Scanning tunneling microscopy is based on the flow of an electrical current and thus cannot be used to directly image insulating material. It has been found, however, that a very thin film of water (about one monolayer) adsorbed to a surface exhibits a surprisingly high conductivity 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 biological specimens deposited on such surfaces, as demonstrated by the scanning tunneling microscope imaging of plasmid DNA on mica.
Uncoated tobacco mosaic virus (TMV) can reliably be imaged in humid air (relative humidity above 30%) at relatively high voltages (typically 7 to 10 V) and at very low currents (typically 0.1 to 0.5 pA). Imaging works best with tips that show a small dependence of working distance on tunneling voltage. With such tips, a thickness of the virus of 10 nm and more is measured. When using tips of unsuitable characteristics, heights much smaller than 10 nm and even negative values are found. From these observations, a voltage drop over the TMV on the order of volts can be deduced, which depends on tunneling current and humidity.
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