Browsing by Subject "Metal"

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    Metal structures for photonics and plasmonics
    (2013-07) Park, Jong Hyuk
    The goal of this thesis is to investigate metal structures for photonics and plasmonics and to provide theoretical and experimental bases for their practical applications. Engineered micro- and nanostructures of a metal can efficiently manipulate surface plasmon polaritons (SPPs) - coupled photon-electron waves propagating along a metal-dielectric interface. Since SPPs are able to contain both characteristics of light and charge, exploiting SPPs can lead to novel optical behaviors, for example, concentration of light below the optical diffraction limit, generating large electric-field enhancements in confined regions. This unique characteristic of SPPs has opened up new opportunities for photonic and plasmonic applications such as surface-enhanced spectroscopy, subwavelength waveguides, optical antennas, solar cells, and thermophotovoltaics. However, while many fabrication techniques have been developed and utilized to prepare metal structures, some applications would still benefit from improved methods because SPPs are extremely sensitive to inhomogeneities on a metallic surface arising from roughness, impurities and even grain boundaries of a metal. To minimize the surface inhomogeneities of the metal structures and thus to exploit SPPs effectively, we introduced novel fabrication methods. First, the template-stripping method was employed to obtain high-quality silver films for SPPs in the visible wavelengths. The template-stripped films showed very smooth surfaces, leading to the improved dielectric function with high electrical conductivity and low optical loss. The dielectric function of the template-stripped films was compared with that of conventional films. As a result, the relation between the surface roughness and dielectric function of metal films could be derived. As another approach to reduce the inhomogeneities on a metal surface, we prepared single-crystalline silver films via epitaxial growth. Under controlled deposition conditions, single-crystalline silver films exhibited ultrasmooth surfaces with a root mean square roughness of 0.2 nm. Moreover, we observed that the absence of the grain boundaries can lead to an increase in SPP propagation length as well as precise patterning for metal structures. Beyond noble metals, we then introduced an effective route to obtain smooth patterned structures of refractory metals, semiconductors, and oxides via template stripping. The smooth structures of such materials can be favorable for many applications including thermal emitters, metamaterials, solar absorbers, and photovoltaics. We demonstrated that a variety of desired materials deposited on a thin noble metal layer can be peeled from silicon templates. After removing the noble metal layer, the revealed surfaces had very small roughness. This approach could easily reproduce structures via reuse of templates, leading to a low-cost and high-throughput process in micro- and nanofabrication. Finally, we showed that thermal excitation of SPPs in patterned metallic structures can provide tailored thermal emission. Typically, SPPs on metal structures are generated by using an optical source and then re-radiated as light, of which the emission angle and wavelength are determined by the geometry of the metal structures. However, since thermal energy can be another excitation source to create SPPs, heating of properly designed metal structures can result in tailored thermal emission. We experimentally demonstrated that at high temperatures, tungsten films with bull's-eye patterns exhibit tailored thermal emission with a unidirectional and monochromatic beam. In addition, since the thermal stability of the structures could be enhanced by coating with a protective oxide layer on the metal surface, the bull's-eye structures can be utilized as a novel radiation source. Overall, we pursued efficient engineering of SPPs in metal structures and development of improved fabrication methods for the metal structures. We believe that these results will promote the practical application of SPPs for electronic, photonic and plasmonic devices.
