Doctor of Philosophy (Ph.D.), Bowling Green State University, 2022, Photochemical Sciences

A new, reversible photochemical transformation of V(V) tartrate clusters into V(IV) tartrate is described. While the initial V(V) tartrate cluster and photoproduct are known, this research uncovers a previously unreported photochemical transformation. Irradiation of V(V) tartrate in buffer at pH 5.8 showed color changes from orange yellow through a green intermediate to purple, developing a UV-vis spectrum characteristic of V(IV) tartrate. X–ray crystallography revealed an initial tetranuclear Na5[V4O8(2R,3R–tart) (2S,3S–tart)] •11H2O, and final photoproduct in solution which crystallized to a Na4[(VO)2(2R,3R–tart) (2S,3S–tart)] •12H2O dimer. EPR spectroscopy confirmed the reduction from diamagnetic V(V) to paramagnetic V(IV) photoproduct. CO2 was also produced during photolysis, indicating oxidation of the tartrate ligand during photolysis. These results highlight how light irradiation tunes the oxidation state of V(V/IV) clusters and thus these species have the potential to control the reactivity of these vanadium-based catalysts using light. Incorporation of the transition metal V(V) into hydrogels has been used to impart photo-responsive behavior which was used to tune materials properties during light irradiation. The photoreaction in the cellulose-agarose hydrogels coordinated with vanadium was evidenced by a clear color change of yellow to blue through a green intermediate. This color change was attributed to the reduction of V(V) to V(IV) as described above. A concomitant oxidative breakdown of the polysaccharide chain was noticeable upon the reduction of V(V) with a decrease in stiffness (G') of the hydrogel material. This reduction of the metal ion and breakdown of polysaccharide chain induce irreversible changes in the microstructure of the hydrogel which enables the controlled delivery of V(IV) and/or encapsulated cargo. Polyaniline (PANI) is a famous conductive polymer. When V(V) was reduced to V(IV) upon irradiation, this ultimately causes a (open full item for complete abstract)

Committee: Alexis D. Ostrowski Ph.D. (Committee Chair); Nathan S. Hensley Ph.D. (Other); Malcolm D.E. Forbes Ph.D. (Committee Member); George S. Bullerjahn Ph.D. (Committee Member) Subjects: Chemistry; Organic Chemistry; Polymer Chemistry

Doctor of Philosophy (Ph.D.), University of Dayton, 2018, Electrical and Computer Engineering

This dissertation reports different topics of VO2 thin films. First, the fabrication process of undoped and W-doped VO2 thin films. The pulsed laser deposition (PLD) was used to fabricate the undoped, 0.34 at. % W-doped, 0.54 at. % W-doped and 1.1 at. % W-doped VO2 thin films on sapphire substrates. Second, the measurements and analysis of undoped and W-doped VO2 thin films. Scanning electron microscope (SEM), X-ray diffraction (XRD) analysis and electrical resistivity of different W-doped VO2 thin films have been provided. Third, undoped and W-doped VO2 thin films based RF switches and reconfigurable antennas are reported. The single pole single throw (SPST) switches and reconfigurable antenna have been designed, fabricated and characterized for undoped and W-doped VO2 thin films integrated into those devices. The SPST switches are designed using coplanar waveguide (CPW) transmission lines, and the switches can be turned on or off by adjusting the temperature. At room temperature (20°C), the switch is in off state, and when the temperature is above 68°C, the switch is in the on state. The return loss (S11) of the switch is less than 1 dB at 20 °C, and it is better than 20 dB at 80°C. The isolation of the switch (S21) is better than 30 dB at 20 °C in the off state, and the insertion loss in the on state is less than 4 dB at 80 °C. The radio frequency (7 GHz) and W-band (93 GHz) CPW reconfigurable antennas have been demonstrated, and the wafers with undoped and tungsten-doped (0.1% to 1% tungsten) VO2 integrated with these antennas have been fabricated and tested. For the 0.54 at. % W-doped VO2 thin film based CPW bowtie antenna, the frequency tuning range is from 6.753 GHz with S11 -25.25 dB (10°C) to 6.346 GHz with S11 -30.26 dB (30°C), and the antenna is inactive when the temperature is above 40°C. For the 93 GHz antenna, when the W-doped VO2 is in low conductivity, the simulation shows that the resonant frequency of the antenna is at 92.9 GHz with S11 -51.4dB. Wh (open full item for complete abstract)

