Physical Sciences and Mathematics Commons^™

Thermal Quenching Of Photoluminescence In Zno And Gan, Nahla Albarakati

Theses and Dissertations

Investigation of the thermal quenching of photoluminescence (PL) in semiconductors provides valuable information on identity and characteristics of point defects in these materials, which helps to better understand and improve the properties of semiconductor materials and devices. Abrupt and tunable thermal quenching (ATQ) of PL is a relatively new phenomenon with an unusual behavior of PL. This mechanism was able to explain what a traditional model failed to explain. Usually, in traditional model used to explain “normal” quenching, the slope of PL quenching in the Arrhenius plot determines the ionization energy of the defect causing the PL band. However, in …

Series Solutions Of Polarized Gowdy Universes, Doniray Brusaferro

Theses and Dissertations

Einstein's field equations are a system of ten partial differential equations. For a special class of spacetimes known as Gowdy spacetimes, the number of equations is reduced due to additional structure of two dimensional isometry groups with mutually orthogonal Killing vectors. In this thesis, we focus on a particular model of Gowdy spacetimes known as the polarized T³ model, and provide an explicit solution to Einstein's equations.

Weighted Quantile Sum Regression For Analyzing Correlated Predictors Acting Through A Mediation Pathway On A Biological Outcome, Bhanu M. Evani

Theses and Dissertations

Abstract

Weighted Quantile Sum Regression for Analyzing Correlated Predictors Acting Through a Mediation Pathway on a Biological Outcome

By

Bhanu M. Evani, Ph.D.

A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University.

Virginia Commonwealth University, 2017.

Major Director: Robert A. Perera, Asst. Professor, Department of Biostatistics

This work examines mediated effects of a set of correlated predictors using the recently developed Weighted Quantile Sum (WQS) regression method. Traditionally, mediation analysis has been conducted using the multiple regression method, first proposed by Baron and Kenny (1986), which has since …

Efficient Integration Of Plasmonic And Excitonic Properties Of Metal And Semiconductor Nanostructures Via Sol-Gel Assembly, Dilhara Liyanage

Theses and Dissertations

Research in nanoscience has gained noteworthy interest over the past three decades. As novel chemical and physical properties that are vastly different from extended solids are realized in nanosized materials, nanotechnology has become the center of attention for material in research community. Much to our amazement, investigations in the past two decades revealed that the nanocrystalline semiconductors are “THE PRIME CANDIDATES” to meet the growing energy demand, sensor development, cellular imaging and a number of other optoelectronic applications. Nonetheless, synthesis of nanostructures with control over physical parameters is not sufficient, yet assembling them into functional nanoarchitectures with unique and tunable …

Separation Of Transition And Heavy Metals Using Stationary Phase Gradients And Chelation Thin Layer Chromatography / Evaluation Of The Effectiveness Of Pogil-Pcl Workshops, Stacy L. Stegall

Theses and Dissertations

Gradient surfaces exhibit a variation in functionality along the length of the surface. One method for preparing gradients is controlled-rate infusion (CRI). In Part 1 of this work, CRI was used to prepare gradients for the purpose of separating transition and heavy metals. Initial work on this project was focused on controlling the retention of the metal ions by varying the number of amine groups, aminoalkoxysilane concentration, and the infusion time. The retention factors of four metal ions varied predictably with increasing number of amine groups, increasing aminoalkoxysilane concentration, and increasing infusion time, producing small but useful changes in the …

Gravitational Microlensing, Wesley M. Barnes

Theses and Dissertations

A gravitational microlensing event occurs when a foreground star passes near our line of site to a background source star. The foreground star acts as a lens, perturbing the image of the source star and amplifying the apparent intensity. Because the lens is in motion relative to the source star, the amplification is a function in time, resulting in a characteristic microlensing light curve. If the lens happens to have a planetary companion, the resulting light curve will be perturbed due the planet and the characteristics of the binary system can be ascertained.

