Analysis of the plasmonic excitations in assemblies of three-dimensional electron clusters

In the quest to built novel metamaterials with unique optical properties, three-dimensional assemblies of metal clusters and nanoparticles are gathering significant attention. Organized geometries, such as tetrahedra and icosahedra, can be built, for example, by using DNA strands or virus capsids as templates. Here we use the jellium model and time-dependent density functional theory to study the plasmonic resonances in different arrangements of eight-electron clusters from the electronic perspective. A charge transfer ratio index based on the induced transition densities is used to quantify the charge transfer nature of the excitations at different energies. We vary the size, shape, and intercluster separation, building systems of four-cluster tetrahedra, 12-cluster icosahedra and cuboctahedra, and 20-cluster dodecahedra. All the studied systems are found to have charge transfer plasmon-type excitations at low energies. Analysis of the electron-hole transitions contributing to the transition dipole moment is further used to characterize these excitations, showing that they have significant screening contributions unlike the higher-energy excitations. The understanding gained for the optical response of these simple model systems can help in interpreting the properties of real, complex cluster systems.