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Browsing by Author "Buchanan, Kristen, committee member"

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    Bayesian approach to the anisotropic EIT problem and effect of structural changes on reconstruction algorithm using 2-D D-bar algorithm
    (Colorado State University. Libraries, 2018) Murthy, Rashmi, author; Mueller, Jennifer L., advisor; Cheney, Margaret, committee member; Pinaud, Oliver, committee member; Buchanan, Kristen, committee member
    Electrical Impedance Tomography (EIT) is a relatively new imaging technique that is non-invasive, low-cost, and non-ionizing with excellent temporal resolution.In EIT, the unknown electrical conductivity in the interior of the medium is determined from the boundary electrical measurements. In this work, we attempt to find a direct reconstruction algorithm to the anisotropic EIT problem based on the well-known Calderón's method. The non-uniqueness of the inverse problem is dealt with assuming that the directions of anisotropy are known. We utilize the quasi-conformal map in the plane to accomplish Calderóns approach. Additionally, we derive a probability distribution for the anisotropic conductivity values using a Bayesian formulation, where the direction of anisotropy is encoded as the prior information. We show that this results in the generalized Tikhonov regularization, where the prior information about the direction of anisotropy is incorporated in the regularization operator. The computations of the anisotropic EIT problem using the Bayesian formulation is conducted on simulated data and the resulting reconstructions for the data are shown. Finally, the work of this thesis is concluded by implementing dynamic changes in boundary of a human data during respiration process successfully in the D-bar algorithm.
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    Calibration of the Pierre Auger Observatory fluorescence detectors and the effect on measurements
    (Colorado State University. Libraries, 2015) Gookin, Ben, author; Harton, John, advisor; Toki, Walter, committee member; Buchanan, Kristen, committee member; Menoni, Carmen, committee member
    The Pierre Auger Observatory is a high-energy cosmic ray observatory located in Malargue, Mendoza, Argentina. It is used to probe the highest energy particles in the Universe, with energies greater than 10¹⁸ eV, which strike the Earth constantly. The observatory uses two techniques to observe the air shower initiated by a cosmic ray: a surface detector composed of an array of more than 1600 water Cherenkov tanks covering 3000 km², and 27 nitrogen fluorescence telescopes overlooking this array. The Cherenkov detectors run all the time and therefore have high statistics on the air showers. The fluorescence detectors run only on clear moonless nights, but observe the longitudinal development of the air shower and make a calorimetric measure of its energy. The energy measurement from the the fluorescence detectors is used to cross calibrate the surface detectors, and makes the measurements made by the Auger Observatory surface detector highly model-independent. The calibration of the fluorescence detectors is then of the utmost importance to the measurements of the Observatory. Described here are the methods of the absolute and multi-wavelength calibration of the fluorescence detectors, and improvements in each leading to a reduction in calibration uncertainties to 4% and 3.5%, respectively. Also presented here are the effects of introducing a new, and more detailed, multi-wavelength calibration on the fluorescence detector energy estimation and the depth of the air shower maximum measurement, leading to a change of 1±0.03% in the absolute energy scale at 10¹⁸ eV, and a negligible change in the measurement on shower maximum.
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    Dark Matter annihilation cross-section limits of dwarf spheroidal galaxies with the high altitude water Cherenkov (HAWC) gamma-ray observatory and on the design of a water Cherenkov detector prototype
    (Colorado State University. Libraries, 2016) Proper, Megan Longo, author; Harton, John, advisor; Mostafa, Miguel, advisor; Buchanan, Kristen, committee member; Roberts, Jacob, committee member; Marconi, Mario, committee member
    I present an indirect search for Dark Matter using the High Altitude Water Cherenkov (HAWC) gamma-ray observatory. There is significant evidence for dark matter within the known Universe, and we can set constraints on the dark matter annihilation cross-section using dark matter rich sources. Dwarf spheroidal galaxies (dSphs) are low luminosity galaxies with little to no gas or dust, or recent star formation. In addition, the total mass of a dwarf spheroidal galaxy, as inferred from gravitational effects observed within the galaxy, is many times more than the luminous mass, making them extremely dark matter rich. For these reasons dSphs are prime targets for indirect dark matter searches with gamma rays. Dark matter annihilation cross-section limits are presented for 14 dSphs within the HAWC field of view, as well as a combined limit with all sources. The limits presented here are for dark matter masses ranging from 0.5 TeV to 1000 TeV. At lower dark matter masses, the HAWC-111 limits are not competitive with other gamma-ray experiments, however it will be shown that HAWC is currently dominating in the higher dark matter mass range. The HAWC observatory is a water Cherenkov detector and consists of 300 Water Cherenkov Detectors (WCDs). The detector is located at 4100 m above sea level in the Sierra Negra region of Mexico at latitude 18deg 59'41" N and longitude 97deg 18'28" W. Each WCD is instrumented with three 8 inch photomultiplier tubes (PMTs) and one 10 inch high efficiency PMT, anchored to the bottom of a 5 m deep by 7.3 m diameter steel tank. The tank contains a multilayer hermetic plastic bag, called a bladder, which holds 200,000 L of ultra-purified water. I will also present the design, deployment, and operation of a WCD prototype for HAWC built at Colorado State University (CSU). The CSU WCD was the only full-size prototype outside of the HAWC site. It was instrumented with 7 HAWC PMTs and scintillator paddles both under and above the volume of water. In addition, the CSU WCD was equipped with the same laser calibration system that is deployed at the HAWC site, as well as the same electronics and data acquisition system. The WCD prototype served as a testbed for the different subsystems of the HAWC observatory. During the three different installations of the prototype, many aspects of the detector design and performance were tested including: tank construction, bladder installation and performance, PMT installation and performance, roof design, water filtration and filling, muon coincidence measurements and calibration system. The experience gained from the CSU prototype was invaluable to the overall design and installation of the HAWC detector.
