Scientists from the Institute of Atomic and Molecular Sciences, Academia Sinica, Taiwan, and the Department of Physics at National Taiwan University have developed a technique to grow a single isolated anodic alumina nanochannel (SIAAN). The team first covers a piece of finely polished aluminium with a very thin alumina-protecting layer. Next the group uses a focused ion beam to open a nanoscale aperture on the surface, which facilitates the growth of a SIAAN in the subsequent anodization process.

Dendritic branching

The SIAAN, highlighted by the red rectangle in the upper panel of the image, starts to branch randomly to form a highly ramified dendritic structure. The growth is a highly non-equilibrium process that could be understood in terms of a diffusion-limited aggregation model. After prolonged anodization, the dendritic branches grow into an assembly of self-aligned nanochannels in a radial direction. Their uniform growth rate results in a smooth hemispherical growth-front consisting of close-packed pores (see image, bottom panel). The morphology is surprisingly similar to that of spherical coral but one million times smaller in scale, suggesting the existence of a similar rule for growth and branching.

The study provides the first overview of the morphological evolution from single to multiple AAO nanochannels and reveals their intrinsic trait to branch. The unique self-alignment of nanochannel arrays on an AAO film is a result of harsh competition that completely suppresses the trait. The basic understanding of the intrinsic growth behaviour of AAO nanochannels is expected to help improve our capability to control the geometry and quality of AAO films that are widely used as a technology platform for creating nanomaterials with novel electronic, magnetic and chemical properties.

The researchers presented their results in the journal Nanotechnology.