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 Electronically tailored functionalization of carbon nanotubes(2014) Piao, Yanmei; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon nanotubes (CNTs) were chemically tailored on the electronic level to enhance their optical and electrical properties. Incorporation of sp3 defects into the sidewalls of CNTs significantly improved quantum efficiency of CNT photoluminescence (PL). Nanotube PL is intrinsically inefficient, usually less than 1%, due to the presence of dark excitons. This low efficiency makes nanotubes impractical for many applications, especially bio-imaging and optoelectronics. The nanotube PL was increased by up to 28 times through the chemical creation of a new defect induced state. This new state is optically allowed and resides below the predicted energy levels of the dark excitons, allowing the dark excitons to be harvested from this new defect state. Emission from the new state generates a distinct, structure-specific, and chemically tunable photoluminescence peak. This new peak is red-shifted by as much as 254 meV from the original excitonic transition and located within the tissue transparent window, which merits bio-imaging and bio-sensing. This work opens the door to harnessing dark excitons and lays the foundation for chemical control of defect quantum states in low dimensional carbon materials. Unlike atom-thick materials such as SWCNTs and graphene which are prone to chemical attacks because all constituent atoms are exposed, double-walled carbon nanotubes (DWCNTs) provide a chemically tailorable surface and an inner-tube with intact electronic properties. Even when the outer walls were selectively functionalized up to 6.9% (percent of carbon that are covalently modified), the inner tubes were electrically intact. Correlated Raman and optical absorption spectroscopy unambiguously confirm that the covalent modification was outer wall-selective. Nearly 50% of the electrical conductivity was retained in thin films of covalently functionalized nanotubes owing to the protected inner-tube conducting channels. Lacking such channels, SWCNTs became insulators after similar functionalization. Further experiments demonstrated that the covalently attached aryl groups could be selectively removed by optical annealing. These results suggest the possibility of high performance DWCNT electronics with important capabilities of tailored surface chemistry on the outer walls while the inner walls are chemically protected.Item Developing Tools for Investigating Chemotaxis Signal Clusters in Bacillus subtilis(2012) Rogers, James Allen; Stewart, Richard C; Cell Biology & Molecular Genetics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Many bacteria make use of a set of dedicated chemoreceptor proteins to control a His-Asp signaling system; this control converts environmental sensory information into instructions that regulate flagellar rotation, enabling chemotaxis. This thesis summarizes my investigations of some of the chemotaxis signaling proteins in Bacillus subtilis, particularly coupling proteins CheW and CheV. Proteins CheA, CheW, CheV, CheY, and FliM were each expressed in B. subtilis as translational fusions with either YFP or CFP. These fusion proteins were then shown to fluoresce in living bacterial cells. Motility experiments were conducted to compare the function of these fusion proteins to their wild type counterparts. This thesis proposes a series of experiments that would use these fluorescent fusion proteins to further explore the idea that these chemotaxis proteins change position when B. subtilis encounters chemostimuli.Item CMOS SINGLE-PHOTON AVALANCHE DIODES AND MICROMACHINED OPTICAL FILTERS FOR INTEGRATED FLUORESCENCE SENSING(2012) Dandin, Marc Peralte; Abshire, Pamela A; Smela, Elisabeth; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation presents a body of work that addresses the two most pressing challenges in the field of integrated fluorescence sensing, namely, the design of integrated optical sensors and the fabrication of high-rejection micro-scale optical filters. Two novel enabling technologies were introduced. They are: the perimeter-gated single-photon avalanche diode (PGSPAD), for on-chip photon counting, and the benzotriazole (BTA)-doped thin-film polymer filter, for on-chip ultraviolet light rejection. Experimental results revealed that the PGSPAD front-end, fabricated in a 0.5 μm standard mixed-signal CMOS process, had the capability of counting photons in the MHz regime. In addition, it was found that a perimeter gate, a structural feature used to suppress edge breakdown in the diode, also maximized the signal-to-noise-ratio in the high-count rate regime whereas it maximized sensitivity at low count rates. On the other hand, BTA-doped filters were demonstrated utilizing three commonly used polymers as hosts. The filters were patternable, utilizing the same procedures traditionally used to pattern the undoped polymer hosts, a key advantage for integration into microsystems. Filter performance was analyzed using a set of metrics developed for optoelectronic characterization of integrated fluorescence sensors; high rejection levels (nearing -40 dB) of UV light were observed in films of only 5 μm in thickness. Ultimately, BTA-doped filters were integrated into a portable sensor, and their use was demonstrated in two types of bioassays.Item SPECTROSCOPIC ENHANCEMENT FROM NOBLE METALLIC NANOPARTICLES(2011) Tsai, Shu-Ju; Phaneuf, Raymond J.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Resonant coupling of localized surface plasmon resonances (LSPRs) in noble metallic nanostructures to incident radiation and the related subject of localized behavior of electromagnetic waves are currently of great interest due to their potential application to sensors, biochemical assays, optical transmission, and photovoltaic devices. My thesis research is made up of two related parts. In part one I examined enhanced fluorescence in dye molecules in proximity to Ag nanostructures. In part two I studied the effect of Au nanostructure arrays on the performance of poly(3-hexylthiophene-2,5-diyl) : [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction (BHJ) organic solar cells (OSCs). Nanostructures were fabricated by two different methods: e-beam lithography (top down) and spray pyrolysis (bottom up). Using e-beam lithography, we produced arrays of nanostructures with well defined shapes, sizes, and spacings. By systematically varying these topographical parameters, we measured their effect on nanometer-sized metallic structure-enhanced fluorescence (nMEF) and on absorption and external quantum efficiency (EQE) in OSC devices as a function of optical wavelength. In analyzing experimental results, we carried out numerical simulations of the local electric field under incident light, across plasmonic resonances. The comparison between the calculated local field squared and measured fluorescence/EQE provides physical insight on the configuration- dependence of these two processes. Our results indicate that local field enhancement near nanostructures is dominant in nMEF, and that the local field is strongly affected by the substrate and device architectures. For the OSCs, both measurements and calculations show that absorbance within the active layer is enhanced only in a narrow band of wavelengths (~640-720 nm) where the active layer is not very absorbing for our prototype nanopillar-patterned devices. The peak enhancement for 180 nm wide Au nanopillars was approximately 60% at 675nm. The corresponding resonance involves both localized surface plasmon excitation and multiple reflections/diffraction within the cavity formed by the electrodes. Finally, we explore the role of the size of the nanostructures in such a device on the optical absorption in the OSC active layer. We find that small Au nanopillars produce strong internal absorption resulting in Joule heating, and suppressing the desired enhancement in EQE in OSC devices.