Scanning Microscopy


We have studied the stability of various adsorbates, including gold, a platinum-iridium alloy, and DNA, on monoatomically flat gold imaged with a scanning tunneling microscope. We find that adsorbates are generally more stable, sometimes dramatically so, if imaged with a hopping trajectory of the tip rather than with the conventional constant-current scanning technique. Gold pits and associated debris formed on flat gold surfaces under saline solution by mechanical impact of the tip with the surface are always much more stable when imaged with hopping. Samples of thin, sub-monoatomic layers (0.1 nm and 0.2 nm thick) of a platinum-iridium alloy evaporated onto gold and imaged in air were not stable with either imaging method but were much noisier with constant-current scanning; samples of thick layers (20 nm and 50 nm) were stable with both methods, while layers 1 and 2 nm thick were generally stable with hopping and usually unstable with constant-current scanning. DNA deposited onto gold as an aqueous solution and then air-dried was usually not stable enough to resolve molecular features with either scanning method, though adsorbed aggregates generally showed stable large-scale structure with hopping but were very unstable with constant-current scanning.

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