Materials Science & Engineering

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3-(4-Amino-1,2,5-oxadiazol-3-yl)-4-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole
(MDPI, 2014-05-22) Pagoria, Philip; Zhang, Maoxi; Racoveanu, Ana; DeHope, Alan; Tsyshevsky, Roman V.; Kuklja, Maija M.
The title compound 3-(4-amino-1,2,5-oxadiazol-3-yl)-4-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (ANFF-1) was synthesized by: (1) by reaction of 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (BNFF-1) with gaseous ammonia in toluene and (2) by partial oxidation of 3,4-bis(4-amino-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (BAFF-1) with 35% H2O2 in concentrated H2SO4.
3D-PRINTED POLYSTYRENE FOR CELL CULTURE
(2019) Lerman, Max Jonah; Fisher, John P; Gillen, Greg; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Efficient methods to expand stem cells ex vivo hold significant promise in many clinical applications. For example, hematological malignancies account for nearly 10% of cancer related deaths in the United States of America and frequently require a transplant to successfully treat the disease. Ex vivo expanded hematopoietic stem cells (HSCs) could help narrow treatment gaps; however, generating viable dosages of HSCs currently fall short of expectations with difficulties in expanding HSCs and the loss of cellular multipotency. Coculture with mesenchymal stem cells (MSCs) aims to provide the necessary intercellular signaling to counteract monoculture deficiencies. Typically, achieving these and other clinical goals have relied on 2D polystyrene (PS) as the fundamental substrate for cell culture. With the emergence of 3D printing, improved biomimicry with 3D culture models are becoming widely available. In this dissertation, we develop a 3D PS culture substrate for adherent and non-adherent cells, working towards a model for the bone marrow niche. To achieve this goal, the objectives of the work were to: (1) develop a 3D printing method for PS and surface functionalization strategy to facilitate extracellular matrix protein and MSC adhesion, (2) assess the effects of the underlying surface functionality on osteogenic differentiation under static and dynamic conditions, and (3) validate the culture model successfully cultures multiple cell types with a model non-adherent cell line, demonstrating validity and translatability as a bone marrow niche model. In converting PS from a 2D culture platform to a 3D printed one, we take steps to increase the biomimicry of in vitro cell culture without sacrificing fundamental PS properties (e.g. optical clarity, cost-effectiveness, disposability). Continued development and of the model would see an efficient method for studying the complex bone marrow niche with applications in pharmacology and cancer diagnostics.
A Review of Metastable Beta Titanium Alloys
(MDPI, 2018-06-30) Kolli, R. Prakash; Devaraj, Arun
In this article, we provide a broad and extensive review of beta titanium alloys. Beta titanium alloys are an important class of alloys that have found use in demanding applications such as aircraft structures and engines, and orthopedic and orthodontic implants. Their high strength, good corrosion resistance, excellent biocompatibility, and ease of fabrication provide significant advantages compared to other high performance alloys. The body-centered cubic (bcc) β-phase is metastable at temperatures below the beta transus temperature, providing these alloys with a wide range of microstructures and mechanical properties through processing and heat treatment. One attribute important for biomedical applications is the ability to adjust the modulus of elasticity through alloying and altering phase volume fractions. Furthermore, since these alloys are metastable, they experience stress-induced transformations in response to deformation. The attributes of these alloys make them the subject of many recent studies. In addition, researchers are pursuing development of new metastable and near-beta Ti alloys for advanced applications. In this article, we review several important topics of these alloys including phase stability, development history, thermo-mechanical processing and heat treatment, and stress-induced transformations. In addition, we address recent developments in new alloys, phase stability, superelasticity, and additive manufacturing.
