Theses and Dissertations
This collection includes both ASU Theses and Dissertations, submitted by graduate students, and the Barrett, Honors College theses submitted by undergraduate students.
Displaying 41 - 50 of 64
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
Wide Bandgap (WBG) semiconductor materials are shaping day-to-day technologyby introducing powerful and more energy responsible devices. These materials have
opened the door for building basic semiconductor devices which are superior in terms of
handling high voltages, high currents, power, and temperature which is not possible using
conventional silicon technology. As the research continues in the field of WBG based
devices, there is a potential chance that the power electronics industry can save billions of
dollars deploying energy-efficient circuits in high power conversion electronics. Diamond,
silicon carbide and gallium nitride are the top three contenders among which diamond can
significantly outmatch others in a variety of properties. However, diamond technology is
still in its early phase of development and there are challenges involved in many aspects of
processing a successful integrated circuit. The work done in this research addresses three
major aspects of problems related to diamond technology. In the first part, the applicability
of compact modeling and Technology Computer-Aided Design (TCAD) modeling
technique for diamond Schottky p-i-n diodes has been demonstrated. The compact model
accurately predicts AC, DC and nonlinear behavior of the diode required for fast circuit
simulation. Secondly, achieving low resistance ohmic contact onto n-type diamond is one
of the major issues that is still an open research problem as it determines the performance
of high-power RF circuits and switching losses in power converters circuits. So, another
portion of this thesis demonstrates the achievement of very low resistance ohmic contact
(~ 10-4 Ω⋅cm2) onto n-type diamond using nano crystalline carbon interface layer. Using
the developed TCAD and compact models for low resistance contacts, circuit level
predictions show improvements in RF performance. Lastly, an initial study of breakdown
characteristics of diamond and cubic boron nitride heterostructure is presented. This study
serves as a first step for making future transistors using diamond and cubic boron nitride –
a very less explored material system in literature yet promising for extreme circuit
applications involving high power and temperature.
ContributorsJHA, VISHAL (Author) / Thornton, Trevor (Thesis advisor) / Goodnick, Stephen (Committee member) / Nemanich, Robert (Committee member) / Alford, Terry (Committee member) / Hoque, Mazhar (Committee member) / Arizona State University (Publisher)
Created2023
Description
Solid-state and non-equilibrium processings are of great interest to researchers due to their ability to control and refine bulk and/or surface microstructure of metallic alloys and push them to surpass their conventional properties limit. In this dissertation, solid-state processing i.e., Shear Assisted Processing and Extrusion (ShAPE), and non-equilibrium processes i.e., surface mechanical attrition (SMAT) and additive manufacturing (AM) techniques were used to process the magnesium and aluminum alloys respectively. A synergistic investigation of processing-induced microstructural modification and its effect on corrosion resistance was performed using various ex-situ, quasi in-situ, and in-situ electrochemical, microscopy, and spectroscopy characterization techniques. To evaluate the effect of the same processing condition on a range of microstructures, a variety of magnesium alloys such as AZ31B, Mg-3Si, ZK60, and Pure Mg were processed using a novel solid-state processing method, namely ShAPE. It induced a significant grain refinement, homogenized distribution of second phases, and low residual strain in AZ31B alloy, which contributed toward a noble breakdown potential, stable protective film, and hence better corrosion resistance compared to the parent extruded counterpart. However, with variations in composition, volume fraction, and distribution of second phases with Mg-3Si and ZK60 magnesium alloy an opposite response was inferred indicating a strong dependence of corrosion on underlying microstructure compared to a processing condition. Non-equilibrium processes, i.e. SMAT and AM were utilized to process high-strength 7xxx series aluminum alloys. Continuous high energy impacts of hard balls in room temperature (RT SMAT) and liquid nitrogen (LN2 SMAT) flow environment generated a gradient nanocrystalline surface layer with the dissolution of inherent second phase and precipitation of new phases in aluminum 7075 alloys. RT SMAT showed a reduced anodic dissolution rate and improved film resistance, which was attributed to the thicker and composite oxide layer along with new nanoscale precipitates. Lastly, reactive AM was used to process aluminum 7075 and 7050 alloys which resulted in a refined and textureless microstructure. A reduction in corrosion resistance was observed with precipitation of excessive reactive particles (Ti and B4C) in AM alloys compared to wrought counterparts.
