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.