Effects of the molecule-electrode interface on electrical and mechanical properties of benzene-based single-molecule junctions

Ever since Aviram and Ratner theoretically proposed that rectifiers could be constructed from single molecules, great interest has been expressed in the development of single-molecular junctions as nano-electronic devices. In addition to their applications for molecular switches, chemical sensors, spin filters, and voltage-driven catalysis, such junctions are excellent prototypes for understanding charge transport at the single-molecule level. As the interface-to-bulk ratio becomes large at the nanoscale, the molecule-electrode interface dictates the properties of single-molecule junctions. To understand the various interfacial effects of the anchor group, bridging molecule, electrode material, and electrode binding site on the conductance, we present a simple and efficient tunneling model for coherent transport that is fully parameterized using density functional theory. Using this model, we calculate several properties, such as the binding energies, binding force constants, conductance, and current-voltage characteristics of junctions containing a benzene-1,4-dithiol or benzene-1,4-diamine molecule bridged between gold, silver, or copper electrodes. Due to the implementation of an effective mass in the tunneling model and the use of a range-separated, hydrid density functional, our predicted properties agree well with results from several independent experimental studies. We found that the conductance of benzene-1,4-dithiol depends sensitively on the binding site at the electrode whereas benzene-1,4-diamine shows no such dependence. Silver electrodes produce a much weaker quantum conductance current than gold electrodes due to a lower work function, whereas junctions with copper electrodes produce conductance and current quite similar to those with silver electrodes.