Gold Nanoparticle Plasmonic Superlattices as Surface-Enhanced Raman Spectroscopy Substrates.

Metal colloids are of great interest in the field of nanophotonics, mainly due to their morphology-dependent optical properties, but also because they are high-quality building blocks for complex plasmonic architectures. Close-packed colloidal supercrystals not only serve for investigating the rich plasmonic resonances arising in strongly coupled arrangements but also enable tailoring the optical response, on both the nano- and the macroscale. Bridging these vastly different length scales at reasonable fabrication costs has remained fundamentally challenging, but is essential for applications in sensing, photovoltaics or optoelectronics, among other fields. We present here a scalable approach to engineer plasmonic supercrystal arrays, based on the template-assisted assembly of gold nanospheres with topographically patterned polydimethylsiloxane molds. Regular square arrays of hexagonally packed supercrystals were achieved, reaching periodicities down to 400 nm and feature sizes around 200 nm, over areas up to 0.5 cm. These two-dimensional supercrystals exhibit well-defined collective plasmon modes that can be tuned from the visible through the near-infrared by simple variation of the lattice parameter. We present electromagnetic modeling of the physical origin of the underlying hybrid modes and demonstrate the application of superlattice arrays as surface-enhanced Raman scattering (SERS) spectroscopy substrates which can be tailored for a specific probe laser. We therefore investigated the influence of the lattice parameter, local degree of order, and cluster architecture to identify the optimal configuration for highly efficient SERS of a nonresonant Raman probe with 785 nm excitation.