Browsing by Author "Stokes, Donna W."
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Item A Dosimetric Evaluation of MiniPIX Performance Using In-situ and Simulated Environments(2020-05) Masek, Reed B.Space weather is becoming increasingly relevant as human activity in space and around grows. Primary contributors to this space radiation are galactic cosmic rays (GCRs) which continue to mystify scientists with their high energies and unknown origins. Despite the unknowns, active monitoring of the radiation environment beyond the Earth’s surface is important for the safety of commercial airlines and astronauts. This thesis examines the use of a MiniPIX camera as a relatively low-cost, portable radiation dosimeter used on-board high altitude balloon flights under the High Altitude Student Platform (HASP). The MiniPIX was housed within a miniature container designed to replicate the structure of the International Space Station (ISS). The goal of this construction is to model a complex and exotic environment, such as the ISS, using a simplified representation in attempt to reduce the high dependence of simulations for monitoring the dose received by human on commercial flights or in space by generalizing this methodology to other applications. Its performance is compared to simulations executed by the FLUKA transport code which strive to replicate the atmospheric environment and GCR sources during the HASP missions. The use of the simulations in this context is to validate the configuration flown on the balloon. The results from the simulations are not directly comparable to those from the balloon, but characteristic features within the simulated data are present. Lastly, results from experiments and simulations performed by others are examined and compared to the results from the HASP mission and the simulations performed in this study.Item A Simplified Balloon Payload For Stratospheric Conductivity Measurements(2021-12-17) Ulinski, Alexandra R. B.There are still many open questions about the Earth's atmosphere, and to answer them scientists need data from experimental observations. Stratospheric conductivity measurements are a vital component of global electric circuit research; however, in the twenty-first century there have been very few experiments designed to advance this area of research or investigate puzzling observations made in the decades before. To address this deficiency, this thesis aimed to design a lightweight, low-cost, balloon instrument that could measure stratospheric conductivity. The goal was to create a design that could be shared and replicated by other student groups, adding fresh experimental observations to the available data; thus, allowing scientists to improve models, explain anomalies, and explore new applications of this knowledge. To design an effective, yet simple instrument, previous balloon payloads were analyzed and modified to reduce complexity, weight, and cost. The design, consisting of two spherical conducting probes separated horizontally by a high resistivity boom was shared via ConductivityResearch.com. A prototype was constructed, weighing only 2.72 kg (not including flight train and telemetry devices), and costing just under $2000 (including balloon, helium, and flight train). Initial tests suggest that the design will be capable of measuring atmospheric conductivity and can be replicated with relative ease. The prototype will soon be fully tested during a balloon campaign in Alaska.Item A Study of Energy Materials via Transmission Electron Microscopy(2021-12) McElhenny, Brian Patrick; Chen, Shuo; Forrest, Rebecca L.; Stokes, Donna W.; Varghese, Oomman K.; Ren, Zhifeng; Bao, JimingThis work recognizes the imperative need for green energy solutions in order to secure a clean and safe future for the planet. Transition from greenhouse producing fossil fuels can’t happen overnight, as green energy technologies will have to be introduced into the global infrastructure in tiers according to their economic feasibility. Intermittent renewable sources like solar and wind are in need of a storable, transportable, and clean energy carrier to expand their contribution to the global energy supply. Hydrogen gas is the most promising candidate to play this role, as H2 production and usage is as clean as the renewable sources themselves. Generation of H2 and O2 from water, or electrolysis, on industrial scales will require water-splitting catalysts to perform as efficiently as possible, so the design of novel materials to serve as these catalysts is an important field of research. Three interesting electrocatalysts are investigated via transmission electron microscopy to gain insights on their properties through their structure and composition. One involves a facile, ultrafast, and energy-inexpensive synthesis method to directly grow the active species on the substrate. Another uses a low-energy synthesis technique to modify and improve the layered double hydroxide structure, which is one of the more promising structures in the field. The last involves a core-shell structure that shows excellent potential especially in seawater splitting. On a smaller tier, thermoelectric generators are more immediately assessable and can improve the energy efficiency of existing technologies by converting waste heat into useable electricity. Some of the most promising candidate materials are Mg3Sb2-based compounds. Understanding of charge carrier mobility is critical toward improving