Microscopic Theory of Externally Tunable Exciton Signatures of Two-Dimensional Materials

Abstract: Atomically thin transition metal dichalcogenides (TMDs) are in the focus of current research due to their efficient light-matter interaction and the remarkably strong Coulomb interaction that leads to tightly bound excitons. Due to their unique band structure, TMDs show a variety of bright and optically inaccessible dark excitonic states. Moreover, the optimal surface-to-volume ratio makes these materials very sensitive to changes in their surroundings, which opens up the possibility of tailoring their optical properties via adsorption of molecules, application of strain, and deposition of defects. The aim of this thesis is to use a microscopic many-particle theory to predict different strategies to externally control the optical fingerprint of TMDs.  We show that specific molecules can activate dark excitons leading to new pronounced resonances in optical spectra. We also find that these dark states are very sensitive to strain, leading to significant energy shifts and intensity changes. This renders 2D materials suitable for optical sensing of molecules and strain. Moreover, we investigate how local defects due to single molecules or local strain can trap excitons. We show direct signatures of localized bright excitonic states as well as indirect phonon-assisted side bands of localized momentum-dark excitons. We find that the visibility of these localized states is determined by the interplay between defect-induced exciton capture and intervalley exciton–phonon scattering. Finally, we investigate the formation dynamics and optical signatures of spatially separated interlayer excitons at interfaces of acene-based molecular crystals and 2D materials, which play a crucial role for conversion of light to electricity in photodetecting devices.  Overall, the work contributes to a better microscopic understanding of exciton optics and its control via strain, molecules, magnetic fields and impurities in atomically thin semiconductors.

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