PhD, University of Cincinnati, 2010, Engineering and Applied Science: Materials Science

The Boron Carbon Nitorgen (B-C-N) ternary system includes materials with exceptional properties such as wide band gap, excellent thermal conductivity, high bulk modulus, extreme hardness and transparency in the optical and UV range that find application in most fields ranging from micro-electronics, bio-sensors, and cutting tools to materials for space age technology. Interesting materials that belong to the B-C-N ternary system include Carbon nano-tubes, Boron Carbide, Boron Carbon Nitride (B-CN), hexagonal Boron Nitride (h-BN), cubic Boron Nitride (c-BN), Diamond and beta Carbon Nitride (β-C3N4). Synthesis of these materials requires precisely controlled and energetically favorable conditions. Chemical vapor deposition is widely used technique for deposition of thin films of ceramics, metals and metal-organic compounds. Microwave plasma enhanced chemical vapor deposition (MPECVD) is especially interesting because of its ability to deposit materials that are meta-stable under the deposition conditions, for e.g. diamond. In the present study, attempt has been made to synthesize beta-carbon nitride (β-C3N4) and cubic-Boron Nitride (c-BN) thin films by MPECVD. Also included is the investigation of dependence of residual stress and thermal conductivity of the diamond thin films, deposited by MPECVD, on substrate pre-treatment and deposition temperature. Si incorporated CNx thin films are synthesized and characterized while attempting to deposit β-C3N4 thin films on Si substrates using Methane (CH4), Nitrogen (N2), and Hydrogen (H2). It is shown that the composition and morphology of Si incorporated CNx thin film can be tailored by controlling the sequence of introduction of the precursor gases in the plasma chamber. Greater than 100µm size hexagonal crystals of N-Si-C are deposited when Nitrogen precursor is introduced first while agglomerates of nano-meter range graphitic needles of C-Si-N are deposited when Carbon precursor is introduced first in the deposition c (open full item for complete abstract)

Committee: Raj Singh ScD (Committee Chair); Relva Buchanan ScD (Committee Member); Vesselin Shanov PhD (Committee Member); Rodney Roseman PhD (Committee Member) Subjects: Materials Science

MS, University of Cincinnati, 2002, Engineering : Materials Science

Polymer/Ceramic composite films were fabricated using Polyimide and PZT (90/10) and studied for their physical and electrical properties. The homogeneous and oriented composite films were fabricated by combining a spin deposition technique for polyimide with the novel metal-organic deposition technique for PZT. A RTA furnace was used to sinter the PZT films without damaging the Polyimide to fabricate an alternate PI/PZT layer structure. The effect of unpolymerised polyimide, polymerized polyimide, Pt/Si substrate and heating rate on the orientation of the PZT film was studied. X-ray diffraction and SEM techniques were used to determine the orientation, microstructure development and the thickness of the resulting composite structure. Thermal behavior of polyimide and PZT precursor solution was determined by using Thermogravimetric Analysis and Differential Scanning Calorimetry. Results indicate that use of RTA furnace with a heating rate of 110°C/s, instead of a regular furnace for sintering of the PZT layer, produced oriented PZT layers without damage to the Polyimide layers. Using a polymerized Polyimide film resulted in [111] and [101] oriented PZT film. The dielectric properties and electrical stability of composite films with different number of layers, or with the same number of layers but different fabrication methodology, was investigated. The Composite films were highly resistive as expected, since polyimide itself is very resistive, (volume resistiviy>10 16 ). All the films sintered in the RTA furnace were stable up to field strength 10 4 V/cm. The film capacitance and dielectric properties followed a logarithmic mixing rule. They all exhibited capacitance values of 0.3-0.4 nfarads, greater than polyimide films but smaller than typical PZT films. Very low loss (0.02-0.06) was also observed. Annealing of the composite structure after fabrication resulted in lowering of the loss values in the unannealed samples. The capacitance values were not significantly (open full item for complete abstract)

Committee: Dr. Relva Buchanan (Advisor) Subjects:

