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Browsing by Author "Shores, Matthew, committee member"

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    Advancements in the chemical recyclability of acrylic polymers through investigation of monomer design
    (Colorado State University. Libraries, 2024) Gilsdorf, Reid Anthony, author; Chen, Eugene, advisor; Miyake, Garret, committee member; Shores, Matthew, committee member; Herrera-Alonso, Margarita, committee member
    Depolymerization is a key avenue of state-of-the-art recycling of polymeric materials. Although many polymers have been investigated for their ability to depolymerize, a subset of polymers has been widely left out of the conversation, polyolefins, or polymers containing C-C bonds in the polymer main-chain. Acrylic polymers are an important class of polyolefins used throughout the world in a variety of applications. One of the key drawbacks of the polymer, however, is their unfavorable depolymerization conditions, requiring high temperatures in expensive reactors. Although much work has been performed on the depolymerization of the most widely used acrylic polymer, poly(methyl methacrylate) (PMMA), there have been few reports on trying to improve upon the recycling methods, such as decreasing depolymerization temperature or gaining control over the depolymerization mechanism. In this work, key mechanistic steps of acrylic polymer depolymerization are investigated to gain fundamental understanding on the limitations faced during depolymerization and try to improve upon them. When poly(α-methylene-γ-butyrolactone) (PMBL) and poly(α-methylene-γ-methyl-γ-butyrolactone) (PMMBL) were investigated, the suppression of side reactions that occurred with PMMA depolymerization were identified, attributed to the pendant lactone tethering radical species together. Employing this tethering effect, the design of new polymers with pendant lactones and lower equilibrium polymerization temperatures (ceiling temperature or TC), was carried out, overall decreasing depolymerization temperatures and improving polymer recyclability. Finally, these new polymers were incorporated into the design of copolymers with PMMA and PMMBL in order to exploit the new polymers' depolymerizability without hindering thermomechanical properties. Overall, this work has shed light onto the importance of polyolefin design in, not just thermomechanical properties, but also polymerization and depolymerization behavior which will benefit the continued development of recyclable-by-design polymers.
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    Brillouin light scattering spectroscopy of phonons, magnons, and magnetoelastic waves
    (Colorado State University. Libraries, 2022) Nygren, Katherine Elise, author; Buchanan, Kristen S., advisor; Field, Stuart, committee member; Brewer, Samuel, committee member; Shores, Matthew, committee member
    This thesis discusses three projects that involve the propagation of waves through the utilization of an optical measurement technique known as Brillouin light scattering (BLS) spectroscopy. BLS spectroscopy measurements were completed using a six pass tandem Fabry-Pérot interferometer to detect light that has inelastically scattered from vibrational, spin, or magnetoelastic waves in a sample. This measurement method is noncontact, so wires do not need to be connected to the sample, nondamaging (unless the laser power is too high, and only for sensitive samples), and can detect nonlinear signals. The first project uses an antenna called an interdigital transducer to produce a surface acoustic wave. This wave travels across a piezoelectric substrate and couples to a spin wave in a nickel film. The coupled wave known as a magnetoelastic wave is then studied using BLS as a function of the external applied magnetic field. These results are used to help us understand how the magnetic resonance contributes to the coupled wave. Further BLS measurements as a function of distance across the nickel film are used to calculate a decay length of the magnetoelastic wave two orders of magnitude larger than the decay length for a pure spin wave in nickel. Second, we explore a device using a thin film of an organic ferrimagnet called vanadium tetracyanoethylene (VTCNE) that is magnetic at room temperature and has low damping, which rivals damping in high quality YIG films commonly used in microwave applications. Because VTCNE is oxygen sensitive it is encapsulated between two pieces of glass using an epoxy. The encapsulation does not change the damping, however due to magnetostriction, the strain of the epoxy may change the magnetic properties of the film. To understand how the epoxy strain can effect this device and others with similar encapsulation, we study thermal phonons in the encapsulation materials using Brillouin light scattering. The thermal phonon measurements along with phonon simulations allow us to calculate both the wave speeds and the elastic properties of the materials. These calculated properties can then be used to model future VTCNE devices. The final major project uses BLS spectroscopy to study spin waves in a Y-shaped structure of an iron nickel alloy. Using an in-plane externally applied magnetic field and an antenna across the top of the Y, we excite magnons in each arm of the Y, which then propagate into the base of the Y. BLS measurements are taken in each arm and the base of the Y, as a function of the driving frequency, and a 2D spatial map of the spin waves in the Y-structure was obtained to gain additional information on the modes that propagate past the junction of the Y. The BLS data in conjunction with simulations, demonstrate an indirect way to efficiently excite Damon-Eshbach spin waves as well as convert low wavevector spin waves in the arms of the Y into higher wavevector spin waves as they propagate into the base of the Y. The wavevector conversion and more efficient method of generating Damon-Eshbach spin waves are tools that can be exploited in magnonic device designs. Three additional spin wave projects are also discussed briefly. The projects include a yttrium iron garnet (YIG) confined structure, a VO2 film with a metal-insulator-transition near room temperature, and a heavy metal-ferrimagnet-heavy metal sample that should have a strong interfacial Dzyaloshinskii-Moriya interaction.
