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Transition Rate Enhancement in Reduced Dimensional Optoelectronic Devices
Get PDFJunctions of dissimilar semiconductors, heterojunctions, have been used in a variety of electronic, photonic, and optoelectronic devices. A prime example is the invention of Double Heterojunction (DH) lasers by H. Kroemer and Z. Alferov which was awarded the Nobel Prize in Physics in 2000 for "laying the foundation of modern information technology." When the distance between the two heterojunctions is reduced to a few nanometers, electron motion is confined to two dimensions resulting in a Quantum Well (QW). QW lasers which followed DH lasers show much better optoelectronic properties such as higher efficiency, narrower linewidth, and smaller threshold current. In this thesis, we analyze the effect of dimensionality on the absorption and emission properties of optoelectronic devices. We compare GaAs/AlGaAs DH, Multiple Quantum Well (MQW) and Quantum Wire Lasers where the active region is made of GaAs in each device. Specifically, we calculate the spontaneous emission rate and optical gain of these devices. This analysis helps to determine the underlying physics for the significant increase in the transition rates and optical gain of these devices due to electron confinement. We examine band-to-band optical transition rates for (a) 3D case of DH lasers, (b) 2D case of QW lasers, and (c) 1D case of Quantum wires, and derive and compare, spontaneous emission spectra and the gain coefficient for the three cases. We identify how dimensionality enhances emission spectra through modification of the Joint Optical Density of States (JODS) and oscillator strength. Finally, this study helps explain why heterojunction-based core-shell nanowires show enhanced optoelectronic properties compared to core-only, or homojunction core-shell nanowires.M.S., Electrical Engineering -- Drexel University, 201
Damage Tolerance and Mechanics of Interfaces in Nanostructured Metals
No full textThe concept of interface driven properties in crystalline metals has been one of the most intensely discussed topics in materials science for decades. Since the 1980s researchers have been exploring the concept of grain boundary engineering as route for tuning properties such as fracture toughness and irradiation resistance. This is especially true in ultra-fine grained and nanocrystalline materials where grain boundary mediated properties become dominant. More recently, materials composed of hierarchical nanostructures, such as amorphous-crystalline nanolaminates, have attracted considerable attention due to their favorable properties, ease of manufacture and highly tunable microstructure. While both grain boundary engineering and hierarchical nanostructures have shown promise there are still questions remaining regarding the role of specific attributes of the microstructure (such as grain boundaries, grain/layer size and inter/intralayer morphology) in determining material properties. This thesis attempts to address these questions by using atomistic simulations to perform deformation and damage loading studies on a series of nanolaminate and bicrystalline structures. During the course of this thesis the roles of layer thickness, interlayer structure and interlayer chemistry on the mechanical properties of Ni-NiX amorphous-crystalline nanolaminates were explored using atomistic simulations. This thesis found that layer thickness/thickness ratio and amorphous layer chemistry play a crucial role in yield strength and Young's modulus. Analysis of the deformation mechanisms at the atomic scale revealed that structures containing single crystalline, crystalline layers undergo plastic deformation when shear transformation zones form in the amorphous layer and impinge on the amorphous-crystalline interface, leading to dislocation emission. However, structures containing nanocrystalline, crystalline layers (both equiaxed and columnar nanocrystalline) undergo plastic deformation through a combination of grain boundary sliding and grain boundary mediated dislocation nucleation. Since grain boundaries were found to play a critical role as sources and sinks for dislocations in amorphous-crystalline nanolaminates a follow-up study on the effect of grain boundary character on damage accumulation/accommodation in copper symmetric tilt grain boundaries was performed. This study found that grain boundaries will become saturated with damage, a state where grain boundary energy and grain boundary free volume oscillate about a plateau during continuous defect loading (vacancy, interstitial and frenkel pair loading were all considered). Further, grain boundary character (specifically equilibrium grain boundary energy) was strongly correlated to the damage accommodation behavior of grain boundaries in copper. Finally, a study that attempted to link grain boundary damage saturation behavior to variations in grain boundary mechanical properties was performed. This study found no direct relationships between grain boundary damage saturation behavior and variations in grain boundary properties. The results of this thesis provide researchers with several strategies for tuning the properties of amorphous-crystalline nanolaminates. These strategies include manipulated bulk attributes such as layer thickness and morphology as well as manipulation of microscale attributes such as grain boundary engineering. Finally, this thesis provides valuable insight into the damage loading/accommodation behavior of FCC symmetric tilt grain boundaries.Ph.D., Materials Science and Engineering -- Drexel University, 201
