Large-Area, Narrow-Gap Plasmonic Nanodimer Metasurfaces Exploiting Colloidal Nanocrystals: Promising Platforms for Refractive Index Sensing

Plasmonic metasurfaces composed of nanodimer repeat units with ultranarrow gaps have strong fields confined to the subwavelength gaps, which can enable resonance wavelength tuning and polarization control. We report a scalable fabrication process that takes advantage of nanoimprint lithography and the solution processability and surface conformability of colloidal nanocrystal dispersions to realize large-area, geometrically engineered nanodimer metasurfaces from a single master template. Geometrical control of the nanodimer gap is achieved through a combination of controlled wet etching of bilayer imprint resists and solution-based nanocrystal deposition and ligand exchange. Using a master template with 50 nm gaps and tailoring the wet etch, nanocrystal concentration, and ligand exchange time, we achieve large-area (1 cm2) metasurfaces with nanogaps tailorable from 72 nm to as narrow as 21 nm, even to the point of fusing the nanorods. We characterize the gap-size-dependent spectral response in the near-infrared, which shows increased electric field confinement and polarization dependence when the gap narrows. This process is promising in its potential for scalable manufacturing and nanogap engineering, using a single master template, of nanodimer metasurfaces, which are of particular interest for applications in refractive index sensing.