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    Catalytically generated acyl triazoliums as versatile acylating reagents and progress toward the total synthesis of okilactomycin
    (Colorado State University. Libraries, 2013) Wheeler, Philip Andrew Merris, author; Rovis, Tomislav, advisor; Shi, Yian, committee member; Ferreira, Eric, committee member; Shores, Matthew, committee member; Fisk, John D., committee member
    The first chapter of this dissertation describes the development of reactions involving the NHC-catalyzed acylation of carbon and nitrogen nucleophiles. The overall goal of this work was to expand the scope of the NHC-redox reaction manifold and improve its applicability to the synthesis of products that would be useful to the organic chemistry community. An efficient and simple procedure for the preparation of amides from amine hydrochloride salts and α,β-unsaturated aldehydes was developed. This procedure was then applied to the asymmetric synthesis of α-fluoroamides which are valuable building blocks for the preparation of fluorinated compounds that are highly sought after in pharmaceutical, material, and agrichemical applications. The second chapter describes efforts toward the total synthesis of the complex polyketide natural product okilactomycin, enabled by the rhodium-catalyzed desymmetrization of 3,5-dimethylglutaric anhydride developed previously by our group. Progress includes construction of the entire carbon skeleton in two fragments, poised to be unified and elaborated to the natural product by closely precedented steps. This progress demonstrates the potential of the catalytic, enantioselective desymmetrization of anhydrides to build complexity in rapid fashion.
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    Compositional tuning, crystal growth, and magnetic properties of iron phosphate oxide
    (Colorado State University. Libraries, 2017) Tarne, Michael, author; Neilson, James, advisor; Shores, Matthew, committee member; Ross, Kate, committee member
    Iron phosphate oxide, Fe3PO4O3, is a crystalline solid featuring magnetic Fe3+ ions on a complex lattice composed of closely-spaced triangles. Previous work from our research group on this compound has proposed a helical magnetic structure below T = 163 K attributed to J1-J2 competing interactions between nearest-neighbor and next-nearest-neighbor iron atoms. This was based on neutron powder diffraction featuring unique broad, flat-topped magnetic reflections due to needle-like magnetic domains. In order to confirm the magnetic structure and origins of frustration, this thesis will expand upon the research focused on this compound. The first chapter focuses on single crystal growth of Fe3PO4O3. While neutron powder diffraction provides insight to the magnetic structure, powder and domain averaging obfuscate a conclusive structure for Fe3PO4O3 and single crystal neutron scattering is necessary. Due to the incongruency of melting, single crystal growth has proven challenging. A number of techniques including flux growth, slow cooling, and optical floating zone growth were attempted and success has been achieved via heterogenous chemical vapor transport from FePO4 using ZrCl4 as a transport agent. These crystals are of sufficient size for single crystal measurements on modern neutron diffractometers. Dilution of the magnetic sublattice in frustrated magnets can also provide insight into the nature of competing spin interactions. Dilution of the Fe3+ lattice in Fe3PO4O3 is accomplished by substituting non-magnetic Ga3+ to form the solid solution series Fe3-xGaxPO4O3 with x = 0, 0.012, 0.06, 0.25, 0.5, 1.0, 1.5. The magnetic susceptibility and neutron powder diffraction data of these compounds are presented. A dramatic decrease of the both the helical pitch length and the domain size is observed with increasing x; for x > 0.5, the compounds lack long range magnetic order. The phases that do exhibit magnetic order show a decrease in helical pitch with increasing x as determined from the magnitude of the magnetic propagation vector. This trend can be qualitatively reproduced by increasing the ratio of J2/J1 in the Heisenberg model. Intriguingly, the domain size extracted from peak broadening of the magnetic reflections is nearly equal to the pitch length for each value of x, which suggests that the two qualities are linked in this unusual antiferromagnet. The last chapter focuses on the oxyfluoride Fe3PO_7-xFx. Through fluorination using low-temperature chimie douce reactions with polytetrafluoroethylene, the magnetic properties show changes in the magnetic susceptibility, isothermal magnetization, and neutron powder diffraction. The magnetic susceptibility shows a peak near T = 13 K and a zero field cooled/field cooled splitting at T = 78 K. The broad, flat-topped magnetic reflections in the powder neutron diffraction exhibit a decrease in width and increase in intensity. The changes in the neutron powder diffraction suggest an increase in correlation length in the ab plane of the fluorinated compound. Iron phosphate oxide is a unique lattice showing a rich magnetic phase diagram in both the gallium-substituted and fluorinated species. While mean-field interactions are sufficient to describe interactions in the solid solution series Fe{3-xGaxPO4O3, the additional magnetic transitions in Fe3PO7-xFx suggest a more complicated set of interactions.