Chromatographic Separation and Stability Analysis of Small Interfering RNA and Lipid Vehicles Using Ion-Pair Reversed Phase Liquid Chromatography
Get PDFChromatographic methods were developed capable of separating and quantitating siRNA, lipids, and their potential breakdown products due to oxidation or hydrolysis. Such methods are essential to developing lipid nanoparticles (LNPs) as a formulation delivery system for siRNAs. Separation of siRNAs was achieved using ion-pair reversed-phase liquid chromatography. Part 1 of the thesis describes the development of an ion-pair reversed-phase HPLC method for the separation of closely related stereoisomers of a chemically modified siRNA duplex. A systematic evaluation of key chromatographic parameters showed that a BEH C18 column with sub-2 [mu]-m particle size, coupled with the use of triethylammonium acetate as the ion-pair reagent and acetonitrile as the strong solvent of the hydro-organic mobile phase, achieved baseline resolution of siRNA stereoisomers and their desulfurization products. A high column temperature, creating a denaturing condition for siRNA, is critical to the separation of stereoisomers. An aprotic organic modifier, such as acetonitrile, can effectively disrupt the hydrogen bonding interaction between the duplex and enable the separation of stereoisomers by promoting hydrophobic interactions between the C18 stationary phase and the stereoisomers. Part 2 of the thesis expands the utility of the ion-pair reversed-phase liquid chromatography to include a simultaneous separation of the main lipid components of an LNP system. Ion-pair reversed-phase separation conditions were developed that can reduce the retention gap between siRNAs and lipids that have significant differences in their physical and chemical properties. Studies showed that a BEH phenyl column could significantly retain siRNA due to a combination of both hydrophobic and [pi] – [pi] interactions. In contrast, the lipids experienced a reduced retention with the phenyl column, a key advantage attributed to the presence of a short alkyl chain component in the stationary phase compared to octyl- or octadecyl-derivatized silica. While the ion-pair reagents had virtually no impact on the separation of the lipids, the retention times of the siRNAs showed a quantitative correlation with the structure of the ion-pair reagents in the mobile phase. The chromatographic separation conditions with a phenyl stationary phase, particularly with dibutylammonium acetate as the ion-pair reagent, markedly reduced the retention gap between the siRNAs and the lipids, achieving a baseline resolution of a complex matrix containing five siRNAs and six lipids in a 20-minute gradient elution method. Finally, the ion-pair reversed-phase method was applied to the degradation products of a model siRNA system. Stress testing showed that the model siRNA developed minimal hydrolysis products at neutral pH. This indicated the importance of chemical modification at the 2’-position in the ribose unit of siRNA molecules. In contrast, the siRNA was prone to oxidation by hydrogen peroxide, with or without trace levels of a transition metal, and to oxidation by a radical initiator. Desulfurization and phosphodiester strand scission were the likely main degradation pathways contributing to the observed oxidative reactivity.Ph.D., Chemistry -- Drexel University, 201
Return-to-Learn for Students Who Experience Concussions: A Case Study of a Western Pennsylvania High School
No full textThe purpose of this case study was to examine the academic return-to-learn procedures implemented by a Western Pennsylvania high school. The case study sought to answer the following questions: 1. What factors led to the development of the Western Pennsylvania high school's academic return-to-learn procedures? 2. How has the implementation of a return-to-learn protocol impacted concussion management in the Western Pennsylvania high school? 3. What do the concussion management team members perceive to be the strengths, challenges, and recommendations for improvement of the Western Pennsylvania high school's return-to-learn protocol? The 12 participants of this study were members of the high school's concussion management team during the 2016-2017 school year. Through individual interviews, observation of a meeting, and a review of documents, this research revealed the process of developing and implementing a return-to-learn protocol in a western Pennsylvania high school. Interview data was transcribed and coded to exhibit emergent themes. Observation data analysis included a review of reflective and descriptive notes. Documents relevant to concussion management were reviewed and integrated with the other data to provide a rich, comprehensive picture of concussion management at the high school. The following conclusions were developed as a result of the implementation of the return-to-learn protocol: (a) the protocol improved communication amongst stakeholders, (b) the protocol provided a consistent plan for staff to follow, (c) the protocol provided clearly defined roles and duties for team members, and (d) the protocol allowed for individualization of accommodations.Ed.D., Educational Leadership and Management -- Drexel University, 201
The Impact of Language Status, Gender, and Ecological Factors on Academic Success of Hispanic Children in Grades 3-5
