Surface and bottom structures of grouped alumina nanopores

Fine alumina nanopores were grouped per 100 nm by using a new two-step anodization method of Al in a 0.3 M oxalic acid (40 V → 10 V), referred to as grouped nanopores formed in Al pretextures (GNAPs). GNAPs are formed by the combination of reducing the applied voltage and the preferential NP nucleation facilitated by hexagonal Al pretextures. Pore formation did not occur at the ridges of the honeycomb cells while NPs were grouped at the center of the hexagonal Al pretextures. The number of NPs per group is 14 - 22, and the size distribution of NPs is very broad (3-15 nm). Interestingly, most of the small-sized NPs were located near the ridges while the larger NPs were located at the center of the hexagonal Al pretextures, which is explained by focusing or defocusing models of the applied electric fields on Al pretextures. The anodizing-time-dependent bottom structures of the GNAPs were also investigated.