Bias-voltage induced surface reaction of para-nitrobenzoic acid adsorbed on Ag/AlOx/Al tunnel junction

Surface-enhanced Raman scattering (SERS) spectra of para-nitrobenzoic acid (PNBA) adsorbed on the Ag electrodes of Ag/AlOx/Al tunnel junctions fabricated on plain and CaF2-covered glass substrates are measured under bias-voltage applications. The SERS bands caused by the adsorbed PNBA- on the Ag electrode increase in intensity with positive bias-voltage and a new band attributable to the azo-group emerges at voltages greater than +2.2 eV. In contrast, the bands remain nearly unaltered under negative bias-voltage. The intensity profile of the newly emerged band is similar to the junction's Tafel plot, in which the tunnel current increases steeply at the same voltage because of Fowler-Nordheim tunneling. The azoband intensity change with elapsed time at +2.0 eV bias voltage is remarkable for the junction on the CaF2 roughened glass substrate. These results suggest that the PNBA reductive coupling reaction might correlate with surface plasmon polariton (SPP) excitations by the generated hot-electrons