<div><div><div><p>Lewis acid zeolites contain framework metal heteroatoms that catalyze sugar iso- merization reactions at different turnover rates depending on the local coordination around metal centers and the polarity of their confining secondary environments. Post-synthetic modification routes that react metal precursors with framework va- cancy defects in dealuminated Beta zeolites (Sn-Beta-PS-OH) are developed as an alternative synthetic strategy to the hydrothermal crystallization of Sn-Beta zeolites (Sn-Beta-HT-F). Post-synthetic routes provide the ability to systematically tailor the structural features of active and defect sites in Sn-zeolites, especially in composition ranges inaccessible to materials crystallized by hydrothermal routes (Si/Sn < 100; > 2 wt.% Sn), yet often result in incomplete or unselective Sn grafting within framework vacancy defects and form extraframework metal oxide domains and residual defect sites. The development of robust post-synthetic routes to prepare Sn-zeolites with intended active and defect structures has been limited by the dearth of characteri- zation techniques to unambiguously detect and quantify such structures present in stannosilicate materials, and of mechanistic links between such structures and the turnover rates of catalytic reactions.</p><p><br></p><p>The presence of framework Sn centers that can expand its coordination shell from four- to six-coordinate structures, and small extraframework tin oxide domains that cannot, were unambiguously detected from diffuse reflectance UV-Visible spectra of stannosilicate materials measured after dehydration treatments (523 K, 0.5 h) to discern ligand-to-metal charge transfer bands for tetrahedrally-coordinated Sn heteroatoms (< 220 nm, > 4.1 eV) and those for tin oxide domains (> 230 nm, < 4.1 eV). Liquid-phase grafting of stannic chloride in dichloromethane reflux (333 K) enables preparing Sn-Beta zeolites with higher framework Sn content (Si/Sn = 30– 144; 1.4–6.1 wt.% Sn) than grafting performed in isopropanol reflux (423 K, Si/Sn > 120; 1.6 wt.% Sn). This reflects competitive adsorption of isopropanol solvents with stannic chloride at framework vacancy defects during grafting procedures, consistent with infrared spectroscopy (IR) and temperature-programmed desorption (TPD) of dealuminated Beta samples after saturation with isopropanol at reflux temperatures (423 K), and not any limitations inherent to the structure of vacancy defects within dealuminated zeolite supports that would prevent reaction with metal precursors as often proposed.</p><p><br></p></div></div></div><div><div><div><p>This insight enabled preparing Sn-Beta zeolites with varying densities of residual defects, via dichloromethane-assisted grafting of stannic chloride to different extents, into dealuminated Beta supports of different initial Al content (Si/Al = 19–180) and mineralizing agent used for hydrothermal crystallization of the parent Al-Beta sam- ple (e.g., fluoride or hydroxide). Preparation of low-defect Sn-Beta zeolites using post-synthetic routes (Sn-Beta-PS-F) first required the synthesis of parent Al-Beta zeolites in fluoride media to minimize residual siloxy defects (OSi−) formed during crystallization, and dilute Al content (Si/Al > 100, < 0.6 Al (unit cell)−1), to min- imize the density of intrapore silanol groups formed after dealumination and high temperature oxidative treatment. The methanol packing density within microporous voids of Sn-Beta zeolites was assessed from relative volumetric uptakes at the point of micropore filling from single-component methanol (293 K) and nitrogen (77 K) adsorption isotherms, and decreased systematically among samples with increasing density of silanol groups. The total density of silanol groups within micropores and at external crystallite surface in Sn-Beta zeolites was quantified by H/D isotopic ex- change during temperature-programmed surface reactions (500–873 K), and within microporous voids from IR spectra measured after saturation of microporous binding sites with CD3CN (2275 cm−1, 303 K). In situ IR spectra collected at low methanol pressures (P/P0 < 0.2, 303 K) provide further evidence that methanol molecules ar- range in localized clusters within Sn-Beta-PS-F, but form extended hydrogen-bonded networks within Sn-Beta-PS-OH.</p><p><br></p></div></div></div><div><div><div><p>Glucose-fructose isomerization rate constants (373 K) were used to probe the lo- cal coordination of Sn heteroatoms and the polarity of the secondary environment as influenced by silanol defects within microporous cavities. Ex situ pyridine titration of Sn-Beta-HT-F samples suppressed isomerization rates (per total Sn, 373 K) after only a subset of Sn sites were poisoned, which correspond to the number of open Sn sites quantified ex situ via CD3CN IR (303 K), providing further evidence that open Sn sites are dominant active sites for glucose isomerization. First-order isomerization rate constants (373 K) decrease with increasing Sn content when normalized by total Sn density, and are invariant when normalized by the number of open Sn sites, be- cause open Sn sites are grafted preferentially within Sn-Beta-PS-OH (Si/Sn = 30–144; 1.4–6.1 wt.% Sn) at low Sn densities. Isomerization rate constants (per open Sn, 373 K), however, are lower by ∼4x and ∼15x on Sn-Beta-PS-F (Si/Sn = 284; 0.7 wt.% Sn) and Sn-Beta-PS-OH, respectively, than on Sn-Beta-HT-F. Open Sn sites catalyze aqueous-phase glucose isomerization at higher turnover rates (373 K) when their mi- croporous surroundings contain silanol defects present in low (hydrophobic) densities than high (hydrophilic) densities, which are characteristic of Sn-Beta-HT-F and Sn- Beta-PS-OH samples, respectively. This reflects reorganization of extended water networks, which are stabilized in high-defect, hydrophilic micropore environments, at kinetically relevant 1,2-hydride shift transition states that incurs entropic penal- ties that lower turnover rates. This thesis highlights the development of synthesis- structure-function relationships to guide the preparation of catalytic materials with intended active and defect site structures within confining reaction environments, the development of characterization techniques for the identification and quantification of such structures, and the influence of such structures on turnover rates of liquid-phase sugar isomerization.</p></div></div></div>

