Probing the In-Plane Near-Field Enhancement Limit in a Plasmonic Particle-on-Film Nanocavity with Surface-Enhanced Raman Spectroscopy of Graphene

When the geometric features of plasmonic nanostructures approach the subnanometric regime, nonlocal screening and charge spill-out of metallic electrons will strongly modify the optical responses of the structures. While quantum tunneling resulting from charge spill-out has been widely discussed in the lit-erature, the near-field enhancement saturation caused by the nonlocal screening effect still lacks a direct experimental verification. In this work, we use surface-enhanced Raman spectroscopy (SERS) of gra-phene to probe the in-plane near-field enhancement limit in gold nanosphere-on-film nanocavities where different layers of graphene are sandwiched between a gold nanosphere and a gold film. Together with advanced transmission electron microscopy (TEM) cross-sectional imaging and nonlocal hydrodynamic theoretical calculations, the cavity gap width correlated SERS and dark-field scattering measurements reveal that the intrinsic nonlocal dielectric response of gold limits the near-field enhancement facto...