Doctor of Philosophy, The Ohio State University, 2018, Physics

This thesis addresses scanning tunneling microscopy (STM) studies of metal defects in GaAs(110). Single-atom defects are useful for their potential applications in developing nano-scale miniaturized or new types of optical, magnetic, and electronic devices, as well as expanding our understanding of atomic-scale interactions. STM is used to measure physical and electronic properties of surface and near-surface materials with atomic resolution, but affects the properties of defects being studied. Zn-doped GaAs(110) is one of the most popular commercially used semiconductor materials, and is well-understood on the macroscopic scale. By studying individual Zn defects at a wider range of doping levels than previously studied, we discovered three types of defect-based conductivity caused by dopant interactions that inform macroscopic doped semiconductor properties and atomic-scale defect properties. Additionally, we tuned these properties using electronic fields and doping level, laying the groundwork for Zn defects in solotronic or nanoscale devices. Additionally, erbium, an element with useful optical properties, has been studied on GaAs(110) for the first time, and been shown to have four types of interaction with the host surface. This improved our understanding of the significance of the interaction strength of atoms with host materials on defect charge state and electronic properties. The techniques developed in studying Zn and Er atoms have yielded preliminary results for new properties of Mn and Co atoms and the first studies of V atoms on GaAs(110).

Committee: Jay Gupta (Advisor); Amy Connolly (Committee Member); Roland Kawakami (Committee Member); Mohit Randeria (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2023, Physics

Wide bandgap semiconductors (WBGS) are an exceptionally important class of materials for next-generation microelectronics that exhibit a diverse set of physical phenomena spanning piezo- and ferroelectricity, ferromagnetism, superconductivity, and 2-dimensional electron and hole gases. Oxide and nitride WBGS are particularly well-positioned to accelerate the advancement of renewable energy technologies and improve power electronic efficiencies and life-cycle costs. The identification and capacity for control of optically and electrically active point defects are central in transitioning these materials from research laboratory to practical applications. This dissertation investigates the optoelectronic property-structure relationship of native point defects in wide bandgap semiconductors. By combining spatially resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy (SPS) with surface sensitive x-ray photoelectron spectroscopy (XPS), native anionic point defects and cation defect complexes are identified in ternary ZnGeN2 and binary ScN. The control and refinement of these defects with material growth is essential for coherent integration with complementary GaN architectures. Direct control of point defects on the nanoscale is demonstrated in oxide semiconducting Ga2O3 vertical devices and ZnO nanowire structures by applying electric fields to stimulate the migration of intrinsic defect species. Redistribution of oxygen vacancies (VO) in high power Ga2O3 is demonstrated for the first time by nanoscale hyperspectral imaging after strong reverse biasing while manipulation of oxygen vacancies in suspended ZnO nanowires is selectively driven by applied bias through nanoscale contacts. The density of positively charged interfacial VO is tuned via electric fields to control Schottky barrier heights and reversibly convert metal-ZnO interfaces between Ohmic and Schottky behavior, highlighting the need to consider intrinsic point defect migration i (open full item for complete abstract)

Committee: Leonard Brillson (Advisor); Ciriyam Jayaprakash (Committee Member); Andrew Heckler (Committee Member); Jay Gupta (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2018, Oral Biology

Large craniofacial defects caused by congenital diseases, surgical resection of pathology, or traumatic injuries can result in major psychological and physiological impacts on patients by affecting their facial aesthetics, speech and mastication functions. Currently, the gold standard treatment for a large craniofacial defect is autogenous bone grafting, which involves a secondary surgery to harvest the donor bone from another body part such as the rib, iliac crest, or tibia. This technique can cause donor site morbidity or graft failure due to graft resorption or inadequate bone sources. To overcome the limitations of autogenous bone graft, a bone tissue engineering technique combining autologous mesenchymal stem cells (MSCs) with a bone substitute is being used as an alternative. In the first study of this dissertation, we confirmed the efficacy of autologous MSCs combined with ß-tricalcium phosphate scaffolds in reconstructing a large mandibular defect in a pig model. Although autologous MSCs are generally safe and possess great bone regeneration capacity, it is not practical to gain a large quantity of high-quality autologous MSCs in senior or medically compromised patients. Therefore, in the second study, we investigated the efficacy of xenogeneic MSCs in regenerating mandibular defects in a T cell-deficient rat model. As its regenerative efficacy turned out to be negative, we then investigated the survival of transplanted MSCs and local inflammation in response to the osteotomy and MSC transplantation. We found that xenogeneic MSC transplantation, even with abolished T-cell immunity, was challenged by rejection mediated by both innate and adaptive immune responses. Future studies should focus on further determining the mechanism of autologous MSCs in positive bone regeneration and improving the efficacy of allogeneic or xenogeneic MSC transplantation by controlling host immune rejection.

