Browsing by Author "Gupta, Arunava"
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Item Accelerated Design of Novel Heusler Compounds for Spintronics Applications(University of Alabama Libraries, 2021) KC, Shambhu; LeClair, Patrick; University of Alabama TuscaloosaMaterial discovery could be defined as the identification of a previously unexplored phases/composition which may exhibit properties that are unique or similar to that of previously explored composition. Historically, this has relied to some extent on serendipity. With the search space getting wider and at the same time an increased global competitiveness, it has become apparent that the material discovery process can be accelerated, which also helps in reducing cost. Spintronics, which utilizes both the spin and charge of an electron, is a technology that has the promise to take over existing charge-based technology. Half-metallic ferromagnets, due to their ability to generate 100% spin polarization, are considered ideal materials to be used in spintronic devices. While many candidate half-metals have been predicted based on theoretical calculations, finding a half-metallic character in experiments is still an open challenge. This provides impetus to search for new candidate materials with robust half-metallic character. In this dissertation, a new substitution scheme has been realized that allows for the design of many new functional materials in a relatively short time. It is also shown that, in many cases, alloy properties can be tuned by counting the total number of valence electrons, which is less dependent on the substitution scheme. Another approach, which paves the way to enhance the magnetic properties of the materials is also discussed. Hence with the identification of new approaches to material design, this dissertation adds value in the quest for the accelerated design of functional materials.Item Applications of methods beyond density functional theory to the study of correlated electron systems(University of Alabama Libraries, 2013) Sims, Hunter Robert; Butler, W. H.; University of Alabama TuscaloosaThe difficulty in accurately treating systems in which electron-electron interactions are the dominant physics has plagued condensed matter physics for decades. Currently, there exist many different computational techniques designed to improve upon density functional theory to varying degrees of accuracy. To date, no unified, parameter-free method exists that is guaranteed to yield the correct answer for all materials. Consequently, proper treatment of such systems often requires a combination of several methods, allowing one to check them against one another when their regions of validity overlap and to expand one's reach when a single method cannot reliably describe all of the physics at work. In this dissertation, I present discussion and, when appropriate, brief derivations of several of the most prominent electronic structure methods currently in use---from the local density approximation through LDA+DMFT. I then present several investigations into the electronic and magnetic structure of materials of potential interest for information technology that also illustrate the current state of affairs in computational condensed matter physics. I explore the intersite exchange interactions in CrO_2 within density functional theory (with and without Hubbard “+U” corrections) and evaluate these results through analytic and numerical means. I study the dependence of the mysterious magnetization of Fe_16 N_2 on crystal and electronic structure and employ a wide range of techniques in an attempt to bring greater rigor and deeper understanding to the widely-varying reports on this material. In conjunction with others' careful experimental analysis, I provide a picture of the band structure of the magnetic insulator NiFe_2 O_4 that reveals a novel hierarchy in its band gaps and suggests applications in spintronics and possibly other areas. Finally, I employ dynamical mean-field theory to study the behavior of impurity states in elemental semiconductors, using H impurities in Ge as a base system.Item Atom probe tomography study of wide bandgap semiconductor materials(University of Alabama Libraries, 2014) Dawahre Olivieri, Nabil Farah; Kung, Patrick; University of Alabama TuscaloosaThis dissertation focuses on developing atom probe tomography (APT) for semiconductors. APT is quickly gaining interest in the field of material characterization because of its unique ability to provide 3D nanoscale studies. APT has been widely used in metals and conductive materials but design changes in the tool in recent years have made atom probe a suitable tool for semiconductor analysis. Because research in atom probe tomography of semiconductors is still in its infancy, it is still unclear whether this characterization method is suitable for semiconductor and how the added knowledge can be different than other accessible tools. This work will utilize APT as a characterization tool for wide bandgap semiconductors, specifically zinc oxide (ZnO) nanowires and GaN-based epitaxial sample. Wide bandgap semiconductor materials have attracted considerable attention in recent years because of the practical applications such as green and blue light emitting and laser diodes, solid-state lighting, photovoltaics, RF and microwave electronics, and gas sensors. Although silicon has remained the industry standard for many of these applications, its limitations have made way into the research of wide bandgap semiconductor materials, such as zinc oxide (ZnO) and gallium nitride (GaN). Because of their