Plasmonic Coupling in Single Silver Nanosphere Assemblies by Polarization-Dependent Dark-Field Scattering Spectroscopy

Abstract

In this paper, we present an experimental and theoretical study of the plasmonic properties of single Ag nanospheres and the plasmon interactions in assemblies of Ag nanosphere dimers and trimers. High-quality Ag nanospheres with small size distribution are synthesized by etching prefabricated Ag nanocubes. We perform a 360° polarization-resolved scattering study on silver nanosphere dimers and trimers, and correlate the scattering anisotropy with nanoparticle structure through correlated dark-field spectroscopy and scanning electron microscopy (SEM) characterization. The polarization-resolved dimer scattering shows a dipolar pattern aligned with the long axis of the dimer. For single Ag nanosphere trimers assembled in an equilateral triangle geometry, we also observe the dipolar scattering pattern to a certain degree, although the dipolar pattern is not preferentially aligned with any sides of the triangle. Theoretical studies using the T-matrix method reveal that if the Ag nanospheres are perfectly spherical and are assembled in a trimer with D3h symmetry, the scattering spectra should be polarization independent, in contrast to the observed experimental results. The same phenomena are demonstrated in Ag nanopshere assemblies in D4h, D5h, and D6h symmetry as well. Using the discrete dipole approximation method, we find that slight elongation (5%) in one of the three axes of the Ag nanospheres can induce a significant anisotropy in the scattering pattern. We here have shown that even small variations in the nanoparticle geometry that are difficult to resolve with SEM can lead to significant effects in the plasmonic coupling, therefore affecting the scattering spectra of the assembled nanostructures.