Committee: Zongyang Sun (Advisor); Sudha Agarwal (Committee Member); Brian Foster (Committee Member); Maiko Suzuki (Committee Member); Yi Zhao (Committee Member) Subjects: Biomedical Engineering; Immunology

PHD, Kent State University, 2015, College of Arts and Sciences / Chemical Physics

Disclination lines play a decisive role in determining the equilibrium structures of topologically constrained liquid crystal systems including cholesteric blue phases, twist grain boundary phases and liquid crystal colloids. The extra energy associated with disclinations is key to stabilizing one particular director structure over another, yet our knowledge of disclination energetics is limited as are characterization methods. In this work, we detail our approach which has focused onbuilding versatile one-of-a-kind instruments for studying liquid crystal systems. This work details the development and use of two novel instruments: an automated maskless photoalignment pattern generator (maskless system) and ad ynamic-cell system that allows for the automated mechanical adjustment of the liquid crystal cell thickness, twist angle and temperature. Both instruments were extensively refined and characterized for maximum performance. In addition, both instruments were designed as versatile platformsfor new research. In this work, we used the maskless system to create novel surface alignments and Pancharatnam-phase devices, and we employed the dynamic-cell system for the generation and characterization of reverse-twist-domain defect loops.

Committee: Hiroshi Yokoyama (Advisor); Philip Bos (Committee Member); Antal Jakli (Committee Member); Elizabeth Mann (Committee Member); Michael Tubergen (Committee Member) Subjects: Chemistry; Physical Chemistry; Physics

Master of Science (MS), Ohio University, 2024, Mechanical Engineering (Engineering and Technology)

In recent years, lithium-ion batteries (LIBs) have played an essential role in nowadays energy storage system, especially electric vehicles (EVs) and portable electronics because of its high energy density and long cycle life [1, 2]. However, one of the biggest challenges is how to guarantee their dependability and trustworthiness. In the present investigation, Acoustic Emission (AE) and Ultrasound Testing (UT) techniques are systematically employed to verify probable critical defects in the LIBs. Where AE technology is able to record the stress waves produced by the growth of the defects, UT uses high-frequency sound waves to penetrate the batteries and provide an indication of the internal voids. The performances of these approaches were systematically tested on as-received, pre-damaged and cold-soaked batteries. Different AE and UT activity patterns were shown in the results under various environmental conditions that influenced battery performance. Combining Acoustic Emission (AE) and Ultrasound Testing (UT) with clustering and outlier analysis machine learning algorithms improved defect detection effectiveness. Such research highlights that AE and UT can be robust noninvasive techniques for on-line health monitoring of LIBs that should aid in maintaining the longevity and operability of LIBs.

Committee: Brian Wisner (Advisor) Subjects: Acoustics; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2024, Physics

Phase transitions occur in many classical and quantum systems, and are the subject of many an open problem in physics. In the past decade, the conformal bootstrap has provided new perspectives for looking at the critical point of a transition. With this formalism, it is possible to exploit the conformal symmetry intrinsically present at the critical point, and derive general results about classes of transitions that obey the same symmetries. This thesis presents the application of this method to two problems of note in classical and quantum phase transitions. The first is a classical model of O(N) spins in the presence of a boundary. We use the numerical conformal bootstrap to prove rigorously the existence of a new boundary phase in three-dimensional Heisenberg (O(3)) and O(4) magnets, deemed the extraordinary-log universality class. The results agree well with a parallel numerical study but are more rigorous due to the bounded nature of the error. The second case is the inherently quantum problem of Anderson transitions between metals and insulators. It has been discovered that at criticality, the wavefunctions describe multifractal objects, that are described by infinitely many fractal dimensions. We use analytical constraints from conformal symmetry to predict the form of these fractal parameters in dimensions greater than two. Our exact prediction, which works in arbitrary dimensions, can be used as a probe for conformal symmetry at Anderson transitions. By studying these two problems, we demonstrate the power of conformal symmetry as a non-perturbative tool in the theory of phase transitions in arbitrary dimensions. Throughout the thesis, we have extended the domain of applicability of traditional bootstrap techniques for the purpose of non-unitary and non-positive systems.

