Orientation-Controlled Single Molecule-Electrode Junctions

Molecular scale electronic devices usually are designed by wiring a single molecule between two metal electrodes via anchoring groups.  Conventional single molecule conductance (SMC) studies focus on molecules with functional groups that provide efficient electronic coupling and bind the organic molecular backbone to the metal electrodes. However, conductance is sensitive to the atomic level details of the molecule-electrode contact so that the anchoring groups end up being resistive spacers between the molecule and the metal, decreasing the single molecule junction conductivity.  Thus creating well-defined, highly conductive molecular junctions to minimize resistance introduced by chemical linkers is a challenging experimental problem especially under ambient conditions.

The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature using a combination of molecular assembly, Scanning Tunneling Microscopy (STM) imaging and STM break junction (STM-BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat-oriented mesitylene on Au(111) to enable direct tip/π contacts, a result that is not possible via conventional methods. The measured conductance of sandwiched Au/π/Au junction is ~0.1G_o, two orders of magnitude higher than the conductance of phenyl rings connected via standard anchoring groups. A comparison with other benzene derivatives, that do not display long-range ordered structure, suggests that such structures, which hold the aromatic ring in place and parallel to the surface, are essential to the increased probability of the formation of orientation-controlled “sandwiched” molecular junctions.