Browsing by Author "Yan, Feng"
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Item Atomic Layer Deposition for Surface Modifications and Solid Film Fabrication(University of Alabama Libraries, 2021) Yan, Haoming; Peng, Qing; University of Alabama TuscaloosaAlong with the unceasing development of the surface and material science, modification of substrates surfaces in nanoscale, to fabricate the functional materials with precisely controlled dimensions, refined composition and desired properties becomes crucial. In this report, atomic layer deposition (ALD), a vapor phase, sequential and self-limiting deposition process, has been used as an alternative strategy to modify the surface of materials and fabricates nanometer or micrometer level of functional materials with precise control. In the first part of this dissertation, ALD was used to modify the surface of the shape-engineered nanocrystals (SENCs), which enhanced the thermal stability of the SENCs from 300˚C to 700˚C and enhanced the catalytic activities of the nanocrystals as well. We also proposed a new reaction mechanism of metal-organic precursor with oxide surface, in which the conventional layered ALD growth does not happen but the oxide surface was modified via controlled metal doping. In the second part of this dissertation, ALD precursors were used to reacting with liquid substrates to fabricate freestanding solid thin films. Benefits from the unique reaction mechanism of the ALD metal-organic precursors, the thickness and the compositions of the fabricated films can be controlled. The fundamental of gas-liquid reaction has been discussed in this study. In the third part of this dissertation, area-selective ALD (AS-ALD) has been reported using carboxylic acid self-assembled monolayer as a growth inhibitor. Excellent selectivity of AS-ALD has been achieved by using this method, which could potentially be used in microfabrication as a substitution step for photolithography.Item Chemical variation induced nanoscale spatial heterogeneity in metallic glasses(Taylor & Francis, 2018) Wang, Neng; Ding, Jun; Luo, Peng; Liu, Yanhui; Li, Lin; Yan, Feng; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; Chinese Academy of Sciences; Institute of Physics, CASMetallic glasses possess amorphous structures with inherent heterogeneity at the nanoscale. A combined experimental and modeling investigation to elucidate the chemical effect on such nanoscale heterogeneity in a Cu-Zr-Al metallic glass system is conducted. By using the dynamic atomic force microscopy, we reveal a reduction of the nanoscale spatial heterogeneity in the local viscoelastic response after introducing Al into the Cu50Zr50 metallic glass. The change of such nanoscale heterogeneity can be contributed to the variation of local atomic structures. The addition of Al increases the population of the icosahedral short-range ordered clusters, thus reducing the structural heterogeneity at the nanoscale. IMPACT STATEMENTThis paper provides a combination between the nanoscale experimental and theoretical understanding of the chemical variation induced spatial heterogeneity in CuZrAl metallic glass and their impacts on the mechanical properties.Item CuSCN as the Back Contact for Efficient ZMO/CdTe Solar Cells(MDPI, 2020) Li, Deng-Bing; Song, Zhaoning; Bista, Sandip S.; Alfadhili, Fadhil K.; Awni, Rasha A.; Shrestha, Niraj; Rhiannon, DeMilt; Phillips, Adam B.; Heben, Michael J.; Ellingson, Randy J.; Yan, Feng; Yan, Yanfa; University of Toledo; University of Alabama TuscaloosaThe replacement of traditional CdS with zinc magnesium oxide (ZMO) has been demonstrated as being helpful to boost power conversion efficiency of cadmium telluride (CdTe) solar cells to over 18%, due to the reduced interface recombination and parasitic light absorption by the buffer layer. However, due to the atmosphere sensitivity of ZMO film, the post treatments of ZMO/CdTe stacks, including CdCl2 treatment, back contact deposition, etc., which are critical for high-performance CdTe solar cells became crucial challenges. To realize the full potential of the ZMO buffer layer, plenty of investigations need to be accomplished. Here, copper thiocyanate (CuSCN) is demonstrated to be a suitable back-contact material with multi-advantages for ZMO/CdTe solar cells. Particularly, ammonium hydroxide as the solvent for CuSCN deposition shows no detrimental impact on the ZMO layer during the post heat treatment. The post annealing temperature as well as the thickness of CuSCN films are investigated. Finally, a champion power conversion efficiency of 16.7% is achieved with an open-circuit voltage of 0.857 V, a short-circuit current density of 26.2 mA/cm(2), and a fill factor of 74.0%.Item Design and discovery of a novel half-Heusler transparent hole conductor made of all-metallic heavy elements(Nature Portfolio, 2015-06-24) Yan, Feng; Zhang, Xiuwen; Yu, Yonggang G.; Yu, Liping; Nagaraja, Arpun; Mason, Thomas