Physics Masters Theses

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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.
A Density Functional Theory and Many Body Perturbation Theory Based Study of Photo-Excited Charge Separation in Doped Silicon Nanowires with Gold Leads: Toy Models for the Photovoltaic Effect
(North Dakota State University, 2020) Walker, Nathan Thomas
We analyze a toy model for p-n junction photovoltaic devices by simulating photoexcited state dynamics in silicon nanowires. One nanowire is approximately circular in cross section with a diameter of d = 1.17 nm. The other has an approximately rhombic cross-section with d1 = 1.16 nm and d2 = 1.71 nm. Both nanowires have been doped with aluminum and phosphorus atoms and capped with gold leads. We use Boltzmann transport equation (BE) that includes phonon emission, carrier multiplication (CM), and exciton transfer. BE rates are computed using non-equilibrium finite-temperature many-body perturbation theory (MBPT) based on Density Functional Theory (DFT) simulations, including excitonic effects from Bethe-Salpeter Equation. We compute total charge transfer amount generated from the initial photoexcitation and find an enhancement when CM is included. In particular, we see between 78% and 79% enhancement in the smaller wire, while we see 116% enhancement in the larger nanowire
Excited State Dynamics in 1D Thermoelectric Materials
(North Dakota State University, 2020) Gima, Kevin Victor
Here, nonadiabatic computations are used to study the thermoelectric effect and evaluate electron relaxation rates in lead telluride nanowires. κ_e = 1/τ_el is defined as the electron relaxation rate. It is directly connected to the thermoelectric figure of merit in a material. This work provides computational evidence in support of the proceeding hypothesis. The hypothesis is the electron relaxation rates will comply with the following band gap law: Ke = Aexp(-αΔE), where Ke is the electronic relaxation rate, A and α are constants, and ΔE is the energy difference between the initial and final states. This work reports results on PbTe (lead telluride) atomistic models doped with sodium and iodine that contain approximately 300 atoms in simulation cells with periodic boundary conditions.
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.
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.
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.
Inspection of Excited State Properties in Defected Carbon Nanotubes from Multiple Exciton Generation to Defect-Defect Interactions
(North Dakota State University, 2020) Weight, Braden Michael
Covalent SP3-hybridization defects in single-walled carbon nanotubes (CNTs) have been prevalent in recent experimental and theoretical studies for their interesting photophysical properties. These systems are able to act as excellent sources of single, infrared photons, even at room temperature, making them marketable for applications to sensing, telecommunications, and quantum information. This work was motivated by recent experimental studies on controllable defect placement and concentration as well as investigating carrier multiplication (CM) using DFT-based many-body perturbation theory (MBPT) methods to describe excitonic relaxation processes. We find that pristine CNTs do not yield appreciable MEG at the minimum threshold of twice the optical gap 2Eg, but covalent functionalization allows for improved MEG at the threshold. Finally, we see that defect-defect interactions within CNT systems can be modeled simply as HJ-aggregates in an effective Hamiltonian model, which is shown to be valid for certain, highly-redshifted defect configurations at low defect-defect separation lengths.
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.
Single-Wall Carbon Nanotube Thin Films: Processing Measurement and Methodology
(North Dakota State University, 2016) Waters, Alex John Brown
As society advances to desire smaller, faster, and cheaper electronics along with stronger materials, the search for novel materials continues. Thin films made from single wall carbon nanotubes (SWCNTs) offer a possible solution to many of the challenges that materials scientists currently face. Here, a methodology for studying the deformation of thin SWCNT films is investigated during their processing for use in applications such as photovoltaic devices. A variety of methods for manipulating and visualizing these thin films are discussed, along with the setbacks encountered along the path to improving process characterization.
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.

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