Structural evolution of (Au2S)n (n = 1–8) clusters from first principles global optimization

Abstract

We explore the structural evolution of (Au₂S)_n (n = 1–8) clusters using a first principles global minimization technique, namely, a genetic algorithm from density functional theory geometry optimization (GA-DFT). The growth sequence and pattern for n from 1 to 8 are analyzed from the perspective of geometric shell formation. The average binding energy, HOMO–LUMO energy gaps, vertical electron affinity, and vertical ionization potential are examined as a function of the cluster size. The global minimum structures are planar at n = 1–3, three-dimensional at n = 4–8. The formation of these structures are attributed to the high stability of S–Au–S structural unit and particularly the Au₃S₃ and Au₄S₄ rings. Chemical bonding analysis reveals that the three-dimensional clusters (n = 4–8) can be viewed as [Au_2n−xS_n]^x−·xAu⁺ in electronic structure. The Au⁺ cations are not involved in any S–Au covalent bond, however, are attracted by only Au⋯Au aurophilic interactions. Direct evidence for the Au⋯Au aurophilicity are given by a noncovalent interaction index analysis. Such Au⋯Au aurophilic interactions play an important role in the stability of (Au₂S)_n clusters.