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Hot Electron Effect in Ultrathin Photovoltaic Junctions
(North Dakota State University, 2012) Mihaylov, Deyan Ivov
The focus of the research work described in the following thesis is increasing the efficiency of photovoltaic devices by reducing hot carrier thermalization losses. In principle this can be achieved by reducing the size of the absorber down to lengths comparable to the thermalization length for hot carriers. With the use of ultrathin absorbers hot carrier can be collected before they have reached thermal equilibrium with the lattice. The theoretical work on the subject is comprised of improving the empirical relationship developed in the most recent publication on the topic by. By making the assumption that the energy loss rate fits the exponential decay model, an expression for the energy as a function of absorber thickness was developed. The experimental work consist of fabricating devices with different absorber thicknesses and testing their ability to show change in performance due to collection of hot electrons.
A Novel Macroscopic Technique to Measure the Nanomechanics of Durable Multifunctional Nanosheets
(North Dakota State University, 2017) Taufique, Abu Md Niamul
In the recent advancement of nanotechnology, carbon nanotubes (CNTs) have shown promise and potential for a wide variety of applications due to their excellent mechanical, electrical, and optical properties. A network of single wall carbon nanotubes (SWCNTs) is of interest due to its potential applications in flexible electronics, composites, constructional materials, and so on. Characterizing the mechanical properties of these thin films will be critical to achieving a full understanding of their behavior, yet relatively few experimental methods exist for querying the deformation mechanics of these films. To provide additional insight into the large-deformation mechanics of these films, we propose a novel method for evaluating the mechanical properties of thin SWCNT films. We provide theoretical background, describe the experimental approach, and use a MATLAB based analysis to extract the film modulus. Finally, we compare our results to existing wrinkling-based measurements, where we find reasonable agreement between the two techniques.
Free Energy Minimization and Multicomponent, Multi-Phase Lattice Boltzmann Simulations of Van Der Waals Fluid Mixtures
(North Dakota State University, 2018) Ridl, Kent Stephen
In this thesis, we develop a general framework for the lattice Boltzmann method to simulate multiphase systems with an arbitrary number of components. Theoretical expectations are easily visualized for binary mixtures, so we focus on characterizing the performance of the method by numerically minimizing the free energy of a binary van der Waals mixture to generate phase diagrams. Our phase diagrams contain very intriguing features that are not well-known in today’s physics community but were understood by van der Waals and his colleagues at the turn of the 20th century. Phase diagrams and lattice Boltzmann simulation results are presented in a density-density plane, which best matches with LB simulations performed at constant volume and temperature. We also demonstrate that the algorithm provides thermodynamically consistent results for mixtures with larger numbers of components and high density ratios. All of the theoretical phase diagrams are recovered well by our lattice Boltzmann method.
Fluctuations in the Lattice Boltzmann Method
(North Dakota State University, 2012) Kaehler, Goetz August
The implementation of fluctuations in the lattice Boltzmann method has made significant progress in the last 10 years. The significance of incorporating noise to all non-conserved degrees of freedom was a significant recent discovery that was based on a simplified Langevin treatment of the linarized Boltzmann equation. However, for non-vanishing mean velocities significant deviations in the correlation functions were observed. In this thesis we show how we can largely alleviate these deviations by incorporating fully velocity dependent moment transforms and thus recover a fluctuation dissipation theorem that is valid for a larger range of velocities. Furthermore we show that the remaining deviations can be attributed to the collision operator of the linearized Boltzmann equation not being identical to the one of the BGK collision which forms the basis of most modern lattice Boltzmann applications. Finally we show that the locally velocity dependent transforms significantly improve the stability of fluctuating lattice Boltzmann simulations at low particle densities.
Computational Modeling of Polymer Crowding: Influence of Solvent Quality and Dimensionality on Conformations
(North Dakota State University, 2018) Davis, Wyatt Julian
The structure and function of polymers in confined environments, e.g., biopolymers in the cytoplasm, are affected by macromolecular crowding. To explore the influence of solvent quality and dimensionality on conformations of crowded polymers, polymers are modeled as penetrable ellipsoids/ellipses, whose shapes are governed by the statistics of random walks. Within this coarse-grained model, Monte Carlo simulations of two and three-dimensional polymer-nanoparticle mixtures, including trial changes in polymer size and shape, are performed. Penetration of polymers by nanoparticles is incorporated via a free energy cost predicted by polymer field theory. Simulation results of polymer conformation are compared with predictions of free-volume/area theory for polymers in good and theta solvents. Results indicate that dimensionality and solvent quality significantly affect crowded conformation, especially in the limit of small crowders. This approach may help to motivate future experimental studies of polymers in crowded environments, with relevance for drug delivery.
