Computational Studies of Compressed Diborane and Engineered Narrow-Gap Semiconductors

The research contained in this thesis is two-fold: understanding the behavior of diborane under pressure, and engineering wide-gap semiconductors in order to promote their optical eciency. Each of these themes are further explained below.Diborane (B2H6), is a prototypical electron-deficient molecule and has received a great deal of attention in recent years due to its unique and peculiar structure, as well as its potential applications as a hydrogen-storage material. At high pressures, vibrational spectroscopy analysis have revealed several changes in the spectral profile that suggest occurrence of polymorphic transformations; however, the new crystal structures at high pressures have not been identified due to experimental challenges. In this study, we employ electronic structure calculations to investigate and assign the pressure-induced polymorphic transformations of crystalline diborane observed by vibrational spectroscopy up to 88 GPa. In particular, our density-functional calculations predict that diborane will remain in molecular form up to near-megabar pressures, above which it should transform into a structure with covalently bonded chains of boron atoms and eventually become metallic around 138 GPa.Zinc oxide (ZnO) and zinc sulfide (ZnS) are abundant and nontoxic compound semiconductors, but their band gaps are too wide for potential use in light-harvesting applications. Integration of these thermally and chemically stable compounds into a bulk heterostructure presents an opportunity for the generation of novel materials with notably different properties from their bulk counterparts. Using screened hybrid density-functional methods, we show that the band gaps of ZnO and ZnS can be dramatically reduced by creating layered ZnO/ZnS bulk heterostructures in which m contiguous monolayers of ZnO alternate with n contiguous monolayers of ZnS. In particular, the band gap decreases by roughly 40% upon substitution of every tenth monolayer of ZnS with a monolayer of ZnO (and vice versa) and becomes as low as 1.5 eV for heterostructures with m = 3 to m = 9 contiguous monolayers of ZnO alternating with n = 10 - m monolayers of ZnS. The predicted band gaps of layered ZnO/ZnS heterostructures span much of the visible spectrum, which makes these materials suitable for photovoltaic device engineering.