Colour centres in 2-D hexagonal boron nitride

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Point defects in semiconductors show a rich spin and optoelectronic physics that can be exploited to fabricate qubits for quantum computing technology as well as for single photon sources for quantum cryptography. This thesis contains a detailed study of emission from point defects in hexagonal boron nitride using density functional theory (DFT) and quantum chemistry approaches with an objective to identify the source of observed single photon emission. A survey of possible defects responsible for observed emission is performed using computationally inexpensive generalized-gradient approximation (GGA), Perdew-Burke-Ernzerhof (PBE) and those defects that form localized states with an energy gap of ~2 eV are picked for calculations using highly non-local Heyd-Scuseria-Ernzerhof hybrid functional (HSE06). The photoluminescence line shape is calculated and compared with experiment to propose the most likely defects causing the observed emission. The standard DFT approaches are found to be inaccurate when compared with ab initio CCSD(T), EOM-CCSD, and CASPT2 approaches in predicting the excited-state energies, especially when dealing with states which have considerable open shell character. Thus, a benchmarking scheme is proposed and correction factors are devised for DFT energies. Finally, limitations of the finite model compound used in ab initio calculations are discussed and a possible solution is presented. A complete optical cycle of the likely defects is predicted using results from the HSE06 DFT approach, that are corrected by applying results from the high-level ab initio calculations. Finally, group theoretical analysis of defects is performed and possible applications in quantum computation technology are proposed.
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