A semi-supervised deep-learning approach for automatic crystal structure classification
(Wiley, 2022) Lolla, Satvik; Liang, Haotang; Kusne, A. Gilad; Takeuchi, Ichiro; Ratcliff, William
The structural solution problem can be a daunting and time-consuming task. Especially in the presence of impurity phases, current methods, such as indexing, become more unstable. In this work, the novel approach of semi-supervised learning is applied towards the problem of identifying the Bravais lattice and the space group of inorganic crystals. The reported semi-supervised generative deep-learning model can train on both labeled data, i.e. diffraction patterns with the associated crystal structure, and unlabeled data, i.e. diffraction patterns that lack this information. This approach allows the models to take advantage of the troves of unlabeled data that current supervised learning approaches cannot, which should result in models that can more accurately generalize to real data. In this work, powder diffraction patterns are classified into all 14 Bravais lattices and 144 space groups (the number is limited due to sparse coverage in crystal structure databases), which covers more crystal classes than other studies. The reported models also outperform current deep-learning approaches for both space group and Bravais lattice classification using fewer training data.
ABNORMAL GRAIN GROWTH IN MAGNETOSTRICTIVE GALFENOL ROLLED SHEET
(2011) Chun, Hyunsuk; Flatau, Alison B; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Highly textured Fe-Ga (Galfenol) rolled sheet with Cube (100)<100> or Goss (110)<100> preferred orientation is under investigation to provide easy magnetization, enhanced magnetostrictive performance and a cost-effective option for production of these alloys for use in applications as sensors and actuators. In this study, 1-2.5% NbC added Galfenol rolled sheet was used because NbC particles enhance the rollability of and abnormal grain growth (AGG) in polycrystalline Galfenol rolled sheet. Driving forces, due to grain boundary energy, surface energy, deformation energy and magnetic fields are generally considered to explain grain growth phenomena. In this dissertation, the effect on grain boundary energy for influencing AGG was studied for the case of high temperature annealing at 1200°C. Both Coincident Site Lattice (CSL) and High Energy Grain Boundary (HEGB) models were investigated as possible mechanisms to explain the contribution of grain boundary energy to Goss-textured AGG. Results support the HEGB model as a suitable model for the observed development of Goss-textured AGG in Galfenol rolled sheet. Next, the effect of deformation energy on AGG was studied by using tension annealing and strain annealing methods in the temperature range of 900°C to 1100°C. This study was built on results from studies of grain boundary energy on other alloys. For the tension annealing investigation, Galfenol rolled sheet was simultaneously subjected to tensile loading during high temperature annealing. No AGG was observed from the tension annealing method. For the strain-annealing investigation, homogeneously recrystallized Galfenol rolled sheet with a taper was subjected to tensile loading under different strain rates and post-strain high temperature anneal conditions to investigate the resultant grain growth phenomena. Different grain growth modes, including Cube- and Goss-textured AGG, were observed in this study. Assessment of the extent of AGG resulting from these was conducted using Electron Backscattering Diffraction (EBSD) patterns that were captured and analyzed using Orientation Imaging Microscope (OIM) software to obtain Inverse Pole Figures (IPF) and Orientation Distribution Function (ODF). Additionally, Ga loss, which lowers the magnetostrictive properties, under different conditions was investigated by Electron Probe Micro Analyzer (EPMA). No significant Ga loss was observed during the annealing process at 1000°C, however, about 2% Ga loss was observed during the annealing process at 1100°C and 1200°C in the areas with a high density of grain boundaries.
ACCELERATED SELF-ASSEMBLY OF PEPTIDE-BASED NANOFIBERS USING NANOMECHANICAL STIMULUS
(2010) Chang, Jonathan Paul; Seog, Joonil; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
One-dimensional nanostructures are ideal building blocks for functional nanoscale assembly. Peptide-based nanofibers have great potential for building smart hierarchical structures due to their tunable structures at a single residue level and their ability to reconfigure themselves in response to environmental stimuli. In this study, it was observed that a pre-adsorbed silk-elastin-based protein polymer self-assembled into nanofibers through a conformational change on the mica substrate. Furthermore, using atomic force microscopy, it was shown that the rate of the self-assembling process was significantly enhanced by applying a nanomechanical stimulus. The orientation of the newly grown nanofiber was mostly perpendicular to the scanning direction, implying that the new nanofiber assembly was locally activated with a directional control. The method developed as a part of this study provides a novel way to prepare a nanofiber patterned substrate using a bottom-up approach.