the power factor, which is needed to increase the output power density of devices. A precise understanding of the thermally activated mobility seen in these materials has not been achieved. This work investigates this phenomena via a microstructural comparison between samples with variation in their room-temperature conductivities. Interesting results are found involving crystalline bismuth segregations in Mg3.2Sb1.5Bi0.5Te0.01 samples that involve lower temperature synthesis conditions. This result could lead to a better understanding of low temperature mobility in this as well as other similar material systems.Item An Inverse Scattering Series (ISS) Data Comprehensive Internal Multiple Attenuation Algorithm That Accommodates Primaries and Internal Multiples in the Input Data(2016-08) Ma, Chao 1988-; Weglein, Arthur B.; Hall, Stuart A.; Pinsky, Lawrence S.; Stokes, Donna W.; Wood, Lowell T.; Chemingui, NizarThe first part of this dissertation contributes to the removal of internal multiples using the Inverse Scattering Series (ISS). The ISS internal multiple attenuator (of a given specific order), inputs the recorded primaries and internal multiples. The primaries in the input data predict internal multiples of that order from all reflectors at once with accurate time and approximate amplitude and without subsurface information. When the internal multiples in the input data enter the ISS attenuator of a given order, they (1) contribute to higher-order internal multiples removal and (2) under certain circumstances, cause false or spurious events to be predicted. Terms in the ISS, which are of higher order than the attenuator, have the purpose and capability of addressing a shortcoming of its lower-order and less-accommodating relative. The new internal multiple algorithm within this dissertation combines the original lower-order attenuation algorithm with the inclusion and assist of the higher-order terms, providing a comprehensive internal multiple attenuator that can accommodate primaries and internal multiples in the input data. That new higher-order algorithm provides all the benefits of the original ISS internal multiple attenuation algorithms without its deficits and shortcomings. In principle, only primaries are called for to determine structure and to identify subsurface properties. However, when the collection of primaries is incomplete, then the predicted multiples can, at times, be used to provide an approximate image of unrecorded primaries. The latter can supplement the subsurface structural image from recorded primaries. The second part of this dissertation contributes to (1) studying the procedure of using multiples to enhance subsurface structural imaging, and (2) examining and illustrating the added-value from that procedure. To summarize, this dissertation contributes to two important topics in exploration seismology, (1) identifying and removing multiples and (2) using multiples. This dissertation shows multiples can be used to provide an approximate image of unrecorded primaries to enhance the subsurface structural from recorded primaries. However, multiples need to be first predicted and removed from the data before imaging the recorded primaries for processing goals that seek to effectively locate and invert reflections.Item Characterization of Intrinsically Disordered Peptide Ensembles(2020-05) Ezerski, Jacob C. 1987-; Cheung, Margaret S.; Stokes, Donna W.; Morrison, Greg; Ratti, Claudia; Briggs, James M.Intrinsically disordered proteins/peptides (IDPs) are a category of proteins that possess a poorly defined equilibrium structure. IDPs have been shown to play a central role in biological systems, however atomistic details about their binding mechanisms, selectivity, and specificity are poorly understood. The structures of IDPs are particularly challenging to study directly using experimental techniques due to their rapid inter-conversion of ensemble conformations. Similarly, theoretical techniques such as molecular dynamics simulations (MD), are typically parameterized via experimentally determined observables of stable proteins and therefore contain inherent biasing. Moreover, MD data refinement requires clustering of uniquely sampled structures in order to produce a non-biased ensemble. The work presented within this dissertation aims to remedy these IDP structure determination challenges by using circular dichroism (CD) to refine the conformations obtained from all-atom MD trajectories, and cluster the resulting conformations to remove degenerate structures in order to produce an unbiased ensemble with atomic resolution. The future application of the generated structure ensembles towards IDP binding mechanism determination using a Markov State Model (MSM) is discussed and outlined.Item CONTROL AND UNDERSTANDING OF FLEXIBLE ORGANIC PHOTOVOLATICS VIA NANO PRINTING(2013-05) Haldar, Amrita 1983-; Curran, Seamus A.; Gunaratne, Gemunu H.; Stokes, Donna W.; Varghese, Oomman K.; Clarke, Mark S. F.Organic photovoltaics (OPVs) based on conjugated polymers and fullerene derivatives offer a great potential as an economically and environmentally friendly technology to utilize solar energy because of their simple fabrication processes and minimal material usage. They have gathered momentum over the last two decades primarily due to the significant advances in this field. The research described in the body of this dissertation aims to investigate the use of new materials and deposition