Generation of guided pores in nanoporous anodic alumina by nanoimprinting

This study shows the fabrication process of monodomain nanoporous anodic alumina templates (MD-NAATs) by nanoimprinting with a pore lattice constant shorter than the master stamp period). Nanoimprinting has been improved since it was first used to fabricate high-aspect ratio MD-NAATs, and it is now one of the most cost-effective and efficient methods of producing MD-NAATs. The nanoimprint process makes it possible to produce perfect patterns of the master stamp on the annealed and electropolished Al substrates which, after direct anodization under suitable conditions, become MD-NAATs. To this end, crucial factors such as the stamping pressure and the anodization conditions first have to be optimized. In this way, perfectly ordered nanoporous anodic alumina templates can be mass fabricated and the durability of the master stamp increased. Additionally, NAATs can be fabricated with an interpore distance shorter than the master stamp period if the anodization process is performed under specific conditions (i.e. specific anodization voltage). A reduction in the pore lattice constant by a factor of 60% was achieved by nanoimprinting from a master stamp with a period of 500 nm. The self-ordering mechanism allows the guided growth of a new pore (i.e. guided pore) by three imprinted pores in a MD-NAAT with a triangular pore lattice. Here, we investigated the combination of the self-ordering mechanism with the nanoimprinting for reducing the pore lattice constant in MD-NAATs using a silicon nitride (Si₃N₄) master stamp with a period of 235 nm. Furthermore, these guided pores have different structural characteristics (i.e. thinner oxide barrier layer at the pore bottom tips), what can be a new advantage towards the fabrication of hybrid materials with applications in a wide range of optoelectronic applications as solar cells or photonic crystals.