Dissertations

Nanoporous sponge structures show fascinating optical properties related to a strong spatial localization of field modes and a resulting strong field enhancement. In this work, a novel efficient method for the generation of three-dimensional nanoporous sponge structures using time-resolved phase-field simulations is presented. The algorithm for creating the geometries and the underlying equations are discussed. Different sponge geometries are generated and compared with sponges that have been experimentally measured using FIB tomography. Meaningful parameters are defined for the comparison of the geometric properties of the random sponge structures. In addition, the optical properties of the simulated sponges are compared with the experimentally measured sponges. It is shown that a description using effective media does not provide a good agreement to the actual spectra. This shows that the optical properties are largely determined by the local structures. In contrast, the numerically obtained spectra of the phase-field sponge models accounting for the real-space structure show excellent agreement with the spectra of the experimentally measured sponges.

Searching for the relationship between the nanostructure and optical properties has always been exciting the researchers in the field of optics (linear optics as well as non-linear optics), energy harvesting (anti-reflective Si solar cells, perovskite solar cells, ..., etc.), and industry (anti-reflection coating on car windows, sunglasses, etc.). In this work, we present an approach for nanostructuring the silicon substrate to silicon photonic crystals. By precisely controlling the etching time and etching path after using nanoimprint lithography, ordered arrays of inverted Si nanopyramids and Si nanopillars with good homogeneity, uniform surface roughness, high reproducibility of pattern transfer, and a controllable aspect ratio are prepared. Experimental investigation of the optical properties indicates that the reflections of these Si nanostructures are mainly determined by the aspect ratio as well as the period of nanostructures. Furthermore, we have experimentally observed visible-light scattering (V-LS) patterns on inverted Si nanopyramids and Si nanopillars, and their corresponding patterns can be precisely controlled by the patterned nanostructures. The V-LS pattern, background, and "ghost peaks" on the angle-resolved scattering results are caused by constructive interference, destructive interference, and the interference situation between both. This controllable nanopatterning on crystalline Si substrates with precisely tunable optical properties shows great potential for applications in many fields, for example, optics, electronics, and energy.

A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased.

Tremendous efforts have been made to fabricate large-scale plasmonic nanostructures, which show wide applications in surface plasmon resonance (SPR) sensing, catalytic conversion, photothermal conversion, optoelectronics, photothermal therapy. However, unable to fabricate over 5 cm^2 plasmonic nanostructures with good controllability hinders their further applications. Here, super large-scale (153 cm^2) 3D AgSiO2 hybrid plasmonic nanostructures with adjustable and ultra-broadband light absorption are fabricated by a simple and controllable two-step approach. The metastable atomic layer deposition (MS-ALD) is combined with physical vapor deposition (PVD) to generate these structures in a self-assembly manner. The structures look like coral tentacles. These excellent properties are attributed to multiple forward scatterings and extinction effects produced by Ag@SiO2 nanostructures. Using 3D Ag@SiO2 plasmonic nanostructures as light absorber for bottom-heating-based evaporation, the water evaporation rate remarkably improves seven times under 1 Sun than that in dark condition. Our results pave the avenue for developing super large-scale Ag-based plasmonic nanostructure with potential applications in solar energy conversion.