step, we elucidate the high impact of surface effects on the atomically thin materials with respect to modifications of the charge carrier density. We demonstrate a photogating effect based on a charge transfer from physisorbed environmental molecules, which can be gradually removed by the exposure to light. Subsequently, control over the charge carrier density via field effect devices is used to study the interaction of excitons with phonons and free charge carriers. We observe a strong Fröhlich exciton-phonon interaction, which can be suppressed by electron doping. Our findings reveal the importance of the Fröhlich exciton-phonon interaction to optical processes in MoS2, in particular for the depolarization of the valley degree of freedom. Finally, we investigate optical interlayer transitions in a detailed photoluminescence study on artificially stacked MoSe2/WSe2 heterobilayers. We observe two emission peaks with long lifetimes of up to hundred nanoseconds which we attribute to momentum direct and indirect interlayer excitons. Our observations give important insights to exciton states in TMD heterostructures.