Committee: Guru Subramanyam Ph.D. (Advisor); Partha Banerjee Ph.D. (Committee Member); Monish Chatterjee Ph.D. (Committee Member); Robert Penno Ph.D. (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Materials Science

PHD, Kent State University, 2009, College of Arts and Sciences / Department of Physics

The study of quantum phase transitions (QPT) provides a new route to find and understand unconventional phases in condensed matter physics. The presently studied alloy, Ni(1-x)Vx, offers an opportunity to investigate a ferromagnetic quantum phase transition, a transition from a ferromagnetic ordered state into a paramagnetic state at T = 0 K, by varying the vanadium concentration, x. Magnetization and transport measurements are used to probe the critical behavior of the phase transition and characterize the onset of “unconventional behavior” such as non-Fermi liquid behavior, which signals a deviation from Fermi liquid theory, a fundamental concept in metals. Towards 11.2 % vanadium, the Curie temperature (Tc) is reduced to zero from its pure nickel value of Tc = 627 K. The critical behavior of the phase transition in samples with the higher nickel content (x < 11%) at a finite Tc essentially follows theories as expected for weak itinerant magnets. The samples with more vanadium (x > 11.2%) do not show a conventional ferromagnetic transition or the typical properties of an ordinary paramagnet. Instead, we see evidence for power laws with unusual exponents in the temperature dependence of the magnetization and the resistivity due to an inhomogeneous magnetic moment distribution. We compare our data findings with recent theories addressing a new critical scenario, quantum phase transitions with disorder. One signature is a Quantum Griffiths' phase which is observed as power laws with non-universal exponents heading towards a T → 0 instability. At very low temperatures, the quantum Griffiths phase in Ni-V leads to the formation of a frozen cluster glass phase. To our knowledge, our compound is the first to experimentally show all signatures of a quantum Griffiths phase in an extended regime, and therefore provides an ideal model system for a disordered itinerant 3-d Heisenberg system.

Committee: Almut Schroeder Dr. (Advisor); Carmen Almasan Dr. (Committee Member); David Allender Dr. (Committee Member); Songping Huang Dr. (Committee Member); Robert Twieg Dr. (Committee Member) Subjects: Physics

Master of Arts, The Ohio State University, 1906, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science (M.S.), University of Dayton, 2024, Electrical Engineering

With the rapid development of 5G technology and the explosive growth of the mobile industry, existing technologies have become increasingly mature. As a result, researchers and manufacturers are gradually focusing on designing more compact and cost-effective devices. The use of tunable devices is one of the promising technologies to achieve a compact size with few extra costs. In this thesis, a novel Sprout-Shaped Defected Ground Structure(DGS) and its tunable modification with thin film vanadium dioxide(VO2) are proposed. It has the advantage of compact size, clear in-band performance, and frequency tunability. The proposed single-stage structure provides -13.5 dB rejection at 4.008 GHz and has no harmonic before 10 GHz. With two-stage cascading, the rejection can be enhanced to -50.12 dB. This study also represents the tunable potential of the proposed DGS structure after modification, which can be tuned from the center frequency of 4.056 GHz to 5.987 GHz.

Committee: Guru Subramanyam (Committee Chair); Monish Chatterjee (Committee Member); Alex Watson (Committee Member); Guru Subramanyam (Advisor) Subjects: Electrical Engineering

Doctor of Philosophy, University of Toledo, 2023, Physics

Low-cost thin film absorber layer materials with high absorption coefficients (> 105 cm-1 in visible spectral range) and bandgap close to the ideal value for efficient photovoltaic conversion efficiency are leading candidates for thin film photovoltaic (PV) applications. This dissertation discusses the fabrication and optical and microstructural properties of magnetron-sputtered glancing angle deposited CdTe thin film absorber layer material and its application as an interlayer in CdS/CdTe solar cells. In addition, optoelectronic properties of non-toxic and earth-abundant absorber layer material, antimony selenide (Sb2Se3), and optimization of polycrystalline VO2 fabrication from amorphous vanadium oxide (VOx) film along with its optical properties have been discussed. Sb2Se3 is a promising candidate as an absorber layer material in PV applications. I have performed optical property characterization of thin film Sb2Se3 and identified electronic losses when used in a PV device. The indirect bandgap, direct bandgap, and Urbach energy have been determined to be 1.12 eV, 1.17 eV, and 21.1 meV, respectively using photothermal deflection spectroscopy. Optical properties of Sb2Se3 in the form of complex dielectric function (ε = ε1 + iε2) spectra in 0.75 to 4 eV spectral range is determined using spectroscopic ellipsometry. The line shape of ε is obtained using a parametric model which incorporates an Urbach tail, a band edge function, and five critical point oscillators. The optical property spectra in ε and structural parameters in terms of the thickness of solar cell layer components are used as input parameters for external quantum efficiency (EQE) simulation to investigate the electronic and optical losses in Sb2Se3-based solar cells. A carrier collection length of ~ 400 nm and a ~97 % carrier collection probability near the heterojunction in the Sb2Se3 solar cell are identified by comparing experimental and simulated EQE. Next, I describe deposition and characterizati (open full item for complete abstract)