techniques in the fabrication of organic photovoltaics. There has been a roadblock in the designing of novel OPV architectures primarily due to the use of brittle ITO which imposes a limit to the flexibility of the OPV. The use of thin metal films such as silver and gold is shown to be viable alternative to ITO with improved mechanical properties. Another great advantage of organic semiconductors is their solution-processability which allows inkjet-printing of the polymers for low-waste deposition on large and flexible substrates. In this work, inkjet-printing is developed as a viable deposition technique for the active layer, the effect of relevant parameters on the opto-electronic properties of the active layer is investigated. Finally, the results are incorporated and the new deposition technique with optimized parameters in combination with a thin metallic anode is applied to the fabrication of devices using the standard photo-active layer representing an important step in the manufacture of fully printed, flexible OPVs.Item Developing and Analyzing Green's Theorem Methods to Satisfy Prerequisites of Inverse Scattering Series Multiple Attenuation for Different Types of Marine Acquisition: Towards Extending Prerequisite Satisfaction Methods for On-Shore Exploration(2014-08) Tang, Lin 1987-; Weglein, Arthur B.; Pinsky, Lawrence S.; Wood, Lowell T.; Stokes, Donna W.; Meier, Mark A.Inverse Scattering Series (ISS) algorithm can directly achieve the objectives of seismic processing without requiring any subsurface information. For achieving the potential capabilities of ISS algorithm, there are prerequisites that need to be satisfied. These prerequisites (including separating the reference wave from the reflected data, estimation of the source wavelet, and deghosting) can be satisfied by using Green's-theorem methods. This dissertation provides three contributions in satisfying the prerequisites for Inverse Scattering Series (ISS) multiple removal algorithm. Chapter 2 examines the impact of a specific seismic-acquisition design (over/under cables) on the wave-separation methods. When the depth difference between the two cables is smaller, the wave-separation results are more accurate and have less errors. In the (x, ω) domain, Green's theorem requires the prediction point to be chosen away from the measurement cable, but it can accommodate a non-at cable (e.g., at ocean bottom). Green's theorem in the (k, ω) domain can predict the separated wavefields on the cable. However, it requires a flat cable to perform Fourier transform over the measurement surface. Chapter 3 presents a method for determining the correct reference velocities. The criteria for finding the correct reference velocities could be the invariances of source wavelet at different output points below the cable for the point source data, or the invariances along one radiation angle for the source array data. The third project investigates and compares three different wavelet estimation methods, including: (1) the Wiener filter method, (2) the spectral division method, and (3) the Green's-theorem method. Comparing with the other two methods, the Green's-theorem method demonstrates strength when the data contains random noise, since it utilizes an integral along the measurement surface, which tends to reduce random noise.Item Development of High Efficiency Low Cost III-V Solar Cells (2017-12) Shervin, Kaveh; Zagozdzon-Wosik, Wanda; Freundlich, Alex; Charlson, Earl J.; Litvinov, Dmitri; Ruchhoeft, Paul; Stokes, Donna W.We have investigated the integration of high efficiency solar cells with crystallographically compatible large grain Ge thin films (fabricated on glass) for cost reduction purposes. We have combined defect tolerant III-V dilute nitride (quantum engineered) solar cells and tunnel diode technologies with low cost poly-Ge substrates (developed by Al-Induced Crystallization method). Hence, it was required to allocate our research into three complementary paths, the fabrication of defect tolerant tunnel diodes, implementation of defect tolerant high efficiency quantum structured solar cells and development of low-cost Ge substrates. First, we theoretically and experimentally evaluated the impact of moderate to large dislocation densities on the performance of GaAs tunnel diodes. We have shown for excessive dislocation densities (ND>1×109 cm-2) I-V characteristics undergo a severe degradation. However, for devices with moderate dislocations (about 2×108cm-2), above what has been perceived as acceptable for PV applications, there is even an improvement in term of peak current densities. Next, we studied ultra-thin defect tolerant solar cells compatible with highly defective low-cost substrates. A 1eV quantum engineered design was investigated for elevated absorption capability to compensate device thickness reduction. Dilute nitride III-V materials were used for their ideal bandgap in 4-junction solar cells (lattice matched with Ge), high absorption coefficient and mere valence band offset with GaAs (as an ideal case for quantum engineering). A resonantly coupled multi-quantum well and supperlattice system were theoretically and experimentally studied for the validation of a thermo-tunneling concept. The results suggest a world record Voc beyond the radiative limit (0.4 eV below bandgap). In the last step, we have empirically investigated a low temperature (<400C) Al induced crystallization of amorphous Ge on glass. We were