Potential-Induced High-Conductance Transport Pathways through Single-Molecule Junctions.

Employing single molecules as electronic circuit building blocks is one promising approach to electronic device miniaturization. We report single-molecule junction formation where the orientation of molecules can be controlled externally by the working electrode potential. The scanning tunneling microscopy break junction (STM-BJ) method is used to bridge tetrafluoroterephthalic acid (TFTPA) and terephthalic acid (TPA) molecules between the Au(111) electrode and the STM tip to measure the single-molecule conductance through the junction. When the Au(111) electrode is at negative potentials (with respect to the zero-charge potential), a highly ordered and flat-oriented superstructure forms, allowing for direct contact between the π system of the benzene ring of the molecules and the Au(111) electrode, leading to junction formation with no anchoring group involvement. Our first-principles nonequilibrium Green's function (NEGF) computation shows a flat configuration yields a conductance that is 3 orders of magnitude larger than for a molecule vertically connected to the electrodes via anchoring groups. Conductances of 0.24 ± 0.04 and 0.22 ± 0.02 G