Committee: Nikolas J. Podraza (Committee Chair); Robert W. Collins (Committee Member); Yanfa Yan (Committee Member); Song Cheng (Committee Member); Terry Bigioni (Committee Member) Subjects: Physics

Master of Science in Engineering, Youngstown State University, 2023, Department of Mechanical, Industrial and Manufacturing Engineering

This thesis presents a comparative analysis of the dissimilar laser welding of Ti6Al4V and Ni-Ti with two different interlayers, namely vanadium and niobium, using a continuous fiber laser welding machine. This study attempts to solve the problem associated with dissimilar welding of the Ni-Ti and Ti6Al4V with the use of interlayers specimen. The objective of this study is to improve the welding strength between Ni-Ti and Ti6Al4V in comparison to previous research and to investigate the effect of interlayer composition on the quality of the weld joint. The welding process was performed using identical laser power, welding speed, and focal position, and the quality of the weld joint was evaluated through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) analysis, hardness testing, and tensile testing. The welding was successfully performed using both interlayers. The tensile strength of the welded samples with niobium interlayer was found to be 100 MPa greater than that of the samples with vanadium interlayer. Scanning electron microscopy images showed that the fracture occurred at the welding region interface between the Ni-Ti-interlayer in both cases due to the dendritic structure, which caused the region to be more brittle. Furthermore, the hardness of the Ni-Ti-interlayer interface was higher, resulting in brittle fracture at the same interface in both cases during tensile testing. The findings suggest that the use of niobium interlayer produces a higher quality weld joint with improved mechanical properties under the same laser welding parameters compared to the vanadium interlayer. These results are significant for designing the laser welding process and selecting the appropriate interlayer for specific applications. Further research can be conducted to optimize the laser welding parameters and explore the impact of different interlayer thicknesses on the welding behavior.

Committee: Jae Joong Ryu PhD (Advisor); Virgil Solomon PhD (Committee Member); Kyosung Choo PhD (Committee Member) Subjects: Materials Science; Mechanical Engineering; Morphology

Doctor of Philosophy, The Ohio State University, 2023, Materials Science and Engineering

Phase transformations are commonly exploited in materials science for a multitude of technologies due to a coinciding property transition. However, the understanding of solid-solid phase transformations is still developing, especially from a local perspective (< 30 A). A suite of characterization and analysis techniques both proceeding and preceding the phase and property transition provides structural information starting at two angstroms. Operando and in situ experimental methodologies have advanced to the point that these structural phase transformations can be observed throughout the entire transition. The development of these advanced experimental characterization techniques with respect to x-ray diffraction, and total x-ray scattering along with large dataset analysis methodologies elucidated highly detailed local structure information. An archetypal first-order metal-insulator transition (MIT) material, vanadium dioxide, was first investigated using these techniques which were then extended to the battery cathode material, titanium trisulfide. From these analyses, the Peierls-Mott hypothesis of MIT origin was supported and local vanadium oxidation upon increasing tungsten-substitution was found to instigate a decrease in the change in properties across the transition temperature. For titanium trisulfide, it was found that kinetically driven Li-entrapment is decreasing the overall gravimetric capacity retention leading to premature battery death.

