Matching Items (306)
Description
Ordered mesoporous materials have tunable pore sizes between 2 and 50 nm and are characterized by ordered pore structures and high surface areas (~1000 m2/g). This makes them particularly favorable for a number of membrane applications such as protein separation, polymer extrusion, nanowire fabrication and membrane reactors. These membranes can be fabricated as top-layers on macroporous supports or as embedded membranes in a dense matrix. The first part of the work deals with the hydrothermal synthesis and water-vapor/oxygen separation properties of supported MCM-48 and a new Al-MCM-48 type membrane for potential use in air conditioning systems. Knudsen-type permeation is observed in these membranes. The combined effect of capillary condensation and the aluminosilicate matrix resulted in the highest separation factor (142) in Al-MCM-48 membranes, with a water vapor permeance of 6×10-8mol/m2Pas. The second part focuses on synthesis of embedded mesoporous silica membranes with helically ordered pores by a novel Counter Diffusion Self-Assembly (CDSA) method. This method is an extension of the interfacial synthesis method for fiber synthesis using tetrabutylorthosilicate (TBOS) and cetyltrimethylammonium bromide (CTAB) as the silica source and surfactant respectively. The initial part of this study determined the effect of TBOS height and humidity on fiber formation. From this study, the range of TBOS heights for best microscopic and macroscopic ordering were established. Next, the CDSA method was used to successfully synthesize membranes, which were characterized to have good support plugging and an ordered pore structure. Factors that influence membrane synthesis and plug microstructure were determined. SEM studies revealed the presence of gaps between the plugs and support pores, which occur due to shrinking of the plug on drying. Development of a novel liquid deposition method to seal these defects constituted the last part of this work. Post sealing, excess silica was removed by etching with hydrofluoric acid. Membrane quality was evaluated at each step using SEM and gas permeation measurements. After surfactant removal by liquid extraction, the membranes exhibited an O2 permeance of 1.65x10-6mol/m2.Pa.s and He/O2 selectivity of 3.30. The successful synthesis of this membrane is an exciting new development in the area of ordered mesoporous membrane technology.
ContributorsSeshadri, Shriya (Author) / Lin, Jerry Y. S. (Thesis advisor) / Dai, Lenore (Committee member) / Rege, Kaushal (Committee member) / Smith, David J. (Committee member) / Vogt, Bryan (Committee member) / Arizona State University (Publisher)
Created2011
Description
III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the progress of green emission light emitting devices has been limited by the incorporation of indium in the alloy, mainly due to phase separation. This difficulty could be addressed by studying the growth and thermodynamics of these alloys. Knowledge of thermodynamic phase stabilities and of pressure - temperature - composition phase diagrams is important for an understanding of the boundary conditions of a variety of growth techniques. In this dissertation a study of the phase separation of indium gallium nitride is conducted using a regular solution model of the ternary alloy system. Graphs of Gibbs free energy of mixing were produced for a range of temperatures. Binodal and spinodal decomposition curves show the stable and unstable regions of the alloy in equilibrium. The growth of gallium nitride and indium gallium nitride was attempted by the reaction of molten gallium - indium alloy with ammonia at atmospheric pressure. Characterization by X-ray diffraction, photoluminescence, and secondary electron microscopy show that the samples produced by this method contain only gallium nitride in the hexagonal phase. The instability of indium nitride at the temperatures required for activation of ammonia accounts for these results. The photoluminescence spectra show a correlation between the intensity of a broad green emission, related to native defects, and indium composition used in the molten alloy. A different growth method was used to grow two columnar-structured gallium nitride films using ammonium chloride and gallium as reactants and nitrogen and ammonia as carrier gasses. Investigation by X-ray diffraction and spatially-resolved cathodoluminescence shows the film grown at higher temperature to be primarily hexagonal with small quantities of cubic crystallites, while the one grown at lower temperature to be pure hexagonal. This was also confirmed by low temperature photoluminescence measurements. The results presented here show that cubic and hexagonal crystallites can coexist, with the cubic phase having a much sharper and stronger luminescence. Controlled growth of the cubic phase GaN crystallites can be of use for high efficiency light detecting and emitting devices. The ammonolysis of a precursor was used to grow InGaN powders with different indium composition. High purity hexagonal GaN and InN were obtained. XRD spectra showed complete phase separation for samples with x < 30%, with ~ 9% indium incorporation in the 30% sample. The presence of InGaN in this sample was confirmed by PL measurements, where luminescence from both GaN and InGaN band edge are observed. The growth of higher indium compositions samples proved to be difficult, with only the presence of InN in the sample. Nonetheless, by controlling parameters like temperature and time may lead to successful growth of this III-nitride alloy by this method.
