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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Item UNVEILING THE SELF-ASSEMBLY OF POLYMER-GRAFTED NANOPARTICLES IN SELECTIVE SOLVENTS(2023) Lamar, Chelsey; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The self-assembly of inorganic nanoparticles (NPs) has garnered considerable attention due to the potential for fabricating functional structures with unique collective properties. In recent years, polymers have emerged as valuable candidates in assisting the organization of NPs into complex architectures with multiple capabilities. Researchers have shown that polymer-grafted nanoparticles (PGNPs) facilitate the use of advanced nanostructures with tailored properties in biomedical applications. Although, continued exploration of the rational design and tailoring of PGNP assemblies is needed to expand our understanding before we can fully realize the potential of these structures in desired applications. My dissertation aims to investigate the fundamental aspects and elucidate the underlying mechanisms in the self-assembly of PGNPs for modern biomedical applications. A facile and versatile solution-based strategy was utilized to explore the individual self-assembly of PGNPs with anisotropic NPs and the co-assembly of binary PGNPs with distinct sizes. We focused on designing, characterizing, and exploring the optical properties of hierarchical assembly structures produced from inorganic NPs tethered with amphiphilic block copolymers (BCPs). Individual PNGPs with anisotropic NPs and binary mixtures of small and large PGNPs produce vesicle structures with well-defined packing arrangements. My work shows how key parameters, including polymer chain length, nanoparticle size, and concentration, influence the self-assembly behavior and the formation of vesicles in each system. Through a combination of experimental observations and theoretical considerations, I highlight the significance of polymer shell shape in dictating the self-assembly behavior of individual anisotropic PGNPs. Moreover, I demonstrate that elevated temperatures impacted the stability and optical responses of the vesicle structures. In co-assembly studies, my work describes the macroscopic segregation of PGNPs with different sizes in the vesicular membrane, which is attributed to the conformation entropy gain of the grafted copolymer ligands. This research will provide valuable insights into the self-assembly behavior and fundamental design of PGNP structures relevant to biomedical applications.Item Statistical Network Analysis of High-Dimensional Neuroimaging Data With Complex Topological Structures(2023) Lu, Tong; Chen, Shuo SC; Mathematical Statistics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation contains three projects that collectively tackle statistical challenges in the field of high-dimensional brain connectome data analysis and enhance our understanding of the intricate workings of the human brain. Project 1 proposes a novel network method for detecting brain-disease-related alterations in voxel-pair-level brain functional connectivity with spatial constraints, thus improving spatial specificity and sensitivity. Its effectiveness is validated through extensive simulations and real data applications in nicotine addiction and schizophrenia studies. Project 2 introduces a multivariate multiple imputation method specifically designed for voxel-level neuroimaging data in high dimensions based on Bayesian models and Markov chain Monte Carlo processes. According to both synthetic data and real neurovascular water exchange data extracted from a neuroimaging dataset in a schizophrenia study, our method indicates high imputation accuracy and computational efficiency. Project 3 develops a multi-level network model based on graph combinatorics that captures vector-to-matrix associations between brain structural imaging measures and functional connectomic networks. The validity of the proposed model is justified through extensive simulations and a real structure-function imaging dataset from UK Biobank. These three projects contribute innovative methodologies and insights that advance neuroimaging data analysis, including improvements in spatial specificity, statistical power, imputation accuracy, and computational efficiency when revealing the brain’s complex neurological patterns.Item INTEGRATION OF CLASSICAL/NONCLASSICAL OPTICAL NONLINEARITIES WITH PHOTONIC CIRCUITS(2023) Buyukkaya, Mustafa A; Waks, Edo; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent developments in nanofabrication have opened opportunities for strong light-matter interactions that can enhance optical nonlinearities, both classical and non-classical, for applications such as optical computing, quantum communication, and quantum computing. However, the challenge lies in integrating these