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Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references. Also available in electronic version.

Thesis (Ph. D.)--University of Oregon, 2006. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 164-170). Also available for download via the World Wide Web; free to University of Oregon users.

Metal halide perovskites have rapidly gained researchers’ attention and become one of the most promising materials today, with exciting properties and multiple optoelectronic applications. Regardless of the shape into which perovskite materials have been fashioned, the nanocrystals that constitute these materials have been the object of extensive research. Perovskite nanocrystals (PNCs) have rapidly developed due to their favorable optical and electronic properties, including high photoluminescence quantum yield, narrow emission bands, and tunable optical band gap. This dissertation discusses several chemical approaches to enhance optical performance properties of PNCs, including their photoluminescence quantum yield and stability.

No / Here we report an investigation of systematic control of crystal phase in the ternary nanocrystal system, dicopper tin triselenide. Optimizing the synthetic parameters allows modulation between nucleation and growth in either the hexagonal or cubic phase. In addition to size controlled single crystals, the particles can be tuned to occur as 1D linear heterostructures or 3D tetrapods with growth in one phase and termination in the alternate. / SFI, IRCSET

Traditionally, supported metal catalysts have been synthesized by reduction of precursors directly over the support. In these techniques, it is challenging to control the metal cluster size, composition and crystal structure. Herein, we have developed a novel approach to design catalysts with controlled morphologies by infusing presynthesized nanocrystals into the supports. High surface area mesoporous materials, including graphitic carbons, have been utilized for obtaining a high degree of metal dispersion to enhance catalyst stabilities and activities. Gold and iridium nanocrystals have been infused in mesoporous silica with loadings up to 2 wt % using supercritical CO₂ as an antisolvent in toluene to enhance the van der Waals interactions between nanocrystals and the silica. The iridium catalysts show high catalytic activity and do not require high temperature annealing for ligand removal, as ligands bind weakly to the iridium surface. To further enhance metal loadings to >10 % in the catalysts, short-ranged interactions between the metal nanocrystals and the support are further strengthened with weakly binding ligands to expose more of the metal surface to the support. For pre-synthesized FePt nanocrystals, coated with oleic acid and oleylamine ligands, high loadings >10 wt % in mesoporous silica are achieved, without using CO₂. The strong metal-support interactions favor FePt adsorption on the support and also enhance stability against sintering at high temperatures. High resistance to sintering favors formation of the FePt intermetallic crystal structure with <4 nm size upon thermal annealing at 700 °C. The fundamental understanding of the metal-support interactions gained from these studies is then utilized in the design of highly stable Pt and Pt-Cu electrocatalysts with controlled size, composition and alloy structure supported on graphitized mesoporous carbons for oxygen reduction. The resistance of the graphitic carbons to oxidation coupled with strong metal-support interactions mitigate nanoparticle isolation from the support, nanoparticle coalescence, Pt dissolution and subsequent Ostwald ripening and thus enhance catalyst stability. The control of the Pt nanocrystal morphology with high concentrations of highly active (111) surface leads to 25% higher activities than commercial Pt catalysts. Furthermore, the catalyst activities obtained for Pt-Cu catalysts are 4-fold higher than Pt catalysts due to strained Pt shell generated from electrochemical dealloying of copper from the nanoparticle surface. / text

No description available.