O.; Zunger, Alex; Northwestern University; University of Colorado System; University of Colorado Boulder; University of Alabama TuscaloosaTransparent conductors combine two generally contradictory physical properties, but there are numerous applications where both functionalities are crucial. Previous searches focused on doping wide-gap metal oxides. Focusing instead on the family of 18 valence electron ternary ABX compounds that consist of elements A, B and X in 1:1:1 stoichiometry, we search theoretically for electronic structures that simultaneously lead to optical transparency while accommodating intrinsic defect structures that produce uncompensated free holes. This leads to the prediction of a stable, never before synthesized TaIrGe compound made of all-metal heavy atom compound. Laboratory synthesis then found it to be stable in the predicted crystal structure and p-type transparent conductor with a strong optical absorption peak at 3.36 eV and remarkably high hole mobility of 2,730 cm(2)V(-1)s(-1) at room temperature. This methodology opens the way to future searches of transparent conductors in unexpected chemical groups.Item Electrospun Cadmium Selenide Nanoparticles-Loaded Cellulose Acetate Fibers for Solar Thermal Application(MDPI, 2020-07-08) Angel, Nicole; Vijayaraghavan, S. N.; Yan, Feng; Kong, Lingyan; University of Alabama TuscaloosaSolar thermal techniques provide a promising method for the direct conversion of solar energy to thermal energy for applications, such as water desalination. To effectively realize the optimal potential of solar thermal conversion, it is desirable to construct an assembly with localized heating. Specifically, photoactive semiconducting nanoparticles, when utilized as independent light absorbers, have successfully demonstrated the ability to increase solar vapor efficiency. Additionally, bio-based fibers have shown low thermal conductive photocorrosion. In this work, cellulose acetate (CA) fibers were loaded with cadmium selenide (CdSe) nanoparticles to be employed for solar thermal conversion and then subsequently evaluated for both their resulting morphology and conversion potential and efficiency. Electrospinning was employed to fabricate the CdSe-loaded CA fibers by adjusting the CA/CdSe ratio for increased solar conversion efficiency. The microstructural and chemical composition of the CdSe-loaded CA fibers were characterized. Additionally, the optical sunlight absorption performance was evaluated, and it was demonstrated that the CdSe nanoparticles-loaded CA fibers have the potential to significantly improve solar energy absorption. The photothermal conversion under 1 sun (100 mW/cm(2)) demonstrated that the CdSe nanoparticles could increase the temperature up to 43 degrees C. The CdSe-loaded CA fibers were shown as a feasible and promising hybrid material for achieving efficient solar thermal conversion.Item Electrospun single-phase spinel magnetic high entropy oxide nanoparticles via low-temperature ambient annealing(Royal Society of Chemistry, 2023) Han, Xiao; Li, Dian; Zhou, Jingyi; Zheng, Yufeng; Kong, Lingyan; Li, Lin; Yan, Feng; University of Alabama Tuscaloosa; University of Nevada Reno; Arizona State University; Arizona State University-TempeHigh entropy oxide nanoparticles (HEO NPs) with multiple component elements possess improved stability and multiple uses for functional applications, including catalysis, data memory, and energy storage. However, the synthesis of homogenous HEO NPs containing five or more immiscible elements with a single-phase structure is still a great challenge due to the strict synthetic conditions. In particular, several synthesis methods of HEO NPs require extremely high temperatures. In this study, we demonstrate a low cost, facile, and effective method to synthesize three- to eight-element HEO nanoparticles by a combination of electrospinning and low-temperature ambient annealing. HEO NPs were generated by annealing nanofibers at 330 degrees C for 30 minutes under air conditions. The average size of the HEO nanoparticles was similar to 30 nm and homogenous element distribution was obtained from post-electrospinning thermal decomposition. The synthesized HEO NPs exhibited magnetic properties with the highest saturation magnetization at 9.588 emu g(-1) and the highest coercivity at 147.175 Oe for HEO NPs with four magnetic elements while integrating more nonmagnetic elements will suppress the magnetic response. This electrospun and low-temperature annealing method provides an easy and flexible design for nanoparticle composition and economic processing pathway, which offers a cost- and energy-effective, and high throughput entropy nanoparticle synthesis on a large scale.Item Enhancing Mechanical Properties and Design of Metallic Glasses Through Nanoscale Heterogeneities and Machine Learning Optimization(University of Alabama Libraries, 2023) Gu, Yucong; Daniewicz, Steven