Properties of Reinfoced Carbon Nanotube and Laser-Crystallized Silicon Films
(North Dakota State University, 2016) Semler, Matthew Roy
Flexible electronics are anticipated to be one of the next technological advancements of electronic devices. The enhanced durability, light-weight nature, and conformity of flexible electronics are desired properties in a variety of fields and are anticipated to reduce production costs. Two promising materials for use in flexible electronics are carbon nanotube (CNT) films and laser-crystallized thin silicon films. CNTs are in their infancy in respect to their presence in electronic devices; however their superb mechanical and electronic properties make them ideal candidates for flexible electronics. Thin silicon films are a natural transition from bulk silicon as bulk silicon has been the preferred material in electronics since the dawn of the transistor. Thin-film silicon retains the well-studied electronic properties of bulk silicon; however, it becomes flexible as it is thinned. Obstacles to the application of both these materials in flexible electronics nonetheless exist. Compressed CNT films undergo strain softening – a mechanism in which the CNT film restructures itself in response to an applied strain, which reduces the Young’s modulus and electronic conductivity. In this dissertation, thin CNT films are capped with a thin polymer layer, with the aim to mitigate strain softening through excluded volume interactions in a bilayer format that serves as a paradigm for more sophisticated device relevant settings. More specifically, metallic and semiconducting CNT films of different thicknesses are capped with a polystyrene film of comparable thickness, and the mechanical and electronic strain response of the capped CNT film is examined and discussed. Ultrathin silicon films cannot be grown as monocrystalline silicon, so amorphous silicon films must be deposited and crystallized. Laser crystallization is an alternative to oven annealing and has a faster throughput. In this dissertation, amorphous silicon films of various thicknesses were deposited on several substrates via plasma enhanced chemical vapor deposition. The films were crystallized with a pulsed Nd:YVO4 laser operating at the third harmonic of 355 nm, and the structural and electronic properties were characterized to determine the effects of film thickness and substrate composition.
Thermodynamic and Kinetic Modeling of Mixed Lipid Membranes and their Interaction with Macromolecules
(North Dakota State University, 2015) Loew, Stephan
Mixed lipid membranes play a crucial role in numerous cellular processes and pharmaceutical applications. Fully understanding the interactions between membranes and biomacromolecules is not possible without gaining insight into underlying physical concepts. In this thesis we develop theoretical models that aim to rationalize a number of experimental findings, all involving lipid layers and their interaction with macromolecules. Our models are phenomenological and employ a minimal set of order parameters, thus focusing on essential physical interactions. We address four major subjects: First, certain mixed model membranes containing cholesterol are able to undergo macroscopic phase separation. Based on a previously suggested thermodynamic model we demonstrate that peripherally adsorbed membrane proteins tend to further facilitate phase separation, especially when they exhibit attractive interactions. Second, we show that the coupling between the two leaflets of a mixed lipid bilayer can influence its phase behavior. To this end, we calculate detailed phase diagrams and argue that their predictions are in principal agreement with experimental observations. Specifically, the coupling can trigger or inhibit phase separation, depending on lipid compositions in each leaflet and coupling strength. Third, we investigate the fundamental question if physiological pH-changes are sufficient -- and can this be employed by cellular processes -- to trigger the adsorption of peripheral proteins. Proposing a model for the previously suggested electrostatic-hydrogen bond switch mechanism, we show that protein adsorption based on electrostatic interactions alone has a weak pH dependence but is rendered pH sensitive by the electrostatic-hydrogen bond switch. Finally, the transfer of hydrophobic drug molecules in model systems from donor liposomes to a target carrier is known from experimental work to typically exhibit a first-order kinetics, sometimes also sigmoidal behavior. We develop a detailed kinetic model for drug transfer that is based on a statistical description of drug occupation numbers in liposomes and includes both drug diffusion and liposome collision mechanisms.
