Optical Properties of Metal Clusters and Cluster Arrangements

Abstract

Metal clusters are nanoparticles that have from two to thousands of metal atoms. The properties of metal clusters are extremely size-dependent, and adding or removing even one atom can make a difference. The optical response of clusters is influenced by their composition, shape, size, charge, and environment. This tunability makes metal clusters and cluster arrangements ideal candidates for several applications ranging from cancer imaging and treatment to photovoltaic devices. Especially clusters with plasmons, strong collective excitations of the valence electrons, are of interest. In this thesis, the plasmon resonance in metal clusters and cluster arrangements is investigated computationally. The density functional theory and the simple jellium model are employed to study the principles of the plasmon resonance from the electronic perspective. The evolution of the localized surface plasmon resonance is followed in clusters with 8–138 valence electrons. The coupling of plasmons of the individual clusters is observed for dimers and larger cluster assemblies. The emergence of charge transfer plasmons at low energies is observed for systems with conductive linking or sufficiently small inter-cluster separation. Several tools, such as transitions contribution maps and visualization of the induced density are used to analyze the features which make an absorption peak plasmonic, and to distinguish different types of plasmons. A new quantitative index is developed to study the charge transfer nature of excitations, helping in the identification of the charge transfer plasmons. The detailed analysis of the optical excitations in these model systems can help to interpret the absorption spectra of more complex, real cluster systems.