Anodic Aluminum Oxide Photonic Crystals: Fabrication And Applications

The unique properties of photonic crystals (PhCs) have inspired scientific interest for along time because of their potential applications in the fields of optical device, sensor, andsolar energy. In recent years, the one-dimensional (1D) anodic aluminum oxide (AAO) PhCswith layer-by-layer structure have attracted much attention due to their high controllability ofthe process, and convenience in fabricating periodic nanostructures over a large area, andgood optical performance. In this dissertation,1D AAO PhCs are fabricated by anodizingmetal aluminum, and the methods for adjustment of the position, deep, and width of thephotonic band gap (PBG) are investigated systematically. In addition, the obtained techniqueis tried to apply in coloring of aluminum and its alloy, and in the field of chemical sensing.In this dissertation, AAO PhCs are fabricated by the voltage-controlled anodizationmethod at low temperature (0~5°C) to obtain samples with highly ordered microstructure anduniform structural colors over a large area. Black metals are deposited into the bottom of theAAO channels by an electro-deposition technique, and the structural colors are visualized onthe aluminum substrate. Moreover, the position of the PBG is adjusted successfully byaltering the pore density and the layer spacing of the AAO films.In this dissertation, the effects of light path contrast and pore density contrast between apair of layers on the optical performance are investigated by simulative calculation. Theresults show that the large pore density contrast is the key to improve the optical performance.By analyzing the etching process of the electrolyte to the AAO channels, it is known that thepore density contrast increases along with the increase of etching duration. Increasing theelectrolyte concentration, the pore density contrast of the top layers is increased effectively,and the AAO PhCs with complete PBG and stronger reflectance peaks can be obtained by thecurrent-controlled anodization method. The goals of structural design and performanceoptimization are achieved.In the previous studies, the structural colors of AAO PhCs can not be directly seen onthe aluminum substrate due to the high reflectivity of the substrate. In this dissertation, thestructural colors are successfully visualized based on the current-controlled method, and itsintrinsic mechanism is discussed. By employing a compensation technique during anodization to compensate the shift of the PBG position caused by the inhomogeneousetching of the electrolyte, the complete PBG is formed. With the complete PBG and sufficientnumber of layers, the path of light with wavelength out of the PBG is much longer than thepath of light in the PBG. This part of light is weakened due to the scatter and absorption ofAAO film. Therefore, a relative black back ground can be formed and the structural colorscan be visualized. The operability of AAO PhCs is much improved and the application rangeis widened to a large extent.A modified voltage-controlled method is presented in this dissertation to solve thecontrollability problem in mass production of the current-controlled method. In this method, anew-type voltage wave pattern is used during anodization to simulate the growth process ofAAO film in the current-controlled method. AAO PhCs with complete PBG, high intensityreflectance peak, and structural colors visualized on the substrate are obtained by employingthis method. At the meantime, the AAO PhCs can be fabricated cheaply without therestriction of sample shape and area by this method, which solve the problem in massproduction.Coloring of aluminum and its alloy is the most direct and fundamental application of theAAO PhCs. Compared with the anodization electrolytic coloring, structural coloring based onthe AAO PhCs is not only a new coloring concept, but also has many advantages intechnology, such as low pollution, simple and stabile process, and wide color range. In thisdissertation, the commercial aluminum alloy components are successfully colored based onthe modified voltage-controlled method. The results demonstrate that this coloring techniquecan be directly transferred from high pure aluminum to its alloy, and therefore has much wideapplication prospects and considerable commercial benefits.The application of1D AAO PhCs in chemical sensing has distinct advantage due to theirhigh controllability in pore density and layer spacing. In this dissertation, the sensingresponses of AAO PhCs to water, ethanol,1-propanol,1-butanol are tested. The results showthat all the samples display good sensing properties. The position of PBG red-shifts linearlywith the increase of the refractive index of the analytes, while the intensity of reflectance andtransmission peaks exhibits nonlinear response. By a chemical etching post-treatment, thewavelength sensitivity can be effectively increased, which far exceeds the data reportedbefore. After the increase of sensitivity, the color changes between the analytes with relatively large difference in refractive index can be distinguished simply by the naked eyes, exhibitinggreat potential used as visualized chemical sensors at the near future.