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 SYNTHESIS AND POLYMER-MEDIATED REGIOSELECTIVE SELF-ASSEMBLY OF SHAPED PLASMONIC NANOPARTICLES(2021) Lin, Xiaoying; Fourkas, John T; Nie, Zhihong; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasmonic nanoparticles with collective excitations of the conduction-band electrons have many potential applications in surfaceenhanced spectroscopies, photocatalysis, photovoltaics, and biomedicine. The plasmonic properties are highly tunable via the size, shape, chemical composition, and surrounding media of individual nanoparticles, as well as by the interactions with other nanoparticles in close proximity. Fabricating plasmonic nanostructures with precisely controlled size, shape, and interparticle distance is critical for harnessing desirable properties. Although top-down lithographic techniques are widely used for fabricating shaped plasmonic nanoparticles and ensembles, the products are limited to 2D planar structures and the cost can be prohibitive. As low-cost and versatile alternatives, colloidal synthesis and self-assembly have been explored to fabricate complex plasmonic nanostructures.In this dissertation, wet-chemistry synthetic and self-assembly approaches are explored for fabricating well-defined plasmonic nanostructures with high structural complexity. Chapter 2 describes a facile synthetic method for circular and triangular gold nanorings with tunable diameters, ring thicknesses, surface roughness, and hence the plasmonic response. The gold nanorings with rough surface show 100-fold higher enhancement factor than solid gold nanoparticles as substrate for surface-enhanced Raman spectroscopy. In chapter 3, we demonstrate regioselective bonding between nanospheres and nanoplates originating from the steric hindrance of polymeric ligand brushes and the anisotropy of nanoparticles. The regioselectivity enables a self-assembly system with precise control over the relative orientation of Au nanospheres on Ag nanoplates and the stoichiometry of reactive groups of copolymeric ligands dictates the number of nanoparticles in one nanocluster. The yield of each assembly was ~70% without further purification. Optical study reveals that different bonding modes affect the plasmonic coupling of assembled structures. In chapter 4, the regioselective bonding is applied to fabricate complex plasmonic nanocluster, nanoflowers and nanobuds, with distinct bonding modes. Compared with nanobuds, nanoflowers with the same number of petals show stronger electric field enhancement and further localized surface plasmon resonance peak shifts. The synthetic and self-assembly methods demonstrated in this dissertation have great potentials and versatility in designing not only plasmonic nanoclusters, but also other inorganic nanoparticle-based functional structures with high complexity.Item IR detection and energy harvesting using antenna coupled MIM tunnel diodes(2012) Yesilkoy, Filiz; Peckerar, Martin; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The infrared (IR) spectrum lies between the microwave and optical frequency ranges, which are well suited for communication and energy harvesting purposes, respectively. The long wavelength IR (LWIR) spectrum, corresponding to wavelengths from 8um to 15um, includes the thermal radiation emitted by objects at room temperature and the Earth's terrestrial radiation. Therefore, LWIR detectors are very appealing for thermal imaging purposes. Thermal detectors developed so far either demand cryogenic operation for fast detection, or they rely on the accumulation of thermal energy in their mass and subsequent measurable changes in material properties. Therefore, they are relatively slow. Quantum detectors allow for tunable and instantaneous detection but are expensive and require complex processes for fabrication. Bolometer detectors are simple and cheap but do not allow for tunability or for rapid detection. Harvesting the LWIR radiation energy sourced by the Earth's heating/cooling cycle is very important for the development of mobile energy resources. While speed is not as significant an issue here, conversion efficiency is an eminent problem for cheap, large area energy transduction. This dissertation addresses the development of tunable, fast, and low cost wave detectors that can operate at room temperature and, when produced in large array format, can harvest Earth's terrestrial radiation energy. This dissertation demonstrates the design, fabrication and testing of Antenna Coupled Metal-Insulator-Metal (ACMIM) tunnel diodes optimized for 10um wavelength radiation detection. ACMIM tunnel diodes operate as electromagnetic wave detectors: the incident radiation is coupled by an antenna and converted