Browsing by Author "Gelfand, Martin, committee member"

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Open Access
A measurement of the double-differential electron antineutrino charged-current inclusive cross section in the NOvA near detector
(Colorado State University. Libraries, 2023) Doyle, Derek, author; Buchanan, Norm, advisor; Gelfand, Martin, committee member; Harton, John, committee member; Norman, Andrew, committee member; Pouchet, Louis-Noël, committee member
The neutrino is a fundamental particle of the universe that was first hypothesized in 1930 by Wolfgang Pauli to explain the observed energy distribution of outgoing electrons produced from beta-decay. Since then, it has been discovered that there are at least three types, or flavors, of neutrinos and that they oscillate between these flavors as they travel through space and time. This discovery proved that neutrinos have a non-zero mass and positioned neutrino oscillations to provide a window into understanding the matter/antimatter asymmetry in the universe. Principle to all neutrino measurements is an accurate and robust interaction model over a large range of energies, and measurements to support the model. Of particular importance to the NuMI Off-axis νe Appearance (NOvA) neutrino oscillation experiment is the energy range from 1 to 10 GeV, where Quasi-Elastic (QE), Meson-Exchange Current (MEC), and Deep Inelastic Scattering (DIS) interactions all contribute significantly. Using neutrinos from the Neutrinos at the Main Injector (NuMI) beam and the NOvA near detector, the first double-differential electron antineutrino charged-current inclusive cross section is measured and compared to various interaction models implemented within the genie Generator framework, version 3. Good agreement is observed between measurement and a genie model tuned to NOvA data.
Open Access
Design strategies for high-efficiency CdTe solar cells
(Colorado State University. Libraries, 2017) Song, Tao, author; Sites, James R., advisor; Kanevce, Ana, committee member; Gelfand, Martin, committee member; Wu, Mingzhong, committee member; Sampath, W. S., committee member
With continuous technology advances over the past years, CdTe solar cells have surged to be a leading contributor in thin-film photovoltaic (PV) field. While empirical material and device optimization has led to considerable progress, further device optimization requires accurate device models that are able to provide an in-depth understanding of CdTe device physics. Consequently, this thesis is intended to develop a comprehensive model system for high-efficiency CdTe devices through applying basic design principles of solar cells with numerical modeling and comparing results with experimental CdTe devices. Four key topics about high-efficiency CdTe cells are covered in this dissertation: (a) material optimization of CdTe absorber, (b) roles of emitter/absorber interface on carrier transport, (c) substrate choices for monocrystalline CdTe cells, and (d) back contact configurations for thin-film polycrystalline CdTe cells. Finally, comparisons between simulation and experiment are carried out to identify both beneficial and detrimental mechanisms for CdTe cell performance and to guide future cell optimization. The CdTe absorber is central to cell performance. Numerical simulation has shown the feasibility of high energy-conversion efficiency (open-circuit voltage VOC > 1000 mV, efficiency η > 25%), which requires both high carrier density (p >1016 cm-3) and long minority carrier lifetime (τn > 100 ns). As the minority carrier lifetime increases (τn > 10 ns), the carrier recombination at the back surface becomes a limitation for cell performance with absorber thickness < 3 µm. Hence, either a thicker absorber or an appropriate back-surface-field layer is a requisite for reducing the back-surface recombination. When integrating layers into devices, more careful design of interfaces is needed. One consideration is the emitter/absorber interface. It is shown that a positive conduction-band offset ΔEC ("spike") at the interface is beneficial to cell performance, since it can induce a large valence-band bending which suppresses the hole injection near the interface for the electron-hole recombination, but too large a spike is detrimental to photocurrent transport. In a heterojunction device with many defects at the emitter/absorber interface (high SIF), a thin and highly-doped emitter can induce strong absorber inversion and hence help maintain good cell performance. Performance losses from acceptor-type interface defects can be significant when interface defect states are located near mid-gap energies. In terms of specific emitter materials, the calculations suggest that the (Mg,Zn)O alloy with 20% Mg, or a similar type-I heterojunction partner with moderate ΔEC (e.g., Cd(S,O) or (Cd,Mg)Te with appropriate oxygen or magnesium ratios) should yield higher voltages and would therefore be better candidates for the CdTe-cell emitter. The CdTe/substrate interface is also of great importance, particularly in the growth of epitaxial monocrystalline CdTe cells. Several substrate materials (CdTe, Si, GaAs, and InSb) have been discussed and all have challenges. These have generally been addressed through the addition of intermediate layers between the substrate and CdTe absorber. InSb is an attractive substrate choice for CdTe devices, because it has a close lattice match with CdTe, it has low resistivity, and it is easy to contact. However, the valence-band alignment between InSb and p-type CdTe, which can both impede hole current and enhance forward electron current, is not favorable. Three strategies to address the band-offset problem are investigated by numerical simulation: (a) heavy doping of the back part of the CdTe layer, (b) incorporation of an intermediate CdMgTe or CdZnTe layer, and (c) formation of an InSb tunnel junction. Each of these strategies is predicted to be helpful for higher cell performance, but a combination of them should be most effective. In addition, the CdTe/back contact interface plays a significant role in carrier transport for conventional polycrystalline thin-film CdTe devices. A significant back-contact barrier φb caused by metallic contact with low work function can block hole transport and enhance the forward current and thus result in a reduced VOC, particularly with fully-depleted CdTe devices. A buffer contact layer between CdTe absorber and metallic contact is strongly needed to mitigate this detrimental impact. The simulation has shown that a thin tellurium (Te) buffer as well as a highly doped p-type CdTe layer can assume such a role by reducing the downward valence-band bending caused by large φb and hence enhancing the extraction of the charge carriers. Finally, experimental CdTe cells are discussed in parallel with the simulation results to identify limiting mechanisms and give guidance for future efficiency improvement. For the monocrystalline CdTe cells made at NREL, it is found that the sputter damage causing large numbers of defect states near the Cd(S,O)/CdTe interface plays an important role in limiting cell performance, particularly for cells with low oxygen Cd(S,O) (with a "cliff" band offset). Other effects, such as the large series resistance and reflection, also reduce the cell performance. A lattice-matched material with less deposition damage and with a type-I interface is suggested to introduce less interfacial recombination in future emitter growth on epitaxial CdTe absorbers. For polycrystalline CdTe solar cells made at CSU, it is demonstrated that an MZO emitter forms a spike at the MZO/CdTe interface and a Te buffer layer mitigates large back-contact barrier φb. Both play very important roles in achieving good cell performance (VOC ~ 860 mV, η ~ 18.3%). The simulation has also shown that the electron reflector would be an effective approach to further increase VOC even with a relative low CdTe carrier concentration (~1014 cm-3).