optical nonlinearities efficiently and practically with fiber-based and silicon-based photonic circuits on a large scale and at low power. In this thesis, we aimed to achieve this integration of classical and quantum optical nonlinearities with fiber-based and silicon-based photonic circuits.For classical optical applications, optical bistability is a well-researched nonlinear optical phenomenon that has hysteresis in the output light intensity, resulting from two stable electromagnetic states. This can be utilized in various applications such as optical switches, memories, and differential amplifiers. However, integrating these applications on a large scale requires low-power optical nonlinearity, fast modulation speeds, and photonic designs with small footprints that are compatible with fiber optics or silicon photonic circuits. Thermo-optic devices are an effective means of producing optical bistability through thermally induced refractive index changes caused by optical absorption. The materials used must have high absorption coefficients and strong thermo-optic effects to realize low-power optical bistability. For this purpose, we choose high-density semiconductor quantum dots as the material platform and engineer nanobeam photonic crystal structures that can efficiently be coupled to an optical fiber while achieving low-power thermo-optical bistability. For applications that require non-classical nonlinearities such as quantum communication and quantum computing, single photons are promising carriers of quantum information due to their ability to propagate over long distances in optical fibers with extremely low loss. However, the efficient coupling of single photons to optical fibers is crucial for the successful transmission of quantum information. Semiconductor quantum dots that emit around telecom wavelengths have emerged as a popular choice for single photon sources due to their ability to produce bright and indistinguishable single photons, and travel long distances in fiber optics. Here, we present our advances in integrating telecom wavelength single photons from semiconductor quantum dots to optical fibers to realize efficient fiber-integrated on-demand single photon sources at telecom wavelengths. Finally, using the same methodology, we demonstrate the integration of these quantum dots with CMOS foundry-made silicon photonic circuits. The foundry chip is designed to individually tune quantum dots using the quantum confined stark shift with localized electric fields at different sections of the chip. This feature could potentially enable the tuning of multiple quantum emitters for large-scale integration of single photon sources for on-chip quantum information processing.Item Solution-Processed Clean SWCNTs and Their Use as Templates for One-Dimensional van der Waals Heterostructures(2022) Zhang, Chiyu; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWCNTs) have shown exceptional electrical, optical, mechanical, and thermal properties. Solution processing is a critical first step to harness these nanomaterials for applications in electronics, biomedicine, and energy technologies. However, dispersion of SWCNTs in solutions requires assistance by surfactants or polymers, which cannot be cleanly removed easily and become unwanted contaminants, resulting in degraded performance of SWCNTs.In this dissertation, I developed strategies to attain clean, solution-processed SWCNTs and further demonstrated their applications as templates for the synthesis of van der Waals heterostructures. We investigated the role of surfactants in dispersing SWCNTs and found that the highest efficiency in dispersing SWCNTs occurs at the critical micelle concentration of surfactants, which is well below the typically required surfactant concentrations. Furthermore, we synthesized a thermally removable surfactant, ammonium deoxycholate (ADC) which can be removed cleanly at a relatively low temperature without damaging the SWCNT structure. Compared to a commonly used surfactant, sodium deoxycholate (DOC), ADC features the same anion, but contains an ammonium (NH4+) cation in place of the metal ion (Na+). ADC exhibits the same high dispersion efficiency for SWCNTs as DOC, but the peak thermal decomposition temperature of ADC is nearly 70 oC lower than that of DOC. A two-step annealing process can remove this new nanotube surfactant while keeping the SWCNTs intact, even with a small diameter of just 0.76 nm. This work also reveals the chemical origin of residues from thermal annealing of surfactant-processed carbon nanomaterials. The clean SWCNTs enable the synthesis of van der Waals heterostructure consisting of pure chiral single-wall carbon nanotubes nested in boron nitride (SWCNT@BN). Transmission electron microscopy and electron energy-loss spectroscopic mapping confirm the successful synthesis of SWCNT@BN