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    Developing and investigating copper and iron chalcogenide nanoparticle syntheses to elucidate the underlying processes of formation
    (Colorado State University. Libraries, 2021) Moloney, Lily June, author; Prieto, Amy L., advisor; Sambur, Justin, committee member; Krummel, Amber, committee member; Buchanan, Kristen, committee member
    Nanoparticle technology is rapidly growing field due to the unique, tunable properties of nanoparticles (NPs) as compared to their bulk counterparts, enabling a wide range of new applications. The key step, however, to applying NP systems to new areas is developing high quality syntheses. Specifically, solution-based methods for synthesizing NPs offer many synthetic handles for tuning structure/property relationships by controlling composition, size, morphology and capping agents. To effectively design syntheses to control the resulting properties, in depth investigation of the underlying fundamental processes are required. Elucidation of these processes would allow trends to be illuminated and a toolkit of synthesis methods could be built. However, a myriad of interactions (organic, inorganic, solid-state) occur during these reactions, and often these interactions are intertwined with each other. Therefore, careful examination of nanoparticle synthesis for various systems are necessary to advance the NP field. In Chapter 1, we review literature reports that exemplify the careful examination of underlying mechanisms and pathways required for developing synthesis methods that allow for control over composition, size, morphology. This chapter is split into two major sections; 1) balancing precursor reactivities and elucidating mechanisms and 2) understanding reaction pathways. The first section is split into anion and cation speciation/reactivity. The anion section focuses on the development of chalcogenide reactivity trends and how these trends have led to advanced nanoparticle control. The cation section considers the use of metal-amide complexes to increase and balance reactivities of cations to produce small, phase pure NPs. In depth mechanism studies of these metal-amide reactions in model unary and binary system led to the utilization of this reagent in more complex ternary systems. The second section focuses on investigation of reaction pathways, discussing how exploration of the reaction phase space as well as determination of intermediates is important for achieving a full picture. We end with a brief discussion on the importance of thorough characterization for describing these reaction mechanisms and pathways accurately. The importance of investigating the reaction pathway was inspiration for the research described in Chapter 2. Few solution-based techniques to synthesize Cu3Si, a material with applications in electronics, batteries, and photovoltaics, exist. This could stem from the limited number of Si precursors viable in solution-based techniques. This led us to explore the reaction between Mg2Si and CuCl2 in oleylamine, as a solution-based metathesis route to form Cu3Si particles. The reaction pathway and the role of the solvent were characterized and elucidated. It was found that the reaction proceeds through a two-step pathway, where a Si matrix, Cu particles, and MgCl2 initially form. Then, the Cu particles diffuse into the Si matrix to form Cu3Si particles encased in a Si matrix (Cu3Si@Si matrix). Additionally, various solvents are tested to understand the importance of the solvent for the reaction to proceed successfully. The coordinating ability of the solvent was important, where an overly non-coordinating or coordinating solvent limited the production of Cu3Si@Si matrix particles. Oleylamine was found to be a “goldy-locks” solvent as the coordination supported both steps of the reaction. In Chapter 3, the mechanistic metal-silylamide studies described in Chapter 1 offered inspiration. Specifically, Rebecca C. Miller in our group was recently able to synthesize Fe2GeS4 by utilizing lithium bis(trimethylsilyl)amide (LiHMDS). The increased understanding of the Fe-Ge-S phase space and the role of the LiHMDS gained from this previous study led to the research presented in Chapter 3. In this chapter, we report the first nanoparticle synthesis of Fe2GeSe4 and Fe2GeS4-xSex (x = 0.8 and 1) via a LiHMDS-assisted hot-injection method. This system allowed the role of LiHMDS in balancing not only the cationic but also anionic species to be investigated. This was done by exploring the synthesis parameters, the pre-injection chalcogen speciation, and the reaction pathways. Finally, in Chapter 4, synthesis methods attempted toward Cu2SiSe3 formation are described and a future direction for this project is proposed. Copper-based chalcogenides have gained much attention due to their exemplary intrinsic and structural properties. Cu2SiSe3 has been theorized to be a potential photovoltaic material, yet a NP synthesis of this material has not been realized. Exploration of hot injection, metathesis, and solvothermal solution phase methods are reported. However, all efforts resulted in binary Cu/Se phases over the formation of the desired ternary. A future direction for this project could be instead focusing on investigating reactivity trends of the group IV elements, Si, Ge, and Sn, using an amide-assisted synthesis of the Cu2IVSe3 materials.
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    Discovery and properties of hybrid materials for potential applications in quantum information science
    (Colorado State University. Libraries, 2022) Lundgren, Crystal J., author; Neilson, James R, advisor; Prieto, Amy, committee member; Buchanan, Kristen, committee member
    Hybrid halide perovskites and their derivatives are sought after for their unique optoelectronic properties, ease of preparation, and highly tunable structure. Some conjugated π-system containing hybrid halide semiconductors derived from hybrid perovskites show a unique primary electronic transition from the inorganic layer (halide) states to the organic layer (π∗) states. This type of charge-transfer semiconductor demonstrates a quantum two-level system between these frontier orbitals, suggesting that these materials may be useful as qubits in quantum computation. For a material to be suitable for a qubit, it must contain a quantum two-level system that can be char- acterized via optically adressable emission. Here, a new family of hybrid halide semiconductors containing 4-amino-1,2,4-triazole (4AMTZ) are discovered. Chapter 2 discusses the synthesis and characterization of 4AMTZBiI4. The crystal structure of 4AMTZBiI4 is solved and con rmed with powder X-ray diffraction. Photoluminescence studies reveal that there is no optically addressable emission from this system, and the iodide congener is thus not usable as a qubit. Chapter 3 discusses the synthesis and photoluminescence emission spectra of 4AMTZBiBr4 and 4AMTZBiCl4. These studies reveal emission from both the chloride and bromide congeners at T = 77 K that is likely due to the primary charge transfer between the halide and organic states based on the blue shifting of 4AMTZBiBr4 (475 nm) relative to that of 4AMTZBiCl4 (415 nm). Another region of emission observed in both 4AMTZBiBr4 and 4AMTZBiCl4 is centered at 660 nm. This region of emission is not shifted between the halide congeners, suggesting the presence of an emissive self- trapped exciton localized on the inorganic lattice. Though these materials emit at T=77K, there is no optically addressable emission at room temperature.