Additive Manufacturing of High-Temperature Hybrid Electronics via Molecular-Decomposed Metals
(Wiley, 2023-10-20) Khuje, Saurabh; Alshatnawi, Firas; Smilgies, Detlef; Alhendi, Mohammed; Islam, Abdullah; Armstrong, Jason; Yu, Jian; Poliks, Mark; Ren, Shenqiang
As the modern electronic technology extends into operating in harsh working conditions, it calls for a system that is capable of uncompromising performance in extreme environments, thus providing a strong motivation to look for advanced materials and electronics with the capability of high-throughput and rapid prototyping. Coupled with additive manufacturing, molecular decomposition metals bypass the challenging oddities of traditional material-limited and thermally initiated metalworking, enabling high throughput and rapid prototyping of stoichiometry and composition-controlled metals. Here, a new paradigm for the design and additive manufacturing of copper metallic alloy materials onto ceramics is described by printing molecular decomposable metal materials, capable of withstanding thermo-mechanical loading, operating in extreme environments in static and dynamic conditions. The resulting printed hybrid electronics are electrically stable for 25 h of aging at 1000 °C. This curious fact paves a way for printed antenna and sensor electronics that reliably operate up to 1000 °C. These results can be further extended to establish other printable molecular decomposable materials as a platform for rapid prototyping of high temperature electronics that are suitable for harsh environments.
Additive Manufacturing of High-Temperature Preceramic-Derived SiOC Hybrid Functional Ceramics
(Wiley, 2023-09-22) Li, Zheng; Khuje, Saurabh; Islam, Abdullah; Ren, Shenqiang
High-temperature capable materials, metals, and ceramics are attracting significant interest for applications in extreme environmental conditions. Herein, a hybrid metal-reinforced ceramic matrix material consisting of preceramic-derived high-temperature SiOC and copper nanoplates is reported, enabling the manufacturing of high-temperature sensing electronics. The preceramic polymer precursors including polydimethylsiloxane and polydimethylsilane, together with copper nanoplates, are thermally converted into durable copper-reinforced SiOC ceramics. The presence of copper in SiOC ceramics enhances its electrical conductivity, while SiOC suppresses oxygen uptake and acts as a shield for oxidation to achieve high-temperature thermal resistance and negative temperature coefficient at high temperatures. A comprehensive electric and sensing performance, combined with cost-effectiveness and scalability, can facilitate the utilization of hybrid Cu and SiOC composites in high-temperature electronics.
Advanced Analytical Microscopy at the Nanoscale: Applications in Wide Bandgap and Solid Oxide Fuel Cell Materials
(2016) Taillon, Joshua Aaron; Salamanca-Riba, Lourdes G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The atomic-level structure and chemistry of materials ultimately dictate their observed macroscopic properties and behavior. As such, an intimate understanding of these characteristics allows for better materials engineering and improvements in the resulting devices. In our work, two material systems were investigated using advanced electron and ion microscopy techniques, relating the measured nanoscale traits to overall device performance. First, transmission electron microscopy and electron energy loss spectroscopy (TEM-EELS) were used to analyze interfacial states at the semiconductor/oxide interface in wide bandgap SiC microelectronics. This interface contains defects that significantly diminish SiC device performance, and their fundamental nature remains generally unresolved. The impacts of various microfabrication techniques were explored, examining both current commercial and next-generation processing strategies. In further investigations, machine learning techniques were applied to the EELS data, revealing previously hidden Si, C, and O bonding states at the interface, which help explain the origins of mobility enhancement in SiC devices. Finally, the impacts of SiC bias temperature stressing on the interfacial region were explored. In the second system, focused ion beam/scanning electron microscopy (FIB/SEM) was used to reconstruct 3D models of solid oxide fuel cell (SOFC) cathodes. Since the specific degradation mechanisms of SOFC cathodes are poorly understood, FIB/SEM and TEM were used to analyze and quantify changes in the microstructure during performance degradation. Novel strategies for microstructure calculation from FIB-nanotomography data were developed and applied to LSM-YSZ and LSCF-GDC composite cathodes, aged with environmental contaminants to promote degradation. In LSM-YSZ, migration of both La and Mn cations to the grain boundaries of YSZ was observed using TEM-EELS. Few substantial changes however, were observed in the overall microstructure of the cells, correlating with a lack of performance degradation induced by the H2O. Using similar strategies, a series of LSCF-GDC cathodes were analyzed, aged in H2O, CO2, and Cr-vapor environments. FIB/SEM observation revealed considerable formation of secondary phases within these cathodes, and quantifiable modifications of the microstructure. In particular, Cr-poisoning was observed to cause substantial byproduct formation, which was correlated with drastic reductions in cell performance.