initially able to achieve polycrystalline Ge, mainly oriented towards [111] direction, with grain sizes as large as 200µm. Further, we studied experimental parameters favoring Ge crystallization towards [110] or [100] crystals direction, suitable for solar cells application. The AlOx layer, between Al and Ge layer, have been found a critical parameter facilitating Ge crystallization. The thickness of Al layer has been known as determining parameter in selecting [110] cubic crystals orientation or [100] rhombohedral Ge structure.Item Dilute Nitride Quantum Engineered Solar Cells: For Next Generation of Multispectral Ultra-High Efficiency Si and III-V Photovoltaics(2018-12) Kharel, Khim Lal 1982-; Stokes, Donna W.; Freundlich, Alex; Ordonez, Carlos; Trombetta, Leonard P.; Chu, Wei-KanEach year, the global photovoltaic markets continue to rapidly grow. However, one of the major challenges faced by photovoltaic manufacturing remains to be the cost of solar cell fabrication. The improvement of a cell’s photo-conversion efficiency is a critical factor in driving down the levelized cost of a solar cell. To improve the efficiency beyond the single junction limit, we must focus on the transmission and thermalization losses because they represent the largest contributions to the efficiency limit. In this research work, we investigate, from a theoretical perspective, a quantum engineered solar cell in tandem with an inexpensive silicon technology, to minimize the major losses. A GaAsyP1-x-yNx/GaP (where GaAsyP1-x-yNx has a lattice constant matched with silicon at y=4.7*x-0.1) based symmetric or asymmetric MQWs solar cell, designed to minimize the effects of the degraded minority carrier transport properties of bulk dilute nitride layers is proposed. Similarly, MBE grown GaAsN/GaAs MQWs solar cell device design has been adopted to lower the thermalization loss which enables a significant sub-GaAs-bandgap photocurrent generation while maintaining a world record-setting open-circuit voltage (Voc) approaching the ideal radiative limit (i.e. Woc=Eg-Voc ~ 0.4 V). Using a drift-diffusion approach, the tandem efficiency of the purposed p-i-n GaAsPN/GaP RTT MQWs solar cell in conjunction with an existing 25.6% HIT silicon device has been simulated under 1 sun and AM 1.5 G spectrum and the results show the possibility of achieving an efficiency of above 33% with this type of device. To gain a better understanding of the carrier transport mechanism in the fabricated devices, the optical and electrical properties were measured and analyzed. Bias-dependent EQE analysis shows 30x faster carrier escape in RTT devices compare to thick barrier MQWs cells. Similarly, extracted barrier heights of carriers from PL measurements in RTT devices are lower than in thick barrier MQWs devices. This suggests the improvement in the carrier-escape mechanisms in RTT devices. Finally, electronic temperatures of carriers were extracted from PL measurements which shows a significantly high (up to 1000 K for 300 K lattice temperature) and unusual carrier temperatures which suggests the presence of a significant hot carrier effect.Item Enhancing the Expressivity of OpenMP API through Task-to-Task Synchronization(2013-12) LaGrone, James 1967-; Chapman, Barbara M.; Gabriel, Edgar; Stokes, Donna W.OpenMP is an application programmer interface that provides a parallel program- ming model that has been successfully used to parallelize sequential applications on shared-memory systems. The current OpenMP specification provides the ability to create independent tasks but lacks a provision for task-to-task synchronization, which limits its expressivity for parallelizing some common algorithms. Tasks can be the basic execution unit of future OpenMP runtime implementations, so the ability to use them in a flexible manner will require increased expressivity and flexibility before adoption by users will become widespread. Enhancements to the API will enable scalability and productivity in existing and future implementations. In this thesis, I present a novel approach to providing task-to-task synchronization in the OpenMP programming model and a corresponding extension to the OpenMP Application Programmer Interface. I include a reference implementation for the run- time library, a basic design for a compiler translation of the proposed constructs, and a case study showing favorable results. This work demonstrates how the OpenMP API can be improved with respect to expressivity of the language.Item EPITAXIAL GROWTH AND REAL-TIME ELECTRON DIFFRACTION ANALYSIS OF ADVANCED III-V SEMICONDUCTOR-BASED QUANTUM DOTS(2015-05) Gunasekera, Manori Vajira Chintha 1981-; Freundlich, Alex; Chu, Wei-Kan; Stokes, Donna W.; Wood, Lowell T.; Zagozdzon-Wosik, WandaThe impact of growth kinetics on structural properties significantly affects optical and electronic performances of self-assembled Stranski-Krastanov (SK) quantum dots. Here and within the framework of the synthesis of archetype InAs/GaAs quantum dot system we have undertaken a systematic study on the evolution of dot facet orientation in terms diffraction characterization by the reflection high-energy electron diffraction (RHEED) technique. The existence of a sharp transition of facet arrangements and shapes near the onset of dot formation is investigated, and a dot structure bound by {2 5 11} facets is proposed. During further calculations, we observed asymmetric ripening of quantum dot facets at initial growth stages, and