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Structural Effects in 2D-Stabilized FAPbI3 Films and ToF-SIMS for Ultra-thin h-BN Fabrication
No full textThe next generation of electronics, photonics, and optoelectronics are based on advancements in semiconductor materials. In such, it is vital to gain understanding of and cultivate solutions for degradation pathways and engineer effective synthesis methods. This work details two thrusts that take a view through the stability and fabrication lens: first, structural changes in a novel method to stabilize halide perovskites, and second, a method for forming ultra-thin van der Waals materials.
Thrust I: The halide perovskite formamidinium lead iodide (FAPbI3) is a prime candidate for photovoltaics due to its excellent optoelectronic properties, but its application has been limited due to its structural instability. The large size of the FA cation results in metastability of the photoactive cubic phase and a facile degradation into the thermodynamically stable hexagonal phase at room temperature. Recently, the incorporation of 2D Ruddlesden-Popper halide perovskite seeds into a FAPbI3 precursor solution has been shown to template the growth of and stabilize cubic FAPbI3. Here, we investigate the nanoscale structural and optoelectronic mechanisms behind the observed
bulk stabilization using synchrotron-based x-ray microscopies. Nanoprobe x-ray diffraction reveals 2D-templated FAPbI3 films exhibit an average compressive strain normal to the substrate of -3.4%, two-fold larger than that of MACl-stabilized FAPbI3. Further, this compression creates locally templated regions comprised of tetragonal-phase FAPbI3 distributed non-uniformly throughout the film with fewer crystalline defects than purely cubic regions. Scanning x-ray excited optical luminescence (x-ray analogue of photoluminescence) reveals that this local templating results in increased radiative recombination and redshifted emission. Our results help better understand the structural phenomena resulting from stabilization methods in FAPbI3 for engineering durable photovoltaics.
Thrust II: In recent years, hexagonal boron nitride (h-BN) has become a promising candidate for next-generation electronics and photonics, such as a gate dielectric in field effect transistors. However, methods for fabrication of ultra-thin materials often lack spatial control or require harsh environment depositions. Here, we report a method to prepare ultra-thin h-BN using the combination of micromechanical cleaving (i.e. Scotch Tape Method) and ion beam etching through time-of-flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS is further employed for 3D reconstruction of h-BN
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Surface Modification of Polycrystalline Diamond
No full textThe dissertation investigates surface modification techniques on polycrystalline diamond surfaces, exploring their impacts on properties relevant to electronic, surface- cleaning, and nanofabricating applications. Firstly, a comparative study elucidates the oxidation of microcrystalline diamond powder (DP) and polycrystalline diamond film (PCD) via wet chemical treatments and dry processes. The investigation reveals that sulfuric/nitric acids (H2SO4/HNO3) treatment at 360°C demonstrates superior oxidation performance, while oxygen (O2) plasma treatment enhances oxygen content on PCD surfaces. This study provides insights into oxidation mechanisms and guides the optimization of diamond surface cleaning conditions. Secondly, a novel strategy for aminating boron-doped diamond (BDD) via UV irradiation in ammonia (NH3) is presented. By employing hydrobromic acid (HBr) treatment, primary amine dominance is achieved, enhancing amination efficiency. The study also demonstrates the influence of preoxidation states on amine group coverage, offering insights into surface cleaning effects and mechanisms through theoretical simulations. Thirdly, the dissertation explores the functionalization of hydrogen (H)-terminated diamond surfaces with nitrogen (N) and
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sulfur (S) heteroatoms, revealing improved electrical conductivity compared to H- terminated diamonds. Pre-functionalization with S promotes sequential amination efficiency on diamond surface, facilitating reduced UV-exposure times. Density functional theory (DFT) simulations indicate downshifts in bandgap upon functionalization, suggesting enhanced surface conductivity for various electronic applications. Finally, a top-down approach for fabricating diamond nanostructures using metal masks and reactive-ion etching (RIE) process is presented. Silver (Ag) mask exhibits distinct etching profiles, where diamond nanorods (DNRs) cluster is preferably formed after etching and preserves single crystallinities with features resembling diamond nanotubes. Preliminary electrical measurements show Schottky-like conductivity features, indicating potential applications in nanodiamond-based electronics. Collectively, these investigations contribute to a deeper understanding of surface modification techniques on polycrystalline diamond surfaces, offering insights into their utilities across diverse technological domains
New technological advances in scalable manufacturing and biophysical characterization of extracellular vesicles in biomedicine
No full textExtracellular Vesicles (EVs) have emerged as important mediators of intercellular
communication that package and disseminate biochemical signals. This newly recognized
mode of communication between the cells has brought unprecedented therapeutic
and diagnostic opportunities making them attractive nanocarriers for clinical and
industrial translation. As the EV industry rapidly grows, there is a rising demand for
strategies that facilitate EV manufacturing. One of the most vexing issues in the field
is a method of EV isolation that can offer reliability, purity, speed, and reproducibility
and meet the stringent manufacturing standards of the pharmaceutical industry.
