Browsing by Subject "thin films"
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Item Dielectric-Loaded Microwave Cavity for High-Gradient Testing of Superconducting Materials(2011-08-08) Pogue, Nathaniel JohnstonA superconducting microwave cavity has been designed to test advanced materials for use in the accelerating structures contained within linear colliders. The electromagnetic design of this cavity produces surface magnetic fields on the sample wafer exceeding the critical limit of Niobium. The ability of this cavity to push up to 4 times the critical field provides, for the first time, a short sample method to reproducibly test these thin films to their ultimate limit. In order for this Wafer Test cavity to function appropriately, the large sapphire at the heart of the cavity must have specific inherent qualities. A second cavity was constructed to test these parameters: dielectric constant, loss tangent, and heat capacity. Several tests were performed and consistent values were obtained. The consequences of these measurements were then applied to the Wafer Cavity, and its performance was evaluated for different power inputs. The Q_0 of the cavity could be as low as 10^7 because of the sapphire heating, therefore removing the ability to measure nano-resistances. However, with additional measurements in a less complex environment, such as the Wafer Test Cavity, the Q_0 could be higher than 10^9.Item Electrochemical hydrogenation of aromatic compounds chemisorbed at polycrystalline and single-crystal Pd surfaces(2009-06-02) Sanabria-Chinchilla, JeanThe chemisorption and electrochemical hydrogenation of hydroquinone (H2Q) at polycrystalline (pc) Pd, well-ordered Pd(100), and Pd-modified Au(hkl) electrodes were studied using a combination of ultra-high vacuum (UHV) surface spectroscopy, electrochemistry (EC), and electrochemical mass spectrometry (EC-MS). H2Q was found to form a slightly tilted flat-oriented quinone (Q) adlayer, when adsorbed from low concentrations; when chemisorbed from high concentrations, an edgewise-oriented H2Q adlayer was indicated. The hydrogenation of the chemisorbed layer is initiated at potentials before the onset of the hydrogen evolution region. As expected, the kinetics increases as the applied potential is increased, but the hydrogenation pathway appears to be independent of the potential. Hydrogenation in the absence of absorbed hydrogen (sub-surface) was studied at ultra-thin Pd films on Au single-crystal substrates. Hydrogenation and/or potential induced desorption were established, although non-volatile and/or hydrophobic products were detected. In comparison, negative excursions with benzene-coated electrodes resulted in nothing more than potential-induced desorption of the starting material. Negative-potential electro-desorption was more facile at terraces than at steps. Vibrational spectroscopic measurements suggested that hydrogenation occurs one molecule at a time to the fullest extent that resulted in desorption of product; that is, partially hydrogenated species do not exist on the surface.Item Interfacial Properties of Ultrathin- Film Metal Electrodes: Studies by Combined Electron Spectroscopy and Electrochemistry(2012-07-16) Cummins, KyleA pair of studies investigating the deposition and surface chemical properties of ultrathin metal films were pursued: (i) Pt-Co alloys on Mo(110); and (ii) Pd on Pt(111). Experimental measurement was based on a combination of electron spectroscopy (low energy ion scattering spectroscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and low energy electron diffraction) and electrochemistry (voltage efficiency, voltammetry, and coulometry). Mixed-metal preparation of Pt-Co films by thermal vapor deposition (TVD) resulted in a thin-film binary alloy. Careful analysis revealed a substantial divergence between the composition at the interface and that in the interior. This outcome was observed for all compositions and allowed for the construction of a ?surface phase diagram?. The proclivities of the alloys of pre-selected compositions towards enhanced catalysis of the oxygen-reduction reaction were assessed in terms of their voltage efficiencies, as manifested by the open-circuit potential (OCP) in O2-saturated dilute sulfuric acid electrolyte. The particular alloy surface, Pt3Co (XPt=3,XCo=1), whether from the thin film or a bulk single crystal, exhibited the highest OCP, a significant improvement over pure Pt but still appreciably lower than the thermodynamic limit. Under test conditions, the degradation of thusly-prepared films was primarily due to Co corrosion. Ultrathin Pd films on well-defined Pt(111) surfaces, with coverages from 0.5 to 8 monolayers (ML), were prepared by surface-limited redox replacement reaction (galvanic exchange) of underpotentially deposited Cu. Spectroscopic data revealed that films prepared in this manner are elementally pure, pseudomorphic to the substrate, and stable, independent of the surface