Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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    ATOMIC LAYER DEPOSITION OF CADMIUM TELLURIDE FOR THE PASSIVATION OF MERCURY CADMIUM TELLURIDE
    (2021) Pattison, James William; Salamanca-Riba, Lourdes G; VanMil, Brenda; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Mercury cadmium telluride (MCT) is an important infrared (IR) detector material due to high quantum efficiency and the ability to tune the bandgap, covering important IR wavelengths from near-infrared (~1 m) to very-long wavelength infrared (>12 m) detection. Focal Plane Arrays (FPAs) are used to image in the infrared and consist of photodiodes that absorb IR photons, generating charge carriers that create an electric signal used to form an image by combining the signals from all of the photodiodes. Decreasing photodiode size increases the resolution of optical systems incorporating MCT FPAs, but challenges current state-of-the-art passivation processes. Passivation is needed to increase the signal-to-noise of a system by rendering benign the charge carrier transport. Physical-vapor-deposited (PVD) CdTe is the incumbent passivation material for MCT, but fails when applied to the next generation of MCT photodiodes because of non-conformal deposition. Atomic layer deposition (ALD) is a superior deposition technique in this regard because the vapor-phase chemicals enable conformal exposure of the surface as opposed to line-of-sight deposition in PVD. ALD of CdTe requires deposition temperature lowering to suppress out-gassing of Hg at elevated temperature, which leads to mercury vacancy formation, reducing signal-to-noise of any eventual detector. Previous demonstration of CdTe ALD was spontaneous above ~200 °C for chemisorbed dimethylcadmium (DMCd) to react with diethyltellurium (DETe). However, this temperature is incompatible with MCT devices, because of the loss of Hg from the material. This dissertation attempts to overcome the low temperature requirement of current CdTe ALD using a novel approach in which argon plasma successfully decomposes the chemisorbed DMCd, replacing temperature induced thermal decomposition, and induced CdTe growth using either DETe or bis(trimethylsilyl)telluride as the tellurium precursors at low temperatures. Film deposition conditions were developed through deposition on silicon substrates, and the process was transferred to MCT samples, demonstrating low temperature deposition, conformal deposition, and passivation of the MCT surface. The films were characterized by in situ spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), x ray diffraction (XRD), and transmission electron microscopy (TEM). Photoconductive decay (PCD) measurements were made of MCT material passivated by CdTe ALD, demonstrating effective passivation through enhanced minority carrier lifetime.
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    ALD-ENABLED CATHODE-CATALYST ARCHITECTURES FOR LI-O2 BATTERIES
    (2015) Schroeder, Marshall Adam; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The Li-O2 electrochemical redox couple is one of the prime candidates for next generation energy storage. Known for its impressive theoretical metric for specific energy, even current practically obtainable values are competitive with state of the art Li-ion intercalation chemistries and the achievable performance of batteries featuring this nascent technology will continue to improve as fundamental scientific challenges in each component of the device are addressed. The positive electrode is particularly complicated by its role as a scaffold for oxygen reduction and evolution, exhibiting sluggish kinetics, poor chemical stability, and limited cyclability due to parasitic side reactions. Fortunately, recent Li-O2 research has shown some success in improving the performance and cyclability of these O2 cathodes by shifting toward nanostructured architectures with catalytic functionalizations. Atomic layer deposition (ALD) is one of the most promising enabling technologies for fabricating these complex heterostructures. Offering precise control of film thickness, morphology, and mass loading with excellent conformality, this vapor-phase deposition technique is applied in this work to deposit thin film and particle morphologies of different catalyst chemistries on mesostructured carbon scaffolds. This thesis dissertation discusses: (1) development of a lab-scale infrastructure for assembly, electrochemical testing, and characterization of Li-O2 battery cathodes including a custom test cell and a state of the art integrated system for fabrication and characterization, (2) design, fabrication, testing, and post-mortem characterization of a unique 3D cathode architecture consisting of vertically aligned carbon nanotubes on an integrated nickel foam current collector, (3) atomic layer deposition of heterogeneous ruthenium-based catalysts on a multi-walled carbon nanotube sponge to produce a freestanding, binder-free, mesoporous Li-O2 cathode with high capacity and long-term cyclability, (4) evaluation of dimethyl sulfoxide as an electrolyte solvent for non-aqueous Li-O2 batteries, and (5) investigation of the relative importance of passivating intrinsic defects in carbon redox scaffolds vs. introduction of heterogeneous OER/ORR catalysts for improving the long-term stability and cyclability of these Li-O2 electrodes.