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    Damping and switching in thin films and hetero-structures of magnetic materials and topological materials
    (Colorado State University. Libraries, 2020) Ding, Jinjun, author; Wu, Mingzhong, advisor; Camley, Robert, committee member; Field, Stuart, committee member; Roberts, Jacob, committee member; Shores, Matthew, committee member
    Yttrium iron garnet (Y3Fe5O12, YIG) materials have been widely used in microwave devices and have also shown high potential for magnonics and spintronics applications. This is because the fact that YIG materials have very low intrinsic damping and is electric insulating. The development of YIG-based spintronics demands YIG films that have a thickness in the nanometer (nm) range and at the same time exhibit low damping comparable to single-crystal YIG bulk materials. In this dissertation, the demonstration of using magnetron sputtering to grow high-quality polycrystalline nm-thick YIG films on gadolinium gallium garnet (Gd3Ga5O12, GGG) substrates is discussed in detail, which is of great technological significance as well as scientific research. The damping constant of the YIG films is the lowest among all the previous reports of nm-thick YIG films grown. Such demonstration of high-quality nm-thick YIG films proves the possibility of nanoscale patterning of YIG films and the future development of YIG-based nanoscale devices. Further, YIG thin films having a thickness of several nanometers and showing both strong perpendicular magnetic anisotropy (PMA) and low magnetic damping are realized in this dissertation. The phenomenon of spin pumping refers to the transfer of spins from precessional moments in a ferromagnet to a non-magnetic material. In a ferromagnetic/non-magnetic bi-layered system, spin pumping manifests itself as two distinct effects: (1) an enhancement in the damping in the ferromagnetic layer and (2) a pure spin current in the non-magnetic layer. This dissertation studies spin pumping effects in a ferromagnetic NiFe thin film associated with topological surface states (TSS) in a neighboring topological Dirac semimetal α-Sn thin film. Large damping enhancement due to the TSS of the Dirac semimetal alpha-Sn thin film is observed. Moreover, the spin current generated in the alpha-Sn film was utilized to switch a magnet through spin-orbit torque (SOT). The switching efficiency is comparable to that in topological insulators, which paves the way for the application of alpha-Sn thin films in future SOT-based magnetic memory. When a topological insulator (TI) is interfaced with a magnetic insulator (MI), it may host the anomalous Hall effect (AHE) and the quantum AHE associated with Berry-phase curvature in momentum space. This dissertation reports a bona fide topological Hall effect (THE) in a single magnetic phase TI/MI heterostructure (Bi2Se3/BaFe12O19) where the electrical transport is exclusively confined to the TI layer. Experimental observations are consistent with a THE originating from skyrmions in BaFe12O19 that are formed due to interfacial Dzyaloshinskii–Moriya interaction.
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    Development and application of conformational methodologies: eliciting enthalpic global minima and reaction pathways
    (Colorado State University. Libraries, 2014) Allison, Joseph T., author; Rappé, Anthony, advisor; Strauss, Stephen, committee member; Levinger, Nancy, committee member; Shores, Matthew, committee member; Slayden, Richard, committee member
    The information granted by assembling the global minimum and low-enthalpy population of a chemical species or ensemble can be utilized to great effect across all fields of chemistry. With this population, otherwise impossible tasks including (but not limited to) reaction pathway characterization, protein folding, protein-ligand docking, and constructing the entropy to characterize free energy surfaces becomes a reasonable undertaking. For very small systems (single molecule with 1-3 torsions) generating the low-enthalpy population is a trivial task. However as the system grows, the task exponentially increases in difficulty. This dissertation will detail the two sides of this problem, generating the low-energy population of larger and more complex species and then utilizing those populations to garner a greater understanding of their systems. The first discussion describes a new model, Surface Editing Molecular Dynamics (SEMD), which aids in accelerating conformational searching by removing minima from the potential energy surface by adding Gaussian functions. Accompanying this new method are a multitude of new tools that can be utilized to aid in molecular dynamics simulations. The first of these tools, named CHILL, performs a projection of unproductive degrees of freedom from the molecular dynamics velocity to smooth atomic motions without artificially constraining those degrees of freedom. Another tool, Conjugate Velocity Molecular Dynamics (CVMD), rigorously generates a list of productive velocities via the biorthogonalization of local modes with a vector representation of previously explored conformational minima. In addition to these tools, a new description of distance in torsional space was developed to provide a robust means of conformational uniqueness. With each of these tools working in concert, the global minimum and associated low-enthalpy population of conformations have been obtained for various benchmark species. The second section discusses the application of conformational searching and the subsequent electronic structure calculations to characterize the reaction pathway for the ruthenium tris(2,2'-bipyridine) photocatalyzed [2+2] cycloaddition of aromatically substituted bis(enones). The APFD hybrid density functional is used along with a 6-311+g* basis and a PCM solvent model. The reaction is computed to proceed through a rate-limited formation of a cyclopentyl intermediate. Lithium tetrafluoroborate is found to facilitate initial bis(enone) reduction as well as final product distribution. In addition, aromatic substituents are found to impact both initial reduction and final product distribution.
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    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.
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    Exploring excited states of transition metal photocatalysts with time dependent density functional theory
    (Colorado State University. Libraries, 2018) Nite, Collette M., author; Rappé, Anthony K., advisor; Shores, Matthew, committee member; Strauss, Steven, committee member; Sites, James, committee member
    Advances in photocatalysis have led to a rise in interests in more sustainable chemistry. It has been shown that visible light can be harnessed through a photocatalyst to promote conventionally unfavorable chemical transformations. Most of these photoreactions rely on a rare metal photocomplex such as Ru(bpy)32+. However, in order to scale these reactions for industrial purposes, rare metals must be replaced with more earth abundant metals. First row transition metals provide an earth abundant alternative that open up new reaction pathways. Due to the differences between first and second and third row transition metals, catalytic design requires complex knowledge of the photophysics and photochemistry of the complex that is not easily obtained with experimental methods. Electronic structure methods can aid in catalytic design. Density functional theory (DFT) and time dependent density functional theory (TDDFT) are methods capable of calculating large molecular systems. TDDFT is a useful tool in studying excited states, providing excited state energies and intensities, probing the photochemistry of the system. However, DFT/TDDFT are by no means black box calculations, especially when calculating first row transition metal complexes with complicated spin state manifolds. Screening different metal ligand scaffolds requires a high level of benchmarking, ensuring functionals and basis sets are optimal for the given system. A higher level of analysis is required in order to go beyond the electronic spectrum to get at the vibronic character of a system. There is also a coupling between the protonation of a complex and the electronic excited state. Understanding the protonation effects of a system is very useful for tuning a catalyst to a given reaction. In addition, specific binding effects of a solvent must be understood in order to corroborate theoretical and experimental data. All of these factors must be considered when studying the character of metals and their relation to their ligand backbone. This dissertation highlights these issues associated with using TDDFT for photocatalytic development, and derives useful conclusions furthering the development of a first row transition metal photocatalyst.