Application Of The Fisher Dimer Model To Dna Condensation, John C. Baker Iii

Theses and Dissertations

This paper considers the statistical mechanics occupation of the edge of a single helix of DNA by simple polymers. Using Fisher's exact closed form solution for dimers on a two-dimensional lattice, a one-dimensional lattice is created mathematically that is occupied by dimers, monomers, and holes. The free energy, entropy, average occupation, and total charge on the lattice are found through the usual statistical methods. The results demonstrate the charge inversion required for a DNA helix to undergo DNA condensation.

Quantitative Analysis And Process Of High Speed Live Cell Interferometry Measurements, Daniel Guest

Theses and Dissertations

The application of auto focus, using an optical beam deflection technique, to existing live cell interferometry measurements was developed and examined. The benefit to relevant experiments, currently being performed, is shown as well as its performance across various magnifications. Enough information is given so that the system can be reproduced to fit any end users needs.

Effect Of Charged Species On The Gradient Properties, Kayesh Ashraf

Theses and Dissertations

Surface chemical gradients are materials that exhibit continuous, gradually varying chemical or physical properties along and across the length of a substrate. As a result, each point on the gradient surface can represent an individual sample. They are broadly classified as chemical and physical gradients depending upon the properties that the gradient exhibits. A physical gradient involves a continuous variation of physical properties such as surface roughness and film porosity on the micrometer scale. Chemical gradients involve a gradual variation of chemical properties such as polarity, acidity and basicity, etc. Such gradients have found various applications in cell adhesion, nanoparticle …

Short-Term Effects Of Nutrients On A Barrier Island Grassland Community, Ashley Moulton

Theses and Dissertations

Increased nutrient availability globally has the potential to affect community functional composition of plants in nutrient limited environments, such as coastal grassland systems. Stability of these systems are threatened worldwide by urbanization, as well as effects of sea level rise and increased frequency and intensity of storms, and atmospheric N deposition, associated with climate change. Annual net primary productivity (ANPP), species composition, and functional traits (community weighted specific leaf area (CWSLA), leaf area index (LAI), growth form and photosynthetic pathway) were measured across four treatments to assess multiple resource limitation of nitrogen (N) and phosphorus (P) and functional community response …

The Generalized Monotone Incremental Forward Stagewise Method For Modeling Longitudinal, Clustered, And Overdispersed Count Data: Application Predicting Nuclear Bud And Micronuclei Frequencies, Rebecca Lehman

Theses and Dissertations

With the influx of high-dimensional data there is an immediate need for statistical methods that are able to handle situations when the number of predictors greatly exceeds the number of samples. One such area of growth is in examining how environmental exposures to toxins impact the body long term. The cytokinesis-block micronucleus assay can measure the genotoxic effect of exposure as a count outcome. To investigate potential biomarkers, high-throughput assays that assess gene expression and methylation have been developed. It is of interest to identify biomarkers or molecular features that are associated with elevated micronuclei (MN) or nuclear bud (Nbud) …

Investigations Into Structure And Properties Of Atomically-Precise Transition Metal-Chalcogenide Clusters Of Crte And Ligated Cr6te8(Pet3)6, Anthony F. Pedicini

Theses and Dissertations

The complete understanding of a clusters electronic structure, the primary mechanisms for its properties and stabilization is necessary in order to functionalize them for use as building blocks within novel materials. First principle theoretical studies have been carried out upon the electronic properties of Cr_xTe_y (x = 1 – 6, y = 0 – 8, x + y ≤ 14), as well as for the larger triethylphosphine (PEt₃) ligated cluster system of Cr₆Te₈(PEt₃)₆. Together, we aim to use the information garnered from the smaller clusters to address …

Study Of The Photoluminescence Spectra Of Mg-Doped Gan, Puranjan Ghimire

Theses and Dissertations

We have studied luminescence properties of Mg-doped GaN grown by hydride vapor phase epitaxy. Steady state photoluminescence (PL) spectra have been analyzed. Exciton, ultraviolet luminescence (UVL) and blue luminescence (BL) bands are the dominant PL bands in the spectra. At low temperature, Exciton and UVL bands show almost no shift with excitation intensity, whereas the BL band blueshifts by almost 0.4 ���� with increasing excitation intensity by seven orders of magnitude. Such shifting nature of bands with excitation intensity is explained by assuming that the BL band is detected from the region of the sample where potential fluctuations are very …

