Ag versus Pb electrodes in inelastic-electron-tunneling spectroscopy

The elastic and inelastic tunneling characteristics for Ag and Pb electrodes on Al-$\mathrm{Al}{\mathrm{O}}_x$ substrates doped with a series of aromatic carboxylic acids have been compared. Model calculations of barrier heights and measurements of ± bias asymmetries in tunnel characteristics and inelastic-electron-tunneling (IETS) mode intensities have been compared. Ag-overlaid electrodes produce a systematic lowering of the effective organic barrier heights by a factor of ∼2 and enhance the nonlinear characteristics of the junction. The large relative variation in effective barrier heights for specific carboxylic acids previously observed for Pb electrodes is also observed for Ag. For effective barrier heights ≤ 1 eV, the loss of IETS mode intensity at high bias voltages is accentuated by Ag. The ± bias asymmetry in IETS mode intensity for Ag is also less than predicted by the long-range molecular interaction potential, in contrast to the agreement with the long-range potential observed for Pb.