as the dot ripens, more symmetric facet arrangement to the incident direction was noted. Further, incorporating the correlation between dot structural properties and RHEED intensity features, we propose structures for quantum dots bounded by {1 3 6} and {1 (3 ) ̅5} facets for both asymmetric and symmetric situations. In addition, development of advanced quantum dot structures to fabricate wetting layer separated InAs quantum dots is implemented. Improvement of photoluminescence emission from InAs quantum dots embedded in a GaAs matrix is presented at the end of the work.Item EVENT–BY–EVENT NET–LAMBDA FLUCTUATIONS IN LEAD–LEAD COLLISIONS AT√sNN= 5.02 TeV WITH THE ALICE DETECTOR AT THE LHC(2020-05) Umaka, Ejiro; Bellwied, Rene; Gorb, Yuliya; Ratti, Claudia; Stokes, Donna W.; Hungerford, Ed V.This dissertation documents the calculation of the cumulants of the net-$\Lambda$ multiplicity distribution in Pb-Pb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV with the ALICE detector at the LHC so as measure, on an event-by-event basis, the effects of quantum number conservation during the phase transition of strongly interacting matter. The Quantum Chromodynamics (QCD) phase diagram depicts a phase transition from a deconfined quark-gluon plasma phase into confined hadronic matter. The quark-gluon plasma (QGP) is created in ultrarelativistic heavy-ion collisions such as the heavy-ion collisions measured in ALICE, which stands for A Large Ion Collider Experiment. ALICE is a general-purpose, heavy-ion detector at the CERN Large Hadron Collider which focuses on QCD, the strong-interaction sector of the Standard Model. Called the QGP for short, the quark-gluon plasma only exists for a short time (10$^{-23}$ seconds), and at LHC energies, it is only slightly bigger than the size of a proton, making direct observation impossible. Despite this, the temperature and baryon chemical potential of the QGP formation can be indirectly characterized by linking theory (thermodynamic susceptibilities calculated in lattice QCD) and phenomenological models with observables created in the collision such as event-by-event net-particle multiplicity fluctuation measurements. In heavy-ion collisions, fluctuations can be as a result of inhomogeneities in the energy and baryon number deposition in the initial state or due to thermal fluctuations in the subsequent evolution of the system. The latter represents the fluctuations under investigation, particularly in the vicinity of a phase transition. The observables studied and documented in this dissertation are the first two cumulants of the net-$\Lambda$ distribution. In particular, the mean and variance of the net-$\Lambda$ distribution and their ratios were calculated and compared to statistical baselines to search for deviation, if any, from Poisson behavior. The pseudorapidity dependence of the ratio of the second cumulant of the net-$\Lambda$ distribution to the sum of the mean of the $\Lambda$ and $\bar\Lambda$ distributions were also calculated to explore global conservation laws. The deviation from Poisson behavior found in the second cumulant is attributed to global baryon number conservation.Item Extending the Inverse Scattering Series Free-Surface Multiple Elimination and Internal Multiple Attenuation Algorithms by Incorporating the Source Wavelet and Radiation Pattern: Examining and Evaluating the Benefit and Added-Value(2014-08) Yang, Jinlong 1981-; Weglein, Arthur B.; Bassler, Kevin E.; Chapman, Barbara M.; Liu, Fang; Stokes, Donna W.As the petroleum industry moves to more complex and challenging offshore and onshore plays, untangling multiples and primaries becomes more challenging. This dissertation is part of a comprehensive strategy to address the outstanding and pressing challenges in exploration seismology. To improve the effectiveness of the inverse scattering series (ISS) methods, the current ISS demultiple methods are modified and extended from an isotropic point source to a general source array by incorporating the source wavelet and radiation pattern. In this dissertation, three projects are discussed and contributed. First, several Green's theorem preprocessing methods are provided. They can in practice provide all the prerequisites (reference wave removal, an estimation of source wavelet and radiation pattern, and deghosting) required by ISS methods. Numerical examples for finding the source wavelet and radiation pattern and deghosting are shown by using Green's theorem methods. Second, the effects of satisfying and not satisfying the prerequisites of the ISS algorithms are exemplified. To improve the experimental description, the current ISS free-surface multiple elimination algorithm is extended and modified from the isotropic point source to accommodate a general source array with a radiation pattern. In tests using synthetic point-source data, the current ISS free-surface elimination algorithm can predict the free-surface multiples accurately and remove them from the data through a simple subtraction without the need for adaptive subtraction. In tests using synthetic source-array data, the extended free-surface multiple elimination algorithm (in principle) has the ability to predict precisely the free-surface multiples when the source corresponds to an array (or a point source has radiation pattern). Third, to enhance the fidelity of the amplitude and phase predictions of internal multiples, the ISS internal multiple attenuation algorithm is