Templated self-assembly of nanoporous alumina : pore formation and ordering mechanisms, methodologies, and applications

Abstract

Porous anodic aluminum oxide (AAO), also known as porous alumina, is a self-ordered nanostructured material well-suited for use in electronic, magnetic, optical and biological applications due to its small pore size (4-200nm) and spacing (10-500nm). Under slightly acidic conditions, both oxidation and dissolution of aluminum leads to the formation of pores. AAO pores form a self-assembled honey-comb structure with short range order over certain ranges of anodic potential and pH. In this work, three key results related to porous AAO science and technology are presented. First, a new theory based on strain-induced instability has been developed from the analysis of results obtained from kinetic studies and stress measurements to explain the formation of AAO pores. Experiments show that excess vacancies of aluminum, created by the dissolution process, generate a large tensile stress and an associated strain energy, which destabilizes the initially flat A1/AAO interface and leads to pore formation. Other factors affecting stability of the Al/AAO interface and the self-assembly process are also presented.

(cont.) Second, templated self-assembly (TSA) of AAO pores, ordered over wafer-scale areas and with controlled spacing and symmetry, have been achieved by pre-patterning the substrate using interference lithography. TSA of AAO pores led to control of pore spacing and order symmetry in ranges not achievable without templating. Independent control of pore spacing and diameter were successfully demonstrated, allowing formation of novel 3-D nanostructures such as nanofunnels, fabricated using periodic variations in the anions and/or electrolyte pH. Using the TSA approach, AAO with ordered pores <35nm in diameter and aspect ratios >50:1 were fabricated on Si substrates. A 1-D array of ordered pores, either in or out of plane with the substrate, was fabricated by confining the growth of AAO pores using silicon oxide masks patterned by lithography techniques. Finally, AAO templates were used to fabricate ordered nanostructures including carbon nanotubes, magnetic nanotubes and antidots, and metallic nanowires and nanoparticles, all of which display properties very different from their bulk counterparts.

(cont.) These results, and other proposed methodologies, provide new techniques for controlled in-plane and out-of-plane growth and organization of nanotubes and nanowires on Si substrates.