from the solution-purified nanotubes. The photoluminescence peak of (7,5)-SWCNT@BN heterostructure is found to redshift by 10 nm relative to that of (7,5)-SWCNT and the Raman G peak of (7,5)-SWCNTs downshift by 10 cm-1 after BN coating.Item QUANTUM CONTROL AND MEASUREMENT ON FLUXONIUMS(2022) Xiong, Haonan; Manucharyan, Vladimir; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Superconducting circuit is a promising platform for quantum computing and quantum simulation. A number of efforts have been made to explore the physics in transmon systems and optimize the qubit performance. Compared to transmon, fluxonium is a relatively new type of qubit and attracts more attention recently due to its high coherence time and large anharmonicity. In this thesis, we summarize recent progress toward high fidelity two-qubit gate and readout for fluxonium qubits. We report improved fluxonium coherence either in cavity or cavityless environment. In the former case, we demonstrate single-shot joint readout for two fluxonium qubits and explore various two-qubit gate schemes such as controlled-Z(CZ) gate, controlled-phase(CP) gate, bSWAP gate and cross-resonance(CR) gate. The CZ gate realized by near-resonantly driving the high transitions exhibits 99.2% fidelity from randomized benchmarking. A continuous CP gate set can be implemented by off-resonantly driving the high transitions and shows an average 99.2% fidelity from the cross-entropy benchmarking technique. Other gates involving only computational states are also explored to further improve the gate fidelity, which can take advantage of the high coherence of the fluxonium lower levels. In the cavityless environment, we demonstrate fluorescence shelving readout with 1.7 MHz radiative decay rate for the readout transition while maintaining 52 us coherence time for the qubit transition. Our research explores the basic elements for fluxonium-based quantum processors. The results suggest that fluxonium can be an excellent candidate for not only universal quantum computation but also quantum network and quantum optics studies.Item Fluorescent Carbon Nanotubes as Molecular Sensors and Color-Center Hosts(2022) Qu, Haoran; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates the application of single-walled carbon nanotubes (SWCNTs) as single-digit nanopores for molecular sieving and addresses a fundamental challenge pertaining to controlled synthesis of organic color-centers (OCCs) on the sp2 carbon lattice of SWCNTs. First, I describe a hyperspectral single-defect photoluminescence imager system that provides both hyperspectral imaging and super-resolution capabilities in the shortwave infrared. Second, I aim to understand the relationship between nanotube photoluminescence and encapsulated molecules. Using carbon nanotubes with sub-1 nm pores, I demonstrate molecular sieving of n-hexane from cyclohexane, which are nearly identical in size. Furthermore, I discovered a light irradiation method to drive structural transformation of OCCs which allow us to narrow the spectral distribution of defect emissions by 26%. Finally, I show that [2+2] cycloaddition can efficiently create OCCs. Remarkably, this novel defect chemistry reduces the number of OCC bonding configurations from six, which are commonly observed with monovalent defect chemistries, to just three. This work may have broad implications to the potential applications of SWCNTs and OCCs in chemical sensing, bioimaging, and quantum information science.Item Hybrid plasmonic tubular nanostructures: synthesis, optical and photoelectrocatalytic property(2022) Zhang, Qian; Lee, Sang Bok; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Utilizing sustainable and clean solar energy in the visible-NIR range to drive chemical transformation has been a key resolution to solve the energy crisis. Recently, rational design of photocatalysts that conjugate plasmonic nanostructures with catalytic metal nanostructures has attracted particular research interests. This dissertation describes the design principles of plasmonic-catalytic hybrid tubular nanostructures and investigates their application in photoelectrocatalytic reactions.First, we introduced the mechanism of plasmon-mediated catalysis and current research of plasmonic photocatalysts. Specifically, managing the energy flow from the plasmonic entity to the catalytic entity is the crucial part for customizable design of photoelectrocatalysts. We also summarized the design principles of general electrocatalysts and synthesis of hollow nanostructures. Second, we reported a facile but highly reproducible synthetic strategy to fabricate catalytic Pt hollow nanotubes (NTs). Scalable fabrication of ultrathin Pt NTs can be achieved through simple mediation of the concentration of the surfactant employed in the reaction. Third, we reported a template-directed synthesis