UC Berkeley

With a rapidly growing population, dwindling resources, and increasing environmental pressures, the need for sustainable technological solutions becomes more urgent. Metal oxides make up much of the earth's crust and are typically inexpensive materials, but poor electrical and optical properties prevent them from being useful for most semiconductor applications. Recent breakthroughs in chemistry and materials science allow for the growth of high-quality materials with nanometer-scale features and high surface-to-volume. These nanostructured materials exhibit new and enhanced properties that lead to acceptable performance from previously unsuitable materials or decreased amounts of high-performance materials. Because the fundamental length scales of optical, electrical, and thermal phenomena lie on the nanometer-scale, nanostructured materials present an opportunity to independently enhance or disrupt these properties. Through the careful selection of material composition, morphology, and dimensions, we can design materials for specific applications, such as optics or energy generation and storage.

The integration of high-bandwidth photonic devices with silicon microchips is essential to continue pace with the increases of processing power predicted by Moore's Law. However, there is a large mismatch in the size of electronic and photonic devices, due to the difficulty of confining infrared wavelengths to the nanometer length scale. Nanowires with diameters as small as 200 nm have been shown to be capable of waveguiding visible and UV wavelengths. Zinc oxide nanodisks were grown using a bottom-up synthesis and investigated as optically-pumped UV laser source with dimensions smaller than the free-space wavelength of light (sub-wavelength). Plasmon-enhanced photoluminescence and p-type doping were also investigated in consideration of fabricating a functional laser diode.

Semiconductor alloying has long been a tool for designing materials with specific optical and electronic properties, but growing tunable alloy nanowires require complex syntheses. As an alternative, solid state conversion chemistry allows for the decoupling of the nanowire synthesis and the alloying procedure. This method allows for for bottom up, large scale preparation of complex metal oxide alloys with a high level of control over nanowire morphology, composition, and properties. Conversion chemistry for complex metal oxide nanowires is demonstrated in two material systems, with potential applications in thermoelectrics and photoelectrochemical water splitting.

Thermoelectric materials have generated interest as a means of increasing the efficiency of power generation through the scavenging of waste heat. Alloys of zinc oxide and indium oxide (and other tri-valent oxides) form a complex crystal structure with inherent features on the nanoscale that enhance phonon scattering and are ideal for thermoelectrics. Conversion chemistry provides simple method to prepare In2-xMxO3(ZnO)n (M = In, Ga, Fe, n = integer) nanowires from ZnO nanowires. Single-nanowire thermal and electrical measurements on In2-xGaxO3(ZnO)n reveal a simultaneous improvement in all contributing factors to the thermoelectric figure of merit, giving an order of magnitude enhancement over similar bulk materials at room temperature.

The increasing environmental concerns associated with fossil fuels motivates the development of technology to inexpensively capture and store solar energy. Photoelectrochemical water splitting uses semiconductor materials to absorb sunlight and produce hydrogen and oxygen from water. Rutile, a form of titanium dioxide, is capable of water photolysis under certain conditions, but its wide band gap prevents it from being a practical solution. Through a conversion chemistry scheme, rutile TiO2 nanowires are reacted with transition metal oxides to form ilmenite nanowires, ATiO3 (A = Mn, Fe, Co, Ni). When A is sub-stoichiometric, ATiO3-decorated TiO2 nanowires or periodic heterojunction nanowires are formed. CoTiO3 and NiTiO3 are evaluated as potential photoanode and catalyst materials for water oxidation.