<p></p><p>Global demand for high performance, low cost, and eco-friendly electronics is ever increasing. Ion/charge transport ability and mechanical adaptability constitute two critical performance metrics of battery and semiconductor materials, which are fundamentally correlated with their structural dynamics under various operating conditions. It is imperative to reach the mechanistic understanding of the structure-property relationships of electronic materials to develop principles of materials design. Nevertheless, the intricate atomic structure and elusive phase behaviors in the operation of devices challenge direct experimental observations. Herein, we employ a spectrum of modeling methods, including quantum chemistry, ab-initio modeling, and molecular dynamics simulation, to systematically study the phase dynamics and physico-mechanical behaviors of multiple electronic materials, ranging from transition-metal cathodes, polymer derived ceramics anodes, to organic semiconductor crystals. The multiscale atomistic modeling enriches the fundamental understanding of the electro-chemo-mechanical behaviors of battery materials, which provides insight on designing state-of-the-art energy materials with high capacity and high structural stability. By leveraging the genetic-algorithm refined molecular modeling and phase transformation theory, we unveil the molecular mechanisms of thermo-, super- and ferroelastic transition in organic semiconductor crystals, thus promoting new avenues of adaptive organic electronics by molecular design. Furthermore, the proposed computational methodologies and theoretical frameworks throughout the thesis can find use in exploring the phase dynamics in a variety of environmentally responsive electronics.</p><p></p>

<p>  </p> <p>Magnetism plays a vital role in the technologies of today, and materials used for magnetic applications largely consist of solid state phases. Intermetallic chalcogenides are one such material which have exhibited a full range of properties useful for a variety of applications requiring soft magnets, superconductors, magnetocalorics, and even rarer magnetic phenomenon such as 1D Heisenburg magnetic chains. Solid state chemists continue to develop new synthesis methods for chalcogenides as they produce both new phases and modifications of existing phases, usually with the express intent of improving their physical and chemical properties. Low dimensional chalcogenides often have predictable structure-property relationships which when understood aids in these efforts of optimizing existing materials.</p> <p>In this work, we have synthesized novel, low-dimensional Tl1-xAxFe3Te3 (A = K, Na)-based magnetocalorics for magnetic refrigeration technologies utilizing a variety of synthetic methods. Doping of alkali metals into the thallium site simultaneously reduces the toxicity and cost of the material, and also modifies their crystal structures leading to changes in their magnetic properties including ordering temperature, magnetic anisotropy, magnetic hysteresis, coercivity, and magnetic entropies. Most notably, the magnetic ordering temperature has been boosted from 220 K of the prior known TlFe3Te3 phase up to 233 K in the new Tl0.68Na0.32Fe2.76Te3.32 phase, further towards room temperature which is required for the commercialization of magnetic refrigerants for home appliances. There exist strong magnetostructural correlations for most of the alterations in the magnetic properties, and relationships have been modelled where trends exist to match the magnetism to the changes in the unit cell of the structure.</p> <p>New synthetic methods were also developed for the ternary TBi4S7 (T = transition metal) phase which exhibits a pseudo-1D structure of Heisenberg antiferromagnetic chains. These synthetic techniques resulted in more consistent high purity of phases than methods reported previously in literature. Attempts at synthesizing new phases were made, and crystallographic and composition analysis methods suggested the synthesis of a new Mn1-xCoxBi4S7 phase, though magnetic impurities prevented characterization of this new material’s magnetic properties. </p>

<p dir="ltr">Zeolites are microporous crystalline aluminosilicates that are widely used as catalysts for upgrading hydrocarbons and oxygenates to higher value chemicals and fuels. The substitution of tetrahedral Si⁴⁺ with Al³⁺ in a charge-neutral silica framework ([SiO_4/2]) generates anionic centers ([AlO_4/2]^-), which charge-compensate Brønsted acid protons (H⁺) that serve as active sites for catalysis. Brønsted acid sites in aluminosilicates of diverse topologies have similar acid strength, but can be located within varying intracrystalline (or internal) microporous environments (0.4‒2 nm diameter) or at extracrystalline (or external) surfaces and mesoporous environments (>2 nm diameter); yet, catalytic diversity exists, <i>even</i> for a fixed zeolite framework topology, because micropores impose constraints on molecular access to and from intracrystalline active sites and provide van der Waals contacts that influence the stabilities of reactive intermediates and transition states. Tailoring the material properties of a given zeolite framework for targeted catalytic applications requires strategies to design both the bulk crystallite properties (e.g., morphology, active site density) that influence intracrystalline diffusion and the secondary environments that surround active sites and influence intrinsic kinetics, and further necessitates molecular-level insights to elucidate the influences of bulk and active site properties on catalysis. In this work, we provide synthetic and post-synthetic strategies to respectively tune active site environments within varying micropore voids and at external surfaces of zeolites, and develop gas-phase toluene methylation and liquid-phase mesitylene benzylation as probe reactions to quantify the catalytic consequences of active site environments on aromatic alkylation catalysis.