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

Like many problems in physics, the interaction between high intensity laser pulses and solid materials depends critically on the relative timescales of the drive (the laser pulse with finite duration) and the material response. This is especially true for Laser-Induced Damage and Ablation (LIDA) of solids, where femtosecond (1 fs = 10e−15 s) laser pulses can achieve extremely high energy densities since there isn't enough time for energy to diffuse away during the laser pulse like there is for picosecond (10e−12 s) and nanoseond (10e−9 s) pulses,for example. The pulse duration dependence of fs-LIDA for Near-Infrared (NIR) pulses less than 100 fs is less well-understood, especially in the Few-Cycle Pulse (FCP) regime (<10fs) where energy is deposited faster than almost all of the processes associated with the material response. In this thesis, the pulse duration dependence of LIDA of transparent dielectric material systems down to the FCP regime is studied using a well-established time-and space-resolved imaging technique as well as high-resolution depth-profiling. LIDA of dielectric solids has large application spaces in precision micro-machining andsurface patterning, as well as improving the LIDA performance of dielectric thin-film opticsto increase the output of high power laser systems. Practical multilayer thin-film opticsintroduce more complexity to the LIDA process due to thin-film interference, so I startedwith a study of FCP-LIDA of the simplest thin-film system: a single layer. I found that dif-ferences in LIDA between two film thicknesses are exacerbated by Few-Cycle Pulses (FCPs)relative to 100 fs pulses. I wrote a Finite-Difference Time-Domain (FDTD) simulation thatmotivates a possible mechanism for this, suggesting FCPs result in a more spatially non-uniform excitation of the films. My results show that the models I used must be extendedto more completely describe my experimental observati (open full item for complete abstract)

Committee: Enam Chowdhury (Advisor); Gregory Lafyatis (Committee Member); Thomas Lemberger (Committee Member); Douglass Schumacher (Committee Member) Subjects: Condensed Matter Physics; Electromagnetism; Optics; Physics; Plasma Physics; Solid State Physics

PhD, University of Cincinnati, 2004, Engineering : Electrical Engineering

Silicon dioxide, silicon-containing polymer, silicon nitride, metal nitride, and germanium thin films were grown by electron cyclotron resonance (ECR) microwave plasma enhanced chemical vapor deposition (PECVD), and multilayer coatings were grown for high hardness and high corrosion resistance. Silicon dioxide was grown from hexamethyldisiloxane (HMDSO), 1,3,5,7-tetramethylcyclotetrasiloxane (TOMCTS ), and octamethylcyclotetrasiloxance (OMCTS) in a oxygen plasma. The grown silicon dioxide thin films were hard and colorless. Silicon nitride was grown from hexamethyldisiloxane (HMDSO) and tetramethylsilane (TMS) in an ammonia (NH3) plasma. The silicon nitride thin films grown from HMDSO were hard and transparent while the silicon nitride thin films grown from TMS were black and hard. Silicon-containing polymer was grown from 100% OMCTS. The polymer thin films are colorless, had relatively low hardness and very good salt-fog corrosion resistance. Titanium nitride, zirconium nitride, and chromium nitride were grown from titanium (IV) isopropoxide and tetrakis(dimethylamino)titanium, zirconium 2-methyl-2-butoxide and zirconium t-butoxide, and bis(ethylbenzene)chromium in an ammonia plasma. The grown titanium nitride and zirconium nitride thin films had characteristic gold coloring and high hardness while the grown chromium nitride thin films were black gray and had high hardness. Germanium thin films were grown from tetramethylgermane (TMG) in a argon plasma. The deposited germanium films were uniform and had polished-like shining surface. X-ray photoelectron spectroscopic (XPS) analyses showed the films contained 97 % germanium atomic concentration with less than 1 % carbon, and X-ray diffraction (XRD) analyses showed the films had the crystal structure of <220>. Hard corrosion-resistant silicon-containing multilayer coatings were grown in a high-density microwave electron cyclotron resonance discharge. The multilayer coatings consist of a relatively soft silicon-contai (open full item for complete abstract)

Committee: Dr. Thomas Mantei (Advisor) Subjects:

Master of Science (MS), Bowling Green State University, 2008, Physics

This project investigated optoelectronic properties of thin film n-type ZnS on p-type Si substrate, which have been fabricated using pulsed-laser deposition (PLD) technique. The photocurrent (PC) response of the above heterostructure was measured using the lock-in technique. The experimental PC response spectrum was compared with the theoretical spectrum using density of states (DOS), modified Urbach rule,and the surface recombination of a thin film. In addition, current-voltage (I/V) characteristics was employed to determine the electronic features of the thin-film sample.