also modified and extended by incorporating the source wavelet and radiation pattern. The tests on synthetic data show that the extended algorithm can improve the amplitude and phase prediction of internal multiples. In summary, this dissertation contributes to our seismic capability by adding the realism and more complete physics of a source that has a radiation pattern into the ISS free-surface multiple elimination and internal multiple attenuation algorithms.Item Fabrication and Characterization of Low Dimensional Metal Oxides and Sulfides for Solar Energy Conversion(2023-05-11) Rana, Dhan; Varghese, Oomman K.; Chen, Shuo; Stokes, Donna W.; Robles Hernandez, Francisco C.; Hosur, PavanIncreasing energy needs and the environmental damage caused by carbon dioxide (CO2) accumulation in the atmosphere due to fossil fuel burning are the two most pressing issues in today’s world. A fast transition to renewable sources-based energy system could address these issues. Solar photovoltaics (PV) and solar photoelectrochemical (PEC) water splitting are two promising renewable energy conversion technologies. Nevertheless, the PV and PEC technologies must offer affordable power and fuels, respectively, for making an impact in the energy market. The primary objective of this dissertation work was to develop and characterize novel nanostructured semiconductors for efficient solar energy conversion. Considering the potential for efficient solar energy conversion, material abundance, environmental compatibility and economic viability, tungsten oxide and copper tin sulfide were selected for the investigation. We employed a colloidal synthesis method to obtain CTS sol. Electrochemical anodization, a low cost and scalable method, was used to fabricate nanostructured tungsten oxide. A major invention that emerged from the dissertation work was the growth of tungsten oxide nanotubes using anodic oxidation. Except for an unconfirmed work, no study had ever shown the fabrication of WO3 nanotubes of length in the micrometer scale. We identified the fabrication conditions favorable for growing ordered nanoporous and nanotube array films of WO3 in a wide thickness range. We carried out a comprehensive investigation of the effects of additives, oxidizers and solvent composition in the electrolyte and other synthesis conditions on the growth of anodic nanostructures of WO3. The photoanodes fabricated using this material for PEC water splitting showed ~75% improvement in the photocurrent compared to the highest reported for a W/WO3 photoanode. CTS thin films were fabricated using a newly developed sol synthesis method. The sol was highly air stable. Performance of CTS solar cells employing WO3 films as window layers was compared with sol TiO2 based CTS cells. Although the PV performance was not highly impressive, the study showed that CTS and WO3 nanotube could be promising for future photovoltaics. The dissertation discusses the details of the development processes of these new materials and their properties relevant to devices for solar energy conversion.Item Fabrication and Design of Sub-Wavelength Periodic Textures for Improving Light Harvesting in Multi-Junction III-V Photovoltaics(2015-12) Wang, Wei 1987-; Freundlich, Alex; Chu, Wei-Kan; Stokes, Donna W.; Ruchhoeft, Paul; Wood, Lowell T.Optimization of non-planar antireflective coating and back- (or front-) surface texturing are widely studied to further reduce the reflection losses and increase the sunlight absorption path in solar cells. In III-V concentrator applications sunlight is focused onto the surface of cell and consequently, light arrives with a wide angular distribution that limits the effectiveness of conventional thin-film antireflective coatings (ARCs). Furthermore, the transmission properties are generally degraded non-uniformly over the electromagnetic spectrum, which in the case of multi-junction solar cells, leads to additional sub-cell current matching-related losses. Here, and in an attempt to identify a better alternative to the conventional dual-layer ARCs, a systematic analysis is undertaken regarding the design of parameters and angular dependent antireflective properties of dielectric grating formed through the implementation of sub-wavelength arrays of 2D pyramidal and hemispherical textures. The study includes evaluation of these properties for several common dielectrics through a careful selection of dielectric material and design. These structures can significantly surpass the performance of planar dual-layer ARCs, and the total number of reflected photons over 380-2000 nm wavelength range can be reduced to less than 2%, by use of single-material textured dielectric. It is also shown that the implementation of these structures for a typical concentrated 3 or 4 junction solar cells with apertures ranging from 0-60 degrees reduces total losses of reflected photons for each sub-cell to less than 4%, and therefore reduces current degradation. Back reflectors have been developed from perfect mirror to textured mirror in order to further increase light path, which can significantly improve the efficiency and allow for much-thinner devices. A Lambertian surface, which has the most random texture, can theoretically raise the light path to 4n2 times that of a smooth surface. It’s a challenge however to fabricate ideal Lambertian texture, especially in a fast and low cost way. In this work, a method is developed to overcome this challenge that combines