Stamping-Assisted Cavity Enhancement of Ag Nanoparticle Films for SERS-Based Sensing

Abstract

Surfaces composed of self-assembled metal nanoparticles, prepared by physical vapor deposition, can generate lithography-free, pristine and easily scalable substrates for surface-enhanced Raman scattering (SERS). These substrates exhibit remarkable SERS enhancement factor (EF) spanning several orders of magnitude. In this study, we present a facile method to achieve further amplification of these EF values through formation of a Fabry–Perot cavity, which effectively enhances the field around the nanoparticles through constructive interference. We have observed that this enhancement by a quarter-wave Fabry–Perot cavity is superior to a near-field enhancement by a plasmonically coupled metallic mirror within a few nm proximities of a metal nanoparticle layer. Using Ag nanoparticles on a 90 nm thick SiO₂ spacer with a metallic mirror, a SERS EF of 1.3 × 10¹⁰ with a detection limit down to 10^–13 M was observed. Furthermore, we have observed even a simpler far-field cavity supporting only specular reflections can yield a 5-fold gain compared to a standalone nanoparticle-on-glass SERS substrate. Based on this observation we have designed a low-cost, reusable mirror platform that can be dry stamped on an Ag nanoparticle coated glass coverslip, yielding a SERS EF of 5.3 × 10⁹ with a detection limit down to 10^–12 M. Such a stamped mirror platform serves a dual purpose. It acts as an external element to enhance the capabilities of any SERS substrate with a flat surface, and also as a long-term protective barrier against airborne contaminants that can degrade the Raman signal quality.