Open Access
Development and advancement of thin CdTe-based solar cells for photovoltaic performance improvements
(Colorado State University. Libraries, 2020) Bothwell, Alexandra, author; Sites, James, advisor; Krueger, David, committee member; Gelfand, Martin, committee member; Sampath, Walajabad, committee member; Topič, Marko, committee member
Photovoltaic technologies, with an essentially infinite energy source, large total capacity, and demonstrated cost competitiveness, are well-positioned to meet growing global demand for clean energy. Cadmium-telluride (CdTe) thin-film photovoltaics is advantageous primarily for its direct optical band gap (approximately 1.48 eV) which is well-matched to the standard AM 1.5G solar spectrum, and its high absorption coefficient. These advantages, in tandem with innovations in fabrication and photovoltaic design in the past decade, have significantly increased CdTe photovoltaic device performance and reduced cost. Major advances in CdTe device performance have been achieved through improved current collection and fill factor, however, the open-circuit voltage (VOC) of CdTe devices remains limited compared to the band gap-determined maximum achievable VOC. The voltage deficit could be minimized through various approaches, and this work addresses it through progressive structural changes to a thin CdTe device. Absorbers of less than 2 µm were pursued for ultimate electron-reflector devices which incorporate a wide band-gap material behind the absorber to induce a back-surface field via a back-side conduction-band offset for improved VOC. An optimized and stable base structure is necessary to quantify characteristics and improvements in progressive devices with additional material layers. Thin, 0.4-1.2 µm CdTe absorber devices were optimized and demonstrated respectable and repeatable performance parameters, and a maximum efficiency of 15.0% was achieved with only 1.2 µm CdTe. Capacitance measurements also showed that thinner devices had fully-depleted absorbers into forward bias. To improve device performance through increased current collection, a 1.4-eV band gap CdSeTe layer was introduced as an additional absorber material preceding CdTe. Prior understanding of the effects of the additional CdSeTe material was incomplete, and this work deepens and expands this understanding. Performance improvement was achieved for thin, 1.5-µm absorber devices with no intentional interdiffusion of the CdSeTe and CdTe. The importance of the CdSeTe thickness was demonstrated, where performance was consistently reduced for CdSeTe thickness greater than CdTe thickness, independent of CdSe composition in the close-space sublimation (CSS) CdSeTe source material. Longer time-resolved photoluminescence (TRPL) tail lifetimes in CdSeTe/CdTe devices compared to CdTe devices suggested better bulk properties, and current loss analysis showed that CdSeTe is the dominant absorber in 0.5-µm CdSeTe/1.0-µm devices. 1.5-µm CdSeTe/CdTe devices demonstrated increased current collection and 30-mV voltage deficit reduction due to the 100-meV narrower band gap of CdSeTe compared to CdTe and passivating effects of selenium, for an ultimate efficiency improvement to 15.6%. Lattice-constant matching to CdTe and wide, ~1.8-eV band-gap requirements directed the selection of CdMgTe as the electron-reflector layer. CdMgTe was incorporated into the CdSeTe/CdTe device structure first through CSS, but sputter deposition was found to be more favorable to address the material complexities of CdMgTe (temperature-induced magnesium diffusion and CdCl2 passivation loss, doping, and MgO formation), and produced higher performing CdMgTe electron-reflector devices. Low substrate temperature achievable in sputtered CdMgTe deposition proved the greatest advantage over CSS-CdMgTe: CdCl2 passivation and magnesium can be appropriately maintained with a corresponding maintenance of device performance, whereas temperature-induced CdCl2 passivation loss or magnesium loss will occur for CSS-deposited CdMgTe with incumbent performance reduction. Through low-temperature depositions, doping optimization, and small structural adjustments, 16.0% efficiency was achieved with CdMgTe sputtered on 0.5-µm CdSeTe/1.0-µm CdTe absorbers, the highest-known CdMgTe electron-reflector device performance. The CdMgTe and non-CdMgTe-containing device VOC's suggested that electron reflection was enacted with partial success for the sputter CdMgTe-incorporated structure, but the significant improvements expected based on simulation have not been realized due to MgO formation and a negative valence-band offset which somewhat impedes hole transport to the back contact. Suggestions to overcome or circumvent these limitations are presented and discussed in the context of progressed understanding of CdMgTe electron-reflector devices.