To overcome this challenge, in the first part of my thesis, I propose a new highyield
and rapid (<20 min) real-time EV isolation method called Size Exclusion –
Fast Performance Liquid Chromatography (SE-FPLC). We show that our method
can effectively isolate EVs from multiple sources, including EVs derived from human
and mouse cells and biofluids. The results indicate that our SE-FPLC platform
can successfully remove highly abundant protein contaminants, such as albumin and
lipoprotein complexes, which currently represent a significant hurdle in the largescale
isolation of EVs for clinical translation. Additionally, the high-yield nature of
SE-FPLC allows for easy industrial upscaling of EV production for various clinical utilities. Moreover, SE-FPLC enables analysis of very small volumes of blood for use
in point-of-care diagnostics in the clinic. Collectively, our platform offers many advantages
over current EV isolation methods and offers rapid clinical utility potential.
Once the EVs are isolated, it is imperative to perform EV physicochemical characterization
as particle shape and particle charge is pivotal in immune cell interaction.
Bulk ensemble methods quantify EV composition but mask heterogeneity. Studying
single-vesicle heterogeneity is vital, especially given their emerging role as therapeutic
cargos. In the second part of my thesis, I developed a label-free method: to
image, perform high-quality biological segmentation using a custom pre-trained neural
network model, and quantify and classify single EVs purified from a diverse set of
samples. Evaluating the heterogeneity of EVs is crucial for unraveling their complex
actions and biodistribution. We identified consistent architectural heterogeneity of
EVs using cryogenic transmission electron microscopy (cryo-TEM). Imaging EVs isolated
using different methodologies from distinct sources such as cancer cells, normal
cells, and body fluids, we identify a structural atlas of their dominantly consistent
shapes. We identify EV architectural attributes by utilizing a segmentation neural
network model. In total, 7,600 individual EVs were imaged and quantified by our
computational pipeline. Across all 7,600 independent EVs, the average eccentricity
was 0.5366, and the average equivalent diameter was 132.43 nm. The architectural
heterogeneity was consistent across all sources of EVs, independent of purification
techniques, and compromised of single spherical (S. Spherical), rod-like or tubular,
and double shapes. This openly accessible data and computation toolkit will serve as
a reference foundation for high-resolution EV images and offer insights into potential
biological impact
Sharp-peaked lanthanide nanocrystals for near-infrared photoacoustic multiplexed differential imaging
No full textPhotoacoustic tomography offers a powerful tool to visualize biologically relevant molecules and understand processes within living systems at high resolution in deep tissue, facilitated by the conversion of incident photons into low-scattering acoustic waves through non-radiative relaxation. Although current endogenous and exogenous photoacoustic contrast agents effectively enable molecular imaging within deep tissues, their broad absorption spectra in the visible to near-infrared (NIR) range limit photoacoustic multiplexed imaging. Here, we exploit the distinct ultrasharp NIR absorption peaks of lanthanides to engineer a series of NIR photoacoustic nanocrystals. This engineering involves precise host and dopant material composition, yielding nanocrystals with sharply peaked photoacoustic absorption spectra (~3.2 nm width) and a ~10-fold enhancement in NIR optical absorption for efficient deep tissue imaging. By combining photoacoustic tomography with these engineered nanocrystals, we demonstrate photoacoustic multiplexed differential imaging with substantially decreased background signals and enhanced precision and contrast
High Order, Entropy Stable, Positivity Preserving Discontinuous Galerkin Discretizations of Compressible Flow
No full textHigh order DG methods offer improved accuracy in turbulent and under-resolved flows due to low numerical dissipation and dispersion, but may lose robustness due to the lack of stabilization. A priori stabilization techniques, such as artificial viscosity, slope limiting, and filtering, require heuristic parameter, which does not provide provable guarantees of robustness and may lead to over-dissipation of solutions.
