Controlled fabrication of multitiered three-dimensional nanostructures in porous alumina

We present the fabrication of multitiered branched porous anodic alumina (PAA) substrates consisting of an array of pores branching into smaller pores in succeeding tiers. The tiered three-dimensional structure is realized by sequentially stepping down the anodization potential while etching of the barrier layer is performed after each step. We establish the key processing parameters that define the tiered porous structure through systematically designed experiments. The characterization of the branched PAA structures reveals that, owing to constriction, the ratio of interpore distance to the anodization potential is smaller than that for pristine films. This ratio varies from 1.8 to 1.3nmV-1 depending on the size of the preceding pores and the succeeding tier anodization potential. Contact angle measurements show that the multitiered branched PAA structures exhibit a marked increased in hydrophilicity over two-dimensional PAA films.