DIFFUSION OF ATOMS ON AU(111) BY THE ELECTRIC-FIELD GRADIENT IN SCANNING-TUNNELING-MICROSCOPY

It has been shown in field ion microscopy experiments that the electri c field gradient drives the diffusion of atoms on a surface, The same idea has been proved to work in a scanning tunneling microscopy experi ment, as was first demonstrated when Cs atoms were induced to move on GaAs(110) by the electric field, taking advantage of the high Cs stati c dipole moment. In this work we demonstrate that this effect is more general and can be used even in systems that have a small dipole momen t, such as the surface Au adatoms of Au(111). The experiment is perfor med in ultrahigh vacuum with a clean surface and tip. By applying a vo ltage increase and maintaining constant the tunneling current we are a ble to produce protrusions under the tip of several atom-heights and s maller clusters occupying large areas. We study the influence of the t ime duration of the pulse and the voltage amplitude. Surface modificat ions are already produced at 0.2 V/Angstrom electric field value. We s uggest that the clusters are produced by an increase of the density of free adatoms in the high electric field region. (C) 1996 American Vac uum Society.