Direct observation of alumina nanowire formation from porous anodic alumina membrane via the droplet etching method

We report the first direct observation of the formation process of an alumina nanowire (ANW) array from a porous anodic alumina (PAA) membrane employing a simple droplet etching method, not a general immersion etching method, using field emission scanning electron spectroscopy (FESEM). The PAA membrane is prepared by the two-step anodization method and subsequent lift-off process. The surface view SEM images of the PAA membrane show an array of highly ordered hexagonal pore distribution within the domains of 1-2 νm size, which are separated from neighbouring domains with different orientations of the pore lattice by grain boundaries. The FESEM analysis of the formation process of the alumina nanowire reveals that the formation process consists of three critical steps. The first step involves an image revealing that the outer parts of pore walls are dissolved. The image can be described by the different etching rates of oxide layers in the PAA membrane. Differences in the etching rate can be explained in terms of the different anion impurity concentration in the oxide layer of the PAA membrane. The second step produces an image of the PAA membrane with triangular triple points, surrounded by three adjacent pores. This image can be interpreted as the combined effects of the thickness difference of pore walls and the thermal annealing of the PAA membrane. The last step shows images where the triangular cross section ANWs transform into circular cross section ones as further etching occurs. This can result from the crystallization of the triple points during the annealing process.