Committee: Stephen Niezgoda (Advisor); Michael Mills (Committee Member); Jinwoo Hwang (Committee Member) Subjects: Chemistry; Engineering; Inorganic Chemistry; Materials Science

Doctor of Philosophy, The Ohio State University, 2023, Electrical and Computer Engineering

This collection of works is an effort toward finding new solutions to challenges in electromagnetic devices, all connected by the implementation of vanadium dioxide (VO2). VO2 is a phase change material (PCM) that exhibits a reversible phase transition at 68 °C between monoclinic insulating and tetragonal conducting states. We designed, simulated, fabricated, and tested various devices in mmWave and optical wavelength domains. As an alternative to complex solid-state switching networks, we chose VO2 for its ability to react to various stimuli as a PCM, low phase transition temperature, and the freedom to design arbitrary geometries for reconfigurable and smart reactive devices. First, we experimented with the growth and characterization of VO2 thin films on sapphire and silicon substrates with Al2O3 buffer layers. Traditionally, VO2 has been deposited on sapphire substrates because of the lattice match between the two. This produces films with high resistivity contrast. However, sapphire is not as versatile a substrate material as silicon, being dielectric rather than semiconductor and extremely difficult to etch. To expand the realm of substrates useful for sputtering high quality VO2, we grew and compared such films on C-plane sapphire and silicon wafers with atomic layer deposited (ALD) alumina (Al2O3) films. Silicon has poor lattice match with VO2, and the alumina eliminates that interface. Furthermore, rapid thermal annealing (RTA) the alumina films before sputtering VO2 provides a basis for quasi-epitaxial films that have similar properties to those on the C-plane sapphire substrates. The figure of merit (FOM) resistivity contrast ratios for these variations are 9.76×104, 3.66×103, and 1.46×104 for C-plane sapphire, as- deposited amorphous ALD alumina on Si, and RTA ALD alumina on Si, respectively. We also characterized the films using X-Ray diffraction, atomic force microscopy, and scanning electron microscopy. In the next step, we examined the material (open full item for complete abstract)

Committee: Nima Ghalichechian (Advisor); Fernando Teixeira (Committee Member); Asimina Kiourti (Committee Co-Chair) Subjects: Electrical Engineering; Electromagnetics; Materials Science; Nanotechnology; Technology

Doctor of Philosophy, The Ohio State University, 2022, Physics

Quantum information science and engineering requires novel low-loss magnetic materials for magnon-based quantum-coherent operations. The search for low-loss magnetic materials, traditionally driven by applications in microwave electronics near room-temperature, has gained additional constraints from the need to operate at cryogenic temperatures for many applications in quantum information science and technology. Whereas yttrium iron garnet (YIG) has been the material of choice for decades, the emergence of molecule-based materials with robust magnetism and ultra-low damping has opened new avenues for exploration. Specifically, thin-films of vanadium tetracyanoethylene (V[TCNE]x) can be patterned into the multiple, connected structures needed for hybrid quantum elements and have shown room-temperature Gilbert damping (α = 4 x 10^(-5)) that rivals the intrinsic (bulk) damping otherwise seen only in highly-polished YIG spheres (far more challenging to integrate into arrays). However, while these properties clearly establish the potential of V[TCNE]x for new applications in traditional microwave electronics, very little is known about its low-temperature magnetization dynamics and therefore its potential for applications in quantum information science and technology. Presented in this thesis a comprehensive and systematic study of the low-temperature magnetization dynamics for V[TCNE]x thin films, with implications for their application in quantum systems. These studies reveal a temperature-driven, strain-dependent magnetic anisotropy that compensates the thin-film shape anisotropy, and the recovery of a magnetic resonance linewidth at 5 K that is comparable to room-temperature values (roughly 2 G at 9.4 GHz). We can account for these variations of the V[TCNE]x linewidth within the context of scattering from very dilute paramagnetic impurities, and anticipate additional linewidth narrowing as the temperature is further reduced. Additionally, ongoing work investigati (open full item for complete abstract)

Committee: Ezekiel Johnston-Halperin Professor (Advisor); Marc Bockrath Professor (Committee Member); Ciriyam Jayaprakash Professor (Committee Member); Christopher Hirata Professor (Committee Member) Subjects: Physics

Master of Science, Miami University, 2022, Chemical, Paper and Biomedical Engineering

