Agnieszka Brzózka

Chemical and Structural Modifications of Nanoporous Alumina and Its Optical Properties

A growing scientific interest in the fabrication of porous anodic aluminum oxide (AAO) films and their further applications for the fabrication of various devices, has given rise to many studies of porous alumina properties. Highly ordered porous alumina exhibits some unique physical and optical characteristics, especially in the visible spectrum. These properties are of technological importance for applications in the fields of micro and nanotechnology.

AAO templates with different patterns and channel shapes

Nanoporous anodic aluminum oxide (AAO) membranes formed by anodic oxidation of metallic Al have become one of the most popular templates for the fabrication of diverse range of nanostructured materials in the form of nanodots, nanowires, and nanotubes. A key advantage of using AAO instead of other templates is a highly ordered and close-packed distribution of pores within the template. In addition, the fabrication of porous AAO membrane via electrochemical anodization is relatively simple and inexpensive. All structural features of anodic oxide templates, such as pore diameter, pore-to-pore distance, thickness of the template, pore density, porosity, etc., can be easily controlled by anodizing conditions, especially type of electrolyte, potential applied, temperature and duration of the process. For the preparation of porous AAO templates, two different approaches can be distinguished: a pre-patterned guided anodization and a self-organized two-step anodization. In the first case, a pre-patterned (pre-textured) aluminum substrate is used for anodization, while the latter is based on a self-organized pre-patterning of Al during the first anodizing step and the removal of irregular oxide layer before the final (second) anodization. The pre-patterned guided anodization results in a perfectly arranged pore lattice formed over a surface area defined by the dimensions of the previously prepared stamp (mold) or indented Al surface. The pre-pattern guided anodization can result in pore shapes and cell configurations different from the pattern with circular and hexagonally arranged pores, which is typically observed for self-organized AAO templates. In particular, complex nontypical regular pore patterns on AAOs (e.g., square, triangle, diamond, triangle–diamond, checkerboard, etc.) can be formed. The self-organization process of AAO formation can occur over a macroscopic surface area, however it always results in a non-perfectly ordered hexagonal arrangement of pores. A typical self-organized anodization results in porous AAO layers with straight cylindrical nanochannels parallel to each other. However, a great variety of procedure modifications was recently proposed. They allow for the fabrication of anodic alumina films with a more complex internal architecture (e.g., Y-branched, multilevel Y-branched, and multi-branched nanochannels) and various nanochannel shapes (e.g., conical, step-shaped, serrated, periodically branched, and modulated nanochannels).