ContributorsBeura, Vikrant Kumar (Author) / Solanki, Kiran N (Thesis advisor) / Peralta, Pedro (Committee member) / Alford, Terry (Committee member) / Ankit, Kumar (Committee member) / Joshi, Vineet V (Committee member) / Arizona State University (Publisher)
Created2022
Description
Doping is the cornerstone of Semiconductor technology, enabling the functionalities of modern digital electronics. Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have tunable direct bandgaps, strong many-body interactions, and promising applications in future quantum information sciences, optoelectronic, spintronic, and valleytronic devices. However, their wafer-scale synthesis and precisely controllable doping are challenging. Moreover, there is no fixed framework to identify the doping concentration, which impedes their process integration for future commercialization. This work utilizes the Neutron Transmutation Doping technique to control the doping uniformly and precisely in TMDCs. Rhenium and Tin dopants are introduced in Tungsten- and Indium-based Chalcogenides, respectively. Fine-tuning over 0.001% doping level is achieved. Precise analytical techniques such as Gamma spectroscopy and Secondary Ion Mass Spectrometry are used to quantify ultra-low doping levels ranging from 0.005-0.01% with minimal error. Dopants in 2D TMDCs often exhibit a broad stokes-shifted emission, with high linewidths, due to extrinsic effects such as substrate disorder and surface adsorbates. A well-defined bound exciton emission induced by Rhenium dopants in monolayer WSe2 and WS2 at liquid nitrogen temperatures is reported along with specific annealing regimes to minimize the defects induced in the Neutron Transmutation process. This work demonstrates a framework for Neutron Doping in 2D materials, which can be a scalable process for controlling doping and doping-induced effects in 2D materials.
ContributorsLakhavade, Sushant Sambhaji (Author) / Tongay, Sefaattin (Thesis advisor) / Alford, Terry (Committee member) / Yang, Sui (Committee member) / Arizona State University (Publisher)
Created2023
Description
In the Rare-earth-Tri-telluride family, (RTe3s) [R=La, Ce, Nd, Sm, Gd, Tb, Dy, Er, Ho, Tm] the emergence of Charge Density Waves, (CDW) has been under investigation for a long time due to broadly tunable properties by either chemical substitution or pressure application. These quasi 2D Layered materials RTe3s undergo Fermi Surface Nesting leading to CDW instability. CDWs are electronic instabilities found in low-dimensional materials with highly anisotropic electronic structures. Since the CDW is predominantly driven by Fermi-surface (FS) nesting, it is especially sensitive to pressure-induced changes in the electronic structure. The FS of RTe3s is a function of p-orbitals of Tellurium atoms, which are arranged in two adjacent planes in the crystal structure. Although the FS and electronic structure possess a nearly four-fold symmetry, RTe3s form an incommensurate CDW.This dissertation is structured as follows: Chapter 1 includes basic ideas of Quantum materials, followed by an introduction to CDW and RTe3s. In Chapter 2, there are fundamentals of crystal growth by Chemical Vapor Transport, including various precursors, transport agent, temperature gradient, and rate of the reaction. After the growth, the crystals were confirmed for lattice vibrations by Raman, for composition by Energy Dispersive Spectroscopy; crystal structure and orientation were confirmed by X-ray Diffraction; magnetic ordering was established by Vibrating sample measurement. Detailed CDW study was done on various RTe3s by Raman spectroscopy. The basic mechanism and instrumentations used in these characterizations are explained in Chapter 3. Chapter 4 includes experimental data for crystal growth and results of these characterizations for Parent RTe3s. Chapter 5 includes fundamental insights on Cationic alloying of RTe3s, along with one alloy system’s crystal growth and characterization. This work tries to explain the behavior of CDW by a Temperature-dependent Raman study of RTe3s established the CDW transition temperature accompanied by Phonon softening; Angle-resolved Raman data confirming the nearly four-fold symmetry; thickness-dependent Raman spectroscopy resulting in the conclusion that as thickness decreases CDW transition temperature increases. Also, CDW transition is analyzed as a function of alloying.
ContributorsAttarde, Yashika (Author) / Tongay, Sefaattin (Thesis advisor) / Botana, Antia (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2021
Description
The characterization of interface properties in molten slag is crucial for understanding the interface phenomenon and the reactions between slag and metal. This study focuses on examining the influence of Cr2O3, an important surface active oxide, on the wettability and surface tension of slag. Industrial Electric Arc Furnace (EAF) slag with two different Cr2O3 contents (1 wt% and 3 wt%) was investigated using the sessile drop measurement technique at a high temperature of 1650°C. For the preparation of 3 wt% Cr2O3-doped slags, the following crucibles were used: Al2O3, Mo, and MgO. The behavior of crucibles, the dissolution process as well as its effect on the slag thermophysical properties were studied. For the evaluation of surface tension, Mo and MgO substrates were used. The contact angle was measured using the sessile drop method, and the surface tension was calculated using the Young-Laplace-based software. The interaction and wettability behavior between the slag and different substrates was studied. The effects of Cr2O3 content, in correlation with Al2O3, Mo, and MgO, as well as temperature, on the surface tension, and phase formation were analyzed using FactSage 8.2. The results indicate an increase in the formation of solid phases with Al2O3 and Mo dissolution into the slag. The composition of the MoO3 is confirmed with the XRF and EDS analysis. Furthermore, an increase in the formation of the spinel phase was observed with the addition of chromium, which is confirmed via XRD. The increase in the CaCrMo-oxide-based spinel led to a decrease in the surface tension of the slag. The surface tension of the slag pre-melted in a Mo, decreases as the Cr2O3 content increases. The effects of the amounts of Cr2O3 in correlation with Al2O3, MgO, and MoO3 on the slag foaming index were determined using the existing models in the literature.