Hexagonal boron nitride (hBN) is a material recently discovered to exhibit surprising catalytic activity for the oxidative dehydrogenation of hydrocarbons. Previous studies indicate that redox sites can be produced on hBN, which suggests that hBN may also have potential in other oxidation reactions. In this work, hBN is instead tested as a catalyst and catalytic support for the oxidation of methanol over a temperature range of 210-360 °C. Methanol oxidation additionally reveals changes in surface active sites due to the formation of characteristic products for acid, basic, and redox active sites. Thermal treatment and sonication of hBN are demonstrated to have significant effects on conversion and product selectivity during methanol oxidation reactions. Dispersion of vanadium oxide on hBN yields increased redox activity and methanol conversion due to formation of VOx groups on the surface of hBN. The supported vanadium catalyst is shown to have superior performance when hBN is also exposed to thermal treatment and sonication. Raman, XRD, and FTIR studies are performed to characterize changes in the catalyst due to treatment or vanadium dispersion. Experiments are also performed to measure changes in active sites due to in situ functionalization of hBN during propane oxidation.

Committee: Keith Hohn (Advisor); Catherine Almquist (Committee Member); Jason Boock (Committee Member) Subjects: Chemical Engineering

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2022, Photochemical Sciences

Nature provides a wide range of biopolymers that have been used over the years to create different materials with different properties. Among these biopolymers, we used polysaccharides to develop sustainable materials with unique properties. To enhance their properties, we tried recreating the hierarchal assemblies found in nature between soft organic and hard inorganic components. In other words, our approach was to use metal coordination to ligand groups present in the polysaccharides to make materials with unique mechanical properties and water stability. We also wanted to be able to use light to modify the properties of these materials or degrade them. We chose to work with Fe(III) and V(V) metal ions because these two metals ions showed photoreactivity with different ligands such as carboxylate-containing polyuronates such as alginate and pectin, and other polysaccharides such as chitosan and cellulose and small hydroxy acids such as tartaric acid. First, we studied the photoreactivity of V(V) with two polysaccharides, alginate and chitosan in aqueous solution. In both solutions, a decrease in viscosity was observed with light irradiation accompanied by a change in color from an initial yellow color to a blue color corresponding to the photochemical reduction of V(V) to V(IV) according to previous studies. Second, we made solid films from pectin and chitosan and improved their properties using V(V) ion coordination. V(V)-coordinated films showed increased strength and water stability compared to V(V)-free films. The photochemical reaction observed in solution was also observed in solid state. Finally, to further understand the photochemical reaction in solid state, we made films by blending two of the three polysaccharides, either pectin and chitosan or pectin and ᴋ-carrageenan with different Fe-species. We learned that rheological properties and photochemical properties can be tuned by changing the blend of polysaccharide (open full item for complete abstract)

Committee: Alexis D. Ostrowski Ph.D. (Committee Chair); Xiangdong Xie Ph.D. (Other); Alexander Tarnovsky Ph.D. (Committee Member); Joseph C. Furgal Ph.D. (Committee Member) Subjects: Chemistry; Polymer Chemistry; Polymers

Doctor of Philosophy, The Ohio State University, 2022, Chemical Engineering

Catalysts play an important role in the chemical industry since they help increase the speed of chemical reactions and reduce waste generation. Catalysts are typically functionalized onto the surface of solid supports such as mesoporous silica materials to ensure easy separation from the product and make the manufacturing process more economically viable. Mesoporous silica materials (pore size > 2 nm) such as SBA-15 and MCM-41 are generally preferred for supporting catalysts because of their thermal stability, robustness, and tunable pore architecture. Whereas such silica supported catalytic materials hold promise, key synthesis-structure-activity relationships remain to be discovered to enable commercial implementation of these laboratory-based catalysts. Such synthesis-structure-activity relationship studies are important to make supported catalysts more industrially viable and better performing than their homogeneous counterparts. The current work focuses on obtaining synthesis-structure-activity relationships by varying design parameters for the mesoporous silica support and catalyst analogue. This is done using two catalytic systems: (i) Knoevenagel condensation catalyzed by tertiary amines functionalized on SBA-15, and (ii) Oxidative dehydrogenation of propane catalyzed by vanadium oxides functionalized on SBA-15. Whereas the SBA-15 support is predominantly mesoporous, it also has secondary micropores (pore size < 2 nm) in its structure. Our results reveal that reducing the micropore volume in the SBA-15 support (NMP SBA-15) enhances the catalytic performance of tertiary amines for Knoevenagel condensation in comparison to their regular micropore counterparts (REG SBA-15). Such performance enhancement is observed for a wide range of tertiary amine densities on the SBA-15 surface. This indicates the generality of the inhibiting effect of micropores on performance of tertiary amines for this reaction. The micropore volume of SBA-15 is observed to significantly d (open full item for complete abstract)