ContributorsHill, Arlinda (Author) / Ponce, Fernando A. (Thesis advisor) / Chamberlin, Ralph V (Committee member) / Sankey, Otto F (Committee member) / Smith, David J. (Committee member) / Tsen, Kong-Thon (Committee member) / Arizona State University (Publisher)
Created2011
Description
Proteins are a fundamental unit in biology. Although proteins have been extensively studied, there is still much to investigate. The mechanism by which proteins fold into their native state, how evolution shapes structural dynamics, and the dynamic mechanisms of many diseases are not well understood. In this thesis, protein folding is explored using a multi-scale modeling method including (i) geometric constraint based simulations that efficiently search for native like topologies and (ii) reservoir replica exchange molecular dynamics, which identify the low free energy structures and refines these structures toward the native conformation. A test set of eight proteins and three ancestral steroid receptor proteins are folded to 2.7Å all-atom RMSD from their experimental crystal structures. Protein evolution and disease associated mutations (DAMs) are most commonly studied by in silico multiple sequence alignment methods. Here, however, the structural dynamics are incorporated to give insight into the evolution of three ancestral proteins and the mechanism of several diseases in human ferritin protein. The differences in conformational dynamics of these evolutionary related, functionally diverged ancestral steroid receptor proteins are investigated by obtaining the most collective motion through essential dynamics. Strikingly, this analysis shows that evolutionary diverged proteins of the same family do not share the same dynamic subspace. Rather, those sharing the same function are simultaneously clustered together and distant from those functionally diverged homologs. This dynamics analysis also identifies 77% of mutations (functional and permissive) necessary to evolve new function. In silico methods for prediction of DAMs rely on differences in evolution rate due to purifying selection and therefore the accuracy of DAM prediction decreases at fast and slow evolvable sites. Here, we investigate structural dynamics through computing the contribution of each residue to the biologically relevant fluctuations and from this define a metric: the dynamic stability index (DSI). Using DSI we study the mechanism for three diseases observed in the human ferritin protein. The T30I and R40G DAMs show a loss of dynamic stability at the C-terminus helix and nearby regulatory loop, agreeing with experimental results implicating the same regulatory loop as a cause in cataracts syndrome.
ContributorsGlembo, Tyler J (Author) / Ozkan, Sefika B (Thesis advisor) / Thorpe, Michael F (Committee member) / Ros, Robert (Committee member) / Kumar, Sudhir (Committee member) / Shumway, John (Committee member) / Arizona State University (Publisher)
Created2011
Description
HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers and HgCdTe-based devices. The microstructure of CdTe surface passivation layers deposited either by hot-wall epitaxy (HWE) or molecular beam epitaxy (MBE) on HgCdTe heterostructures was evaluated. The as-deposited CdTe passivation layers were polycrystalline and columnar. The CdTe grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well-textured with mostly vertical grain boundaries. Observations and measurements using several TEM techniques showed that the CdTe/HgCdTe interface became considerably more abrupt after annealing, and the crystallinity of the CdTe layer was also improved. The microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures grown by MBE was investigated. Many inclined {111}-type stacking faults were present throughout the thin ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Lattice parameter measurement and elemental profiles indicated that some local intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd, Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively. Initial observations of CdTe(211)B/GaAs(211) heterostructures indicated much reduced defect densities near the vicinity of the substrate and within the CdTe epilayers. HgCdTe epilayers grown on CdTe(211)B/GaAs(211) composite substrate were generally of high quality, despite the presence of precipitates at the HgCdTe/CdTe interface. The microstructure of HgCdSe thin films grown by MBE on ZnTe/Si(112) and GaSb(112) substrates were investigated. The quality of the HgCdSe growth was dependent on the growth temperature and materials flux, independent of the substrate. The materials grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and high dislocation densities. For epitaxial growth of HgCdSe on GaSb substrates, better preparation of the GaSb buffer layer will be essential in order to ensure that high-quality HgCdSe can be grown.