Committee: Dr. Bruno Ullrich PhD (Advisor); Dr. Lewis Fulcher PhD (Committee Member); Dr. Mikhail Zamkov PhD (Committee Member) Subjects: Optics; Physics

Doctor of Philosophy, University of Akron, 2007, Polymer Science

The main objective of the work presented in this thesis is to understand the surface and interface structure and dynamics of diblock copolymer brushes (DCBs). DCBs are stimuli-responsive materials and the surface properties of a DCB can be changed from those characteristic of one polymer block to those characteristic of the other one by treating the DCB with a solvent selective for one of its blocks. For this purpose, polystyrene-block-polyacrylate or polyacrylate-block-polystyrene brushes were synthesized using the “grafting from” technique in combination with atom transfer radical polymerization (ATRP). In the first part of this project the internal structure of DCBs after the synthesis and surface rearrangement were investigated using neutron reflectivity (NR) and grazing incidence small angle X-ray scattering (GISAXS). It was found that the internal brush structure depends strongly on the synthesis sequence of polymer blocks and the value of xN. For small values of xN (xN is smaller than 11), a model of two layers with an interfacial region of finite width provides a good description of the data. The interface width is found to be larger for DCBs which have the polymer block with the lower surface energy synthesized next to the substrate. A three layer model must be used to describe the structure of DCBs of larger xN values (xN is greater than 23) and of sufficiently asymmetric composition. The necessity of including a third layer is consistent with the presence of a lateral ordering of some type in the center of the brush, as evidenced by correlation peaks in the GISAXS data. The spacing of the in-plane ordering varies with the thickness of the poly(methyl acrylate) (PMA) block. After a DCB is treated with a selective solvent, Bragg rods appear in the GISAXS pattern. The appearance of Bragg rods indicates the formation of a new 2D structure which has a lateral spacing on the order of the total thickness of the brush. The Bragg rods disappear upon heating to 80 (open full item for complete abstract)

Committee: Mark Foster (Advisor) Subjects:

Master of Science, The Ohio State University, 2005, Graduate School

Committee: Not Provided (Other) Subjects:

Master of Science, The Ohio State University, 2005, Graduate School

Committee: Not Provided (Other) Subjects:

Doctor of Philosophy in Materials Science and Engineering, Youngstown State University, 2024, Materials Science

Semiconductor materials have played a huge role in advancing today's technology through the electronic and photonic devices ushered in over the years. The advancement has been driven in part by society's growing need for electronic devices capable of handling higher power, higher temperature, and higher frequency. Current research efforts are expanding to ultra-wide bandgap semiconductors such as gallium oxide Ga2O3). The principal goal of this dissertation is to obtain high quality β-Ga2O3 films with controlled conductivity by magnetron sputtering deposition. The specific objectives are the following: To grow β-Ga2O3 films on sapphire substrates (section 5.2) and on native β-Ga2O3 by rf sputtering (section 5.3), to produce doped and undoped β-Ga2O3 films (Section 5.4). Additionally, to grow Lu2O3/ Ga2O3 and B2O3/Ga2O3 alloy films on (-2 0 1) UID or Sn-doped Ga2O3 and Al2O3 substrates to tune Ga2O3 original bandgap (Section 5.5). To obtain microstructural, morphological, compositional, and optical data from XRD, AFM, SEM, EDS, and UV-Vis characterization methods for all the experiments mentioned above. From this data, correlate the effects of the varying parameters for the optimization of the films, to use the developed films to fabricate Schottky barrier diodes and proceed with the electrical characterization of the fabricated devices (section 5.6).

Committee: Tom Oder PhD (Advisor); Clovis Linkous PhD (Committee Member); Constantin Solomon PhD (Committee Member); Michael Crescimanno PhD (Committee Member); Donald Priour PhD (Committee Member) Subjects: Electrical Engineering; Engineering; Experiments; Materials Science; Optics; Physics; Technology

Master of Science in Electrical Engineering, University of Dayton, 2023, Electrical and Computer Engineering