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    Enhanced surface functionality via plasma modification and plasma deposition techniques to create more biologically relevant materials
    (Colorado State University. Libraries, 2013) Shearer, Jeffrey C., author; Fisher, Ellen R., advisor; Henry, Charles, committee member; Szamel, Grzegorz, committee member; Bailey, Travis, committee member; Buchanan, Kristen, committee member
    Functionalizing nanoparticles and other unusually shaped substrates to create more biologically relevant materials has become central to a wide range of research programs. One of the primary challenges in this field is creating highly functionalized surfaces without modifying the underlying bulk material. Traditional wet chemistry techniques utilize thin film depositions to functionalize nanomaterials with oxygen and nitrogen containing functional groups, such as -OH and -NHx. These functional groups can serve to create surfaces that are amenable to cell adhesion or can act as reactive groups for further attachment of larger structures, such as macromolecules or antiviral agents. Additional layers, such as SiO2, are often added between the nanomaterial and the functionalized coating to act as a barrier films, adhesion layers, and to increase overall hydrophilicity. However, some wet chemistry techniques can damage the bulk material during processing. This dissertation examines the use of plasma processing as an alternative method for producing these highly functionalized surfaces on nanoparticles and polymeric scaffolds through the use of plasma modification and plasma enhanced chemical vapor deposition techniques. Specifically, this dissertation will focus on (1) plasma deposition of SiO2 barrier films on nanoparticle substrates; (2) surface functionalization of amine and alcohol groups through (a) plasma co-polymerization and (b) plasma modification; and (3) the design and construction of plasma hardware to facilitate plasma processing of nanoparticles and polymeric scaffolds. The body of work presented herein first examines the fabrication of composite nanoparticles by plasma processing. SiOxCy and hexylamine films were coated onto TiO2 nanoparticles to demonstrate enhanced water dispersion properties. Continuous wave and pulsed allyl alcohol plasmas were used to produce highly functionalized Fe2O3 supported nanoparticles. Specifically, film composition was correlated to gas-phase excited state species and the pulsing duty cycle to better understand the mechanisms of allyl alcohol deposition in our plasma systems. While these studies specifically examined supported nanoparticle substrates, some applications might require the complete functionalization of the entire nanoparticle surface. To overcome this challenge, a rotating drum plasma reactor was designed as a method for functionalizing the surface of individual Fe2O3 nanoparticles. Specifically, data show how the rotating motion of the reactor is beneficial for increasing the alcohol surface functionality of the nanoparticles when treated with pulsed allyl alcohol plasmas. Plasma copolymerization was used to deposit films rich in both oxygen and nitrogen containing functional groups using allyl alcohol and allyl amine plasma systems. Functional group retention and surface wettability was maximized under pulsed plasma conditions, and films produced under pulsed plasma conditions did not exhibit hydrophobic recovery or experience loss of nitrogen as the films aged. Plasma surface modification with N2/H2O and NH3/H2O, and plasma deposition with allyl alcohol and allyl amine, were used to increase the wettability of poly(caprolactone) scaffolds while simultaneously implanting functional groups onto the scaffold surface and into the scaffold core. While plasma deposition methods did not modify the internal core of the scaffold as much as modification methods, it afforded the ability to have higher water absorption rates after a three week aging period. Additionally, cell viability studies were conducted with N2/H2O plasma treated scaffolds and showed enhanced cell growth on plasma treated scaffolds over non plasma-treated scaffolds.
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    Exotic phenomena in rare-earth based geometrically frustrated magnets
    (Colorado State University. Libraries, 2022) Yahne, Danielle Rose, author; Ross, Kate A., advisor; Bradley, Mark, committee member; Buchanan, Kristen, committee member; Zadrozny, Joe, committee member
    Rare-earth (RE) based frustrated magnets are ideal systems to explore quantum effects in materials, which are paramount for the development of quantum computers, MRAM, and other next-generation technology. RE based materials are of specific interest due to the strong spin-orbit coupling and crystal electric field effects, which split the degenerate 4f angular momentum states, often leading to an effective spin-1/2 doublet with anisotropic effective exchange models. For this reason, RE materials are paramount to investigating the effects of anisotropic exchange on exotic ground states or quantum phases. Exchange frustration refers to when a system cannot simultaneously satisfy competing interactions, which can lead to a macroscopic degeneracy in the ground state of the system. Materials with geometric frustration, where competing interactions occur due to the crystal geometry alone, have been shown to host a wealth of exotic phenomena, including spin ice phases, quasi-particle excitations, order-by-disorder, and the highly entangled quantum spin liquid (QSL) state, to name a few. In this thesis, we will discuss three RE systems that exhibit geometric frustration in addition to exchange frustration: two RE pyrochlore oxides (RE2TM2O7) and a 2D isosceles triangular lattice material K3Er(VO4)2. Spin-1/2 antiferromagnetic (AFM) 2D triangular lattice magnets are an archetype of geometric frustration. While these materials are theorized to host a variety of different ground states and exotic phases depending on the anisotropies of the system, only a handful of RE material examples have been explored. We report the first deep dive into one such system, K3Er(VO4)2. We have determined the ordered magnetic structure of K3Er(VO4)2, finding an unusual structure with alternating layers comprised of AFM aligned and zero moment. We theorize this unique structure is due to the strong XY single-ion anisotropy, suggested from magnetometry measurements, which acts to suppress (to the point of vanishing completely) the out-of-plane pseudo-spin-1/2 magnetic moments. Next, we explored the effects of phase competition in a well-studied effective spin-1/2 RE pyrochlore oxide, Er2Sn2O7. Previous polycrystalline work has found Er2Sn2O7 to possess a suppressed critical temperature and an AFM Palmer-Chalker ground state. The determined exchange and single-ion anisotropy of Er2Sn2O7 find the ground state lies in close proximity to a competing AFM phase. Through extensive single crystal heat capacity measurements, we discovered a reentrant field vs. temperature phase diagram, where a system that has developed order returns to the original, less ordered (paramagnetic) state as some external parameter (field) is tuned continuously. We investigated the underlying mechanisms behind the reentrance by utilizing Monte Carlo simulations, mean field theory, and classical linear spin-wave calculations. This theory suggests that reentrance is linked to soft modes arising from phase competition, either from enhanced competition of the proximal AFM phase or from competing T=0 field-evolved ground states, depending on the specific applied field direction. In both cases, the soft modes enhance thermal fluctuations which cause the specific ordered phase to be entropically stabilized, thus forming a reentrant phase diagram. Finally, we report recent elastic neutron diffraction results on a RE pyrochlore oxide and candidate octupolar spin-ice, Ce2Sn2O7. The pseudo-spin-1/2 moments in Ce2Sn2O7 are known to possess dipolar-octupolar character and a large parameter space within the phase diagram is theorized to host novel QSL states. Previous powder neutron diffraction found diffuse scattering at high scattering vectors associated with magnetic octupoles. However, our undertaking of a similar measurement on nominally the same sample, found strikingly different results. Our neutron diffraction resulted in a broad, diffuse signal at low scattering vectors, reminiscent of a dipolar spin-ice. Neutron diffraction and atomic PDF measurements have not turned up obvious sample deformities or evidence of oxidation that could explain the differences in the diffuse signals. Further atomic studies and significant theory work is necessary to fully understand the results of this measurements, but the similarities to sister compound Ce2Zr2O7 suggest that Ce2Sn2O7 could lie on a phase boundary that is sensitive to minor distortions.
