Nano-patterning using ultra-thin alumina membranes

With the mass production of well-controlled and low-cost nanostructures on the horizon, considerable attention has been given to porous anodic alumina (PAA) templates to assist in the fabrication of both individual and ordered nanostructured objects – particles, rods, wires, and holes – with applications in electronics, data storage, bioengineering, and nanomedicine. The fabrication of free-standing PAA templates, several microns thick, as well as their applications, have been largely described in the literature. In recent years, research has focused on the synthesis of ultra-thin anodic alumina membranes (UTAMs), making them compatible with top-down fabrication and large-scale production. The ability to obtain these nanostructures on different surfaces, including glass, silicon wafers, or flexible substrates, extends their range of applications, enabling the integration of nanostructured materials on top of thin layers and allowing for the precise tuning of the physical and chemical properties of the materials. This review focuses on this new and promising nanopatterning approach to fabricate large areas of ordered nanostructures using UTAMs as patterning masks. We report the most recent advances in the synthesis of UTAMs, focusing on two different approaches: in-situ anodization of thin aluminum films on various substrates and deterministic transfer of UTAMs onto a desired substrate. In the first case, we collect information regarding substrates, buffer layers, growth of Al films, anodization, and the post-treatment of the UTAMs. In the second case, we focus the review on the synthesis of UTAMs and, especially, on the transfer process to the substrate. For both methods, we compare the results regarding the nanostructure’s self-organization and the control of size, shape, and spacing. Finally, we will review several applications in which the use of UTAMs plays a key role in the performance of nanostructured devices.