Scalable Multilayered Plasmonic Nanoporous Films for Surface-Enhanced Raman Spectroscopy

The nanoporous film is a well-known plasmonic material that is widely used for improving the performance of surface-enhanced Raman spectroscopy (SERS) sensors because of its large surface area and versatile porous structure. Here, we propose a simple and cost-effective method to develop scalable nanoporous films, which enable wafer-scale fabrication of multilayered Ag-Au plasmonic nanoporous films. In contrast to conventional techniques such as dealloying, templating, and oblique angle deposition, we use a three-step fabrication process that involves thin film deposition, annealing, and etching. The structural and optical properties of the film can be tuned by changing the number of layers and the annealing temperature. We show the existence of longitudinal localized plasmon resonances in the film by polarized absorption measurements. As a potential application of the film, we demonstrate SERS with enhanced sensitivity using rhodamine 6G molecules. Compared to a single-layer nanoporous film, two orders of magnitude higher SERS enhancement is achieved for the multilayer nanoporous film. The porous nature of the film with a higher Ag nanoparticle density in the multilayer film increases the overlap between the local field and the immobilized molecules.