Open Access
Device characterization of cadmium telluride photovoltaics
(Colorado State University. Libraries, 2014) Geisthardt, Russell M., author; Sites, James, advisor; Gelfand, Martin, committee member; Topič, Marko, committee member; Sampath, W. S., committee member
Thin-film photovoltaics have the potential to make a large impact on the world energy supply. They can provide clean, affordable energy for the world. Understanding the device physics and behavior will enable increases in efficiency which will increase their impact. This work presents novel approaches for evaluating efficiency, as well as a set of tools for in-depth whole-cell and uniformity characterization. The understanding of efficiency losses is essential for reducing or eliminating the losses. The efficiency can be characterized by a breakdown into three categories: solar spectrum, optical, and electronic efficiency. For several record devices, there is little difference in the solar spectrum efficiency, modest difference in the optical efficiency, and large difference in the electronic efficiency. The losses within each category can also be further characterized. The losses due to the broad solar spectrum and finite temperature are well understood from a thermodynamic physics perspective. Optical losses can be fully characterized using quantum efficiency and optical measurements. Losses in fill factor can be quantified from series and shunt resistance, as well as the expected fill factor from the measured V oc and A. Open-circuit voltage losses are the most significant, but are also be the hardest to understand, as well as the most technology-dependent. Characterization of the whole cell helps to understand the behavior, performance, and properties of the cell. Several different tools can be used for whole-cell characterization, including current-voltage, quantum efficiency, and capacitance measurements. Each of these tools give specific information about the behavior of the cell. When combined, they can lead to a more complete understanding of the cell performance than when taken individually. These tools were applied to several specific CdTe experiments. They have helped to characterize the baseline performance of both the deposition tool and the measurement systems. Characterization of plasma-cleaned cells show an improvement in performance, even at thinner CdS layer thickness. Measurements of thinning CdTe samples reveal additional optical losses, likely caused by the increasing importance of the back diode. Characterization of Cd(S,O) devices show improved performance, both from improved optical properties and theorized improvement in band alignment properties. Uniformity can have an effect on whole-cell performance, but can also be an important parameter to characterize on its own. Light-beam-induced current is a powerful tool for characterizing uniformity. The LBIC tool was upgraded to improve its accuracy, functionality, and speed. The improved LBIC system aids in the collection of uniformity data. A number of parameters can be varied to provide in-depth uniformity information and help identify causes of nonuniformity. The wavelength can be varied to provide information on different layers. This can help identify variations in CdS thickness and local CdTe band gap. An applied voltage bias can be used to identify locations with weak diode properties. The resolution can also be varied to provide information on nonuniformities at different scales, from variations across the whole cell to variations on the size of several grains. LBIC can also be paired with electroluminescence to create a powerful nonuniformity characterization suite. The two can be paired with EL used as a screening tool to identify cells or areas which need further characterization from LBIC.
Open Access
Donor-appended sensitizers and further exploration of cobalt polypyridyl mediators: behavior and consequences in dye-sensitized solar cells
(Colorado State University. Libraries, 2014) Ashbrook, Lance, author; Rappé, Anthony, advisor; Gelfand, Martin, committee member; Kennan, Alan, committee member; Ladanyi, Branka, committee member; Shores, Matthew, committee member
Dye-sensitized solar cells (DSCs) have been thoroughly investigated over the past two decades as viable alternatives to traditional silicon solar cells. Fueling this research is the potential for DSCs to exhibit comparable efficiencies to silicon but at a fraction of the cost due to the generally cheaper materials employed. This dissertation presents studies conducted with cobalt polypyridyl mediators as substitutes for the more commonly employed I-/I3-. In addition, several novel sensitizers are synthesized incorporating electron donors in order to separate the injected electron and subsequent hole on the dye. Chapter 1 reviews a brief history of DSC development and the relevant processes in an operational cell. The interplay of these processes is discussed. Commonly employed materials are presented as well as alternatives used in the literature and in the work throughout this dissertation. Instrumentation and methods utilized throughout this work are also discussed. The use of copper polypyridyl dyes in DSCs is discussed in Chapter 2. While there is literature precedent for these materials as sensitizers, very few studies exist due to inherent issues to the sensitizers that are not shared with the more traditional ruthenium dyes. These problems are highlighted and discussed in the context of sensitizer design. One of the primary issues is the coordination of mediator additives to the oxidized copper center, rendering it unable to participate in further photoexcitation. Studies are presented that show the incorporation of a phenothiazine-type electron donor into the sensitizer results in rapid reduction of the copper center and prevents additional coordination. Electrochemical and cell testing studies are presented in Chapter 3 that partially explain why the addition of lithium ion to the mediator solution results in better DSC current values, particularly with cobalt mediators. The electrochemistry of the Co2+/3+ couple on FTO appears to be highly dependent on cations present in solution. Li+ present in solution results in current being "shut off" at the FTO surface. Thus, Li+ addition leads to an additional charge transfer resistance at the anode which leads to a reduction in undesired electron scavenging. Although platinum films or platinized FTO are the usual materials of choice for DSC cathodes, they generally perform better when used in conjunction with I-/I3-. The cobalt complexes employed as alternative mediators tend to exhibit more reversible electrochemistry on gold, but gold cathodes have historically been difficult to fabricate reproducibly. Chapter 4 probes a sulfide modification technique that appears to improve gold cathode performance. Based on the data presented, the mediator additive t-butyl pyridine weakly adsorbs to the gold surface which disrupts the electronic coupling with an oxidized cobalt complex. Modification with sulfide ion results in a lower charge transfer resistance at the surface which translates to a better fill factor. Finally, the last chapter further explores the use of incorporating a phenothiazine electron donor into the sensitizer. In this chapter, novel ruthenium dyes are synthesized and evaluated against some commonly employed sensitizers in the literature. The relevant processes are more difficult to elucidate in these systems than in the copper systems due to the similar absorption profiles of the Ru → ligand MLCT and oxidized phenothiazine. This makes the important technique of transient absorption more problematic to employ. Therefore, the effect of the donor is evaluated based primarily off cell testing data. The de-convolution of mass transport and donor effects is attempted by comparing with Z-907, which is a commonly used sterically demanding sensitizer. Additional experiments are also suggested which would offer more insight into this competition.
