Small morphology variations effects on plasmonic nanoparticle dimer hotspots

Plasmonic nanoparticle (NP) dimer structures, forming highly intense areas of field enhancement called hotspots, have been the focus of extensive investigations due to their phenomenal light manipulating abilities. However, the actual morphology of the NP hotspot is usually distinct from the ideal nanosphere dimer model. In this study, we demonstrate numerically that small morphology variations in the presence of nanobridge, nanocrevice, nanofacet or nanoroughness, can have a major impact on the plasmonic properties of the whole system. The resonance wavelength and magnitude of the near-field enhancement are found to acutely depend on the interparticle gap geometry. The hotspot may become degenerated or regenerated. We also observe that the hybridized modes excited under longitudinal polarizations, including the bonding dipole plasmon (BDP) and charge transfer plasmon (CTP) modes, can be assigned to the bonding longitudinal antenna plasmon (LAP) modes for all gap geometries. These results provide means to understand and justify the ongoing poor reproducibility of surface enhanced Raman scattering (SERS) substrates, stressing the importance of precision plasmonics.