ContributorsZhao, Wenfeng (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha (Committee member) / Carpenter, Ray (Committee member) / Bennett, Peter (Committee member) / Treacy, Michael J. (Committee member) / Arizona State University (Publisher)
Created2011
Description
Multi-task learning (MTL) aims to improve the generalization performance (of the resulting classifiers) by learning multiple related tasks simultaneously. Specifically, MTL exploits the intrinsic task relatedness, based on which the informative domain knowledge from each task can be shared across multiple tasks and thus facilitate the individual task learning. It is particularly desirable to share the domain knowledge (among the tasks) when there are a number of related tasks but only limited training data is available for each task. Modeling the relationship of multiple tasks is critical to the generalization performance of the MTL algorithms. In this dissertation, I propose a series of MTL approaches which assume that multiple tasks are intrinsically related via a shared low-dimensional feature space. The proposed MTL approaches are developed to deal with different scenarios and settings; they are respectively formulated as mathematical optimization problems of minimizing the empirical loss regularized by different structures. For all proposed MTL formulations, I develop the associated optimization algorithms to find their globally optimal solution efficiently. I also conduct theoretical analysis for certain MTL approaches by deriving the globally optimal solution recovery condition and the performance bound. To demonstrate the practical performance, I apply the proposed MTL approaches on different real-world applications: (1) Automated annotation of the Drosophila gene expression pattern images; (2) Categorization of the Yahoo web pages. Our experimental results demonstrate the efficiency and effectiveness of the proposed algorithms.
ContributorsChen, Jianhui (Author) / Ye, Jieping (Thesis advisor) / Kumar, Sudhir (Committee member) / Liu, Huan (Committee member) / Xue, Guoliang (Committee member) / Arizona State University (Publisher)
Created2011
Description
Sparsity has become an important modeling tool in areas such as genetics, signal and audio processing, medical image processing, etc. Via the penalization of l-1 norm based regularization, the structured sparse learning algorithms can produce highly accurate models while imposing various predefined structures on the data, such as feature groups or graphs. In this thesis, I first propose to solve a sparse learning model with a general group structure, where the predefined groups may overlap with each other. Then, I present three real world applications which can benefit from the group structured sparse learning technique. In the first application, I study the Alzheimer's Disease diagnosis problem using multi-modality neuroimaging data. In this dataset, not every subject has all data sources available, exhibiting an unique and challenging block-wise missing pattern. In the second application, I study the automatic annotation and retrieval of fruit-fly gene expression pattern images. Combined with the spatial information, sparse learning techniques can be used to construct effective representation of the expression images. In the third application, I present a new computational approach to annotate developmental stage for Drosophila embryos in the gene expression images. In addition, it provides a stage score that enables one to more finely annotate each embryo so that they are divided into early and late periods of development within standard stage demarcations. Stage scores help us to illuminate global gene activities and changes much better, and more refined stage annotations improve our ability to better interpret results when expression pattern matches are discovered between genes.