For the compressible Euler and Navier-Stokes equations, physically meaningful solutions must maintain positive physical quantities and satisfy entropy stability. The primary objective of this dissertation is to develop high order DG methods that are provably entropy stable and preserve positivity through a posteriori limiting techniques
We first introduce a more general discretization of the viscous terms in the compressible Navier-Stokes equations, which also enables a simple and explicit imposition of entropy stable wall boundary conditions.
A positivity limiting strategy for entropy-stable discontinuous Galerkin spectral element (ESDGSEM) discretizations is then introduced. The strategy is constructed by blending high order solutions with a low order positivity-preserving and semi-discretely entropy stable discretization through an elementwise limiter. The proposed limiting strategy is both semi-discretely entropy stable and positivity preserving for the compressible Navier-Stokes equations under an appropriate CFL condition.
Another approach we propose is an entropy stable limiting strategy for discontinuous Galerkin spectral element (DGSEM) discretizations. The strategy is an extension to the subcell limiting strategy that satisfies the semi-discrete cell entropy inequality by formulating the limiting factors as solutions to an optimization problem. The optimization problem is efficiently solved using a deterministic greedy algorithm. We discuss the extension of the proposed subcell limiting strategy to preserve positivity and general convex constraints.
Numerical experiments confirm the high order accuracy, entropy stability, and robustness of the proposed strategies
Building Secure Runtime Programmable Networked Systems
No full textOur modern lives rely on a variety of network services, which are powered by large-scale networked systems spanning the globe. These systems constantly evolve to match the growing demand for variegated applications and services. The most recent advance in this domain is programmable network devices (e.g., programmable switches, SmartNICs, and FPGAs). Built with specialized hardware, these new devices can deliver high performance while enabling greater operational flexibility. Researchers have seized this opportunity to build efficient and agile networked systems by redesigning the network protocols and applications. On these grounds, in this thesis, we consider the research question of what next-generation networked systems should entail. While we view network programmability as a positive initial step, we believe future networked systems should extend beyond that. Specifically, we investigate two crucial properties that are absent from today's networked systems: security and runtime programmability.
Security is a long-absent network property, as has been proved by the escalating network attacks. Although those attacks are launched through or directly targeting the network, the current network infrastructure has not played an active role in defense. We believe the future network infrastructure should treat security as a first-class goal just as it does routing so that while routing traffic end-to-end, the network also applies a variety of security defenses to eliminate threats within the traffic. The increasing programmability provides an unprecedented opportunity to architect security into the network without incurring intrusive infrastructure changes. In this thesis, we envision enhancing the network infrastructure with security functionalities in two steps. We first turn a programmable switch into a defense platform where a range of defenses can be activated according to the type of threat. Next, we turn the whole network into a defense fleet that conducts security functions while performing end-to-end routing. Following this roadmap, we design NetWarden, a performance-preserving covert channel defense on a single switch, and Ripple, a programmable, decentralized link-flooding defense on multiple switches across the network.
