The Role of Oligomeric Gold-Thiolate Units in Single-Molecule Junctions of Thiol-Anchored Molecules

Using the break-junction technique, we show that "Au(RS)(2)" units play a significant role in thiol-terminated molecular junctions formed on gold. We have studied a range of thiol-terminated compounds, with the sulfur atoms either in direct conjugation with a phenyl core or bonded to saturated methylene groups. For all molecules we observe at least two distinct groups of conductance plateaus. By a careful analysis of the length behavior of these plateaus, comparing the behavior across the different cores and with methyl sulfide anchor groups, we demonstrate that the lower conductance groups correspond to the incorporation of Au(RS)(2) oligomeric units at the contacts. These structural motifs are found on the surface of gold nanoparticles, but they have not before been shown to exist in molecular-break junctions. The results, while exemplifying the complex nature of thiol chemistry on gold, moreover clarify the conductance of 1,4-benzenedithiol on gold. We show that true Au S Ph S Au junctions have a relatively narrow conductance distribution, centered at a conductance of log(G/G(0)) = -1.7 (+/- 0.4).