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    Frustration driven emergent phenomena in quantum and classical magnets
    (Colorado State University. Libraries, 2021) Sarkis, Colin L., author; Ross, Kathryn, advisor; Buchanan, Kristen, committee member; Gelfand, Martin, committee member; Shores, Matt, committee member; de la Venta, Jose, committee member
    Frustrated and quantum magnets remain a fascinating and broad area of physics with applications ranging from information science to commercial applications. The wide breadth of possible behavior, caused through the combination of frustration, anisotropy, and many-body physics allow for a large number of exotic phenomena to be realized within these systems. In this dissertation, I cover work on three compounds which all exhibit unusual properties in their low temperature magnetic phases. For Fe3PO4O3, the low temperature static magnetic structure shows partial magnetic ordering, where the system orders commensurately along the c-axis and retains a well-defined ordering wavevector in magnitude but not direction within the ab-plane. Within a simple Heisenberg J1-J2 model, Luttinger-Tisa ground state calculations show the existence of a quasi-degenerate well of lowest energy states coinciding with the rings of scattering observed in neutron diffraction. Taken with polycrystalline data, a small correlation size in the ab-plane suggests a large number of topological defects present in Fe3PO4O3. A few possible magnetic textures which could produce the observed behavior in Fe3PO4O3 are discussed. In the antiferromagnetic pyrochlore Yb2Ge2O7, continuum excitations were previously found through neutron scattering below this material's long range magnetic ordering temperature. By comparing field polarized inelastic neutron scattering data to linear spin wave theory we extract the four unique exchange parameters and place Yb2Ge2O7 within a classical phase diagram. We find Yb2Ge2O7 lies in close proximity to the boundary between an antiferromagnetic and ferromagnetic state leads to a phenomenon known as phase competition, where the excitations are poorly defined because of the influence of the neighboring state. Finally non-equilibrium dynamics in CoNb2O6 show the existence of a frozen state existing within its commensurate antiferromagnetic long range ordered state. This frozen state introduce aging effects at low temperatures in CoNb2O6, complicating its behavior. Following quenches of a magnetic field transverse to all moments in this material, we observe a relaxation below its field-induced phase transition into the commensurate antiferromagnetic state. Quenches of a transverse field exhibit a scaling behavior as a function of quench rate remarkably similar to a Kibble Zurek mechanism, although in our experiments, this behavior can be traced back to systematic effects. Each of these materials exemplify distinct unusual behaviors possible in low temperature quantum and frustrated magnetism.
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    Hot injection synthesis and characterization of copper antimony selenide non-canonical nanomaterials toward earth-abundant renewable energy conversion
    (Colorado State University. Libraries, 2018) Agocs, Daniel B., author; Prieto, Amy L., advisor; Buchanan, Kristen, committee member; Sambur, Justin, committee member; Sites, James R., committee member; Van Orden, Alan, committee member
    Renewable and carbon-free energy generation has become a critically important field as the global population continues to increase. Further, the ample supply afforded by natural resources such as sunlight and geothermal heat are attractive options that can be harnessed using technologies like photovoltaics and thermoelectrics. There is a growing interest in searching for novel materials that exhibit high efficiencies in these devices, ideally composed of earth abundant, non-toxic materials. This search is aided by theory, which has identified several families of compounds with interesting structure types that may exhibit properties amenable to incorporation in high efficiency devices. However, many of these materials have not yet been thoroughly evaluated for photovoltaics or thermoelectrics. This dissertation is focused on developing the synthesis and describing the basic characterization of nanoparticles of members of the compounds in the Cu-Sb-Se series, of which syntheses have been developed for Cu3SbSe4 and Cu3SbSe3 and are described in this dissertation. Herein, we describe a hot-injection route for the formation of Cu3SbSe4 and Cu3SbSe3 nanocrystals. In order to place this work in context, the first chapter of this dissertation provides a detailed summary of the literature investigating the Cu-Sb-Se family of compounds. Here, the highest thermoelectric efficiencies have been achieved for Cu3SbSe4 while Cu3SbSe3 is not yet comparable thermoelectrically to Cu3SbSe4 nor as efficient as the photovoltaic material CuSbSe2. The second chapter details the development of a hot injection synthesis of Cu3SbSe4 nanocrystals. In order for these materials to be applied as electronic materials in real devices, their stability and function under ambient conditions is of interest. Therefore, we studied the changes in electronic conductivity as a function of exposure to atmosphere. The conductivity increase was attributed to a hole mobility increase, and this was further correlated to structural oxidations. Chapter 3 details development of a synthesis for phase-pure Cu3SbSe3 nanodiscs. This material has become of interest recently for photovoltaic applications due to its acceptable band gap for solar absorption. While the synthesis of nanoscale Cu3SbSe3 has been reported, these results have not been reproduced, and property measurements among these limited works vary. Therefore, a robust synthesis was developed and initial optical and photoelectrochemical properties were measured and are reported in this dissertation that demonstrate photoactivity in thin films of the Cu3SbSe3 nanodiscs. In the fourth chapter, a more vigorous exploration of the nanodisc morphology observed in Cu3SbSe3 is reported. As a degree of self-assembly is observed in stacks of the nanodiscs, the morphology is investigated to understand how tuning nanocrystal morphology, size, and surface might affect the resulting particle interactions. To this end, a double injection synthesis was developed wherein the products exhibit optoelectronic properties similar to those of the original single injection reaction. Chapter 5 entails the electrochemical investigation of the copper antimony selenide nanostructures. Electrochemical measurements to experimentally elucidate the electronic structure are reported, and a photovoltaic architecture is proposed for a Cu3SbSe3-absorber layer device. Further, the presence of a thiol has been demonstrated to be critical to not only morphology within the Cu3SbSe3 synthesis but also the product phase formation. Therefore, initial measurements and challenges with in-situ electrochemical exploration of precursor reactivity are reported. Finally, chapter 6 briefly emphasizes the major findings within this dissertation. The experimental results for both Cu3SbSe4 and Cu3SbSe3 syntheses are reiterated. Further, additional directions for future work with this system are suggested. These primarily focus on fabrication of a Cu3SbSe3 photovoltaic cell to begin understanding photogenerated carrier transport. This can be extended through applying knowledge gained by understanding disc stacking to improve film deposition and electronic properties within Cu3SbSe3 materials. Finally, development of an electrochemical measurement system for use in oleylamine media would allow a new perspective on investigation of colloidal nanocrystalline formation. These proposed experiments would contribute to their respective fields in the broader context of expanding search criteria for novel photovoltaic materials, addressing the challenge of grain boundary recombination sites in photovoltaic nanocrystals, and providing tools for exploring nanoparticle synthesis.