of PtAu NTs with tunable localized surface plasmon resonance (LSPR) bands in the visible-near infrared region (NIR). The geometric dependent LSPR band shift was systematically studied based off the spectra from both experiment and finite-difference time-domain (FDTD) simulations. It was found that the PtAu NTs exhibited the LSPR characters of both rodlike and hollow nanostructures. Finally, we investigated the photoelectrocatalytic performance of the PtAu NTs under visible-NIR light irradiation. The optimized photocatalytic activity of the PtAu NTs towards the electrooxidation of methanol was achieved by maximizing their LSPR absorption cross sections at longer wavelengths in the visible-NIR region. Meanwhile, combining the superior intrinsic electrocatalytic performance of PtAu NTs towards methanol oxidation, we expected the bimetallic PtAu tubular nanostructure could effectively convert visible light energy to drive the electrochemical transformation.Item Developing New Experimental Techniques to Understand Neuronal Networks(2021) Aghayee, Samira Sadat; Losert, Wolfgang; Biophysics (BIPH); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Studying the propagation of action potentials across neuronal networks and how information is stored and accessed in their dynamic firing patterns has always been the essence of neuroscience. Emerging evidence shows that information in the brain is encoded in the simultaneous or avalanche-like firing of multiple, spatially separated groups of neurons. Thus, understanding the collective behavior of neurons is essential for understanding how the brain processes information and encodes memory. Since its discovery, the advent of optogenetics has brought upon a revolution in neuroscience, where individual neuronal circuits are able to be selectively probed and their connections decoded. This ability has been used by many groups to great effect, with some groups even using optogenetic stimulation to create phantom sensations, which are typically encoded in the functional activity of distinct neuronal populations. However, in-vivo optogenetic excitation relies inherently on the quality and accuracy of the stimulation method, with many problems arising due to biological effects such as animal motion, the scattering nature of brain tissue, and cell health. Typically, groups either use digital micromirror arrays or spatial light modulators, with the former lacking transmission efficiency and the latter having a high technical skill barrier due to its propensity to induce artifacts into intended patterns of light. This dissertation attempts to reduce the barrier towards the use of spatial light modulators in optogenetics by improving targeting accuracy, reducing the effects of unmodulated light and related artifacts, and developing new methods of stimulation which reduce the power density directed at neurons. To accomplish the first step, improving targeting accuracy, I created and demonstrated a real-time capable particle-based motion tracking algorithm to correct for animal motion. To reduce the effects of optical artifacts, I developed and patented a method of using Fresnel lenses convolved with intended light patterns to project higher orders of diffraction and un-diffracted light axially away from the object plane. To improve cell health during stimulation, I researched the use of optical vortices to stimulate neurons, allowing for ion channel activation with reduced power per unit area. Finally, I set the stage for new science by creating neuroimaging platforms integrating these techniques and capable of imaging activity across multiple scales. Other avenues for improvement are outlined as well in this dissertation, as well as new scientific questions that can be asked, leveraging these developments contained within.Item ATOMISTIC EXPLORATION OF DENSELY-GRAFTED POLYELECTROLYTE BRUSHES: EFFECT OF APPLIED ELECTRIC FIELD AND MULTIVALENT SCREENING COUNTERIONS(2022) Pial, Md Turash Haque; Das, Siddhartha; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polyelectrolyte (PE) or charged polymers are ubiquitous under biological and synthetic conditions, ranging from DNA to advanced technologies. PE chains can be grafted on a surface and they extend into solution to form a "brush"-like configuration if the grafting density is high. PE brushes respond to external stimuli by changing their conformation and chemical details, which make them very attractive for numerous applications. Multivalent counterions (neutralizing PE charges) and external electric fields are known to significantly affect the brush behavior. Obtaining fundamental insights into PE brush’s response to ions and electric filed is of utmost importance for both industrial and academic research. In this dissertation, we use atomistic tools to improve our understanding of the PE brushes grafted on a single