<p> Optical metamaterials have triggered extensive studies driven by their fascinating electromagnetic properties that are not observed in natural materials. Aside from the extraordinary progress, challenges remain in scalable processing and material performance which limit the adoption of metamaterial towards practical applications. The goal of this dissertation is to design and fabricate nanocomposite thin films by combining nitrides with a tunable secondary phase to realize controllable multi-functionalities towards potential device applications. Transition metal nitrides are selected for this study due to the inherit material durability and low-loss plasmonic properties that offer stable two-phase hybridization for potential high temperature optical applications. Using a pulsed laser deposition technique, the nitride-metal nanocomposites are self-assembled into various geometries including pillar-in-matrix, embedded nanoinclusions or complex multilayers, that possess large surface coverage, high epitaxial quality, and sharp phase boundary. The nanostructures can be further engineered upon precise control of growth parameters. </p><p> This dissertation is composed of a general review of related background and experimental approaches, followed by four chapters of detailed research chapters. The first two research chapters involve hybrid metal (Au, Ag) - titanium nitride (TiN) nanocomposite thin films where the metal phase is self-assembled into sub-20 nm nanopillars and further tailored in terms of packing density and tilting angles. The tuning of plasmonic resonance and dielectric constant have been achieved by changing the concentration of Au nanopillars, or the tuning of optical anisotropy and angular selectivity by changing the tilting angle of Ag nanopillars. Towards applications, the protruded Au nanopillars are demonstrated to be highly functional for chemical bonding detection or surface enhanced sensing, whereas the embedded Ag nanopillars exhibit enhanced thermal and mechanical stabilities that are promising for high temperature plasmonic applications. In the last two chapters, dissimilar materials candidates beyond plasmonics have been incorporated to extend the electromagnetic properties, include coupling metal nanoinclusions into a wide bandgap semiconducting aluminum nitride matrix, as well as inserting a dielectric spacer between the hybrid plasmonic claddings for geometrical tuning and electric field enhancement. As a summary, these studies present approaches in addressing material and fabrication challenges in the field of plasmonic metamaterials from fundamental materials perspective. As demonstrated in the following chapters, these hybrid plasmonic nanocomposites provide multiple advantages towards tunable optical or biomedical sensing, high temperature plasmonics, controllable metadevices or nanophotonic chips.</p><div><br></div>

The aim of the work was a processing of ceramic material based on BST for ferroelectric application. Wet chemical techniques based on precipitations and sol-gel methods with ultrasound, hydrothermal or mechanochemical treatment supporting deaglomeration and reducing particle growth were used for BST ceramic powder synthesis. Suitable powders were selected by the evaluation of particle morphology, size and agglomeration, from these powders BST bulk ceramic with defined phase composition and morphology applicable for ferroelectric applications was prepared. It was found, that the most suitable method for preparation of phase pure and nanosized BST powder was sol-gel synthesis with solvothermal treatment (200 °C/48 h). Ceramic with relative density of 85 % TD and with the average grain size of about 1, 22 µm was prepared by pressing and sintering of the powder synthesized by the sol-gel method.