ALD PROCESSES AND APPLICATIONS TO NANOSTRUCTURED ELECTROCHEMICAL ENERGY STORAGE DEVECES
(2013) Chen, Xinyi; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Next generation Li-ion batteries (LIB) are expected to display high power densities (i.e. high rate performance, or fast energy storage) while maintaining high energy densities and stable cycling performance. The key to fast energy storage is the efficient management of electron conduction, Li diffusion, and Li-ion migration in the electrode systems, which requires tailored material and structural engineering in nanometer scale. Atomic layer deposition (ALD) is a unique technique for nanostructure fabrications due to its precise thickness control, unprecedented conformality, and wide variety of available materials. This research aims at using ALD to fabricate materials, electrodes, and devices for fast electrochemical energy storage. First, we performed a detailed study of ALD V2O5 as a high capacity cathode material, using vanadium tri-isopropoxide (VTOP) precursor with both O3 and H2O as oxidant. The new O3-based process produces polycrystalline films with generally higher storage capacity than the amorphous films resulting from the traditional H2O-based process. We identified the crucial tradeoff between higher gravimetric capacity with thinner films and higher material mass with thicker films. For the thickness regime 10-120 nm, we chose areal energy and power density as a useful metric for this tradeoff and found that it is optimized at 60 nm for the O3-VTOP ALD V2O5 films. In order to increase material loading on fixed footprint area, we explored various 3-dimentional (3D) substrates. In the first example, we used multiwall carbon nanotube (MWCNT) sponge as scaffold and current collector. The core/shell MWCNT/V2O5 sponge delivers a stable high areal capacity of 816 μAh/cm2 for 2 Li/V2O5 voltage range (4.0-2.1 V) at 1C rate (nC means charge/discharge in 1/n hour), 450 times that of a planar V2O5 thin film cathode. Due to low density of MWCNT and thin V2O5 layer, the sponge cathode also delivers high gravimetric power density in device level that shows 5X higher power density than commercial LIBs. In the other example, Li-storage paper cathodes, functionalized of conductivity from CNT and Li-storage capability from V2O5¬, presented remarkably high rate performance due to the hierarchical porosity in paper for Li+ migration. The specific capacity of V2O5 is as high as 410 mAh/g at 1C rate, and retained 116 mAh/g at high rate of 100C. We found V2O5 capacities decreased by about 30% at high rates of 5C-100C after blocking the mesopores in cellulose fiber, which serves to be the first confirmative evidence of the critical role of mesoporosity in paper fibers for high-rate electrochemical devices. Finally, we made high density well-aligned nanoporous electrodes (2 billion/cm2) using anodic alumina template (AAO). ALD materials were deposited into the nanopores sequentially - Ru or TiN for current collection, and V2O5 for Li-storage. Ru metal by ALD shows high conductivity and conformality, and serves best as the current collector for V2O5. The capacity of V2O5 reaches about 88% of its theoretic value at high rate of 50C. Such electrodes can be cycled for 1000 times with 78% capacity retention.
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.
Analysis of Flow-Based Microfluidic Gradient Generators for the Study of Bacterial Chemotaxis
(2015) Wolfram, Christopher James; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Chemotaxis is a phenomenon which enables cells to sense concentrations of certain chemical species in their microenvironment and move towards chemically favorable regions. This behavior is best understood in the bacteria Escherichia coli, which exhibits chemotaxis towards a variety of energy sources and signaling molecules. Recent advances in microbiology have engineered the chemotactic properties of bacteria to perform novel functions, but traditional methods of characterizing chemotaxis are not sufficient for such complex applications. The field of microfluidics offers solutions in the form of gradient generators. Many of these gradient generators are flow-based, where a chemical species diffuses across a solution moving through a microchannel. A microfluidic gradient generator was explored as a chemotaxis platform. Sources of error during experimental operation and methods of mitigating this error were demonstrated, and the fundamental theory behind these devices was examined. These devices were determined to be inadequate for the study of bacterial chemotaxis.
