Browsing by Subject "Nanowires"
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Item Electrophoresis of large DNA with a sparse zinc oxide nanowire array.(2010-05) Araki, NoritoshiWe developed a simple inexpensive method to integrate ZnO nanowires into a microchannel using a combination of aqueous solution synthesis of ZnO nanowires and photolithography, which is used as a nanowire-embedded microfluidic device. The density of ZnO nanowires inside the microchannel is controllable by simply changing the concentration of the seed solution. We conducted a study of dynamic interactions between electric field driven !DNA and a single isolated ZnO nanowire using single molecule spectroscopy technique. The study shows that the hooking time is exponentially dependent on b/Rg, in agreement with a prediction by simulation work. We also find that the hooking probability for small values of b/Rg increases as the electric field strength increases.Item Localized Programmable Gas Phase Electrodeposition and Its Applications in Functional Nanomaterials and Devices(2016-04) Fang, JunThis thesis focuses on development and application of a gas phase nanomaterial integration concept. We developed and demonstrated a novel gas phase electrodeposition method to control material flux transported and deposited at desired points on a patterned biased substrate based on the Coulomb force. The thesis is divided into two sections: (A) a corona based analyte charging method and an electrodynamic nanolens based analyte concentration concept to effectively transport airborne analytes to sensing points to improve the response time of existing gas sensor designs, and (B) a gas phase electrodeposition process to grow free-standing point-to-point electrical nanowire connections spanning a distance of up to 10 µm. Section A introduces a new general approach which uses a corona based charging method in combination with an electrodynamic lens based collection concept to transport particles to precise points on a surface. We discovered that the transport is faster than diffusion based transport commonly used. The faster transport and speed was then applied to the field of nanosensors of airborne particles. Specifically, we were able to reduce the response time of existing airborne sensor designs by several orders of magnitude. The process, referred to as “corona/lens-based-collection”, enables us to transport nanomaterials and airborne analytes from a space that is centimeters away to specific sensing points on a surface with a minimal spot size approaching 100 nm. We find that the collection rate is several orders of magnitudes higher than the case where the corona/lens-based-collection is turned off and collection is driven by diffusion only. The collection scheme is integrated on an existing SERS based sensor that is sensitive to the adsorption of small molecules. We compare the results with and without corona/lens-based-collection and find that SERS signal is enhanced by three orders of magnitudes as a result of increased collection efficiency. In terms of response time, the process is able to detect analytes at 9 ppm (parts per million) within 1 second. As a comparison, 1 hour is required to approach the same signal intensity in the case where diffusion-only-transport is used. Section B presents a gas phase electrodeposition process to grow free-standing point-to-point electrical nanowire connections spanning a distance of up to 10 µm. The gas phase electrodeposition process uses a patterned resist with openings to a conductor to guide the deposition of charged nanoparticles. Nanowire growth occurs at charge dissipating contacts which are accessible due to the openings in the resist. The formation of interconnects between contacts or bridges across a trench is possible through nearest neighbor interaction. The growing nanowires are composed of metallic nanoparticles. We discovered that a reduction of the primary nanoparticles size to the 1-5 nm range is required to achieve electrical conductive and mechanically stable bondwires. The annealing temperature has been reduced to 250°C due to the small particle size. The diameter of the nanowires depends on the growth duration and the size of the openings. The adjustable range is 50 nm-1 µm. Mechanically stable bondwires have a typical diameter of 250 nm. A 5 µm long interconnects with a radius of 250 nm had a resistance of 85 Ω.Item Synthesis of titanium dioxide and zinc oxide nanowires for excitonic solar cells.(2009-08) Boercker, Janice ElaineDye-sensitized, quantum-dot sensitized, quantum-dot, and hybrid organic/inorganic solar cells are promising excitonic photovoltaic devices for the generation of low cost, carbon free energy. Wide-band gap semiconductor nanowire photoanodes have the potential to increase the efficiencies of these excitonic solar cells. Controlling and tailoring the dimensions of the nanowires (i.e. nanowire height, diameter, and planar number density) for each solar cell type is important for efficiency improvement. Obtaining such control will require a detailed and fundamental understanding of the nanowire growth process. Towards this end, the synthesis of TiO2 and ZnO nanowire films in aqueous solutions was studied. Anatase TiO2 nanowire films were grown on flexible titanium foil substrates using a three step hydrothermal synthesis. First, the top surface of the titanium foil was transformed to Na2Ti2O4(OH)2 nanotubes through hydrothermal