Tuning Optimization Software Parameters For Mixed Integer Programming Problems, Toni P. Sorrell

Theses and Dissertations

The tuning of optimization software is of key interest to researchers solving mixed integer programming (MIP) problems. The efficiency of the optimization software can be greatly impacted by the solver’s parameter settings and the structure of the MIP. A designed experiment approach is used to fit a statistical model that would suggest settings of the parameters that provided the largest reduction in the primal integral metric. Tuning exemplars of six and 59 factors (parameters) of optimization software, experimentation takes place on three classes of MIPs: survivable fixed telecommunication network design, a formulation of the support vector machine with the ramp …

Theoretical And Experimental Analysis Of Optical Properties Of Defects In Gan:, Ibrahima Castillo Diallo

Theses and Dissertations

Using the Heyd-Scuseria-Ernzherof (HSE06) hybrid functional method along with photoluminescence experimental measurements, we analyze the properties of intrinsic defects such as vacancies, interstitials, antisites, and common complexes. By using configurational coordinate diagrams, we estimate the likelihood of defects to be radiative or non-radiative. Our calculations show that gallium vacancies exhibit a large magnetic moment in the neutral charge state and are most likely non-radiative. We also investigate the correlation between the observed infrared PL bands created in 2.5 MeV electron-irradiated GaN samples and the formation of native defects. It is found that gallium-nitrogen divacancies are possible sources of the broad …

Synthesis, Characterization, And Enhanced Magnetic Properties Of Iron Carbide Nanomaterials, Brent M. Williams

Theses and Dissertations

Permanent magnets are classified as hard magnetic materials with the main purpose of generating flux for applications such as electric motors, turbines, and hard drives. High coercivity, magnetic remanence, and saturation values with high stability are some of the requirements for permanent magnets. Rare-earth magnets including neodymium and samarium based magnets are known to have superior magnetic properties due to their high magnetocrystalline anisotropy. However, due to the price of rare-earth materials development of alternate permanent magnets composed of inexpensive materials is an ongoing process. Previously cobalt carbide (Co_xC) have shown promise as a potential rare-earth free magnet …

Computational Fluid Dynamics In A Terminal Alveolated Bronchiole Duct With Expanding Walls: Proof-Of-Concept In Openfoam, Jeremy Myers

Theses and Dissertations

Mathematical Biology has found recent success applying Computational Fluid Dynamics (CFD) to model airflow in the human lung. Detailed modeling of flow patterns in the alveoli, where the oxygen-carbon dioxide gas exchange occurs, has provided data that is useful in treating illnesses and designing drug-delivery systems. Unfortunately, many CFD software packages have high licensing fees that are out of reach for independent researchers. This thesis uses three open-source software packages, Gmsh, OpenFOAM, and ParaView, to design a mesh, create a simulation, and visualize the results of an idealized terminal alveolar sac model. This model successfully demonstrates that OpenFOAM can be …

Adversarial Decision Making In Counterterrorism Applications, Dogucan Mazicioglu

Theses and Dissertations

Our main objective is to improve decision making in counterterrorism applications by implementing expected utility for prescriptive decision making and prospect theory for descriptive modeling. The areas that we aim to improve are behavioral modeling of adversaries with multi objectives in counterterrorism applications and incorporating risk attitudes of decision makers to risk matrices in assessing risk within an adversarial counterterrorism framework. Traditionally, counterterrorism applications have been approached on a single attribute basis. We utilize a multi-attribute prospect theory approach to more realistically model the attacker’s behavior, while using expected utility theory to prescribe the appropriate actions to the defender. We …

Preparation And Application Of Catalysts For The Stereospecific Reduction And Photooxygenation Of Olefins In Continuous Operations: A Novel Method For The Production Of Artemisinin, Daniel C. Fisher

Theses and Dissertations

Over the last two centuries, the discovery and application of catalysts has had a substantial impact on how and what chemicals are produced.Given their broad significance, our group has focused on developing new catalyst systems that are recoverable and reusable, in an attempt to reduce concomitant costs.

