Highly Conducting pi-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

摘要

We measure electronic conductance through single conjugated molecules bondedto Au metal electrodes with direct Au-C covalent bonds using the scanningtunneling microscope based break-junction technique. We start with moleculesterminated with trimethyltin end groups that cleave off in situ resulting information of a direct covalent sigma bond between the carbon backbone and thegold metal electrodes. The molecular carbon backbone used in this study consistof a conjugated pi-system that has one terminal methylene group on each end,which bonds to the electrodes, achieving large electronic coupling of theelectrodes to the pi-system. The junctions formed with the prototypical exampleof 1,4-dimethylenebenzene show a conductance approaching one conductancequantum (G0 = 2e2/h). Junctions formed with methylene terminated oligophenylswith two to four phenyl units show a hundred-fold increase in conductancecompared with junctions formed with amine-linked oligophenyls. The conductionmechanism for these longer oligophenyls is tunneling as they exhibit anexponential dependence of conductance with oligomer length. In addition,density functional theory based calculations for the Au-xylylene-Au junctionshow near-resonant transmission with a cross-over to tunneling for the longeroligomers.