Memristors are among the leading devices for non-volatile memory applications due to its in-memory computing capability, power efficiency and high endurance, as well as the speed at which memory states can be written. Tantalum oxide is currently one of the most promising materials in memristor design. Tantalum oxide based memristor devices operate via oxygen vacancy migration in the TaOx layer forming a filament which then allows current flow once formed. Due to the migration of the vacancies through the material to form this chain, there is permanent structural damage to the device over the course of operation which eventually results in reduced performance of the devices and less change between the SET/RESET states. In this paper, we analyze the impact of zirconium doping concentrations on oxygen vacancy migration by studying the electric field required to induce switching in the devices and device endurance. We test the power efficiency and ON/OFF resistance ratios of numerous TiN/TaOx/Ta/TiN devices made with zirconium oxide doping layers. The results are then analyzed to understand the effects of the TaOx/ZrO2 bilayer structure and the impact on the total power efficiency and endurance of the devices. The devices did show increased conductivity but switching performance and endurance were reduced. TEM imaging was performed to determine the cause, but no definitive cause could be identified.

Committee: Guru Subramanyam (Committee Chair); Andrew Sarangan (Committee Member); Ganguli Sabyasachi (Committee Member) 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

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

Beta-phase gallium oxide (β-Ga2O3), with its ultrawide band gap energy (~4.8 eV), high predicted breakdown field strength (6-8 MV/cm), controllable n-type doping and availability of large area, melt-grown, differently oriented native substrates, has spurred substantial interest for future applications in power electronics and ultraviolet optoelectronics. The ability to support bandgap engineering by alloying with Al2O3 also extends β-(AlxGa1-x)2O3 based electronic and optoelectronic applications into new regime with even higher critical field strength that is currently unachievable from SiC-, GaN- or AlxGa1-xN- (for a large range of alloy compositions) based devices. However, the integration of β-(AlxGa1-x)2O3 alloys into prospective applications will largely depend on the epitaxial growth of high quality materials with high Al composition. This is considerably important as higher Al composition in β-(AlxGa1-x)2O3/Ga2O3 heterojunctions can gain advantages of its large conduction band offsets in order to simultaneously achieve maximized mobility and high carrier density in lateral devices through modulation doping. However, due to the relative immaturity of β-(AlxGa1-x)2O3 alloy system, knowledge of the synthesis and fundamental material properties such as the solubility limits, band gaps, band offsets as well as the structural defects and their influence on electrical characteristics is still very limited. Hence, this research aims to pursue a comprehensive investigation of synthesis of β-(AlxGa1-x)2O3 thin films via metal organic chemical vapor deposition (MOCVD) growth methods, building from the growth on mostly investigated (010) β-Ga2O3 substrate to other orientations such as (100), (001) and (-201), as well as exploring other polymorphs, such as alpha (α) and kappa (κ) phases of Ga2O3 and (AlxGa1-x)2O3 to provide a pathway for bandgap engineering of Ga2O3 using Al for high performance device applications. Using a wide range of material characterization techniqu (open full item for complete abstract)

Committee: Hongping Zhao (Advisor); Siddharth Rajan (Committee Member); Steven A. Ringel (Committee Member); Sanjay Krishna (Committee Member) Subjects: Condensed Matter Physics; Electrical Engineering; Engineering; Materials Science; Nanoscience; Nanotechnology; Physics

Master of Science (MS), Bowling Green State University, 2023, Physics

Gallium oxide has been recognized as a promising Ultrawide-Bandgap(UWBG) semiconductor with a band gap ranging from 4.3-5.3eV. Beta gallium oxide, a polymorph of Gallium Oxide, has an ultra-wide bandgap of 4.9eV making it a great candidate for next-generation power electronics. In addition, UWBG semiconductors are capable of large breakdown voltage and has the potential of becoming a high-effciency switching device. The electrical characteristics are then analyzed using a Hall effect. Some samples are often annealed or doped to improve their electrical properties. The work presented in this thesis shows the process of recently developed gallium oxide flms with a focus on indium gallium oxide samples. The result of our research shows that signifcant changes to the electrical properties of gallium oxide films can be achieved through doping and annealing. In order to utilize the powerful properties of gallium oxide, this work incorporated indium to grow indium gallium oxide. Homogeneous and heterogeneous samples of indium gallium oxide were grown on the MOCVD with a unique recipe that allowed for controlled precursor injection. Furthermore, growth parameters were varied to test their effectiveness in the interplay between indium oxide and gallium oxide growth. The samples were then characterized using X-ray Diffraction(XRD) and Hall effect system. The XRD system allows for further understanding between the growth parameters and the resulting structure of the material. The combination of the XRD system and Hall effects allows for a great correlation between the MOCVD growth parameters and the resulting film qualities. Samples are annealed in hydrogen to enhance their electrical properties. Our experiments demonstrate enhanced electrical characteristics and even p-type conductivity can be produced with effective growth processing.

