Self-organized formation of hexagonal nanopore arrays in anodic alumina

Conditions for a self-organized formation of ordered hexagonal structure in anodic alumina were investigated, using oxalic or sulphuric acid as an ele ctrolyte. Highly-ordered nanopore arrays with pore densities of 9 x 10(9)-6 .5 x 10(10)cm(-2) and high aspect ratios over 3000 were fabricated by a two -step anodization process. The array exhibits characteristics analogous to a two-dimensional polycrystalline structure of a few micrometres in size. T he interpore distance can be controlled by changing the electrolyte and/or the applied voltage. The formation mechanism of ordered arrays is consisten t with a previously proposed mechanical stress model, i.e., the repulsive f orces between neighbouring pores at the metal/oxide interface promote the f ormation of hexagonally ordered pores during the oxidation process.