Furthermore, for most of the programmable network devices available on the market, their programmability is restricted by a practical yet fundamental barrier: device functions are only programmable at compile time, but they effectively become fixed functions at runtime. We believe future networked systems need not only compile-time programmability but also runtime programmability---the ability to seamlessly incorporate function changes at any time. Runtime programmable networked systems can shapeshift in response to real-time change; they can be optimally tuned for the current requirements and traffic workloads. This requires runtime programming of individual devices as a building block including both switches and NICs. In this thesis, we first realize runtime programmability for switches by designing FlexCore, a whole-stack design for runtime programmable switches. Next, we explore runtime programmability on SmartNICs in Pipeleon, where packet processing performance varies with traffic patterns. Leveraging runtime programmability, we address the problem by adjusting the implementation according to runtime traffic profiles
Social and Emotional Learning (SEL) Skills and Their Relationship to Campus Discipline Outcomes in Aldine ISD
No full textPast research has shown that efforts to improve social and emotional learning (SEL) can improve campus culture and disciplinary practices. The Kinder Institute for Urban Research’s Houston Education Research Consortium partnered with Aldine ISD, an urban school district serving predominantly economically disadvantaged and minority students, to better understand how self-reported teacher and student SEL skills were associated with campus disciplinary practices
Transcriptional responses to direct and indirect TGFB1 stimulation in cancerous and noncancerous mammary epithelial cells
No full textTransforming growth factor beta (TGFβ) is important for the morphogenesis and secretory function of the mammary gland. It is one of the main activators of the epithelial–mesenchymal transition (EMT), a process important for tissue remodeling and regeneration. It also provides cells with the plasticity to form metastases during tumor progression. Noncancerous and cancer cells respond differently to TGFβ. However, knowledge of the cellular signaling cascades triggered by TGFβ in various cell types is still limited
Challenges in photocatalysis using covalent organic frameworks
No full textPhotocatalysis is an attractive, energy-efficient technology for organic transformations, polymer synthesis, and degradation of environmental pollutants. There is a need for new photocatalysts stable in different media and that can be tailored for specific applications. Covalent organic frameworks (COF) are crystalline, nanoporous materials with π-conjugated backbone monomers, representing versatile platforms as heterogeneous, metal-free photocatalysts. The backbone structure can be tailored to achieve desired photocatalytic properties, side-chains can mediate adsorption, and the nanoporous structure provides large surface area for molecular adsorption. While these properties make COFs attractive as photocatalysts, several fundamental questions remain regarding mechanisms for different photocatalytic transformations, reactant transport into porous COF structures, and both structural and chemical stability in various environments. In this perspective, we provide a brief overview of COF photocatalysts and identify challenges that should be addressed in future research seeking to employ COFs as photocatalysts. We close with an outlook and perspective on future research directions in the area of COF photocatalysts
Electrically driven plasmonic processes: hot carriers and strong coupling
No full textPlasmonic modes confined to metallic nanostructures at the atomic and molecular scale push the boundaries of light-matter interactions. Within these extreme plasmonic structures of ultrathin nanogaps and tunnelling junctions, new physical phenomena arise when plasmon resonances couple to electronic, exitonic, or vibrational excitations, as well as the generation of non-radiative hot carriers. This thesis will summarize experimental and theoretical advances in the regime of extreme nanoplasmonics, with an emphasis on plasmon-induced hot carriers, strong plexitonic effects, and electrically driven processes at the molecular scale. We examine above-threshold light emission in electromigrated tunnel junctions, which is consistent with a suggested theoretical model describing hot-carrier dynamics driven by nonradiative decay of electrically excited localized plasmons. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through different possibilities for the first time, finding quantitative agreement with theory in each regime. Using plasmonic tunnel junctions as a platform supporting both electrically and optically excited localized surface plasmons, we report a much greater (over 1000× ) plasmonic light emission at upconverted photon energies under combined electro-optical excitation, compared with electrical or optical excitation separately. We use electroluminescence to probe plasmon-exciton coupling in hybrid structures each consisting of a nanoscale plasmonic tunnel junction and few-layer two-dimensional transition-metal dichalcogenide transferred onto the junction. The resulting hybrid states act as a novel dielectric environment to affect the radiative recombination of hot carriers in the plasmonic nanostructure. Further potential of this work and possible future research directions will also be discussed