SCANNING-TUNNELING-MICROSCOPY OF INSULATORS AND BIOLOGICAL SPECIMENS BASED ON LATERAL CONDUCTIVITY OF ULTRATHIN WATER FILMS

Scanning tunneling microscopy is based on the flow of an electrical cu rrent and thus cannot be used to directly image insulating material. I t has been found, however, that a very thin film of water (about one m onolayer) adsorbed to a surface exhibits a surprisingly high conductiv ity that is sufficient to allow scanning tunneling microscope imaging at currents below 1 picoampere. Hydrophilic insulators, such as glass and mica, can thus be imaged in humid air. The same is true for biolog ical specimens deposited on such surfaces, as demonstrated by the scan ning tunneling microscope imaging of plasmid DNA on mica.