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In this dissertation, the surface interactions of fluorine were studied during atomic layer deposition (ALD) and atomic layer etching (ALE) of wide band gap materials. To enable this research two high vacuum reactors were designed and constructed for thermal and plasma enhanced ALD and ALE, and they were equipped for in-situ process monitoring. Fluorine surface interactions were first studied in a comparison of thermal and plasma enhanced ALD (TALD and PEALD) of AlF3 thin films prepared using hydrogen fluoride (HF), trimethylaluminum (TMA), and H2-plasma. The ALD AlF3 films were compared ¬in-situ using ellipsometry and X-ray photoelectron spectroscopy (XPS). Ellipsometry showed a growth rate of 1.1 Å/ cycle and 0.7 Å/ cycle, at 100°C, for the TALD and PEALD AlF3 processes, respectively. XPS indicated the presence of Al-rich clusters within the PEALD film. The formation of the Al-rich clusters is thought to originate during the H2-plasma step of the PEALD process. The Al-rich clusters were not detected in the TALD AlF3 films. This study provided valuable insight on the role of fluorine in an ALD process.
Reactive ion etching is a common dry chemical etch process for fabricating GaN devices. However, the use of ions can induce various defects, which can degrade device performance. The development of low-damage post etch processes are essential for mitigating plasma induced damage. As such, two multistep ALE methods were implemented for GaN based on oxidation, fluorination, and ligand exchange. First, GaN surfaces were oxidized using either water vapor or O2-plasma exposures to produce a thin oxide layer. The oxide layer was addressed using alternating exposures of HF and TMG, which etch Ga2O3 films. Each ALE process was characterized using in-situ using ellipsometry and XPS and ex-situ transmission electron microscopy (TEM). XPS indicated F and O impurities remained on the etched surfaces. Ellipsometry and TEM showed a slight reduction in thickness. The very low ALE rate was interpreted as the inability of the Ga2O3 ALE process to fluorinate the ordered surface oxide on GaN (0001).
Overall, these results indicate HF is effective for the ALD of metal fluorides and the ALE of metal oxides.
ContributorsMessina, Daniel C (Author) / Nemanich, Robert J (Thesis advisor) / Goodnick, Stephen (Committee member) / Ponce, Fernando A (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2021
Description
Past experiments have revealed several unusual properties about interstitial hydrogen atoms in niobium. Absorption isotherms showed that niobium absorbs a large amount of hydrogen without changing its crystal structure. These isotherms also revealed that the interactions between hydrogen atoms in niobium are a combination of long-range attraction and short-range repulsion and exhibit many-body characteristics. Other experiments reported the facile thermal diffusion of hydrogen and deuterium in niobium. Contrary to the classical theory of diffusion, these experiments revealed a break in the activation energy of hydrogen diffusion at low temperatures, but no such break was reported for deuterium. Finally, experiments report a phenomenon called electromigration, where hydrogen atoms inside niobium respond to weak electric fields as if they had a positive effective charge. These experimental results date back to when tools like density functional theory (DFT) and modern high-performance computing abilities did not exist. Therefore, the current understanding of these properties is primarily based on inferences from experimental results. Understanding these properties at a deeper level, besides being scientifically important, can profoundly affect various applications involving hydrogen separation and transport. The high-level goal of this work is to use first-principles methods to explain the discussed properties of interstitial hydrogen in niobium. DFT calculations were used to study hydrogen atoms' site preference in niobium and its effect on the cell shape and volume of the host cell. The nature and origin of the interactions between hydrogen atoms were studied through interaction energy, structural, partial charge, and electronic densities of state analysis. A phenomenological model with fewer parameters than traditional models was developed and fit to the experimental absorption data. Thermodynamic quantities such as the enthalpy and entropy of hydrogen dissolution in niobium were derived from this model. The enthalpy of hydrogen dissolution in niobium was also calculated using DFT by sampling different geometric configurations and performing an ensemble-based averaging. Further work is required to explain the observed isotope effects for hydrogen diffusion in niobium and the electromigration phenomena. Applications of the niobium-hydrogen system require studying hydrogen's behavior on niobium's surface.
