Superatom Model for Ag-S Nanocluster with Delocalized Electrons

Several Ag-S nanoclusters where the cluster core comprises mixed metal (main component) and sulfur atoms show superatomic orbitals in the conduction band edge. However, there are no superatomic states, i.e., delocalized electrons, in the valence band, and the clusters in question can be labeled as "zerovalent". We show here an example of an Ag-S cluster which fulfills the superatom model and has delocalized electrons: The recently synthesized and characterized [Ag₆₂S₁₂(StBu)₃₂]²⁺ cluster has four delocalized valence electrons based on a simple counting rule, and we compare it to the zerovalent cluster [Ag₆₂S₁₃(StBu)₃₂]⁴⁺. Our electronic structure analysis confirms the existence of superatomic states in the valence and conduction bands, but the locations of these states do not agree completely with the conventional prediction based on the spherical Jellium model. [Ag₆₂S₁₂(StBu)₃₂]²⁺ displays the 1S² electronic shell closure at the Fermi energy instead of the 1S²1P² configuration as suggested by its electron count. This shift of energy levels and electron shell closing has been introduced by the core-shell structure of the cluster. Our optical absorption simulation can reproduce the features observed in the experiments, and we assign these features to the transitions involving superatomic states within the conduction band.