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Plasmonic coupling of gold nanoparticles to a gold surface creates intense plasmonic hot spots with large electromagnetic field-enhancements within the cavity formed by the two metallic surfaces. The localised field in such structures is extremely sensitive to morphological fluctuations and subtle changes in the dielectric properties of the cavity contents. Here, we present an optical method that pins down the properties of the gap contents with high sensitivity, termed normalising plasmon resonance (NPR) spectroscopy. We use this on a variety of ultrathin molecular spacers such as filled and empty cucurbiturils, and graphene. Clear differences in the spectral positions and intensities of plasmonic modes observed in the scattering spectrum resolve thickness differences of 0.1 nm, and refractive index changes from molecular filling. [ABSTRACT FROM AUTHOR]

Grapheneis used as the thinnest possible spacer between gold nanoparticlesand a gold substrate. This creates a robust, repeatable, and stablesubnanometer gap for massive plasmonic field enhancements. White lightspectroscopy of single 80 nm gold nanoparticles reveals plasmoniccoupling between the particle and its image within the gold substrate.While for a single graphene layer, spectral doublets from coupleddimer modes are observed shifted into the near-infrared, these disappearfor increasing numbers of layers. These doublets arise from charger-transfer-sensitivegap plasmons, allowing optical measurement to access out-of-planeconductivity in such layered systems. Gating the graphene can thusdirectly produce plasmon tuning. [ABSTRACT FROM AUTHOR]