the use of laser interference lithography and selective wet etching. The approach allows for a rapid wafer scale texture processing with subwavelength (nano-) scale control of the pattern and the pitch. The technique appears as being particularly attractive for the development of ultra-thin III-V devices, or in overcoming the weak sub-bandgap absorption in devices incorporating quantum dots or quantum wells (QWs). The design and fabrication process on the application of the technique for the development of back reflectors for MQWs solar cells are presented. Depending on the growth order (inverted or up-right growth), the two-side device-metallization incorporated with lift-off process are designed differently due to the fragile ultra-thin (~ 2 µm) active layer, and the strain from embedded QWs. Another approach is done through thinning the substrate (~ 15-20 µm), texturing the substrate as an incoherent reflector, and metallization, which won’t affect active layers.Item Formation of Proton Translocating Water Channels in ATP Synthase(2019-08) Maric, Sladjana 1980-; Briggs, James M.; Bassler, Kevin E.; Gunaratne, Gemunu H.; Stokes, Donna W.The F1FO – ATP synthase has been the object of study in the scientific community both from theory and experiment over the past couple of decades. The ATP synthase is a protein complex in the mitochondrial membrane that efficiently converts the cell’s transmembrane proton gradient into chemical energy stored as adenosine triphosphate (ATP). The protein is made of two molecular motors, F1 and FO, coupled by the central stalk. The free energy used for the synthesis of ATP is in the form of protons moving down the electrochemical gradient from the inner-membrane space to the mitochondrial matrix via the two offset half-channels. The membrane bound part of ATP synthase, FO, converts the transmembrane electrochemical potential into mechanical rotation of the rotor in FO and the stalk physically connected to it. Mutations in a gene encoding ATP synthase are proven to affect its function and cause severe syndromes related to energy deficiency. In this dissertation, we study, using molecular dynamics (MD) simulations, the formation of the half-channels within the stator part of FO. These half-channels enable the proton translocation to and from the rotor portion, known as the c-ring. Combining MD with the protein structure network paths and hydrogen-bonding network analysis, we were able to observe clear evidence for proton pathways and compare our results with previous experimental results. We also report studies of leucine-arginine and leucine-proline amino acid replacements, encoded by the T-G and T-C point mutations at locus 8993 of mtDNA. Our results suggest, for the first time, that these mutations adversely affect water half-channels, and consequently impair the ability of the ATP synthase to produce ATP.Item Hermite-Gauss Quadrature with Generalized Hermite Weight Functions and Small Sample Sets for Sparse Polynomials(2020-04) Vu, Brian-Tinh D.This thesis derives a Gaussian quadrature rule from a complete set of orthogonal lacunary polynomials. The resulting quadrature formula is exact for polynomials whose even part skips powers, with a set of sample values that is much smaller than the degree. The weight for these quadratures is a generalized Gaussian, whose negative logarithm is an even monomial; the powers of this monomial make up the even part of the polynomial to be integrated. We first present Rodrigues formulas for generalized Hermite polynomials (GHPs) that are complete and orthogonal with respect to the generalized Gaussian. From the Rodrigues formula for even GHPs we establish a three-term recursion relation and find the normalization constants. We present a slight modification to the Christoffel-Darboux identity and the Lagrange interpolation polynomials, and proceed to derive the roots, weights, and estimate of the error for the generalized Hermite-Gauss quadrature rule applied to sufficiently smooth functions. We illustrate the quadrature rule by applying it to two examples. Finally, we apply a major result from compressive sensing relating a matrix's coherence and sparse recovery guarantees to the quadrature setting.Item I. Preprocessing for Towed Streamer, Ocean Bottom and Onshore Acquisition, for Horizontal or Non-Horizontal Acquisition Surface: Implications for Multiple Removal, Structural Determination and Amplitude Analysis; II. Inverse Scattering Series Internal Multiple Attenuation in an Absorptive Dispersive Earth, without Knowing, Needing or Estimating Elastic or Anelastic Subsurface Properties(2017-08) Wu, Jing 1987-; Weglein, Arthur B.; Wood, Lowell T.; Stokes, Donna W.; Kostov, ClementThe first part of this dissertation advances Green's theorem wave separation methods for separating the reference wave and reflection data and for deghosting. There are several contributions within this first topic area. Firstly, note that a depth-variable acquisition surface can frequently occur either with a feathered cable in water or with a complicated topography in onshore or ocean bottom acquisition. Under these circumstances, directly applying Green's theorem deghosting method cannot deghost the recorded data on the acquisition surface. This dissertation proposes a new approach which is able to effectively solve this problem. Secondly, Green's theorem wave separation is a mature application in marine towed streamer acquisition. This dissertation