R.; Li, LinMetallic glasses (MGs) are a type of metal alloy that has a disordered atomic structure similar to glass. They possess unique properties such as high strength, elasticity, and corrosion resistance. However, their susceptibility to catastrophic failure through shear banding has limited their widespread use. Recently, the local ordering in MGs has been identified within the amorphous structure, which can influence the physical and mechanical properties of these materials. This dissertation investigates the effect of local ordering and nanoscale heterogeneities on shear band behaviors and optimizes MG designs accordingly. Firstly, a mesoscale shear transformation zone (STZ) dynamic model is employed to investigate the deformation behaviors of MGs. The presence of nanoscale heterogeneity results in a Hall-Petch-like relationship between yield stress and spatial correlation length of heterogeneity. Secondly, dynamic mechanical responses of MGs are studied via experiments and simulations on thin film MGs with various nanoscale heterogeneities. The strain rate sensitivity transition is attributed to a shift in deformation mechanisms from structure-dictated strain localization to stress-dictated strain percolation into a shear band. Finally, a data-driven design framework is developed using artificial neural networks (ANN) and a genetic algorithm (GA) to optimize dual-phase MG designs. The ANN models are trained using simulation data from the STZ dynamic model, and the GA is used to guide the development of new dual-phase MGs with improved mechanical properties. Additionally, the data-driven design framework is extended to optimize material performance under both mechanical and electrochemical processes, minimizing material degradation under the simultaneous action of wear and corrosion based on hierarchical ANN models trained on multiphysics simulations. This dissertation integrates machine learning with multiscale and multiphysics models to uncover the mechanisms that explain the emergence of mechanical behaviors of MGs and explores a massive design space for property optimization. These findings provide valuable insights into the material structure-property relationships, enabling informed decision-making for material design.Item Going Beyond Shockley-Queisser Limit Perovskite Chalcogenides Tandem Solar Cell(University of Alabama Libraries, 2023) Gokul Menon, Harigovind; Yan, Feng; Daniewicz, StevenThe efficiencies of single junction solar cell (SJ SC) technologies like silicon and cadmium telluride have been rapidly increasing and are nearing their fundamental Shockley Queisser (SQ) efficiency limit. New novel methods must be developed to go beyond the limitations of SJ SCs. A promising approach is the combination of an efficient narrow bandgap (NBG) absorber with a wide bandgap (WBG) absorber to form a tandem solar cell that can effectively utilize the solar spectrum. The aim of this work is the realization of mechanically stacked perovskite-antimony selenide (Sb2Se3) and perovskite-cadmium telluride (CdTe) 4T tandem solar cells. Perovskite materials with their excellent performance and tunable bandgap make it an ideal candidate for tandem solar cells. Sb2Se3 with a bandgap of 1.2eV and a theoretical efficiency limit of over 30% and CdTe with a bandgap of 1.5eV and an efficiency over 22% make them potential NBG absorbers. In this work, we first simulated 4T tandem cells pairing a 1.6eV WBG Perovskite solar cell (PSC) with a 1.2 eV NBG Sb2Se3 cell and a 1.6eV WBG PSC with a 1.5 eV NBG CdTe cell using Solar Cell Capacitance Simulator (SCAPS). We obtained a simulated tandem PCE of 23.14 % for the perovskite Sb2Se3 tandem and a PCE of 23.37% for the perovskite CdTe tandem. We then developed two WBG perovskite top cells with a bandgap of 1.6eV and 1.77eV to study the impact of bandgap on the tandem architecture. We also developed a CdTe solar cell with Cu doping which had a PCE of 17.94% and a Sb2Se3 solar cell with a PCE of 5.76%. As a substitute for the opaque metal back electrode for the top cell, we developed an indium tin oxide (ITO) transparent electrode using DC sputtering to fabricate a semitransparent top cell. By mechanically stacking the sub-cells, we obtained an excellent 4T tandem PCE of 16.13% for the perovskite-Sb2Se3 tandem and a tandem PCE of 19.41% for the perovskite-CdTe tandem. The experimental results show promising tandem cell performance and pave the way to go beyond the SQ limit.Item Growth, Characterization, and Properties of Bismuth Ferrite-Based Multiferroic Complex Oxides(University of Alabama Libraries, 2022) Joshi, Chhatra Raj; Gupta, Arunava; Mankey, Gary; University of Alabama TuscaloosaMaterials that have at least two coupled electric, magnetic, and structural order parameters resulting in simultaneous ferroelectricity, ferromagnetism, and ferroelasticity are known as multiferroic materials. Bismuth ferrite (BiFeO3) is one of the most heavily studied room temperature single-phase multiferroic material. The simultaneous existence of ferroelectricity and antiferromagnetism with cross-coupling between these order parameters has driven intense research to accomplish electric field control of magnetism. To utilize these materials in electronic applications it is desirable to increase the magnetization and magnetoelectric coupling while reducing the switching voltage and leakage current. Tuning these responses can be achieved via strain and/or elemental engineering techniques. As the former is limited by the availability of suitable high-quality substrates for control of strain state, the latter is a more flexible technique. This thesis focuses on a systematic study of growth, structural, electrical, and magnetic characterizations of epitaxial thin films of multiferroic BiFeO3 (BFO) and Fe-site substituted BiFeO3. High-quality multiferroic epitaxial films of BiFeO3 on SrRuO3 buffered(001)-oriented SrTiO3 substrates fabricated using pulsed laser deposition are investigated. Switching dynamics of BiFeO3 have been explored using three fundamental scaling laws: Kittel's law, Kay-Dunn law, and the Ishibashi-Orihara model to acquire a complete description of the dynamical behavior and its relationship to the microstructure of the films. The primary goal of this work is to explore Fe-site substitution with magnetic elements Co and Mn, and non-magnetic element Al in BFO over a wide range of compositions. The enhancement of piezoelectric properties, electrical conductivity, and magnetic properties have been achieved through cobalt substitution. On the other hand, reduction in leakage current, and enhancement in magnetic and piezoelectric properties have been achieved through Al substitution. Moreover, we analyzed the switching dynamics in the time domain and corroborated the findings with the domain structure via a microscopy technique. It is demonstrated that Fe-site substitution of BFO is indeed a viable option to achieve improved characteristics required for device use. By identifying thebenefits of Fe-site substitution, this dissertation provides a pathway to explore BFO-based alternate magnetoelectric materials with desired properties for device applications.Item High-Efficiency and Stable Carbon-Based Planar Perovskite Solar Cells(University of Alabama Libraries, 2023) Sankaranarayanan Nair, Vijayaraghavan; Daniewicz, StevePerovskite solar cells (PSCs) have attracted much attention both in research and industrial domains. The power conversion efficiencies (PCE) of PSCs have been improved from 3.8% to 25.8% in just over a decade, rivaling that of silicon solar cells. Hybrid perovskite materials have exceptional optoelectrical properties and can be processed using cost-effective solution-based methods. In contrast, the fabrication of silicon solar cells requires high-vacuum, high-temperature, and energy-intensive processes. The combination of excellent optoelectrical properties and cost-effective manufacturing makes hybrid perovskite a winning candidate for solar cells.As the PCE of PSCs improves and their long-term stability increases, one of the crucial hurdles to be addressed is the cost-effective scalable fabrication and its long-term stability. Most high-efficiency solar cells require an energy-consuming method of depositing the cathode to complete the cell. These high-efficiency devices also require highly expensive noble metal electrodes. Herein, following recommendations from the literature, carbon-based nanomaterials are utilized, and their effects on the efficiency, fill factor (FF), open-circuit voltage (Voc), and short circuit density (Jsc) are analyzed. The PSCs and carbon counter electrodes are fabricated at low temperatures (~100 degrees Celsius). As per the literature, carbon-based devices exhibited an efficiency of >19%. This endeavor further buttresses the fact that carbon nanomaterials are promising candidates for the future of low-cost, high-performance, and scalable production of PSCs. Thus, complex vacuum deposition of expensive cathodes to complete the cells might be eliminated.In this thesis, I focus on several novel interface engineering techniques, that can reduce the surface roughness of the carbon electrode, and improve the interface it forms with the underlying layer. These interfacial engineering techniques improved the charge collection efficiency of the carbon, and thereby reduced the recombination happening at the interface. As a result, the PCE could be enhanced from ~13% to >18%.In addition to these techniques, a high-quality narrow band gap perovskite layer was developed with low dimensional 2D perovskite passivation to further improve the device performance. Together with the improved perovskite film quality and optimized film composition, along with interface engineered carbon electrode, an impressive PCE of >21% was attained.Item Improve