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    Investigation into catalyst interactions in a dye-sensitized photoelectrochemical cell for water oxidation catalysis
    (Colorado State University. Libraries, 2022) Jewell, Carly Francis, author; Finke, Richard, advisor; Shores, Matthew, committee member; Krummel, Amber, committee member; Sampath, Walajabad, committee member
    Solar energy has the potential to contribute significantly to solving the global energy crisis. However, solar energy is both diffuse and intermittent, meaning the capture and storage of this energy is critical. One method of storing this energy is the generation of storable hydrogen fuel via photoelectrochemical water-splitting, that is, storing energy in chemical bonds, specifically the H-H bond. However, the efficiency of the water-splitting process is limited by the water oxidation reaction, a four-electron process occurring at the anode. As such, water splitting devices, and more specifically water-oxidation devices, have been the focus of research for several decades. One such strategy, employed herein, uses molecular light-harvesting dyes and associated materials to capture and convert energy from the sun into chemical bonds. The work presented in this dissertation examines one such water-oxidation dye-sensitized photoelectrochemical cell (DS-PEC) with the goal of better understanding how charge-carrier interactions in the system are impacted by varying the system's catalyst, architecture and device composition. Throughout this dissertation a photoanode consisting of nanostructured SnO2 coated in perylene diimide dye N,N'-bis(phosphonomethyl)-3,4,9,10-perylenediimide plus photoelectrochemically deposited cobalt oxide (CoOx) is examined. Chapter I provides an in-depth overview to water-oxidation catalysis with a focus on the state of DS-PECs in the literature. Chapter II looks to understand the impact of an alumina overlayer on this DS-PEC system, with the specific goal of better understanding why the addition of the CoOx catalyst decreases photocurrent and increases recombination, a so-called "anti-catalyst" effect. The studies presented in Chapter II demonstrate that the presence of an ultrathin alumina overlayer by atomic layer deposition (ALD) increases photocurrents and decreases recombination in the device, although the addition of CoOx catalyst still decreases photocurrent. Chapter III examines the same system with the continued goal of identifying the source of increased recombination and decreased photocurrents with CoOx catalyst addition. Through a series of controls, residual carbon attributable to organic stabilizer used in the nanostructured SnO2 synthesis is discovered to be the culprit of this "anti-catalysis" effect. Anodes made using more carbon-free SnO2 deposited by ALD, rather than the nanostructured SnO2 with residual carbon, show an increase in photocurrents with CoOx addition. Subsequently, Chapter IV looks at two methods of overcoming and outcompeting the recombination attributable to residual carbon in the device. The effect of the residual carbon is shown to be mitigated through both the use of a more active iridium-based catalyst, amorphous Li-IrOx, rather than CoOx, and then through the use of a more carbon-free ALD-SnO2, without organic stabilizer, rather than nanostructured SnO2. The planar ALD-SnO2 is compared to the nanostructured SnO2 both on a per dye basis and on an electrochemically active surface area basis. The results presented in this dissertation offer fundamental insights into achieving both a better understanding, and an improved performance, of DS-PECs for water-oxidation catalysis that is a critical component of solar energy capture and storage.
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    Investigation of the dynamics of magnetic vortices and antivortices using micromagnetic simulations
    (Colorado State University. Libraries, 2017) Asmat-Uceda, Martin Antonio, author; Buchanan, Kristen S., advisor; Gelfand, Martin P., committee member; Wu, Mingzhong, committee member; Shores, Matthew, committee member
    This thesis is focused on investigating the dynamic properties of spin textures in patterned magnetic structures by using micromagnetic simulations. These textures become particularly relevant at sub-micron length scales where the interplay between magnetostatic and exchange energy leads to unique properties that are of great interest both from a fundamental perspective and for the development of new technologies. Two different systems, a magnetic antivortex (AV) stabilized in the intersection of perpendicular microwires, and three interacting vortices in an equilateral arrangement, were considered for this study. For the first system, the AV, the formation process and the excitation spectra were investigated. Since the AV is a metastable state, the design of a host structure capable of stabilizing it requires careful consideration and it is desirable to have general guidelines that could help to optimize the AV formation rate. The role of the shape anisotropy and the field dependence of the AV formation process is discussed in detail. Micromagnetic simulations along with magneto-optical Kerr effect and magnetic force microscopy measurements demonstrated that the asymmetry in the structure can be used to promote the formation of such AV's and that regions with lower shape anisotropy lead the reversal process, while simulations of the dynamic response show that when the system is excited with in-plane and out-of-plane external magnetic fields, normal modes with azimuthal and radial characteristics are found, respectively, in addition to the low frequency gyrotropic mode. The modes are influenced by the spin texture in the intersection, which offers additional possibilities for manipulating spin waves (SW). For the second system, three interacting vortices are simulated and compared with a simple analytical model that considers only dipolar interactions. It was found that when a fitting parameter is introduced to the model, the main features of the simulations are captured better than more complex models, which suggest that this simple framework can be used to accurately model more complex vortex networks.