Get PDFThe ability to read is an essential component of the educational process and is strongly correlated with academic success. The literature reveals that, in general, Hispanic elementary school students in third through fifth grade consistently perform below their school-age peers in the academic arena, and that once below, remain below throughout their schooling. The purpose of the present study was to more clearly identify the ecological factors that are closely associated with the academic success of Hispanic third through fifth grade elementary school students who are identified as English Language Learners (ELL). Two research questions guided this study: 1.) Are there differences between third to fifth grade Hispanic boys and girls who are identified as ELL on reading and math achievement as measured by the Smarter Balanced Assessments, and 2.) Are there differences in achievement as measured by the Smarter Balanced reading and math assessment among ecological variables (Neighborhood, School, Friends, Family, Parent Education Involvement, Health and Well-Being, Social Behavior at Home and School, and School Performance) for third to fifth grade boys and girls identified as ELL? The sample consisted of 65 Hispanic third through fifth grade students who were identified as English Language Learners. They were assessed using the Elementary Student Success Profile (ESSP); this tool resulted in triangulated data from three different surveys (from parents, the children, and their teachers). The ESSP was then used to identify the factors associated with academic achievement in reading and math, as measured by the Smarter Balanced Assessment. Statistical analysis yielded non-significant findings for both questions. However, when additional analysis was completed, significant associations were found between school environment and reading scores, friends and math scores, and health and well-being with both reading and math scores. There was no difference by gender.Ed.D., Educational Leadership and Management -- Drexel University, 201
Emerging Roles of the Protein Kinase MK2 in Spermatogenesis
Get PDFDeveloping male germ cells are highly sensitive to environmental stresses such as heat and oxidative damage. The inability of these cells to mitigate such insults often results in germ cell apoptosis, a decline in the number of functional sperm, and reduced fertility. Cells respond and recover from stress, in part, by activating specific intracellular signaling pathways. Foremost amongst these is the p38 MAP kinase (MAPK) pathway, a stress-induced signal transduction pathway with well-established roles in eliciting protective responses in many cell types exposed to damage. Upon activation, p38 phosphorylates and activates multiple substrates including the serine/threonine kinase MAPKAP kinase 2 (MK2); however, the precise role that MK2 plays in regulating spermatogenesis is unknown. In this work, we have identified two new germ cell-specific substrates of MK2, the RNA-binding protein Dazl (deleted in azoospermia-like) and HspA1L, a testis-specific heat shock protein of the Hsp70 family. We have determined the physiological functions of MK2-mediated phosphorylation of these proteins and the possible roles that these post-translational modifications may play in germ cell biology. These findings firmly establish a novel role for this kinase in regulating the stress response in developing male germ cells. Moreover, our results lend support to a model in which spatiotemporal activation of MK2 may either promote or inhibit the process of spermatogenesis in response to stress.Ph.D., Molecular and Cell Biology and Genetics -- Drexel University, 201
Interaction Between the AAA+ ATPase p97 and Its Cofactor Ataxin3 in Health and Disease
Get PDFp97 is an essential ATPase associated with various cellular activities (AAA+) that functions as a segregase in diverse cellular processes, including the maintenance of proteostasis. p97 interacts with different cofactors that target it to distinct pathways; an important example is the deubiquitinase ataxin3, which collaborates with p97 in endoplasmic reticulum associated degradation. However, the molecular details of this interaction have been unclear. We have characterized the binding of ataxin3 to p97, showing that ataxin3 binds with low-micromolar affinity to both wild-type p97 and mutants linked to degenerative disorders known as multisystem proteinopathy 1 (MSP1); we further showed that the stoichiometry of binding is one ataxin3 molecule per p97 hexamer. We have mapped the binding determinants on each protein, demonstrating that ataxin3’s p97/VCP-binding motif (VBM) interacts with the inter-lobe cleft in the N-domain of p97. We also probed the nucleotide dependence of this interaction, confirming that ataxin3 and p97 associate in the presence of ATP and in the absence of nucleotide, but not in the presence of ADP. Our experiments suggest that an ADP-driven downward movement of the p97 N-terminal domain dislodges ataxin3 by inducing a steric clash between the D1-domain and ataxin3’s C-terminus. In contrast, MSP1 mutants of p97 bind ataxin3 irrespective of their nucleotide state, indicating a failure by these mutants to translate ADP binding into a movement of the N-terminal domain. Our model provides a mechanistic explanation for how nucleotides regulate the p97-ataxin3 interaction and why atypical cofactor binding is observed with MSP1 mutants.Ph.D., Biochemistry -- Drexel University, 201
Creating Online Training Courses for the Corporate Environment: What Have Been the Experiences of Instructional Designers?