large direct bandgap, these materials show excellent promise in the field of optoelectronics, high frequency, high temperature and high power applications. First, we understand the behavior of the material to achieve field evaporation under APT conditions and the mechanisms behind, as well as ways to overcome the different artifacts introduced during sample preparation and data collection. Following this understanding, we can begin to apply APT to device structures to understand the effects of radiation on materials at the atomic scale, as well as the cluster formation of some of the elements along the material. At the conclusion of this dissertation, APT will deliver the results necessary to maximizing device efficiency as well as build the pathway for future APT analysis.Item Bacterium E. coli- and phage P22-templated synthesis of semiconductor nanostructures(University of Alabama Libraries, 2010) Shen, Liming; Gupta, Arunava; University of Alabama TuscaloosaThe properties of inorganic materials in the nanoscale are found to be size- and shape-dependent due to quantum confinement effects, and thereby nanomaterials possess properties very different from those of single molecules as well as those of bulk materials. Assembling monodispersed nanoparticles into highly ordered hierarchical architectures is expected to generate novel collective properties for potential applications in catalysis, energy, biomedicine, etc. The major challenge in the assembly of nanoparticles lies in the development of controllable synthetic strategies that enable the growth and assembly of nanoparticles with high selectivity and good controllability. Biological matter possesses robust and precisely ordered structures that exist in a large variety of shapes and sizes, providing an ideal platform for synthesizing high-performance nanostructures. The primary goal of this thesis work has been to develop rational synthetic strategies for high-performance nanostructured materials using biological templates, which are difficult to achieve through traditional chemical synthetic methods. These approaches can serve as general bio-inspired approaches for synthesizing nanoparticle assemblies with desired components and architectures. CdS- and TiO_2 -binding peptides have been identified using phage display biopanning technique and the mechanism behind the specific affinity between the selected peptides and inorganic substrates are analyzed. The ZnS- and CdS-binding peptides, identified by the phage display biopanning, are utilized for the selective nucleation and growth of sulfides over self-assembled genetically engineered P22 coat proteins, resulting in ordered nanostructures of sulfide nanocrystal assemblies. The synthetic strategy can be extended to the fabrication of a variety of other nanostructures. A simple sonochemical route for the synthesis and assembly of CdS nanostructures with high yield under ambient conditions has been developed by exploiting the chemical characteristics and structure of permeabilized E. coli bacteria. The crystal phase, morphology, micro/nanostructure, optical absorption, and photocatalytic properties of the CdS nanostructures are tailored over a wide range by merely changing the synthetic conditions. Photoanodes fabricated using the nanoporous hollow CdS microrods exhibit excellent performance for the photocatalytic hydrogen production. This facile approach has been extended to the synthesis and assembly of other semiconducting sulfides, including PbS, ZnS, and HgS.Item Bulk Single Crystal-Like Structural and Magnetic Characteristics of Epitaxial Spinel Ferrite Thin Films with Elimination of Antiphase Boundaries(Wiley-VCH, 2017) Singh, Amit V.; Khodadadi, Behrouz; Mohammadi, Jamileh Beik; Keshavarz, Sahar; Mewes, Tim; Negi, Devendra Singh; Datta, Ranjan; Galazka, Zbigniew; Uecker, Reinhard; Gupta, Arunava; University of Alabama Tuscaloosa; Department of Science & Technology (India); Jawaharlal Nehru Center for Advanced Scientific Research (JNCASR); Leibniz Institut fur Kristallzuchtung (IKZ)Spinel ferrite NiFe2O4 thin films have been grown on three isostructural substrates, MgAl2O4, MgGa2O4, and CoGa2O4 using pulsed laser deposition. These substrates have lattice mismatches of 3.1%, 0.8%, and 0.2%, respectively, with NiFe2O4. As expected, the films grown on MgAl2O4 substrate show the presence of the antiphase boundary defects. However, no antiphase boundaries (APBs) are observed for films grown on near-lattice-matched substrates MgGa2O4 and CoGa2O4. This demonstrates that by using isostructural and lattice-matched substrates, the formation of APBs can be avoided in NiFe2O4 thin films. Consequently, static and dynamic magnetic properties comparable with the bulk can be realized. Initial results indicate similar improvements in film quality and magnetic properties due to the elimination of APBs in other members of the spinel ferrite family, such as Fe3O4 and CoFe2O4, which have similar crystallographic structure and lattice constants as NiFe2O4.Item Characterization of bismuth telluride two-dimensional nanosheets for thermoelectric applications(University of Alabama Libraries, 2015) Guo, Lingling; Wang, Hung-Ta; University of Alabama TuscaloosaSolid-state thermoelectric devices are compact, scalable, quiet, and environmentally friendly, which are widely used as thermal engines or refrigerators. Bismuth telluride (Bi2Te3) and other V-VI group chalcogenides are known as one of the best thermoelectric materials specifically for applications in a temperature environment from room temperature to 300 ℃. Recently, the unique topological surface