Committee: Farida Selim Ph.D (Committee Chair); Marco Nardone Ph.D (Committee Member); Alexey Zayak Ph.D (Committee Member) Subjects: Physics

PhD, University of Cincinnati, 2023, Engineering and Applied Science: Electrical Engineering

Thermal detectors have been gaining a lot of attention because their operation is independent of wavelength and bandgap limitations. Thermal detectors are seen as viable candidates capable of long wavelength detection specifically where conventional semiconductor photodetectors fail to provide room-temperature high responsivities in the field of optical astronomy, space exploration study, satellite imaging, 3D sensing, and LiDAR based automobile safety systems. Previously, metallic films such as Nickel, Bismuth, Platinum, and Nickel-Chromium, have been studied as possible radiation sensing materials for bolometer thermal detectors. So far, the metallic film bolometer thermal detectors have been struggling with thermal mass and heat dissipation. The objective of this dissertation is to establish room temperature thin film linear array bolometer devices as thermal detectors. A large % TCR near room-temperature is achieved for both Nickel and Bismuth thin films. The large % TCR is responsible for achieving room-temperature high responsivity of 0.11 V/W for Bismuth thin film linear array bolometer thermal detectors. This dissertation reports the first demonstration of an enhancement in room temperature responsivities of thin film linear array bolometer devices achieved by a novel heterostructure radiation sensing layer of Carbon Nitride thin films over the Nickel and Bismuth thin films. A higher responsivity of 0.14 V/W is achieved at room temperature operation with a much-reduced thermal time constant of 142 ms for the heterostructure radiation sensing layer of Carbon Nitride films over the Nickel thin film for the thin film linear array bolometer devices. Thus, through this dissertation, it is demonstrated that Carbon Nitride thin film linear array bolometer devices as thermal detectors can achieve higher room temperature responsivities by overcoming the current limitations of thermal noise, thermal mass, and heat dissipation. These results also establis (open full item for complete abstract)

Committee: Marc Cahay Ph.D. (Committee Chair); Je-Hyeong Bahk Ph.D. (Committee Member); Vesselin Shanov Ph.D. (Committee Member); P. Terrence Murray Ph.D. (Committee Member); Mark Schulz Ph.D. (Committee Member); Punit Boolchand Ph.D. (Committee Member) Subjects: Electrical Engineering

Master of Science in Chemistry, Youngstown State University, 2023, Department of Biological Sciences and Chemistry

Metal oxide semiconductor materials such as tungsten oxide are promising candidates for use as photoanodes in solar water splitting. Tungsten oxide is an n-type semiconductor that was prepared on stainless steel 304 substrate and subsequently studied for water-splitting applications. This study investigated the effect of the annealing temperature and substrate cleaning reagents on the photoelectrochemical (PEC) properties of tungsten oxide thin films. The main method of synthesis employed was the sol-gel method. Tungsten oxide thin films were deposited from a precursor solution of peroxotungstic acid by doctor blading. The as-deposited amorphous WO3 films were further subjected to heat treatment at various annealing temperatures (200 ℃, 300 ℃, 400 ℃, and 500 ℃) to transform the amorphous material into polycrystalline WO3 nanostructures. Surface morphology, the crystallinity of the film, the thickness of the film, and photoelectrochemical properties were investigated using scanning electron microscopy, (SEM), X-ray diffractometry (XRD), stylus profilometry, cyclic voltammetry (CV), and linear sweep voltammetry (LSV). The optimal WO3 film, at a thickness of 5 µm and annealed at 400 ℃, achieved a photocurrent density of 98.0 µA/cm2 at an applied voltage of 0.53 V vs Ag/AgCl. It is essential to treat the substrate with HNO3 to passivate the surface of the stainless-steel substrate with the Cr2O3 layer.