Applying theoretical frameworks from cognitive psychology to assess faculty professional development and student reasoning in physics
(North Dakota State University, 2024) McInerny, Alistair
Understanding human behavior and reasoning is essential for developing successful instruction. Discipline-based education researchers have examined how students learn, informing the development of successful instructional strategies. Researchers have also identified barriers to the successful implementation of such strategies. This work utilizes two theoretical frameworks from psychology to further examine: 1) efforts to enact instructional change and 2) the effectiveness of instructional approaches to improve students' reasoning in physics. The Theory of Planned Behavior (TPB) is used to assess professional development supporting the successful implementation of evidence-based instructional strategies. The Dual Process Theories of Reasoning and Decision-making (DPToR) are used to model human reasoning and explain persistent inconsistencies in student responses. Guided by the TPB, an assessment instrument was created, validated, and implemented to evaluate instructor’s beliefs and intentions about active-learning methodologies. A semi-novel research methodology was also applied to address response-shift bias, a phenomenon common in professional development self-reported assessments. The validation of the instrument and the utility of the retrospective pretest methodology are reported, together with initial assessment results, demonstrating the value of both the TPB and the retrospective pretest in the context of professional development. The second half of this work discusses inconsistent student reasoning, where students correctly apply conceptual understanding in one context but fail to do so in similar situations. This phenomenon is examined using the Dual Process Theories of Reasoning, which describes reasoning in terms of two processes: a fast, automatic process 1 and a slow, resource-intensive process 2. Process 1 is quick but frequently inaccurate. Process 2 is analytical but time-consuming and effortful. Four reasoning hazards are identified and examined through the lens of DPToR. Three different types of interventions are implemented to help students develop skills to navigate reasoning hazards: 1) Collaborative exams are used to trigger socially-mediated-metacognition in a high-stakes environment, modeling process 2 activation through group reasoning, 2) a multi-stage guided individual intervention followed by a classroom discussion, and 3) explicit discussion of human reasoning modeled by DPToR. The impacts of these interventions are assessed by comparing results from the treatment (intervention) and controlled (alternative intervention) groups.
Synthesis of Small Silicon Carbide Nanocrystals in Low Pressure Nonthermal Plasma
(North Dakota State University, 2021) Petersen, Reed Jeffrey
Nanoparticles have attracted much attention because of their unusual physical properties. This work represents original, incipient research into small crystalline silicon carbide nanoparticles synthesized in a low-pressure nonthermal plasma reactor. The nonthermal plasma technique offers a route for size-tunable synthesis of high-purity silicon carbide nanocrystals. Even though it has a high sublimation point, silicon carbide is synthesized in crystalline form in a nonthermal plasma reactor since nanoparticles are intensely heated by exothermic surface reactions on a nanoscale level. Using vaporized tetramethylsilane as a precursor and molecular hydrogen as an additive, both silicon carbide and silicon- or carbon-coated silicon carbide were created. Since plasma synthesis is a ligand-free process and charges on particles prevent agglomeration, silicon carbide is soluble in short-chain alcohols.
Guiding Self-Assembly of Functionalized Nanoparticles by Computational Modeling of Effective Interactions
(North Dakota State University, 2018) Shah, Vijay
Nanoparticles have attracted much attention because of their unusual physical properties, which allow them to be used in many practical applications. The self-assembly of nanocrystals into crystalline arrays can be facilitated by functionalizing the nanocrystals with ligand brushes, allowing for bulk dispersions to be sterically stabilized against aggregation. Studies have been conducted to study the clustering of gold nanoparticle dispersions. To study the self-assembly of gold nanoparticle dispersions based on nanocrystal volume fraction and ligand coverage, we performed Monte Carlo simulations and characterized the ability of the nanoparticle dispersions to self-assemble into crystalline arrays. Experiments have shown that silver nanoparticles can self- assemble into equilibrium superlattices in the presence of free ligands. To better understand the role of adsorbed and free ligands in self-assembly, we extracted the effective pressure between two flat, ligated plates through molecular dynamics simulations. Our results are compared to the theoretical prediction and discrepancies are discussed.

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