Dissertation (Ph.D.) -- University of Texas Southwestern Medical Center at Dallas, 2006. / Vita. Bibliography: p.210-222

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Electrical and Computer Engineering." Includes bibliographical references (p. 150-157).

Thesis (Ph. D.)--Duke University, 2008. / Includes bibliographical references.

Thesis (doctoral)--Ludwig-Maximilians-Universität München, 2003. / Accompanying CD-ROM includes color images of all figures. Includes bibliographical references (p. 130-135). Original thesis in German available online via the Internet.

Thesis (Ph. D. in Physics)--Vanderbilt University, May 2006. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 246-276).

Orientador: Alessandra Nara de Souza Rastelli / Resumo: Estudos recentes relatam que resinas compostas contendo nanopartículas (NPs) de óxidos metálicos, tais como óxido de zinco (ZnO) e dióxido de titânio (TiO2) têm potencial antibacteriano e podem controlar a formação do biofilme oral cariogênico. O objetivo desse estudo foi avaliar a capacidade antibacteriana de uma resina composta modificada por NPs de óxido de zinco e dióxido de titânio puras e decoradas com prata (ZnO, ZnO/Ag, TiO2 e TiO2/Ag), bem como, avaliar as propriedades de resistência à compressão e à tensão diametral, estabilidade de cor, rugosidade superficial e grau de conversão após modificação da resina composta com as NPs. As NPs foram sintetizadas pelos métodos dos precursores poliméricos e hidrotermal assistido por micro-ondas, e caracterizadas por DRX, área de superfície BET, FTIR e MET. A resina FiltekTM Z350XT modificada com 0,5; 1 e 2% (em massa) foi testada sobre Streptococcus mutans por meio do teste do contato direto (UFC/mL) para determinação da menor concentração inibitória e então avaliada em biofilme de 7 dias por meio decontagem das unidades formadoras de colônias (UFC/mL). As resistências à compressão e à tração diametral da resina composta modificada (n=40) foram avaliadas utilizando-se máquina de ensaio universal (EMIC). O grau de conversão (n=25) foi realizado por análise em FTIR e a leitura da rugosidade superficial (n=50) foi realizada utilizando-se rugosímetro portátil. A estabilidade de cor (n=180) foi avaliada por leitura espectrofotométri... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Different methods to inhibit biofilm formation on dental restorative materials have been studied for decades and recent studies report that composite resins containing metal oxide nanoparticles (NPs), such as zinc oxide (ZnO) and titanium dioxide (TiO2), have antibacterial potential and can control the formation of cariogenic oral biofilm. In this way, the purpose of this study was to evaluate the antibacterial capacity of a resin modified by pure and silver decorated ZnO and TiO2 (ZnO, ZnO/Ag, TiO2 and TiO2/Ag) NPs and to evaluate the compressive and diametral tensile strength, color stability, surface roughness and degree of conversion after modification of the composite resin with the NPs. The NPs were synthesized by polymeric precursor and microwave-assisted hydrothermal methods, characterized by XRD, BET surface area, FTIR and MET. The direct contact test with Filtek ™ Z350XT modified with 0.5; 1 and 2% (by mass) of NPs against Streptococcus mutans was performed in order to choose the minor concentration to perform the other tests. The modified resin was tested against the 7- days S. mutans biofilm. The compressive strength and diametral tensile strength of the modified composite resin (n = 40) was tested using a universal test machine (EMIC). The degree of conversion (n = 25) was performed by FTIR analysis and the surface roughness reading (n = 50) was performed using a portable surface roughness tester. The color stability (n=180) was evaluated by spectrophotometric readings after storage in coffee solution and artificial saliva. The data was analyzed using the software IBM SPSS Statistics 20.0 (SPSS Inc. Chicago, USA). One-way ANOVA were performed to antibacterial and mechanical tests data, two-way ANOVA for degree of conversion and a mixed model repeated measurements ANOVA and a post hoc test for repeated measures with adjustment... (Complete abstract electronic access below) / Doutor