Committee: Ilya Gruzberg (Advisor); Marc Bockrath (Committee Member); Samir Mathur (Committee Member); Yuanming Lu (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, Case Western Reserve University, 2024, Physics

In this work we study magnetization dynamics and pinning in thin film permalloy (Py) microdisks. Thin, micron-scale Py disks magnetize into a vortex state in which the magnetization curls around a central core. To study pinning, we raster scan a vortex core within a thin Py disk using in-plane magnetic fields, and monitor the displacement using the magneto-optical Kerr effect. The resulting measurements yield a map of how defects and inhomogeneities in the material serve to trap high energy-density magnetization configurations such as vortex cores or domain walls. Next, we study magnetic vortex magnetization dynamics as a function of applied in-plane static field and AC driving frequency by optically monitoring a nearby nitrogen vacancy (NV) defect center spin. Despite driving the AC field at frequencies far detuned from an NV spin transition, we observe off-resonant NV spin relaxation in ODMR spectra, due to two different dynamic vortex modes. First, the low frequency gyrotropic mode of the vortex core couples to the NV spin non-linearly due to higher frequencies in the spectrum of the magnetic fringe field arising from the soliton-like nature of the gyrotropic mode. And second, when an in-plane magnetic bias field is applied to the Py disk we observe off-resonant NV spin relaxation due to coupling of a confined magnon mode to an NV spin transition through a parallel pumping effect. Each of these findings is supported through micromagnetic simulations. The results of this research help make progress towards magnetic and spintronic technology that rely on nanoscale control of magnetization dynamics in magnetic spin textures.

Committee: Jesse Berezovsky (Advisor); Anna Samia (Committee Member); Shulei Zhang (Committee Member); Xuan Gao (Committee Member) Subjects: Condensed Matter Physics

Doctor of Philosophy (PhD), Ohio University, 2024, Mechanical and Systems Engineering (Engineering and Technology)

The world is currently experiencing a shortage of semiconductor chips. This shortage is affecting different industries that rely on electronic components that involve semiconductor chips to manufacture their products. Due to the shortage of chips, manufacturers are unable to complete the final assembly of their products, resulting in a delay in delivering the finished products to their customers. To address this issue, the US Congress passed the "Creating Helpful Incentives to Produce Semiconductors (CHIPS) and Science Act of 2022" on 9th August, 2022. This act aims to improve the competitiveness, innovation, and national security of the US. This dissertation focuses on addressing the chip shortage through the reduction of chronic semiconductor manufacturing process quality problems caused by wafer map surface defects. The proposed solution involves detecting mixed-type wafer map surface defect patterns using Ensemble Deformable Convolutional Neural Networks. The framework for defect detection proposed in this dissertation outperforms other machine learning models from literature, such as Conv-Pool-CNN, All-CNN, NIN-CNN, DCNN-v1, and DCNN-v2, in terms of F1-score. The proposed framework uses an industrial wafer map dataset (MixedWM38) from a semiconductor wafer manufacturing process to train the base models for the ensemble method. The results show that the proposed framework accurately identifies multi-pattern defects from the surface of wafer maps. This dissertation will contribute to advancing academic literature for the new field of detecting mixed-type defect patterns from the surface of wafer maps. Defects are indicators of process problems, and preventing quality defects in advance is the best approach to achieving positive yield. The efficient and accurate detection of wafer map mixed-type surface defect patterns is important for addressing chronic manufacturing process quality problems. The proposed framework can be used by semiconductor manufacturer (open full item for complete abstract)

