Tunable subnanometer gap plasmonics using 2D superlattices of ligand capped gold nanospheres

Plasmonic nanoparticles can couple to and confine light below the diffraction limit, resulting in significant enhancement of the fields, leading to the development of many exciting optical properties. To maximize these properties, classical theory predicts the local enhancement becomes infinite as the interparticle gap between the nanoparticles approaches zero, which is physically impossible, therefore breakdown of the field must occur. To investigate this limit, we created centimeter-scale area superlattices from a hexagonally close packed monolayer of gold nanospheres coated with tunable alkanethiol ligand shells using directed assembly. The tunable interparticle gap ranged from 2.8 nm to 0.45 nm. Experimental results reveal the optical response of the superlattices agrees well with classical models, and breakdown of this classical field in the gap due to nonlocal effects is minor. Using spectroscopic ellipsometry, we find the effective real part of the refractive index ranges from 1.0 at 555 nm up to 5.0 at 743 nm, sustaining values greater than 3.5 far out into the near infrared. Furthermore, we showed that changing the terminal group and conjugation of these ligands in the superlattice does little to the film’s optical properties.