Interface Feedback Effect in Molecular Tunnel Junctions

Despite numerous prior studies on molecular tunnel junctions, many important questions remain about the nature of metal-molecule contacts. Using the conducting probe atomic force microscope (CP-AFM) platform, we report here an investigation of electrical contact effects in junctions based on oligophenylene and alkyl dithiols (OPDn, n = 1, 2, 3 and CnDT, n = 8, 9, 10) linked via thiol anchoring groups to dissimilar Ag, Au, and Pt metal electrodes. Our data reveal a peculiar effect: the two metal-molecule interfaces "talk" to each other, i.e., the choice of metal for the tip (t) electrode substantially changes the metal-HOMO electronic coupling Γ associated with the substrate (s) electrode, and vice versa. The metal-HOMO couplings Γ_t and Γ_s are not independent quantities. Their interdependence does not correlate with metal work function, chemisorption-driven work function change, or metal electronegativity, i.e., properties characterizing charge transfer at the molecule-metal interface. Overall, our results reveal an undiscovered complexity associated with electrical contacts in molecular tunnel junctions that must be considered in theoretical descriptions and ongoing efforts to design junctions with specific electronic functions.