Adsorbates in ambient operated scanning tunneling microscopy: Their appearance, mobility during scanning, and role in surface diffusion measurements

Abstract

Adsorbed species are commonly observed when operating a scanning tunneling microscope in an air environment. Often the quality of images is poor, characteristically appearing streaked, as the tip instantaneously interacts with the adsorbates during scanning. On other occasions, the adsorbates are imaged readily and by changing the tunneling parameters, may be forced to relocate, or in the extreme, may be evaporated by the action of the tunnel current. Furthermore, the subtleties of scanning tunneling microscope imaging allow the possibility of adsorbed species appearing not only as mounds, but as holes as well, depending on the value of the adsorbate work function relative to that of the underlying substrate. It is found that when a combination of both ‘‘mound-like’’ and ‘‘hole-like’’ adsorbate is present, there is a bias-dependent preference for the tip to interact with a particular type. A threshold field is also apparent for the passive removal of adsorbates. It is also found that the presence of adsorbates hinders the surface diffusion process.