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    Imaging as characterization techniques for thin-film cadmium telluride photovoltaics
    (Colorado State University. Libraries, 2014) Zaunbrecher, Katherine, author; Sites, James R., advisor; Gelfand, Martin, committee member; Buchanan, Kristen, committee member; Sampath, W. S., committee member
    The goal of increasing the efficiency of solar cell devices is a universal one. Increased photovoltaic (PV) performance means an increase in competition with other energy technologies. One way to improve PV technologies is to develop rapid, accurate characterization tools for quality control. Imaging techniques developed over the past decade are beginning to fill that role. Electroluminescence (EL), photoluminescence (PL), and lock-in thermography are three types of imaging implemented in this study to provide a multifaceted approach to studying imaging as applied to thin-film CdTe solar cells. Images provide spatial information about cell operation, which in turn can be used to identify defects that limit performance. This study began with developing EL, PL, and dark lock-in thermography (DLIT) for CdTe. Once imaging data were acquired, luminescence and thermography signatures of non-uniformities that disrupt the generation and collection of carriers were identified and cataloged. Additional data acquisition and analysis were used to determine luminescence response to varying operating conditions. This includes acquiring spectral data, varying excitation conditions, and correlating luminescence to device performance. EL measurements show variations in a cell's local voltage, which include inhomogeneities in the transparent-conductive oxide (TCO) front contact, CdS window layer, and CdTe absorber layer. EL signatures include large gradients, local reduction of luminescence, and local increases in luminescence on the interior of the device as well as bright spots located on the cell edges. The voltage bias and spectral response were analyzed to determine the response of these non-uniformities and surrounding areas. PL images of CdTe have not shown the same level of detail and features compared to their EL counterparts. Many of the signatures arise from reflections and severe inhomogeneities, but the technique is limited by the external illumination source used to excite carriers. Measurements on unfinished CdS and CdTe films reveal changes in signal after post-deposition processing treatments. DLIT images contained heat signatures arising from defect-related current crowding. Forward- and reverse-bias measurements revealed hot spots related to shunt and weak-diode defects. Modeling and previous studies done on Cu(In,Ga)Se2 thin-film solar cells aided in identifying the physical causes of these thermographic and luminescence signatures. Imaging data were also coupled with other characterization techniques to provide a more comprehensive examination of nonuniform features and their origins and effects on device performance. These techniques included light-beam-induced-current (LBIC) measurements, which provide spatial quantum efficiency maps of the cell at varying resolutions, as well as time-resolved photoluminescence and spectral PL mapping. Local drops in quantum efficiency seen in LBIC typically corresponded with reductions in EL signal while minority-carrier lifetime values acquired by time-resolved PL measurements correlate with PL intensity.
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    Mantle velocity variations under the northern Canadian Cordillera through body wave tomography
    (Colorado State University. Libraries, 2019) Khare, Aditya U., author; Schutt, Derek, advisor; Buchanan, Kristen, committee member; Egenhoff, Sven, committee member
    The Mackenzie Mountains (MM) in the northern Canadian Cordillera (NCC) are an actively uplifting mountain range and an excellent location to investigate the causes of intra-plate orogeny. The orogen is situated almost ~750 km inboard of the active Pacific plate boundary, and little deformation is occurring between the MM and the Pacific Coast, except within the Coast Ranges. To investigate the causes of this orogeny, the Mackenzie Mountains Earthscope Project (MMEP) deployed 40 broadband seismographs and 4 continuous GPS instruments in a linear array from near the Pacific Coast to the Slave craton. Here we present results of teleseismic body wave tomography in the NCC that were obtained by using data from 37 of these MM stations as well as 67 other stations in the region surrounding the MM. Results show a sharp sub-vertical transition between low velocity in the Cordillera (ΔV -2%) and high velocity in the craton (ΔV +2%) about 100 km southwest of the Mackenzie River. The locations of Miocene to Present volcanism in the region also coincide well with the low velocity zones suggesting the presence of melt and/or anomalous temperatures. Two notable high velocity anomalies are seen beneath the Cordillera. The first is present under the Tintina Fault (ΔV +1.5%) and may be indicative of a lower crustal compositional anomaly. The other is at 600 km depth below the Cordillera (ΔVp +2%) which we interpret as delaminated lithosphere. The delamination possibly resulted from mantle upwelling due to the opening of the slab window ~20 Ma.
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    Measurement of cadmium telluride bilayer solar cells
    (Colorado State University. Libraries, 2024) Chime, Chinecherem Agnes, author; Sites, James, advisor; Buchanan, Kristen, committee member; Sampath, Walajabad, committee member
    Photovoltaic (PV) technology is a green technology that uses devices and semiconducting materials to generate power by converting the absorbed energy from solar to electrical energy. Understanding the performance and behavior of a fabricated device is essential for enhancing their efficiency for future commercialization. Cadmium-telluride (CdTe) technology is a PV technology that uses CdTe as the semiconductor layer for absorbing and converting sunlight into electricity. Incorporating a bilayer of cadmium selenium telluride (CdSexTe1-x) alloy and CdTe into solar cell devices have shown particularly good performance, enhanced passivation, and higher efficiency. In this research, cadmium telluride solar cells were fabricated with a focus on improving the performance of the absorber layers. Radio frequency (RF) magnetron sputtering and close-space sublimation were adopted in preparing the front and back contact layers respectively. The fabricated device comprises of Tec-10 glass/100-nm magnesium-doped zinc oxide (MZO)/0.5-μm CST40/2.5-µm CdTe/ cadmium-chloride passivation/ Cu-doping/ 40-nm Te/ carbon and nickel paint back contact. As part of the performance improvement measures, the bilayer surface was passivated with cadmium chloride (CdCl2) and doped afterwards with copper. The fabricated CdSexTe1-x/CdTe device was subjected to room temperature and low temperature current density-voltage (J-V), capacitance, phase angle, quantum efficiency (QE), reflectance, electroluminescence (EL), and photoluminescence (PL) measurements. The J-V characteristics gave 15% device efficiency and showed diode curves which rolled over at lower temperatures, but were more ideal at higher temperatures. Capacitance measurements gave a hole density of 4x1014 cm-3 and a phase angle of 88o. The cells recorded high quantum efficiency of about 85% which is indicative of reduced recombination rate. Few defects were observed from the EL images while the PL emission peaks were obtained at 875 nm corresponding to an approximate energy band gap value of 1.42 eV. The measurement results show good performance for use in commercial solar cells for energy sustainability. Future implications encompass module fabrication, flexible devices, and affordability for enhancing green energy production and minimizing environmental pollution. Prospects envisage fabricating CdTe devices with higher efficiencies which would continue to compete successfully with other solar cell technologies.
