Self-Assembly Route Controls Nanoscale Structural Order and SERS Sensing Performance in Ag Film-over-Nanosphere Plasmonic Substrates

Abstract Colloidal crystals, ordered two- or three-dimensional arrays of colloidal nanospheres, exhibit intriguing optical properties arising from photonic interactions within their periodic nanoscale structures. Beyond their intrinsic photonic behavior, these assemblies serve as versatile templates for fabricating nanostructured plasmonic metal films whose optical response is governed by nanometric structural features, including the size of monocrystalline domains and the geometry of nanoscale interstices or gaps between neighboring metal features. With the goal of identifying the optimal fabrication route for large-area SERS sensing substrates, we compare two major self-assembly methods for producing large-area, uniform two-dimensional colloidal nanosphere lattices: convective self-assembly (CSA) and air–water interface self-assembly (ISA). Monolayers of 460 nm polystyrene spheres were prepared by both methods and characterized by scanning electron microscopy (SEM) as well as optical microscopy. Optical properties were investigated by optical (micro)spectroscopy and FDTD electromagnetic simulations. The CSA-derived films exhibited slightly denser sphere packing than those obtained via ISA, resulting in narrower and more intense photonic transmission bands in the visible range. Subsequent deposition of silver films produced plasmonic Ag film over nanosphere (AgFoN) substrates, whose optical response was further analyzed. The extraordinary optical transmission (EOT) associated with propagating surface plasmons was found to be stronger and spectrally sharper for CSA-based samples, whereas reflectance spectra dominated by localized surface plasmons showed only subtle differences, with a slightly narrower resonance of CSA-derived AgFoN. Correspondingly, the SERS activity was slightly enhanced for AgFoN substrates fabricated from CSA-prepared colloidal crystals. These findings underline the critical influence of the self-assembly method on the nanoscale structural order and plasmonic performance of colloidal-template-based nanostructures, providing fabrication guidelines for optimizing large-area AgFoN nanostructures as reproducible SERS sensing platforms.