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 mu-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 G(0) (G(0) = 2e(2)/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.