Our efforts have centered on constructing a recyclable chiral heterogeneous catalyst capable of effecting asymmetric hydrogenations of olefins with high stereoselectivity. A class of phosphinoimidazoline ligands, developed by researchers at Boehringer-Ingelheim, known as BIPI ligands, have proven efficacious in the asymmetric reduction of alkenes. However, these chiral ligands are homogeneous and coordinated to …

Mathematical Models Of The Inflammatory Response In The Lungs, Sarah B. Minucci

Theses and Dissertations

Inflammation in the lungs can occur for many reasons, from bacterial infections to stretch by mechanical ventilation. In this work we compare and contrast various mathematical models for lung injuries in the categories of acute infection, latent versus active infection, and particulate inhalation. We focus on systems of ordinary differential equations (ODEs), agent-based models (ABMs), and Boolean networks. Each type of model provides different insight into the immune response to damage in the lungs. This knowledge includes a better understanding of the complex dynamics of immune cells, proteins, and cytokines, recommendations for treatment with antibiotics, and a foundation for more …

Developing Conceptual Understanding And Procedural Fluency In Algebra For High School Students With Intellectual Disability, Andrew J. Wojcik

Theses and Dissertations

Teaching students with Intellectual Disability (ID) is a relatively new endeavor. Beginning in 2001 with the passage of the No Child Left Behind Act, the general education curriculum integrated algebra across the K-12 curriculum (Kendall, 2011; National Governors Association Center for Best Practices & Council of Chief State School Officers, 2010), and expansion of the curriculum included five intertwined skills (productive disposition, procedural fluency, strategic competence, adaptive reasoning, and conceptual understanding) (Kilpatrick, Swafford, & Findell, 2001). Researchers are just beginning to explore the potential of students with ID with algebra (Browder, Spooner, Ahlgrim-Delzell, Harris & Wakeman, 2008; Creech-Galloway, Collins, Knight, …

Edge-Transitive Bipartite Direct Products, Cameron M. Crenshaw

Theses and Dissertations

In their recent paper ``Edge-transitive products," Hammack, Imrich, and Klavzar showed that the direct product of connected, non-bipartite graphs is edge-transitive if and only if both factors are edge-transitive, and at least one is arc-transitive. However, little is known when the product is bipartite. This thesis extends this result (in part) for the case of bipartite graphs using a new technique called "stacking." For R-thin, connected, bipartite graphs A and B, we show that A x B is arc-transitive if and only if A and B are both arc-transitive. Further, we show A x B is edge-transitive only …

Synthesis And Characterization Of Metallic Nanoparticles For Catalytic Applications, Sarah Smith

Theses and Dissertations

In recent years, research has focused on reducing the cost of catalysts in a variety of ways including using less expensive materials, improving the synthetic method, and increasing the catalytic activity, recovery, and recyclability. Typically with nanoparticles, the size, shape, composition, and surface coating have an effect on catalytic activity.^1-2 In this work, we focused on reducing the cost of precious metal based catalysts by altering the synthetic methods.

One way to lower the cost of synthesizing precious metal nanoparticles is by debasing the precious metal with a second cheaper more abundant metal. CuPd nanoparticles were synthesized in oleylamine …

A Novel Synthesis And Subsequent Decyclization Of Iminothiozolidinones: Expansion Of Thiourea Chemistry For Biological Applications, Constance D. Franklin

Theses and Dissertations

Small molecule synthesis has become a valuable tool in the study of biological systems. Biologically active compounds can be designed based on well-characterized endogenous systems or they can be found through the screening of large libraries of small molecules. This work involves the development of a small library of cyclic thiourea-based small molecules by use of an unreported synthetic pathway. Briefly, parent thioureas were cyclized by reaction with bromoacetyl bromide, and one or two isomeric heterocycles were found to form. Further studies indicated that the reaction could be easily manipulated by temperature or solvent to effectively control the product distribution. …

Quantitative Analysis Of Tobacco-Specific Nitrosamines And Their Precursor Alkaloids In Tobacco Extracts, Celeste T. Wilkinson

Theses and Dissertations

Tobacco-specific nitrosamines (TSNA) are carcinogenic constituents derived from alkaloids in tobacco. Researchers are actively exploring several avenues to reduce TSNA levels in tobacco products like moist snuff tobacco. The focus of the research presented within is the quantitative analysis of TSNA in tobacco, specifically N’-nitrosonornicotine (NNN), 4-(methylnitrosamino)-1(3-pyridyl)-1-butanone (NNK), N’-nitrosoanatabine (NAT), and N’-nitrosoanabasine (NAB).

