Atomistic Characterization of Plasmonic Dimers in the Quantum Size Regime

Plasmonic dimer systems show great promise in a wide range of applications because of their unique optical and electronic properties that arise from the coupling of monomer plasmons. To determine the origin of each plasmonic mode and understand the plasmon coupling, atomistic quantum mechanical simulations are required. Here, we adopt a Hirshfeld partitioning scheme of atomic charges and polarizabilities within the time-dependent density functional theory framework to study the plasmonic properties of plasmonic dimers. We are able to separate the charge-transfer plasmons because of electron tunneling from local-resonance plasmons by the partitioned polarizabilities and induced charges. We find that the strength of charge-transfer plasmons is limited by the charge-flow pathways and dependent on the chemical species. New plasmonic modes for a series of tetrahedral dimers are identified by mapping the induced charges. This approach allows for intuitive and consistent characterizations of strongly coupled plasmonic systems.