Mountain Scholar :: Browsing by Author "Neilson, James R., advisor"

Browsing by Author "Neilson, James R., advisor"

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Open Access
Defect tolerance, anharmonicity, and organic-inorganic coupling in hybrid organic-inorganic semiconductors
(Colorado State University. Libraries, 2018) Maughan, Annalise E., author; Neilson, James R., advisor; Prieto, Amy L., committee member; Reynolds, Melissa M., committee member; Sites, James R., committee member
Implementing and improving sustainable energy technologies is predicated upon the discovery and design of new semiconducting materials. Perovskite halides represent a paradigm shift in solar photovoltaic technologies, as devices utilizing perovskites as the active semiconductor can achieve power conversion efficiencies rivaling those of commercial solar cells after less than a decade of dedicated research. In contrast to conventional semiconductors, perovskites are unique in that they exhibit excellent photovoltaic performance despite the presence of significant materials disorder. This disorder manifests as (1) a large concentration of crystallographic defects introduced by low-temperature processing, and (2) as dynamic disorder due to the deformable metal-halide framework and the presence of dynamic organic species within the crystalline voids. Vacancy ordered double perovskites of the general formula A2BX6 are a defect-ordered variant of the archetypal perovskite structure comprised of isolated [BX6] units bridged by cationic species at the A-site. The presence of ordered vacancies and relatively decoupled octahedral units presents an ideal system to investigate defects and lattice dynamics as they pertain to optical and electronic properties of perovskite halide semiconductors. This work aims to illuminate the fundamental structure-dynamics-property relationships in vacancy-ordered double perovskite and hybrid organic-inorganic semiconductors through a combination of advanced structural characterization, optical and electrical measurements, and insight from computation. We begin with a study of the Cs2Sn1-xTexI6 series of vacancy-ordered double perovskites to inform the chemical and bonding characteristics that impact defect chemistry in vacancy-ordered double perovskites. While the electronic properties of Cs2SnI6 are tolerant to the presence of crystallographic defects, introducing tellurium at the B-site yields an electronic structure that renders Cs2TeI6 defect-intolerant, indicating the importance of the B-site chemistry in dictating the optoelectronic properties in these materials. Next, we elucidate the interplay of the A-site cation with the octahedral framework and the subsequent influence upon lattice dynamics and optoelectronic properties of several tin-iodide based vacancy-ordered double perovskites. The coordination and bonding preferences of the A-site drive the structural and dynamic behavior of the surrounding octahedra and in turn dictate charge transport. A-site cations that are too small produce structures with cooperative octahedral tilting, while organic-inorganic coupling via hydrogen bonding yields soft, anharmonic lattice dynamics characterized by random octahedral rotations. Both regimes yield stronger electron-phonon coupling interactions that inhibit charge transport relative to undistorted analogs. The final study presented here details the discovery of two hybrid organic-inorganic semiconductors containing the organic tropylium cation within metal iodide frameworks. In C7H7PbI3, the tropylium electronic states couple to those of the lead iodide framework through organic-inorganic charge transfer. Electronic coupling between the organic and inorganic sublattices within a singular material provides an avenue to elicit unique optical and electronic properties unavailable to either components individually. The above work is then placed in context of other recent studies of vacancy-ordered double perovskite semiconductors, and a set of design principles are constructed. Future avenues of research are proposed. These structure-dynamics-property relationships represent an important step towards rational design of vacancy-ordered double perovskite semiconductors for potential optoelectronic applications.
