Monolayer (ML) WS2 exhibits multiple excitonic states arising from the strong Coulomb interactions induced by two-dimensional quantum confinement effect and reduced screening effect, making it an ideal system for exploring excitonic phenomena and their potential applications. Moreover, its heterostructures with zero-dimensional quantum dots (QDs) allow the combination of the advantages and properties of both materials. Photoluminescence (PL) spectroscopy is a powerful tool for investigating the optical properties of monolayer WS2, as it is a non-destructive and highly sensitive probe of the material's electronic structure. In this thesis, the optical properties detected by the PL spectra of the ML WS2 and its heterostructures with InGaN quantum dots (QDs) are studied.

Understanding the nature of these excitonic states and their response to external stimuli is crucial for the development of opto-electronic devices based on TMDCs. We detected multiple excitonic peaks in ML WS2 at cryogenic temperature using PL spectra and successfully assigned them. Additionally, we systematically studied the properties of the excitonic states in ML WS2 under different excitation power, temperature, and changing electron doping and magnetic field. By investigating and assigning the excitonic states, we established a foundation for further studies of excitonic states and their applications.

Among the excitonic states in WS2, the charged states, i.e., trions and negative biexcitons, can drift under an external electric field. However, the formation of the trions is still in controversary. We studied the polarization behaviors of the singlet trions and triplet trions in the magnetic field, which show negative and positive dependence on the magnetic field, respectively, in a non-degenerate regime. With a higher electron density, the slope of singlet trions becomes positive and that of triplet trions changes to larger magnitude. A model is proposed to elucidate the polarization behavior, considering the bimolecular formation in both non-degenerate and weakly-degenerate regimes, as well as the formation of Fermi-polaron in degenerate regime. Furthermore, the temperature-dependent polarization behaviors of trions and negative biexcitons can also be explained by the model.

The ML WS2-InGaN QDs heterostructure provides an ideal platform for studying the interfacial coupling between the 2D and 0D materials. We measured the PL spectra of WS2 in the coupled region and uncoupled region, observing a decrease in exciton and trion emission and an enhancement of negative biexciton emission in the coupled region. An interfacial bound negative biexciton state, combined by a bright trion in WS2 and an exciton in QDs, is revealed to form with an emission energy similar to the negative biexciton in ML WS2. Its formation is further confirmed by its different power factor, binding energy and g-factor from those of the negative biexciton. Most important, the ratio of the interfacial bound negative biexcitons can be effectively tuned by the excitation energy. Moreover, electron transfer from QDs to WS2 is detected in the coupled region.