Open Access
Dose reconstruction in the large Japanese field mouse using electron paramagnetic resonance spectroscopy of tooth enamel
(Colorado State University. Libraries, 2020) Davis, Mariah, author; Johnson, Thomas, advisor; Brandl, Alexander, committee member; Gelfand, Martin, committee member
Electron spin resonance (ESR) analysis of tooth enamel is recognized as a reliable method for lifetime dose reconstruction, particularly in human tooth enamel. While the use of ESR to reconstruct dose is well understood for human tooth enamel, the reliability and usefulness of dose reconstruction using ESR in mouse tooth enamel has not been as thoroughly studied. This paper aims to resolve this gap in knowledge concerning the use of the tooth enamel from the Large Japanese Field Mouse as acting dosimeter using EPR spectroscopy. Methods of tooth preparation were analyzed to find a preparation method that resolved a baseline shift or slope in output signals of preliminary samples. Use of purity EDTA (ethylenedinitrilotetraacetic acid, disodium salt dihydrate) was initially found to reduce an observed baseline shift and slope in the output spectrum. Subsequent samples treated with EDTA, however, again saw baseline shifts. More needs to be done to analyze appropriate methodology to reduce the baseline shift, and to further determine the suitability of mouse teeth for ESR spectroscopy for reconstruction of lifetime dose.
Open Access
Electroluminescence of thin-film CdTe solar cells and modules
(Colorado State University. Libraries, 2015) Raguse, John Michael, author; Sites, James, advisor; Gelfand, Martin, committee member; Topič, Marko, committee member; Sampath, W.S., committee member; de la Venta, Jose, committee member
Thin-film photovoltaics has the potential to be a major source of world electricity. Mitigation of non-uniformities in thin-film solar cells and modules may help improve photovoltaic conversion efficiencies. In this manuscript, a measurement technique is discussed in detail which has the capability of detecting such non-uniformities in a form useful for analysis. Thin-film solar cells emit radiation while operating at forward electrical bias, analogous to an LED, a phenomena known as electroluminescence (EL). This process relatively is inefficient for polycrystalline CdTe devices, on the order of 10⁻⁴%, as most of the energy is converted into heat, but still strong enough for many valuable measurements. A EL system was built at the Colorado State University Photovoltaics Laboratory to measure EL from CdTe cells and modules. EL intensity normalized to exposure time and injection current density has been found to correlate very well with the difference between ideal and measured open-circuit voltage from devices that include a GaAs cell, an AlGaAs LED, and several CdTe cells with variations in manufacturing. Furthermore, these data points were found to be in good agreement when overlaid with calibrated data from two additional sources. The magnitude of the inverse slope of the fit is in agreement with the thermal voltage and the intercept was found to have a value near unity, in agreement with theory. The expanded data set consists of devices made from one of seven different band gaps and spans eight decades of EQELED efficiencies. As expected, cells which exhibit major failure of light-dark J-V superposition did not follow trend of well-behaved cells. EL images of selected defects from CdTe cells and modules are discussed and images are shown to be highly sensitive to defects in devices, since the intensity depends exponentially on the cells' voltages. The EL technique has proven to be a useful high-throughput tool for screening of cells. In addition to EL images, other opto-electronics characterization techniques were used to analyze defects in cells and modules such as weak-diode areas, cell delineation near substrate edge, non-uniform chlorine passivation, holes in back contact, high-resistance foreign layer, high back-contact sheet resistance, a discontinuous P3 line scribe (intercell shunt) and shunt through a cell (intracell shunt). Although EL images are proficient at illustrating the location and severity of defects with potentially high spatial resolution and short measurement times, their ability to identify the cause of such defects is limited. EL in concert with Light-Beam-Induced Current (LBIC), however, makes for a powerful ensemble as LBIC can probe different film layers at arbitrary voltage bias conditions, albeit with increased measurement times and potentially reduced spatial resolution.
