Spectral Tuning of a Nanoparticle-on-Mirror System by Graphene Doping and Gap Control with Nitric Acid

Nanoparticle-on-mirror systems are a stable, robust,and reproduciblemethod of squeezing light into sub-nanometer volumes. Graphene isa particularly interesting material to use as a spacer in such systemsas it is the thinnest possible 2D material and can be doped both chemicallyand electrically to modulate the plasmonic modes. We investigate asimple nanoparticle-on-mirror system, consisting of a Au nanosphereon top of an Au mirror, separated by a monolayer of graphene. Withthis system, we demonstrate, with both experiments and numerical simulations,how the doping of the graphene and the control of the gap size canbe controlled to tune the plasmonic response of the coupled nanosphereusing nitric acid. The coupling of the Au nanosphere and Au thin filmreveals multipolar modes which can be tuned by adjusting the gap sizeor doping an intermediate graphene monolayer. At high doping levels,the interaction between the charge-transfer plasmon and gap plasmonleads to splitting of the plasmon energies. The study provides evidencefor the unification of theories proposed by previous works investigatingsimilar systems.