Precise and tunable control in electronic states of metal surfaces should be linked to a novel catalytic reaction. Hot-electrons, which have energies greater than the Fermi level, are able to access adsorbates on metal surfaces and, thereby, may induce new surface reactions of the adsorbate. Metal-insulator-metal (MIM) tunnel junction would be one of the suitable devices for the hot-electron generation. The aim of this study is to clarify the tunnel junction surface works as a promoter of the chemical reaction of the adsorbate. We investigate the hot-electron-induced surface reaction of p-nitrobenzoic acid (PNBA) on the Ag top electrode of the Ag/AlOx/Al tunnel junctions by surface enhanced Raman scattering (SERS).
The Ag/AlOx/Al tunnel junction was fabricated as follows. First, Al (25nm in thickness) was deposited on a Pyrex glass plate and CaF2-covered-glass substrates and, then, the deposited Al surface was oxidized in an atmosphere of 1atm O2. Finally, 18nm-thick Ag was evaporated onto the AlOx film surface. PNBA was deposited onto the Ag top electrode by spin-coating. Raman light excited by He-Ne laser (8 mW) was dispersed by a polychrometer and detected with a CCD detector.
The SERS bands caused by PNBA adsorbed on the Ag electrode surface increased in intensity with increasing positive-bias voltage to the electrode. In particular, the band intensities increased significantly above +2.2eV, accompanied by emergence of a new band due to a stretching vibration of azo-group. Dependence of the new band's intensity on bias-voltage was quite similar to the junction's I-V curve, in which the tunnel current increased steeply above +2.2eV (Fowler-Nordheim tunneling). In contrast, when the Ag electrode was biased negatively, the SERS bands remained nearly unchanged irrespective of the substrates. Further, it should be worth mentioning that the azo-band intensity change with elapsed time is remarkable for the junction on the CaF2 roughened substrate. These results suggest that the PNBA reductive coupling reaction may correlate with surface plasmon polariton (SPP) excitations by the generated hot-electrons at the electrode surface.
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