Bonding Asymmetry and Adatoms in Low-Density Self-Assembled Monolayers of Dithiols on Au(111)

Abstract

The involvement of Au adatoms in the interfacial structure of monothiols on Au(111) is now established, but the same issue is virtually unexplored in the case of dithiols. To gain more insight into the latter, we have studied by scanning tunneling microscopy the self-assembly of two prototypic symmetric dithiols (1,6-hexanedithiol and biphenyl-4,4′-dimethanethiol) from dilute aqueous solutions and correlated their growth with the deconstruction of the Au(111) herringbone pattern known to produce adatoms. For both molecules, we determine an initial low-density monolayer where the molecules are lying down and paired by 0.45 A tall protrusions, assigned to Au adatoms. The other terminal group is imaged differently, revealing a strong asymmetry in the dithiol bonding. The formation of vacancy islands and, thus, the extraction of additional adatoms from terraces are detected only after substantial molecular rearrangement and loss of bonding asymmetry.