The development of the ordered channel array in the anodic porous alumina was initiated by the textured pattern of the surface made by the molding process, and growth of an almost defect-free channel array can be achieved throughout the textured area. The long-range-ordered channel array with dimensions on the order of millimeters with a channel density of 1010 cm−2 was obtained, and the aspect ratio was over 150. The master for molding could be used many times, which makes it possible to overcome problems in the conventional nanolithographic technique, such as low through-put and high cost.

The controlling factor of self-ordering of anodic porous alumina was investigated by focusing on the current density during film growth. The homogeneity of cell size was improved with increasing formation voltage accompanied by the exponential increase in current density. The maximum anodizing voltage for proceeding uniform oxide growth while avoiding extremely high current accompanied by gas evolution was identical with the previously established self-ordering voltage. With the increase in formation voltage up to the self-ordering voltage, the ratio of pore diameter to cell diameter d pore /d cell lowered and converged to ϳ0.3 regardless of the electrolyte type. Moreover, domains of highly self-ordered pore arrays were found in the film formed during burning, where extremely high current was locally concentrated. This suggests that the condition inducing film growth under high current density, i.e., high electric field strength is the key controlling factor of self-ordering. Based on the above knowledge, a new self-ordered porous alumina with a 600 nm pore interval was fabricated in citric acid just under the critical voltage of burning.

No abstract

The generation of hole array architectures with, for example, square or triangular patterns (see Figure) is achieved by controlling the spatial definition of the indentation sites for hole development in anodic porous alumina. The technique has potential for the fabrication of two‐dimensional photonic crystals, or carbon nanotubes with triangular or square cross section.

The conditions for the fabrication of ideally ordered anodic porous alumina with a high aspect ratio were examined using pretextured Al in oxalic acid solution. The obtained anodic porous alumina has a defect-free array of straight parallel channels perpendicular to the surface. The channel interval could be controlled by changing the interval of the pretextured pattern and the applied voltage. However, the depth at which perfect ordering could be maintained depended on the anodizing conditions, that is, the hole array with a high aspect ratio could be obtained only under the appropriate anodizing voltage, which corresponded to that of the long-range ordering conditions in the oxalic acid solution. Under the most appropriate condition, ideally ordered channels with an aspect ratio of over 500 could be obtained. From these results, it was concluded that the long-range ordering conditions significantly influenced the growth of channels in anodic porous alumina even in/on the pretextured Al.