Tapping mode atomic force microscopy in liquids gives a substantial im
provement in imaging quality and stability over standard contact mode.
In tapping mode the probe-sample separation is modulated as the probe
scans over the sample. This modulation causes the probe to tap on the
surface only at the extreme of each modulation cycle and therefore mi
nimizes frictional forces that are present when the probe is constantl
y in contact with the surface. This imaging mode increases resolution
and reduces sample damage on soft samples. For our initial experiments
we used a tapping frequency of 17 kHz to image deoxyribonucleic acid
plasmids on mica in water. When we imaged the same sample region with
the same cantilever, the plasmids appeared 18 nm wide in