the stability of organic-inorganic hybrid perovskite by vapor-solid reaction(University of Alabama Libraries, 2019) Yu, Xiaozhou; Peng, Qing; University of Alabama TuscaloosaAbstract Organic-inorganic hybrid perovskites, such as CH3NH3PbI3 and NH2CH=NH2PbI3, emerge as a new class of low-cost semiconductors that have the potential applications in high-efficiency photovoltaic cells, light emitting diodes, lasers, and sensors. However, hybrid perovskites can be easily degraded by H2O, O2, and light in ambient conditions. To improve the stability of hybrid perovskites, we carried out a comprehensive study including the degradation kinetics and surface modification by vapor-solid reactions for encapsulation. The degradation kinetics of perovskites were studied by using in situ methods. We found CH3NH3PbI3 perovskite degrades slowly at 85°C. This result indicates hybrid perovskites alone is not stable in the working conditions. We enhanced the stability of perovskites by surface modification through studying the surface reaction mechanism on perovskites. We found that by increasing the partial pressure of vapor reactants such as pyridine, the vapor-perovskite reactions will change from surface terminated reaction to bulk transformation reactions. A thin pin-hole free oxide barrier layer cannot only block H2O and O2 from meeting perovskites but also encapsulate the gas byproducts from the degradation reactions to stop the reversible degradation reaction. Atomic layer deposition (ALD) is a promising method to deposit a pinhole-free metal oxide barrier film onto perovskites. Although there are numerous reports in applying ALD on hybrid perovskites, the nucleation mechanism of ALD on these perovskites are poorly understood. Herein, we will present our findings about the atomic level surface reaction mechanism during ALD on perovskite-related substrates. Collectively, we are able to create a couple of new pathways to improve the stability of perovskite materials.Item In-situ observation of trapped carriers in organic metal halide perovskite films with ultra-fast temporal and ultra-high energetic resolutions(Nature Portfolio, 2021) Kobbekaduwa, Kanishka; Shrestha, Shreetu; Adhikari, Pan; Liu, Exian; Coleman, Lawrence; Zhang, Jianbing; Shi, Ying; Zhou, Yuanyuan; Bekenstein, Yehonadav; Yan, Feng; Rao, Apparao M.; Tsai, Hsinhan; Beard, Matthew C.; Nie, Wanyi; Gao, Jianbo; Clemson University; United States Department of Energy (DOE); Los Alamos National Laboratory; Huazhong University of Science & Technology; Jilin University; Hong Kong Baptist University; Technion Israel Institute of Technology; University of Alabama Tuscaloosa; National Renewable Energy Laboratory - USAWe in-situ observe the ultrafast dynamics of trapped carriers in organic methyl ammonium lead halide perovskite thin films by ultrafast photocurrent spectroscopy with a sub-25 picosecond time resolution. Upon ultrafast laser excitation, trapped carriers follow a phonon assisted tunneling mechanism and a hopping transport mechanism along ultra-shallow to shallow trap states ranging from 1.72-11.51 millielectronvolts and is demonstrated by time-dependent and independent activation energies. Using temperature as an energetic ruler, we map trap states with ultra-high energy resolution down to < 0.01 millielectronvolt. In addition to carrier mobility of similar to 4 cm(2)V(-1)s(-1) and lifetime of similar to 1 nanosecond, we validate the above transport mechanisms by highlighting trap state dynamics, including trapping rates, de-trapping rates and trap properties, such as trap density, trap levels, and capture-cross sections. In this work we establish a foundation for trap dynamics in high defect-tolerant perovskites with ultra-fast temporal and ultra-high energetic resolution.Item Local charge writing in epitaxial SmNiO3 thin films(2014-02-28) Yan, Feng; Schoofs, Frank; Shi, Jian; Ha, Sieu D.; Jaramillo, R.; Ramanathan, Shriram; University of Alabama TuscaloosaWe have investigated the evolution of work function in epitaxial correlated perovskite SmNiO3 (SNO) thin film spanning the metal-insulator transition (MIT) by Kelvin probe force microscopy (KPFM). Combining contact-mode atomic force microscopy, KPFM and electrostatic force microscopy (EFM), we present charge writing processes associated with point defect engineering in the SNO thin films. Surface potential tuning in two-terminal devices is demonstrated and compared to thermal control by proximity to the phase transition boundary. The charge distribution, retention, and diffusion on SNO were systematically examined. Local compositional changes by AFM-tip induced electric fields is shown to be a viable approach to spatially engineer electronic properties of correlated oxides towards eventual applications in electronics.Item Local probing of magnetoelectric coupling and magnetoelastic control of switching in BiFeO3-CoFe2O4 thin-film nanocomposite(American