Physics Doctoral Work

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Buckling Instabilities of Nanoscale Polymer Films and Colloidal Particle Layers
(North Dakota State University, 2015) Gurmessa, Bekele Jemama
Nanoscale polymer films have numerous potential applications such as protective coatings, flexible electronics, energy harvesting devices, and drug delivery systems. For realization of these potential applications, the mechanical properties of these materials and the underlying physics need to be understood. This dissertation focuses on understanding the responses of nanoscale films to mechanical deformations. In this regard, an elastic instability was exploited to locally bend and impart a local tensile stress in a nanoscale polystyrene film, and directly measure the resulting residual stress caused by the bending. Our results indicate that the onset of permanent deformation for thin polystyrene films is an order of magnitude smaller than what has been reported for the bulk value. In addition, not only is the onset of failure strain found to be small but also it increases with increased confinement. Using similar processing techniques, the yield strain of a more complex material - poly(styrene-b-divinylpyridine) - was studied. Similar to the polystyrene films, failure in polystyrene-b-poly(2-vinylpyridine) is also initiated at extremely low strain and is influenced by thin film confinement effects. In addition, we have demonstrated that internal nanostructure of self-assembled polystyreneb- poly(2-vinylpyridine) affects the onset of failure strain. Having introduced an idealized heterogeneity to a sample through ultraviolet/ozone treatment, we have created samples ranging from continuous thin films to sets of isolated plates. We demonstrated that, when subjected to mechanical deformation, the unbounded plates form isotropic undulations that persist even beyond high strain. In contrast, isolated plates undergo non-isotropic undulations in the range of high strains. The non-isotropic undulation shape has been described through a simple numerical modeling subjected to controlled boundary conditions. The agreement between experiment and numerical modeling is remarkable. Finally, through an integrated experimental methods and theoretical modeling, the response of discrete colloidal layers to mechanical deformations have been exploited. The buckling profiles measured experimentally demonstrate a great insight that the continuum model may not be able to predict the response of discrete systems. Theoretically, a granular model was constructed and structural stability analysis was investigated to predict the experimental observations. The overall agreement of the experiment and the modeling was good.
Electrostatic Interactions at Dielectric Interfaces: From Colloids to Membranes
(North Dakota State University, 2017) Volpe Bossa, Guilherme
In this thesis we have investigated electrostatic interactions at dielectric interfaces using theoretical models based on the non-linear Poisson-Boltzmann theory and its extensions. We have focused on three major topics: (1) modeling the energetics and interactions of charged nanoparticles trapped at the air-water interface; (2) calculation of the line tension between domains in charged lipid membranes, lipid-lipid correlations, and how membrane curvature is influenced by charged peptides; and (3) extensions of the classical Poisson-Boltzmann theory by accounting for the influence of ion-specific solvent-mediated interactions. More precisely, ion-specificity has been accounted for using the Poisson-Helmholtz-Boltzmann formalism, which adds to the bare Coulombic interactions a Yukawa-like potential that accounts for the interacting hydration shells of ions. Motivated by recent experimental and computational results, all projects present here aim to provide a deeper understanding of fundamental physical properties of charged dielectric interfaces.
Fundamental Studies of Interfacial Forces Acting on Thin Films
(North Dakota State University, 2021) Twohig, Timothy John
A thin film is a material that is many orders of magnitude thinner than it is long or wide. They are commonly found in many forms and have been adapted to a wide variety of uses. The art of origami uses thin films(sheets of paper) and precise folding to create complex, three-dimensional shapes out of flat, quasi two-dimensional sheets, and has emerged as a unique way to solve problems in engineering and science. As technology and devices are scaled to smaller sizes new understanding of origami methods are required to work at these small scales. The interactions between thin films and liquids, substrates that films exist on, and other thin films is the focus of this dissertation. Capillary interactions are used to manipulate and fold thin films that are too thin to be actuated with hands or everyday tools. The relation between the macroscopic and the microscopic interactions at the point where the capillary liquid and the film meet is explored. We show how films can be manipulated by capillary drops and how exactly the force is applied to the film. The adhesive interactions of the film were studied as a method of precisely placing folds for elastic film origami. The capillary peel of a film from a substrate drove folds to desired locations. Adhesion of a film to itself was used to lock these bends in place in lieu of the permanent creases commonly used in plastic systems such as paper. The combination of these two methods enabled the creation of stable, multi-step origami systems from reusable elastic films. This research culminates in the discussion of fundamentally new origami-like designs that rely only on adhesion of the film to itself, which we call kuttsukugami (sticky+paper from Japanese). This new form allows for the creation of shapes that are nearly impossible to create with traditional origami methods such as loops, tubes, and cones. Advances made in capillary and adhesive thin film studies allow for kuttsukugami shapes to be scaled down to microscopic sizes for a huge array of applications including drug delivery, thin electronics, encapsulation, and more.
The Nature of Single-Wall Carbon Nanotube-Silicon Heterojunction Solar Cells
(North Dakota State University, 2015) Harris, John Michael
Since their inception in 2007, nanotube-silicon heterojunction solar cells have experienced rapid improvement due to the diligent work of several research groups. These devices have quickly reached a point where they might begin to possibly compete with current well-established silicon solar technologies; however the development of industrial-scale nanotube synthesis and purification capabilities remains problematic. Although there has been significant recent progress in improving performance, the precise classification of nanotube-silicon heterojunctions has remained ambiguous. In this thesis, I use type, chirality and length purified single-wall carbon nanotubes to clarify the nature of this particular class of solar cell. The junctions that I assembled were made from freestanding nanotube sheets that showed remarkable stability in response to repeated crumpling and folding during fluid processing, which suggests that the films could be well suited to flexible device platforms. Despite modest ideality factors, the best diodes created in this study met or exceeded state-of-the-art device characteristics, but with a surprising lack of any significant dependence on sample type. The data further suggest that these devices might be simultaneously categorized as either Schottky or p-n junctions. More importantly, the results of this study demonstrate the manner in which band-gap engineering can optimize these devices while emphasizing the important role of the junction morphology.
