Tunable Subnanometer Gaps in Self-Assembled Monolayer Gold Nanoparticle Superlattices Enabling Strong Plasmonic Field Confinement

Nanoparticle superlattices produced with controllableinterparticlegap distances down to the subnanometer range are of superior significancefor applications in electronic and plasmonic devices as well as inoptical metasurfaces. In this work, a method to fabricate large-area(& SIM;1 cm(2)) gold nanoparticle (GNP) superlattices witha typical size of single domains at several micrometers and high-densitynanogaps of tunable distances (from 2.3 to 0.1 nm) as well as variableconstituents (from organothiols to inorganic S2-) is demonstrated. Our approach is based on the combination of interfacialnanoparticle self-assembly, subphase exchange, and free-floating ligandexchange. Electrical transport measurements on our GNP superlatticesreveal variations in the nanogap conductance of more than 6 ordersof magnitude. Meanwhile, nanoscopic modifications in the surface potentiallandscape of active GNP devices have been observed following engineerednanogaps. In situ optical reflectance measurementsduring free-floating ligand exchange show a gradual enhancement ofplasmonic capacitive coupling with a diminishing average interparticlegap distance down to 0.1 nm, as continuously red-shifted localizedsurface plasmon resonances with increasing intensity have been observed.Optical metasurfaces consisting of such GNP superlattices exhibittunable effective refractive index over a broad wavelength range.Maximal real part of the effective refractive index, n (max), reaching 5.4 is obtained as a result of the extremefield confinement in the high-density subnanometer plasmonic gaps.