Part 1. Femtosecond transient absorption studies of metal and semiconductor nanostructures Part 2. Synthesis and characterization of complex hydride materials for hydrogen storage applications

Part I. Femtosecond transient absorption studies of hollow gold nanostructures and silicon nanowires. Nanomaterials have unique optical and electronic properties due to their large surface to volume ratio and electron confinement. Both metal and semiconductor nanomaterials can be tuned, although the mechanisms are different. For metals, the coherent oscillation of conduction band electrons, or surface plasmon resonance (SPR), induced by a resonant wavelength of electromagnetic radiation results in strong visible light absorption for gold, silver and copper nanomaterials. Semiconductor nanomaterials absorb and fluoresce visible light because of quantum confinement effects. Ultrafast pump-probe transient absorption spectroscopy was used to investigate the relaxation of the SPR in unique, optically tunable, hollow gold nanostructures (HGNs) as well as in porous silicon nanowires. Despite having significant fundamental differences, both displayed very fast relaxation dynamics. After the initial 1 ps electron-phonon relaxation for HGNs, coherent vibrational oscillations were observed. A discrepancy between the theoretical prediction and experimental measurement of these oscillations was rationalized in terms of the degree of polycrystallinity, where increasing disorder decreased the vibrational frequency. The porous silicon nanowires, which are one-dimensional semiconducting nanomaterials, showed a ∼4 ps decay component attributed to intraband recombination. Understanding the ultrafast relaxation dynamics of nanomaterials is essential for developing more efficient nanomaterials for photothermal ablation. In the case of porous silicon, knowledge of relaxation dynamics may lead to understanding the origin of the PL and how to better optimize and exploit that property for future optoelectronic applications.;Part 2. Synthesis and characterization of complex hydrides for hydrogen storage applications. Hydrogen has long been considered as an alternative to hydrocarbon-based fuels because it is abundant, environmentally benign and has an energy density three times that of gasoline. Light-weight complex metal hydrides such as group I and II borohydrides have extremely high volumetric and gravimetric densities of hydrogen and are therefore highly attractive fir solid-state hydrogen storage applications. In particular, magnesium borohydride is a promising candidate because of its theoretical hydrogen weight percentage of 14.9 % and an estimated dehydrogenation enthalpy that permits H2 desorption at reasonable temperatures, among other reasons. However, the decomposition pathway is complicated and was recently shown to involve stable [B12H12]- intermediates. We employed a variety of characterization techniques including Raman Spectroscopy, powder X-ray diffraction and NMR to gain a more fundamental understanding of the multi-step decomposition pathway with and without additives. By employing metal halide additives, the kinetics of this system were substantially improved. Additionally, stable light-weight alkali [B12H12] - intermediates were synthesized and shown to be reversible to the [BH4]- species. These studies represent advancements in both the applicability and understanding of light-weight borohydrides as effective hydrogen storage materials.