Understanding the Effect of Symmetry Breaking on Plasmon Coupling from TDDFT
Fahri Alkan, Christine M. Aikens
Abstract
We perform a time-dependent density functional theory (TDDFT) investigation for the optical properties of nanorod assemblies for different sizes (Ag10, Ag59, and Ag139), interparticle distances, and orientations with a focus on the effect of symmetry breaking via an angle on plasmon coupling. For the model systems, the angle (θ) between the particles is varied between 0 and 180°, where θ = 0° and θ = 180° correspond to symmetric side-by-side and end-to-end orientations of the nanorods, respectively. Our analysis reveals that for a sufficiently large interparticle distance (r > 0.7 nm), where the wave-function overlap between monomers is negligible, TDDFT results agree quite well with the predictions of the dipole–dipole interaction model for the intensity of the different modes of coupled plasmons. For smaller gap distances (0.4–0.5 nm), a charge-transfer plasmon (CTP) mode occurs for the symmetry broken case of the Ag10 dimer. For the assemblies of larger nanorods, however, the CTP mode is predicted to be less pronounced, especially for the cases where the deviation from the end-to-end geometry is larger than 30°. The orbital overlap and configuration–interaction analyses show that these results are related to the fact that the relative overlap strength between monomeric energy levels is significantly reduced for symmetry-broken orientations of larger nanorods.