Committee: Nicholas Brunelli (Advisor); Aravind Asthagiri (Committee Member); Jessica Winter (Committee Member) Subjects: Chemical Engineering

Master of Science (MS), Wright State University, 2021, Biochemistry and Molecular Biology

Silk fibroin from the silkworm, Bombyx mori, is a unique biomaterial that has been extensively studied for a variety of applications due to its promising properties such as controllable self-assembly, robust mechanical properties, and biological compatibility. Previously, there have been numerous methods describing the chemical modification of silk fibroin that utilize synthetic or enzymatic means that do not use halogens as a means of functionalization. Herein, a halogenation strategy is presented to modify silk fibroin with the aim of developing a novel functional material through the carbon-halogen (C-X) bond. Modification with NaX (X = Cl, Br, and I) salts, hydrogen peroxide (H2O2), and a vanadium dependent haloperoxidase (VHPO) from Curvularia inaequalis produced halogenated tyrosine residues along the protein's amorphous regions. Halogenation was confirmed using various methods including 1D and 2D 1H NMR and a chymotrypsin digest with LCMS. Secondary structure was analyzed by FTIR-ATR, circular dichroism (CD), and Raman spectroscopy which revealed that halogenated silk fibroin prefers helical conformations in solution and beta sheet structures when made into dried films but still has random coil content. Addition of halogens increased hydrophilicity on silk fibroin films evaluated by contact angle measurements. Finally, to showcase the C-X bond as a route for functionalization, Suzuki-Miyaura coupling was employed to add a fluorescent molecule, fluorescein, through a palladium catalyzed reaction scheme. Although coupling efficiency was observed to vary for chlorine, bromine, and iodine, the results demonstrate that this strategy can be used to add new functional groups on to silk fibroin which can potentially modulate the material's characteristics and enhance function.

Committee: Patrick Dennis Ph.D. (Committee Chair); Weiwen Long Ph.D. (Committee Member); Michael Leffak Ph.D. (Committee Member) Subjects: Biochemistry; Materials Science

Doctor of Philosophy, The Ohio State University, 2021, Mechanical Engineering

With the advantages of high bandwidth and abilities to see through opaque materials, millimeter wave (mmW) band (30 to 300 GHz) has been intensively explored in recent years. Although there are increasing demands for reconfigurable mmW systems for their potential applications in defense, switching, imaging, and sensing, overcoming the limitations such as high losses and large power consumption in mmW systems is still a challenge. Phase change materials (PCM) like vanadium dioxide (VO2), which have novel and tunable physical properties such as electrical resistivity and optical transmittance, are appealing choices for mmW reconfiguration to provide faster operation speed and lower loss microsystems. One aspect of VO2 thin film that is not fully exploited is the metal-insulator transition (MIT) region, where the electrical resistivity changes about four orders of magnitude with external stimuli. In this work, we present a highly sensitive antenna-coupled VO2 microbolometer for mmW imaging. The proposed microbolometer takes advantage of the large thermal coefficient of resistance (TCR) of VO2 at the non-linear region. The thermal resistance of the device is significantly improved by micro-electro-mechanical systems (MEMS) techniques to suspend the device above the substrate, compared with non-suspended microbolometers. The finite element method is employed to analyze the electrothermal and electromagnetic performance of the device. The frequency range of operation is 65 to 85 GHz, and the realized gain at broadside is > 1.0 dB. Simulation results indicate a high responsivity of 1.72x10^3 V/W and a low noise equivalent power (NEP) of 33 pW/√Hz. Targeting for broader applications, it is highly desired to deposit VO2 thin films on silicon (Si) substrate. Here, we employ the annealed alumina (Al2O3) buffer layers to obtain high-contrast VO2 thin films. The fabrication details for the Al2O3 buffer layers using atomic layer deposition (ALD) and VO2 thin films using DC sput (open full item for complete abstract)

Committee: Nima Ghalichechian (Advisor); Hanna Cho (Committee Member); Renee Zhao (Committee Member) Subjects: Electrical Engineering; Materials Science; Mechanical Engineering; Nanotechnology