ContributorsMeena, Neha (Author) / Seetharaman, Sridhar (Thesis advisor) / Alford, Terry (Committee member) / Korobeinikov, Yuri (Committee member) / Arizona State University (Publisher)
Created2023
Description
Organic light-emitting diodes (OLEDs) have been successfully implemented in various display applications owing to rapid advancements in material design and device architecture. Their success in the display industry has ignited a rising interest in applying OLEDs for solid-state lighting applications through the development of white OLEDs (WOLEDs). However, to enter the market as a serious competitor, WOLEDs must achieve excellent color quality, high external quantum efficiency (EQE) as well as a long operational lifetime. In this research, novel materials and device architectures were explored to improve the performance of single-stack WOLEDs. A new Pt-based phosphorescent emitter, Pt2O2-p2m, was examined as a single emissive emitter for the development of a stable and efficient single-doped WOLED. A bilayer structure was employed to balance the charges carriers within the emissive layer resulting in low efficiency roll-off at high brightness, realizing a peak EQE of 21.5% and EQEs of 20% at 1000 cd m-2 and 15.3% at 7592 cd m-2. A novel phosphorescent/fluorescent, or hybrid, WOLED device architecture was also proposed. To gather a thorough understanding of blue fluorescent OLEDs prior to its use in a WOLED, a study was conducted to investigate the impact of the material selection on the device performance. The use of an anthracene type host demonstrated an improvement to the operational stability of the blue OLED by reducing the occurrence of degradation events. Additionally, various dopant concentrations and blocking materials revealed vastly different efficiency and lifetime results. Finally, a Pd (II) complex, Pd3O8-Py5, with efficient amber-colored aggregate emission was employed to produce a WOLED. Various host materials were investigated to achieve balanced white emission and the addition of an interlayer composed of a high triplet energy material was used to reduce quenching effects. Through this strategy, a color stable WOLED device with a peak EQE of 45% and an estimated LT95 over 50,000 hours at 1000 cd m-2 was realized. The comprehensive performance of the proposed device architecture competes with WOLED devices that are commercially available and reported within the literature domain, providing a strong foundation to further advance the development of highly efficient and stable single-stack WOLEDs.
ContributorsAmeri, Lydia (Author) / Li, Jian (Thesis advisor) / Adams, James (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2022
Description
Electron Paramagnetic Resonance (EPR) has facilitated great scientific advancements in many fields, like material science, engineering, medicine, biology, and health. EPR provided the ability to investigate samples on molecular level to detect chemical composition and identify harmful substances like free radicals. This thesis aims to explore current health and diagnostics EPR research and investigate the free radical content in related paramagnetic centers. Examining paramagnetic diagnostic markers of Cancer, Sicklecell disease, oxidative stress, and food oxidation. After exploring current literature on EPR, an experiment is designed and conducted to test seven different coffee samples (Turkish coffee, Espresso Coffee, European Coffee, Ground Arabic Coffee, American Coffee, Roasted Arabic Coffee, and Green Arabic Coffee), using Bruker ELEXSYS E580 spectrometer at x-band and under both room temperature (298 K) and low temperature (106 -113 K). Several microwave powers (1, mW, 0.25 mW, 0.16 mW, 0.06 mW, 0.04 mW) and different modulation frequency (10 G, 5 G, 3 G) are used. The results revealed average g-value was 2.009, highest linewidth was 16.312. Espresso coffee had the highest concentration of radicals, and green Arabic coffee beans had the lowest. Obtained spectra showed signals of Reactive Oxygen Species (ROS) radicals; believed to be result of natural oxidation process, as well as trace amounts of Fe3+ and other transition metals impurities, likely to be naturally found in coffee or resulting from the process of coffee production.