Committee: Tao Yuan (Advisor); Gary Weckman (Committee Member); Ashley Metcalf (Committee Member); William Young (Committee Member); Saeed Ghanbartehrani (Committee Member); Omar Alhawari (Committee Member) Subjects: Artificial Intelligence; Computer Science; Engineering; Industrial Engineering; Mathematics; Mechanical Engineering; Nanotechnology; Operations Research; Statistics; Systems Design

Doctor of Philosophy, The Ohio State University, 2022, Electrical and Computer Engineering

Gallium oxide in its beta phase (β-Ga2O3) has compelling material properties that have generated an intense worldwide research interest for applications in high-voltage and high-power electronics, radio frequency (RF) electronics, and ultraviolet optoelectronics. The ultra-wide bandgap (UWBG) of 4.8 eV and large breakdown field of 8 MV/cm lead to a potentially superior performance compared to contemporary wide bandgap materials (SiC, GaN) in high-power and high-frequency applications, as well as in harsh radiation environments, due to the predicted improved radiation hardness. The potentially transformative performance advantages of β-Ga2O3 are further enhanced by the ability to grow low-cost, large-area native substrates through melt-based growth methods. This enables high-quality homoepitaxially grown layers for improved device reliability compared to non-native substrates from the significant reduction in mismatch-related defects such as dislocations. This dissertation is focused on accelerating β-Ga2O3 material and device technology through the characterization and understanding of how atomic-level defects impact the performance, reliability, and radiation hardness of cutting-edge β-Ga2O3 transistors and provide pathways to reduce their effects by a combined understanding of growth, defect engineering, and device engineering. This is done by systematically evaluating the effects of the Fe deep acceptor in molecular beam epitaxy (MBE) grown devices, which is then compared with the use of the Mg deep acceptor in metalorganic chemical vapor deposition (MOCVD) grown devices. Furthermore, the exploration of high energy particle irradiation effects and the implications of the evolving defect spectrum from irradiation are studied for materials and devices. The defects causing dispersion in MBE grown δ-doped MESFETs are determined to be located in the buffer. The close proximity of the Fe from the substrate that also surface rides into the epitaxially grown buffer (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Siddharth Rajan (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering; Nanotechnology

MS, University of Cincinnati, 2022, Medicine: Genetic Counseling

Background: Genetic evaluation is indicated for individuals with congenital heart disease (CHD) for the purpose of medical management and assessment of reproductive risk, especially individuals with extracardiac comorbidities. Approximately 11% of the pediatric CHD population has a genetic diagnosis and 20-30% have noncardiac congenital malformations, while little is known regarding how many adults with CHD have a clinical genetic diagnosis or extracardiac comorbidities indicating need for genetic evaluation. The aims of this study were to determine the prevalence of genetic diagnosis, testing, and referral patterns, compare prevalence of genetic diagnoses in adult vs. pediatric CHD populations, and identify patient characteristics associated with genetic diagnosis, testing, and referral. Methods: We performed a retrospective chart review on a sample of adults with congenital heart disease seen at a single institution from 2010-2021. Patients with any of the following diagnoses in isolation were excluded: bicuspid aortic valve and/or thoracic aortic aneurysm, isolated aortic dilation, cardiomyopathies, or arrhythmias. Data was stored in a REDcap database and included cardiac lesion, congenital and neurodevelopmental comorbidities, and genetic clinical evaluation and testing. Associations of selected traits against genetic diagnoses, testing, and referrals were tested using Fisher's exact test. Results: In 233 patients, the mean age was 37.5 years. Fourteen percent had genetic diagnoses, 13.7% had genetic testing, and 11.6% had genetic referrals. Of 36 patients with a genetic diagnosis, the most common diagnoses were Down syndrome (47%) and 22q11.2 microdeletion syndrome (19%). Patients were more likely to have genetic diagnosis (p<0.001), referral (p<0.001), or testing (p<0.001) if at least one congenital and/or neurodevelopmental comorbidity was present. Thirty-four percent had at least one congenital and/or neurodevelopmental comorbidity. Of 170 pat (open full item for complete abstract)

