Ultra-uniform high-quality plasmonic metasurfaces through electrostatic self-assembly of gold nanoparticles on chemically unmodified glass†

Abstract

We present a novel method for the fabrication of ultra-uniform metasurfaces through the direct electrostatic self-assembly of positively charged gold nanoparticles (AuNPs) on chemically unmodified glass. The method was successfully applied to two types of ultra-uniform AuNPs like nanospheres and nanocubes differing in shape, size, and cationic surfactant ligands, proving the versatility of the proposed methods. Unlike previous studies, we found that the AuNP clustering was due to improper drying of the metasurfaces after the deposition and not to instability of the colloids. Our fabrication methods resulted in metasurfaces of high densities and ultra-uniform arrangements with negligible clustering at both the microscale and macroscale, as confirmed by microscopic, spectroscopic, and nanophotonic analyses. Furthermore, thanks to far-field dipole couplings, the plasmon resonances of metasurfaces were significantly narrower (and blueshifted) compared to the corresponding colloid. Combined with the ultra-uniformity feature, these plasmon phenomena increased the quality factors (Q) of metasurfaces up to ∼15. The densities, uniformities, and Q-factors of our metasurfaces are among the highest reported until now for similar nanostructures realized through the electrostatic self-assembly technique. Our findings demonstrate new possibilities to achieve higher Q-factors through simple, scalable, and cost-effective electrostatic self-assembly processes, with practical implications in optical sensing and nanophotonics. Moreover, the ultra-uniformity achieved by our methods opens up new opportunities to study the far-field dipole couplings in random arrays of anisotropic AuNPs.