Committee: Clovis Linkous PhD (Advisor); Timothy Wagner PhD (Committee Member); Christopher Arntsen PhD (Committee Member) Subjects: Alternative Energy; Analytical Chemistry; Chemistry; Energy; Environmental Science; Nanotechnology

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

Photoelectrochemical (PEC) water splitting is a reliable method to produce hydrogen as a sustainable source of energy from water and sunlight. Amorphous titanium oxide (a-TiO2) thin films are able to protect silicon photo-anodes in PEC cell and stabilize the oxidation reaction on the electrode surface. Atomic layer deposition (ALD) can grow ultra-thin conformal pinhole-free a-TiO2 films on Si photo-anodes. However, variations in ALD conditions and post-ALD treatments can strongly influence PEC performance of a-TiO2 films due to mechanisms which have not been well-understood yet. Understanding a-TiO2 structure and nature of variations in its PEC performance could advance a-TiO2 growth in order to achieve (30,000 hour) industrial requirements for the photo-anode lifetime. Therefore, a combination of experimental characterization and computational modeling is required to obtain structure-property relationship in this disordered complex system. In this work, statistical analyses based on four dimensional scanning transmission electron microscopy (4D-STEM) were used in-situ and ex-situ to study characteristics of medium range ordering (MRO) in a-TiO2 films. High resolution scanning transmission electron microscopy (HR-STEM) and electron energy loss spectroscopy (EELS) were utilized to uncover atomic structure of orderings within the amorphous film. Moreover, experimentally-driven StructOpt optimization was applied to determine atomic structure of a-TiO2. First, the evolution of MRO and formation of nano-crystals within a-TiO2 films were studied by 4D-STEM and HR-STEM in ex-situ (ALD growth) and in-situ environments. The characteristics of a-TiO2 MRO were uncovered, and its relationship with crystalline nucleation was disclosed. It was revealed that MRO regions are preferential sites for crystalline nucleation. Stability and structural relaxation of MRO at different temperatures was discovered. Nucleation of anatase nanocrystals was detected, which could be linked to (open full item for complete abstract)

Committee: Jinwoo Hwang (Advisor); Sheikh Akbar (Committee Member); Stephen Niezgoda (Committee Member) Subjects: Materials Science

Master of Science, University of Toledo, 2022, Mechanical Engineering

A large amount of living and industrial waste gases is released into the environment every day. Monitoring and control of such environmental gases and industrial emissions are key targets for many domestic activities and industrial applications. The development of high-performance gas sensors is therefore crucial for the detection of those hazardous gases and to keep them under control. Decades of research and development have proven metal oxides to be the potential materials for gas sensor application owing to their high stability, high sensitivity, and outstanding ability to detect a wide range of target gases. However, challenges remain inevitable when it comes to overall sensing performance based on selectivity, response time, recovery time, and operating temperature requirements. Therefore, optimization of the existing metal oxide gas sensors by different techniques is important. In this study, possible attempts to enhance the performance of zinc oxide (ZnO) and nickel oxide (NiO) based gas sensors by impurity doping and incorporating heterostructures were investigated. Zinc oxide is an n-type semiconductor with a wide bandgap of 3.37 eV, large excitation binding energy of 60 meV. Therefore, it is considered one of the most promising semiconductor materials for gas sensor applications. With thermal activation, we can obtain more carriers on the surface of ZnO at high temperatures and increase the effective adsorption sites on the surface, thus increasing the adsorption. Doping with impurity elements is one of the best ways to enhance the sensing performance of ZnO. Iron-doped zinc oxide thin films were prepared via the sol-gel process and the effects of dopant on the microstructure, surface morphology, electrical properties, and gas sensitivity of ZnO were investigated. The optimum gas sensing performance was obtained from 6.0 at.% Fe-doped ZnO sample at 250 °C. Similarly, nitrogen-doped zinc oxide thin films were fabricated through radio frequency (rf) m (open full item for complete abstract)