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    Kinetic control of solid state metathesis reactions
    (Colorado State University. Libraries, 2017) Martinolich, Andrew J., author; Neilson, James, advisor; Prieto, Amy, committee member; Krummel, Amber, committee member; Shores, Matthew, committee member; de la Venta, Jose, committee member
    The control of solid state reaction pathways will enable the design and discovery of new functional inorganic materials. A range of synthetic approaches have been used to shift solid state chemistry away from thermodynamic control, in which the most energetically favorable product forms, towards a regime of kinetic control, so that metastable materials can be controllably produced. This work focuses on the use of solid state metathesis in the preparation of transition metal sulfides and selenides, and understanding the reaction pathways through which these reactions proceed. Through a range of structural probes combined with thermal analysis techniques, the reaction pathways are identified. The challenge of changing the pathway is then tackled, aiming to maximize mixing in the reaction mixtures to overcome the classical diffusion limitations in solids at low temperatures. Changing the reaction pathway promotes the formation of metastable intermediates and products, highlighted by the formation of the superconducting cubic polymorph of CuSe2. Future work is suggested, surrounding the idea of maximizing diffusion and mixing at low temperatures. Understanding the properties of reactants, intermediates, and products to direct the reaction pathway is paramount in controlling the pathways through which reactions occur. This will progress the field of synthetic solid state chemistry towards the ability to design materials and reactions that are not limited by thermodynamics, in turn yielding the discovery of a range of new, functional compounds.
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    Part I: Synthesis and characterization of titania and magnesium nanoparticles for hydrogen production and storage. Part II: Characterization and growth of branched silicon nanowires grown via a simultaneous vapor-liquid-solid and vapor-solid-solid mechanism
    (Colorado State University. Libraries, 2015) Shissler, Daniel Jay, author; Prieto, Amy, advisor; Shores, Matthew, committee member; Rappé, Anthony, committee member; Van Orden, Alan, committee member; Dandy, David, committee member
    To view the abstract, please see the full text of the document.
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    Quantum magnetism in the rare-earth pyrosilicates
    (Colorado State University. Libraries, 2021) Hester, Gavin L., author; Ross, Kate, advisor; Chen, Hua, committee member; Gelfand, Martin, committee member; Shores, Matthew, committee member
    In recent years, both physicists and non-physicists have shown immense interest in the burgeoning field of quantum computing and the possible applications a quantum computer could be used for [1]. However, current quantum computers suffer from issues of decoherence: where the quantum state used for computation is broken by external noise. A new possible avenue for quantum computation would be to use systems that are intrinsically protected from some level of noise, such as topologically protected states. Topological states are inherently protected from small perturbations due to their topological nature. However, to exploit this feature of topologically protected systems more experimental realizations are needed to better understand the underlying mechanisms. This has motivated a surge in interest of condensed matter systems with topologically protected states, such as the quantum spin liquid or fractional quantum Hall systems. A current focus in the subfield of quantum magnetism has focused on using the anisotropic exchange properties of the rare-earth (La - Lu) ions to find quantum spin liquid states, such as the Kitaev spin liquid that is predicted for systems exhibiting a honeycomb lattice. The Kitaev model is an exactly solvable model with a quantum spin liquid ground state, allowing for precise comparison between experiment and theory. Currently, no system has been rigorously proven to be a Kitaev spin liquid but developing our understanding of the underlying physical mechanisms in these systems may allow for the "engineering" of systems that are likely to be Kitaev spin liquids. The desire to understand the underlying mechanisms for quantum spin liquids and other quantum ground states led to the study of the three-honeycomb rare-earth pyrosilicate compounds discussed in this dissertation. The first compound, Yb2Si2O7, is a quantum dimer magnet system with the first evidence for a rare-earth based triplon Bose-Einstein condensate. Inelastic neutronscattering, specific heat, and ultrasound velocity measurements showed a characteristic (for triplon Bose-Einstein condensates) dome in the field-temperature phase diagram and provided evidence for predominantly isotropic exchange, something that is not typically expected for rare-earth systems. Following this work on Yb2Si2O7, our focused turned to two of the Er3+ rare-earth pyrosilicates. The first of these Er3+-based pyrosilicates measured was D-Er2Si2O7. Previous work on D-Er2Si2O7 discovered a highly anisotropic g-tensor, an antiferromagnetic ground state, and modeled some of the magnetic field induced transitions via Monte-Carlo simulations [2]. Our work followed up on this with AC susceptibility, powder inelastic neutron scattering, and powder neutron diffraction measurements to further investigate the ground state of this quantum magnet. Through this we discovered that the system enters an antiferromagnetic state with the spins almost aligned along the previously determined local Ising-axis [2]. The inelastic neutron scattering spectrum show a gapped excitation at zero field - consistent with Ising-like exchange. Transverse field AC susceptibility shows a change in the susceptibility at 2.65 T. These signatures indicate that D-Er2Si2O7 exhibits predominantly Ising-like exchange and that a transition can be induced by a field applied transverse to the Ising axis. This allows for the possibility of D-Er2Si2O7 bein g a new experimental realization of the Transverse Field Ising Model (TFIM). The TFIM is a simple, anisotropic exchange, theoretically tractable model exhibiting quantum criticality with few experimental examples, making new experimental examples of this model highly desired. These intriguing results on D-Er2Si2O7 and Yb2Si2O7 led to an interest in the polymorph formed at lower synthesis temperatures, C-Er2Si2O7, which happens to be isostructural to Yb2Si2O7. Measurements of the neutron diffraction, specific heat, and magnetization/susceptibility in this system allowed for us to determine that C-Er2Si2O7 magnetically orders at 2.3 K into an antiferromagnetic Néel state. While this is the expected ground state for an isotropically exchange coupled honeycomb system, C-Er2Si2O7 does not form a "perfect" honeycomb lattice and it is interesting that C-Er2Si2O7 magnetically orders while Yb2Si2O7 does not. Understanding the ground state for C-Er2Si2O7 will allow for bettering our understanding of Yb2Si2O7 and rare-earth quantum magnet ground states by comparing the properties of the two systems. Overall, the work on these three compounds required numerous experimental techniques, models, and theoretical understanding. It is my hope that the preliminary understanding for these three pyrosilicates will motivate future work within the rare-earth pyrosilicate family and provide a family of rare-earth quantum magnets that can be studied to improve our understanding of novel quantum states.
