Excitons in two-dimensional heterostructures - from single quantum emitters to many-body states

The emerging class of two-dimensional materials exhibits fascinating new and exciting properties that have been explored in the last two decades. Among these are the semiconducting transition metal dichalcogenides (TMDs), such as MoS2, MoSe2, WS2, and WSe2. The particular interest in this material group arises from the transition to a direct band gap semiconductor in the monolayer limit. Moreover, the weak dielectric screening and strong confinement in two dimensions give rise to extraordinarily strong Coulomb interactions. As a result, the interaction of TMDs with light is dominated by excitons with binding energies on the order of hundreds of meV.
This thesis presents an experimental study on excitons in TMD heterostructures in two extreme cases. First, strongly localized excitons at artificially created defects in atomically thin MoS2. We demonstrate that site-selective helium ion irradiation is a highly scalable approach to create quantum emitters on-demand in monolayers. We show that encapsulation of MoS2 monolayers in hexagonal boron nitride significantly reduces adsorbate-related background luminescence. Furthermore, we evidence single-photon emission, revealed by an antibunching behavior of individual and spectrally clean quantum emitters. From a statistical analysis, we determine the creation yield of such quantum emitters as a function of the helium ion dose. We find a maximum probability of 37 % for the generation of a single emitter by local irradiation. Our findings present a promising methodology to integrate single-photon emitters in as-fabricated field-switchable van der Waals devices and may envision these emitters as a platform for coherently coupled sources of indistinguishable photons.
In the second part of the thesis, we focus on interlayer excitons (IXs) in MoSe2-WSe2 heterostructures, that are characterized by spatial separation of electron and hole wave functions in adjacent monolayers. We use low-temperature PL spectroscopy to show IX formation. Furthermore, we find extended population decay times as expected from the reduced wave function overlap. Spatially resolved PL imaging reveals long-range transport on the order of μm, driven by a repulsive dipolar interaction between the IXs. Due to their bosonic nature, IXs are motivated as an ideal system to study many-body effects, such as high-temperature exciton condensation. In the next step, we explore the phase diagram of photogenerated IX ensembles and report on signatures of a degenerate many-body state. We find that below the degeneracy temperature, the dephasing (and consistently also the homogeneous broadening) is drastically reduced, which cannot be attributed to classical processes. Intriguingly, the homogeneous broadening decreases even for an increasing density of excitons, as it is expected for a many-body state approaching quantum degeneracy.
Moreover, we investigate the transition dipole of IXs in different MoSe2-WSe2 heterostacks. We use low-temperature back focal plane imaging to detect the far-field photoluminescence intensity distribution. To interpret our observations, we model the radiative exciton recombination by classic dipole radiation. The presence of a layered dielectric environment alters the radiation pattern and it is considered by a transfer matrix method. From the model, we can determine the relative contributions of in- and out-of-plane transition dipole moments associated with the photon emission. We find an upper limit for the out-of-plane contribution to be 2 % of the total oscillator strength, independent of the stacking type and the investigated excitation powers and temperatures.