UC Berkeley

The optical and electronic properties of semiconductor nanomaterials have long attracted significant interest due to the strong absorption and tunable spectra caused by quantum confinement. These materials have potential applications ranging from solar energy conversion and lighting to single photon sources and quantum computing. However, to realize any of these applications the role of the atomistic detail of these materials cannot be ignored. Understanding role of defects, traps, and structural distortion on the excited states of these materials remains a great challenge for modern computational science. Semiemperical pseudopotential models represent the leading way to understand the complexity of nanoscale systems in atomistic detail. In this dissertation we expand the applicability of these models to new materials where additional effects, like strong spin-orbit coupling must be considered. We also use these methods to help examine the dynamics of excited states in these nanomaterials, revealing the crucial roles of defects, distortions, and traps.

We develop a formulation of the semiempirical pseudopotential method that includes the effects of spin-orbit coupling and other nonlocal terms in the potential. By using a separable form of these non-local terms we maintain a favorable computational scaling and thus keep the ability to investigate nanomaterials of experimentally relevant sizes. We apply this method to lead halide perovskite nanocrystals (NCs), promising materials for solar energy conversion that are known for their strong spin-orbit coupling. The atomistic study of these systems allows for an understanding of how distortion of the NC structure impacted the exciton fine structure, determining that contrary to some suggestions the ground state exciton is a dark state.

The results of atomistic electronic structure methods also aid in developing kinetic models of excited state species in various nanomaterials. The dynamics of the transfer of holes from multi-excitonic II-VI NCs is explored as a competition between transfer, trapping, and non-radiative Auger recombination (AR). Pseudopotential calculations provide crucial insight in the AR rates and how those are impacted by the presence of trapped species. A similar kinetic model describing carrier recombination in few-layer black phosphorous is informed by density functional theory calculations of surface oxygen defects.

This dissertation shows both the expansion of the semiemperical pseudopotential method and the application of the method to inform studies of material properties and design principles. The combination of theoretical development and experimental collaboration shows the utility of these models to solve practical problems of broad scientific import. By expanding the applicability of these methods to new materials, these detail atomistic calculations can now be applied to even more experimentally relevant systems.