</p><p dir="ltr">The MFI framework (orthorhombic phase) consists of 12 crystallographic distinct tetrahedral-sites and 26 unique framework oxygen atoms located around channels (~0.55 nm diameter) or channel intersections (~0.70 nm diameter). The synthesis of MFI zeolites using the conventional tetra-<i>n</i>-propylammonium (TPA⁺) organic structure directing agent (OSDA) is known to place framework Al and their attendant H⁺ sites within the larger intersection environments, because electrostatic interactions are favorable between such locations of [AlO_4/2]^- and the quaternary N⁺ center in TPA⁺ that becomes positioned rigidly within channel intersections during crystallization. The methylation of toluene by dimethyl ether (DME; 403 K) on MFI-TPA zeolites of fixed active site densities (~2 Al per unit cell) result in <i>ortho</i>-xylene (<i>o</i>-X; ~65%) as the major product over <i>para</i>-xylene (<i>p</i>-X; ~27%) and <i>meta</i>-xylene (<i>m</i>-X; ~8%). In contrast, toluene methylation on MFI zeolites (~2 Al per unit cell) synthesized using non-conventional OSDAs, such as ethylenediamine (EDA) or 1,4-diazabicyclo[2.2.2]octane (DABCO), predominantly forms <i>p</i>-X (~75%) over <i>o</i>-X (~23%) and <i>m</i>-X (~2%). Within the subsets of MFI-TPA and MFI-EDA/DABCO zeolites, measured xylene formation rates and isomer selectivities are independent of crystallite sizes (0.1‒13 µm), toluene conversions (0.02‒2.0%) and external H⁺ content (up to 9% external H⁺ per total Al), indicating negligible effects of diffusion-enhanced secondary xylene isomerization reactions at intracrystalline or extracrystalline domains. The invariance of xylene isomer selectivity with reactant pressures (0.2‒9 kPa toluene, 25‒66 kPa DME) or methylating agent (1‒4 kPa methanol) indicate that differences in reactivity of toluene to form each xylene isomer reflects differences in the stabilities of their respective kinetically relevant transition states that share the same reactive intermediate. Measured xylene isomer formation rate constants and rate constant ratios, obtained from mechanism-derived rate expressions and interpreted using transition state theory formalisms, are used alongside density functional theory (DFT) calculations to reveal that intersection void environments (~0.70 nm diameter) similarly stabilize all three xylene transition states over unconfined surfaces (>2 nm diameter) without altering the established aromatic substitution patterns, while channel void environments (~0.55 nm diameter) preferentially destabilize bulkier <i>o</i>-X and <i>m</i>-X transition states thereby resulting in high intrinsic <i>p</i>-X selectivity. DFT calculations reveal that the ability of protonated DABCO complexes to reorient within MFI intersections and participate in additional hydrogen-bonding interactions with anionic Al centers during synthesis, facilitates the placement of Al in smaller channel environments that are less favored by TPA⁺. These molecular-level details, enabled by combining synthesis, characterization, kinetics and DFT, establish a mechanistic link between OSDA structure, active site placement and transition state stability, and provide active site design strategies orthogonal to crystallite design approaches that rely on complex reaction-diffusion phenomena.</p><p dir="ltr">For various reactions including toluene methylation at higher reaction temperatures (573‒773 K) and toluene conversions (>10%), extracrystalline H⁺ sites in MFI zeolites are reported to influence reactivity, selectivity, and deactivation behavior during catalysis in undesired ways. Post-synthetic chemical treatments to passivate external H⁺ sites on MFI zeolites result in unintended (but not always undesirable) changes to bulk structural properties and Al and H⁺ contents. The number of extracrystalline H⁺ sites is difficult to quantify using conventional spectroscopic or titrimetric methods, especially when present in dilute amounts on samples whose surfaces have been passivated. The systematic treatment of MFI zeolites (2.4, 5.7 and 7.1 Al per unit cell) using ammonium hexafluorosilicate (AHFS) at varying treatment duration times, AHFS concentrations and number of successive treatments resulted in MFI zeolites that retain their bulk structural properties and total Al and H⁺ contents, except for one parent MFI sample containing a significant amount of non-framework Al species. The benzylation of mesitylene by dibenzyl ether (363 K) occurs exclusively at external H⁺ sites because the bulky 1,3,5-trimethyl-2-benzylbenzene product is sterically prevented from forming at intracrystalline H⁺ sites. The intrinsic zero-order rate constant (per external H⁺) for mesitylene benzylation is extracted from rate measurements (per total Al) on a suite of untreated MFI samples with known amounts of external H⁺ sites (1‒15% external H⁺ per total Al) quantified using bulky 2,6-di-<i>tert</i>-butylpyridine base titrants. Measured zero-order rate constants on AHFS-treated MFI zeolites are used to quantify the extent to which AHFS treatments passivate external H⁺ sites, revealing efficacies that depend on the specific treatment conditions and the parent sample used. The developed kinetic methods demonstrate the utility of catalytic probes, when compared to stoichiometric probes based on spectroscopic or titration methods, in amplifying and quantifying dilute concentrations of external H⁺ sites on zeolites. The methods enable comparisons of the efficacy of various post-synthetic passivation strategies and permit rigorous assessments of the influence of external H⁺ during acid catalysis.</p><p dir="ltr">Overall, this work provides (post-)synthetic strategies to tune active site environments within intracrystalline micropores or at extracrystalline surfaces and develops quantitative kinetic probes that enable a molecular-level understanding of catalytic consequences of active site environments on aromatic alkylation reactions. Taken together, the methodology and findings of this study have broader implications in zeolite catalyst design for selectively upgrading traditional fossil feedstocks (crude oil and shale gas) and emerging feedstocks (biomass and waste plastics).</p>

