Theses and Dissertations - Department of Physics & Astronomy

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Browsing Theses and Dissertations - Department of Physics & Astronomy by Author "Bao, Yuping"

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    Bacterium E. coli- and phage P22-templated synthesis of semiconductor nanostructures
    (University of Alabama Libraries, 2010) Shen, Liming; Gupta, Arunava; University of Alabama Tuscaloosa
    The properties of inorganic materials in the nanoscale are found to be size- and shape-dependent due to quantum confinement effects, and thereby nanomaterials possess properties very different from those of single molecules as well as those of bulk materials. Assembling monodispersed nanoparticles into highly ordered hierarchical architectures is expected to generate novel collective properties for potential applications in catalysis, energy, biomedicine, etc. The major challenge in the assembly of nanoparticles lies in the development of controllable synthetic strategies that enable the growth and assembly of nanoparticles with high selectivity and good controllability. Biological matter possesses robust and precisely ordered structures that exist in a large variety of shapes and sizes, providing an ideal platform for synthesizing high-performance nanostructures. The primary goal of this thesis work has been to develop rational synthetic strategies for high-performance nanostructured materials using biological templates, which are difficult to achieve through traditional chemical synthetic methods. These approaches can serve as general bio-inspired approaches for synthesizing nanoparticle assemblies with desired components and architectures. CdS- and TiO_2 -binding peptides have been identified using phage display biopanning technique and the mechanism behind the specific affinity between the selected peptides and inorganic substrates are analyzed. The ZnS- and CdS-binding peptides, identified by the phage display biopanning, are utilized for the selective nucleation and growth of sulfides over self-assembled genetically engineered P22 coat proteins, resulting in ordered nanostructures of sulfide nanocrystal assemblies. The synthetic strategy can be extended to the fabrication of a variety of other nanostructures. A simple sonochemical route for the synthesis and assembly of CdS nanostructures with high yield under ambient conditions has been developed by exploiting the chemical characteristics and structure of permeabilized E. coli bacteria. The crystal phase, morphology, micro/nanostructure, optical absorption, and photocatalytic properties of the CdS nanostructures are tailored over a wide range by merely changing the synthetic conditions. Photoanodes fabricated using the nanoporous hollow CdS microrods exhibit excellent performance for the photocatalytic hydrogen production. This facile approach has been extended to the synthesis and assembly of other semiconducting sulfides, including PbS, ZnS, and HgS.
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    Search for astrophysical tau neutrinos in three years of IceCube data
    (University of Alabama Libraries, 2015) Xu, Donglian; Williams, Dawn R.; University of Alabama Tuscaloosa
    High-energy cosmic neutrinos are expected to be produced in extremely energetic astrophysical sources such as active galactic nuclei (AGNs) and gamma ray bursts (GRBs). The IceCube Neutrino Observatory has recently detected a diffuse astrophysical neutrino flux at 5.7σ significance. One of the outstanding questions regarding astrophysical neutrinos is their flavor composition. Most standard oscillation scenarios predict tau neutrinos (υ_τ) in the astrophysical flux, which have a negligible background from cosmic ray induced atmospheric neutrinos. This work reports a search for high-energy astrophysical υ_τ with the IceCube neutrino observatory. This analysis has devised a new method to search for astrophysical υ_τ in the IceCube waveforms, and it is the first υ_τ analysis in IceCube that is more sensitive to a υ_τ flux than other neutrino flavors. A total of three years of data was used, yielding a sensitivity of 5.1 x 10^-8 GeV cm^-2 s^-1 sr^-1 near the PeV region assuming a flux limit of E^2 Φ_υ < 1.0 x 10^-8 GeV cm^-2 s^-1 sr^-1 per neutrino flavor. Zero events were found. Differential upper limits for astrophysical υ_τ are derived in the Ο(100) TeV to Ο(10) PeV regime based on the zero findings.