states were discovered in Bi2Te3 family materials, and these novel surface states are arisen from a strong spin-orbit coupling in topological insulators. Topological surface states are protected against time-reversal perturbations (i.e., non-magnetic impurities or surface defects), making the electronic transport essentially dissipation-less. Such unique transport behavior with zero energy loss provides new opportunities to enhance thermoelectric properties. Although the promise in thermoelectric properties of topological insulators have been shown in theoretical reports, there is a lack of experimental investigations for a better understanding of their basic properties. This research work focuses on the characterizations of fundamental properties of Bi2Te3 two-dimensional (2D) nanosheets. Samples were prepared via respective solvothermal synthesis and van der Waals epitaxy. The charged surface properties of Bi2Te3 2D nanosheets were investigated using kelvin probe force microscopy. The measured electrical potential difference between aminosilane self-assembled monolayer and Bi2Te3 nanosheet surfaces is found to be ∼650 mV, which is larger than that (∼400 mV) between the silicon oxide substrate and Bi2Te3 nanosheet surface. The elastic properties of Bi2Te3 2D nanosheets (i.e., Young’s modulus and prestress) were acquired by analyzing the thickness dependence of 2D nanosheet deformations creating by atomic force microscopy tips. The Young's modulus by fitting linear elastic behaviors of 26 samples is found only 11.7–25.7 GPa, significantly smaller than the bulk in-plane Young's modulus (50–55 GPa). Furthermore, the thermoelectric properties of Bi2Te3 2D nanosheets were characterized in the cryostat system at a temperature range of 20-400 K. The results reveal that electrical conductivity of 2D nanosheets decreases with increasing temperature and thickness, while the measured Seebeck coefficient does not show a strong thickness dependence and the value is smaller than bulk Bi2Te3. These fundamental properties would help improve the basic understanding of topological surface states towards practical applications.Item Chemical Vapor Deposition of Bismuth Ferrite-Based Multiferroics for Device Applications(University of Alabama Libraries, 2022) Acharya, Mahendra; Gupta, Arunava; University of Alabama TuscaloosaEpitaxial bismuth ferrite and substituted bismuth ferrite films have attracted significant attention due to their potential application in energy-efficient memory and logic devices. However, bismuth ferrite suffers from processing and application issues that can be addressed by improving and optimizing synthetic techniques. Chemical vapor deposition (CVD) is one of the suitable techniques for the high-volume manufacturing of bismuth ferrite. CVD can quickly produce conformal coating over a large area and still produce a superior-quality epitaxial layer. However, development in the CVD of bismuth ferrite has been slow and fragmentary. Very limited works have grown good-quality epitaxial bismuth ferrite films with robust ferroelectric properties using CVD. In this dissertation, a systematic study has been carried out to understand the problems underlying the synthesis of bismuth ferrite by CVD. Moreover, the crucial role of stoichiometry and lattice misfit strain in controlling ferroelectric switching has been elucidated, and the effect of vapor flow dynamics in controlling ferroelectric domain orientation and, subsequently, ferroelectric switching properties has been explained.The importance of the miniaturization of devices and the need for high-density, high-speed, and energy-efficient material systems for memory and logic applications has been mounting. Hence, understanding the application of the finite-size effect in CVD-grown bismuth ferrite is critical. The finite-size effect in CVD-grown bismuth ferrite has been validated by confirming the application of the Kay-Dunn law in the thickness scaling of switching voltage in the material. The insufficiency of thickness scaling to lower the switching voltage of the material to the desired range prompted us to explore the efficacy of samarium substitution in lowering the switching voltage of bismuth ferrite. A 50% reduction in switching voltage has been achieved by samarium substitution at the bismuth site.Item Chemical vapor deposition of thin film materials for electronic and magnetic applications(University of Alabama Libraries, 2011) Li, Ning; Klein, Tonya M.; University of Alabama TuscaloosaChemical vapor deposition (CVD) has been employed to pursue high quality thin film growth for four different materials with excellent electronic or magnetic properties for certain device applications. The relationship between CVD processing conditions and various thin film properties has been systematically studied. Plasma enhanced atomic layer deposition (PEALD) is a special type of CVD technique and can be used for the deposition of very thin (few nanometers) and highly conformal thin films. PEALD of hafnium nitride (HfN) thin film is studied by using tetrakis (dimethylamido) hafnium (IV) (TDMAH) and hydrogen plasma. Prior to thin film deposition, TDMAH adsorption and reaction on hydrogenated Si(100) surface has been investigated by in-situ ATR-FTIR. It has been found that between 100˚C and 150˚C surface adsorbed TDMAH molecules start to decompose based on the ß-hydride elimination mechanism. The decomposition species on the surface has been found hard to desorb at 150˚C, which can contaminate the thin film if the purging/pumping time is insufficient. Uniform and moderately conductive HfNxCy