<div>Photoconductive Semiconductor Switches (PCSS) are metal-semiconductor-metal devices used to switch an electrical signal through photoconduction. Rapidly switched PCSS under high bias voltages have shown remarkable potential for high power electronic and electromagnetic wave generation, but are dependent on precise optoelectronic material parameters such as defect ionization energy and optical absorption. These properties can be measured but are difficult to attribute definitively to specific defects and materials without the aid of high-accuracy, predictive modeling and simulation. This work combines well-established methods for first principles electronic structure calculations such as Density Functional Theory (DFT) with novel modern approaches such as Local Moment Counter Charge (LMCC) boundary conditions to adequately describe charge states and Maximally Localized Wannier Functions (MLWF) to render the summation of optical excitation paths as computationally tractable. This approach is demonstrated to overcome previous barriers to obtaining reliable qualitative or quantitative results, such as DFT band gap narrowing and the prohibitive computational cost of coupled electron-phonon processes. This work contributes electronic structure calculations of 4H-SiC doped with first-row transition metals (V through Ni) that are consistent with prior published work where applicable and add new possibilities for prospective semi-insulating metal-semiconductor systems where investigating new dopant possibilities. The results indicate a spectrum of highly localized, mid-gap, spin-dependent defect energy levels which suggest a wider range of potential amphoteric dopants suitable for producing semi-insulating material. Additionally, this work contributes MLWF-based calculations of phonon-resolved optical properties in 3C and 4H-SiC, indirect gap semiconductors, which accurately produce the expected onset of optical absorption informed by experiment. These results were further expanded upon with small V-doped cells of 4H-SiC, which while not fully converged in terms of cell size still provided a qualitative point of comparison to the ground state results for determining the true optical excitation energy required for substantial photoconductivity. The subsequent speculative analysis suggests the importance of anisotropic absorption and alternative metal defects for optimizing high current optoelectronic devices such as PCSS.</div>

<p>Co and Ni are two major elements in high temperature structural alloys that include superalloys for turbine engines and hard metals for cutting tools. The recent development of complex concentrated alloys (CCAs), loosely defined as alloys without a single principal element (e.g. CoNiFeMn), offers additional opportunities in designing new alloys through extensive composition and structure modifications. Within CCAs and Co- and Ni-based superalloys, solid-solution strengthening and stacking fault energy engineering are two of the most important strengthening mechanisms. While studied for decades, the potency and quantitative materials properties of these mechanisms remain elusive. </p> <p><br></p> <p>Solid-solution strengthening originates from stress field interactions between dislocations and solute of various species in the alloy. These stress fields can be engineered by composition modification in CCAs, and therefore a wide range of alloys with promising mechanical strength may be designed. This thesis initially reports on experimental and computational validation of newly developed theories for solid-solution strengthening in 3d transition metal (MnFeCoNi) alloys. The strengthening effects of Al, Ti, V, Cr, Cu and Mo as alloying elements are quantified by coupling the Labusch-type strengthening model and experimental measurements. With large atomic misfits with the base alloy, Al, Ti, Mo, and Cr present strong strengthening effects comparable to other Cantor alloys. </p> <p> </p> <p>Stacking fault energy engineering can enable novel deformation mechanisms and exceptional strength in face-centered cubic (FCC) materials such as austenitic TRIP/TWIP steels and CoNi-based superalloys exhibiting local phase transformation strengthening via Suzuki segregation. We employed first-principles calculations to investigate the Suzuki segregation and stacking fault energy of the FCC Co-Ni binary alloys at finite temperatures and concentrations. We quantitatively predicted the Co segregation in the innermost plane of the intrinsic stacking fault (ISF). We further quantified the decrease of stacking fault energy due to segregation.  </p> <p><br></p> <p>We further investigated the driving force of segregation and the origin of the segregation behaviors of 3d, 4d and 5d elements in the Co- and Ni-alloys. Using first-principles calculations, we calculated the ground-state solute-ISF interaction energies and revealed the trends across the periodic table. We discussed the relationships between the interaction energies and the local lattice distortions, charge density redistribution, density of states and local magnetization of the solutes. </p> <p><br></p> <p>Finally, this thesis reports on new methodologies to accelerate first-principles calculations utilizing active learning techniques, such as Bayesian optimization, to efficiently search for the ground-state energy line of the system with limited computational resources. Based on the expected improvement method, new acquisition strategies were developed and will be compared and presented. </p>