Master of Science (M.S.), University of Dayton, 2021, Materials Engineering

VO2 is a type of phase change material (PCM) that can switch between a metallic state and a semiconducting state at a temperature of around 68 °C. This produces a large change in electrical resistance (almost three orders of magnitude) and large optical changes. Since this phase change occurs close to room temperature, VO2 has a large number of potential applications, such as thermally activated switches, optical modulators and optical limiters. Due to the multiple oxidation states of vanadium, VO2 thin films are typically difficult to produce. Traditionally, they are produced by reactive physical vapor deposition on heated substrates. In our research group, we have developed a different method where VO2 thin films are fabricated by thermal oxidation of PVD-deposited metallic vanadium films. Due to the high reactivity of vanadium, even small changes in the oxidation conditions will result in significant variations in the oxide films. In this thesis, we have examined the uniformity of VO2 films using stylus profiling, SEM, and 4-point probe measurements. The thickness expansion of the films due to oxidation was calculated and verified against experimental data. We also characterized the temperature profile inside the oxidation furnace. In addition, following an approach similar to the thermal oxidation of silicon, a vanadium oxidation model combining multiple oxidation states has been proposed and developed.

Committee: Andrew Sarangan Ph.D., M.A., P.E. (Advisor); Terrence Murray Ph.D. (Committee Member); Christopher Muratore Ph.D. (Committee Member); Robert Wilkens Ph.D., P.E. (Other); Eddy Rojas Ph.D., M.A., P.E. (Other) Subjects: Engineering; Materials Science; Optics

Doctor of Philosophy, The Ohio State University, 2020, Physics

Magnonics is a promising field revolving around using angular momentum – instead of charge – to transmit, store, and process information. Similar to organic based counterparts to inorganic electronics, magnonics provides the possibility to expand the application space of electronics. It can do this via reduced heating, nonlinear effects allowing for complex, unimaginable manipulations of microwave signals, and miniaturization of microwave components. In order to effectively study the properties of magnons, we need to find and characterize the best materials. For the transduction and transportation of information at microwave frequencies, low magnon loss materials are crucial to prevent the generation of spurious signals and maintain coherence. The two leaders of this low-loss criteria are YIG and V[TCNE]x. Two important factors have hindered the full success of these materials, however, difficulty patterning laterally at micron length scales and characterization and understanding of the damping and band structure within V[TCNE]x. In this dissertation, I explore successful methods for patterning these two low-loss materials and further characterize damping and band structure manipulations in V[TCNE]x. First I investigate patterning of YIG thin films. High-quality micron-scale YIG bars on GGG are achieved via the liftoff of magnetron sputtered YIG with a patterned bilayer of PMMA resists followed by an annealing step. The high quality of the patterned bars is assessed through x-ray diffraction, angle-dependent hysteresis,and angle-dependent FMR. These results advance the application space of YIG by introducing patterned YIG to the market, allowing for the creation of low-damping YIG features with manipulatable anisotropy fields and fine-tuned active areas for device creation. Second I investigate patterning of V[TCNE]x thin films. High-quality micron-scale V[TCNE]x features are achieved via liftoff of CVD grown V[TCNE]x with a patterned bilayer of PMMA resists. (open full item for complete abstract)

Committee: Ezekiel Johnston-Halperin (Advisor); Jay Gupta (Committee Member); Yuanming Lu (Committee Member); Louis DiMauro (Committee Member) Subjects: Physics

Doctor of Philosophy, Miami University, 2020, Geology and Environmental Earth Science

Modern civilization as we know it today could not exist without heterogeneous catalysis as it underpins all development in applied chemistry, materials science, and environmental geochemistry. Catalysts allow important chemical reactions such as the production of agricultural fertilizer, the formation of petrochemicals, and the breakdown of environmental pollutants to occur with lower energy thresholds. Catalysts comprise a diverse group of materials but none quite as important as porous solids, which optimize surface area, facilitate catalyst recyclability, and impart important selective properties. Many porous materials are naturally occurring minerals such as transition metal oxides, phyllosilicates, zeolites or complex compounds of minerals. This dissertation relates efforts to create and explore novel mineral catalysts whose catalytic properties are directly related to their microporosity, that is, minerals whose pores are less than 2 nanometers in diameter. In the first project, cryptomelane - a manganese oxide with a square tunnel-based framework - was synthesized in the absence and presence of an aqueous growth medium and doped with europium during and after crystallization to investigate Eu's effects on cryptomelane's physical, chemical, and catalytic properties in a temperature-dependent oxidation reaction. It was determined that the position of Eu in the cryptomelane framework strongly affected its catalytic activity. In the second project, sepiolite - a phyllosilicate mineral with a ribbon-like crystal structure and high specific surface area - was modified with manganese and europium prior to in-situ growth and deposition of nanocrystalline titanium dioxide to investigate the effects of support-doping vs. catalyst-doping in a photocatalytic reaction under different ultraviolet radiations. It was determined that phyllosilicate support promoters have variable behavior during multi-step catalyst preparation which ultimately impacted the photocatalytic acti (open full item for complete abstract)