films are deposited on hydrogen terminated Si(100) and thermally grown SiO2 (on Si) substrates by PEALD process. The dependence of thin film resistivity on plasma power is found to be related to the change of surface chemical composition. In vacuo XPS depth profile analysis showed the existence of hafnium carbide phase, which to a certain degree can improve the film conductivity. Direct liquid injection chemical vapor deposition (DLI-CVD) has been utilized for epitaxial growth of nickel ferrite (NiFe2O4), lithium ferrite (LiFe5O8) and barium titanate (BaTiO3) films on various lattice match substrates. For the deposition of nickel ferrite, anhydrous Ni(acac)2 and Fe(acac)3 (acac = acetylacetonate) are used as precursor sources dissolved in N,N-dimethyl formamide (DMF) for the DLI vaporizer system. Epitaxial nickel ferrite films of stoichiometric composition are obtained in the temperature range of 500-800 ºC on both MgO(100) and MgAl2O4(100). Film morphology is found to be dependent on the deposition temperature with atomically smooth films being obtained for deposition temperature of 600 and 700 ºC. Magnetic measurements reveal an increase in the saturation magnetization for the films with increasing growth temperature, which correlates well with the trend for improved epitaxial growth. Nickel ferrite films deposited on MgAl2O4 (100) at 800ºC exhibit saturation magnetization very close to the bulk value of 300 emu/cm3. Out-of-plane FMR measurement shows the narrowest FMR line width of ~160 Oe for films deposited at 600˚C. For lithium ferrite deposition, anhydrous Li(acac) and Fe(acac)3 are dissolved in DMF in a molar ratio of 1:5. Epitaxial growth of lithium ferrite films on MgO(100) are observed in the temperature range of 500˚C to 800˚C. The as grown films show increasing saturation magnetization with increasing deposition temperature due to the improved degree of crystal texture. For barium titanate thin film deposition, Ba(hfa)2*tetraglyme and Ti(thd)2(OPri)2 are dissolved in toluene in a molar ratio of 1:1. Epitaxial growth of barium titanate on MgO(100) has been found at the temperature of 750˚C. Film with a thickness of ~500 nm has a relatively large roughness of ~20 nm. Small amount of F elements, which exists in Ba-F bonds, has been detected in the thin film by XPS.Item Colloidal Synthesis, Characterization, and Photoconductivity of Quasi-Layered CuCrS2 Nanosheets(MDPI, 2022) Rodriguez, Jose J. Sanchez J.; Leon, Andrea N. Nunez N.; Abbasi, Jabeen; Shinde, Pravin S.; Fedin, Igor; Gupta, Arunava; University of Alabama TuscaloosaThe current need to accelerate the adoption of photovoltaic (PV) systems has increased the need to explore new nanomaterials that can harvest and convert solar energy into electricity. Transition metal dichalcogenides (TMDCs) are good candidates because of their tunable physical and chemical properties. CuCrS2 has shown good electrical and thermoelectrical properties; however, its optical and photoconductivity properties remain unexplored. In this study, we synthesized CuCrS2 nanosheets with average dimensions of 43.6 +/- 6.7 nm in length and 25.6 +/- 4.1 nm in width using a heat-up synthesis approach and fabricated films by the spray-coating method to probe their photoresponse. This method yielded CuCrS2 nanosheets with an optical bandgap of similar to 1.21 eV. The fabricated film had an average thickness of similar to 570 nm, exhibiting a net current conversion efficiency of similar to 11.3%. These results demonstrate the potential use of CuCrS2 as an absorber layer in solar cells.Item Computational studies of Lewis acidic gas adsorption to transition metal oxide nanoclusters and metal organic frameworks(University of Alabama Libraries, 2017) Flores, Luis Antonio; Dixon, David A.; University of Alabama TuscaloosaComputational studies of the interaction of Lewis acid gases with metal oxide clusters and metal organic frameworks show how these gases interact with and degrade these materials at the molecular level. The calculations were done at the levels of density functional theory and correlated molecular orbital theory ((CCSD(T))). Group VI metal oxides clusters physisorb CO_2 near or below to 298K, and chemisorption of CO_2 by carbonate formation is an endothermic process. SO_2 physisorbs to Group VI clusters near or below 298K. Group VI metal oxides chemisorb SO_2 by forming sulfites with positive free energies of binding at 298K. The formation of sulfates is thermodynamically allowed for Cr clusters because Cr clusters have a higher reducibility than do Mo or W clusters. Group IV metal oxide clusters prefer chemisorption of both gases by carbonate and sulfite formation. Mo and W oxides may function as long lived sorbents for these gases, whereas Cr and Group IV metal oxides would degrade upon exposure to these gases as sulfites, sulfates, or carbonates form on their surfaces. Uranium trioxide clusters are predicted to chemisorb CO_2 by uranyl carbonate formation. The exposure of nuclear waste to CO_2 could cause uranium oxides to degrade leading to ground water contamination. The physisorption of the Lewis acid gases (CO_2, SO_2, H_2O, H_2S, CO, and NO_2) to M-MOF-2 systems (M = Zn, Cu, Co), was investigated. The MOFs are predicted to bind H_2O, H_2S, and SO_2 more strongly than the other gases. The binding energies are larger for Zn and Co than for Cu. Zn-MOF-2 clusters will degrade faster than Cu.Item Controlled Synthesis and Characterization of Magnetic Chalcospinels Nanocrystals(University of Alabama Libraries, 2020) Akbari Afkhami, Farhad; Gupta, Arunava; University of Alabama TuscaloosaBinary