A Scalable, Nanocrystal-Based Process to Narrow the Gap in Plasmonic Nano-Dimer Metasurfaces

Plasmonic metasurfaces composed of nano-dimer repeat units with ultra-narrow gaps have been of particular interest over the years due to the strong electric field enhancement that is possible in these gaps. However, these nano-gap metasurfaces have largely been fabricated using expensive, conventional cleanroom techniques such as electron-beam lithography and evaporation of bulk metals, which limit the achievable nanostructures. We report a facile, bi-layer nanoimprint lithography (NIL)-based fabrication process which takes advantage of the solution processability and surface conformability of metallic colloidal nanocrystals (NCs) to control the critical dimension, i.e. the gap, achieving gold nano-dimer metasurfaces of varying gaps from the same master template. The fabrication process involves a novel combination of wet-etching of the bi-layer imprint resists and NC deposition and ligand exchange to systemically control the gap. Using a master template with 50 nm gaps and tuning the wet etch and NC concentration/ligand exchange parameters, we achieve large-area (>100 mm²) metasurfaces with nano-gaps tailorable from 70 nm, larger than the the master template gaps, to as narrow as 20 nm, to the point of fusing the nanorods. The metasurfaces have a gap-size dependent spectral response in the near-infrared, with a narrower linewidth and an increased polarization dependence as the gap narrows, consistent with the higher electric field confinement in the gap. This fabrication process can be readily extended to a wider nano-dimer geometry and NC material library enabling nano-gap engineering with a single master template and the scalable manufacturing of large-area nanoimprinted metasurfaces for plasmonic sensing and flat optics applications.