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-CtC-C6H4-CtC-X unit, with eight different capping X groups: AuP(OMe)3 (1), H (2), SiMe3 (3), Au-P(cy)3 (4), Au-PMe2Ph (5), Au-PMePh2 (6), Au-PMe3 (7), and AuPPh3 (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.