Analytical Microscopy Applications to Wide-Bandgap Semiconductors and Nanocarbon-Metal Composite Materials
(2020) Klingshirn, Christopher J.; Salamanca-Riba, Lourdes G.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Understanding the atomic structure of materials lies at the heart of materials science. Electron microscopy offers myriad techniques to both probe processing-structure-property relationships in materials, and to manipulate those relationships directly. In this thesis, analytical transmission electron microscopy (TEM) was used to investigate two distinct material systems with applications to energy-efficient technologies: wide-bandgap semiconductors and nanocarbon-metal composites. In the first project, TEM and electron energy loss spectroscopy were used to investigate the structure, composition and bonding of metal-oxide-semiconductor devices based on silicon carbide (SiC) and gallium oxide (Ga2O3). The performance of SiC falls short of ideal due to electrically active interfacial defect states. This work confirms that boron doping at the SiC/SiO2 interface is feasible and improves the device channel mobility likely through a stress-relaxation mechanism. Separately, no adverse structural effects were found after antimony ion implantation into the SiC substrate, which independently raises mobility via a counter-doping mechanism. Few atomic-scale studies on Ga2O3 have been reported to date; this thesis aims to bridge the knowledge gap by investigating gate oxide materials and process conditions from a structural perspective. Elevated annealing temperatures reduced interface quality for both SiO2 and Al2O3 gate oxides. Separately, amorphous Al2O3 layers were crystallized under moderate electron irradiation in TEM. One-fourth the dose was required for crystallization with 100-keV electrons compared to 200 keV, indicating an ionization-induced atomic rearrangement mechanism. This unexpected phenomenon will have implications for devices operating in extreme environments. The second project investigated structure-property relationships in novel nano-carbon metal-matrix composites called covetics, which exploit the superior mechanical and electrical properties of carbon nanostructures such as graphene. Aluminum covetics were characterized using TEM and various spectroscopy techniques; complementary quantum-mechanical and effective-medium models were used to predict the performance of covetics with a range of structures. The models suggest that an electrical conductivity enhancement of ≈10% is feasible with a 5 vol.% carbon loading, but oxides and poor Al/C contact often diminish the performance of real covetics.
Artificial Kagome Spin Ice
(2008-08-04) Qi, Yi; Cumings, John; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Geometrical frustration is known to significantly modify the properties of many materials. Pyrochlore spin ice and hexagonal water ice are canonical systems that show the effects of frustration in both heat capacity and dynamical response. In both instances, microscopic ordering principles on the lattice lead to a macroscopic degeneracy of configurations. This degeneracy in spin ice may also be modified or lifted by lattice imperfections, external pressure, or magnetic field. Unfortunately, these effects are difficult to model or predict, because existing experimental techniques cannot directly observe the local ordering, near lattice defects or otherwise. To address this long outstanding problem, recent interest has focused on fabricating systems that allow the effects of frustration to be physically modeled and the resulting local configurations to be directly observed. In this dissertation, I present an artificial approach to kagome lattice. The kagome lattice is a two-dimensional structure composed of corner-sharing triangles and is an essential component of the pyrochlore spin ice structure. Our artificial kagome spin ice, constructed by magnetic nano-bar elements, mimics spin ice in 2D. The realized system rigorously obeys the ice rule (2-in 1-out or 1-in 2-out configuration at a vertex of three elements), thus providing a sought-after model system appropriate for further studies. To study the ground state of the artificial kagome system and to validate the artificial approach for spin ice study, we demagnetize the samples using rotating field and observe spin configurations using Lorentz TEM. The ice rule, short-range ordering and absence of long-range disorder, as well as the relatively low remnant magnetization are found in the system, which are signatures of spin ice materials in their ground states. To model our system and relate it to other spin study, we introduce magnetic charge model and Shannon entropy concept. The calculated charge correlation (charge ordering coefficient) and Shannon entropy suggest that the degeneracy of our lattice is lifted from a completely disordered kagome spin ice system, and close to a "true" ground state that is usually found as the kagome plateau in pyrochlore spin ice when applying a field in <111> direction. We also study the effects of external perturbations. When applying a magnetic field, chain-like spin flipping is found in the system, which can be explained by the magnetic charge model. When distorting the lattice by introducing an artificial strain, we observe partial ordering or symmetry breaking in the system, which is similar to the pressure effects in real spin ice. In the Appendix, I also introduce another study I have done, i.e. multiferroic thin film measurements. The focus of that chapter is the dielectric measurement for BaTiO3 (BTO) -CoFe2O4 (CFO) thin film material using a microwave microscope. The measurement has a quantitative spatial resolution of approximately 5 m, and it provides a method for film quality check and the basis for a proposed ME coupling measurement.
