Studies of aromatic-ring compounds adsorbed on alumina and magnesia using inelastic electron tunneling

Inelastic-electron-tunneling mode intensities have been studied for a wide range of aromatic ring compounds adsorbed on both Al-$\mathrm{Al}{\mathrm{O}}_x$-Pb and Mg-MgO-Pb tunnel junctions. For adsorption on both the alumina and magnesia barriers, large differences in the relative inelastic-tunneling intensities are observed for different substituted groups on the rings. These differences can be related to changes in the surface adsorption mechanisms due to changes in the electronic structure of the molecule, and fall into two main categories. For substituents that act as strong acids with proton donation to the surface intensities of ring modes are very strong and the spectra are characteristic of the free molecule except for specific modifications due to the adsorbed side group. For compounds without acid groups the ring-mode intensities are strongly depressed, and the basic behavior suggests a $\pi$-complex formation with the surface-active sites. This reaction is always accompanied by a softening of the aromatic C-H stretching mode corresponding to a frequency downshift of ∼ 150 ${\mathrm{cm}}^{-1\mathrm{}}$. The elastic-tunneling characteristics of the doped junctions also show wide variations for the range of substituents and compounds studied. The $I-V$ characteristics have been modeled using a two-barrier model and a computer program has been used to evaluate the parameters. The effective organic barrier heights vary from less than 1 V to ∼ 8 V. For the low-barrier heights an inflection point in the dynamic resistance curves and a corresponding maximum in the $\frac{d^{2}V}{d{I}^2}$ curves are observed. This experimental behavior is expected for voltages approaching the barrier height where the onset of Fowler-Nordheim tunneling can be expected. Inelastic modes lying above this inflection point are completely quenched. Representative cases observed on both alumina and magnesia will be discussed.