Embargo
Investigations of low-temperature reaction pathways in solid-state reactions
(Colorado State University. Libraries, 2024) Tran, Gia Thinh, author; Neilson, James R., advisor; Prieto, Amy L., committee member; Sambur, Justin B., committee member; Chen, Hua, committee member
Advances in our technology are limited by our knowledge of functional materials, and access to new, possibly better, functional materials is limited by our synthesis methods. This dissertation discusses different synthesis methods for a variety of solid state materials. At the core of this thesis are metathesis reactions i.e. double displacement reactions. Metathesis reactions allow for control over product selectivity and reaction kinetics with choice of the spectating ions. We demonstrate these characteristics with different spectating ions in metathesis and cometathesis (e.g., combining 2 halides) reactions. LaMnO3 was chosen to probe the product selectivity of anion cometathesis towards specific off-stoichiometries of LaMnO3. The metathesis reaction for BiFeO3 illustrates that prediction of thermodynamic selectivity is important, but reaction kinetics remain important as well. Kinetic studies of metathesis reactions that form YMnO3 demonstrate the importance of crystalline intermediates to modulate the reaction rates. The complexity of solid-state kinetics their kinetic regimes within a reaction can be identified through synchrotron X-ray diffraction. We attempted to synthesize LiMoO2 as precursors for the proposed phase LaMoO3. We demonstrate our considerations on the synthesis challenges and offer gained insights into alternative Mo-based systems (nitrides). Aside from metathesis reactions, we employ learned concepts to flux reactions to influence the chemical potential of N2. Synthesis of Li-Fe-O-N and Li-Mn-O-N phases was attempted under the hypothesis that alkali halide salt mixtures solubilize nitrogen and pin nitrogen's chemical potential to prevent N2 formation. Cs2SbCl6 was chosen as a single crystal target to gain clearer insights into the electronic structure. Single crystals were synthesized via hydrothermal synthesis, but preliminary conductivity measurements suggest that Cs2SbCl6 has a photoconductance below our limit detection.
Open Access
Local structure studies in functional materials and self-regulated learning interventions in general chemistry courses
(Colorado State University. Libraries, 2020) Paecklar, Arnold A., author; Neilson, James R., advisor; Reynolds, Melissa M., advisor; Rhodes, Matthew G., committee member; Finke, Richard G., committee member; Menoni, Carmen S., committee member
The first part of this dissertation is dedicated to understanding how the origin of the chemical and physical properties of functional materials is correlated to their structure. The standard approach to determining the structure of a crystalline material is to measure the average structure of regular, repeating units. However, this approach is not sufficient for more complex compounds including disorder. Hence, to fully understand the structure-property relationships of these advanced materials, identifying the local structure is crucial. This work focuses on designing approaches for optimizing the measurement of local structure data based on X-ray and neutron total scattering techniques as well as computational approaches for analyzing and understanding these data sets. The main focus lies in designing a novel system for collecting neutron total scattering data involving the controlled exposure of gasses to solid samples. Combining this setup with a Steady-State Isotopic Transient Kinetic Analysis system further enables the collection of kinetics data simultaneously with the structural data. This system was successfully used for studying and identifying the adsorption and reaction sites in porous materials such as zeolites and metal-organic frameworks. The disorder in these systems is based on the adsorbate which is a major contributor to the structure. However, there are also materials in which a single solid phase itself contains all the disorder. Some examples for disordered materials, covered in this work, are semiconducting perovskite materials with the general formula A2BX6. Computational approaches ranging from single to high-throughput Reverse Monte Carlo modeling were developed to gain more insight into anharmonicity and the interplay of the local structural features. Understanding how these specific local structural features influence desired physical properties will help guide the design of new materials covering a wide range of applications ranging from photovoltaics to biomedical devices. While the creation of such new knowledge in material science is important, we must also ensure that this knowledge is understood and transferred effectively. This effort does not only contain educating the general public but also fostering their curiosity and providing them the tools needed to learn that content knowledge. Succeeding in these endeavors is especially important during the first exposure to science courses. The second part of this dissertation focuses on the aspect of learning by looking at educational interventions in two different introductory general chemistry courses. The effectiveness of these interventions was evaluated based on data collected with paper-based, in-class surveys over the course of the semester. A multitude of self-regulated learning (SRL) measures were assessed and range from extrinsic motivation over self-efficacy to help seeking. Statistical analyses were used to identify differences between entire courses and individual sections exposed to the interventions. Additionally, the students' combined grades were also compared. Identifying the effective tools for helping students in chemistry courses is expected to have a major impact on changing the rate of failing students in such courses. This is the step needed for students to decide to become the next researchers contributing to the field with new scientific discoveries themselves.
