Revealing Quantum Tunneling in a Nanocavity with Monolayer Graphene as a Stair

Exploring the quantum mechanical effects of the nanomaterials at the atomic length scale is vital to understanding their inherent functions, expediting the potential novel applications based on quantum plasmonics. Here, graphene is sandwiched as a stair layer in a Au nanoparticle-on-mirror (Au NPoM) structure. Layer-dependent scattering spectra and the theoretical model confirm that electron tunneling occurs in the Au NPoM/monolayer graphene. This is because monolayer graphene is similar to a staircase that reduces the barrier height of the electron tunneling, thereby facilitating the occurrence of electron tunneling. The layer-dependent SERS enhancement factor of graphene in the Au NPoM system agrees with the prediction by the quantum-corrected model. Thus, the plasmonic field limits have been probed in a single-atom-layer gap. This work provides important guidance for further understanding quantum mechanical effects, enabling the potential for novel applications based on quantum plasmonics.