Research Background and Motivation

Abstract

Anodic porous alumina, which exhibits a characteristic nano-honeycomb structure, has received increasing attention both experimentally and theoretically . Due to the quasi-periodic arrangement of the nanopore channels, narrow distribution of pore sizes, and interpore distances, relative ease to control the porous scales and self-ordering qualities by anodization conditions, excellent thermal stability, and very low-cost anodic porous alumina has been extensively used as templates for fabrication of various nanostructured materials such as nanodots [22][23][24], nanowires [25][26][27][28][29], nanotubes [30][31][32], and many other types [33][34][35], especially to realize the collective functioning of arrays of nanoelements which may not be realized by individual nanoelements [5,36], for applications in high-density magnetic media [37][38][39][40][41][42] [95,96]. For neutral electrolytes with pH in the range of 5-7, such as boric acid solution, ammonium borate, or tartrate aqueous solution, only barrier-type anodic alumina films with a uniform thickness will be formed by anodization [1,97]. The configuration of anodic porous alumina is composed of closely packed arrays of nanopore channels perpendicular to the aluminum substrate with pore diameters on the order of several to hundreds of nanometers [58,[93][94][95]98]. When viewed from the top, the nanopores are usually arranged in a quasi-hexagonal porous pattern. The in-plane arrangement of pore channels generally exhibits local variations with ordered zones separated with disordered zone boundaries, which is much similar to the crystallographic grains and grain boundaries of the Al substrate. However, the perfect ordered zones are at most several micrometers big even for highly self-ordered patterns, while the grain