Open Access
Energy transfer interactions with single molecule phenomena in small clusters of quantum dots
(Colorado State University. Libraries, 2014) Whitcomb, Kevin James, author; Van Orden, Alan, advisor; Bernstein, Elliot, committee member; Levinger, Nancy, committee member; Chen, Eugene, committee member; Gelfand, Martin, committee member
This dissertation describes the observed interactions between energy transfer in small clusters of nominally monodisperse semiconductor nanocrystals (quantum dots, QDs) and single molecule phenomena such as fluorescence intermittency (blinking) and antibunching. The relevant literature on energy transfer between QDs has typically invoked the Förster energy transfer mechanism to explain the observations in ensemble measurements. The size dispersion in QDs results in a dispersion in the electronic and optical properties of QDs due to size dependent confinement effects on photogenerated carriers. This size dispersion is thought to be the reason for energy transfer among nominally monodisperse QDs as in the single molecule work in this dissertation. The single molecule measurements in this dissertation were done using confocal microscopy and correlated atomic force microscopy (AFM). The experimental setup is described in detail. Confocal microscopy is used to excite a small region on a surface of sparsely deposited QDs or QD clusters. This allows for observation of individual QDs or individual clusters at a time. The fluorescence from these samples is collected through the microscope objective and spatially filtered using confocal techniques, i.e. spatially filtering the fluorescence with a pinhole. The excitation region can be correlated with a nanoscale topographical image using the light that is backscattered through the microscope objective by an atomic force microscope tip. This provides an additional method for distinguishing individual QDs from QD clusters. Methods for setup, alignment, maintenance of the instruments used will be described with sample preparation and practical measurement considerations. The interaction of energy transfer and QD blinking will be discussed in detail. The major findings are that the mechanism of energy transfer does not affect the individual blinking properties of QDs in a cluster, nor does the close proximity of other quantum dots. The findings will also show evidence that an individual QD governs the fluorescence state of the cluster through energy transfer. The clusters in this work were primarily identified and analyzed using fluorescence properties. The threshold in clusters is not as obvious as in individual QDs so an intensity threshold is set using a model of energy transfer that sets a threshold based on the lifetime. The findings impact future studies of QD clusters and applications that utilize QDs in close proximity to each other. The interaction of energy transfer and photon antibunching will also be discussed in detail. A simple model of energy transfer will be used to model the degree of antibunching in small clusters of QDs. The degree of antibunching observed in QD clusters is more characteristic of an individual emitter than multiple emitters which is a surprising find because it indicates that all QDs interact through energy transfer even in small nominally monodisperse aggregates. This work was done with correlated AFM to be sure that one QD or QD cluster is observed at a time. It is extremely important that only one emitter is in the excitation region because multiple independent emitters confound the analysis of antibunching and the observation of antibunching from multiple emitters heavily impacts single molecule study of QDs. Antibunching is thought to be the single definitive evidence that a single emitter is being probed but this is not so in the case of close proximity QDs even if the QDs are nominally the same size.
Open Access
Experimental realization of two-isotope collision-assisted Zeeman cooling
(Colorado State University. Libraries, 2013) Hamilton, Mathew, author; Roberts, Jacob, advisor; Lundeen, Stephen, committee member; Gelfand, Martin, committee member; Bartels, Randy, committee member
The work presented in this thesis focuses on the demonstration and initial evaluation of a novel non-evaporative cooling method called collision-assisted Zeeman cooling. For this realization, an ultracold gas consisting of a mixture of 87Rb and 8Rb was used. Cooling was accomplished through interisotope inelastic spin-exchange collisions that converted kinetic energy into magnetic energy. Continual optical pumping spin polarized the 85Rb which ensured that only kinetic energy reducing collisions occurred and the scattered pump photons carried entropy out of the system. Thus, cooling of the ultracold gas can be achieved without requiring the loss of any atoms in order to do so. This represents a theoretical advantage over forced evaporative cooling, which is the current state-of-the-art cooling technique in most experiments. This thesis discusses the details of collision-assisted Zeeman cooling, as well as how the theory of the technique has been extended from cooling a single species to cooling with two species. There are many predicted advantages from using two rather than one species of atom in this type of cooling: greater flexibility in finding favorable spin-exchange collision rates, easier requirements on the magnetic fields that must be used, and an additional means to mitigate reabsorption (the primary limitation in many if not most non-evaporative cooling techniques). The experimental considerations needed to prepare a system that simultaneously trapped two isotopes to be able to perform collision-assisted Zeeman cooling are discussed. Because this cooling scheme is highly reliant on the initial conditions of the system, a focused experiment examining the loading of the optical trap with both isotopes of Rb was conducted and the results of that experiment are described here. The first experimental observations of spin-exchange collisions in an ultracold gas mixture of Rb are described as a part of this work. The experiments where collision-assisted Zeeman cooling were demonstrated are then described and evaluated. In this first implementation of the cooling technique the initial densities were too low and optical-pump-induced heating and loss too high for achieving the full predicted performance of the cooling technique. Through additional modeling, these limitations were understood and the necessary improvements for the next iteration of CAZ cooling experiments are laid out at the end of this work.
Open Access
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.