Committee: Nicole Weaver M.D. (Committee Member); Nicole Brown M.D. (Committee Member); Xue Zhang Ph.D. (Committee Member); Amy Shikany MS (Committee Member); Alexander Opotowsky M.P.H. M.D. M.M.Sc. (Committee Member) Subjects: Genetics

Doctor of Philosophy, The Ohio State University, 2022, Microbiology

Leishmaniasis is a neglected protozoan disease affecting over 12 million people globally. Cutaneous leishmaniasis (CL) is the most common form, characterized by chronic skin lesions. Currently, there are no approved vaccines for human use. We have generated centrin knock out Leishmania (L.) mexicana (LmexCen-/-) mutants using CRISPR/Cas9. Centrin is a cytoskeletal protein required only for intracellular amastigote replication in Leishmania. Here, we investigated the safety, immunogenicity, and efficacy of LmexCen-/- parasites in vitro and in vivo. Our data shows that LmexCen-/- amastigotes present a growth defect, which results in significantly lower parasitic burdens and increased protective cytokine production in infected macrophages and dendritic cells, compared to LmexWT. Furthermore, LmexCen-/- parasites are safe in susceptible mouse models and efficacious against challenge with LmexWT in genetically different BALB/c and C57BL/6 mice. Vaccinated mice did not develop cutaneous lesions, displayed protective immunity, and showed significantly lower parasitic burdens compared to the controls. Overall, we demonstrate that LmexCen-/- parasites are a promising candidate vaccine against CL in pre-clinical models. Next, we explored the metabolic drivers of these vaccine-mediated immunological profiles. Metabolomics are emerging as a useful tool to uncover unknown networks that govern immune regulation and determine functional specialization. We analyzed the metabolic changes occurring after immunization with LmexCen-/- and compared them with LmexWT infection. Our results show enriched aspartate metabolism and pentose phosphate pathway (PPP) in ears immunized with LmexCen-/- parasites. These pathways are both known to promote M1 polarization in macrophages, and PPP in particular induces nitric oxide production in macrophages cultured with LmexCen-/-, suggesting a shift to a pro-inflammatory phenotype following immunization. Furthermore, immunized mice showed enriched t (open full item for complete abstract)

Committee: Abhay Satoskar (Advisor); Pravin Kaumaya (Committee Member); Steve Oghumu (Committee Member); Jesse Kwiek (Committee Member) Subjects: Immunology; Microbiology; Neurosciences; Parasitology

Master of Science, The Ohio State University, 2022, Chemical Physics

Ultra-Fast Scanning Tunneling Microscopy (UFSTM) is a novel imaging technique that uses high intensity femtosecond laser light to generate atomic defect states called "traps" and provide subsequent atomic surface imaging between pulses. Many surface engineering applications are possible through femtosecond laser induced damage (fs-LID), a process where a strong non-perturbing laser electric field generates a high density of charge carriers that eventually thermalize and impart energy into a material's lattice changing the surface morphology. The number of pulses during illumination plays a large role in the resulting morphology with current theory predicting the formation of trap states in-between pulses. The trap states are too subtle to observe using traditional methods (ie. scanning electron microscopy or optical based microscopy). This thesis presents a coordinated effort in developing a novel scanning tunneling microscope system capable of detecting atomic trap states. In particular, a discussion of the instrumentation challenges associated with UFSTM techniques are presented. This work also includes a brief summary of the X-ray STM imaging technique that can also be used for elemental resolved surface imaging. The instrumentation challenges associated with coupling x-ray light with a standard STM system is included.

Committee: Jay Gupta (Advisor); Enam Chowdhury (Committee Member) Subjects: Materials Science; Morphology; Optics; Physics

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2021, Photochemical Sciences