The synthesis of n-alkylamine-bound CdSe-NH2Rʹ nanocrystals from carboxylateterminated CdSe-Cd(O2CR)2/HO2CR requires the removal of acidic impurities prior to the addition of primary amine. Otherwise, the formation and subsequent tight binding of n-alkylammonium carboxylate ion pairs prevents quantitative removal of carboxylate species. Dimethylcadmium and diethylzinc were used as reagents to deprotonate acidic impurities, which either causes methylation (with a surface density of 0.04−0.22 nm−2) and photoinduced reduction of the nanocrystal core or X-type ligand exchange with ethyl species, respectively. The acid-scavenged nanocrystals could be completely isolated from displaced carboxylate ligands (≤ 0.01 carboxylates nm-2). In addition to traditional selective precipitation procedures, gel permeation and silica chromatography were investigated as alternative purification methods for the isolation of CdSe-NH2Rʹ. Both demonstrated no improvement compared to the more convenient precipitation process. Thin films fabricated from CdSe-NH2C4H9 show little to no grain growth upon thermal annealing at 250 ºC, maintaining domains (~10 nm) despite complete desorption of n-butylamine from the nanocrystal surface above 150 ºC. Despite no passivation of the surface and a high density of grain boundaries, thin film transistors of CdSe-NH2C4H9 fabricated on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with an average electron mobility of 12 ± 1 cm2 V-1s-1, a low threshold voltage hysteresis (4.0 ± 0.6 Vth), and an on/off ratio of 8x104. Colloidal dispersions of amine bound nanocrystals (CdSe−NH2Rʹ) are indefinitely stable at amine concentrations of 0.1 M or higher and slowly aggregate at lower concentrations. Dissociation and evaporation of the amine ligands in 4-ethylpyridine, tri-n-butylphosphine, or molten tri-n-octylphosphine oxide solution results in nanocrystal aggregation. Greater stability can be achieved using dimethyl-n-octadecylphosphine as the L-type ligand, yielding soluble CdSe- PMe2C18H37 nanocrystals with a phosphine coverage of 1.8 nm-2. CdSe-PMe2C18H37 is the first stable nanocrystal sample bound solely by neutral phosphines. Z-type rebinding was investigated with metal oleate species (Mn+(O2CR)n, M = Cd2+, Zn2+, Pb2+, In3+), and a relative binding affinity of these complexes can be established. Rebinding of metal oleate species at 25 ºC yield lower coverages, yet can reach saturation upon heating to 100 ºC. The rebinding of cadmium chloride to aggregated CdSe-PBu3 stabilizes the particle and aids in their redissolution. L-type ligand exchange and subsequent Z-type rebinding was employed towards the synthesis of a new model compound passivated by dimethyl-n-octadecylphosphine and cadmium trifluoroacetate ligands, CdSe-Cd(O2CCF3)2/PMe2C18H37, which is characterized by UV-Vis, 1H, 19F, and 31P NMR spectroscopies. The findings of this dissertation demonstrate the importance of ion-pair species in the colloidal stabilization of colloidal nanocrystal systems. It also indicates the utility of stoichiometric, amine and phosphine-bound CdSe-L to act as both reporter complexes and as a clean reactive reagent for synthesis of novel CdSe-MX2/L systems to study the molecular interactions at nanocrystal surfaces.

In recent years, the fundamental researches on nanocrystals have been received increasing attentions for the novel applications, especially the nonvolatile memory technology. Adoption of nanocrystals technology could solve the serious limitation suffered by the conventional nonvolatile memory, flash, while scaling down. Once the thin tunneling oxide of flash device has been created a leaky path, all the stored charge in the floating gate will be lost after numerous counts of data reading and writing. Hence, the thinning of tunneling oxide will become one of important keys to the scaling limitation. Furthermore, if the tunneling oxide can not be thinned any more, both the operation voltage and speed of memory can not be improved. These drawbacks will restrict the development of nonvolatile memory. Replacement of floating gate structure with nanocrystals could effectively avoid the data losing due to the leaky path in the thin tunneling oxide. All stored charges can¡¦t be lost through the few leaky paths since the charges are stored in distributed nanocrystals. The charges stored nearby the leaky path will be lost, but others are still kept in the distributed and independent nanocrystals. The advantages of metal nanocrystals has have higher density of states around Fermi level, stronger coupling with conduction channel, wide range of available work functions and smaller energy perturbation due to carrier confinement. So metal nanocrystals can reduce operate voltage, and increase write/erase speed and endurance. In this thesis, we will study of cobalt and cobalt-silicide as the memory storage element. The nanocrystals were formed by high temperature oxidation or metal rapid thermal annealing with all kinds of conditions. And we analyze the effect of electron storage at metal nanocrystals by means of material and electrical analysis.