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    Model-based Analysis of Atomic Layer Deposition Growth Kinetics and Multiscale Process Dynamics
    (2014) Travis, Curtisha Denise; Adomaitis, Raymond A.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A first principles model describing the reaction kinetics and surface species dynamics for the trimethylaluminum (TMA) and water half-reactions of alumina atomic layer deposition (ALD) is coupled with a dynamic film growth model and reactor-scale species transport model. The reaction kinetics model is based on reported enthalpies and transition state structures from published quantum-chemical computational studies; these data are used to determine kinetic parameters using statistical thermodynamics and absolute reaction rate theory. Several TMA half-reactions were modeled to account for TMA adsorption and subsequent reaction on a range of growth surfaces spanning bare to fully hydroxylated states. Several water reactions were also considered. By coupling the reaction rate models with surface species conservation equations, a dynamic model is created which is useful for examining the relative rates of competing surface reactions. To describe the continuous cyclic operation of the deposition reaction system, a numerical procedure to discretize limit-cycle solutions is developed and used to distinguish saturating growth per cycle from non-saturating conditions. The transition between the two regimes is studied as a function of precursor partial pressure, exposure times, and temperature. Finally, a cross-flow tubular ALD reactor system model is derived with components describing the precursor thermophysical properties, precursor delivery system, reactor-scale gas-phase dynamics, and surface reaction kinetics derived from absolute reaction rate theory. These model components are integrated to simulate the complete multiscale ALD process. Limit-cycle solutions defining continuous cyclic ALD reactor operation are computed with a fixed point algorithm based on temporal and spatial discretization within the reactor, resulting in an unambiguous definition of film growth per cycle. The use of the simulator for assisting in process design decisions and optimization frameworks is presented.
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    NANOPOROUS AAO: A PLATFORM FOR REGULAR HETEROGENEOUS NANOSTRUCTURES AND ENERGY STORAGE DEVICES
    (2009) Perez, Israel; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Nanoporous anodic aluminum oxide (AAO) has vast implications as a tool for nanoscience research and as a nanostructure in which nanoscale devices can be fabricated because of its regular and ordered nanopores. Self-assembly plays a critical role in pore ordering, causing nanopores to grow parallel with one another in high density. The mild electrochemical conditions in which porous AAO grows along with its relatively cheap starting materials makes this nanomaterial a cost effective alternative to advanced photolithography techniques for forming high surface area nanostructures over large areas. In this research, atomic layer deposition (ALD) was used to deposit conformal films within in nanoporous AAO with hopes to 1) develop methodologies to characterize ALD depositions within its high aspect ratio nanopores and 2) to better understand how to use nanoporous AAO templates as a scaffold for energy devices, specifically Metal-Insulator-Metal (MIM) capacitors. Using the nanotube template synthesis method, ALD films were deposited onto nanoporous AAO, later removing the films deposited within the templates nanopores for characterization in TEM. This nanotube metrology characterization involves first obtaining images of full length ALD-AAO nanotubes, and then measuring wall thickness as a function of depth within the nanopore. MIM nanocapacitors were also constructed in vertical AAO nanopores by deposition of multilayer ALD films. MIM stacks were patterned into micro-scale capacitors for electrical characterization.