Open Access
GPU-accelerated computational study of block copolymer self-assembly with advanced polymer theories
(Colorado State University. Libraries, 2024) He, Juntong, author; Wang, Qiang, advisor; Prasad, Ashok, committee member; Bailey, Travis, committee member; Gelfand, Martin, committee member
A high-performance GPU-accelerated software package for self-consistent field (SCF) calculations of block copolymer assembly, PSCF+, has been developed. PSCF+ allows various combinations of chain-connectivity models (including the continuous Gaussian chains, discrete Gaussian chains, and freely jointed chains), non-bonded isotropic pair (including the Dirac δ-function, soft-sphere, dissipative particle dynamics, and Gaussian) potentials and system compressibility (incompressible vs. compressible). The Richardson-extrapolated pseudo-spectral methods, the crystallographic fast Fourier transform, the "slice" algorithm, and the automated calculation-along-a-path are implemented in PSCF+, which not only speed up the SCF calculations and reduce the GPU memory usage significantly, but also make it very efficient in constructing phase diagrams. Given the wide use and great success of SCF calculations in understanding and predicting the self-assembled structures of block copolymer, PSCF+ will be an invaluable computational tool for the polymer community. Using PSCF+, we studied the stability of various Frank-Kasper phases formed by neat diblock copolymer (DBC) A-B melts using the "standard" model and the dissipative particle dynamics chain model and found that in general the SCF phase diagrams of these two models are qualitatively the same but with important differences. We also studied the stability of various Frank-Kasper phases formed by binary DBC blends using the "standard" model and found that the relative stability among the Frank-Kasper phases is dominated by their internal-energy densities. Finally, we performed high-accuracy SCF calculations to study the stability of all known tiling patterns formed by symmetrically interacting ABC miktoarm star triblock terpolymers.
Open Access
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.
Open Access
Improved methods for calculating the multifractal spectrum for small data sets
(Colorado State University. Libraries, 2014) Anderson, Leif, author; Eykholt, Richard, advisor; Robinson, Raymond Steve, committee member; Gelfand, Martin, committee member; Wu, Mingzhong, committee member; Shipman, Patrick, committee member
There are multiple definitions for fractal dimension, and those definitions disagree. The multifractal spectrum provides a unifying framework for a broad family of definitions of dimension, but it requires large amounts of data to compute. We provide a description of the multifractal spectrum, one existing improvement that improves convergence for small data sets: the Extended Box Algorithm (EBA), and develop several further improvements: Local and Global Norm modifications to the EBA, the a-Norm, the Variable Box size Algorithm (VBA), and the Patchwork method.
Open Access
Investigating the origins of slow magnetic relaxation of S = ½ Ni(III) cyclams
(Colorado State University. Libraries, 2023) Morrison, Thomas L., author; Shores, Matthew P., advisor; Zadrozny, Joe, committee member; Kennan, Alan, committee member; Gelfand, Martin, committee member
This dissertation describes the syntheses and characterizations of several Ni(III) and Ni(II) complexes in an attempt to better understand the origin of slow magnetic relaxation, or spin reversal, in S = ½ systems by utilizing Ni(III) cyclam (1,4,8,11-tetraazacyclotetradecane) as a toy model system. The content is organized as follows: Chapter 1 provides the historical context and theory surrounding the class of materials called single molecule magnets (SMMs). Therein I describe the prototypical SMM and its primary figures of merit and characteristics, such as S and D, followed by the observation of how S = ½ systems, which have previously been shown to act as SMMs, do not fit within the context currently provided by the literature. The choice of using the Ni(III) cyclam system is then elaborated upon, along with its quirks and foibles. In Chapter 2 I describe the synthesis and magnetic characterization of three Ni(III) cyclams. The first two contain halides in the axial positions, which are 100% abundant in isotopes containing nuclear spin, and the third complex has perchlorate bound in the axial position, where oxygen is nearly nuclear spin free. Neither halide systems showed slow magnetic relaxation, but it was not clear whether it was due to the superhyperfine coupling between the nuclear and electronic spins or due to the antiferromagnetic interactions present at low temperatures. The perchlorate containing complex did show slow magnetic relaxation, consistent with the literature and our predictions. Chapter three describes the crystallographic tuning tools and corresponding magnetic properties of novel S = ½ Ni(III) cyclam complex salts: strong antiferromagnetic coupling in sulfate-bridged chain {[Ni(cyclam)(µ2-SO4)]ClO4·H2O}n and field-, temperature-, and size-dependent slow magnetic relaxation in molecular [Ni(cyclam)(HSO4)2]HSO4. I have reported two methods of manipulating the dynamic magnetic response of these coordination molecules: particle size selection and deuteration. I find that particle size dependency, which I attribute to the phonon bottleneck effect, for the magnetic dynamics in the parent protiated compound is removed in deuterated isotopologue, revealing only the faster molecular relaxation mode(s). Chapter 4 describes the synthesis and characterization of four novel Ni(III) cyclams utilizing neutral ligands in the axial positions as opposed to the anionic ones considered previously, namely [Ni(cyclam)(acetonitrile)2]X3 (X = OTf, ClO4, BF4) and [Ni(cyclam)(butyronitrile)2]OTf3. Through these complexes we probe the role of ligand charge, identity, and subtle differences in the hydrogen-bonding network on the slow magnetic relaxation of the Ni(III) ion. Chapter 5 describes the solution phase studies of [Ni(cyclam)(MeCN)2]OTf3 and [Ni(cyclam)(butyronitrile)2]OTf3 in glassy and non-glassy solvents, as well as their suitability for studying other novel species in situ that may not be able to be synthesized and measured traditionally. We find that there are significant differences in the magnetic relaxation of the Ni(III) cyclams between glassy and non-glassy solutions and discuss the possibilities these findings present. In Chapter 6 I summarize the key findings from Chapters 2-5 and propose new avenues of research for further investigating this phenomenon. Finally, in Chapter 7 I describe a different ligand involving intra-ligand π-π interactions and explore the feasibility of using such interactions for intelligently controlling and tuning the first coordination sphere geometry and electronic structure. By introducing new substituents, changes to the aromaticity, and oxidation of the ligand we are able to exhibit rational control over the crystallographic and electronic structure of the metal center.
