Preview

PDF, English Download (18MB) | Terms of use

Abstract

Two-dimensional materials have gained tremendous interest over the last decade due to their unique properties and their potential to be employed in future electronic or optoelectronic devices. One class of materials that are subject of ongoing research are transition metal dichalcogenides (TMDs) such as MoS2 or WS2. In their 2H-phase, these layered Van der Waals (VdW) crystals are semiconducting and undergo a transition from an indirect to a direct band gap, when the layer number is decreased to the monolayer (ML) limit, giving rise to emerging photoluminescence. One of the research areas where ML TMDs are promising materials due to their high oscillator strength and large exciton binding energies is strong light-matter coupling, where the energy exchange between photons and matter (for example excitons) in optical microcavities results in the formation of hybrid light-matter quasi particles, so-called polaritons. These polaritons exhibit properties of both light and matter, making them interesting for their own sake but also for future polaritonic devices, such as polariton lasers. One of the most commonly applied techniques for the production of two-dimensional nanoobjects is exfoliation of bulk crystals, for example in the liquid phase by sonication assisted liquid phase exfoliation (LPE), where the interlayer binding energies of the bulk crystals are overcome by sonication. However, this process is still not fully understood, and exfoliated nanosheets are relatively small in their lateral dimensions due to scission events within the layers. Additionally, the deposition of the nanosheets from dispersion into homogeneous thin films with preservation of the ML properties remains a major challenge. Therefore, most demonstrations of TMD based applications are designed with single flakes of TMDs, which restricts the scale-up. For example, the formation of polaritons in homogeneous films of TMDs has not been demonstrated yet. The first part of this thesis focuses on the optimization and understanding of the LPE process. It was demonstrated that purity, particle size, and defectiveness do not impact the yield or dimensions of LPE produced nanosheets. However, differences in the PL properties were observed, which might be related to the defectiveness of the starting material. The exfoliation efficiency and the dimensions of the nanosheets can be altered by pretreatment of the starting material, leading to intercalation of the pretreatment agents and reduction of the interlayer binding strength, but the effect is small when high power sonication conditions are chosen. In the next part of the thesis, thin films of TMDs were produced by different strategies, including spin coating of WS2-polymer composite films and deposition of WS2 in Langmuir-type films, that were formed at liquid liquid phase interfaces. The films were characterized and assessed regarding their usability for the implementation in optical microcavities. Here, the Langmuir films were superior to the composite films due to stronger PL, better homogeneity, and lower thickness for a given optical density. The production of high-quality composite films was only possible with low WS2 concentrations, resulting in insufficient optical density or high film thickness. Both composite and Langmuir films were implemented in microcavities, but strong light-matter interaction was only observed in the cavities based on the Langmuir films. While this is the first time that strong coupling is demonstrated based on angle-dependent reflectivity in homogeneous and large-scale thin films of TMDs, it was not possible to measure the PL emission of the cavities, due to low signal intensity. In the last part of this thesis, hydrogen and methyl derivatized germanene (Ge H and Ge Me) were subjected to LPE, since these materials are known for higher PL intensities and are also fluorescent in the bulk structure. The fluorescence properties of Ge-H were not preserved after sonication, but the more stable Ge-Me nanosheet dispersions showed promising properties, including strong PL. However, Ge-Me is susceptible to basal plane degradation under ambient conditions and film preparation was only possible with relatively large nanosheets. In these films, the fluorescence properties were preserved, but no working light-emitting devices could be built, which was attributed to inhomogeneities that are related to the large nanosheets with broad size distribution.