Committee: Ahalapitiya Jayatissa Dr. (Advisor) Subjects: Mechanical Engineering

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

The effect of aluminum doping on the electrical and photoluminescent (PL) properties of flux-grown single crystal ZnGa2O4 was investigated. The undoped and Al-doped (0.55 at% Al) single crystal samples, with maximum dimensions of 6 mm and 4.5 mm (respectively), were both extremely electrically insulating. Low-temperature photoluminescence of single crystal ZnGa2O4 was measured for the first time. XRF, Raman, and photoluminescence analyses suggest the presence of a ZnO impurity phase in both the undoped and Al-doped single crystal samples, contributing to a strong UV-blue PL emission. A sharp PL peak at 500 nm and an atypical yellow-orange broadband emission, possibly related to impurities and either VO or Oi defects (respectively), were also observed for both samples. A PL peak at 621 nm unique to the Al-doped sample was observed. The effects of forming gas annealing, substrate material, and various PLD process conditions on the electrical properties of thin film ZnGa2O4 were also studied. Electrical resistivity decreased with oxygen partial pressure and increased with substrate temperature. The presence of zinc-rich impurity phases seemingly led to lower resistivities. Annealing in forming gas at 500°C generally led to a reduction in resistivity, while annealing at 700°C led to extreme electrical insulation for all samples.

Committee: Lei Kerr (Advisor); Kevin Leedy (Committee Member); Shashi Lalvani (Committee Member) Subjects: Chemical Engineering; Materials Science

Doctor of Philosophy, University of Akron, 2022, Polymer Science

Melanin, a ubiquitous dark pigment (biopolymer) present in various living organisms, is well-known for exhibiting multiple functions including structural coloration and photoprotection in many biological systems. The two unique optical properties of melanin - high refractive index and broadband absorption, and the organization of these melanin particles contribute to vibrant structural colors, as observed in bird feathers. This design strategy has been of significant interest to produce structural colors using synthetic melanin nanoparticles via colloidal nanoparticle-based self-assembly techniques. Nevertheless, we lack a quantitative model to explain color generation in these highly absorbing systems and accounting for multiple scattering in the dense assembly of nanoparticles. Moreover, there is a lack of accurate measurement of the optical constants of melanin in the literature. In my research, we have combined a variety of approaches to develop a comprehensive understanding of structural color generation in melanin-based colloidal assemblies. First, we have built a framework to address the ambiguity surrounding melanin's optical constants using spectroscopic ellipsometry. Second, using the freshly measured melanin's optical constants, we have employed a combined coarse-grained molecular dynamics (CG-MD) simulations and finite-difference time-domain (FDTD) approach to elucidate how melanin modulates structural color generation in one-component (melanin) and two-component (melanin + silica) nanoparticle-based supra-assemblies. Third, we have described robust tools to model structural color generation in densely packed disordered colloidal nanoparticle assemblies by a) thoroughly characterizing the structural information to facilitate 3D reconstruction of the colloidal assembly using small-angle scattering (SAS) techniques and computational reverse-engineering analysis for scattering experiments (CREASE) method, and b) performing FDTD calculations on these 3D (open full item for complete abstract)

Committee: Ali Dhinojwala (Advisor); Mesfin Tsige (Committee Chair); Matthew Shawkey (Committee Member); Hunter King (Committee Member); Jutta Luettmer-Strathmann (Committee Member) Subjects: Chemistry; Materials Science; Nanoscience; Nanotechnology; Optics; Polymers; Sustainability

Doctor of Philosophy, The Ohio State University, 2022, Materials Science and Engineering

Novel spintronic devices based on magnetic skyrmions, a non-volatile nanoscale topological excitation, are attractive as a new paradigm for high data processing speeds and low operating power, addressing the fundamental scaling restrictions in conventional electronics. Identifying materials capable of hosting small skyrmions and being able to image and manipulate skyrmions in these materials is essential as a path to design and develop such devices. Meanwhile, revealing and understanding the mechanisms underlying emergent spintronic phenomenon in these materials will further facilitate the development of next-generation spintronic devices with application in information storage and processing. The focus of this study is to explore and apply different magnetic characterization techniques in the transmission electron microscope (TEM) to investigate the creation, annihilation and manipulation of magnetic skyrmions in potential spintronic materials and devices. The first phase of this research was committed to the establishment of imaging techniques using Lorentz TEM and STEM. In the second phase, these methods have been employed to explore promising skyrmion hosting materials, including single crystal and novel thin films with broken bulk inversion, oxide interfaces with broken surface/mirror inversion, and centrosymmetric materials with geometric confinement. The study's last step entails the design and investigation of skyrmion dynamics in prototype devices using in-situ sample holders.

Committee: David McComb (Advisor); Jinwoo Hwang (Committee Member); Roberto Myers (Committee Member) Subjects: Condensed Matter Physics; Materials Science