Silver Sulfide Nanoclusters and the Superatom Model

The superatom model of electron-shell closings has been widely used to explain the stability of noble-metal nanoclusters of few nanometers, including thiolate-protected Au and Ag nanoclusters. The presence of core sulfur atoms in silver sulfide (Ag–S) nanoclusters renders them a class of clusters with distinctive properties as compared to typical noble-metal clusters. Here, it is natural to ask whether the superatom model is still applicable for the Ag–S nanoclusters with mixed metal and nonmetal core atoms. To address this question, we applied density functional simulations to analyze a series of Ag–S nanoclusters: Ag₁₄S­(SPh)₁₂(PPh₃)₈, Ag₁₄(SC₆H₃F₂)₁₂(PPh₃)₈, Ag₇₀S₁₆(SPh)₃₄(PhCO₂)₄(triphos)₄, and [Ag₁₂₃S₃₅(S<i>t</i>Bu)₅₀]³⁺. We observed that superatomic orbitals are still present in the conduction band of these Ag–S clusters where the cluster cores comprise mostly silver atoms. Our Bader charge analysis illustrates that thiolates play a significant role in withdrawing charge (electron density) from the core Ag atoms. The simulated optical absorption properties of the selected Ag–S clusters reflect the substantial band gaps associated with typical molecular orbitals on both sides. Apart from Ag₁₄S­(SPh)₁₂(PPh₃)₈, which has a central sulfur atom in the cluster core, superatomic orbitals of the Ag–S clusters can have contributions for individual transitions in the conduction band