Electric field effects on the nanometer-level surface modification of Au(111) surfaces

We report observations of contrasting surface modification behavior of the Au(lll) surface in the presence of an electric field and field-emission currents using interfacial force microscopy (IFM) and scanning tunneling microscopy (STM). Our experiments consist of surface modification procedures which allow for large tip-sample gaps, in contrast to fast voltage pulses (applied at tunneling distances) employed by previous STM investigations. Dramatic surface distortions are observed when a 200 nm-radius tip, biased at -100 V, is brought toward the Au surface at a field emission current level of 400 nA and then retracted. In other experiments, we raise the sample voltage to field-emission levels while maintaining a constant current. STM images, measured in a time-resolved manner after each such procedure, show that the presence of a higher electric field (similar to 0.07 V/Angstrom) results in step retraction and the disappearance of small islands on the Au(111) surface followed by the formation of vacancy islands in the area directly beneath the apex of the tip where the field is highest. We discuss the implications of these contrasting surface modifications in terms of the various key parameters and in relation to previous studies using voltage pulses in the STM.