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Fabrication of Functional Nano-Structured Materials and Devices Using Supercritical Fluids

Nano-structured materials possess unconventional properties and enable miniaturization of devices. However, fabrications of such materials and devices are challenging and in many cases cumbersome, and development of nanofabrication techniques are essential to realizing novel designs and commercializing scientific ideas. Supercritical fluids possess a unique combination of gas-like diffusion properties and liquid-like dissolution power, and are suitable reaction media for fabricating materials at the nanometer scale. This dissertation focuses on developing supercritical fluids-related techniques for fast and large-scale fabrication of novel composite materials and devices. Using supercritical fluid deposition (SFD) technique, cobalt thin films were deposited on a variety of substrates, with a cobalt(II) metal-organic precursor dissolved in supercritical CO2. The deposition was found to be more facile compared to similar cobalt(III) precursors, and yielded full coverage, high purity cobalt thin films with good conductivity. Cobalt coatings on copper surfaces also effectively prevented copper oxidation in air at elevated temperatures. SFD technique was further utilized for scalable fabrication of gold-coated woodpile structures as wavelength-selective thermal emitters in the mid-infrared region. High purity, full coverage gold thin films were coated conformally on 3D TiO2 woodpile structures with perfect step-coverage and controlled surface roughness, and thermal emitters with emissivity enhancement at specific wavelength were obtained. Samples gold-deposited using cold-wall reactor were found to be more effective than using hot-wall reactor. Soft-imprint-based scalable fabrication of TiO2 woodpile structures was also optimized for micrometer patterns. Besides using supercritical CO2 for metal deposition, supercritical ethanol was utilized for continuous solvothermal synthesis of manganese oxide nanoparticles with small particle size as active material for supercapacitor electrodes. A mixture of MnO and Mn3O4 crystal structures was obtained and the ligand-capped nanoparticles form stable dispersion in ethanol at high concentrations. The stably dispersed of nanoparticles can be impregnaed into mesoporous carbon films, enabling high through-put fabrication of electrodes for flexible supercapacitors.