into a 30 terahertz signal that is rectified by a fast tunneling MIM diode. For efficient IR radiation coupling, the antenna geometry and its critical dimensions are studied using a commercial finite-element based multi-physics simulation tool, and the half-wave dipole-like bow-tie antennas are fabricated using simulation-optimized geometries. The major challenge of this work is designing and fabricating MIM diodes and coupled antennas with internal capacitances and resistances small enough to allow response in the desired frequency range (~30 THz) and yet capable of efficiently coupling to the incident radiation. It is crucial to keep the RC time constant of the tunnel junction small to achieve the requisite cut-off frequency and adequate rectification efficiency. Moreover, a low junction resistance is necessary to load the coupled AC power across the MIM junction. For energy harvesting applications, the device has to operate without an external bias, which requires asymmetry at the zero bias operation point. To address these requirements, the MIM tunnel junction is established so that one electrode has a field enhancing sharp tip (cathode) and the other is a rectangular patch. This asymmetric geometry not only offers asymmetric current-voltage behavior at the zero bias point, but also it decouples the junction resistance and capacitance by concentrating the charge transport in a small volume around the tip. Various fabrication methods are developed in order to create small junction area (= low parasitic capacitance), low junction resistance (= effective power coupling through antenna), asymmetry (= zero bias operation), high fabrication yield and low cost ACMIM tunnel diodes. High resolution fabrication needs are accomplished by electron beam lithography and nano-accuracy in the junction area is achieved by employing dose modifying proximity effect correction and critical alignment methods. Our Ni/NiOx/Ni ACMIM diodes with an optimized insulation layer created with O2 plasma oxidation are the most successful devices presented to date. A novel fabrication technique called "strain assisted self lift-off process" is used to achieve small junction area devices without relying on lithographic resolution. This technique eliminates the rival parasitic capacitance issue of today's ACMIM tunnel diodes and does not rely on extreme-high resolution lithography technologies.Item CARBON NANOTUBE THIN FILM AS AN ELECTRONIC MATERIAL(2009) Sangwan, Vinod Kumar; Williams, Ellen D; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Carbon nanotubes (CNT) are potential candidates for next-generation nanoelectronics devices. An individual CNT possesses excellent electrical properties, but it has been extremely challenging to integrate them on a large-scale. Alternatively, CNT thin films have shown great potential as electronic materials in low cost, large area transparent and flexible electronics. The primary focus of this dissertation is patterning, assembling, characterization and assessment of CNT thin films as electronic material. Since a CNT thin film contains both metallic and semiconducting CNTs, it can be used as an active layer as well as an electrode material by controlling the growth density and device geometry. The growth density is controlled by chemical vapor deposition and airbrushing methods. The device geometry is controlled by employing a transfer printing method to assemble CNT thin film transistors (TFT) on plastic substrates. Electrical transport properties of CNT TFTs are characterized by their conductance, transconductance and on/off ratio. Optimized device performance of CNT TFTs is realized by controlling percolation effects in a random network. Transport properties of CNTs are affected by the local environment. To study the intrinsic properties of CNTs, the environmental effects, such as those due to contact with the dielectric layer and processing chemicals, need to be eliminated. A facile fabrication method is used to mass produce as-grown suspended CNTs to study the transport properties of CNTs with minimal effects from the local environment. Transport and low-frequency noise measurements are conducted to probe the intrinsic properties of CNTs. Lastly, the unique contrast mechanism of the photoelectron emission microscopy (PEEM) is used to characterize the electric field effects in a CNT field effect transistor (FET). The voltage contrast mechanism in PEEM is first characterized by comparing measurements with simulations of a model system. Then the voltage contrast is used to probe the local field effects on a single CNT and a CNT thin film. This real-time imaging method is assessed for potential applications in testing of micron sized devices integrated in large scale.