<div>The present thesis was completed to satisfy two functions in our laboratory: (1) explore carbon-black (CB) as an additive for electrodeposited intrinsically conductive polymers (ICPs) to improve electrical properties across the electrode-electrolyte interface for use in neuromodulation; and (2) design a histology protocol that will analyze peripheral nerve system (PNS) tissue following implantation of conventional metal and modified conventional metal electrodes with the ICP poly(3,4-ethylenedioxythiophere):poly(styrenesulfonate)/carbon-black (PEDOT:PSS/CB). It would appear that the functions explored may seem unrelated, however, these two topics play a crucial role in designing a viable electrode for use in acute and chronic neuromodulation and the subsequent analysis required to determine the mechanical properties and overall biocompatibility of design.</div><div><br></div><div><div>A series of experiments with different PEDOT:PSS solutions containing varying amounts of suspended CB (n=19; 0 mg/mL to 2 mg/mL) were explored. Solutions were characterized using cyclic voltammetry (CV) using the intended electrode for deposition, composed of stainless steel (SS), as the working electrode (WE) to determine respective redox potentials. SS was chosen because of its inherently bad electrochemical properties, meaning that improved functionality post electrodeposition would be easy to identify. Immediately following CV, stainless-steel electrodes were electrodeposited using one of two techniques: (1) potentiostat, allowing the cell to rest at the redox potential required for bipolaron formation (0.9 V); or (2) galvanostat, where the electrode was submitted to a constant current of 200 mA and allowed to coat. Rapid electrochemical impedance spectroscopy was performed prior to and immediately following coating to determine the pre-electrochemical and post-electrochemical impedance characteristics. Results indicate that there was a positive relationship between the amount of CB additive and the relative impedance drop between the uncoated and coated counterparts. Furthermore, the modified electrochemical interfaces are substantially improved for use in frequency ranges of 10 Hz to 50 kHz, which encompass the ranges of our labs recently discovered low frequency alternating current (LFAC) for use in neuromodulation; thus indicating that PEDOT:PSS/CB modification may be used to improve impedance characteristics during our future LFAC experiments. This protocol, the one that contains the ideal concentration of carbon-black, was then recorded and will be used in our lab.</div></div><div><br></div><div><div>Histology protocols were developed to improve our labs capabilities of post-mortem analysis of PNS tissue. Processing and embedding preparations that explored included paraffin, acrylic, and frozen. Subsequently, staining protocols were developed; however, they varied as a function of the embedding media used; staining protocols developed incorporated progressive and regressive hematoxylin and eosin (H&E) staining as well as toluidine blue (TB). Tissue was sectioned and observed using light microscopy.</div></div>

<div> <div> <div> <p> </p><div> <div> <div> <div> <div> <div> <p> </p><div> <div> <div> <p>Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) and transition metal carbides/nitrides (MXenes), have been recently receiving attention for gas sensing applications due to their high specific area and rich surface functionalities. However, using pristine 2D materials for gas-sensing applications presents some drawbacks, including high operation temperatures, low gas response, and poor selectivity, limiting their practical sensing applications. Moreover, one of the long-standing challenges of MXenes is their poor stability against hydration and oxidation in a humid environment, which negatively influences their long- term storage and applications. Many studies have reported that the sensitivity and selectivity of 2D materials can be improved by surface functionalization and hybridization with other materials.</p><p>In this work, the effects of surface functionalization and/or hybridization of these two materials classes (TMDCs and MXenes) on their gas sensing performance have been investigated. In one of the lines of research, 2D MoS2 nanoflakes were functionalized with Au nanoparticles as a sensing material, providing a performance enhancement towards sensing of volatile organic compounds (VOCs) at room temperature. Next, a nanocomposite film composed of exfoliated MoS2, single-walled carbon nanotubes, and Cu(I)−tris(mercaptoimidazolyl)borate complexes was the sensing material used for the design of a chemiresistive sensor for the selective detection of ethylene (C2H4). Moreover, the hybridization of MXene (Ti3C2Tx) and TMDC (WSe2) as gas-sensing materials was also proposed. The Ti3C2Tx/WSe2 hybrid sensor reveals high sensitivity, good selectivity, low noise level, and ultrafast response/recovery times for the detection of various VOCs. Lastly, we demonstrated a surface functionalization strategy for Ti3C2Tx with fluoroalkylsilane (FOTS) molecules, providing a superhydrophobic surface, mechanical/environmental stability, and excellent sensing performance. The strategies presented here can be an effective solution for not only improving materials' stability, but also enhancing sensor performance, shedding light on the development of next-generation field-deployable sensors.</p> </div> </div> </div><div><div><div><div><div><div> </div> </div> </div> </div> </div> </div></div></div></div> </div> </div> </div></div></div></div><div><div><div> </div> </div> </div>