Transparent Semiconducting oxides (TSO) belong to a special group of wide bandgap oxide materials that have high optical transmittance and high conductivity at the same time. Wide bandgap semiconductors are extremely important for their use in numerous electronic/ optoelectronic devices including MOSFETs, Photo diodes, solar cell, LED, Laser diode, sensors, etc. Recently, wide bandgap oxide materials, especially Ga2O3 have attracted a great deal of attention from the scientific community. β-Ga2O3 is the most stable polymorphs of Ga2O3 with an ultra-wide bandgap of 4.9 eV, high breakdown voltage, and high Baliga's Figure of Merit (BFM) that make it an ideal candidate for the next generation high power devices. A comprehensive study of material properties of β-Ga2O3 is needed to fabricate high-performance devices. Unfortunately, our understanding of β-Ga2O3 as a semiconductor material is not comprehensive. Point defects (e.g., Cation or anion vacancies, interstitials, etc.) that significantly affect the electrical and optical properties of this material are not yet fully understood. Proper understanding, characterizing and modification of defects can lead to its application in semiconductor-based devices. Moreover, finding suitable donors and acceptors for β-Ga2O3 to tune its electrical conductivity is crucial for its use in electronic devices. In this thesis, different aspects of β-Ga2O3 are addressed as a semiconductor material. We have studied optical, electrical, and structural properties of β-Ga2O3 single crystals and epitaxial thin films grown by several techniques. Major point defects in β-Ga2O3 were investigated using several novel techniques. We have identified and characterized major electronic traps and investigated their effects on the optical, structural, and electrical properties of β-Ga2O3. We discovered an innovative way to dope β-Ga2O3 providing high free carrier density and good mobility while maintaining low defect concentration. A novel spectromete (open full item for complete abstract)

Committee: Farida Selim Ph.D. (Advisor); Amelia Carr Ph.D. (Other); Alexander Tarnovsky Ph.D. (Committee Member); Alexey Zayak Ph.D. (Committee Member) Subjects: Chemistry; Materials Science; Physics

PhD, University of Cincinnati, 2020, Engineering and Applied Science: Materials Science

Advances in computing have lead to a sudden burst in simulations involving fully atomistic models. A popular approach for these simulations is the molecular dynamics (MD) technique. Studies employing MD methodology aim to recreate experiments such as nanoindentation, with an aim to extract more information about the model than is possible with experiments. However, while MD does provide a wealth of information such as positions and velocities of all atoms, which can be used to directly study deformation mechanisms as well as provide direct visualization, it has severe drawbacks. The most fundamental restriction of MD is its length-scale limitation, which even on high-performance computing clusters is restricted to the nano-meter range. This makes it difficult to perform one-to-one comparison with experiments. An alternative approach which addresses this length-scale restriction is known as the partitioned-domain method which couples different methods in the same model. One such approach is the quasicontinuum (QC) method which couples atomistic and continuum regions. In the current study we first evaluate the effect of fundamental MD parameters namely: thermostat variables and size of the time-step, on the indentation response of Nickel. We then proceed to show that the length-scales prevalent in MD for nanoindentation simulations have an effect on its response therefore they cannot be used for a direct comparison with larger experimental systems. We then introduce the QC method and improve its computational efficiency by using a hybrid-linear elasticity and Cauchy-Born approach in the continuum region. The QC method is then extended to 3-dimensions and finite temperature for simple and complex lattice systems.

Committee: Woo Kyun Kim Ph.D. (Committee Chair); Donglu Shi Ph.D. (Committee Member); Vijay Vasudevan Ph.D. (Committee Member); Kumar Vemaganti Ph.D. (Committee Member) Subjects: Materials Science

Doctor of Philosophy, The Ohio State University, 2019, Physics

Magnetic skyrmions have attracted substantial interest due to their potential for use in spintronics devices and next-generation information storage, and due to the novel phenomena that result from the topological winding of skyrmions. In this dissertation we theoretically study the properties of magnetic skyrmions in chiral magnets. We focus on two key parameters that must be optimized to achieve maximum device performance: skyrmion size, and skyrmion stability. Skyrmions are stabilized in materials where inversion symmetry is broken. We show that the skyrmion crystal phase is more stable in systems with broken mirror inversion symmetry compared with systems where only bulk inversion symmetry is broken. To understand this effect we study a system where both mirror and bulk inversion symmetry are broken. We show that broken bulk inversion symmetry tends to stabilize a conical phase, and this phase becomes progressively less stable when broken mirror inversion symmetry is introduced. The phase diagram reveals a large region of skyrmion crystal stability, as well as a stable elliptic cone phase, and a square skyrmion crystal phase. In addition to crystal structures with broken inversion symmetry, the presence of an interface in magnetic films introduces a source of broken mirror symmetry. We show that this added source of symmetry breaking enhances the stability of the skyrmion crystal phase. Films surfaces and interfaces also stabilize a novel phase of matter called a chiral bobber crystal. This phase can be uniquely identified by the presence of a two-dimensional lattice of singular points in the magnetization field called Bloch points. We present experimental evidence for the observation of a chiral bobber crystal using magnetization data. Skyrmions can also be found outside the skyrmion crystal phase as metastable, particle-like excitations with a finite lifetime. We show that the lifetime and size of skyrmions have a strong interdependence. Putting limits o (open full item for complete abstract)