Open Access
Investigation of chiral porphyrin aggregates with heterodyne-detected vibrational sum frequency generation spectroscopy
(Colorado State University. Libraries, 2018) Lindberg, Kathryn A., author; Krummel, Amber, advisor; Levinger, Nancy, committee member; Sambur, Justin, committee member; Gelfand, Martin, committee member
In nature, photosynthetic organisms harvest and transport solar energy through the finely-tuned interplay between vibrational, electronic, and excitonic characteristics within photosynthetic reaction centers. These characteristics depend intimately on the precise arrangement of the reaction centers' molecular building blocks. Further knowledge of the relationship between structure and function in these natural systems is key to advancing synthetic solar technologies like dye-sensitized solar cells and artificial photosynthesis. Photosynthetic pigments, such as chlorophyll and bacteriochlorophyll, are of particular interest since their absorptive role is the first step in the solar harvesting process. Porphyrins, a group of macrocyclic organic compounds closely related to these pigments, have gained attention as simpler models for their more complicated natural counterparts. Tetra(4-sulfonatophenyl) porphyrin (TSPP), which closely resembles bacteriochlorophyll, is particularly valuable because it forms molecular aggregates analogous to the highly quantum-efficient light-harvesting "antennae" present in green sulfur bacteria chlorosomes. Imaging and spectroscopic studies indicate that the helical nanotubular TSPP aggregates are chiral and have distinct exciton contributions along different axes. However, the precise arrangement of TSPP monomers within the aggregate walls is still debated, prompting further, more detailed studies. Heterodyne-detected vibrational sum frequency generation (HD-VSFG) spectroscopy is a phase-sensitive, second-order nonlinear technique which probes the vibrational characteristics of noncentrosymmetric molecular environments. HD-VSFG experiments can also probe excitonic and vibronic characteristics via experimental double resonance. By use of polarization conditions, theoretical modeling, and computational fitting, detailed information on the orientation of vibrational, vibronic, and excitonic transition dipoles can be extracted from HD-VSFG spectra. This work presents doubly-resonant HD-VSFG spectra of TSPP thin films drop-cast on gold, which demonstrates the technique's sensitivity to the relationship between complex phase and excitonic versus monomeric characteristics. HD-VSFG is then used to compare spectra of TSPP thin films prepared from racemic and chiral aqueous solutions. This comparison includes a polarization condition sensitive to only chiral environments, further demonstrating HD-VSFG as a valuable tool in the structural investigation of TSPP aggregates.
Open Access
Measurement of the inclusive electron neutrino charged-current cross section in the NOvA near detector
(Colorado State University. Libraries, 2019) Judah, Matthew A., author; Buchanan, Norm, advisor; Gelfand, Martin, committee member; Harton, John, committee member; Pallickara, Sangmi, committee member
This thesis describes the methods used to extract the inclusive νₑ charged-current cross section in the NOνA near detector using data collected from November 2014 to February 2017, corresponding to an exposure of $8.09 x 10²⁰ protons-on-target of a primarily neutrino beam. The near detector is located at Fermilab, 800 m from the primary target. The neutrino beam peaks near 2 GeV and is able to probe a variety of different neutrino-nucleus interactions through their final-state characteristics. The flux-integrated double-differential cross section is measured with respect to the final-state electron kinematics, as well as the total cross-section as a function of neutrino energy integrated over the same phase space used for the double-differential measurement.
Open Access
Modeling and analysis of nanoscale surface patterns produced by broad beam ion bombardment
(Colorado State University. Libraries, 2020) Loew, Kevin M., author; Bradley, R. Mark, advisor; Gelfand, Martin, committee member; Shipman, Patrick, committee member; Wu, Mingzhong, committee member
When a solid surface is exposed to broad beam ion bombardment, nanoscale patterns may spontaneously form. This physical phenomenon is of interest to both the academic and nanofabrication communities. Ion bombardment has the potential to provide a cost-efficient method of producing nanoscale patterns over a large area. As such, it has gathered substantial interest and has been the focus of numerous studies, both experimental and theoretical. However, despite more than half a century of study, there are still many unknowns which limit the application of this method to fabrication. In this dissertation, I present contributions to the field of ion bombarded surfaces (IBS). The first is the development of a Python module which facilitates the rapid production and analysis of simulations. This module provides a well-documented tool to allow collaborators to numerically integrate a user-defined partial differential equation, specifically with IBS in mind. Second is a study of dispersive effects on IBS. Dispersion can lead to the formation of raised and depressed triangular regions traversed by parallel-mode ripples, highly ordered parallel-mode ripples, protrusions and depressions that are elongated along the projected beam direction even when there is no transverse instability, and needle-like protrusions that are visually similar to structures observed in experimental studies. Finally, we applied deep learning techniques to estimate the parameters in the underlying equation of motion from an image of a surface exposed to broad beam ion bombardment at a particular fluence. Our trained neural network will allow experimentalists to quickly ascertain the parameters for a given sputtering experiment.
