Tunneling Decay Constant of Alkanedicarboxylic Acids: Different Dependence on the Metal Electrodes between Air and Electrochemistry

In this paper, the different tunneling decay constants βN of alkanedicarboxylic acids depending on the metal electrodes in various surroundings are discussed. The conductance of alkanedicarboxylic acids (HOOC–(CH2)n–COOH, n = 2–4) contacting to Pd and Ag were systematically measured by the scanning tunneling microscopy (STM) break junction (STM-BJ) in air. A similar tunneling decay constant βN of about 1 per −CH2 was found for both metals, which might arise from the Fermi energy of electrodes pinning with the energy positions of molecular states under ambient atmosphere. However, the pinning effect can be destroyed by potential control in electrochemistry, and the βN is determined by the alignment of the molecular energy levels relative to the Fermi energy level of the electrodes, which well explains the order of βN,Pd < βN,Ag < βN,Cu in electrochemistry. The current work shows the important role of the surroundings in electron transport through molecular junctions.