Osmotic Swelling Behavior of Ionic Microgels
(North Dakota State University, 2020) Alziyadi, Mohammed Obaid
This dissertation studies the thermodynamic and structural properties of aqueous dispersions of ionic microgels – soft colloidal particles composed of cross-linked polymer gels that swell in a good solvent. Starting from a coarse-grained model of microgel particles, we perform computer simulations and theoretical calculations using two complementary implementations of Poisson- Boltzmann (PB) theory. Within the framework of a cell model, the nonlinear PB equation is exactly solved and used to compute counterion distributions and osmotic pressures. By varying the free energy with respect to microgel size, we obtain exact statistical mechanical relations for the electrostatic component of the single-particle osmotic pressure. Explicit results are presented for equilibrium swelling ratios of charged microcapsules and of charged cylindrical and spherical microgels with fixed charge uniformly distributed over the surface or volume of the particle. Molecular dynamics simulations validate the theoretical findings. In the second method, within a one-component model framework, based on a linear-response approximation for effective electro- static interactions, we develop Monte Carlo (MC) simulations to compute the equilibrium swelling ratio, bulk osmotic pressure, radial distribution function, and static structure factor. Results presented in this dissertation demonstrate that swelling of ionic microgels increases with increasing microgel charge and decreases with increasing concentration of salt and microgels. In addition, results demonstrate that the microion distributions and osmotic pressure determine equilibrium swelling of microgels. Cell model predictions for bulk osmotic pressure agree well with data from MC simulations of the one-component model. The MC simulations also provide access to structural properties and to swelling behavior of microgels in highly concentrated suspensions. Taken together, results obtained in this work provide insight into factors of importance for practical use of microgels as drug delivery systems, in tissue engineering, and for other biomedical applications.
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.
Single-Molecule Studies of Intermolecular Kinetics Using Nano-Electronics Circuits
(North Dakota State University, 2020) Froberg, James Steven
As science and medicine advance, it becomes ever more important to be able to control and analyze smaller and smaller bioparticles all the way down to single molecules. In this dissertation several studies aimed at improving our ability to manipulate and monitor single biomolecules will be discussed. First, we will discuss a study on developing a way to map dielectrophoresis with nanoscale resolution using a novel atomic force microscopy technique. Dielectrophoresis can be applied on nanoparticles through micron-scale electrodes to separate and control said particles. Therefore, this new method of mapping this force will greatly improve our ability to manipulate single biomolecules through dielectrophoresis. The next two studies discussed will be aimed at using carbon nanotube nanocircuits to monitor single protein kinetics in real time. Drug development and delivery methods rely on the precise understanding of protein interactions, thus creating the need for information on single protein dynamics that our techniques provides. The proteins studied in these sections are MMP1 and HDAC8, both of which are known targets of anti-cancer drugs. Finally, we developed a new strategy for diagnosing pancreatic cancer. Our strategy involves using graphene nanotransistors to detect exosomes released from the pancreatic tumor. The ability to reliably diagnose pancreatic cancer before it reaches metastasis would greatly improve the life expectancy of patients who develop this condition. We were able to test our technique on samples from a number of patients and were successfully able to distinguish patients with pancreatic cancer from noncancerous patients.
The Thiol-ene Encapsulation and Photo-physical Characterization of Colloidal Silicon Nanocrystals Synthesized with Si6H12 Using Non-thermal Plasma Reactor
(North Dakota State University, 2021) Sefannaser, Mahmud Ayad
Silicon nanocrystals (SiNCs) are nanometer-sized semiconducting materials. Their small size endows them with unique photophysical properties. Efficient photoluminescence (PL) from silicon nanocrystal (SiNC) composites has important implications for emerging solar-energy collection technologies, yet a detailed understanding of PL relaxation in non-colloidal SiNCs is still materializing. In this dissertation, we examine the photophysical properties of silicon nanocrystal/off-stoichiometry thiol-ene composites (SiNCs/OSTE hybrids). The dissertation begins with an introduction to the photophysical properties of SiNCs, their photophysical properties, how SiNC/polymer composites are made, the various SiNC preparation techniques, and the most likely application areas for these nanocrystals. A description of experimental methods such as PL spectroscopy and transmission electron microscopy (TEM) follows, and SiNC/OSTE polymer preparation methods are then explained in detail. In the first study, TEM and photophysical characterization were performed on selected polydisperse SiNCs samples. These samples were synthesized in a nonthermal plasma reactor, using Si6H12 as precursor, and functionalized with R (where R is 1-dodecene). These SiNCs were dispersed in mesitylene:1-dodecene (5:1) as a colloid. Optical absorption, quantum efficiency, and PL lifetime of SiNCs were then investigated, as well as the relationship between quantum yield, lifetime, and PL peak. In the second study, we selected samples for size separation via the density gradient ultracentrifugation method (DGU). We successfully applied this technique to separate silicon nanocrystals with sizes from 2 nm to 4 nm from the ensemble samples using an engineered density medium layer stack, and photophysical characterization was performed on the DGU size–separated SiNCs. Lastly, we explored details of PL relaxation in photo-polymerized off-stoichiometric polymer/nanocrystal hybrids. We found time- and air-stable emission from dilute composites with up to 70% QY, and we investigated PL relaxation in the parameter space of nanocrystal size and temperature. In light of previous work, our results reveal similarities between the impacts of crosslinking and cooling to cryogenic temperature, but of which are characterized by a relative reduction in the available of phonons.

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