ContributorsRamcahandran, Arvind (Author) / Lackner, Klaus S. (Thesis advisor) / Zhuang, Houlong (Thesis advisor) / Muhich, Christopher (Committee member) / Singh, Arunima (Committee member) / Arizona State University (Publisher)
Created2021
Description
Hydrogen is considered one of the most potential fuels due to its highest gravimetric energy density with no pollutant emission during the energy cycle. Among several techniques for hydrogen generation, the promising photoelectrochemical water oxidation is considered a long-term solar pathway by splitting water. The system contains a photoanode and a cathode immersed in an aqueous electrolyte where charge separation takes place in the bulk of the semiconducting material on light absorption, leading to water oxidation/reduction at the surface of the photoelectrodes/cathode. It is imperative to develop materials that demonstrate high light absorption in the wide spectrum along with photoelectrochemical stability. N-type Monoclinic scheelite bismuth vanadate (BiVO4) is selected due to its incredible light absorption capabilities, direct bandgap (Eg ∼ 2.4-2.5 eV) and relatively better photoelectrochemical stability. However, BiVO4 encounters huge electron-hole recombination due to smaller diffusion lengths and positive conduction bands that cause slow charge dynamics and sluggish water oxidation kinetics. In order to improve the illustrated drawbacks, four strategies were discussed. Chapter 1 describe the fundamental understanding of photoelectrochemical cell and BiVO4. Chapter 2 illustrates details of the experimental procedure and state-of-the-art material characterization. Chapter 3 provide the impact of alkali metal placement in the crystal structure of BiVO4 systematically that exhibited ~20 times more performance than intrinsic BiVO4, almost complete bulk charge separation and enhancement in the diffusion length. Detailed characterization determined that the alkali metal getting placed in the interstitial void of BiVO4 lattice and multiple interbands formation enhanced the charge dynamics. Chapter 4 contains stoichiometric doping of Y3+ or Er3+ or Yb3+ at the Bi3+ site, leading to an extended absorption region, whereas non-stoichiometric W6+ doping at the V5+ site minimizes defects and increased charge carriers. To further enhance the performance, type-II heterojunction with WO3 along p-n junction with Fe:NiO enhance light absorption and charge dynamics close to the theoretical performance. Chapter 5 provides a comprehensive study of a uniquely developed sulfur modified Bi2O3 interface layer to facilitate charge dynamics and carrier lifetime improvement by effectively passivating the WO3/BiVO4 heterojunction interface. Finally, chapter 6 summarized the major findings, conclusion and outlook in developing BiVO4 as an efficient photoanode material.
ContributorsPrasad, Umesh (Author) / Kannan, Arunachala Mada (Thesis advisor) / Azeredo, Bruno (Committee member) / Chan, Candace (Committee member) / Segura, Sergio Garcia (Committee member) / Arizona State University (Publisher)
Created2021
Description
The current Li-ion batteries with organic liquid electrolytes are limited by their safety and energy density. Therefore, ceramic electrolytes are proposed in developing next-generation, energy-dense Li-metal batteries by replacing organic liquid electrolytes to improve safety and performance. Among numerous ceramic Li-ion conductors, garnet-based solid electrolyte c-Li7La3Zr2O12 (c-LLZO) is considered one of the most promising candidates to enable Li metal batteries due to its high ionic conductivity, chemical stability, and wide electrochemical stability window against Li metal. However, synthesis and processing of c-LLZO through conventional solid-sate reaction methods requires long periods of calcination (> 6 h) at high reaction temperatures (> 1000 °C). The need for high reaction temperature results to attain cubic-LLZO phase results in large aggerated LLZO particles and causes Li-loss from the garnet structure, making them unfavorable to process further as bulk pellets or thin films. To overcome processing challenges with solid-state reaction method, two novel facile synthesis approaches molten salt (flux growth method), and solution combustion are employed to produce submicron-sized LLZO powders at low reaction temperatures (< 1000 °C) in a short time. In the first case, molten salt synthesis method with LiCl-KCl eutectic mixture is employed to produce sub-micron sized Ta-doped LLZO (LLZTO) powders at low temperatures (900 °C, 4 h). In addition, a detailed investigation on effect of sintering medium and sintering additives on the structural, microstructural, chemical, and Li-ion transport behavior of the LLZTO pellets are investigated. Sintered LLZTO pellets prepared using molten salt synthesis route exhibited high Li-ion conductivity up to 0.6 mS cm-1 and high relative density (> 95 %) using Pt-crucible. In the second case, a facile solution-combustion technique using an amide-based fuel source CH6N4O is utilized to produce submicron-sized Al-doped LLZO (Al-LLZO) powders at low reaction temperatures 600-800 °C in a short duration of 4 h. In addition, effect of fuel to oxidizer ratio on phase purity, particle growth size, and formation mechanism of conductive Al-LLZO are reported and discussed. The Al-LLZO pellets sintered at 1100 °C/ 6 h exhibited high Li-ion conductivity up to 0.45 mS cm-1 with relative densities (> 90 %).