ContributorsYuan, Lei (Author) / Ye, Jieping (Thesis advisor) / Wang, Yalin (Committee member) / Xue, Guoliang (Committee member) / Kumar, Sudhir (Committee member) / Arizona State University (Publisher)
Created2013
Description
HgCdTe is currently the dominant material for infrared sensing and imaging, and is usually grown on lattice-matched bulk CdZnTe (CZT) substrates. There have been significant recent efforts to identify alternative substrates to CZT as well as alternative detector materials to HgCdTe. In this dissertation research, a wide range of transmission electron microscopy (TEM) imaging and analytical techniques was used in the characterization of epitaxial HgCdTe and related materials and substrates for third generation IR detectors. ZnTe layers grown on Si substrates are considered to be promising candidates for lattice-matched, large-area, and low-cost composite substrates for deposition of II-VI and III-V compound semiconductors with lattice constants near 6.1 Å. After optimizing MBE growth conditions including substrate pretreatment prior to film growth, as well as nucleation and growth temperatures, thick ZnTe/Si films with high crystallinity, low defect density, and excellent surface morphology were achieved. Changes in the Zn/Te flux ratio used during growth were also investigated. Small-probe microanalysis confirmed that a small amount of As was present at the ZnTe/Si interface. A microstructural study of HgCdTe/CdTe/GaAs (211)B and CdTe/GaAs (211)B heterostructures grown using MBE was carried out. High quality MBE-grown CdTe on GaAs(211)B substrates was demonstrated to be a viable composite substrate platform for HgCdTe growth. In addition, analysis of interfacial misfit dislocations and residual strain showed that the CdTe/GaAs interface was fully relaxed. In the case of HgCdTe/CdTe/ GaAs(211)B, thin HgTe buffer layers between HgCdTe and CdTe were also investigated for improving the HgCdTe crystal quality. A set of ZnTe layers epitaxially grown on GaSb(211)B substrates using MBE was studied using high resolution X-ray diffraction (HRXRD) measurements and TEM characterization in order to investigate conditions for defect-free growth. HRXRD results gave critical thickness estimates between 350 nm and 375 nm, in good agreement with theoretical predictions. Moreover, TEM results confirmed that ZnTe layers with thicknesses of 350 nm had highly coherent interfaces and very low dislocation densities, unlike samples with the thicker ZnTe layers.
ContributorsKim, Jae Jin (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha R. (Committee member) / Alford, Terry L. (Committee member) / Crozier, Peter A. (Committee member) / Arizona State University (Publisher)
Created2012
Description
The entire history of HIV-1 is hidden in its ten thousand bases, where information regarding its evolutionary traversal through the human population can only be unlocked with fine-scale sequence analysis. Measurable footprints of mutation and recombination have imparted upon us a wealth of knowledge, from multiple chimpanzee-to-human transmissions to patterns of neutralizing antibody and drug resistance. Extracting maximum understanding from such diverse data can only be accomplished by analyzing the viral population from many angles. This body of work explores two primary aspects of HIV sequence evolution, point mutation and recombination, through cross-sectional (inter-individual) and longitudinal (intra-individual) investigations, respectively. Cross-sectional Analysis: The role of Haiti in the subtype B pandemic has been hotly debated for years; while there have been many studies, up to this point, no one has incorporated the well-known mechanism of retroviral recombination into their biological model. Prior to the use of recombination detection, multiple analyses produced trees where subtype B appears to have first entered Haiti, followed by a jump into the rest of the world. The results presented here contest the Haiti-first theory of the pandemic and instead suggest simultaneous entries of subtype B into Haiti and the rest of the world. Longitudinal Analysis: Potential N-linked glycosylation sites (PNGS) are the most evolutionarily dynamic component of one of the most evolutionarily dynamic proteins known to date. While the number of mutations associated with the increase or decrease of PNGS frequency over time is high, there are a set of relatively stable sites that persist within and between longitudinally sampled individuals. Here, I identify the most conserved stable PNGSs and suggest their potential roles in host-virus interplay. In addition, I have identified, for the first time, what may be a gp-120-based environmental preference for N-linked glycosylation sites.
