Structural and electronic properties ofAun(n=2–10)clusters and their interactions with single S atoms:Ab initiomolecular dynamics simulations

Abstract

The structure and electronic properties of ${\mathrm{Au}}_{n}$ and ${\mathrm{Au}}_{n}\mathrm{S}$ $(n=2--10)$ clusters has been demonstrated using an ab initio ultralsoft pseudopotential strategy. The geometry optimization of different isomeric structures was performed under the framework of the spin-polarized density functional theory using the generalized gradient approximation for the exchange correlation energy. The comparison of the ground state geometries between ${\mathrm{Au}}_{n}$ and ${\mathrm{Au}}_{n}\mathrm{S}$ clusters revealed that while free ${\mathrm{Au}}_{n}$ clusters favor planar configuration, the ${\mathrm{Au}}_{n}\mathrm{S}$ clusters adopt three-dimensional structures starting from $n=6$ onwards. Up to $n=5$, the ${\mathrm{Au}}_{n}\mathrm{S}$ clusters show similar structural conformations as ${\mathrm{Au}}_{n+1}$ clusters by replacing Au atoms with S. The relative stability of ${\mathrm{Au}}_{n}$ clusters calculated based on binding energies, atom attachment energy, and a second order difference in energy shows oscillations due to the odd-even electron pairing effect, which differs in the presence of the S atom. In contrast to this, the energy gap between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO) shows strong oscillations for both ${\mathrm{Au}}_{n}$ and ${\mathrm{Au}}_{n}\mathrm{S}$ clusters reflecting the contribution of the Au atomic orbitals to construct the HOMO and LUMO energy levels. Finally, the higher stability of ${\mathrm{Au}}_{6}$ and ${\mathrm{Au}}_{5}\mathrm{S}$ clusters in spite of having even and odd number of electrons, respectively, suggests the dominance of geometrical stability over the electronic shell closure effect.