abstract: The research of this dissertation has involved the nanoscale quantitative characterization of patterned magnetic nanostructures and devices using off-axis electron holography and Lorentz microscopy. The investigation focused on different materials of interest, including monolayer Co nanorings, multilayer Co/Cu/Py (Permalloy, Ni81Fe19) spin-valve nanorings, and notched Py nanowires, which were fabricated via a standard electron-beam lithography (EBL) and lift-off process. Magnetization configurations and reversal processes of Co nanorings, with and without slots, were observed. Vortex-controlled switching behavior with stepped hysteresis loops was identified, with clearly defined onion states, vortex states, flux-closure (FC) states, and Omega states. Two distinct switching mechanisms for the slotted nanorings, depending on applied field directions relative to the slot orientations, were attributed to the vortex chirality and shape anisotropy. Micromagnetic simulations were in good agreement with electron holography observations of the Co nanorings, also confirming the switching field of 700-800 Oe. Co/Cu/Py spin-valve slotted nanorings exhibited different remanent states and switching behavior as a function of the different directions of the applied field relative to the slots. At remanent state, the magnetizations of Co and Py layers were preferentially aligned in antiparallel coupled configuration, with predominant configurations in FC or onion states. Two-step and three-step hysteresis loops were quantitatively determined for nanorings with slots perpendicular, or parallel to the applied field direction, respectively, due to the intrinsic coercivity difference and interlayer magnetic coupling between Co and Py layers. The field to reverse both layers was on the order of ~800 Oe. Domain-wall (DW) motion within Py nanowires (NWs) driven by an in situ magnetic field was visualized and quantified. Different aspects of DW behavior, including nucleation, injection, pinning, depinning, relaxation, and annihilation, occurred depending on applied field strength. A unique asymmetrical DW pinning behavior was recognized, depending on DW chirality relative to the sense of rotation around the notch. The transverse DWs relaxed into vortex DWs, followed by annihilation in a reversed field, which was in agreement with micromagnetic simulations. Overall, the success of these studies demonstrated the capability of off-axis electron holography to provide valuable insights for understanding magnetic behavior on the nanoscale. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2010

abstract: The research of this dissertation involved quantitative characterization of electrostatic potential and charge distribution of semiconductor nanostructures using off-axis electron holography, as well as other electron microscopy techniques. The investigated nanostructures included Ge quantum dots, Ge/Si core/shell nanowires, and polytype heterostructures in ZnSe nanobelts. Hole densities were calculated for the first two systems, and the spontaneous polarization for wurtzite ZnSe was determined. Epitaxial Ge quantum dots (QDs) embedded in boron-doped silicon were studied. Reconstructed phase images showed extra phase shifts near the base of the QDs, which was attributed to hole accumulation in these regions. The resulting charge density was (0.03±0.003) holes/nm3, which corresponded to about 30 holes localized to a pyramidal, 25-nm-wide Ge QD. This value was in reasonable agreement with the average number of holes confined to each Ge dot determined using a capacitance-voltage measurement. Hole accumulation in Ge/Si core/shell nanowires was observed and quantified using off-axis electron holography and other electron microscopy techniques. High-angle annular-dark-field scanning transmission electron microscopy images and electron holograms were obtained from specific nanowires. The intensities of the former were utilized to calculate the projected thicknesses for both the Ge core and the Si shell. The excess phase shifts measured by electron holography across the nanowires indicated the presence of holes inside the Ge cores. The hole density in the core regions was calculated to be (0.4±0.2) /nm3 based on a simplified coaxial cylindrical model. Homogeneous zincblende/wurtzite heterostructure junctions in ZnSe nanobelts were studied. The observed electrostatic fields and charge accumulation were attributed to spontaneous polarization present in the wurtzite regions since the contributions from piezoelectric polarization were shown to be insignificant based on geometric phase analysis. The spontaneous polarization for the wurtzite ZnSe was calculated to be psp = -(0.0029±0.00013) C/m2, whereas a first principles' calculation gave psp = -0.0063 C/m2. The atomic arrangements and polarity continuity at the zincblende/wurtzite interface were determined through aberration-corrected high-angle annular-dark-field imaging, which revealed no polarity reversal across the interface. Overall, the successful outcomes of these studies confirmed the capability of off-axis electron holography to provide quantitative electrostatic information for nanostructured materials. / Dissertation/Thesis / Ph.D. Physics 2011

abstract: As existing solar cell technologies come closer to their theoretical efficiency, new concepts that overcome the Shockley-Queisser limit and exceed 50% efficiency need to be explored. New materials systems are often investigated to achieve this, but the use of existing solar cell materials in advanced concept approaches is compelling for multiple theoretical and practical reasons. In order to include advanced concept approaches into existing materials, nanostructures are used as they alter the physical properties of these materials. To explore advanced nanostructured concepts with existing materials such as III-V alloys, silicon and/or silicon/germanium and associated alloys, fundamental aspects of using these materials in advanced concept nanostructured solar cells must be understood. Chief among these is the determination and predication of optimum electronic band structures, including effects such as strain on the band structure, and the material's opto-electronic properties. Nanostructures have a large impact on band structure and electronic properties through quantum confinement. An additional large effect is the change in band structure due to elastic strain caused by lattice mismatch between the barrier and nanostructured (usually self-assembled QDs) materials. To develop a material model for advanced concept solar cells, the band structure is calculated for single as well as vertical array of quantum dots with the realistic effects such as strain, associated with the epitaxial growth of these materials. The results show significant effect of strain in band structure. More importantly, the band diagram of a vertical array of QDs with different spacer layer thickness show significant change in band offsets, especially for heavy and light hole valence bands when the spacer layer thickness is reduced. These results, ultimately, have significance to develop a material model for advance concept solar cells that use the QD nanostructures as absorbing medium. The band structure calculations serve as the basis for multiple other calculations. Chief among these is that the model allows the design of a practical QD advanced concept solar cell, which meets key design criteria such as a negligible valence band offset between the QD/barrier materials and close to optimum band gaps, resulting in the predication of optimum material combinations. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011