ASSEMBLY OF SILVER NANOCUBE CLUSTERS AND TUNING OF SURFACE PLASMON RESONANCES FOR SURFACE-ENHANCED RAMAN SCATTERING
(2012) Lee, Seung Yong; Rabin, Oded; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
To prepare surface-enhanced Raman scattering (SERS) substrate with metal nanoparticle clusters, various deposition methods were used: (1) vertical deposition (VertD), (2) electrophoretic deposition (EPD), and (3) block-copolymer templated deposition (BCPTD). The EPD is a high throughput process. Substrates for VertD can be reused. The BCPTD does not require the use of any lithography technique. By means of these three deposition methods, metal nanoparticles, including silver nanocubes and gold nanospheres, were positioned at pre-determined substrate sites. Various parameters, such as angle between cluster axis and laser polarization, gap size, number of cubes in clusters and cluster configurations, were investigated for their effect on SERS. Hexagonal arrays of gold nanoparticles with precisely controlled gap were investigated. The proximity of the plasmon resonance to the laser wavelength was correlated to the SERS enhancement. A database of SERS enhancement of silver nanocube clusters from monomers to tetramers was generated. The analysis of the structures and enhancements in this database reveals that clusters should be aligned with the laser polarization to achieve high SERS enhancement. Increasing cluster size from dimers to tetramers improves the reproducibility of enhancement factor values. Non face-to-face clusters are better in SERS enhancement than face-to-face clusters. Sharp corners at the junctions are essential for high SERS enhancement.
Atomic Layer Deposition Conformality and Process Optimization: Transitioning from 2-Dimensional Planar Systems to 3-Dimensional Nanostructures
(2010) Robertson Cleveland, Erin Darcy; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Conformal coatings are becoming increasingly important as technology heads towards the nanoscale. The exceptional thickness control (atomic scale) and conformality (uniformity over nanoscale 3D features) of atomic layer deposition (ALD) has made it the process of choice for numerous applications found in microelectronics and nanotechnology with a wide variety of ALD processes and resulting materials. While its benefits derive from self-limited saturating surface reactions of alternating gas precursors, process optimization for ALD conformality is often difficult as process parameters, such as dosage, purge, temperature and pressure are often interdependent with one another, especially within the confines of an ultra-high aspect ratio nanopore. Therefore, processes must be optimized to achieve self-limiting saturated surfaces and avoid parasitic CVD-like reactions in order to maintain thickness control and achieve uniformity and conformality at the atomic level while preserving the desired materials' properties (electrical, optical, compositional, etc.). This work investigates novel approaches to optimize ALD conformality when transitioning from a 2D planar system to a 3D ultra-high aspect ratio nanopore in the context of a cross-flow wafer-scale reactor used to highlight deviations from ideal ALD behavior. Porous anodic alumina (PAA) is used as a versatile platform to analyze TiO₂ ALD profiles via ex-situ SEM, EDS and TEM. Results of TiO₂ ALD illustrate enhanced growth rates that can occur when the precursors titanium tetraisopropoxide and ozone were used at minimal saturation doses for ALD and for considerably higher doses. The results also demonstrate that ALD process recipes that achieve excellent across-wafer uniformity across full 100 mm wafers do not produce conformal films in ultra-high aspect ratio nanopores. The results further demonstrate that conformality is determined by precursor dose, surface residence time, and purge time, creating large depletion gradients down the length of the nanopore. Also, deposition of ALD films over sharp surface features are very uniform, and verified by profile evolution modeling. This behavior, in contrast to that in high aspect ratio structures, suggests strongly that detailed dynamics, local flow conditions (e.g. viscous vs molecular), surface residence time, and ALD surface reaction kinetics play a complex role in determining ALD profiles for high aspect ratio features.
