Optical studies on quantum nanosystems built from emergent two-dimensional transition metal dichalcogenides

In the course of this thesis, we will investigate optical and excitonic properties of selected sulphur and selenide-based transition-metal dichalcogenides (TMDs) constituting novel nanosystems.
In the first part, we introduce the van-der-Waals crystals MoS2, MoSe2 as well as WSe2 as direct bandgap semiconductors in the monolayer limit and discuss their basic photoluminescence and Raman scattering properties (Chapter 2). We then turn to the electrical control of excitons in few-layer MoS2 in novel lithographically defined micro-capacitor devices. By applying strong electric fields E ~ 3.5 MVcm-1 oriented perpendicular to the basal plane of the crystal, we show the control of emission energy through the quantum confined Stark effect (QCSE). Moreover, we show a layer-independent exciton polarisability of = (0.58 ±0.25)×10¬-8 DmV-1 indicating robust confinement of excitons in single layers of a crystal (Chapter 3). After a brief introduction to symmetry concepts in monolayer TMDs exhibiting strong effects on the luminescence properties, we demonstrate the control of valley polarisation 0.2 < n < 0.6 in MoS2 bilayers via inversion symmetry breaking through externally applied electric fields. We explain our observations with a non-linear electric field coupling to the single particle bilayer Hamiltonian (Chapter 4).
In the second part, we establish hexagonal boron nitride (hBN) as an ideal dielectric for encapsulation of few-layer TMDs by suppressing inhomogeneous broadening effects by a factor of up to 3× for excitonic transitions in MoS2 and a decrease of the Fermi-level due to the screening of charged defects. We extend this heterostructure approach to stacking binary TMD nanostructures. Subsequently, we examine the emergence of interlayer excitons (IX) MoSe2/WSe2 hetero-bilayers due to a type-II band alignment of the constituents. We find excitonic luminescence lifetimes up to three orders of magnitude longer ~ 2 ns compared to free excitons in monolayer TMDs as a consequence of the spatial separation of electrons and holes. By employing second-harmonic generation (SHG) in MoSe2/MoSe2 homo-bilayers, we show the accurate (< 0.1°) measurement of the relative rotation angle between artificially stacked TMDs and determine the relative SHG efficiencies for natural mono- and bilayers as well as artificial bi- and trilayers providing a tool for reproducible sample fabrication. Furthermore, we show the influence of relative rotation on the luminescence properties at low temperatures hinting multiple phonon scattering processes (Chapter 5).
In the final part of the thesis, we focus on curvature-induced spectrally sharp ( ~ 1 meV) quantum emitters in hBN/WSe2/hBN heterostructures. In the hBN dielectric environment, we show luminescence lifetimes exceeding ~ 20 nm and second-order coherence values of g2(0) = 0.42 indicating the generation of sub-poissonian light. Polarization resolved PL experiments reveal a non-zero fine structure splitting on the order of 500 µeV. Detailed lifetime studies show the emergence of Auger-type depopulation processes with a characteristic P-2/3 excitation power dependence. Using solid immersion lenses to increase the spatial resolution by a factor of 4×, we demonstrate an increased spectral selectivity of individual quantum emitters. We conclude this work by introducing a micro-capacitor prototype structure fully consisting of van-der-Waals crystals with graphene electrodes in order to provide electrical control of quantum emitters in WSe2 (Chapter 6).