32 Rost, Stefan Hermann Blügel, Stefan Mayer, Joachim Dunin-Borkowska, Beata Ewa 137510 130000 Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag Jülich 978-3-95806-682-3 English 1 Online-Ressource : Illustrationen, Diagramme 92 Single photons are expected to play an essential role in the transfer of quantum information in modern quantum technology. Monolayers of transition-metal dichalcogenides (TMDs) with implanted defects are promising candidates for single photon sources that allow us to control the polarization of the emitted photons due to their unique spin-valley locking property. This work provides a computational study of a variety of possible materials for implantation, which could possibly be used to confine the photon emission in real space allowing for single photon emission. Several of the tools necessary to carry out the study have been developed and implemented in the course of this work. Here, the focus is on the prediction of electron energy loss spectra and band structure calculations for crystals including a defect. The latter is not straight forward, since the calculation of such defects is done in a supercell consisting of multiple pristine unit cells. The band structure for a supercell is completely different from the pristine one unless the developed unfolding method is applied. This method reveals the defect as a small perturbation to the pristine system, allowing to calculate band structures of defect systems that are comparable to experiments. Experimental electron energy loss spectroscopy (EELS) measurements have a finite ${k}$ resolution due to the finite size of the collection aperture in the transition electron microscope. This spectrum deviates significantly from the EELS spectrum for zero ${k}$ momentum, as it is commonly calculated using the theoretical dielectric function. Here, a stable method for integrating the theoretical spectra over ${k}$ is developed that solves the added difficulty of the integrand varying over several (typically six) orders of magnitude around $k=0$. This scheme is extended by an extrapolation method for a fast convergence of the introduced vacuum, necessary for calculating monolayers in the context of 3D periodic boundary conditions. The combination of both shows a very favorable agreement with experimental spectra, when applied to three prototypical two-dimensional systems. Density-functional theory (DFT) studies of structural relaxations, band structures, and EELS spectra are carried out for a variety of TMDs (MoS$_2$, MoSe$_2$, WSe$_2$) with defects (P, Cr, S, Se, vacancy), applying the developed methods. The investigated system of MoSe$_2$+S turns out to lower the conduction band state at the direct band gap compared to the pristine MoSe$_2$ while preserving the spin-valley locking property. The optical transition in a small island of this material surrounded by MoSe$_2$ is therefore a promising candidate for single photon emission. Also doping with chromium, which introduces an addition state within the band gap, is a promising candidate to localize the photon emission. Druckausgabe: 2023. - Onlineausgabe: 2023. - Auch veröffentlicht auf dem Publikationsserver der RWTH Aachen University ; Dissertation, RWTH Aachen University, 2023 ; 2, ; PUB:(DE-HGF)11, ; PUB:(DE-HGF)3, ; Dissertation / PhD Thesis Dissertation RWTH Aachen University 2023 2023 RWTH-2023-01241 2023 https://publications.rwth-aachen.de/record/918299