Committee: Mohit Randeria Professor (Advisor); Nandini Trivedi Professor (Committee Member); Samir Mathur Professor (Committee Member); Fengyuan Yang Professor (Committee Member) Subjects: Condensed Matter Physics

Doctor of Philosophy, The Ohio State University, 2019, Materials Science and Engineering

Metal casting is a manufacturing process of solidifying molten metal in a mold to make a product with a desired shape. Based on its own unique fabrication benefits, it is one of the most widely used manufacturing processes to economically produce parts with complex geometries in modern industry, especially for transportation and heavy equipment industries where mass production is needed. However, various types of defects typically exist in the as-cast components during the casting processes, which may make it difficult for post-processing and limit the service life and further application of products. It becomes imperative to analyze the processes in actual manufacturing conditions to predict and prevent those casting defects. Since it can be quite time consuming and costly to assess the processes experimentally, a computer-aided approach is highly desirable for product development and process optimization. In recent decades, computer-aided engineering (CAE) techniques have been rapidly developed to simulate different casting processes, which have great benefits to tackle casting defects in a more practical and efficient way. This work focuses on using ProCAST®, a finite element analysis (FEA) software, together with other necessary simulation and modeling techniques, including Computer-Aided Design (CAD), Calculation of Phase Diagrams (CALPHAD) and Cellular Automaton (CA), to study relevant defects in actual metal casting foundries. Specifically, three different cases have been mainly investigated, including (i) veining defect caused by thermal cracking in resin-bonded silica sand molds/inserts for sand casting process; (ii) thermal fatigue cracking in H13 steel dies/inserts for high pressure die casting process; and (iii) Hydrogen-induced gas porosity in A356 castings for gravity casting process with permanent molds. For each case, CAD model was designed and FEA model was constructed with validated materials database based on CALPHAD simulation, experimen (open full item for complete abstract)

Committee: Alan Luo (Advisor); Glenn Daehn (Committee Member); Wei Zhang (Committee Member) Subjects: Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2019, Physics

In this work, scanning probe microscopy (SPM) methods are developed and extended to spatially resolve performance-hampering electrically-active defects, known as traps, present in AlGaN/GaN Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs). Commercial devices used in these studies were cross-sectioned to expose electrically-active regions which are traditionally inaccessible to SPM techniques. Surface potential transients (SPTs) are collected over the cross-sectioned faces of devices using nanometer-scale scanning probe deep-level transient spectroscopy (SP-DLTS), a millisecond time-resolved derivative technique of scanning Kelvin probe microscopy (SKPM) that was implemented with a custom system designed to study SBDs and HEMTs in cross-section. Detected SPTs are indicative of carrier emission from bulk defect-related trap states. In conjunction with similar measurements of these trap states using macroscopic techniques, finite-element simulations provide strong, corroborating evidence that observable SPTs are produced by traps located in the bulk of these samples and are therefore not a result of surface states or surface-related phenomena. GaN-based materials offer advantages over many alternatives in high-frequency and high-voltage applications. Features including a wide bandgap and a large breakdown voltage often translate to improved efficiency, performance, and cost in many electronic systems. However, GaN-based material research is still maturing, and charge trapping may be a limiting factor in GaN electrical performance and therefore hinder its widespread application and adoption. Determining the signatures and spatial distributions of active traps in GaN devices is critical for understanding trap-related mechanisms of device failure as well as the growth or fabrication steps which may be responsible for introducing these defect states. Powerful techniques like deep-level transient spectroscopy (DLTS) exist for identifying specifi (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor); Ezekiel Johnston-Halperin (Committee Member); Richard Kass (Committee Member); Mohit Randeria (Committee Member) Subjects: Electrical Engineering; Physics

