Porous alumina templates with various shaped nanochannels

We report the growth characteristics of porous alumina templates (PATs) with various shaped nanopores, including straight, step-shaped, and Y-branched nanopores. Self-organization of a two-dimensional array of nanopores, which was formed by anodic oxidation of aluminum, was investigated quantitatively by using field emission scanning electron microscopy images and their fast Fourier transformation analysis. It was found that the ordered pore arrangement showed a domain structure analogous to a two-dimensional polycrystalline structure. The highly ordered array with an average domain size of about 5 μm² was attained after anodization for 36 h at 40 V in a 0.3 M oxalic acid solution. Various shaped nanochannels, such as step-shaped and Y-branched nanochannels, were fabricated by employing a pore-widening treatment in the middle of anodic oxidation and reducing the anodizing voltage by a factor of 1/√2, respectively. Ferroelectric PbTiO₃ nanowires were successfully fabricated by way of spin-coating, putting a sol-gel solution into pores of PAT, with removal of the PAT by using wet chemical etching. The template-directed method has high efficiency for fabricating building blocks of various shaped nanomaterials for device applications and for doing fundamental research on their shape-sensitive electrical, optical, and magnetic properties.