Embargo
Navigating the thermodynamic landscape in search of synthetic routes to ternary nitrides
(Colorado State University. Libraries, 2022) Rom, Christopher Linfield, author; Neilson, James R., advisor; Prieto, Amy L., advisor; Sambur, Justin, committee member; Szamel, Grzegorz, committee member; Buchanan, Kristen, committee member
Ternary nitride materials—a class of ceramics composed of two different metals bound with anionic nitrogen (N3-) as a solid—are underexplored because they are difficult to make. Nitrides rarely occur in nature, as the oxygen in the air (O2) is more reactive towards metals than the nitrogen (N2). Consequently, oxide minerals dominate the earth's crust while nitride minerals are extremely rare. Almost all ternary nitrides that have been discovered have synthesized, usually with rigorously air-free conditions. Despite much effort in the past century, the number of known ternary nitrides (approximately 450) pales in comparison to that of ternary oxides (over 4,000). Yet there are world-changing materials within this small number of compounds, like the (In,Ga)N alloys that underpin efficient blue light emitting diodes. Fortunately, recent computational work has predicted a number of theoretically stable ternary nitrides, providing targets for synthesis. This dissertation focuses on the synthesis of new ternary nitrides. Guided by increasingly user-friendly computational tools, these chapters describe syntheses overcome the thermodynamic barriers that often inhibit the formation of new ternary nitrides. Along the way, several new materials are discovered and characterized for promising magnetic and semiconducting properties: MnSnN2, MgWN2 in two structure types, Mg3WN4, MgZrN2, CaZrN2, and CaHfN2. These adventures in synthesis not only report new compounds, but also highlight promising strategies for future explorations of uncharted nitride phase space.
Open Access
Organic cation dynamics and property relationships in layered perovskite derivatives
(Colorado State University. Libraries, 2022) Koegel, Alexandra A., author; Neilson, James R., advisor; Ackerson, Christopher, committee member; Kennan, Alan, committee member; Sites, James, committee member
Layered hybrid halide perovskites are materials with applications in solid-state lighting due to their intrinsic white light emission. Layered hybrid perovskite derivates typically have the composition, (A')2 (A)n − 1BnX3n + 1, where A' = R−NH + 3 containing organic cation, A = methylammonium (CH3NH + 3, MA), B = Sn, Pb, X = Cl, Br, I, and n = number of inorganic octahedral layers. They are called "hybrid" materials because of the inclusion of both organic and inorganic moeities in the material. Studies on the three-dimensional perovskite family have shown correlations between restricted rotational motion of the organic cation and structural phase transitions, and electronic properties. However, several questions remain about the coupling between structure, optoelectronic properties, and organic cation dynamics in layered perovskites. Here, I show that the restriction of the organic cation dynamics influences the static inorganic structure. The relevant excited states that produce the observed white light emission are also impacted by the cation dynamics. Chapter One is an overview of layered perovskites and how their structural diversity influences myriad properties. The excited state dynamics proposed in the literature are examined with respect to broad emission. Chapter Two goes in depth and describes the interplay between organic cation dynamics and broadband emission. Quasi-elastic neutron scattering elucidates the dynamic radii of the organic cation ammonium head groups and their role in tilting the inorganic octahedral structure. The smaller crystallographic ii volumes resulting from restricted cation dynamics induces further out-of-plane octahedral tilting. This tilting gives rise to the observed white light emission by the formation of self-trapped excitons. The ammonium headgroup rotations happen on a time-scale that is faster than the recombination of the self-trapped excitons, providing multiple environments for the excited state to sample, leading to inhomogeneous broadening of the white light. In perovskite derivatives, chemical substitution provides an opportunity to change the physical structure. Chapter Three demonstrates how changing the number of inorganic layers influences the cation dynamics. The methylammonium residence times, determined from quasi-elastic neutron scattering, are shorter in the layered perovskite with more inorganic layers. The dielectric screening provided by the increased number of methylammonium cations in the material with thicker inorganic enables the faster molecular motions to occupy larger crystallographic volumes. The inorganic layer hosts the relevant frontier electronic states necessary for broad emission. The population of these frontier states is influenced by a number of