and ternary metal chalcogenides have become well-known materials among chemists, physicists, material scientists, and other researchers of the field, and they have attracted significant attention because of their novel chemical, magnetic, electronic, mechanical and optical properties. Among the metal chalcogenides, chromium-based chalcospinels ACr2X4 (A = Cu, Co, Fe, Cd, and Hg; X = S, Se, and Te) have gained significant attention because they are a notable class of magnetic materials such as semiconductors, magnetic metals, and insulators. In this work, a general overview of binary and ternary metal chalcogenides and their nanocrystals has been provided. We have also provided an overview of the wet-chemical colloidal methods as an important approach to size and shape-controlled synthesize of nanocrystals. We have also discussed the importance of metal doping reactions as a pathway to create previously unavailable multielemental materials for high-performance applications. In this set of studies, colloidal nanocrystals of chromium-based chalcospinels of CuCr2S4 and CuCr2Se4 have been synthesized via hot-injection and heat-up methods and were characterized using experimental methodology comprised of different microstructural and structural tests. The magnetic properties of these nanocrystals have also been studied. The next studied system was Cr-doped pyrite CuSe2 nanocrystals, eventually leading to the observation of significant enhancement of ferromagnetic moment by Cr-doping in octahedral sites of the pyrite structure. We performed a unique reaction in which nanocrystals of CrxCu1-xSe2 (x = 0.1-0.5) formed in the pyrite phase, which is not stable in bulk form. The host p-CuSe2 nanocubes did also undergo a degradation influenced by the reaction temperature and the doping of Cr3+ ions in the pyrite crystal structure. The Cr-doped nanocrystals of the pyrite phase were formed during the heat-up procedure and by increasing the reaction temperature transformed to CuCr2Se4 spinel nanocrystals. To the best of our knowledge, no cationic substitution of chromium for copper has been reported on pyrite CuSe2 systems so far, likely due to the significant size difference between chromium and copper. Therefore, the results of this work are a powerful approach for the design and fabrication of new multielemental materials that may not be stable in the bulk form.Item Determination of Dzyaloshinskii-Moriya Interaction and Higher Order Magnetic Anisotropy in Ultrathin Films and Magnetic Multilayers(University of Alabama Libraries, 2022) Pokhrel, Ashok; Mewes, Tim; University of Alabama TuscaloosaThe exotic properties of magnetic materials provide multiple avenues for research as well as the possibility for energy-efficient, faster and cheaper technologies. Especially, earlier few decades had been crucial for the advancement of memory devices in-terms of size, speed, price and availability. Material science scientific community has been dedicated for developing next generation memory devices. The presence of a strong perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) in magnetic materials serve as building blocks for the formation of skyrmions which are promising candidates for next generation memory devices. This dissertation discusses the fundamentals of Dzyaloshinskii-Moriya interaction (DMI) which is responsible for the formation of skyrmions in magnetic multilayers. Especially this dissertation discusses the First Principle calculation technique and micromagnetic simulations to estimate the magnitude and sign of DMI and estimation of higher-order perpendicular anisotropy in ferromagnetic multilayers.In the first study presented in this dissertation, the basis of DMI, spin spirals and penalty energy parameters are discussed and the magnitude and strength of DMI vector in Pt/Co multilayers is estimated and related with proximity effect. In the second study presented, the origin of higher-order contribution of perpendicular magnetic anisotropy (PMA) are studied in ferromagnetic bilayers using micromagnetic simulations. These simulations showed that spatial inhomogeneities of the first-order perpendicular anisotropy may be the origin of higher-order anisotropies in magnetic multilayers.Item Dissolution and processing of cellulosic materials with ionic liquids: fundamentals and applications(University of Alabama Libraries, 2010) Sun, Ning; Rogers, Robin D.; University of Alabama TuscaloosaWith the inevitable depletion of petroleum-based resources, there has been an increasing worldwide interest in renewable resources such as biomass. One reason for the current approaches being taken to utilize biomass is the difficulty in processing lignocellulosic materials and the energy needed for separation of the components. The three major components of biomass are covalently bonded together, which makes dissolution and further separation of the three major components difficult and this has been recognized as the grand challenge for biomass utilization. This dissertation describes research efforts in processing of lignocellulosic biomass using ionic liquids (ILs) as solvents. ILs are salts with melting points below 100 oC, which possess many advantage properties. Cellulose composite fibers have been prepared based on IL solution with dispersion of the additives. Wood and bagasse have been completely dissolved in ILs. Partial separation of the components has been obtained using selected reconstitution solvents. High temperature and fast dissolution was found to be an efficient method for both dissolution and separation of biomass components. Biomass composite