ContributorsMaki, Husain (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2022
Description
Microstructure refinement and alloy additions are considered potential routes to increase high temperature performance of existing metallic superalloys used under extreme conditions. Nanocrystalline (NC) Cu-10at%Ta exhibits such improvements over microstructurally unstable NC metals, leading to enhanced creep behavior compared to its coarse-grained (CG) counterparts. However, the low melting point of Cu compared to other FCC metals, e.g., Ni, might lead to an early onset of diffusional creep mechanisms. Thus, this research seeks to study the thermo-mechanical behavior and stability of hierarchical (prepared using arc-melting) and NC (prepared by collaborators through powder pressing and annealing) Ni-Y-Zr alloys where Zr is expected to provide solid solution and grain boundary strengthening in hierarchical and NC alloys, respectively, while Ni-Y and Ni-Zr intermetallic precipitates (IMCs) would provide kinetic stability. Hierarchical alloys had microstructures stable up to 1100 °C with ultrafine eutectic of ~300 nm, dendritic arm spacing of ~10 μm, and grain size ~1-2 mm. Room temperature hardness tests along with uniaxial compression performed at 25 and 600 °C revealed that microhardness and yield strength of hierarchical alloys with small amounts of Y (0.5-1wt%) and Zr (1.5-3 wt%) were comparable to Ni-superalloys, due to the hierarchical microstructure and potential presence of nanoscale IMCs. In contrast, NC alloys of the same composition were found to be twice as hard as the hierarchical alloys. Creep tests at 0.5 homologous temperature showed active Coble creep mechanisms in hierarchical alloys at low stresses with creep rates slower than Fe-based superalloys and dislocation creep mechanisms at higher stresses. Creep in NC alloys at lower stresses was only 20 times faster than hierarchical alloys, with the difference in grain size ranging from 10^3 to 10^6 times at the same temperature. These NC alloys showed enhanced creep properties over other NC metals and are expected to have rates equal to or improved over the CG hierarchical alloys with ECAP processing techniques. Lastly, the in-situ wide-angle x-ray scattering (WAXS) measurements during quasi-static and creep tests implied stresses being carried mostly by the matrix before yielding and in the primary creep stage, respectively, while relaxation was observed in Ni5Zr for both hierarchical and NC alloys. Beyond yielding and in the secondary creep stage, lattice strains reached a steady state, thereby, an equilibrium between plastic strain rates was achieved across different phases, so that deformation reaches a saturation state where strain hardening effects are compensated by recovery mechanisms.
ContributorsSharma, Shruti (Author) / Peralta, Pedro (Thesis advisor) / Alford, Terry (Committee member) / Jiao, Yang (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2022
Description
In the past decade, 2D materials especially transition metal dichalcogenides (TMDc), have been studied extensively for their remarkable optical and electrical properties arising from their reduced dimensionality. A new class of materials developed based on 2D TMDc that has gained great interest in recent years is Janus crystals. In contrast to TMDc, Janus monolayer consists of two different chalcogen atomic layers between which the transition metal layer is sandwiched. This structural asymmetry causes strain buildup or a vertically oriented electric field to form within the monolayer. The presence of strain brings questions about the materials' synthesis approach, particularly when strain begins to accumulate and whether it causes defects within monolayers.The initial research demonstrated that Janus materials could be synthesized at high temperatures inside a chemical vapor deposition (CVD) furnace. Recently, a new method (selective epitaxy atomic replacement - SEAR) for plasma-based room temperature Janus crystal synthesis was proposed. In this method etching and replacing top layer chalcogen atoms of the TMDc monolayer happens with reactive hydrogen and sulfur radicals. Based on Raman and photoluminescence studies, the SEAR method produces high-quality Janus materials. Another method used to create Janus materials was the pulsed laser deposition (PLD) technique, which utilizes the interaction of sulfur/selenium plume with monolayer to replace the top chalcogen atomic layer in a single step.
The goal of this analysis is to characterize microscale defects that appear in 2D Janus materials after they are synthesized using SEAR and PLD techniques. Various microscopic techniques were used for this purpose, as well as to understand the
mechanism of defect formation. The main mechanism of defect formation was proposed to be strain release phenomena. Furthermore, different chalcogen atom positions within the monolayer result in different types of defects, such as the appearance of cracks or wrinkles across monolayers. In addition to investigating sample topography, Kelvin probe force microscopy (KPFM) was used to examine its electrical properties to see if the formation of defects impacts work function. Further study directions have been suggested for identifying and characterizing defects and their formation mechanism in the Janus crystals to understand their fundamental properties.
ContributorsSinha, Shantanu (Author) / Tongay, Sefaattin (Thesis advisor) / Alford, Terry (Committee member) / Yang, Sui (Committee member) / Arizona State University (Publisher)
Created2022