<div>The global demand for energy of mankind, the ever-increasing cost of energy, and the expected depletion of fossil energy carriers within the next centuries urge the exploration of alternative and more sustainable ways to provide energy. The current quest for energy-efficient technologies for the replacement of existing cooling devices has made the magnetocaloric effect a field of current scientific interest. Cooling technologies based on magnetic refrigerants are expected to have a better environmental impact compared with those based on the gas compression-expansion cycle. This technology provides an alternative for refrigeration applications with advantages, such as high energy efficiency, environmental friendliness, and low power consumption. In search of promising magnetocaloric materials, several rare earth-depleted transition metal-based materials were designed and investigated.</div><div>In this work, RCrxAl2-x and RZnAl (R = Gd, Tb, Dy, Ho) belonging to the ternary rare-earth transition-metal Laves phases, were chosen as the starting point to establish the effect of valence electron concentration (VEC) on the magnetic behavior and magnetocaloric effect. Our result and the results from the previously studied RTAl phases (T = Cu, Ni, Co, Fe, Mn) shows that the perturbation of the valence electron concentration at the Fermi level is found to be the driving force that dictates the crystal structure, magnetocaloric and magnetic properties of these systems. Most notably, the decrease in the valence electron concentration at the Fermi level leads to an increase in the curie temperature.</div><div>In addition, we have further extended this theory to GdNiAl2 systems. GdNiAl2 is a known magnetocaloric material which exhibits an isothermal magnetic entropy change of ΔSM = 16.0 Jkg-1K-1 at TC = 28K under a magnetic field change from 0-5T. However, the low TC limits its application as a room temperature refrigerant. We, therefore, substituted Co for (Ni/Al) in the structure of GdNiAl2, intending to substantially perturb the position of the Fermi level of Ni since that will lead to a decrease in the VEC and hence elevate the TC. The study was also extended to another Icosagenides (Ga,), which saw the substitution of Ga for Al in GdNiAl2 and its Co substituted analogs. The Ga analogs exhibit complex magnetic behavior with a cascade (multiple) of magnetic transitions, as opposed to the rather simple magnetism of their Al congeners.</div>

Transition-metal chemistry in ionic liquids (IL) has achieved intrinsic fascination in the last few years. The use of an IL as environmental friendly solvent, offers many advantages over traditional materials synthesis methods. The change from molecular to ionic reaction media leads to new types of materials being accessible. Room-temperature IL have been found to be excellent media for stabilising transition-metal clusters in solution and to crystallise homo- and heteronuclear transition-metal complexes and clusters. Furthermore, the use of IL as solvent provides the option to replace high-temperature routes, such as crystallisation from the melt or gas-phase deposition, by convenient room- or low-temperature syntheses. Inorganic IL composed of alkali metal cations and polynuclear transition-metal cluster anions are also known. Each of these areas will be discussed briefly in this contribution. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

