Surface Plasmon Coupling in Dimers of Gold Nanoparticles: Experiment and Theory for Ideal (Spherical) and Nonideal (Faceted) Building Blocks

The surface plasmon (SP) coupling in dimers of spherical and faceted gold nanoparticles was investigated experimentally and computationally at the single-particle level. Individual ideal dimers of two spherical gold nanoparticles with a constant gap, filled by a self-assembled monolayer of 1,8-octanedithiol linker molecules, exhibit highly uniform dark-field (DF) scattering spectra. In contrast, single nonideal dimers of two faceted gold nanoparticles with the same constant gap exhibit a high degree of spectral nonuniformity. We attribute this significant spectral heterogeneity to the many possible gap morphologies, that is, the many possible configurations of the crystal facet orientations at the junction of the two faceted particles in nonideal dimers. This configurational complexity is reduced in so-called hybrid dimers, which comprise an ideal spherical particle and a nonideal faceted particle. Hybrid dimers were therefore investigated theoretically and experimentally for revealing the influence of crystal facet orientation in the gap. Computed scattering spectra of ideal dimers (two spheres) and three different configurations of hybrid dimers (sphere and icosahedron) with face, edge, or point contact, respectively, were obtained using the finite-difference time-domain method. Theory predicts a blue shift of the longitudinal bonding dipolar plasmon coupling mode for hybrid dimers with point and edge contacts, but a red shift for face contact due to the stronger SP coupling. Experimental DF scattering spectra of individual hybrid dimers were measured for comparison with the predictions from theory. All single-particle DF scattering spectra of hybrid dimers exhibit either a blue shift (two distinct classes) or a red shift relative to the corresponding ideal dimers, in agreement with the predictions from computer simulations.