oxidation in NaOH. Next, the Na2Ti2O4(OH)2 nanotubes were converted to H2Ti2O4(OH)2 nanotubes by ion exchange. Finally, the H2Ti2O4(OH)2 nanotubes were converted to polycrystalline anatase nanowires through a topotactic transformation. The film morphology evolution, crystal structure transformations, and growth mechanism were examined in detail. Dye-sensitized solar cells (DSSCs) were assembled from these TiO2 nanowire films. Transient photocurrent and photovoltage spectroscopies were used to measure the electron transport and recombination rates in these solar cells. Compared to TiO2 nanoparticle DSSCs the electron collection efficiency in the TiO2 nanowire DSSCs was increased due to decreased electron recombination. However, the electron transport in the nanowire DSSCs was similar to that of TiO2 nanoparticle DSSCs. The synthesis of ZnO nanowires from aqueous solutions of methenamine (HMT) and zinc nitrate hexahydrate on substrates was studied in detail. A ZnO nanowire growth mechanism was proposed which predicts that the precursor is a zinc-methenamine complex (Zn-HMT2+ and Zn-HMT-Zn4+) which reacts at the hydroxyl terminated nanowire surface. This growth mechanism was supported by examining the growth with several experimental techniques, kinetic modeling, and thermodynamic calculations. In addition, the ZnO nanowire film growth, on ZnO seeded substrates, was found to be mass transport limited. This results in an inverse relationship between the nanowire planar number density and the height and diameter of the nanowires. By stirring the solution the growth rate is increased by approximately a factor of four.Item Titanium dioxide nanostructures for photovoltaics and photocatalysis.(2011-08) Liu, BinThe dye-sensitized solar cell (DSSC) is a promising low cost photovoltaic device for the generation of carbon-free energy. DSSC consists of two conducting glass electrodes in a sandwich configuration, with a redox electrolyte filling the free space in between. During illumination of the cell, the dye molecules inject electrons into the semiconductor film and the injected electrons diffuse through the semiconductor nanoparticle network through hopping from particle to particle until being collected at the electron collecting photoanode. Meanwhile, the charged dye is regenerated by an electrochemical reaction with a redox couple in the electrolyte. The oxidized ionic species diffuse towards the counter photocathode and are reduced by electrons that have traveled from the photocathode through the load to complete the circuit. To date, DSSCs with light-to-electric conversion efficiencies of ~7 to 11% have been demonstrated with ~10 mm thick electrodes made of 10-30 nm diameter TiO2 nanoparticles sensitized with ruthenium-based dyes, but further device improvement is limited due to the competition between electron transport and recombination. Wide bandgap semiconductor nanowire electrodes have the potential to increase the DSSC performance by increasing the electron transport rate while keeping the electron recombination rate unaltered. Towards this end, the synthesis of single-crystalline TiO2 nanowires on substrates was studied. Mesoporous anatase TiO2 microspheres composed of abutted TiO2 nanoparticles were synthesized through a two-step hydrothermal method. Photoanodes assembled from alternating layers of these mesoporous TiO2 microspheres and TiO2 nanoparticles increase the overall power conversion efficiencies of DSSCs by as much as 26%. This increase is due to enhanced light scattering by porous TiO2 microspheres and is achieved without sacrificing the specific surface area. Single-crystalline TiO2 nanowire arrays were grown on flexible titanium foil using a three-step solution synthesis. The synthesis method relies on the ability to grow single crystal sodium titanate (Na2Ti2O5·H2O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth, the Na2Ti2O5·H2O nanowires are converted to protonated bititanate (H2Ti2O5·H2O) nanowires through an ion-exchange reaction without changing their morphology or crystal structure. Finally, the protonated bititanate nanowires are converted to single crystalline anatase TiO2 nanowires through a topotactic transformation by calcination. These three sequential steps yield a carpet of 2– 50 μm long single crystalline nanowires oriented in the [100] direction and primarily normal to the titanium foil. DSSC assembled from 12 μm thick TiO2 nanowire film gives a light-to-electric conversion efficiency of ~ 1.4%. Further improvements in the cell efficiency should be possible with longer nanowires. Single-crystalline rutile TiO2 nanorods were grown on transparent conductive fluorine-doped tin oxide (FTO) substrates using a facile, hydrothermal method. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, acidity, and additives. The epitaxial relation between the FTO substrate and rutile TiO2 with a small lattice mismatch plays a key role in driving the nucleation and growth of the rutile TiO2 nanorods on FTO. With TiCl4-treatment, a light-to-electricity conversion efficiency of 3% could be achieved by using 4 μm-long TiO2 nanorod films as the photoanode in a DSSC. Single crystal anatase TiO2 nanorods/nanoflakes were grown on FTO substrates though a TiCl4 evaporation-condensation-hydrolyzation process, following by a subsequent thermal treatment. DSSCs assembled from 1 μm long TiO2 nanorod and nanoflake films give a light to electricity conversion efficiency of ~ 2.1%.