Dependence of the conductance change on the molecular orbitals in Ag and Au electrodes

The conductance change in single-molecule junctions due to the molecular orbitals and the metal’s Fermi energy was investigated by using a scanning tunneling microscopy break-junction technique with Ag and Au electrodes. 4,4′-diaminobiphenyl and 4,4′-dicyanobiphenyl as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) junctions were used. The amine-linked HOMO-conducting junction had a lower conductance, and cyano-linked LUMO-conducting junction had a higher conductance for Ag electrode compared to Au electrode. These results are attributed to the difference between the metal electrode Fermi energy and the molecular orbital level in the metal-molecule junction. Furthermore, 2,7-diaminofluorene exhibited a higher conductance but the identical molecular plateau length for the Ag electrodes compared to that of 4,4′-diaminobiphenyl indicating that the twist angle of the molecular backbone affects the conductance.