Institute of Physics, 2013-07-25) Yan, Feng; Chen, Guannan; Lu, Li; Finkel, Peter; Spanier, Jonathan E.; University of Pennsylvania; National University of Singapore; University of Alabama TuscaloosaWe report on the combination of piezoresponse force microscopy (PFM), magnetic force microscopy, and local ferroelectric switching with magnetic field for the study of a thin-film magnetoelectric (ME) nanocomposite. The collection of PFM under an applied variable magnetic field within a polycrystalline perovskite-spinel BiFeO3-CoFe2O4 (BFO-CFO) 0-3 type thin-film nanocomposite enables quantitative and proximal measurement of magnetoelastic strain-driven ME response. Combination of measurement of the as-grown strain state with local measurements of microstructure and macroscopic magnetization permits local mapping of ME coupling. (C) 2013 AIP Publishing LLC.Item Multi-scale modeling of spatial heterogeneity effect on the shear banding behaviors in metallic glasses(University of Alabama Libraries, 2018) Wang, Neng; Li, Lin; University of Alabama TuscaloosaStronger than steels but able to be shaped and molded like plastics, the mechanical properties of metallic glasses (MGs) are proven to be scientific interest and potential applications in industry. However, MGs suffer from negligible plasticity prior to catastrophic failure in the form of a single shear band at room temperature, which precludes their immediate application as structural components. The structural and property heterogeneity have been found recently in MG. These nanoscale heterogeneities may influence the initiation and propagation of the shear band, which would thus improve the plasticity. To understand the structural and property heterogeneity effect on the shear band behaviors, this thesis is divided into three sections. First, Activation-relaxation technique (ART) and dynamic atomic force microscopy (DAFM) are employed to prove the existence of the nanoscale heterogeneity. ART discovers that the activation energy possesses a normal distribution, reflecting the non-uniform local structure. This non-uniformity should correspond to the different motifs found in the molecular dynamics simulation. The energy dissipation resulted from the DAFM also exhibits a normal distribution, thus the inelastic spatial heterogeneity is confirmed. Furthermore, the correlation length of the inelasticity is identified based on the 2D scanning figures from DAFM. Second, a mesoscale modeling technique, shear transformation zone dynamics (STZD) will be employed. A series of configurations with the spatial elastic heterogeneity will be built up. We find that the organization of such nanometer-scale shear transformation events into shear-band patterns is dependent on the spatial heterogeneity of the local shear moduli. A critical spatial correlation length of elastic heterogeneity is identified for the simulated MGs to achieve the best tensile ductility, which is associated with a transition of shear-band formation mechanisms, from stress-dictated nucleation and growth to structure-dictated strain percolation, as well as a saturation of elastically soft sites participating in the plastic flow. Third, a state variable, excess free volume, is incorporated into STZD model in order to introduce the strain softening which is typical during MG deformation. The stress ‘overshoot’ and cyclic hardening of MGs have been successfully captured by the model. We found that it is the dynamic competition between free volume creation and annihilation that give rise to the signature stress overshoot in the homogeneous deformation regime at elevated temperatures during tension and cause the removal of large free volume sites in the confined deformation region during nanoindentation.Item Processing and Physical Functionality of Multicomponent High Entropy Materials(University of Alabama Libraries, 2023) Han, Xiao; Li, Lin; Daniewicz, StevenHigh entropy materials (HEMs) consist of five or more elements in near equal-atomic percentages and stabilize their structures due to high configurational entropy. The interactions between the manifold incorporated elements could give rise to unusual and often outstanding properties, such as excellent mechanical properties, electric properties, and magnetic properties, making them promising for applications in electric devices, semiconductors, engines, hard disks, and sensors. This study aims to synthesize high entropy materials and investigate the relationships between chemical compositions, microstructures, and functional properties. To fabricate various metallic thin films, including metallic glasses and high entropy alloys (HEAs), we employ magnetron sputtering and extensively study the processing-microstructure-property relationship to optimize thin film performance. Additionally, we successfully fabricate a high entropy selenide thin film using a combination of magnetron sputtering and chemical vapor deposition (CVD). Finally, we demonstrate a low-cost, facile, and effective method for synthesizing three- to eight-element single-phase high entropy oxide (HEO) nanoparticles using electrospinning. Chapter 3 investigates the impact of processing-induced local nanoscale heterogeneity of CuZr thin film metallic glasses (TFMGs) on the mechanical and electrical properties by single-target sputtering and co-sputtering methods. Chapter 4 explores the enhanced coercivity of magnetron sputtered Alnico α1 films on Pt/TiO2/SiO2/Si substrates, resulting from the Pt interdiffusion between the α1 phase and Pt buffer layer. In Chapter 5, we develop a two-step vapor deposition method to grow a new (FeCoNiCrMo)Sex high entropy selenide thin film by magnetron sputtering of HEA films and followed by selenization with CVD, which exhibits excellent electrical and optical properties. In Chapter 6, we demonstrate a low-cost, facile, and effective method to synthesize three- to eight-element single-phase spinel magnetic HEO nanoparticles by electrospinning and low-temperature ambient annealing. HEMs offer a promising avenue for achieving extraordinary material properties beyond traditional dilute materials by expanding the multi-dimensional compositional space to a gigantic stoichiometry. This dissertation work demonstrates various successful synthesis methods for fabricating HEMs, which can be modified or combined to tailor the synthesis of HEMs to specific applications. The resulting materials can then be characterized and optimized for their desired properties.Item Pyridine Modified Gold Nanoparticles for Electrocatalytic Carbon Dioxide Reduction and Methanol Oxidation Reactions(University of Alabama Libraries, 2021) Kang, Xin; Pan, Shanlin; University of Alabama TuscaloosaThe global consumption of fossil fuels produces an enormous amount of carbon dioxide, causing a series of environmental problems such as glacial melting, food production reduction, and sea-level rise. Because it can potentially help address the global energy challenges and environmental issues from fossil fuels, CO2 harvesting, storage, and conversion to chemical fuels are of great research interest. CO2 can be transformed into chemical fuels (e.g., CO, CH4, and methanol) by electrochemical reduction reaction. Catalysts play critical roles in enhancing the selectivity and lowering the overpotential of the CO2 reduction reaction. For example, the main product of the carbon dioxide reduction reaction with the gold electrode is CO due to its low adsorption on the gold surface. Pyridine has been used to promote CO2 reduction by controlling the local pH of an electrode to enhance catalytic efficiency and selectivity. This research project primarily focuses on developing pyridine-modified Au nanoparticles for catalyzing electrochemical CO2 reduction and methanol Oxidation. The effects of the gold-pyridine conjugate formula, reduction potential with a rotating ring disc electrode (RRDE) and RRDE rotation speed on CO2 reduction current density and product collection efficiency, and electrode stability are investigated. Pyridine-modified nanoparticles improve the CO2 reduction catalytic reaction. Au/Cu nanoparticles are supposed to have better catalytic performance CO2 reduction than pure Pt nanoparticles due to the synergistic effect of Au and Cu promoting efficient reduction and desorption of CO from its surface. This study also investigated the MOR activities of pyridine-modified Au nanostructures. Au/Pt nanoparticles are supposed to have better catalytic performance for methanol oxidation than pure Au nanoparticles.Item Rare-Earth-Free Ferromagnetic Magnetic Materials and Spoke-Type Permanent Magnet Synchronous Motor(University of Alabama Libraries, 2023) Choi, Minyeong; Hong, Yang-KiThis dissertation consists of two parts. The first part focuses on rare-earth (RE)-free ferromagnetic materials for permanent magnets (PMs) and heat-assisted magnetic recording media (HAMR) applications, and the other part presents spoke-type permanent magnet synchronous motor (PMSM) for electric vehicles and hybrid electric vehicles applications. The objective of this dissertation is (i) to design RE-free PMs improving the stability of the 𝜏-phase of MnAl and low-temperature phase (LTP) of MnBi alloys, therefore, high saturation magnetization (Ms), remanent magnetic flux density (Br), and magnetocrystalline anisotropy constant (K), resulting in high maximum energy product ((BH)max), (ii) to calculate Curie temperature (TC) of the designed RE-free ferromagnetic materials, and (iii) to optimize carbon concentration of Fe-Pt-C alloy system for magnetic information data storage density of 4Tb/in2, and (iv) to apply the designed RE-free permanent and soft magnets to spoke-type PMSM for motor performance evaluation. Our theoretical results show that a 2.33 at.