fibers can be prepared directly from such biomass solutions. With selected catalysts in solution, improved dissolution and separation has been achieved, making the delignification and pulp yield comparable to the kraft pulping process.Item Dynamic transport measurements of vo2 thin films through the metal-to-insulator transition(University of Alabama Libraries, 2018) Jones, Joshua Michael; LeClair, Patrick R.; University of Alabama TuscaloosaVO2 is a transition metal oxide material well known for its high magnitude metal-to-insulator transition (MIT) with a corresponding change in crystal structure [1]. At room temperature, VO2 is found in an insulating monoclinic phase (P21/c) that upon heating through the transition temperature (Tc, ~341 K in bulk material) changes to a metallic rutile phase (P42/mnm) [2]. The MIT can be activated thermally by heating or cooling through Tc, but has also been shown to be sensitive to electric field [3], infrared radiation [4], pressure [5], and strain [6]. The value of Tc is also highly tunable through doping [7] and growth of strained epitaxial thin films [8]. The massive 3-4 order of magnitude change in electrical resistivity (ρ) has drawn interest for possible device level applications. The transition is characterized by the coexistence of rutile metallic domains and a monoclinic insulating matrix that results in a smooth progression of the DC transport and dielectric properties as the MIT is induced. In this thesis, we present an overview of three novel transport experiments all of which involve epitaxial TiO2/VO2 films grown in a home-built low-pressure chemical vapor deposition system. The first experiment looks at the time evolution of the film resistance and capacitance as it settles for an extended period very near Tc. We report evidence that this settling process is characterized by at least two underlying relaxation processes. The second experiment involves the deposition and ferromagnetic resonance (FMR) characterization of TiO2/VO2/Ru/Py heterostructures. Our analysis indicates enhanced spin pumping into the VO2 layer when in the metallic state that is associated with an increase in the effective Gilbert damping parameter. Finally, we discuss the results of 1/f noise spectroscopy measurements collected on Hall-bar patterned VO2(100) films. We show that the processes governing noise along both crystallographic axes are identical and, in the metallic rutile state, follows a unique R-3 scaling behavior.Item The effect of fluoride on the crystallinity and photoactivity of titania(University of Alabama Libraries, 2012) Brauer, Jonathan Isaac; Szulczewski, Gregory J.; University of Alabama TuscaloosaThis dissertation describes the synthesis and characterization of fluorinated N-doped TiO_2 nanoparticles under ambient conditions. Samples were synthesized by sol-gel methods that utilized the controlled hydrolysis of titanium(IV) tetra-isopropoxide in acidic solutions. Nitrogen doping was achieved by two different methods. In one scheme triethylamine (TEA) was added post-synthesis to the nanoparticle formation. In the other scheme, ammonium chloride (NH_4 Cl) was added during the acid catalyzed hydrolysis reaction. A freeze-drying process of the sol-gel was used to prevent aggregation during dehydration and was found to retain the high surface area of the powder. Post-synthesis the hydroxyl groups on the surface were exchanged with fluoride by stirring the powders in acidic solution of NaF. Using this synthetic approach the amount of nitrogen and fluoride could be independently controlled. The nanoparticles were characterized by numerous spectroscopic techniques including DRS, FTIR, Raman, and XPS. Vibrational spectroscopy shows that the particles contain significant amounts of organic impurities after doping with TEA. In contrast, particles synthesized with NH_4 Cl showed much less contamination. XPS analysis revealed that a single nitrogen species with a binding energy of 400.6 eV when using TEA as the N precursor. In contrast, when NH_4 Cl was used as the nitrogen precursor two nitrogen species were observed with binding energies at 402.6 and 401.2 eV. These latter peaks are assigned to interstitial nitrogen in the N^(1+) and N^0 oxidation states. The as-synthesized nanoparticles also show a significant differences in their optical properties. In general, the particles doped from TEA and NH_4 Cl were yellow and white, respectively, despite containing approximately the same amount of nitrogen (~5% with respect to Ti). The difference is attributed to a high fraction of oxygen-vacancies in the TEA doped nanoparticles. XRD and Raman measurements determined that the as-synthesized samples were amorphous, but could be converted to the anatase phase by two different methods. Thermal annealing was shown to convert the amorphous particles to the anatase polymorph. The presence of surface fluoride was found to significantly lower the temperature to observe the amorphous to anatase transition. In the second method, stirring the powders in acidic solutions of NaF at room temperature for 12-168 hours produced the anatase phase with an average crystallite size of 4 nm. It was found that the phase transition only occurs when the pH is below the point of zero charge of the particles. The photoactivity of the nitrogen and nitrogen / fluoride-doped particles was tested for their ability to degrade methylene blue (MB) with visible light (> 400 nm). In general the particles with a surface fluoride were more photoactive that those without. In addition, particles with nitrogen were more photoactive than pure