High-Efficiency SERS of 4-Mercaptobenzoic Acid and Biphenyl-4,4'-Dithiol via Nanoparticle-on-Mirror Plasmonic Nanocavities.

Surface-enhanced Raman scattering (SERS) technology has important applications in many fields, such as biomedicine, environmental monitoring, and food safety. Plasmonic nanocavities have the ability to superdiffract localized light and enhance light-matter interactions. As a key SERS active substrate, research on plasmonic nanocavities has made significant progress regarding the enhancement mechanism, the utilization of hotspots for the detection of specific molecular groups, and practical applications. However, challenges related to improving the enhancement factor of nanocavity SERS, enhancing the stability and reproducibility of hotspots, and enabling the detection of single-molecule layers remain. In this study, we adopt a bottom-up approach to construct a silver microplate-molecule-multi-sized silver nanosphere nanoparticle-on-mirror (NPoM) nanocavity and achieve the efficient stable enhancement of Raman scattering from 4-mercaptobenzoic acid and biphenyl-4,4'-dithiol molecules via the electromagnetic mechanism. By characterizing the fabricated nanocavity using dark-field scattering and micro-confocal Raman scattering, we observed that the Raman scattering intensity in the NPoM nanocavity was enhanced by a factor of 10 compared to that of individual silver nanospheres. Furthermore, we achieved the efficient stabilization of SERS by precisely tuning the size of the silver nanospheres to match their resonance frequency with the Raman shift of the target molecules. This approach offers a valuable reference for the detection of various single-molecule layers and demonstrates significant potential for applications in biosensing and chemical analysis.