ContributorsHepp, Crystal Marie, 1981- (Author) / Rosenberg, Michael S. (Thesis advisor) / Hedrick, Philip (Committee member) / Escalante, Ananias (Committee member) / Kumar, Sudhir (Committee member) / Arizona State University (Publisher)
Created2013
Description
High electron mobility transistors (HEMTs) based on Group III-nitride heterostructures have been characterized by advanced electron microscopy methods including off-axis electron holography, nanoscale chemical analysis, and electrical measurements, as well as other techniques. The dissertation was organized primarily into three topical areas: (1) characterization of near-gate defects in electrically stressed AlGaN/GaN HEMTs, (2) microstructural and chemical analysis of the gate/buffer interface of AlN/GaN HEMTs, and (3) studies of the impact of laser-liftoff processing on AlGaN/GaN HEMTs. The electrical performance of stressed AlGaN/GaN HEMTs was measured and the devices binned accordingly. Source- and drain-side degraded, undegraded, and unstressed devices were then prepared via focused-ion-beam milling for examination. Defects in the near-gate region were identified and their correlation to electrical measurements analyzed. Increased gate leakage after electrical stressing is typically attributed to "V"-shaped defects at the gate edge. However, strong evidence was found for gate metal diffusion into the barrier layer as another contributing factor. AlN/GaN HEMTs grown on sapphire substrates were found to have high electrical performance which is attributed to the AlN barrier layer, and robust ohmic and gate contact processes. TEM analysis identified oxidation at the gate metal/AlN buffer layer interface. This thin a-oxide gate insulator was further characterized by energy-dispersive x-ray spectroscopy and energy-filtered TEM. Attributed to this previously unidentified layer, high reverse gate bias up to −30 V was demonstrated and drain-induced gate leakage was suppressed to values of less than 10−6 A/mm. In addition, extrinsic gm and ft * LG were improved to the highest reported values for AlN/GaN HEMTs fabricated on sapphire substrates. Laser-liftoff (LLO) processing was used to separate the active layers from sapphire substrates for several GaN-based HEMT devices, including AlGaN/GaN and InAlN/GaN heterostructures. Warpage of the LLO samples resulted from relaxation of the as-grown strain and strain arising from dielectric and metal depositions, and this strain was quantified by both Newton's rings and Raman spectroscopy methods. TEM analysis demonstrated that the LLO processing produced no detrimental effects on the quality of the epitaxial layers. TEM micrographs showed no evidence of either damage to the ~2 μm GaN epilayer generated threading defects.
ContributorsJohnson, Michael R. (Author) / Mccartney, Martha R (Thesis advisor) / Smith, David J. (Committee member) / Goodnick, Stephen (Committee member) / Shumway, John (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2012
Description
This dissertation is on the study of structural and optical properties of some III-V and II-VI compound semiconductors. The first part of this dissertation is a study of the deformation mechanisms associated with nanoindentation and nanoscratching of InP, GaN, and ZnO crystals. The second part is an investigation of some fundamental issues regarding compositional fluctuations and microstructure in GaInNAs and InAlN alloys. In the first part, the microstructure of (001) InP scratched in an atomic force microscope with a small diamond tip has been studied as a function of applied normal force and crystalline direction in order to understand at the nanometer scale the deformation mechanisms in the zinc-blende structure. TEM images show deeper dislocation propagation for scratches along <110> compared to <100>. High strain fields were observed in <100> scratches, indicating hardening due to locking of dislocations gliding on different slip planes. Reverse plastic flow have been observed in <110> scratches in the form of pop-up events that result from recovery of stored elastic strain. In a separate study, nanoindentation-induced plastic deformation has been studied in c-, a-, and m-plane ZnO single crystals and c-plane GaN respectively, to study the deformation mechanism in wurtzite hexagonal structures. TEM results reveal that the prime deformation mechanism is slip on basal planes and in some cases, on pyramidal planes, and strain built up along particular directions. No evidence of phase transformation or cracking was observed in both materials. CL imaging reveals quenching of near band-edge emission by dislocations. In the second part, compositional inhomogeneity in quaternary GaInNAs and ternary InAlN alloys has been studied using TEM. It is shown that exposure to antimony during growth of GaInNAs results in uniform chemical composition in the epilayer, as antimony suppresses the surface mobility of adatoms that otherwise leads to two-dimensional growth and elemental segregation. In a separate study, compositional instability is observed in lattice-matched InAlN films grown on GaN, for growth beyond a certain thickness. Beyond 200 nm of thickness, two sub-layers with different indium content are observed, the top one with lower indium content.
ContributorsHuang, Jingyi (Author) / Ponce, Fernando A. (Thesis advisor) / Carpenter, Ray W (Committee member) / Smith, David J. (Committee member) / Yu, Hongbin (Committee member) / Treacy, Michael Mj (Committee member) / Arizona State University (Publisher)
Created2013