Understanding the Electronic Structure Properties of Bare Silver Clusters as Models for Plasmonic Excitation

We present a detailed study of the optical properties of tetrahedral silver clusters ranging from Ag-10 to Ag-220 using frequency domain (FD) and real-time (RT) time-dependent density functional theory. We compare the electronic structure and optical properties of the clusters calculated with different exchange-correlation functionals, different basis sets, and different DFT software packages. We also present an analysis of the orbital contributions to the density of states, which for the larger clusters can be decomposed into surface and bulk contributions. We find that the description of optical properties is nearly insensitive to the choice of exchange-correleation functional and results are consistent for FD and RT implementations. Optical properties are sensitive to basis set selection however, and it is critical that the basis set correctly describes d-orbitals. We show that FD-TDDFT provides insights into the collective excitation nature of a plasmonic nanoparticle allowing us to investigate the hot electron distribution produced immediately after plasmonic excitation. This analysis shows that the electron distribution is largely a flat function of electron energy in the range between zero and the photon energy for a plasmonic transition whereas it is strongly peaked close to zero for an interband transition.