Preparation of ordered anodic porous alumina masks with large areas by a multistep anodization process using multiple electrolytes and their application to Ag nanodot array formation

Anodic porous alumina (APA) is a promising material for deposition masks to prepare metal nanodot arrays with controlled nanodot diameter and spacing on substrates. However, a method for fabricating large-area, thin, and fragile APA masks on a substrate has not yet been established. In this study, we investigated a novel fabrication process to realize large-area APA mask formation. Thin APA films with a thickness of 1 µm or less are extremely fragile, making them difficult to handle as freestanding membranes. However, APA films supported by a thick APA layer exhibited sufficient mechanical strength to be handled as freestanding membranes, even over large areas. After fixing a multilayer APA membrane composed of a mask and a support layer onto a substrate, a 5 cm × 5 cm APA mask was formed on the substrate surface by separating the mask layer from the support layer. By depositing Ag on the obtained sample and removing the APA mask, we obtained a 5 cm × 5 cm Ag nanodot array on the substrate surface. A drop of 4-4'-bipyridyl solution was applied onto Ag nanodots, and Raman spectroscopy measurements confirmed the enhancement of Raman peaks due to surface-enhanced Raman scattering (SERS). This method enables the fabrication of large-area SERS substrates, making the resulting Ag nanodot arrays promising for high-sensitivity Raman imaging.