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

Abstract

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 sim...