extends the deghosting method to ocean bottom data. Thirdly, the current filtering methods for onshore ground roll removal may often damage reflection data; this dissertation develops Green's theorem wave separation algorithm which can satisfactorily address this issue. In a further step, this dissertation proposes a simplified algorithm to achieve onshore wave separation with a reduced data requirement. These solutions can enhance the capability of Green's theorem wave separation method and provide an adequate satisfaction of prerequisites for the subsequent multiple removal, structural determination, and amplitude analysis. The second part of this dissertation investigates the performance of the inverse scattering series internal multiple attenuation method for an anelastic medium with absorption and dispersion. Both analytical and numerical tests demonstrate the attenuation method retains its effectiveness to predict the internal multiple with the right time and approximate amplitude, and without requiring any elastic or anelastic property. For an anelastic medium, the approximate amplitude predicted by the inverse scattering internal multiple attenuation algorithm is further from the exact amplitude than in the corresponding acoustic/elastic circumstance. When the anelastic property can be provided or estimated, a new algorithm is developed to improve the predicted amplitude.Item III-V DILUTE NITRIDE MULTI-QUANTUM WELL SOLAR CELLS FOR HIGH EFFICIENCY PHOTOVOLTAICS(2014-12) Vijaya, Gopi Krishna 1986-; Freundlich, Alex; Bassler, Kevin E.; Cheung, Margaret S.; Stokes, Donna W.; Trombetta, Leonard P.Addition of a few percent of nitrogen to conventional III-V semiconductor alloys creates a surprising reduction of the host band-gap. Due to their unusual band-gap characteristics, dilute nitride III-V semiconductor alloys such as Ga(In)AsN are suitable candidates to serve as a 1eV 3rd junction sub-cell in current high efficiency multi-junction solar cells, and are projected to achieve solar cell efficiencies of ~40%. In order to overcome material constraints such as poor minority carrier lifetime and doping issues in dilute nitrides, prior work has involved insertion of quantum well (QW) nanostructures of dilute nitrides within the intrinsic region of a solar cell, nevertheless, issues of carrier extraction have persisted. In order to improve upon this design, in this work, the band structure and energy levels of dilute nitride multi-quantum-well (MQW) system are calculated, from which the absorption coefficient is evaluated as a function of strain, nitrogen concentration, and electric field strength. Fitting the simulated value of absorption to experimental results showed incomplete extraction of carriers in the multi-quantum well region. Experimental characterization of previously grown dilute nitrides revealed the presence of a parasitic nitridation of the GaAsN/GaAs interface. Improvement of QW quality was done by using a modified run-vent system reduced nitridation. Improvement of carrier extraction was proposed based on the calculations of thermally assisted resonant tunneling QW design, and preliminary devices were grown to test this design. Results included a record open circuit voltage of 636 mV, and improved carrier extraction from resonant tunneling MQW solar cells of ~1eV, enabling future fabrication of multi-junction devices with efficiencies of ~39% (1 sun) and ~49% (500 sun conc).Item Impact of the Topography of the Acquisition Surface on Preprocessing and Subsequent Free Surface Multiple Elimination and Depth Migration: Examining the Issue and Providing a Preprocessing Response That Accommodates a Variable Topography - Thereby Allowing Subsequent Multiple Removal and Imaging Methods to Deliver Their Promise and Potential(2017-08) Zhang, Zhen 1990-; Weglein, Arthur B.; Pinsky, Lawrence S.; Stokes, Donna W.; Morrison, Greg; Hoelting, Cory J.; Liang, DongThis dissertation studies the influence of the topography of the seismic acquisition surface on seismic processing. This is important because in real off-shore and on-shore acquisition, there are many cases in which seismic data have to be acquired along a surface that can be far from horizontal. This dissertation provides three advances and contributions. The first examines the issues in preprocessing when the acquisition surface is assumed to be horizontal and it is actually non-horizontal. To address and solve those issues, a new preprocessing formula is derived which accommodates the topography of the measurement surface. Numerical examples compare the preprocessing results that ignore the acquisition topography, and the preprocessing results that accommodate the acquisition topography. The second investigates the effectiveness of inverse scattering series (ISS) free surface multiple elimination that requires deghosted data, where the deghosted data is input with and without the assumption of horizontal acquisition. Comparison with numerical examples demonstrates that effective deghosting, that includes and accommodates the acquisition surface, is a prerequisite for free surface multiple elimination to deliver its capability. The third looks at the subsequent effectiveness of depth imaging, that uses different free surface multiple elimination results from the second advance as input. Quantitative analysis is provided that defines the positive effect of accommodating acquisition topography in preprocessing steps on depth imaging results.