Scanning tunneling microscopy based on the conductivity of surface adsorbed water. Charge transfer between tip and sample via electrochemistry in a water meniscus or via tunneling?

Thanks to the recently discovered high conductivity of molecular thin water films one can investigate poor conductors and even hydrophilic insulators in the scanning tunneling microscope (STM). Tip approach experiments have looked into the question of whether a permanent water meniscus between the tunneling tip and sample surface must be present while scanning across a water covered surface. For glow discharged Pt/C. gold and freshly cleaved mica we investigated whether the current increases continuously when approaching the tip or if jumps in the current distance behavior can be observed. Jumps in the current-distance curves indicate the buildup of a water meniscus. We found that the tendency of adsorbed water on Pt/C to build up a water bridge between tip and sample is very high at humidities above 55%. When the tip was retracted, the water bridge was pulled out to a length of several tens of nanometers. On gold no comparable effects were observed up to 75% relative humidity. In the case of mica, where surface adsorbed water is the only charge transmitter across the sample, the current was found to increase exponentially with decreasing tip-sample distance. This implies that STM imaging that uses the conductivity of thin surface adsorbed water is based on the tunneling effect and not on a permanent water bridge. (C) 1995 American Vacuum Society.