ATOMIC LAYER DEPOSITION OF ALKALI PHOSPHORUS OXYNITRIDE ELECTROLYTES FOR BEYOND-LITHIUM NANOSCALE BATTERIES
(2022) Nuwayhid, Ramsay Blake; Rubloff, Gary; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Lithium-ion batteries dominate portable energy storage systems today due to their light weight and high performance. However, with the continuing demand for battery capacity projected to outstrip the supply of lithium, alternative energy storage systems based on the more abundant Na and K alkali metals are attractive from both a resource perspective and their similar charge storage mechanism. Beyond limited lithium resources, there remains significant opportunity for innovation to improve battery architecture and thus performance. Nanostructured solid-state batteries (SSBs) are poised to meet the demands of next-generation energy storage technologies, with atomic layer deposition (ALD) being a powerful tool enabling high-performance nanostructured SSBs that offer competitive performance with their liquid-based counterparts. This dissertation has two main objectives: First, the development of the first reported ALD solid-state Na+ and K+ conductors are presented. Second, by leveraging the work on developing new solid- state Na+ ion conductors, a proof-of-principle nanoscale Na-SSB is fabricated and tested.ALD processes are developed for the Na and K based analogues of the well-known solid- state electrolyte (SSE) lithium phosphorus oxynitride (LiPON). In this case; NaPON and KPON. A comprehensive comparison of the structure, electrochemical, and processing parameters between the APON (A = Li, Na, K) family of materials is presented. The structure of NaPON closely resembles that of ALD LiPON, both possessing a N/P of 1, classifying them as alkali polyphosphazenes. Interestingly, KPON exhibits similar ALD process parameters to NaPON and LiPON, but the resulting film composition is quite different, showing little nitrogen incorporation and more closely resembling a phosphate glass. NaPON is determined to be a promising SSE with an ionic conductivity of 1.0 ́ 10-7 S/cm at 25 °C and a wide electrochemical stability window of 0-6.0V vs. Na/Na+. The electrochemical stability and performance of NaPON as a SSE is tested in liquid-based and all solid-state battery configurations comprised of a V2O5 cathode and Na metal anode. Electrochemical analysis suggests intermixing of the NaPON/V2O5 layers during the ALD NaPON deposition, and further reaction during the Na metal evaporation step. The reaction during the ALD NaPON deposition on V2O5 is determined to be two-fold: (1) reduction of V2O5 to VO2 and (2) Na+ insertion into VO2 to form NaxVO2. The Na metal evaporation process is found to exacerbate this reactivity, resulting in the formation of irreversible interphases leading to poor SSB performance. Despite the relatively poor performance, this work represents the first report of a nanoscale Na-SSB and showcases cryo- TEM as a powerful characterization technique to further the understanding of nanoscale SSBs. Looking forward, the intermixing during the ALD NaPON deposition does not impact the cycling of the NaxVO2 electrode in liquid-based cells, with NaPON-coated electrodes outperforming unsodiated V2O5 electrodes. This may be advantageous for the fabrication of SSBs, as the SSE deposition simultaneously could pre-sodiate a stable cathode material, excluding the need for ex-situ sodiation in liquid solutions or depositing a pre-sodiated electrode material. Strategies to pair this NaxVO2/NaPON cathode/electrolyte with a stable anode are discussed, with a focus on the ultimate realization of a high-performance Na-SSB. This work highlights the high reactivity of Na compared to Li based battery chemistries, not only necessitating the need for interfacial coatings in Na SSBs, but also the extreme caution required during fabrication of Na-SSBs or liquid sodium- ion batteries.
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.
