Single Molecule Conductance of Carboxylic Acids Contacting Ag and
Cu Electrodes

In this work, the single molecule conductance of alkanedicarboxylic acid (HOOC–(CH2)n–COOH, n = 1–5) binding to Cu and Ag electrodes is systematically studied by using the electrochemical jump-to-contact scanning tunneling microscopy break junction approach (ECSTM-BJ). The results show that the conductance depends on molecular length and the electrode materials, which give a decay constant βN of 0.95 ± 0.02 per (−CH2) unit for Cu electrodes and 0.71 ± 0.03 for Ag electrodes. The contact conductance shows the order of Gn=0,Cu > Gn=0,Ag. These differences can be attributed to the different electronic coupling efficiencies between molecules and electrodes. The conductance of ultrashort molecular junctions is also studied using oxalic acid as the target molecule, the results revealing that the through-space mechanism (TS) should be considered when the distance between two electrodes is very short. The present work demonstrates that electrode materials play an important role on the molecular conductance, contact conductance, and also the tunneling decay constant.