coverage (?) of palladium. Analysis of the voltammetric profiles in the hydrogen evolution region revealed unique properties of hydrogen adsorption unseen in bulk electrodes. Notably, at 1 ML coverage, a step-free film was produced that did not exhibit hydrogen absorption. At higher coverages, digital (layer-by-layer) deposition gave way to 3D islands in a Stranski- Krastanov growth mode; under these conditions, onset of bulk-like behavior was observed. This method makes possible the synthesis of well-ordered noble-metal films in the absence of high-temperature treatmentItem Layer-by-layer Assembly of Nanobrick Wall Ultrathin Transparent Gas Barrier Films(2012-07-16) Priolo, Morgan AlexanderThin layers with high barrier to oxygen and other gases are a key component to many packaging applications, such as flexible electronics, food, and pharmaceuticals. Vapor deposited thin films provide significant gas barrier, but are prone to cracking when flexed, require special, non-ambient processing environments, and can involve complex fabrication when layered with polymers. The addition of clay into polymers can enhance barrier properties relative to the neat polymer; however, these composites are subject to clay aggregation at high loadings, which leads to increased opacity and random platelet alignment that ultimately reduce barrier improvement. Layer-by-layer (LbL) assembly is capable of producing thin films that exhibit super gas barrier properties, while remaining flexible and completely transparent. Montmorillonite (MMT) clay and branched polyethylenimine (PEI) were deposited via LbL assembly to create gas barrier films that can be tailored by altering the pH of the PEI deposition solution or the concentration of the MMT suspension. Films grow linearly as a function of layers deposited, where increasing PEI pH increases spacing between clay layers and increasing MMT concentration increases thin film clay content. An oxygen transmission rate (OTR) below the detection limit of commercial instrumentation (< 0.005 cm3/m2?day?atm) is observed after 70 layers of 0.2 wt % MMT or 24 layers of 2 wt % MMT are deposited with pH 10 PEI onto 179 ?m thick poly(ethylene terephthalate) (PET) film. Three-component films of PEI, poly(acrylic acid) (PAA), and MMT grow exponentially as a function of PEI/PAA/PEI/MMT quadlayers deposited. A transparent, ultrathin film of only four quadlayers deposited onto PET exhibits the lowest oxygen permeability ever reported for any thin film material, at only 51 nm thick. Finally, the first example of LbL assembly using large aspect ratio vermiculite (VMT) clay was performed. PEI/VMT films grow linearly as a function of layers deposited and exhibit 95 % light transmission with 97 wt % VMT. The barrier of these films is due to the highly aligned nanobrick wall structure that creates a tortuous path for permeating molecules. Coupling high flexibility, transparency, and barrier, these coatings are good candidates for a variety of packaging applications.Item Molecular Dynamic Simulation of Thermo-Mechanical Properties of Ultra-Thin Poly(methyl methacrylate) Films(2011-08-08) Silva Hernandez, Carlos Ardenis A.The thermal conductivity of PMMA films with thicknesses from 5 to 50 nanometers and layered over a treated silicon substrate is explored numerically by the application of the reverse non-equilibrium molecular dynamics (NEMD) technique and the development of a coarse-grained model for PMMA, which allows for the simulation time of hundreds of nanoseconds required for the study of large polymer systems. The results showed a constant average thermal conductivity of 0.135 W/m_K for films thickness ranging from 15 to 50 nm, while films under 15 nm in thickness showed a reduction of 30% in their conductivity. It was also observed that polymer samples with a degree of polymerization equal to 25% of the entanglement length had 50% less thermal conductivity than films made of longer chains. The temperature profiles through the films thickness were as predicted by the Fourier equation of heat transfer. The relative agreement between the thermal conductivity from experiments (0.212 W/m_K for bulk PMMA) and the results from this investigation shows that with the proper interpretation of results, the coarse-grained NEMD is a useful technique to study transport coefficients in systems at larger nano scales.Item Polycrystalline Silicon Solar Cells Fabricated by Pulsed Rapid Thermal Annealing of Amorphous Silicon(2014-05-07) Lee, I-SyuanThe PECVD intrinsic, n^(+), and p^(+) a-Si:H thin film deposition processes have been studied by the optical emission spectroscope to monitor the plasma phase chemistry. Process parameters, such as the plasma power, pressure, and gas flow rate, were correlated to SiH*, H_(a), and H_(?) optical intensities. The power, pressure, and hydrogen dilution effect on radical intensities and intensities ratio such as H_(a)/SiH* and H_(?)