factors, namely the out-of-plane tilt angle. Chemical substitution of the inorganic layer affects the out-of-plane tilting; therefore, it is necessary to control the tilt angle as a variable in order to determine a more direct correlation between cation dynamics and white light. Chapter Four discusses the effect of isotopic substitution of the organic cation as a way to understand the influence of dynamics independent of tilt angle. Calculations using a harmonic oscillator approximation show the deuteration of the ammonium headgroup is iii closely coupled to the inorganic lattice, does not have much effect on the residence times of hydrogen motion. Halide substitution in the three-dimensional perovskites leads to reduced organic cation rotation residence times and further correlates to changes in electronic properties. Neutron spectroscopy presented in Chapter Five demonstrates how substitution of the halide site influences the cation dynamics and broadband emission in layered perovskites. Materials with broad emission have a lesser extent of hydrogen rotational motion, which follows previous trends in the literature. Chapter Six further demonstrates the effect of chemical substitution on broad emission and cation dynamics. The formation of solid solutions in the three-dimensional materials influence cation dynamics and phase transitions. White light emission at room temperature is achievable with solid solutions of layered perovskite derivatives. The extent of hydrogen motion determined from neutron scattering does not follow what is previously discussed in Chapter Two. Cation dynamics modify the static inorganic structure and optoelectronic properties in complex, excited state-mediated pathways. The identity of the organic cation dictates the overall perovskite structure and influences the tilting of the octahedra. The cation dynamics influence the broad emission in layered perovskite derivatives. Characterization of these coupled behaviors enable design principles for solid-state lighting applications.
Open Access
Organic fluxes as a tool for solid-state synthesis
(Colorado State University. Libraries, 2022) Fallon, M. Jewels, author; Neilson, James R., advisor; Finke, Richard G., committee member; Menoni, Carmen S., committee member; Buchanan, Kristen S., committee member
Solid-state materials allow us to charge our phones, store information on a computer, and harvest energy from the sun, among many other applications. They are the backbone of many modern technologies. However, making solid-state materials remains challenging. Traditional solid-state synthesis involves heating materials up to high temperatures to promote reactivity. These high temperatures make controlling the reactions and directing product formation difficult, as they generally form products that are stable at those high temperatures. There are limited techniques to make solid-materials, especially those that are not stable at high temperatures. In order to advance modern technologies based on solid-state materials, more well-understood, controllable synthetic techniques are necessary. This thesis describes a new technique for making solid-state materials. This technique is based on using molten organic materials, called organic fluxes, to enable selective reactivity between solids at lower temperatures. Owing to the lower reaction temperatures, this synthesis can form materials that are traditionally more difficult to make. The concept of an organic flux is introduced through a case study where triphenylphosphine, the organic flux, is used to make the low-temperature phase of iron selenide. This study demonstrates the efficacy of organic fluxes and provides insight to their mechanism of reactivity. Then, triphenylphosphine fluxes are further explored through reactions involving other metal chalcogenide binaries. By analyzing a variety of systems, the guiding principles behind the reactivity of triphenylphosphine fluxes are determined. Next, the ability of organic fluxes to aid materials discovery is shown through the formation of a new cobalt-selenium-triphenylphosphine complex. Finally, preliminary work exploring other organic fluxes and the future prospects for this synthetic scheme are discussed. This research introduces a new technique to target low-temperature materials. The tunability of organic fluxes enables the design of synthesis for selective reactivity in the solid-state. Adding to the library of synthetic tools available to solid-state chemists is a step towards materials discovery and the advancement of technologies based on solid-state materials.
Open Access
Organic-inorganic dipolar and quadrupolar coupling underlies the structure and properties of hybrid perovskites
(Colorado State University. Libraries, 2020) Mozur, Eve M., author; Neilson, James R., advisor; Prieto, Amy, committee member; Krummel, Amber, committee member; Sites, James, committee member
To view the abstract, please see the full text of the document.