TiO_2 . By analyzing the decomposition products with electrospray ionization mass spectrometry and UV-Vis spectroscopy, it was possible to elucidate a different decomposition pathway for the nitrogen-doped samples. When TEA was the dopant precursor, MB primarily decomposed by a ring-cleavage pathway using superoxide. In contrast, when NH_4 Cl was the dopant precursor, MB decomposed through demethylation pathway induced by hydroxyl radicals. The as-synthesized particles were found to be more photoactive those thermally annealed. The loss of photoactivity could be ascribed to two main factors: (1) loss of nitrogen and fluoride and (2) loss of surface area by sintering.Item Efficient Thermolysis Route to Monodisperse Cu2ZnSnS4 Nanocrystals with Controlled Shape and Structure(Nature Portfolio, 2014) Zhang, Xiaoyan; Guo, Guobiao; Ji, Cheng; Huang, Kai; Zha, Chenyang; Wang, Yifeng; Shen, Liming; Gupta, Arunava; Bao, Ningzhong; Nanjing Tech University; University of Alabama TuscaloosaMonodisperse Cu2ZnSnS4 (CZTS) nanocrystals with tunable shape, crystalline phase, and composition are synthesized by efficient thermolysis of a single source precursor of mixed metal-oleate complexes in hot organic solvents with dissolved sulfur sources. Suitable tuning of the synthetic conditions and the Cu/(Zn + Sn) ratio of the precursor has enabled precise control of the crystalline phase in the form of kesterite, or a newly observed wurtzite structure. Nanocrystals with morphology in the form of spherical, rice-like, or rod-like shapes are obtained over a wide range of compositions (0.5 <= Cu/(Zn + Sn) <= 1.2). Both the final products and intermediates for each shape exhibit consistent composition and structure, indicating homogenous nucleation and growth of single-phase nanocrystals. Thin films prepared from colloidal nanocrystal suspensions display interesting shape-dependent photoresponse behavior under white light illumination from a solar simulator.Item Elastic distortion determining conduction in BiFeO3 phase boundaries(Royal Society of Chemistry, 2020) Holsgrove, Kristina M.; Duchamp, Martial; Moreno, M. Sergio; Bernier, Nicolas; Naden, Aaron B.; Guy, Joseph G. M.; Browne, Niall; Gupta, Arunava; Gregg, J. Marty; Kumar, Amit; Arredondo, Miryam; Queens University Belfast; Nanyang Technological University & National Institute of Education (NIE) Singapore; Nanyang Technological University; CEA; Communaute Universite Grenoble Alpes; UDICE-French Research Universities; Universite Grenoble Alpes (UGA); University of St Andrews; University of Alabama TuscaloosaIt is now well-established that boundaries separating tetragonal-like (T) and rhombohedral-like (R) phases in BiFeO3 thin films can show enhanced electrical conductivity. However, the origin of this conductivity remains elusive. Here, we study mixed-phase BiFeO3 thin films, where local populations of T and R can be readily altered using stress and electric fields. We observe that phase boundary electrical conductivity in regions which have undergone stress-writing is significantly greater than in the virgin microstructure. We use high-end electron microscopy techniques to identify key differences between the R-T boundaries present in stress-written and as-grown microstructures, to gain a better understanding of the mechanism responsible for electrical conduction. We find that point defects (and associated mixed valence states) are present in both electrically conducting and non-conducting regions; crucially, in both cases, the spatial distribution of defects is relatively homogeneous: there is no evidence of phase boundary defect aggregation. Atomic resolution imaging reveals that the only significant difference between non-conducting and conducting boundaries is the elastic distortion evident - detailed analysis of localised crystallography shows that the strain accommodation across the R-T boundaries is much more extensive in stress-written than in as-grown microstructures; this has a substantial effect on the straightening of local bonds within regions seen to electrically conduct. This work therefore offers distinct evidence that the elastic distortion is more important than point defect accumulation in determining the phase boundary conduction properties in mixed-phase BiFeO3.Item Electrochemical and spectroscopic studies of bodipy dyes and nanostructured electrodes for solar energy harvesting and conversion(University of Alabama Libraries, 2018) Kaneza, Nelly; Pan, Shanlin; University of Alabama TuscaloosaThe use of fossil fuel-based technologies has contributed to the increase in the concentration of greenhouse gases, especially CO2, causing global climate change. To achieve the global energy demand, advanced energy technologies for renewable and sustainability applications have been gaining much attention. Among several promising approaches, stable organic chromophores and nanostructured materials have provided new opportunities in their application in solar energy harvesting and conversion and storage. This dissertation explores the emerging technologies to harvest solar energy to generate (1) electricity using boron dipyrromethene (BODIPY)-thiophene- triphenylamine (TPA) dye-sensitized solar cells, (2) hydrogen fuel using an unbiased Z-scheme tandem cell, and (3) carbon-free fuels by reducing atmospheric CO2 in the presence of water. Nanostructured electrode materials are used for all these three major projects described in this dissertation because of their surface to volume ratios, tunable light absorption, and enhanced charge transport and transfer. By engineering the structural properties of