Open Access
Modeling and controlling nanoscale patterns formed by bombardment with a broad ion beam
(Colorado State University. Libraries, 2017) Harrison, Matthew Paul, author; Bradley, R. Mark, advisor; Gelfand, Martin, committee member; Shipman, Patrick, committee member; Field, Stuart, committee member
For over half a century it has been known that bombarding a solid surface with a broad ion beam can produce periodic nanoscale structures. Given the virtually limitless promise of nanotechnology, the potential of ion bombardment to produce nanopatterned surfaces over large areas in a simple and economical way has attracted substantial interest. In the decades since its discovery, there has been a wealth of experimental and theoretical work examining the phenomenon in detail, with the eventual goal of using ion beam sputtering (IBS) to produce useful nanostructures. Despite the body of work, there are many open questions and unsurmounted challenges remain- ing. In this thesis, I present work that I have conducted in collaboration with my advisor, Mark Bradley, with whom I addressed some of these challenges. I show how we developed a formalism which connects information about single ion impacts to the evolution of a surface which sustains > 1016 such impacts per square centimeter. We have also produced theoretical results for the case of a binary material being bombarded while rotated azimuthally, with some unexpected findings. I also discuss some very exciting theoretical predictions for the case in which an elemental target is bombarded while the polar angle of ion incidence periodically changes. In this case we find the temporal driving can induce a surface pattern which is nearly perfectly periodic in the long time limit. I also discuss our work on using templated surfaces in conjunction with IBS to produce ii high quality blazed gratings and multilayer blazed gratings. This work is the subject of a current collaboration with Carmen Menoni and her students.
Open Access
Non-equilibrium states of disordered systems: from low-frequency properties of glasses to distribution function of active Ornstein-Uhlenbeck particles
(Colorado State University. Libraries, 2022) Shakerpoor, Alireza, author; Szamel, Grzegorz, advisor; Van Orden, Alan, committee member; Kim, Seonah, committee member; Gelfand, Martin, committee member
This dissertation focuses on stationary and dynamical properties of non-equilibrium systems of disordered matter. In particular, we discuss the correlation between the stability of ultra-stable to moderately stable amorphous solids and the structural fluctuations of the elastic field at low frequencies. We report a strong correlation between the stability and the structural homogeneity which we demonstrate numerically through the calculation of local elastic moduli of the solid. Notably, we do not identify any significant length scale associated with elastic correlations which bears specific implications for the wave attenuation in amorphous solids. In the second part of the dissertation, we shift our focus to the disordered systems of active matter. We derive a formal expression for the stationary probability density function of a tagged active particle in an interacting system of active Ornstein-Uhlenbeck particles. We further identify an effective temperature in the probability density function which allows for the subsequent numerical validation of our theoretical results beyond the linear response regime. We show that the effective temperature defined through the violation of the Einstein relation (or equivalently the fluctuation-dissipation theorem), can predict the tagged active particle's density distribution. Lastly, we derive theoretical expressions for the stationary probability density distribution and the current of a non-interacting active Ornstein-Uhlenbeck particle in a tilted periodic potential. We demonstrate the quantitative agreement of these expressions with our numerical results for small to moderate correlation times of the colored-noise. We further explore the dependence of the diffusive motion on the strength of tilting force. We observe a giant enhancement in the diffusion of the particle which becomes more pronounced with increasing the persistence time.
Open Access
Off-resonant RF heating of strongly magnetized electrons in ultracold neutral plasma
(Colorado State University. Libraries, 2021) Guthrie, John M., author; Roberts, Jacob, advisor; Fairbank, William, Jr., committee member; Gelfand, Martin, committee member; Wilson, Jesse, committee member
Magnetic fields are common in many plasma systems. Ultracold neutral plasmas (UCPs) are capable of not only accessing strong Coulomb coupling physics but also strong and extreme electron magnetization regimes, as well. These magnetization regimes, as defined by Baalrud and Daligault [S. Baalrud and J. Daligault, Phys. Rev. E, 96, 043202 (2017)], are predicted to modify screening or binary collision properties as the electron cyclotron radius approaches or subceeds the relevant plasma length scales. UCPs provide an advantageous testing ground for measuring magnetized electron-ion interactions, such as collisional heating induced by applied off-resonant RF fields. The experiments described in this thesis are focused on observations of RF heating in a UCP made from a photoionized cloud of ultracold 85Rb at three electron magnetization strengths that span the weakly-strongly magnetized boundary to the strongly-extremely magnetized boundary. Relative comparisons between heating rates at different magnetic fields were measured with ~20% precision, and an absolute determination of the heating rate near the weak-strong magnetization boundary is determined with ~40% precision. The results from these experiments were compared to theoretical predictions we developed that account for the finite-RF amplitude conditions used in the UCP measurements. This finite-amplitude heating rate theory is shown to be an extension of low-amplitude magnetized AC conductivity treatments as well as unmagnetized nonlinear collisional radiation absorption treatments. Mixed agreement was discovered between our observations and the theory for the three magnetic fields investigated: 10.6, 65, and 134 G. The measured absolute RF heating rate at 10.6 G and the relative rate between 134 and 10.6 G are in agreement with predictions within uncertainty; the relative rate between 65 and 10.6 G was observed to be a factor of ~3 lower than the predictions, with an absolute difference---in terms of the measurement uncertainty---on the order of 10σ. The implications of this disagreement are discussed, and future measurements that can be conducted with this technique are presented.