Charge Transfer through Thin Layers of Water Investigated by STM, AFM, and QCM

Charge transfer through thin layers of water between a Pt/Ir tip and a gold surface has been investigated by using a scanning tunneling microscopy (STM) technique at the temperature of 21 ± 1 °C. The amount of the water layer inside the STM or an atomic force microscopy (AFM) junction was controlled by relative humidity. It was found from STM, AFM, and quartz crystal microbalance experiments that the thin water film was formed on the gold sample when relative humidity was in the range up to 80%. Charge transfer across the interfacial water layer was found to originate mostly from electron tunneling. The value of the barrier height of the electron tunneling was determined to be 0.95 eV from the current vs distance curve, which was independent of the tip−sample distance and the bias voltage. At relative humidity above 90%, the surface was covered with a thick water layer. In such a case the decay for charge transfer was strongly dependent on the bias voltage. For a low bias of 0.03 V charge transfer was similar to that at low relative humidity. On the contrary, the current was found to decay nonexponentially in the bias range of |0.1|−|0.5| V, where the plateau currents were observed in the long distance. The plateau current at 0.5 V was observed to flow up to several hundred nanometers. The magnitude of the plateau currents depended on the applied electrode potential as well as its polarity. The results were rationalized by means of electron tunneling and electrochemical process through the thick water layer.