Optical spectra of silver clusters and nanoparticles of all sizes from the TDDFT+U method

Abstract

The localized surface-plasmon resonances (LSPRs) of coinage-metal clusters and nanoparticles provide the basis for a great number of applications, the conception and necessary optimization of which require precise theoretical description and understanding. However, for the size range from clusters of a few atoms through nanoparticles of a few nanometers, where quantum effects and atomistic structure play a significant role, none of the methods employed to date has been able to provide high-quality spectra for all sizes. The main problem is the description of the filled shells of d electrons which influence the optical response decisively. In the present work we show that the DFT+U method, employed with real-time time-dependent density-functional theory calculations (RT-TDDFT), provides spectra in good agreement with experiment for silver clusters ranging from 4 to 923 atoms, the latter representing a nanoparticle with a diameter of 3 nm. Both the electron-hole-type discrete spectra of the smallest clusters and the broad plasmon resonances of the larger sizes are obtained. All calculations have been carried out using the same value of the effective U parameter that has been found to provide good results in bulk silver. The agreement with experiment for all sizes shows that the U parameter is surprisingly transferable. Our results open the pathway for TDDFT calculations of many practically relevant systems including clusters coupled to bio-molecules or to other nano-objects.