No full textOnline training is a mainstay of the corporate environment in the twenty-first century. However, creating online instruction presents different challenges than those for traditional classroom-based learning. Instructional design for the online environment is considered to be in its formative stage, which suggests that there is no set path, model, or formula for instructional designers to follow when creating online training. The study explores the experiences of instructional designers who have created online training for the corporate training environment, focusing specifically on the instructional designers' experiences regarding any processes or models they may have followed when creating online instruction. By exploring the experiences of these instructional designers who created online courses, future instructional designers may better understand the processes or models available to them as they create their own online instruction. Three questions were addressed for this research: (1) what were the experiences of the instructional designers regarding the processes or models they followed to create online training; (2) what criteria did instructional designers use when determining whether a course's content was suitable for the online training environment; and (3) what process or instructional design model did the instructional designers follow in order to create online courses, and if no specific instructional design model was used, what was the reason for not using one. This study was conducted utilizing a collective case study method. This study was designed to explore, compare, and understand the varied experiences of the study participants within the context of their profession; and attempted to differentiate the details of each participant's experience in order to better understand what an instructional designer considers when creating an online educational or training course. Research participants' responses were categorized into four major themes: (1) Models versus Experience, (2) Choosing a Familiar Instructional Design Model, (3) Instructional Design Occurs in Teams, and (4) Personal Best Practices. All of the study participants agreed that having instructional design models to follow early in their careers is helpful, but strict adherence to those models tends to fall away as experience takes over.Ed.D., Education -- Drexel University, 201
Combining Strategic and Operational Decision Making in Liquefied Natural Gas (LNG) Logistics
Get PDFRecent increase in energy prices, concern regarding CO2 emissions, exploration of new energy sources, and some conventional methods of Liquefied Natural Gas transportation have a significant impact on LNG trade to make it more competitive in the energy market. This results in a lot of investment for LNG value chain. For profitable operations such LNG logistics, it is necessary to find the optimal design in terms of the supply chain associated with it. Of special interests are finding an optimal schedule for LNG delivery by ships from production terminal to regas terminals and satisfying inventory and port constraints by minimizing total cost and selecting an optimal combination of contracts and suppliers. This can be possible by modelling a combination of the inventory routing problem (LNG-IRP) and the model to minimize procurement cost by selection of LNG contracts that varies in price formulation, duration, quality etc. These various cost factors in the objective makes the combined model more challenging. To find the lowest cost solution for the model, optimization-based approaches can be very useful. Therefore, in this paper, we address these circumstances by proposing a mixed-integer linear programming model that helps the buyers select the best combination of suppliers and contract, and based on selecting amount of contract, buyer’s demand (inventory capacity) in each regas terminal is satisfied by minimizing stock out, unmet demand, and losts production.M.S., Supply Chain Management and Logistics -- Drexel University, 201
Physical Properties of Surface Modified PVDF Fibers and PVDF Template Carbon-Carbon Shish Kebab for Nanocomposites
No full textBecause of its large range of sizes and its customizability, porous polymer materials have gained interest in many applications including energy storage , tissue engineering, and filtration, and environmental cleanup. One vast subfield of macroporous polymer materials is electrospun fibrous materials which are porous membranes made of a network of nano- to micron-sized fibers. Although there has been reports on using polymer blends to functionalize the fiber surface in order to create highly porous activated carbon fiber, there have been very few reports of activated carbon fibers with highly ordered hierarchical structures with the same control as with comparative polymeric hierarchically ordered microstructures such as the polymer Nano Fiber Shish Kebab (NFSK) structures. For the first time reported, PVDF-PVDF NFSKs were created and the crystalline structure was characterized. These PVDF-PVDF NFSKs were also used as a precursor for carbonization and the successful maintenance of the original NFSK morphology allowed for the creation of the first reported fully carbon NFSK. With future research into precise control of PVDF-PVDF NFSK dimension and this successful procedure of creating a fully carbon highly ordered hierarchical structured NFSK via carbonization can lead to creation of activated carbon materials with highly desirable, finely tuned, meso- and macroporous microstructure for multifunctionality.M.S., Materials Science and Engineering -- Drexel University, 201