Efficacy of Au−Au contacts for scanning tunneling microscopy molecular conductance measurements

We measured conductance traces while breaking gold point contacts in a solution of molecules containing the μ-p-phenylenediethynyl X−C⋮C−C6H4−C⋮C−X unit, with eight different capping X groups:  Au−P(OMe)3 (1), H (2), SiMe3 (3), Au−P(cy)3 (4), Au−PMe2Ph (5), Au−PMePh2 (6), Au−PMe3 (7), and Au−PPh3 (8). Our goal with this work was to achieve a direct Au−C link with a conjugated organic group, potentially forming a molecular junction without chemical link groups that typically decrease junction conductances, such as thiols or amines. Conductance traces collected in the presence of molecules 1, 2, 3, 5, and 7 reveal additional steps at conductances as high as 0.1 G0 (G0 = 2e2/h) down to the measurable limits of the experimental setup. Conductance histograms generated from these traces therefore show a broad increase of counts when compared to a control histogram collected in the solvent alone suggesting the binding of the molecules to the broken Au contacts. The histograms for molecules 1, 5, 7, and 2 were not distinguishable, although that of molecule 3 had considerably fewer counts over the entire conductance range, suggesting that the steric bulk of the SiMe3 prevented frequent junction formation. The histograms collected in a solution of molecules 4, 6, or 8 did not differ from that of the control histogram probably because of the steric bulk of the Au−PR3 capping groups prevented the formation a molecular junction.