Embargo
Synthesis and discovery of mixed-anion nitride materials
(Colorado State University. Libraries, 2023) Storck, Emily N., author; Neilson, James R., advisor; Ackerson, Chris J., committee member; Ma, Kaka, committee member
The ability to synthesize heteroanionic (or mixed-anion) materials is an important area in solid-state chemistry research. Mixed-anion compounds offer the potential to provide more desirable functionality compared to single-anion systems. However, mixed-anion systems are underexplored compared to single-anions. This is especially true for nitride materials when compared to oxides, because nitrides are difficult to make. The ease of making most oxides is due to the reactivity of oxygen and the thermodynamic stability of metal oxides, whereas the strong triple bond of N2 leads to its low reactivity and therefore difficulty in making nitrides and oxynitrides. Therefore, improved synthetic routes to produce these mixed-anion compounds are needed to unlock the potential of this underexplored phase space. This thesis describes the use of solid-state metathesis reactions to produce heteroanionic ZrNCl through reaction between AyNCl (A = Zn, Mg, or Li) precursors and ZrCl4. This thesis also highlights the use of flux reactions in attempts to synthesize new oxynitride materials based on the hypothesis that alkali halide salts have the ability to solublize nitrogen and raise its chemical potential relative to the chemical potential of nitrogen in traditional solid-state reactions to produce nitrides and oxynitrides, allowing for incorporation into products to form an oxynitride material. Here, a eutectic flux mix, LiCl-KCl, was used in the reaction between V2O3 and Li3N to synthesize vanadium containing compounds along with preliminary experiments to ascertain their stiochiometry.
Open Access
Towards understanding the atomistic disorder of synthetic bone mineral
(Colorado State University. Libraries, 2018) Marisa, Mary E., author; Neilson, James R., advisor; Bernstein, Elliot, committee member; Popat, Ketul, committee member
Biominerals are an interesting class of materials due to their complex structures and superior properties as compared to similar materials produced under laboratory settings. These complex structures often demonstrate a high level of control from the nano- to macroscopic scale. As a result, it is very difficult to create mimetic materials with hierarchical structures under laboratory conditions. Bone mineral, nominally calcium hydroxyapatite [Ca10(PO4)6(OH)2], shows a distinct, well known hierarchical structure from the individual nanoparticles of hydroxyapatite in the collagen matrix to the macroscopic bone. However, the atomistic structure of the apatite is not as well understood. This is due to the high level of chemical substitution and atomistic disorder. One of the most common chemical substitutions in bone mineral is the replacement of the tetrahedral phosphate ion with a planar carbonate ion. While several studies have attempted to understand this chemical substitution, there is not a consensus on the orientation of the carbonate ion in the phosphate site. Using X-ray or neutron diffraction as a structural determination tool is very useful for highly crystalline materials. However, the usefulness of these diffraction techniques decreases with increased disorder due to broadening of reflections which can obscure structural information. Instead, a total scattering technique, such as pair distribution function analysis, can be used to obtain an understanding of the local coordination environment. This, in conjunction with calculations of energy, can be used to identify the most likely substitution orientation. Using this method of structural determination, it is possible to conclude that the lowest energy substitution is the substitution of the planar ion into the mirror plane of the tetrahedral phosphate. Many biominerals formed in aqueous media, such as those found in bone, are synthesized via metastable or amorphous precursors. Crystallization pathways can be dependent on the species initially present in solution and other chemical factors such as pH. Bone mineral is of importance because of the medical implications in connection with various bone tissue diseases. Understanding the pathway through which biomimetic bone mineral is formed may inform targets for bone disease or improve processing for synthetic grafting materials. Here, the crystallization of biomimetic bone mineral is monitored via ex situ X-ray diffraction to determine the precursor phases. Samples prepared with and without exogenous carbonate are studied to determine possible factors which influence the rate of crystallization. Carbonate is chosen because of the known substitution for phosphate in bone mineral. This synthesis pathway from low pH to high pH shows that brushite, a hydrated calcium phosphate phase, is initially formed prior to precipitation of the desired apatite phase. However, the apatite phase appears more slowly in the carbonated samples. Analysis of the phosphate concentration via an ammonium molybdate assay shows that the non-carbonated synthesis has a steady decline of the phosphate throughout the reaction while the carbonated synthesis shows an induction period during which the phosphate concentration remains steady before having a sharp decrease.

Browsing by Author "Neilson, James R., advisor"

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