First principle study on the structures and properties of Agm(Ag2S)6 (m = 3–12) clusters

Genetic algorithm (GA) combined density functional theory (DFT) method is used to obtain the structures of Agm(Ag2S)6 (m = 3–12) clusters. The size of the studied clusters is less than 2 nm. The computational results reveal that the six S atoms in Agm(Ag2S)6 (m = 3–12) clusters exist in S–Ag-S units, and the S–Ag-S units further form Ag3S3 or Ag4S4 units. The S atoms all bind with three or four Ag atoms to form μ3 or μ4-S. Other Ag atoms not in S–Ag-S units tend to form Ag3, Ag4, Ag5 or Ag6 cores to stabilize the structure. The evolution of the global minimum (GM) structures manifests that as the addition of Ag atom to Agm(Ag2S)6 cluster one by one, the number of Agn cores increases and the molecular framework unit changes from Ag3S3 to Ag4S4. When m ≤ 10, the silver cores gather at one side of the cluster and are wrapped by the cage molecular framework formed by Ag3S3 and Ag4S4 from one side. When m is bigger than 10, the molecular framework is not big enough to wrap the silver cores. Then, the silver cores are separated on the left and right sides of the molecular framework. The stability and ionization potentials of the clusters present odd–even oscillation, because the number of valence electrons is odd or even. Ag8(Ag2S)6 and Ag10(Ag2S)6 clusters are more stable than other clusters because of their high second-order difference energies. Ag7(Ag2S)6 and Ag11(Ag2S)6 clusters are easy to get electrons to form anionic clusters, resulting in big electron affinities. The sulfur atoms in the clusters obtain electrons to become negatively charged, whereas silver atoms loose valence electrons to become positively or negatively charged. The direction of electron transfer in the clusters is from silver to sulfur.