ContributorsBadami, Pavan Pramod (Author) / Kannan, Arunachalandar Mada (Thesis advisor) / Chan, Candace (Thesis advisor) / Song, Kenan (Committee member) / Arizona State University (Publisher)
Created2021
Description
Over the past three decades, significant progress in the development of organic light-emitting diodes (OLEDs) has been achieved, enabling OLEDs to become a main component in state-of-the-art displays and next generation solid-state lighting. As this technology advances, it is highly desirable to further improve the device efficiency and operational stability to drive the success of OLEDs in future display and lighting applications. This dissertation aims at developing novel organic emitting materials covering visible and near-infrared (NIR) emissions for efficient and table OLEDs. Firstly, a series of tetradentate Pd(II) complexes, which have attractive phosphorescent aggregate emission performance especially at high brightness level in device settings, have been developed. The luminescent lifetime of Pd(II) complex aggregates was demonstrated to be shorter than 1 μs with a close-to-unity photoluminescence quantum yield. Moreover, a systematic study regarding structure-property relationship was conducted on four tetradentate Pd(II) complexes, i.e., Pd3O3, Pd3O8-P, Pd3O8-Py2, and Pd3O8-Py5, featuring aggregate emission. As a result, an extremely efficient and stable OLED device utilizing Pd3O8-Py5 was achieved. It demonstrated a peak external quantum efficiency (EQE) of 37.3% with a reduced efficiency roll-off retaining a high EQE of 32.5% at 10000 cd m-2, and an estimated LT95 lifetime (time to 95% of the initial luminance) of 48246 h at 1000 cd m-2. Secondly, there is an increasing demand for NIR OLEDs with emission spectra beyond 900 nm to expand their applications in biometric authentication, night vision display, and telecommunication, etc. A stable and efficient NIR Pt(II) porphyrin complex named PtTPTNP-F8 was developed, and exhibited an electroluminescent spectrum at 920 nm. By carefully choosing the host materials, an PtTPTNP-F8 based NIR OLED achieved a EQE of 1.9%. Furthermore, an PtTPTNP-F8 OLED fabricated in a stable device structure demonstrated extraordinary operational stability with LT99 of >1000 h at 20 mA cm-2. Lastly, a series of imidazole-based blue Pt(II) complexes were developed and studied. Results indicated that structural modification of ligand molecules effectively tuned the emission spectral wavelength and bandwidth. Two blue complexes, i.e., Pt2O2 P2M and Pt2O2-PPy5-M, emitting at 472 and 476 nm respectively, exhibited narrow-band emission spectra with a full width at half maximum of 16 nm.
ContributorsCao, Linyu (Author) / Li, Jian (Thesis advisor) / Adams, James (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2021
Description
The increasing demand for structural materials with superior mechanical properties has provided a strong impetus to the discovery of novel materials, and innovations in processing techniques to improve the properties of existing materials. Methods like severe plastic deformation (SPD) and surface mechanical attrition treatment (SMAT) have led to significant enhancement in the strength of traditional structural materials like Al and Fe based alloys via microstructural refinement. However, the nanocrystalline materials produced using these techniques exhibit poor ductility due to the lack of effective strain hardening mechanisms, and as a result the well-known strength-ductility trade-off persists. To overcome this trade-off, researchers have proposed the concept of heterostructured materials, which are composed of domains ranging in size from a few nanometers to several micrometers. Over the last two decades, there has been intense research on the development of new methods to synthesize heterostructured materials. However, none of these methods is capable of providing precise control over key microstructural parameters such as average grain size, grain morphology, and volume fraction and connectivity of coarse and fine grains. Due to the lack of microstructural control, the relationship between these parameters and the deformation behavior of heterostructured materials cannot be investigated systematically, and hence designing heterostructured materials with optimized properties is currently infeasible. This work aims to address this scientific and technological challenge and is composed of two distinct but interrelated parts. The first part concerns the development of a broadly applicable synthesis method to produce heterostructured metallic films with precisely defined architectures. This method exploits two forms of film growth (epitaxial and Volmer-Weber) to generate heterostructured metallic films. The second part investigates the effect of different microstructural parameters on the deformation behavior of heterostructured metallic films with the aim of elucidating their structure-property relationships. Towards this end, freestanding heterostructured Fe films with different architectures were fabricated and uniaxially deformed using MEMS stages. The results from these experiments are presented and their implications for the mechanical properties of heterostructured materials is discussed.