surface and two inner walls of a nanochannel under these two stimuli.We start by developing an all-atom molecular dynamics simulation framework to test the behavior of the PE brushes (grafted on a single surface) in the presence of externally applied electric fields. It is discovered that the charge density of PE monomers can have significant influence on their response; a smaller monomer charge density helps the brush to tilts along the electric field, while the PE brush with higher monomer charge density bends and shrinks. We found that counterion condensation to PE chains has a substantial impact in controlling these responses. In the subsequent study we discuss the effect of counterion size and valence in dictating counterion mediated bridging interaction of two or more negative monomers. By examining the solvation behavior, we identify that bridging interactions are not a sole function of the counterion valence. Rather, it depends on the counterion condensation on the PE chain, as well as the size of the counterion solvation shell. We also test the dynamic properties of the counterions and associated bridges. Later, we proceeded to simulate PE brush-grafted nanochannels to explore equilibrium and flow behavior in presence of nanoconfinement. We identify the onset of overscreening: there are a greater number of coions than counterions in the bulk liquid outside the brush layer. This specific ion distribution ensures that the overall electroosmotic flow is along the direction of the coions. Furthermore, for a large electric field, some of the counterions leave the PE brush layer into the bulk, resulting in disappearance of overscreening. If the number of counterions is greater than coions, electroosmotic flow reverses its direction and follows the motion of counterions. Finally, we discover that counterion-monomer interactions control the ion distribution. As a result, a diverse range of electroosmotic flow is found for counterions with different valence and size.Item CONFINED PHOTOTHERMAL HEATING OF NANOPARTICLE DISPLAYED BIOMATERIALS(2021) Hastman, David A; Medintz, Igor L; Aranda-Espinoza, Helim; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Controlling the temperature of biological systems has long been utilized as a tool for regulating their subsequent biological activity. Recently, photothermal heating of gold nanoparticles (AuNPs) has emerged as an efficient and remote method to heat proximal biological materials. Moreover, this technique has tremendous potential for controlling biological systems at the subcellular level, as specific components within the system can be heated while the larger system remains unaffected. The small size, biocompatiblilty, and optical properties of AuNPs make them attractive nanoscale heat sources for controlling biological systems. While the utility of photothermal heating has significantly advanced through the optimization of AuNP size, shape, and composition, the choice of incident light source utilized has largely been unexplored. One of the more interesting excitation sources is a femtosecond (fs) pulsed laser, as the subsequent temperature increase lasts for only a few nanoseconds and is confined to the nanoscale. However, it is not yet clear how biological materials respond to these short-lived and ultra-confined nanoscale spaciotemporal temperature increases. In this dissertation, we utilize fs laser pulse excitation to locally heat biological materials displayed on the surface of AuNPs in order to understand the corresponding heating profiles and, in turn, interpret how this can be used to modulate biological activity. Due to its unique temperature sensitive hybridization properties, we exploit double-stranded deoxyribonucleic acid (dsDNA) as our prototypical biological material and demonstrate precise control over the rate of dsDNA denaturation by controlling the laser pulse radiant exposure, dsDNA melting temperature, bulk solution temperature, and the distance between the dsDNA and AuNP surface. The rate of dsDNA denaturation was well fit by a modified DNA dissociation equation from which a “sensed” temperature value could be obtained. Evaluating this sensed temperature in the context of the theoretical temperature profile revealed that the ultra-high temperatures near the AuNP surface play a significant role in denaturation. Additionally, we evaluate this technique as a potential means to enhance enzyme activity and report that enhancement is governed by the laser repetition rate, pulse width, and the enzyme’s inherent turnover number. Overall, we demonstrate that the confined and nanosecond duration temperature increase achievable around AuNPs with fs laser pulse excitation can be used to precisely control biological function and establish important design considerations for coupling this technique to more complex biological systems.