Optimizing Gold Nanoparticle Cluster Configurations (n ≤ 7) for Array Applications

Nanoparticle duster arrays (NCAs) are novel electromagnetic materials whose properties depend on the size and shape of the constituent nanopartide clusters. A rational design of NCAs with defined optical properties requires a thorough understanding of the geometry-dependent optical response of the building blocks. Herein, we systematically investigate the near- and far-field responses of clusters of closely packed 60 nm gold nanoparticles (n <= 7) as a function of size and cluster geometry through a combination of experimental spectroscopy and generalized Mie theory calculations. From all of the investigated cluster configurations, nanoparticle trimers with D-3h geometry and heptamers in D-6h geometry stand out due to their polarization-insensitive responses and high electric (E) field intensity enhancements, making them building blocks of choice in this size range. The near-field intensity maximum of the D-6h heptamer is red-shifted with regard to the D-3h trimer by 125 nm, which confirms the possibility of a rational tuning of the near-field response in NCAs through the choice of the constituent nanoparticle clusters. For the nanoparticle trimer we investigate the influence of the cluster geometry on the optical response in detail and map near- and far-field spectra associated with the transition of the cluster configuration from D-3h into D-infinity h.