Two-dimensional semiconductors are a topic of intense research and very attractive materials for new developments in different elds of semiconductor technology. These materials are promising candidates to satisfy the demand for faster and more compact electronics. They make new technological possibilities feasible, such as the realization and the commercial development of exible and semitransparent electronics. For these purposes, a deep knowledge of their electronic and optical properties is required. Besides the technological interest, numerous discoveries of fundamental physics were made with many others still to come. Recently for instance, superconductivity has been achieved in twisted bilayer graphene and high-temperature exciton condensation was observed in transition metal dichalcogenide heterostructures 1. The scope of this PhD is to investigate the infrared and optical properties of different two-dimensional semiconductor systems. To this end, various spectroscopic and time-resolved investigations on transition metal dichalcogenide monolayers and few-layer InSe crystals will be presented. First of all, the fabrication of exfoliated samples and van derWaals heterostructures has been successfully carried out and is described in detail. With this knowledge, the exciton physics of MoSe2 monolayer was studied. In particular, the effects of adsorbed gas molecules on the monolayer surface is discussed. It has been demonstrated that these adsorbates can localize excitons at low temperatures and that laser irradiation can release the binding of the physisorbed gas molecules. These results are of fundamental interest for spectroscopic investigations as well as relevant for opto-electronic devices as for instance gas sensors. Thereafter, several experiments were carried out with the use of the infrared free-electron laser FELBE. The general idea was to investigate the response of transition metal dichalcogenide monolayers in the far-infrared frequency range. An effect that was observed is a redshift of the trion induced by non-resonant infrared absorption. In fact, after the absorption of infrared radiation by free carriers, the energy and the momentum of the heated electron gas are transferred to the trion population, leading to a redshift of the trion resonance. By measuring the dynamics of this process, the cooling time of the electron-hole population and the far-infrared absorption of MoSe2 monolayer were extrapolated. The experiments conducted on few-layer InSe will be also presented in this thesis. The effects of hBN-encapsulation on the optical properties of InSe are discussed. The encapsulation in hBN does not only prevent the material from degradation, but also improves the optical quality by reducing the disorder potential in the crystal. Furthermore, the photoluminescence dynamics was investigated as a function of layer thickness and temperature. A bi-exponential decay was observed and the two contributions are attributed to the direct bandgap electron-hole transition and the defect assisted radiative recombination. Because of the direct-to-indirect bandgap crossover driven by the sample thickness, the dynamics gets slower while decreasing the number of layers. In particular, the fast component, i.e. the direct bandgap recombination, tends to disappear for thin InSe samples. Moreover, the photoluminescence lifetime decreases at high temperatures as a consequence of more effcient non-radiative recombination. Finally, heterostructures of MoSe2/WSe2 monolayer were fabricated and the rst spectroscopic results are presented. The interlayer exciton was observed and its dynamics was investigated.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:73364 |
| Date | 18 January 2021 |
| Creators | Venanzi, Tommaso |
| Contributors | Helm, Manfred, Bratschitsch, Rudolf, HZDR |
| Publisher | Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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