The "staple" motif: A key to stability of thiolate-protected gold nanoclusters

Recently obtained single-crystal structure of a thiolate-protected gold cluster shows that all thiolate groups form "staple" motifs on the cluster surface. To find out the driving force for such a formation, we use first-principles density functional theory simulations to model formation of "staple" motifs on an Au₃₈ cluster from zero to full coverage. By geometry optimization, molecular dynamics, and simulated annealing, we show that formation of "staples" is strongly preferred on a cluster surface and helps stabilize the cluster by pinning the surface Au atoms and increasing the HOMO-LUMO gap. We devise a method to generate initial structural models for thiolate-protected gold clusters by adding "staples" to the cluster surface. Using this method, we obtain a staple-covered, low-energy structure for Au₃₈(SCH₃)₂₄, a much studied cluster whose structure is not yet known. Optical band-edge energy computed from time-dependent DFT for our Au₃₈(SCH₃)₂₄ structure shows good agreement with experiment.