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Abstract
First-principles studies, grounded in quantum mechanics, offer a profound understanding of materials and chemical systems at the most fundamental level without relying on empirical data. By providing detailed insights into electronic structures, atomic interactions, and potential energy surfaces, first-principles calculations have become indispensable in the exploration and design of new materials and molecules. These studies not only enhance our comprehension of existing systems but also pave the way for innovative applications in fields ranging from energy storage and catalysis to pharmaceuticals and nanotechnology. In this dissertation, I will present several advancements in the development and application of quantum mechanical methods for first-principles simulations of solid-state systems, focusing on their excited states and optical properties.
First, I will present the development and implementation of the linear-response time-dependent density functional theory and its analytical nuclear forces. A multilevel parallelization scheme, numerical approximations, and GPU accelerations enable the study of excited state energy and atomic geometry relaxation of solid-state systems containing thousands of electrons at the level of hybrid functional. We demonstrated its capability by studying the excited states of several point defects in semiconductors and insulators.
Next, I will present our effort in developing and implementing the generating function approach to compute the vibrationally resolved optical spectra for point defects in solids. We assessed the validity of theoretical and numerical approximations used in first-principles calculations by computing the photoluminescence spectra for prototypical point defects, the divacancy centers in SiC, and the nitrogen vacancy in diamond and compared them with experimental results. We also addressed the importance of correcting for finite-size effects.
Following the development, implementation, and validation of several theoretical approaches, I then present an application study on the vibrationally resolved optical spectra for the nitrogen vacancy center in diamond, focusing on the optical transitions between the highly correlated singlet states. Our development and implementation of the spin-flip time-dependent density functional theory and analytical nuclear forces enable us to compute the energy and vibrations in the highly correlated singlet states, and our development and implementation of the generating function approach enable us to compute the vibrationally resolved absorption spectrum between the highly correlated singlet states, resulting in good agreement with the experiments.
I will then present several applications of first-principles methods for the study of spin defects in semiconductors, including the study of the photoionization process of the neutral divacancy center in silicon carbide and the neutral substitutional nitrogen in diamond. These studies are instrumental in the interpretation of experimental results and the design of new spin defects for quantum technology applications.
Finally, I will present a theoretical study of several inorganic metal-halide perovskite systems, focusing on the formation of the self-trapped exciton and the associated broadband emission. We investigated the exciton-phonon coupling and multiphonon optical processes. This study provides insights for designing metal-halide perovskites with tailored emission properties.