Study on fabrications of hierarchical nanostructures and its application

Abstract

Among the methods of imposing the functionalities on the surface of the material, bio-inspired technology is a technique to use engineering method such as simulating and applying the surface structure, principle and mechanism of the living organisms present in nature. This thesis describe how to make a layered nanostructure inspired by nature in silicon, glass, and aluminum, and its application. The first chapter describe how to fabricate layered nanostructures on silicon substrates by plasma etching. A self-aligned surface mask can help reduced the etching process and add a flake structure to the boiling water treatment to create a hierarchical structure. Specifically, we have developed a novel method to fabricate ultra-low reflective Si surfaces having nano-scale hierarchical structures by combination of AlOOH, or boehmite, nanoflake having 100-200 nm in width nested on plasma-etched Si nanostructures with 100 nm in diameter and from 500 to 1500 nm in height. By using CF4 plasma etching, Si surfaces were nanostructured with pillar-like structures by selective etching mechanism with self-masking by fluorocarbon residues. AlOOH nanoflakes were formed by Al thin film coating with various thicknesses on Si surfaces with nanopillars and subsequent boiling water immersion, which control its configuration from needle to flake by formation. Hierarchical structures coated with a low-surface-energy material have higher water wetting angle over 150o while contact angle hysteresis shows very low, implying the self-cleaning surface. Reflectance was significantly reduced down to 0.5% on hierarchical nanostructures through synergetic effect by Si nanopillar structures for lowering reflection for shorter than 600 nm in wavelength and AlOOH nanoflakes for longer than 600 nm. This structure will help enhance solar cell performance. In the second chapter, we describe the production of transparent functional glass that can be applied to the exterior glass of a car, a ship, or a building or to an electronic device. The superhydrophobic/hydrophilic surface is mainly accompanied with low reflection function and antifogging effect can be expected. However, it is very difficult to making roughness surface by plasma etching method because it contains alkaline ion which is strong in reactivity. To overcome this difficulty, we report a novel method to fabricate nanostructures on a glass by a non-lithographic method that introduces a sacrificial SiO2 layer on glass for anisotropic plasma etching. The nanopillars were first formed on a SiO2 layer coated glass through preferential etching by CF4 plasma. With continuous plasma etching, SiO2 pillars become local etching-resistant masks on glass, so that the glass regions covered with SiO2 pillars are etched slowly, while the regions with no SiO2 pillars etched rapidly, resulting in nanopatterns on glass. The glass surface, just etched with CF4 plasma, becomes superhydrophilic due to its high surface energy as well as nanoscale roughness with high aspect ratio. With a subsequent hydrophobic coating on nanostructured glasses, a superhydrophobic surface was also achieved. A water contact angle was measured over 160° and a contact angle hysteresis of < 5o. Furthermore, strong anti-dewing effect was confirmed by water condensation experiments on superhydrophilic glass under condition that the temperature raised from -15 to 25 oC in water vapor condition. It was measured that the transmission of the light on glasses was relatively not affected by the nanostructures, while the reflectance was significantly reduced with the increase the roughness of the nanopatterns on glass. In the third chapter, we describe a study of the capture of water by applying nanostructures to Kirigami which is a method of expanding formability by applying a simple cut to the material. When a deformation is applied to the flat Kirigami structure, a tilting angle is generated and the path of the water is forced, which can be efficient in water collection. By controlling the wettability by surface roughness, it is possible to maximize the amount of water harvest. When water is collected in the superhydrophilic region and then the water is easily roll off on the superhydrophobic region, the amount of water uptake increases by about three times that of the untreated surface. Surface roughness was achieved by boiling water treatment using aluminum substrate and TiO2 nanoparticle was mixed to maximize the roughness to ensure superhydrophobicity.