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    Mesoscopic revelations: studying the shape of AOT reverse micelles
    (Colorado State University. Libraries, 2024) Gale, Christopher D., author; Levinger, Nancy E., advisor; Krummel, Amber, committee member; Prieto, Amy, committee member; Buchanan, Kristen, committee member
    Aerosol-OT (AOT) reverse micelles are a quintessential model system for studying nanoconfinement, creating consistent reverse micelles with a repeatable and very small size (~1-10 nm) using just 3 components. These reverse micelles have been used for studying the behavior of water and solutes in nanoconfinement, modeling the behavior of key solutes and proteins in a system more analogous to in vivo work, synthesizing nanoparticles, and even as a vehicle for suspending proteins in a low-viscosity solvent for high quality NMR experiments. Despite their usefulness, AOT reverse micelle's shape is poorly understood, but important to understanding behavior within a reverse micelle. Interfacial properties have been found to be key to many aspects of behavior within AOT reverse micelles and distance from the interface as well as the actual amount of interface present are highly dependent on shape. Therefore, the study of shape is key to a better understanding of AOT reverse micelles and behavior in nanonconfinement. In this work, I develop a series of metrics for shape--- coordinate-pair eccentricity (CPE), convexity, and the curvature distribution--- and apply them to several simulations of AOT reverse micelles. The simulations were designed to test the impact of the force field on the shape and behavior of the reverse micelles, including the first parameterization of AOT into the OPLS force field. The system was extensively checked to ensure equilibration was achieved and the system was not biased by the starting configuration. To aid in the shape analysis, I have developed a model and a formal proof to predict how the CPE changes for an arbitrary shape as it grows to model the shape behavior of general core-shell structures. Additionally, I measured the dipole moment of AOT, the rotational anisotropy decay of water, and several radial distribution functions to provide experimental verification where possible and further explore the behavior of the AOT reverse micelle system. Several key findings emerge from this work. Most notably, I find that AOT reverse micelles are significantly aspherical and non-convex over every force field tested, providing robust evidence that AOT reverse micelles are aspherical at any given moment in time. This provides strong evidence in support of the idea that experimental observations of spherical particles are the result of ensemble averaging. I also observe that the shape at the AOT/oil interface is comparatively more spherical with a "Goldilock's" value of convexity, neither too high nor too low, compared to the water/AOT interface. My model predicts that the CPE should fall with the addition of a shell, here provided by the AOT surfactant layer, suggesting this is largely the result of geometry. There is great variation between simulations and metrics in their dynamics, but in general, the shape appears to change on the order of 10 ns. This provides a useful method of deducing which values may or may not be impacted by shape, based on the time scale. For instance, it can reasonably be said that shape likely has no impact on water dynamics based on the roughly four orders of magnitude difference in the time scales of each process, which is supported by my own findings. Across all metrics studied, there are noticeable differences between simulations, but none of the differences are consistent. I believe this observation has important implications for both the behavior and simulation of AOT reverse micelles. First, this implies that the forces and interactions giving rise to different aspects of the reverse micelle are complex and largely independent, and that there is a disconnect between molecular-level measurements like radial distribution functions and and mesoscopic-level measurements like shape. Second, this implies that any simulation parameterized on one measure has no guarantee that it reproduces any other aspect of the reverse micelle accurately.
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    Navigating the thermodynamic landscape in search of synthetic routes to ternary nitrides
    (Colorado State University. Libraries, 2022) Rom, Christopher Linfield, author; Neilson, James R., advisor; Prieto, Amy L., advisor; Sambur, Justin, committee member; Szamel, Grzegorz, committee member; Buchanan, Kristen, committee member
    Ternary nitride materials—a class of ceramics composed of two different metals bound with anionic nitrogen (N3-) as a solid—are underexplored because they are difficult to make. Nitrides rarely occur in nature, as the oxygen in the air (O2) is more reactive towards metals than the nitrogen (N2). Consequently, oxide minerals dominate the earth's crust while nitride minerals are extremely rare. Almost all ternary nitrides that have been discovered have synthesized, usually with rigorously air-free conditions. Despite much effort in the past century, the number of known ternary nitrides (approximately 450) pales in comparison to that of ternary oxides (over 4,000). Yet there are world-changing materials within this small number of compounds, like the (In,Ga)N alloys that underpin efficient blue light emitting diodes. Fortunately, recent computational work has predicted a number of theoretically stable ternary nitrides, providing targets for synthesis. This dissertation focuses on the synthesis of new ternary nitrides. Guided by increasingly user-friendly computational tools, these chapters describe syntheses overcome the thermodynamic barriers that often inhibit the formation of new ternary nitrides. Along the way, several new materials are discovered and characterized for promising magnetic and semiconducting properties: MnSnN2, MgWN2 in two structure types, Mg3WN4, MgZrN2, CaZrN2, and CaHfN2. These adventures in synthesis not only report new compounds, but also highlight promising strategies for future explorations of uncharted nitride phase space.
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    Optofluidic intracavity spectroscopy for spatially, temperature, and wavelength dependent refractometry
    (Colorado State University. Libraries, 2012) Kindt, Joel D., author; Lear, Kevin L., advisor; Buchanan, Kristen, committee member; Notaros, Branislav, committee member
    A microfluidic refractometer was designed based on previous optofluidic intracavity spectroscopy (OFIS) chips utilized to distinguish healthy and cancerous cells. The optofluidic cavity is realized by adding high reflectivity dielectric mirrors to the top and bottom of a microfluidic channel. This creates a plane-plane Fabry-Perot optical cavity in which the resonant wavelengths are highly dependent on the optical path length inside the cavity. Refractometry is a useful method to determine the nature of fluids, including the concentration of a solute in a solvent as well as the temperature of the fluid. Advantages of microfluidic systems are the easy integration with lab-on-chip devices and the need for only small volumes of fluid. The unique abilities of the microfluidic refractometer in this thesis include its spatial, temperature, and wavelength dependence. Spatial dependence of the transmission spectrum is inherent through a spatial filtering process implemented with an optical fiber and microscope objective. A sequence of experimental observations guided the change from using the OFIS chip as a cell discrimination device to a complimentary refractometer. First, it was noted the electrode structure within the microfluidic channel, designed to trap and manipulate biological cells with dielectrophoretic (DEP) forces, caused the resonant wavelengths to blue-shift when the electrodes were energized. This phenomenon is consistent with the negative dn/dT property of water and water-based solutions. Next, it was necessary to develop a method to separate the optical path length into physical path length and refractive index. Air holes were placed near the microfluidic channel to exclusively measure the cavity length with the known refractive index of air. The cavity length was then interpolated across the microfluidic channel, allowing any mechanical changes to be taken into account. After the separation of physical path length and refractive index, it was of interest to characterize the temperature dependent refractive index relationship, n(T), for phosphate buffered saline. Phosphate buffered saline(PBS) is a water-based solution used with our biological cells because it maintains an ion concentration similar to that found in body fluids. The n(T) characterization was performed using a custom-built isothermal apparatus in which the temperature could be controlled. To check for the accuracy of the PBS refractive index measurements, water was also measured and compared with known values in the literature. The literature source of choice has affiliations to NIST and a formulation of refractive index involving temperature and wavelength dependence, two parameters which are necessary for our specialized infrared wavelength range. From the NIST formula, linear approximations were found to be dn/dT = -1.4×10-4 RIU °C-1 and dn/dλ = -1.5×10-5 RIU nm-1 for water. A comparison with the formulated refractive indices of water indicated the measured values were off. This was attributed to the fact that light penetration into the HfO2/SiO2 dielectric mirrors had not been considered. Once accounted for, the refractive indices of water were consistent with the literature, and the values for PBS are believed to be accurate. A further discovery was the refractive index values at the discrete resonant wavelengths were monotonically decreasing, such that the dn/dλ slope for water was considerably close to the NIST formula. Thus, n(T,λ) was characterized for both water and PBS. A refractive index relationship for PBS with spatial, temperature, and wavelength dependence is particularly useful for non-uniform temperature distributions caused by DEP electrodes. First, a maximum temperature can be inferred, which is the desired measurement for cell viability concerns. In addition, a lateral refractive index distribution can be measured to help quantify the gradient index lenses that are formed by the energized electrodes. The non-uniform temperature distribution was also simulated with a finite element analysis software package. This simulated temperature distribution was converted to a refractive index distribution, and focal lengths were calculated for positive and negative gradient index lenses to a smallest possible length of about 10mm.