ContributorsBerlia, Rohit (Author) / Rajagopalan, Jagannathan (Thesis advisor) / Sieradzki, Karl (Committee member) / Peralta, Pedro (Committee member) / Crozier, Peter (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2021
Description
ABSTRACTWith the National Aeronautics and Space Administration (NASA) Psyche Mission, humans will soon have the first opportunity to explore a new kind of planetary body: one composed mostly of metal as opposed to stony minerals or ices. Identifying the composition of asteroids from Earth-based observations has been an ongoing challenge. Although optical reflectance spectra, radar, and orbital dynamics can constrain an asteroid’s mineralogy and bulk density, in many cases there is not a clear or precise match with analogous materials such as meteorites. Additionally, the surfaces of asteroids and other small, airless planetary bodies can be heavily modified over geologic time by exposure to the space environment. To accurately interpret remote sensing observations of metal-rich asteroids, it is therefore necessary to understand how the processes active on asteroid surfaces affect metallic materials. This dissertation represents a first step toward that understanding. In collaboration with many colleagues, I have performed laboratory experiments on iron meteorites to simulate solar wind ion irradiation, surface heating, micrometeoroid bombardment, and high-velocity impacts. Characterizing the meteorite surface’s physical and chemical properties before and after each experiment can constrain the effects of each process on a metal-rich surface in space. While additional work will be needed for a complete understanding, it is nevertheless possible to make some early predictions of what (16) Psyche’s surface regolith might look like when humans observe it up close. Moreover, the results of these experiments will inform future exploration beyond asteroid Psyche as humans attempt to understand how Earth’s celestial neighborhood came to be.
ContributorsChristoph, John Morgan M. (Author) / Elkins-Tanton, Linda (Thesis advisor) / Williams, David (Committee member) / Dukes, Catherine (Committee member) / Sharp, Thomas (Committee member) / Bell III, James (Committee member) / Arizona State University (Publisher)
Created2023
Description
Electrochemical technologies emerge as a feasible solution to monitor and treat pollutants. Although electrochemical technologies have garnered widespread attention, their commercial applications are still constrained by the use of expensive electrocatalysts, and the bulky and rigid plate design of electrodes that restricts electrochemical reactor design to systems with poor electrode surface/ volume treated ratios. By making electrodes flexible, more compact designs that maximize electrode surface per volume treated might become a reality. This dissertation encompasses the successful fabrication of flexible nanocomposite electrodes for electrocatalysis and electroanalysis applications.First, nano boron-doped diamond electrodes (BDD) were prepared as an inexpensive alternative to commercial boron-doped diamond electrodes. Comparative detailed surface and electrochemical characterization was conducted. Empirical study showed that replacing commercial BDD electrodes with nano-BDD electrodes can result in a cost reduction of roughly 1000x while maintaining the same electrochemical performance.
Next, self-standing electrodes were fabricated through the electropolymerization of conducing polymer, polypyrrole. Surface characterizations, such as SEM, FTIR and XPS proved the successful fabrication of these self-standing electrodes. High mechanical stability and bending flexibility demonstrated the ability to use these electrodes in different designs, such as roll-to-roll membranes. Electrochemical nitrite reduction was employed to demonstrate the viability of using self-standing nanocomposite electrodes for electrocatalytic applications reducing hazardous nitrogen oxyanions (i.e., nitrite) towards innocuous species such as nitrogen gas. A high faradaic efficiency of 78% was achieved, with high selectivity of 91% towards nitrogen gas.
To further enhance the conductivity and charge transfer properties of self-standing polypyrrole electrodes, three different nanoparticles, including copper (Cu), gold (Au), and platinum (Pt), were incorporated in the polypyrrole matrix. Effect of nanoparticle wt% and interaction between metal nanoparticles and polypyrrole matrix was investigated for electroanalytical applications, specifically dopamine sensing. Flexible nanocomposite electrodes showed outstanding performance as electrochemical sensors with PPy-Cu 120s exhibiting a low limit of detection (LOD) of 1.19 µM and PPy-Au 120s exhibiting a high linear range of 5 µM - 300 µM.
This dissertation outlines a method of fabricating self-standing electrodes and provides a pathway of using self-standing electrodes based on polypyrrole and polypyrrole-metal nanocomposites for various applications in wastewater treatment and electroanalytical sensing.