Committee: Mark Krekeler (Advisor); John Rakovan (Committee Member); Claire McLeod (Committee Member); Catherine Almquist (Committee Member); Peter Heaney (Committee Member) Subjects: Chemical Engineering; Environmental Geology; Materials Science; Mineralogy

Doctor of Philosophy, The Ohio State University, 2019, Physics

The study of coherent magnonic interactions relies implicitly on the ability to excite and exploit long lived spin wave excitations in a magnetic material. That requirement has led to the nearly universal reliance on yittrium iron garnet (YIG), which for half a century has reigned as the unchallenged leader in low-loss magnetic resonance despite extensive efforts to identify alternative materials. Surprisingly, the organic-based ferrimagnet vanadium tetracyanoethylene (V[TCNE]x; x ~ 2) has recently emerged as a compelling alternative to YIG. In contrast to other organic-based materials, V[TCNE]x exhibits robust magnetism, has a single-peaked, narrow magnetic resonance feature (less than 1 G at 10 GHz), and has a Curie temperature of over 600 K with sharp hysteresis switching to full saturation at room temperature. On the other hand, progress in the field of organic electronics has yielded significant advances in the development and application of organic light emitting diodes (OLEDs), organic photovoltaics (OPVs), and organic field effect transistors (OFETs). The success of these device applications suggests that further expansion of the field to include magnetic functionality offers promising opportunities. At the same time, the emergence of optimized thin-film growth of and successful encapsulation strategies for organic-based magnetic materials allows for long term stability of high-quality magnets under ambient conditions. Presented here is the synthesis of a new class of organic-based magnetic nanostructures consisting of nanowires of V[TCNE]x that assemble along the ridges of a grooved substrate. These nanowires exhibit uniaxial magnetic anisotropy with an in-plane easy axis perpendicular to the nanowires, which is in direct contrast to the isotropic in-plane response of typical thin-films. These nanostructures support the excitation of multiple modes, and when these different magnon modes are brought into resonance by varying the orientation of an in-plane (open full item for complete abstract)

Committee: Ezekiel Johnston-Halperin PhD (Advisor); Richard Furnstahl PhD (Committee Member); Mohit Randeria PhD (Committee Member); Rolando Valdés Aguilar PhD (Committee Member) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy, The Ohio State University, 2018, Physics

This thesis addresses scanning tunneling microscopy (STM) studies of metal defects in GaAs(110). Single-atom defects are useful for their potential applications in developing nano-scale miniaturized or new types of optical, magnetic, and electronic devices, as well as expanding our understanding of atomic-scale interactions. STM is used to measure physical and electronic properties of surface and near-surface materials with atomic resolution, but affects the properties of defects being studied. Zn-doped GaAs(110) is one of the most popular commercially used semiconductor materials, and is well-understood on the macroscopic scale. By studying individual Zn defects at a wider range of doping levels than previously studied, we discovered three types of defect-based conductivity caused by dopant interactions that inform macroscopic doped semiconductor properties and atomic-scale defect properties. Additionally, we tuned these properties using electronic fields and doping level, laying the groundwork for Zn defects in solotronic or nanoscale devices. Additionally, erbium, an element with useful optical properties, has been studied on GaAs(110) for the first time, and been shown to have four types of interaction with the host surface. This improved our understanding of the significance of the interaction strength of atoms with host materials on defect charge state and electronic properties. The techniques developed in studying Zn and Er atoms have yielded preliminary results for new properties of Mn and Co atoms and the first studies of V atoms on GaAs(110).

Committee: Jay Gupta (Advisor); Amy Connolly (Committee Member); Roland Kawakami (Committee Member); Mohit Randeria (Committee Member) Subjects: Condensed Matter Physics; Physics