/H_(a) were investigated. For all films, the deposition rate increases with the rise of the SiH* intensity. For the doped films, the H_(?)/SiH* ratio is a critical factor affecting the resistivity. Changes of the free radicals intensities can be used to explain variation of film characteristics under different deposition conditions. The n^(+) and p^(+) layer were optimized by analyzing the optical emission spectra. The i-layer was optimized by assembling it with n^(+) and p^(+) to a PIN diode and measuring the current-voltage characterization under solar light illumination. H_(a)*/SiH* ratio is an important clue to study the effects of H content and Si-H binding mode on solar light conversion efficiency. The moderate hydrogenation can improve the film quality by passivating the dangling bonds, which acts like recombination centers in the film. However, too much H content leads to the increasing amount of SiH_(2) in the film, which causes the dangling bonds and deteriorates the film. The pattern size and the thickness of light-absorbed layer (i-layer) were also optimized. The novel nickel-induced crystallization with low thermal budget was demonstrated. Polycrystalline silicon thin films were formed from the amorphous silicon thin films by the pulsed rapid thermal annealing process enhanced with a thin nickel seed layer through the vertical crystallization mechanism. The crystalline volume fraction, grain size, and the material properties of polycrystalline i-, n^(+), and p^(+) silicon layers were reported. The influence of PRTA frequency and the dopant effect were also investigated. It has been demonstrated that a 30 nm thick amorphous silicon could be transformed into polycrystalline with 70%-80% of crystalline volume fraction in a short time. The thin-film polycrystalline silicon solar cells were produced using nickel-induced crystallization with amorphous silicon film and a thin Ni layer. The conversion efficiency was improved by transforming amorphous n^(+) to polycrystalline n^(+). The n^(+)/i- and i-/p^(+) interfaces trapped Ni atoms and deteriorated the diode property after crystallizing the films on the top of n^(+). The current-voltage characterizations in the dark, under red, green, blue, and solar light illumination were measured and investigated.Item Scanning tunneling microscopic studies of SiO2 thin film supported metal nano-clusters(Texas A&M University, 2005-11-01) Min, Byoung KounThis dissertation is focused on understanding heterogeneous metal catalysts supported on oxides using a model catalyst system of SiO2 thin film supported metal nano-clusters. The primary technique applied to this study is scanning tunneling microscopy (STM). The most important constituent of this model catalyst system is the SiO2 thin film, as it must be thin and homogeneous enough to apply electron or ion based surface science techniques as well as STM. Ultra-thin SiO2 films were successfully synthesized on a Mo(112) single crystal. The electronic and geometric structure of the SiO2 thin film was investigated by STM combined with LEED, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The relationship between defects on the SiO2 thin film and the nucleation and growth of metal nano-clusters was also investigated. By monitoring morphology changes during thermal annealing, it was found that the metal-support interaction is strongly dependent on the type of metal as well as on the defect density of the SiO2 thin film. Especially, it was found that oxygen vacancies and Si impurities play an important role in the formation of Pd-silicide. By substituting Ti atoms into the SiO2 thin film network, an atomically mixed TiO2-SiO2 thin film was synthesized. Furthermore, these Ti atoms play a role as heterogeneous defects, resulting in the creation of nucleation sites for Au nano-clusters. A marked increase in Au cluster density due to Ti defects was observed in STM. A TiO2-SiO2 thin film consisting of atomic Ti as well as TiOx islands was also synthesized by using higher amounts of Ti (17 %). More importantly, this oxide surface was found to have sinter resistant properties for Au nano-clusters, which are desirable in order to make highly active Au nano-clusters more stable under reaction conditions.Item Synthesis of NiCoMnX (X = In, Al) Heusler-type Magnetic Shape Memory Alloy Thin Films(2014-08-13) Rios, Steven EliMagnetic shape memory alloys are a class of shape memory alloys, and therefore exhibit a thermoelastic martensite phase transformation between symmetric and asymmetric crystalline states induced by appropriate temperature and/or stress changes. Shape memory alloys are able to recover strain when stress is applied, which can generate higher actuation forces and displacements compared to piezoelectrics and magnetostrictive materials when the material is constrained. While shape memory alloys have found applications in biomedical and aerospace industries, actuator applications are limited to relatively low frequencies compared to piezoelectric materials. The slow response of shape