ATOMIC LAYER DEPOSITION OF LEAD ZIRCONATE-TITANATE AND OTHER LEAD-BASED PEROVSKITES
(2019) Strnad, Nicholas Anthony; Phaneuf, Raymond J; Polcawich, Ronald G; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Lead-based perovskites, especially lead zirconate-titanate (PbZrxTi1-xO3, or PZT), have been of great technological interest since they were discovered in the early 1950s to exhibit large electronic polarization. Atomic layer deposition (ALD) is a thin-film growth technique capable of uniformly coating high aspect-ratio structures due to the self-limited nature of the precursor chemisorption steps in the deposition sequence. In this thesis, a suite of related processes to grow lead-based perovskites by ALD are presented. First, a new process to grow ferroelectric lead titanate (PbTiO3, or PTO) by ALD using lead bis(3-N,N-dimethyl-2-methyl-2-propanoxide) [Pb(DMAMP)2] and tetrakis dimethylamino titanium [TDMAT] as the lead and titanium cation precursors, respectively, is discussed. A 360-nm thick PTO film grown by ALD displayed a maximum polarization of 48 µC/cm2 and remanent polarization of ±30 µC/cm2. Second, a new process (similar to the ALD PTO process) to grow PZT by ALD is demonstrated by partial substitution of TDMAT with either tetrakis dimethylamino zirconium or zirconium tert-butoxide. The 200 nm-thick ALD PZT films exhibited a maximum polarization of 50 µC/cm2 and zero-field dielectric constant of 545 with leakage current density < 0.7 µA/cm2. Third, a new ALD process for antiferroelectric lead hafnate (PbHfO3, or PHO) is presented along with electrical characterization showing a field-induced antiferroelectric to ferroelectric phase transition with applications for capacitive energy storage. Finally, ALD lead hafnate-titanate (PbHfxTi1-xO3, or PHT), considered to be an isomorph of PZT, is demonstrated by combining the process for PTO and PHO. The thin-film PHT grown by ALD is shown to have electronic properties that rival PZT grown at compositions near the morphotropic phase boundary (MPB). The processes for both ALD PZT and PHT are shown to yield films with promising properties for microelectromechanical systems (MEMS) actuators and may help to dramatically increase the areal work density of such devices.
Atomic Layer Deposition of Ru and RuO2: New Process Development, Fabrication of Heterostructured Nanoelectrodes, and Applications in Energy Storage
(2013) Gregorczyk, Keith E.; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The ability to fabricate heterostructured nanomaterials with each layer of the structure having some specific function, i.e. energy storage, charge collection, etc., has recently attracted great interest. Of the techniques capable of this type of process, atomic layer deposition (ALD) remains unique due to its monolayer thickness control, extreme conformality, and wide variety of available materials. This work aims at using ALD to fabricate fully integrated heterostructured nanomaterials. To that end, two ALD processes, using a new and novel precursor, bis(2,6,6-trimethyl-cyclohexadienyl)ruthenium, were developed for Ru and RuO2 showing stable growth rates of 0.5 Å/cycle and 0.4 Å/cycle respectively. Both process are discussed and compared to similar processes reported in the literature. The Ru process is shown to have significantly lower nucleation while the RuO2 is the first fully characterized ALD process known. Using the fully developed RuO2 ALD process, thin film batteries were fabricated and tested in standard coin cell configurations. These cells showed high first cycle gravimetric capacities of ~1400 mAh/g, which significantly degraded after ~40 cycles. Rate performance was also studied and showed a decrease in 1st cycle capacity as a function of increased rate. These results represent the first reports of any RuO2 battery studied beyond 3 cycles. To understand the degradation mechanisms witnessed in the thin film studies in-situ TEM experiments were conducted. Single crystal RuO2 nanowires were grown using a vapor transport method. These nanowires were cycled inside a TEM using Li2O as an electrolyte and showed a ~95% volume expansion after lithiation, ~26% of which was irreversible. Furthermore, a chemical irreversibility was also witnessed, where the reaction products Ru and Li2O remain even after full delithiation. With these mechanisms in mind heterostructured nanowires were fabricated in an attempt to improve the cycling performance. Core/shell TiN/RuO2 and MWCNT/RuO2 structures were fabricating using the ALD process developed in this work. While the TiN/RuO2 structures did not show improved cycling performance due to connection issues, the MWCNT/RuO2 structure showed a stable areal capacity of ~600 μAh/cm2 after ~20 cycles and were easily cycled 100 times.

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