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    Simulation of nanoscale patterns yielded by ion bombardment of solid surfaces
    (Colorado State University. Libraries, 2018) Pearson, Daniel A., author; Bradley, Mark, advisor; Buchanan, Kristen, committee member; Gelfand, Martin, committee member; Shipman, Patrick, committee member
    This thesis includes numerical investigations into two topics of self-organized topographies produced on solid surfaces that are bombarded with a broad ion beam. The first topic is the formation of terraces. When a surface is bombarded at relatively large angles of incidence, the surface often develops facets that are characterized by large regions of nearly constant gradient in height, which are called terraces. The second topic is related to the observation that when the surface of a nominally flat binary material is bombarded with a broad, normally-incident ion beam, disordered hexagonal arrays of nanodots can form. Shipman and Bradley have derived equations of motion that govern the coupled dynamics of the height and composition of such a surface [P. D. Shipman and R. M. Bradley, Phys. Rev. B 84, 085420 (2011)]. We investigate the influence of initial conditions on the hexagonal order yielded by integration of those equations of motion. In our work on terrace formation, we introduce a model that includes an improved approximation to the sputter yield and that produces a terraced surface morphology at long times for a wide range of parameter values. Numerical integrations of our equation of motion reveal that the terraces coarsen for a finite amount of time after which the coarsening is interrupted, just as observed experimentally. We also show that the terrace propagation direction can reverse as the amplitude of the surface disturbance grows. This highlights the important role higher order nonlinearities play in determining the propagation velocity at high fluences. We study the nanoscale terraced topographies that arise when a solid surface is bombarded with a broad ion beam that has a relatively high angle of incidence θ. Our simulations establish that the surfaces exhibit interrupted coarsening, i.e., the characteristic width and height of the surface disturbance grow for a time but ultimately asymptote to finite values as the fully terraced state develops. In addition, as θ is reduced, the surface can undergo a transition from a terraced morphology that changes little with time as it propagates over the surface to an unterraced state that appears to exhibit spatiotemporal chaos. For different ranges of the parameters, our equation of motion produces terraced topographies that are remarkably similar to those seen in various experiments, including pyramidal structures that are elongated along the projected beam direction and isolated lenticular depressions. For our study of the influence of prepatterning surfaces governed by the Bradley-Shipman equations, the initial conditions studied are hexagonal and sinusoidal templates, straight scratches and nominally flat surfaces. Our simulations indicate that each of the prepatterned surfaces can lead to marked improvements in the hexagonal order compared to what is obtained from the nominally flat surfaces. For the hexagonal and sinusoidal templates with amplitude approximately equal to one hundredth of the amplitude of the pattern obtained at late times, the greatest improvement in order is obtained if the initial wavelength is approximately equal to or double the linearly selected wavelength. Our simulations of sinusoidal templates demonstrate that increasing the amplitude of the template can improve the effectiveness of templates with longer wavelengths. Scratches enhance the hexagonal order in their vicinity if their width is close to or less than the linearly selected wavelength. Our results suggest that prepatterning a binary material can dramatically increase the hexagonal order achieved at large ion fluences.
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    Studies of tuning magnetic properties of ferromagnetic heterostructures
    (Colorado State University. Libraries, 2020) Lauzier, Joshua, author; de la Venta Granda, Jose, advisor; Buchanan, Kristen, committee member; Gelfand, Martin, committee member; Field, Stuart, committee member; Menoni, Carmen, committee member
    The magnetic properties of hybrid systems have increasingly become an area of intense focus in both fundamental research and technological application due to the inherent flexibility in material properties by mixing and matching various constituent components. One particularly interesting choice is hybrid heterostructures that consist of ferromagnetic (FM) materials and materials that undergo phase transitions, coupled via structural, electronic, and/or magnetic coupling. Two canonical examples of phase transition materials are vanadium dioxide (VO2) and iron rhodium (Fe50Rh50, abbreviated FeRh). Both materials undergo structural phase transitions (SPT). With increasing temperature, VO2 transitions from a low temperature monoclinic to high temperature rutile structure at 340 K. The SPT is concurrent with a 4-5 orders of magnitude metal to insulator transition (MIT) from a low temperature insulating phase to a high temperature metallic phase. Similarly, FeRh undergoes an isotropic 1% volume expansion at 370 K with increasing temperature. Coincident with the SPT, FeRh also undergoes a magnetic transition from a low temperature antiferromagnetic (AF) to a high temperature ferromagnetic (FM) phase, which is unusual for magnetic materials. The delicate nature of these transitions makes them sensitive to parameters such as stoichiometry, growth conditions, and external stimuli, which allows for high tunability of their respective phase transitions. In this thesis, we first show in Chapter 3 that the surface morphology and MIT properties of sputtered VO2 thin films can be tuned via deposition conditions such as deposition temperature and O2 flow rate during the sputtering process while maintaining the quality of the VO2 transition. Films grown at higher temperatures (>525 ℃) and low O2 flow rate show sub 2 nm surface roughness. Higher temperatures lead to a 'melted'-like surface morphology along with a 5 orders of magnitude MIT, comparable to single crystals. Choice of substrate allows another avenue to strongly tune both the morphology and the MIT characteristics while maintaining a strong VO2 transition due to lattice mismatch. In Chapter 4, we turn to a discussion of VO2/Ni bilayer structures, where the temperature induced VO2 SPT will impart a strain across the interface into the FM layer, which will then influence the magnetic properties via magnetoelastic coupling. Due to an inverse magnetostrictive effect the coercivity and magnetization of the FM layer can be strongly modified. Tuning the VO2 SPT via growth conditions or substrate choice then allows for tuning the coupled magnetic properties of the FM. For sufficiently smooth films, there is a strong enhancement in the coercivity localized close to their respective SPT Tc due to phase coexistence in the SPT material. This chapter is largely based on work previously published as "Coercivity enhancement in VO2/Ni bilayers due to interfacial stress" in Journal of Applied Physics.1 VO2/FM hybrid films also show a dependence on the growth conditions during the FM deposition, which is explored in Chapter 5. Films with the FM deposited above the VO2 phase transition critical temperature (Tc) show a high coercivity below Tc and a low coercivity above Tc, whereas films deposited below Tc show the opposite behavior. Films deposited below Tc also show an irreversibility in their magnetic properties the first time they are thermally cycled. A similar irreversibility is observed in the resistance vs. temperature (R vs. T) properties of bare VO2 films, and cracking as the VO2 crosses the SPT is proposed as a common mechanism. The plausibility of cracking as a mechanism is investigated via computational modeling of the R vs. T properties in a random resistor network, as well as probed directly via Atomic Force Microscopy (AFM). The work shown in this chapter has been previously published under the title "Magnetic irreversibility in VO2/Ni bilayers" in Journal of Physics: Condensed Matter.2 Sputtered FeRh/FM bilayer films show a similar sensitivity as the VO2/FM system to the growth conditions, with the coercivity below Tc tunable whether the FM is initially deposited above or below Tc. Above Tc, the magnetic FeRh phase adds an additional complication, dominating the magnetic response via exchange coupling. This effect is explored in FeRh/Ni bilayer systems in Chapter 6. Polarized neutron reflectometry (PNR) allows for depth dependent structural and magnetic characterization with nanometer resolution. PNR measurements show that the bilayer's magnetic behavior below Tc is likely driven by magnetoelastic effects due to the structural transition of the FeRh, rather than simple magnetic coupling or a pinned interfacial FM layer. The overall magnetic properties of the bilayers are therefore a product of both structural and magnetic coupling between the FeRh and the FM Ni layer. The results of this chapter have been previously published as "Using structural phase transitions to enhance the coercivity of ferromagnetic films" in Applied Physics Letters Materials.