<p>Impressive examples of the generation of hierarchically-patterned, three-dimensional (3-D) structures for the control of light can be found throughout nature. <i>Morpho rhetenor</i> butterflies, for example, possess scales with periodic parallel ridges, each of which consists of a stack of thin (nanoscale) layers (lamellae). The bright blue color of <i>Morpho</i> butterflies has been attributed to controlled scattering of the incident light by the lamellae of the wing scales. Another stunning example is the frustule (microshell) of the <i>Coscinodiscus wailesii</i> diatom, which is capable of focusing red light without possessing a traditional lens morphology. The photonic structures and the optical behaviors of <i>Morpho</i> butterflies and <i>Coscinodiscus wailesii</i> diatoms have been extensively studied. However, no work has been conducted to shift such light manipulation from the visible to the infrared (IR) range via shape-preserving conversion of such biogenic structures. Controlling IR radiation (i.e., heat) utilizing biogenic or biomimetic structures can be of significant utility for the development of energy-harvesting devices. In order to enhance the optical interaction in the IR range, inorganic replicas of biogenic structures comprised of high-refractive-index materials have been generated in this work. Such replicas of <i>Morpho</i> <i>rhetenor</i> scales were fabricated via a combination of sol-gel solution coating, organic pyrolysis, and gas/solid reaction methods. Diatomimetic structures have also been generated via sol-gel coating, gas/solid reaction, and then patterning of pore arrays using focused ion beam (FIB) milling.</p> Throughout the sol-gel solution coating and chemical conversion steps of the processes developed in this study, attention was paid to preserve the starting shapes of the nanopatterned, microscale biogenic or biomimetic structures. Factors affecting such shape preservation included the thicknesses and uniformities of coatings applied to the biogenic or biomimetic templates, nano/microstructural evolution during thermal treatment, and reaction-induced volume changes. A conformal surface sol-gel (SSG) coating process was developed in this work to generate oxide replicas of <i>Morpho rhetenor</i> butterfly scales with precisely-controlled coating thicknesses. The adsorption kinetics and relevant adsorption isotherm of the SSG process were investigated utilizing a quartz crystal microbalance. Analyses of thermodynamic driving forces, rate-limiting kinetic steps, and volume changes associated with various chemical reactions were used to tailor processing parameters for optimized shape preservation.

<div>Despite the abundant current applications for ionic hydrogels, much about the stimuli-responsive behavior of these materials remains poorly understood. Due to the soft nature of these materials, the number of traditional characterization methods which can be applied to these systems is limited. Many studies have been conducted to characterize bulk property responses of these materials, and experimental studies have been produced examining the distribution of free ions around single polyelectrolyte chains. However, little experimental work has been published in which molecular-scale interactions are elucidated in confined polyelectrolyte networks. Furthermore, the way in which responsive properties, other than bulk swelling capacity, scale with ionic fraction in mixed polyelectrolyte-non-polyelectrolyte hydrogel systems has not been thoroughly investigated.</div><div>The distribution and strength of polymer-counter-ion bonds has a remarkable effect on hydrogel properties such as absorption capacity, mechanical strength, and size and chemical selectivity. In order to tailor these properties for targeted applications in ionic environments, it is imperative that we thoroughly understand the character of these polymer-ion interactions and their arrangement within the bulk hydrogel. In order to do so, however, non-traditional methods of analysis must be employed.</div><div>This dissertation focuses on a model part-ionic hydrogel system, poly(sodium acrylate-co-acrylamide), in order to assess not only the polymer-counter-ion interactions but also the impact of gel ionic fraction on these interactions and the responses which they induce in gel performance properties. A model alkali (NaCl), alkaline earth (CaCl2), and transition (CuSO4) metal salt are employed to investigate changes in polymer properties from the macroscale to the nanoscale. The aim of this dissertation is to lay the foundation for the development of fundamental structure-property relationships by which we may fully understand the ionically-induced performance properties of polyelectrolyte networks.</div>

<div> <div> <div> <p>Transient liquid phase sintering (TLPS) technologies are potential low-temperature solders for sustainable replacements of lead-based solders and high-temperature lead-free solders. Compared to solid-state sintering and lead-free solders, TLPS uses lower temperatures and is, thus, suitable for assembling temperature-sensitive components. TLPS is a non- equilibrium process and determining the kinetics is critical to the estimation of processing times needed for good joining. The tin-indium (Sn-In) system with a eutectic temperature of 119°C is being considered as the basis for a TLPS system when combined with tin. Most models of TLPS include interdiffusion, dissolution, isothermal solidification, and homogenization and are based on simple binary alloys without intermediate phases. The Sn-In system has two intermediate phases and thus the reaction kinetics require additional terms in the modeling. Differential Scanning Calorimetry (DSC) has been used to measure the response of Sn-In alloys during the transient liquid phase reaction. Preparation of tin indium alloys for microstructural analysis is challenging due to their very low hardness. This study uses freeze-fracturing of the tin indium alloys to obtain sections for microstructural analysis. The combination of DSC and microstructure analysis provides information on the reaction kinetics. It was observed that the solid/liquid reaction does not proceed as quickly as desired, that is, substantial liquid remains after annealing even though the overall composition is in the single-phase region in the phase diagram. </p> </div> </div> </div>