MS, University of Cincinnati, 2019, Medicine: Clinical and Translational Research

Background/Purpose: Overall morbidity and mortality in the giant omphalocele population is complicated not only by the large size of the defect and the associated physiologic aberrancies but also by the propensity for these patients to have other congenital anomalies. Appropriate understanding of these characteristics in addition to surgical options and outcomes is crucial to optimizing patient care. Methods: A retrospective chart review from June 2009 to December of 2018 at a single academic children's hospital identified 35 patients with giant omphalocele who met criteria for further analysis. Patients were categorized into favorable (n=20) or unfavorable (n=15) outcomes based on mortality and operative morbidity. Odds ratios were then used to analyze potential prenatal and postnatal predictors of overall outcome. Survivors were then further stratified into staged (n=11), delayed (n=8), and primary (n=6) cohorts for subgroup analysis of surgery specific outcomes. Results: Unfavorable outcomes were most notably associated with other major congenital anomalies, sac rupture, major cardiac anomalies, and prematurity and were significantly less likely with increasing birth weight (p=0.0002). In the survivors, the staged repair group had significantly more trips to the operating room prior to fascial closure (p=0.0046), the primary group was younger at the time of repair (p=0.0007) and had shorter length of stay (hospital p=0.0161, NICU p=0.0046); however no other outcomes were found to be significantly different between the three groups including sepsis, ventilator days, or recurrent hernia. Conclusion: Predictions of overall outcomes in the giant omphalocele population requires analysis of multiple patient variables. In those that survive to repair, more research is necessary to determine superiority between repair types.

Committee: Patrick Ryan Ph.D. (Committee Chair); Todd Jenkins Ph.D. (Committee Member); Foong-Yen Lim (Committee Member) Subjects: Surgery

Master of Science, University of Toledo, 2018, Mechanical Engineering

Two-dimensional (2D) nano-materials are finding numerous applications in next generation electronics, consumer goods, energy generation and storage, and healthcare. The rapid rise of utility and applications for 2D nano-materials necessitates developing means for their mass production. Here, we detail a new compressible flow exfoliation (CFE) method for producing 2D nano-materials using a multiphase flow of 2D layered materials suspended in a high pressure gas undergoing expansion. The expanded gas-solid mixture is sprayed in a suitable solvent, where a significant portion (up to 10% yield) of the initial hexagonal boron nitride material is found to be exfoliated with a mean thickness of 4.2 nm. The exfoliation is attributed to the high shear rates ( >105 s-1) generated by supersonic flow of compressible gases inside narrow orifices and converging-diverging channels. This method has significant advantages over current 2D material exfoliation methods, such as chemical intercalation and exfoliation, as well as liquid phase shear exfoliation, with the most obvious benefit being the fast, continuous nature of the process. Another significant advantage of this process is producing low defect layered materials. The existing methods like liquid sonication, shear exfoliation and ball milling processes are aggressive techniques which induce significant amount of defect in the 2D materials structure. In the second part of this work, we proved that the compressible flow exfoliation induces less defect than other popular methods. The existing methods are also highly solvent dependent. In our work, we provided evidence that our process is solvent independent and efficient exfoliation can be achieved in a broader range of solvents. Thus, scaling this process will reduce the cost of exfoliation a lot while maintaining a better fake quality.

Committee: Dr. Reza Rizvi (Committee Chair); Dr. Matthew Liberatore (Committee Member); Dr. Ahalapitiya Jayatissa (Committee Member) Subjects: Mechanical Engineering

PHD, Kent State University, 2018, College of Arts and Sciences / Department of Physics

In this dissertation research, we propose and develop a new type of photomasks, named plasmonic metamasks (PMMs), for high resolution and high throughput photopatterning of LC molecules into highly complex two-dimensional (2D) or three-dimensional (3D) molecule orientations. Enabled by this new technique, we also explore its applications in engineering topological defects and flat optical elements, and the defect-mediated phase transitions.

Committee: Qi-Huo Wei (Committee Chair); Elizabeth Mann (Committee Co-Chair) Subjects: Optics; Physics