Self-assembly of isolated plasmonic dimers with sub-5 nm gaps on a metallic mirror

Realizing plasmonic nanogaps with a refractive index (n = 1) environment in metallic nanoparticle (NP) structures is highly attractive for a wide range of applications. So far in self-assembly-based approaches, without surface functionalization of metallic NPs, achieving such extremely small nanogaps is challenging. Surface functionalization introduces changes in the refractive index at nanogaps, which in turn deteriorates the desired plasmonic properties. In addition, fabrication of low-density dimer NP designs with smaller nanogaps poses a big challenge. Here, we introduce a simple and straightforward self-assembly-based strategy for the fabrication of low-density, isolated dimer gold nanoparticles in a nano-particle-on-metallic-mirror (NPoM) platform. A minimum interparticle gap distance between NPs of ∼3 nm is achieved without surface functionalization. This is possible by utilizing the M13 bacteriophage as the spacer layer instead of SiO₂ in NPoM. Density functional theory calculations on Au atom adsorption on SiO₂ and M13 bacteriophage surface constituents trace the NP assembly on the latter to a comparatively weak interaction with the substrate. Our study offers an attractive route for fabricating low density plasmonic dimer structures featuring small nanogaps and will enrich structure specific/isolated studies benefitting a variety of optical, actuator, and sensing applications.