Control\nover Near-Ballistic Electron Transport through Formation of Parallel\nPathways in a Single-Molecule Wire

This\npaper reports highly efficient coherent tunneling in single-molecule\nwires of oligo-ferrocenes with one to three Fc units. The Fc units\nwere directly coupled to the electrodes, i.e., without chemical anchoring\ngroups between the Fc units and the terminal electrodes. We found\nthat a single Fc unit readily interacts with the metal electrodes\nof an STM break junction (STM = scanning tunneling microscope) and\nthat the zero-voltage bias conductance of an individual Fc molecular\njunction increased 5-fold, up to 80% of the conductance quantum G0 (77.4 μS), when the length of the molecular\nwire was increased from one to three connected Fc units. Our compendium\nof experimental evidence combined with nonequilibrium Green function\ncalculations contemplate a plausible scenario to explain the exceedingly\nhigh measured conductance based on the electrode/molecule contact\nvia multiple Fc units. The oligo-Fc backbone is initially connected\nthrough all Fc units, and, as one of the junction electrodes is pulled\naway, each Fc unit is sequentially disconnected from one of the junction\nterminals, resulting in several distinct conductance features proportional\nto the number of Fc units in the backbone. The conductance values\nare independent of the applied temperature (−10 to 85 °C),\nwhich indicates that the mechanism of charge transport is coherent\ntunneling for all measured configurations. These measurements show\nthe direct Fc–electrode coupling provides highly efficient\nmolecular conduits with very low barrier for electron tunneling and\nwhose conductivity can be modulated near the ballistic regime through\nthe number of Fc units able to bridge and the energy position of the\nfrontier molecular orbital.