Abstract.This study presents an experimental approach for fabricating super-hydrophobic coatings based on a dual roughness structure composed of zinc oxide nanorod arrays coated with a sputtered zinc oxide nano-layer. The ZnO nanorod arrays were grown by mean of low-temperature electrochemical deposition technique (75°C) on FTO substrates with pre-sputtered ZnO seed layer of 30 nm thickness. The ZnO nanorods show a (002) orientation along the c-axis, and have a hexagonal structure, with an average length of 710 nm, and average width of 156 nm. On the other hand, the crystallite size of the top-coating sputtered ZnO layer is of 30 nm. The as-deposited ZnO nanorod arrays exhibited a hydrophobic behavior, with a surface water contact angle of 108°, whereas the dual-scale roughness ZnO nanorods coated with sputtered ZnO exhibited a superhydrophobic behavior, with a surface water contact angle of 157° and high water droplet adhesion. The photo-catalytic degradation of methylene blue in the presence of both the single roughness ZnO nanorod arrays, and dual roughness structured ZnO. The ZnO nanorod arrays showed good activity, with a degradation efficiency of 50% and a degradation constant of (k = 0.00225 min -1 ) after 5 hours of UV illumination. On the other hand, the ZnO dual roughness structure prepared by sputtered ZnO on top of the ZnO nanorod arrays showed minimal activity, with a degradation efficiency of 16% and a degradation constant of (k = 0.000586 min -1 ) after 5 hours of UV exposure. The double structured films exhibited high sensitivity to UV light, with a UV-induced switching behavior from super-hydrophobic to super-hydrophilic after only 30 minutes of UV exposure.Key Words: Super-hydrophobic, water contact angle, zinc oxide, nanorod arrays, rose petal effect, photo-catalysis. Highlights Hydrophobic ZnO nanorod arrays with high water-adhesion are achieved. Sputtered ZnO top-coating is used to reach super-hydrophobicity. Switchable wettability for ZnO single and dual roughness films under UV exposure. Strong pinning to the surface, similar to the rose-petal effect. The higher WCA is responsible for the decrease in the photo-catalysis activity.

This work elaborates on the high scattering which dielectric nanorods exhibit and how it can be exploited to control light propagation across material interfaces. A detailed overview of how dielectric nanorods interact with light through a combination of dipolar scattering and leaky modes is performed via outward power flux calculations. We establish and account for design parameters that best result in light magnification owing to resonant behavior of nanorods. Impact of material parameters on scattering and their dispersion have been calculated to establish that low loss dielectric oxides like ZnO when nanostructured show excellent antenna like resonances which can be used to control light coupling and propagation. Interfacial scattering calculations demonstrate the high forward directivity of nanorods for various dielectric interfaces. A systematic analysis for different configurations of single and periodic nanorods on air dielectric interface emphasizes the light coupling tendencies exhibited by nanorods to and from a dielectric. Spatial characteristics of the localized field enhancement of the nanorod array on an air dielectric interface show focusing attributes of the nanorod array. We give a detailed account to tailor and selectively increase light propagation across an interface with good spectral and spatial control.

Efficient light management in optoelectronic devices requires nanosystems where high optical qualities coincide with suitable device integration. The requirement of chemical and electrical passivation for integrating nanostrutures in e.g. thin film solar cells points towards the use of insulating and stable dielectric material, which however has to provide high scattering and near-fields as well. We investigate metal@dielectric core-shell nanoparticles and dielectric nanorods. Whereas core-shell nanoparticles can be simulated using Mie theory, nanorods of finite length are studied with the finite element method. We reveal that a metallic core within a thin dielectric shell can help to enhance scattering and near-field cross sections compared to a bare dielectric nanoparticle of the same radius. A dielectric nanorod has the benefit over a dielectric nanosphere in that it can generate much higher scattering cross sections and also give rise to a high near-field enhancement along its whole length. Electrical benefits of e.g. Ag@oxide nanoparticles in thin-film solar cells and ZnO nanorods in hybrid devices lie in reduction of recombination centers or close contact of the nanorod material with the surrounding organics, respectively. The optical benefit of dielectric shell material and elongated dielectric nanostructures is highlighted in this paper.

No abstract

Oxygen shows significant absorption lines in the millimeter wave spectrum. Resonators are widely used to achieve a strong absorption even with a short absorption paths length for concentration measurements. A sensor system based on a Fabry-Pérot resonator for oxygen measurements at ambient pressure is presented here. The Fabry-Pérot resonator consists of two metal mirrors with a diameter of 50 mm. For purpose of oxygen detection the resonator covers a frequency range between 55 GHz and 65 GHz with a resonant peak density between 1 GHz and 1.5 GHz, depending on the mirror distance, and a quality factor of approximately 7000. To achieve a compact sensor system the concept envisages two integrated transceiver circuits directly coupling to coaxial ports in the metal mirrors of the resonator. The integrated SiGe front-end addresses a frequency band from 50 GHz to 75 GHz. They are realized as heterodyne structures with integrated directional couplers, thus it is possible to measure scattering parameters. For first oxygen concentration measurements, the resonator sample was coupled to a commercially available Vector Network Analyzer. The cavity was filled with oxygen concentrations of 0% vol. and 20% vol. at ambient pressure and temperature resulting in a significant change of the quality factor for frequencies close to the oxygen absorption line at 60.6 GHz. The sensor does not contain hot components. This is an advantage compared to other oxygen sensors, like electrochemical or metal-oxide sensors