Tobacco alkaloids and nitrosamines in tobacco are currently analyzed by different instrumentation due to orders of magnitude difference in their concentrations, chromatographic separation challenges due to structural similarities, and similar mass fragmentation patterns. An analytical column using silica and 1,2-bis(siloxy)ethane hybrid particles of 1.7 µm size is …

Comparing The Structural Components Variance Estimator And U-Statistics Variance Estimator When Assessing The Difference Between Correlated Aucs With Finite Samples, Anna L. Bosse

Theses and Dissertations

Introduction: The structural components variance estimator proposed by DeLong et al. (1988) is a popular approach used when comparing two correlated AUCs. However, this variance estimator is biased and could be problematic with small sample sizes.

Methods: A U-statistics based variance estimator approach is presented and compared with the structural components variance estimator through a large-scale simulation study under different finite-sample size configurations.

Results: The U-statistics variance estimator was unbiased for the true variance of the difference between correlated AUCs regardless of the sample size and had lower RMSE than the structural components variance estimator, providing better type 1 error …

Big Data Visualization Platform For Mixed Reality, Aliakbar Panahi

Theses and Dissertations

The visualization of data helps to provide faster and deeper insight into the data. In this work, a system for visualizing and analyzing big data in an interactive mixed reality environment is proposed. Such a system can be used for representing different types of data such as temporal, geospatial, network graph, and high dimensional. Also, an implementation of this system for four different data types are created. The data types include network data, volumetric data, high dimensional, and spectral data for different mixed reality devices such as Microsoft HoloLens, Oculus Rift, Samsung Gear VR, and Android ARCore were created. It …

Network Analytics For The Mirna Regulome And Mirna-Disease Interactions, Joseph Jayakar Nalluri

Theses and Dissertations

miRNAs are non-coding RNAs of approx. 22 nucleotides in length that inhibit gene expression at the post-transcriptional level. By virtue of this gene regulation mechanism, miRNAs play a critical role in several biological processes and patho-physiological conditions, including cancers. miRNA behavior is a result of a multi-level complex interaction network involving miRNA-mRNA, TF-miRNA-gene, and miRNA-chemical interactions; hence the precise patterns through which a miRNA regulates a certain disease(s) are still elusive. Herein, I have developed an integrative genomics methods/pipeline to (i) build a miRNA regulomics and data analytics repository, (ii) create/model these interactions into networks and use optimization techniques, motif …

Parametric, Nonparametric And Semiparametric Approaches In Profile Monitoring Of Poisson Data, Sepehr Piri

Theses and Dissertations

Profile monitoring is a relatively new approach in quality control best used when the process data follow a profile (or curve). The majority of previous studies in profile monitoring focused on the parametric modeling of either linear or nonlinear profiles under the assumption of the correct model specification. Our work considers those cases where the parametric model for the family of profiles is unknown or, at least uncertain. Consequently, we consider monitoring Poisson profiles via three methods, a nonparametric (NP) method using penalized splines, a nonparametric (NP) method using wavelets and a semi parametric (SP) procedure that combines both parametric …

A Multi-Method Exploration Of The Genetic And Environmental Risks Contributing To Tobacco Use Behaviors In Young Adulthood, Elizabeth K. Do

Theses and Dissertations

Tobacco use remains the leading preventable cause of morbidity and mortality in both the United States and worldwide. Twin and family studies have demonstrated that both genetic and environmental factors are important contributors to tobacco use behaviors. Understanding how genes, the environment, and their interactions is critical to the development of public health interventions that focus on the reduction of tobacco related morbidity and mortality. However, few studies have examined the transition from adolescent to young adulthood – the time when many individuals are experimenting with and developing patterns of tobacco use. This dissertation thesis seeks to provide a comprehensive …