Particle Size-Dependent Onset of the Quantum Regime in Ideal Dimers of Gold Nanospheres

We report on the nanoparticle-size-dependent onset of quantum tunneling of electrons across the sub-nanometer gaps in three different sizes (30, 50, and 80 nm) of highly uniform gold nanosphere dimers. For precision plasmonics, the gap distance is systematically controlled at the level of single C-C bonds via a series of alkanedithiol linkers (C2-C16). The corresponding single-particle scattering spectra reveal that for the larger dimers the onset of quantum effects occurs at larger gap distances: C6 for 80 nm, C5 for 50 nm, and C4 for 30 nm dimers. 2D non-local and quantum-corrected model (QCM) calculations reveal the physical origin for this experimental observation: the lower curvature of the larger particles leads to a higher tunneling current due to a larger effective conductivity volume in the gap. Our results have possible implications in scenarios where precise geometrical control over plasmonics properties is crucial such as in hybrid (molecule-metal) and/or quantum plasmonic devices.