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    Spin multiplets: theory and application
    (Colorado State University. Libraries, 2018) Nite, Jacob M., author; Rappé, Anthony K., advisor; Shores, Matthew, committee member; Shi, Yian, committee member; Lee, Siu Au, committee member
    Transition metal complexes have seen an increased use as photocatalysts for organic reactions in recent literature, mostly involving the Ru(II)(bpy)3 family of catalysts. Due to the rarity of ruthenium in the Earth's crust, alternative catalysts using Earth abundant materials are desirable. Recent literature has shown that chromium based catalysts show great promise as a replacement for ruthenium for some reactions. The mechanisms of these first-row transition metal complexes are significantly more complex than those of the second and third row. The excited state complexities of first-row transition metal complexes are challenges for both experimental and theoretical research. The complexities of the excited states require theoretical methods beyond the standard single reference methods commonly used in the literature. Through the use of recent multi-reference post Hartree Fock (HF) methods as well as a new multi-reference density functional theory (DFT), insights into the character of chromium-based photocatalysts were examined. A new multi-determinant DFT method named few-determinant density functional theory (FD-DFT) was described. FD-DFT incorporates multiple DFT determinants using a finite difference approach to calculate the exchanges between multiple determinants for open shell multiplets. The method is implemented in a generalized bond valence (GVB) wave function, and can be converged through an SCF procedure. The system was benchmarked using oxygen atom and diatomic oxygen as well as atomic systems with more open shell orbitals. The benchmarking shows stability across many different functional choices, and gives good excitation energies with and without SCF convergence. The Cr(III)(AcAc)3 system has been long studied for its unique excited state properties that defy the standard cascade model for excited state relaxation. The tris(1,3-propanedionato)chromium(iii) (Cr(III)(PDO)3) complex was studied as an analog to the Cr(III)(AcAc)3 system to understand the excited state pathway between the initial excited 4T2g state and the long lived 2Eg state. Using the FD-DFT method as well as the multi-reference spectroscopy oriented configuration interaction (SORCI) method, the initial excited state energies were studied compared to previous perturbation theory (PT) approaches. Both SORCI and FD-DFT calculate reasonable 2Eg excitation energies, an improvement over earlier results. The SORCI method was also used to map the potential energy curve between the initial 4T2g excited state and its fully relaxed distorted structure. The pathway agrees with previous experimental and theoretical studies showing that a transitionless path exists between the quartet and doublet states, but spin-orbit coupling calculations suggest that a direct path between the 4T2g and 2Eg is possible rather than needing a internal conversion step to the lowest 2Eg state. Chromium-based photocatalysts have been recently studied in the literature as having a competitive mechanism between the reaction substrate and O2 whereby the O2 quenches the excited catalyst. Using the combined Cr(III)(PDO)3 • O2 system, the likely states by which this quenching event occurs were studied with FD-DFT as well as recent multi-reference PT approaches. Comparing the excited state calculated using the multi-reference based methods to standards DFT calculations shows the inability of single-determinant methods to correctly produce the proper excited state character even when obtaining somewhat reasonable energies. The excited state responsible for the quenching of the excited complex is identified using spin density plots of the CASSCF calculations. The search for suitable first-row transition metals requires a search across possible ligands and metal centers. Using the success of chromium-based catalysts, isoelectronic vanadium catalysts were studied to identify any potential differences between the complexes as well as identify the utility of vanadium-based catalysts. Using a variety of methods, including TDDFT-based absorption spectra, vibrational component plots of the excited state distortions, and SORCI potential energy curves (PEC), the differences between the chromium and vanadium catalysts were examined. It was found that vanadium catalysts absorptions are shifted significantly from chromium complexes and the vanadium excited states disperse the unpaired electron over the complex instead of localizing it on the metal center. The distortions in the chromium-based catalysts have a greater amount of asymmetric vibrational character compared to vanadium, which shows mostly symmetric behavior. Lastly, the SORCI PECs show that, unlike chromium, the doublet curves do not intersect the quartet curves, making a transition to a long lived doublet state a significantly slower process. The results highlight significant differences between the complexes even with ligand structure is controlled.
