Optical properties of transition metal dichalcogenide monolayers, heterostructures and alloys

Recent progress in the fabrication of two-dimensional (2D) materials attracts our attention to the study of such structures. Most of them are typically van-der-Waals-like with weak interlayer and strong intralayer bonding. Nowadays, single-layer semiconducting transition metal dichalcogenides (TMDCs), namely molybdenum and tungsten selenides and sulphides, have the biggest interest in optoelectonics due to the remarkable optical properties: the giant spin-orbit coupling, the inversion symmetry breaking, the large exciton binding energy and large oscillator strength. This thesis discusses the investigations in the samples based on the above compounds using photoluminescence (PL), photoluminescence excitation (PLE) and reflectance contrast (RC) spectroscopies. We observe prominent changes in the PL spectral parameters, such as integrated intensities, linewidths, energy positions of peaks and distances between peak maxima, with the alloy composition in the mono-, bi- and trilayer MoxW1-xS2 crystals at room temperature. The bowing parameters from parabolic dependences of the band-edge PL and RC on the Mo concentration are extracted. The modifications of the PL and RC spectral peaks with the number of layers and the indirect-to-direct band-gap crossover are also first demonstrated in those materials. Analysing gate-voltage- and temperature-dependent PL spectra, we make evaluations of exciton-electron and exciton-phonon interactions in the single-layer MoSe2. Additionally, the influence of localization and recharging processes is established. In the MoSe2/WSe2 heterostructures (HSs), the behaviour of localized interlayer excitonic (IX) complexes is investigated by means of different experimental techniques, specifically polarization-resolved, power-, gate-voltage-, temperature-dependent PL and PLE. The impact of the HfO2/hBN encapsulation on the structure and polarization of the IX emission is studied for the first time. Finally, the specific band alignment of the MoSe2/WS2 HS appears to cause the extraordinary PL signal emitted from the states which are likely originated from the interlayer electron hybridization.