Influence of adsorbates on the electronic density of metal electrodes
The electronic structure of a metal surface is modified in the presence of adsorbats. This was shown in UHV by several techniques ranging from ultraviolet photoelectronemission spectroscopy (UPS) to local measurement of the density of states by scanning tunneling spectroscopy (STS). In electrochemical environment, however, the measurement of the electronic density of states, e.g., by photoelectron spectroscopy, is hampered by the presence of electrolyte. We therefore want to employ in situ scanning tunneling spectroscopy in electrochemical environment. The variation of the tunneling current with the applied tunneling voltage directly reflects the electronic density of states in the vicinity of the STM tip. Usually in electrochemical environment such studies suffer from the bad stability of the tunneling junction and undesirable electrochemical side reactions at tip and surface. In preliminary experiments, we studied cyanid and thiol layers on Au(111) under ambient conditions with an improved experimental set up. Due to humidity, a water layer formed in the tip-surface gap, which simulated “electrochemical environment”, however, without explicit control of the electric potential of the electrode.
The STM-image on the left shows a ~100x100 Å area of a CN-monolayer deposited on an annealed gold surface. The cyanid is ordered in a 1.15x R-30° structure and crosses two monoatomic steps of the underlying substrate. In addition the associated tunneling spectrum, i.e. derivative dIT/dUT is shown. The dI/dU-signal rises remarkably for UT > 0.3V, indicating empty electronic states at that energy.
 T. Yamada, R. Sekine, T. Sawaguchi, The Journal of Chemical Physics 2000, 113, 1217.