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    Synthetic control of magnetic resonance properties towards metal-based electron paramagnetic resonance imaging
    (Colorado State University. Libraries, 2023) Campanella, Anthony John, author; Zadrozny, Joseph, advisor; Shores, Matthew, committee member; Bandar, Jeff, committee member; Wu, Mingzhong, committee member
    Electron paramagnetic resonance imaging (EPRI) is the electron-spin analogue to conventional biological (nuclear) magnetic resonance imaging (MRI) whereby unpaired electron spins are probed in order to generate an image. The greater sensitivity of electron spins to their environment can thus be leveraged to capture detailed chemical information from the surroundings, producing an image of the local physiology that adds an extra dimension to the already powerful anatomical information gained from MRI. To move EPRI a step closer to common utilization, paramagnetic probes must be developed to sense the local environment using safe low-frequency microwaves at high (ca. 1.5 T) magnetic fields. Paramagnetic metal complexes are ideal candidates due to their electronic structures but have not been investigated for such purposes. The goal of this dissertation is to develop fundamental design principles to improve the utility of metal complexes as EPRI probes. Presented herein is the first comprehensive collection of experimental investigations to this end. Firstly, a method for improving signal sharpness is investigated, where exhaustive spectroscopic and computational studies suggest differences in relaxation dynamics as being a key factor in spectral linewidth (Chapters 2 and 3). A highly tunable clathrochelate structure is developed, inducing an unusual coordination geometry around the Ni(II) ion affording an 11 cm−1control of zero-field splitting (Chapter 4). The temperature dependence of zero-field splitting is examined in a series of Mn(II) complexes where an unusually high temperature sensitivity is found in the solid state (Chapter 5). Finally, the utility of metal complexes as environmental sensors is demonstrated with a pair of Mn(II) complexes showing that increasing magnetic anisotropy is a design strategy for enhancing microviscosity sensitivity (Chapter 6). The learned design principles will serve as a foundation for the design of metal-based EPRI agents towards improving the non-invasive diagnostic capabilities.
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    Understanding the solid electrolyte interface (SEI) on alloying anodes: development of a methodology for SEI sample preparation and x-ray photoelectron spectroscopy characterization and studies of the SEI on electrodeposited thin film intermetallic anodes for Li-ion batteries
    (Colorado State University. Libraries, 2020) Kraynak, Leslie A., author; Prieto, Amy L., advisor; Shores, Matthew, committee member; Strauss, Steven, committee member; Bandhauer, Todd, committee member
    The solid electrolyte interface (SEI) is an important component of Li-ion rechargeable batteries that forms due to the potential stability limits of the organic electrolyte falling within the large operating potential window of the battery. It plays a crucial role in battery performance by passivating the electrode surface; it also affects the safety, Li-ion consumption/inventory, and Li-ion transport rates of the battery. Despite decades of study, there is still much that is unknown about the SEI, especially how to intentionally modify the composition and properties of the SEI in order to obtain better performance as measured by metrics that include reversible capacity and cycle lifetime. The gaps in understanding of the SEI are even more pronounced for alloying anode materials, and the mechanical and chemical instability of electrode surfaces and the SEI formed from conventional secondary battery electrolytes is one of the bottlenecks in the development of next generation battery technologies. The first chapter of this dissertation is an overview of studies from the past two decades concerning the SEI formed on metallic alloying anodes, examining SEI formation, the evolution of the SEI over long term cycling, and improvements to the SEI through the use of additives and novel electrolytes. Compared to the body of literature on the SEI on other anode materials such as graphite, Li metal, and silicon, there has been relatively little published about the SEI on metallic alloying anodes such as tin, antimony, and intermetallics, especially considering the scope of these types of anode materials. However, a comparison of the existing literature concerning the SEI on alloying anodes reveals interesting similarities and difference between the SEI formation and evolution on metallic alloying anodes and highlights some critical gaps in knowledge for the field. The second chapter concerns the development of a methodology to study the SEI formed on alloying anodes, and in particular binder- and additive-free thin film electrodes. The formation, composition, and properties of the SEI are dependent on a number of experimental variables, which makes it difficult understand the factors that affect SEI performance and limits progress towards the goal of more controlled or intentional SEI formation for better battery performance. One of the first steps towards this goal is to be able to make and characterize SEI samples in a reproducible manner. This chapter outlines some of the important considerations for SEI sample preparation that are not widely discussed in the battery community in addition to some of the important considerations for using X-ray photoelectron spectroscopy to characterize the SEI. The third and fourth chapters are about using the methodology described in Chapter 2 to characterize the SEI formed on intermetallic thin film anodes. The third chapter examines the role that vinylene carbonate, a conventional SEI-improving electrolyte additive, plays in passivating the surface and extending the cycle lifetime of Cu2Sb electrodes. The fourth chapter is concerned with understanding what role the SEI plays in the cycle performance of pure phase SnSb thin film electrodes. Studying changes in the SEI on SnSb over different stages of cycling can help elucidate whether the SEI plays a role in the capacity retention and long cycle lifetime of SnSb and whether it ultimately contributes to the failure of the electrode.