the proposed functional nanomaterials with respect to increasing solar energy conversion and at a low cost, earth abundant and environmental friendly, metal oxide nanomaterials were synthesized and characterized using different analytical techniques. Firstly, this study investigated a series of BODIPY-based dye-sensitized solar cells (DSSCs). Due to their promising potential as efficient photosensitizers, the synthesized BODIPY-based dyes (Dyes 1-5) containing thiophene and/or triphenylamine as electron donors were studied using optical and electrochemical techniques. Although the highest power efficiency achieved was low, correlation between the BODIPY dye structure and properties were established. Secondly, inspired by nature’s photosynthesis, a Z-scheme solar water splitting system comprised of carbon-modified cuprous oxide (C /Cu2O) nanoneedles and oxygen-deficient titanium dioxide (TiO2-x) nanorods in tandem cell was established to enhance charge carrier-separation for unassisted solar water splitting. Although the overall tandem performance is still limited by the C/Cu2O NNs performance, the proposed tandem cell exhibited a photo-induced catalytic activity of 64.7 µA cm-2 that unfortunately gradually decreases over time. Lastly, a photocatalytic anode material, cobalt-doped WO3/BiVO4, was combined with CuO-based nanoneedles to demonstrate a low cost approach to reduce CO2 selectively and water oxidation under sunlight. In addition to highlighting the optical, electrochemical, and spectroscopic advantages of the proposed nanostructured materials, their limitations and challenges were also addressed.Item Electron tunneling in the tight-binding approximation(University of Alabama Libraries, 2016) Mackey, Frederick D.; Butler, W. H.; University of Alabama TuscaloosaIn this thesis, we treat tunneling similar to a scattering problem in which an incident wave on a barrier is partially transmitted and partially reflected. The transmission probability will be related to the conductance using a model due to Landauer. Previously tunneling has been treated using a simple barrier model, which assumes the electron dispersion is that of free electrons. In this model it is not possible to investigate tunneling in the gap between a valence band and a conduction band. We shall remedy this limitation by using the tight-binding model to generate a barrier with a gap separating a valence band and a conduction band. To do this, we constructed a model consisting of semi-infinite chains of A atoms on either side of a semi-infinite chain of B-C molecules. The B-C chain has a gap extending between the onsite energy for the B atom and the onsite energy for the C atom. Tunneling through the gap has been calculated and plotted. We present exact closed form solutions for the following tunneling systems: (i) A-B interface, (ii) A-(B-C) interface, (iii) A-B-A tunnel barrier, (iv) A-(B-C) interface with the orbitals on B having s-symmetry and those on C having p-symmetry, (v) A-(B-C)-A tunnel barrier.Item Electron-deficient conjugated materials(University of Alabama Libraries, 2018) Cao, Hongda; Rupar, Paul A.; University of Alabama TuscaloosaThe development of new conjugated materials with tailored properties is in high demand for applications in organic electronics. This has led to conjugated materials receiving a great deal of research attention over the last few decades. One efficient method to prepare these novel conjugated small molecules and polymers is the incorporation of p-block elements into conjugated systems. In Chapter 2, a new phosphafluorene is synthesized and copolymerized to produce a donor-acceptor (D-A) conjugated copolymer PPF-BDTT. Direct post-polymerization modification of PPF-BDTT was performed on the phosphorus center to prepare the phosphine sulfide polymer PPF-BDTT-S and phosphine gold chloride polymer PPF-BDTT-Au. These D-A polymers have been fully characterized, and their applications in organic solar cells was also investigated. In Chapter 3, three air-stable difuran small molecules were synthesized by the incorporation of phosphorus, germanium, and silicon. The good stability is partially due to the σ*-π* conjugation interaction between p-block elements and the pi systems. These molecules show strong absorption in the UV region and intense emissions. The phosphorus bridged difuran was also copolymerized with fluorene to produce a D-A conjugated polymer. In Chapter 4, bithiazole (BTz) was functionalized with a 9-borabicyclononane (BBN) moiety to produce a boronium molecule BTz-BBN. With similar synthetic methods, two conjugated organoboronium polymers PFOBPy-BBN and PFOBTz-BBN were developed, which demonstrated a novel method to prepare conjugated boronium polymers in high yield. PFOBPy-BBN and PFOBTz-BBN were studied optically and electrochemically. In Chapter 5, benzothiophene dioxide was selected to prepare the non-fullerene acceptor ITBC with an acceptor-donor-acceptor structure. The strong electron-withdrawing sulfonyl acceptor units lead to extended UV-Vis absorption and low frontier molecular orbital energy levels with a narrow bandgap. A power conversion efficiency of 4.17 % was achieved by fabricating organic solar cells with polymer FTAZ as the donor and ITBC as the acceptor. In Chapter 6, a chlorinated bifuran small molecule ClBF was synthesized, and a series of random copolymers (PNDI-ClBFx) using ClBF and naphthalene diimide were prepared. These polymers exhibit strong and broad absorption, and the low-lying frontier energy levels of PNDI-ClBFx are suitable as polymer acceptors in all-polymer solar cells applications.