Structures and properties of [Ag(Ag2S)n]+ clusters with n = 1–9: a density functional theory study

Silver sulfide clusters with size less than 2 nm are of much interest in photoelectric devices, such as photoconductive devices, infrared detectors and superconductors. The electronic structures of [Ag(Ag2S)n]+ (n = 1–9) clusters are investigated using the combined method of genetic algorithm (GA) and density functional theory (DFT). Results reveal that S atoms prefer to be at the tip of the structure, and Ag atoms tend to form S-Ag-S unit between two S atoms. With the increase of cluster size, the open structure evolves into closed centre-hollow structure. According to the stability analysis, [Ag(Ag2S)6]+ is a magic number cluster due to its high HOMO-LUMO gap, big average energy and high second-order energy difference. Ag-S bonds, S-Ag-S and triangular Ag3S3 units play key roles in stabilizing [Ag(Ag2S)n]+ clusters. According to the electron affinity energy analysis, the clusters are easier to get electrons when n = 1, 3, 5 and 9. Based on the charge analysis of the studied clusters, the charges on Ag and S atoms are positive and negative, respectively, which indicates that the charge transfer from Ag to S occurs in cluster. Infrared and Raman spectra are dependent on the symmetries of the clusters. The molecular orbitals of [Ag(Ag2S)6]+ and [Ag(Ag2S)8]+ present superatomic properties. We hope that the clusters studied here could provide valuable data for future experimental and theoretical study of cationic silver sulfide clusters.