ContributorsBansal, Rishabh (Author) / Garcia-Segura, Sergio (Thesis advisor) / Westerhoff, Paul (Committee member) / Perreault, Francois (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2023
Description
Perovskite solar cells are one of the rising stars in the solar cell industry. This thesis explores several approaches to enhance the properties of the perovskite layer and the solar cell devices in which they operate. They include studies of different antisolvent additives during spin coating of triple cation perovskites, the use of surfactants to improve the quality of perovskite film microstructures, the applicability of a new fabrication process, and the value of post-deposition thermal and chemical annealing processes.This thesis experimentally analyzes different antisolvents, viz., ethyl acetate, isopropyl alcohol, toluene, and chlorobenzene. It focuses on the antisolvent-assisted crystallization method to achieve homogenous nucleation of the perovskite film. Of all the antisolvents, ethyl acetate-treated films gave the best-performing device, achieving a power conversion efficiency of 15.5%.
This thesis also analyzes the effects of mixed antisolvents on the qualities of triple-cation perovskites. Different solution concentrations of chlorobenzene in ethyl acetate and isopropyl alcohol in ethyl acetate are optimized for optimal supersaturation to achieve enlarged perovskite grains. Evaluations are discussed in the context of solution polarity and boiling point of the antisolvents, where 25% chlorobenzene in ethyl acetate antisolvent mixture shows the best film properties.
Another study discusses a new fabrication process called electrical field-assisted direct ink deposition for large-scale printing of perovskite solar cells. This process involves the formation of nanodroplets under an electrical field deposited onto ITO/glass substrates. As a result, smooth Poly (3,4-ethylene dioxythiophene) polystyrene sulfonate layers are
ii
produced with an average effective electrical resistivity of 4.15104 0.26 -m compared to that of spin-coated films.
A successive chapter discusses the studies of the electrical field-assisted direct ink deposition of the photoactive CH3NH3PbI2 (MAPbI3) layer. Its focus is on the post-deposition chemical annealing of the MAPbI3 films in methylamine gas, termed as methylamine gas-assisted healing and growth of perovskite films. This treatment improved the smoothness, reduced porosity, increased density, and generated more uniform grain sizes. Moreover, it improved the inter-grain boundary contacts by eliminating secondary, fine-grained boundary structures. Mechanisms behind the initial liquefaction of the MAPbI3 film's subsequent re-solidification are discussed.
ContributorsGogoi, Banashree (Author) / Alford, Terry (Thesis advisor) / Petuskey, William (Thesis advisor) / Gould, Ian (Committee member) / Li, Jian (Committee member) / Arizona State University (Publisher)
Created2023
Description
Cellular metamaterials arouse broad scientific interests due to the combination of host material and structure together to achieve a wide range of physical properties rarely found in nature. Stochastic foam as one subset has been considered as a competitive candidate for versatile applications including heat exchangers, battery electrodes, automotive, catalyst devices, magnetic shielding, etc. For the engineering of the cellular foam architectures, closed-form models that can be used to predict the mechanical and thermal properties of foams are highly desired especially for the recently developed ultralight weight shellular architectures. Herein, for the first time, a novel packing three-dimensional (3D) hollow pentagonal dodecahedron (HPD) model is proposed to simulate the cellular architecture with hollow struts. An electrochemical deposition process is utilized to manufacture the metallic hollow foam architecture. Mechanical and thermal testing of the as-manufactured foams are carried out to compare with the HPD model. Timoshenko beam theory is utilized to verify and explain the derived power coefficient relation. Our HPD model is proved to accurately capture both the topology and the physical properties of hollow stochastic foam. Understanding how the novel HPD model packing helps break the conventional impression that 3D pentagonal topology cannot fulfill the space as a representative volume element. Moreover, the developed HPD model can predict the mechanical and thermal properties of the manufactured hollow metallic foams and elucidating of how the inevitable manufacturing defects affect the physical properties of the hollow metallic foams. Despite of the macro-scale stochastic foam architecture, nano gradient gyroid lattices are studied using Molecular Dynamics (MD) simulation. The simulation result reveals that, unlike homogeneous architecture, gradient gyroid not only shows novel layer-by-layer deformation behavior, but also processes significantly better energy absorption ability. The deformation behavior and energy absorption are predictable and designable, which demonstrate its highly programmable potential.
ContributorsDai, Rui (Author) / Nian, Qiong (Thesis advisor) / Jiao, Yang (Committee member) / Kwon, Beomjin (Committee member) / Liu, Yongming (Committee member) / Phelan, Patrick (Committee member) / Arizona State University (Publisher)
Created2021