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    Stability of reduced carbon in the mantle
    (2013-01) Hastings, Patrick Timothy Jr
    Reduced carbon in the mantle is commonly thought to be chiefly in diamond, but experiments suggest that at >250 km the mantle contains small amounts (0.1-1 %) of FeNi alloy. [1, 2, 3]. Thus, alloy may be a significant host of reduced C [4], but little is known about C solubility of FeNi alloy under mantle conditions. To determine the carbon solubility in FeNi alloy and melt, we conducted experiments in the system Fe-Ni-C with bulk compositions having 5 wt. % C and variable Fe/(Fe+Ni) at 3 to 7 GPa and 1000 - 1400°C. Experiments at 3 GPa and 1000-1250 °C were performed in an end loaded piston cylinder apparatus; those at 5 and 7 GPa and 1200-1400°C were performed in a 1000 ton Walker-style octahedral multianvil. At 3 GPa, Fe-rich melts contain up to 4.5 wt. % C, but Ni-rich (>25 mole% Ni) compositions remain subsolidus at 1250°C. The solubility of carbon in pure Fe and Ni metal are 2 wt. % and 1 wt. % respectively, but in the alloy passes through a minimum of 0.4 wt. % for Fe0.2Ni0.8. Assuming that these concentrations apply at higher temperatures and pressures (as will be tested by future experiments) allows a first estimate of the potential storage of C in FeNi alloy in the mantle. If the mantle at 250 km contains 0.1-0.2 wt.% Ni-rich (Fe0.4Ni0.6) alloy, increasing with depth to 1 wt.% Fe-rich (Fe0.88Ni0.12) alloy at 700 km [1,2], then maximum storage of C in alloy rises from 5 ppm in the deep upper mantle to 180 ppm in the shallow lower mantle. For mantle similar to the MORB source, with ~10-30 ppm C [5], alloy cannot store all C in the deep upper mantle but can in the lower mantle. For OIB sources with 33-500 ppm C [5], complete storage in alloy is less likely. Additional phases will be diamond in the upper mantle, as our experiments and previous work [6] indicate that carbide is not stable in equilibrium with Ni-rich alloy, and carbide melt in the lower mantle. [1] Frost et al (2004) Nature 428 409-412 [2] Frost and McCammon (2008) EPSL 36 389-420 [3] Rohrbach et al. (2011) J.Petrol 52 #717-731 [4] Dasgupta et al. (2009) GCA 73 6678-6691 [5] Dasgupta and Hirschmann (2010) EPSL 298 1-13 [6] Romig and Goldstein (1978) Metal Trans. Met. AIME 9a 1599-1609.
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    Synthetic Control and Characterization of NU-1000
    (2019-12) Webber, Tom
    The production and release of greenhouse gasses has become a major issue in today’s society. Methane is a powerful greenhouse gas and is the main component of natural gas. Natural gas is often collected and transported to be used as a fuel, but leaks result in release of some of that methane into the atmosphere. Work is underway to develop an efficient catalyst capable of selective oxidation of methane to methanol. Metal-organic frameworks have become popular candidates for catalysts and catalyst scaffolds. The Zr-based metal-organic framework NU-1000 is a robust, mesoporous material that can be used in a variety of applications, including catalysis, sensing, gas separation and storage, and scaffolds. It can be synthesized by combining a solution of hexazirconium nodes ([Zr6O16H16]8+) and organic acid modulator in N,N-dimethylformamide with a solution of linker (1,3,6,8-tetrakis(p-benzoic acid)pyrene) and aging at elevated temperature. The typical product is 1-3 μm crystals that are primarily composed of NU-1000 but that contain domains of an impurity phase called NU-901 that is a polymorph of NU-1000. The ideal NU-1000 synthesis will yield phase-pure particles and enable control over crystal size. The structural differences between NU-1000 and NU-901 lead to a hypothesis that changing the organic acid modulator from benzoic acid to a larger and more rigid carboxylic acid might lead to steric interactions between the modulator coordinating on the node and linkers bound to nodes, inhibiting growth of the more dense NU-901-like material and resulting in phase-pure NU-1000. Side-by-side reactions comparing the products of synthesis using benzoic acid or biphenyl-4-carboxylic acid as organic acid modulator produce structurally heterogeneous crystals and phase-pure NU-1000 crystals, respectively. NU-1000 particles synthesized in the range of 1-3 μm, while useful for many applications, are not large enough for single-crystal X-ray diffraction and are not small enough for nanomaterial applications like drug delivery. The synthesis of NU-1000 provides a variety of experimental handles that can be tuned to produce a wide range of particle sizes. For example, the rate of nucleation and growth is closely tied to the concentration of modulator. This is because NU-1000 is formed via a competitive reaction between modulator and linker molecules for the binding sites on the hexa-Zr nodes. By changing the concentration of the linker, modulator, and any additives, the nucleation and growth rates can be altered to produce the desired particle size. The choice of Zr precursor between ZrOCl2 • 8 H2O and ZrCl4 also plays a significant role in determining the resulting particle size. People acquire a wide range of data like crystal size and morphology, crystallographic information, and elemental quantification and distribution using techniques like transmission electron microscopy and energy-dispersive X-ray spectroscopy. The characterization of size, size distribution, crystallinity, and chemical composition are critical to studying the catalytic properties of product materials. However, due to the delicate nature of MOFs, gathering this data can be very challenging. MOFs commonly undergo radiation damage under a focused electron beam causing a loss of crystallinity. While various techniques can circumvent this damage like cryogenic transmission electron microscopy and low-dose electron microscopy, this dissertation focuses on analyzing the damage and ensuring the data collected remains reliable.