Controllable Multi-branched Channels Inside Anodic Aluminum Oxide: Fabrication And Applications

Low-dimension nanoscale materials, such as nanotubes and nanowires, were widelyapplied in nano-devices because of their unique structures and corresponding physical andchemical properties. However, with the development of nanotechnology, nano-structuredmaterials of higher complexity are required for the exploitation of new nanoscale functionaldevices. Nano-template materials have significant potential in the fabrication of variousnano-structured materials as replicas. AAO membranes with branched channels arecompetitive candidates for such application due to their controllable complex structure,flexible fabrication method, and low cost. In this dissertation, dynamic voltage is applied togrow branched channels inside AAO membranes. Intensive discussions are carried out to findout the methods to control the complex morphologies of the nanostructure precisely. Practicalapplications of the complex structured AAO membranes obtained are approved in somenano-devices, such as photonic crystals (PhCs) and humidity sensors that investigated in thisdissertation.Former researchers found that, during steady growing process, both the thicknesses ofbarrier layers and the inter-pore distances are proportional to the anodizing voltage. Therefore,the morphologies of AAO membranes were attributed to the anodizing voltage, whereas theinfluence from the space charge that accumulated in the barrier layer was always ignored. Infact, the parameters that determine the morphologies of the AAO membranes are the inducedelectric field by the space charge and the electric field in the barrier layer. During steadygrowing process, these two parameters are proportional to the anodizing voltage and all oftheir influences on the anodizing process can be considered as a single constant. However,space charge will play an important role under dynamic growing process because theadjustment of space charge is nonsynchronous with the applied voltage. Under suchcircumstance, the relationship between the anodizing voltages and the resulting morphologiesbecomes complicated. In this dissertation, experiments are designed to demonstrate theinfluence of the space charge on the anodizing process, and the real equal-field strength modelis proposed for the growth of AAO membranes, especially in dynamic anodizing process.Meng G. et al. grew n branched channels inside AAO membranes by reducing theanodizing voltage by a factor of 1/ n . They claimed that, by this method, controlledfabrication of arbitrary branched channels can be achieved. It is expected that reducing theanodizing voltage by a factor of 1 n will result in n times as many channels appearing inorder to maintain the original total area of the template. However, this is a simplified growth model and the geometric arrangement of the branched channels grown from the stem channelsshould be taking into account in the growth of branched channels as well as the anodizingvoltage. Furthermore, disorder morphologies can be generated by the competitive growth ofthe branched channels. In this dissertation, we establish geometric distribution model andcompetitive growth model for the growth of branched channels inside AAO membranes.Specific numbers of branched channels were chosen to meet the requirement of the geometricarrangement and the starting thickness of the barrier layer for the branched channels wasajusted by chemical etching to reduce competitive growth. Finally, ordered controllablebranched channels can be grown inside AAO membranes.Various morphologies of channels inside AAO membranes can be grown by gradualreduction of the anodizing voltage. Moreover, flexible morphologies can be obtained byadjusting the waveform of anodizing voltage. However, the growth mechanism underdynamic voltage is complicated and little systemic report appears in the literature. Withoutfully understand the fabrication technique, controllable complex structures of AAO channelscan not be grown precisely by dynamic anodization. In this dissertation, the influence of spacecharge in the barrier layers is introduced to the growth model established by former researcher,and uniform layer-by-layer structured AAO membranes can be obtained according to thiscorrected growth model.Wang B. et al. fabricated PhCs by applying periodical signal of anodizing voltage in thegrowing process of AAO membranes to form a layer-by-layer structure. In their experiments,the position of band gap can be adjusted by the duration of chemical etching. Their researchattached much attention, but the original of the optical properties can not be well explained. Inthis dissertation, the conception of one dimensional PhCs in layer-by-layer structured AAOmembranes is proposed, thus, Bragg equation can be used to calculate the position of bandgap of the device. And then, the position of the band gap can be arbitrary set by adjusting theeffective diffraction rate and the layer distance. Moreover, samples which contain varioustypes of layers with different distances can generate various band gaps at arbitrary positionwith desirable width in the range of infrared and visible light wave length.Because of the huge surface area, AAO membranes are ideal materials for the fabricationof humidity sensors and they are widely used already. However, only single side of the AAOchannels are utilized to exchange vapors in the application of former researchers. As a result,the response time of the sensors is relative long and catalyst can not be filled in the channelsto improve their properties. In this dissertation, humidity sensors with through-hole structurewere fabricated by RF magnetron sputtering. Porous gold electrodes were deposited on the surface of the AAO membranes. Experimental results show that this type of humidity sensorshave good properties, such as high sensitivities, fast response time and wide sensitive range.At specific testing frequency, linear |Z|-RH curve was found. AAO membranes withlayer-by-layer structure were introduced to fabricate humidity sensors as well. The propertiesof the sensors with such structure can be greatly increased after inorganic salt filled in thechannels. We believe that this structure can also be utilized in the application of various kindsof gas sensors.