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    Using antimony as a model anode to study the chemical and mechanical stability of electrodes in Li-ion and next generation batteries
    (Colorado State University. Libraries, 2019) Schulze, Maxwell Connor, author; Prieto, Amy, advisor; Shores, Matthew, committee member; Neilson, James, committee member; Weinberger, Chris, committee member
    As humanity grapples with the ever-increasing global demand for electrical energy, we are concurrently trying to curb global greenhouse gas emissions on massive scales to avoid potentially catastrophic changes in the global climate. Strategies to address these problems include transitioning away from a fossil fuel powered society where electrical grid energy is instead generated from renewable sources and internal combustion engine vehicles are replaced with electrified ones. Both of these transitions require energy storage technologies that can deliver high efficiencies, large energy densities, large power outputs, long lifetimes, and good safety factors all while remaining affordable and sustainable to produce. Li-ion batteries have already proven their merit as an effective energy storage technology with high enough energy densities, low enough costs, and long enough lifetimes to be ubiquitous in powering portable electronic devices. While the performance metrics of Li-ion batteries have also started to allow all-electric vehicles and grid-level energy storage to become commercially feasible, limitations in their cycle lifetimes and safety concerns arising from their flammable nature still limit their widespread implementation for these application. Ultimately, the interactions between constituent materials of a battery and the modes of their degradation limit a battery's performance. As such, research to understand and mitigate the degradation of battery materials, including those that move beyond Li-ion battery chemistry, is necessary to promote the widespread, tunable, and diverse use of batteries in overcoming the challenges discussed. Herein, I present a study that uses antimony as a model anode material to develop an understanding of the critical limiting factors of next-generation battery materials. Antimony-based anodes exhibit degradation and concomitant short cycle-lifetimes that are typical of many promising next-generation battery materials, including those that move beyond Li-ion chemistries. Thus, antimony-based model anodes can be used to study such degradation, which is primarily due to chemical and mechanical instability of the electrode and its interfaces with other battery cell components. In the following chapters, strategies to improve the chemical or mechanical stability of the antimony-anode and its interfaces are developed and can be more generally applied to other promising next-generation electrode materials. The following is a journal format dissertation, with each chapter being a document that is published, submitted, or in preparation to a peer-reviewed journal. The first chapter reviews the basic operating principles of rechargeable batteries as well as critically discusses the electrochemical experiments that are common in battery materials research. In particular, the first chapter emphasizes the limits of testing half-cell configurations in representing the cycle lifetimes of full-cell batteries, the key metric needed for long cycle lifetimes in full-cells being extremely high coulombic efficiencies. Chapter two explores and develops mitigation strategies for detrimental mechano-chemical interactions at the interface between the active Cu-Sb anode and the current collector that arise from the existence of a ternary Li-Cu-Sb phase with structural similarity to both Cu2Sb and Li3Sb. While the existence of the ternary phase results in good reversibility of Cu-Sb electrodes when cycled in Li-ion batteries, it also results in the formation of voids at Cu-Sb interfaces that exacerbates delamination during cycling to result in short cycle lifetimes. Chapter three develops a procedure for the electrodeposition of antimony carbon nanotube composites as a strategy to address the bulk mechanical instability of the anode during cycling in Li- and Na-ion batteries. Results of chapter three reveal significant chemical instability at the anode-electrolyte interface and motivate much of the work performed in chapter four, which departs from focusing on antimony as an anode material and instead uses antimony to explore the properties of anode coatings. Chapter four is a systematic study that explores how annealing conditions affect properties of polyacrylonitrile coatings relevant to the chemical stabilization of the electrode-electrolyte interface. This study reveals that ion diffusion in annealed polyacrylonitrile films is correlated to the delocalization of electrons in conjugated domains within the polyacrylonitrile films. Finally, chapter five reviews the materials properties that have made the Li-ion battery so successful, such as the mechanically and chemically stable interfacial layers that form at the electrode-electrolyte interfaces. The chapter additionally highlights some recent progress in the battery materials field and suggests that electrolyte additives, interfacial coatings, and solid-state electrolytes as the most impactful types of materials to continue researching and developing for the future.
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    Using electrochemical methods to synthesize and understand energy dense anodes for lithium-ion and "beyond" battery technologies
    (Colorado State University. Libraries, 2021) Ma, Jeffrey, author; Prieto, Amy L., advisor; Shores, Matthew, committee member; Finke, Richard, committee member; Weinberger, Chris, committee member
    To view the abstract, please see the full text of the document.
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    Utilizing silicon for the synthesis of tri- and tetrasubstituted olefins
    (Colorado State University. Libraries, 2013) Rooke, Douglas Alexander, author; Ferreira, Eric, advisor; Wood, John, committee member; Fisk, John D., committee member; Shores, Matthew, committee member; Peersen, Olve, committee member
    Functionalized organosilanes serve an important role as reactive precursors for a number of synthetic transformations. Consequently there is still great use for the development of new methods that allow for facile and efficient generation of organosilicon compounds. Herein, a number of such methods are described. The stereoselective syntheses of α-silylenones using catalytic PtCl2 are reported. Via alkyne activation, α-hydroxypropargylsilanes are converted to (Z)-silylenones through a highly selective silicon migration. A trans halosilylation of alkynes is also reported. Both the PtCl2 catalyzed silyl migration the halosilylation reaction proceed through a 1,2-silicon shift onto the activated alkyne intermediate in an anti fashion relative to the activating agent. Both reactions afford excellent yields and selectivity for the product tri- and tetrasubstituted alkenes. The high yielding Pt catalyzed hydrosilylation reactions of internal alkynes are described with a focus on understanding the factors that govern the regioselectivity of the process. Electronic, steric, and functional group properties all influence the selectivity, an understanding of which allows the selective formation of trisubstituted vinylsilanes, which are synthetically useful compounds for accessing stereodefined alkenes. Finally, efforts to show the synthetic utility of tri- and tetrasubstituted vinylsilanes for the formation of C-C bonds using Hiyama coupling and halodesilylation reactions are reported. Hiyama couplings of tetraorganosilanes with and without the use of fluoride activators are thoroughly evaluated. Coupling reactions with vinylsiloxanes are also shown. Finally, stereoretentive halodesilylation reactions are explored with the product vinylhalides subsequently subjected to Suzuki cross coupling conditions affording high yields of highly substituted all-carbon alkenes with good retention of alkene geometry.