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 Electrical Properties of a Tube-in-a-Tube Semiconductor(2016) Ng, Allen Lee; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tube-in-a-tube (Tube^2) nanostructures were synthesized through the outer-wall selective covalent functionalization of double-walled carbon nanotubes (DWCNTs) at high functional densities. Upon functionalization, the properties of individual walls within the structure decouple resulting in an electrically insulating functional outer tube while the inner tube retains exceptional CNT properties. The exceptional electrical properties of Tube^2 semiconductor structures were demonstrated for applications that include molecular and biological sensors and patterning of CNTbased structures with electronic type specificity. Tube^2 thin film transistor (TFT) sensors exhibited simultaneous ultrahigh sensitivity and selectivity towards chemical and biological targets. Carboxylic acid terminated Tube^2 sensors displayed an NH3 sensitivity of 60 nM, which is comparable with small molecule aqueous solution detection using state-of-the-art TFT sensors while simultaneously attaining 6,000 times higher chemical selectivity towards a variety of amine containing analyte molecules over carboxylic acids. Similarly, 23-base ii oligonucleotide terminated Tube^2 sensors demonstrated concomitant sensitivity down to 5 nM towards their complementary sequence without amplification techniques and single mismatch selectivity without the use of a gate electrode. Unique sensor architectures can be designed with the requirement of a gate electrode, such as the creation of millimeter-scale point sensors. The optical features and unique structural features of Tube^2 thin films were also exploited to address the challenge of patterning CNT nanostructures with electronic type specificity. Patterned dot arrays and conductive pathways were created on an initially insulating Tube^2 thin film by tuning the resonance of the direct-writing laser with the electronic type of the inner tube (i.e., metallic or semiconducting). The successful patterning of Tube^2 thin films was unambiguously confirmed with in situ Raman spectral imaging and electrical characterization. Furthermore, a hybrid 2-D carbon nanostructure comprised of a functionalized graphene that covers a semiconducting (6,5) SWCNT network (fG/sSWCNT) was developed. The hybrid fG/sSWCNT nanostructure exhibits similar structural and electrical properties as a semiconducting Tube^2 thin film, but possesses a transconductance that is an order of magnitude larger than Tube^2 and ON/OFF ratios as high as 5400 without the useful of further processing steps such as electrical breakdown.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 Novel Interactions of Liquid Crystals with Coated Nanoparticles(2013) Taylor, Jefferson; Martinez-Miranda, Luz J; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Functionalized nanoparticles have a wide range of applications in liquid crystal systems, including displays, photovoltaics, and drug delivery. We need to understand the interactions between the nanoparticles and the liquid crystal molecules in order to utilize them fully and safely. We investigate the short-range interaction of coated nanoparticles with a liquid crystal membrane or bulk sample through the use of atomic force microscopy (AFM) and X-ray scattering techniques. We identify the role the functionalization plays in the phase behavior of the liquid crystal both as a thin film and in bulk. Our research produced three results. We identify differing behavior in thin film samples of liquid crystal and coated nanoparticles dependent upon particle functionalization using AFM. Using X-ray scattering we measure the alignment and smectic layer formation in the presence of coated nanoparticles, even above the smectic-A to nematic transition temperature. We find evidence of a "halo" that forms around coated nanoparticles, particularly with longer coating molecules.