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    Vortex rectification and phase slips in superconducting granular aluminum
    (Colorado State University. Libraries, 2020) Maughan, Weston F., II, author; Field, Stuart B., advisor; Gelfand, Martin, committee member; Buchanan, Kristen, committee member; Neilson, James R., committee member
    Superconductivity is a unique and interesting phenomenon that manifests as a new phase of matter in a wide variety of materials. The most well-known property of superconductors is that they exhibit perfect conductivity when cooled below a critical temperature Tc. In addition to their perfect conductivity, superconductors exhibit the equally fundamental Meissner effect that expels magnetic fields from the interior of the material. While applications of a material that exhibits perfect conductivity, such as generating large magnetic fields via electromagnets or transmitting a large current with zero dissipation, are highly desired, the subtle details of flux penetration into mesoscopic samples may also be exploited to realize useful devices, or as a testbed to understand one-dimensional superconductivity. In this work, the nature of superconductivity in granular aluminum was explored in two mesoscopic sample classes: first, by studying Abrikosov vortices in films, and then by studying dissipation from phase slips in one-dimensional nanowires. The penetration of an applied field is possible in film sample geometries, even though the Meissner effect generally expels flux. This penetration occurs in type-II superconductors via quantized flux bundles through normal regions or domains of the superconductor called vortices. The behavior and dynamics of these vortices are of significant interest as they can be exploited to realize fluxonic devices that perform circuit operations analogous to the operations performed with electrons in electronics. One method to influence the motion of vortices within a superconductor in order to realize a fluxonic device is to introduce a periodic potential landscape that causes an easy and a hard direction for vortex motion. In other words, the vortex motion is rectified. By realizing a so-called vortex ratchet with such a potential landscape, vortices may easily be introduced or removed from the superconductor by driving them in the easy or hard directions respectively. We begin by studying the rectification properties of both symmetrically and asymmetrically thickness-modulated superconducting films. These thickness modulations were fabricated with an elegant method of angle-sputter deposition of granular aluminum onto a glass substrate that has a sinusoidal modulation in its thickness. We then explored the rectification of these symmetric and asymmetric films by studying the motion of vortices using cryogenic transport measurements. In these measurements, vortices are driven in both directions across a modulated sample and the resulting voltages are measured. Differences in the voltages corresponding to motion in opposite directions imply that the vortices move more readily in one direction, that is, that there is an overall rectification in their motion. While these measurements performed with the symmetric washboard film seemed to exhibit reversibility in the transport properties, the asymmetric washboard exhibited a mild asymmetry that was much smaller than expected. This result indicates that the potential landscape is influenced by another source in addition to the asymmetric thickness modulation. To better understand these effects, we tested the influence of the sample edges on the nucleation of vortices with two multi-segment films. These multi-segment films were fabricated in either an 8- or 14-probe geometry where each segment shares a vertical reference edge, while the opposing edges between pairs of voltage leads contain tapers of varying lengths which were fabricated lithographically. Clear rectification effects are observed with cryogenic transport measurements of these samples, with enhanced rectification for longer taper lengths showing the importance of the sample edge geometry on vortex motion. Following these studies in superconducting films, we explored the nature of dissipation in one-dimensional superconducting nanowires. Recent advancements in laboratory fabrication techniques have reduced the accessible size scale of superconducting samples into the nanometer regime. As a result, superconductors can be fabricated that exhibit one-dimensional superconductivity, in which the complex superconducting order parameter ψ is restricted to fluctuations along the length of the nanowire because its cross-sectional dimensions are smaller than ξ. Experiments performed with these nanowires exhibit a non-zero resistance even when the samples were cooled below Tc. This dissipation is understood as due to thermal fluctuations that cause |ψ| to vanish in a small segment of the wire of length ∼ξ, allowing the superconducting phase to "slip" by ±2π, resulting in a voltage pulse. However, several experimental studies have observed excess nanowire resistance at low temperatures that cannot be described with this thermal fluctuation model alone. Some researchers have proposed that macroscopic quantum tunneling events lead to the excess resistance, while other studies claim that nanowire inhomogeneities influence the thermally activated phase slip rate. In order to provide insight into the origin of the excess nanowire resistance, we performed cryogenic scanning experiments to map the local phase-slip rate along a superconducting nanowire. This was achieved by scanning either a dielectric or a magnetic tip with a home- built cryogenic atomic force microscope (cryo-AFM) to locally perturb superconductivity along a granular aluminum nanowire, while simultaneously measuring the nanowire resistance. This required the construction and characterization of the cryo-AFM along with a method of locating nanowire samples at cryogenic temperatures. We then fabricated one-dimensional granular aluminum nanowires with electron beam lithographic (EBL) techniques. We scanned these nanowires with the cryo-AFM and found that a dielectric tip does not locally perturb superconductivity enough to cause a measurable change in the wire resistance. However, repeating this experiment with either a magnetic tip or another material may plausibly elucidate the origins of the low-temperature nanowire dissipation.