memory alloys is associated with heating or cooling the material from an external source. Compared to traditional shape memory alloys, the coupling of structural and magnetic ordering result in magnetic and structural transformations that increase the functional properties in magnetic shape memory alloys, such as magnetic field-induced rapid martensite transformation (forward and reversed), giant magnetoresistance, and the magnetocaloric effect. While bulk MSMAs can be used for structural components, in many cases MSMA thin films are preferred for device applications, such as miniaturized actuators, small scale propulsion devices, and micro-electro-mechanical systems (MEMS). This thesis focuses on the synthesis of NiCoMnX (X=In, Al) Heusler-type magnetic shape memory alloy thin films via physical vapor deposition, and details the challenges associated with controlling film composition, precipitation, microstructure, residual stress, and mechanical properties. As-deposited films were found to contain a mixture of amorphous and nanocrystalline microstructure, and thus, did not exhibit a martensitic transformation. Appropriate post-deposition heat treatments were required to crystallize the films, tailor the grain size, and reduce the formation of precipitates. Crystallized films exhibited martensitic transformations that showed a grain size dependence. An analytical model that uses a thermodynamic framework was developed to explain the suppression of the martensitic transformations for films with submicron-sized grains. Hence, in addition to chemical composition, sub-micron grain size can be used to tailor the martensitic transformation temperature of NiCoMnX (X=In, Al) thin films for device applications. Additionally, the analytical model may reduce the uncertainty associated with a direct scale-up of thin film compositions used for combinatorial investigations of magnetic shape memory alloys.Item Transparent and Conductive Carbon Nanotube Multilayer Thin Films Suitable as an Indium Tin Oxide Replacement(2012-07-16) Park, Yong TaeTransparent electrodes made from metal oxides suffer from poor flexibility and durability. Highly transparent and electrically conductive thin films based on carbon nanotubes (CNTs) were assembled as a potential indium tin oxide (ITO) replacement using layer-by-layer (LbL) assembly. The ultimate objective of this dissertation work is to produce CNT-based assemblies with sheet resistance below 100 Omega/sq and visible light transmission greater than 85 percent. The alternate deposition of positively charged poly(diallyldimethylammonium chloride) [PDDA] and CNTs stabilized with negatively charged deoxycholate (DOC) exhibit linear film growth and thin film properties can be precisely tuned. Ellipsometry, quartz crystal microbalance, and UV-vis were used to measure the growth of these films as a function of PDDA-CNT bilayers deposited, while TEM, SEM, and AFM were used to visualize the nanostructure of these films. Following a literature review describing potential ITO substitutes and LbL technology, the influence of CNT type on optoelectronic performance of LbL assemblies is described. Three different types of nanotubes were investigated: (1) multiwalled carbon nanotubes (MWNTs), (2) few-walled carbon nanotubes (FWNT), and (3) purified single-walled carbon nanotubes (SWNTs). SWNTs produced the most transparent (>85 percent visible light transmittance) and electrically conductive (148 S/cm, 1.62 kOmega/sq) 20-bilayer films with a 41.6 nm thickness, while MWNT-based films are much thicker and more opaque. A 20-bilayer PDDA/(MWNT DOC) film is approximately 103 nm thick, with a conductivity of 36 S/cm and a transmittance of 30 percent. In an effort to improve both transparency and electrical conductivity, heat and acid treatments were studied. Heating films to 300 degree C reduced sheet resistance to 701 Omega/sq (618 S/cm conductivity, 38.4 nm thickness), with no change in transparency, owing to the removal of insulating component in the film. Despite improving conductivity, heating is not compatible with most plastic substrates, so acid doping was investigated as an alternate means to enhance properties. Exposing SWNT-based assemblies to HNO3 vapor reduced sheet resistance of a 10 BL film to 227 Omega/sq. Replacing SWNTs with double walled carbon nanotubes (DWNTs) provided further reduction in sheet resistance due to the greater metallic of DWNT. A 5 BL DWNT film exhibited the lowest 104 Omega/sq sheet resistance (4200 S/cm conductivity, 22.9 nm thickness) with 84 percent transmittance after nitric acid treatment. DWNT-based assemblies maintained their low sheet resistance after repeated bending and also showed electrochemical stability relative to ITO. This work demonstrates the excellent optoelectronic performance, mechanical flexibility, and electrochemical stability of CNT-based assemblies, which are potentially useful as flexible transparent electrodes for a variety of flexible electronics.