% of carbon stabilizes 𝜏-phase MnAl. Partial Fe substitution for Mn of MnAl increases K, and Co or Ni substitution changes the magnetic anisotropy direction to the in-plane from the out-of-plane. When Cu was inserted in the interstitial sites of MnBi, the magnetocrystalline anisotropy changed to the out-of-plane from the in-plane direction at 14 at.% Cu. This indicates that interstitial Cu can prevent the Mn atom from its diffusion into the interstitial sites, resulting in improved LTP stability. The partial Sn substitution for Bi of MnBi increased K and reoriented spin direction. Amoropus Fe-P-C-Ge alloys were designed for the motor's stator and rotor core applications. Four atomic % of Ge increases the saturation magnetization at 300 K. HAMR needs 25Tgrains/in2 to achieve an areal data density of 4T/in2. Carbon atoms refine grains of FePt thin film. Thus, we have inserted carbon atoms into the interstitial sites of Fe-Pt and optimized carbon concentration of 12 at.% Lastly, we designed a spoke-type PMSM with commercial and our theoretically designed PMs and simulated motor performance. It was found that regardless of (BH)max, coercivity (Hc) plays a dominant role in motor performance. A motor performs best when the ratio of Hc to Br equals one.Item Scalable and Highly Efficient Antimony Selenide Thin Film Solar Cells by Close Spaced Sublimation(University of Alabama Libraries, 2021) Guo, Liping; Yan, Feng; University of Alabama TuscaloosaSb2Se3 has emerged as a promising absorber material for solar cells owing to its superior optical and electronic properties, such as high absorption coefficient and suitable bandgap. It is a simple binary compound comprised of environmentally friendly and earth-abundant constituents, with one stable phase under normal conditions. More importantly, the crystal structure of Sb2Se3 is the unique one-dimensional ribbons that are stacked together by weak van der Waals force. The grain boundary is inert without dangling bonds, leading to significantly reduced combination centers. Furthermore, based on the S-Q limit, the theoretical maximum power conversion efficiency of Sb2Se3 can achieve as high as ~32%, implying the great potential to be employed for highly efficient solar cells. The goal of this dissertation work is to fabricate Sb2Se3 thin-film solar cells using close-spaced sublimation (CSS), which falls into three parts: (1) growth of high-quality Sb2Se3 thin films with CSS for thin-film solar cells, (2) investigation of the various buffer layer, and (3) hole-transport layer exploration for Sb2Se3 thin-film solar cells. In the first study, we systematically studied the growth behavior of Sb2Se3 with the CSS, particularly the dependence of the grain orientation on growth conditions, e.g., substrate temperature, thickness, etc. After optimization, we obtained Sb2Se3 films with desired crystal texture, i.e., (211)- and (221)-preferred orientation, thereby realizing efficiency of 4.27%. This work lays the foundation for our future work further optimizing the Sb2Se3 solar cells. The second study focuses on buffer layer engineering for the Sb2Se3 solar cells. Here we tried three different buffer layers: the oxygenated CdS via chemical bath deposition, oxygenated CdS via sputtering and sputtered CdSe. We carefully studied the optical response, band alignment with Sb2Se3, element diffusion, and the impact on grain orientation for each buffer layer. After careful engineering, we achieved 6.3%, 7.01%, and 4.5% for each buffer layer, respectively. Lastly, we applied NiOx as the hole-transport layer (HTL) on Sb2Se3 devices to construct a p-in configuration. The addition of NiOx HTL benefits charge extraction and helps reduce recombination at the backside via surface passivation. The thickness of the NiOx is a critical parameter for device performance. After the systematic screening, a decent efficiency enhancement from 6.6% to 7.3% was achieved with optimized NiOx HTL.Item Solution-processed doping in CdTe thin film solar cells(University of Alabama Libraries, 2019) Montgomery, Angeqlique; Yan, Feng; University of Alabama TuscaloosaCadmium Telluride (CdTe) is one of the leading photovoltaic (PV) technologies in the world with a world record ~ 22.1%. With a bandgap of 1.45eV and a high absorption coefficient, the theoretical power conversion efficiency limit of 32% is limited by recombination and high resistivities in CdTe devices. Doping CdTe is a necessary way to improve device performance. In this work, it is demonstrated that a cost-effective solution-processed Group I and Group V doping in CdTe thin film solar cells allows for efficiency increases and stability improvement in CdTe devices. By varying the doping concentration, activation annealing temperatures, and deposition parameters, the root cause for the increased power conversion efficiency was investigated. Group I and Group V dopants showed smoother films indicating less resistance through the back contact leading to an increase in power conversion efficiencies.