Time-Resolved Photoluminescence in Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are inorganic compounds with the chemical formula MX2, where M is a transition metal atom (e.g., Mo, W), and X is a chalcogen atom (S, Se, Te). The bulk crystal of TMDs consists of layers connected by van der Waals bonds. A monolayer of TMD consists of X-M-X sublayers, where M atoms form covalent bonds with X atoms. Monolayers of TMDs were first obtained by mechanical exfoliation from bulk crystals using tape. Bulk TMDs are indirect bandgap semiconductors, whereas their monolayers are direct bandgap semiconductors. The optical properties of TMDs vary significantly with sample thickness. In the photoluminescence spectrum of flakes containing several layers, three peaks are observed: two are associated with direct excitons formed at the K points of the Brillouin zone due to strong spin-orbit coupling, and the third is associated with an indirect exciton. The variation in optical properties with a decreasing number of layers, the presence of excitons in the spectrum at room temperature, and the possibility of mechanically assembling heterostructures make TMDs unique materials, attractive for optoelectronic applications. This work aims to develop a methodology and investigate spatially resolved photoluminescence in layers and structures based on TMDs. The tasks of this study include developing techniques for measuring time-integrated and time-resolved micro-photoluminescence spectra, assembling hBN/WS2 heterostructures, characterizing them using established methods and Raman spectroscopy, and conducting measurements with temporal resolution on the obtained samples over a wide range of temperatures (10K-300K) and excitation power densities (0.01 kW cm(^{-2}) - 5 kW cm(^{-2})). Two experimental setups were assembled for measuring micro-photoluminescence spectra. The first setup is used for primary sample characterization and allows two types of measurements: spectroscopy at a single point on the sample and mapping. An optical scheme was assembled, and programs were written to automate the experiment. The second setup, based on a femtosecond laser and streak camera, is used to measure time-averaged photoluminescence spectra and time-resolved photoluminescence spectra. The optical scheme was aligned during the work. Monolayers of WS2 were obtained by a modified mechanical exfoliation method using a metallic film. hBN/WS2 heterostructures were assembled using the dry hot transfer method. In stationary PL spectra of hBN/WS2 heterostructures at different temperatures, wide peaks were observed, gradually narrowing with decreasing temperature. The integrated intensity decreased with decreasing temperature, explained by the dark nature of the exciton/trion ground state in TMDs where W is metal. Meanwhile, the maximum intensity increased with decreasing temperature, characteristic of the trion peak. Time-resolved PL measurements showed very fast dynamics, changing little with temperature. The rapid dynamics also indicate the trion nature of the main PL peak. It can be concluded that monolayers obtained by mechanical exfoliation using a metallic film are heavily doped. The developed methodology will allow the assembly of structures based on TMDs and controlling electrodes, patterned or located on textured substrates. Measurements of spectra and photoluminescence dynamics at different temperatures will be the main tools for investigating such new metamaterials.