Step-induced tip polarity reversal investigated by dynamic force microscopy on KBr(001).

We present atomic resolution images of monatomic step edges on the KBr(001) surface imaged by dynamic force microscopy operated in the non-contact mode. Under certain experimental conditions, we observe a systematic and reversible change of the atomic contrast when the tip crosses the step. This change is attributed to the reversal of the polarity of the ionic tip under the influence of the tip-substrate interaction in the immediate vicinity of the step edge. This polarity reversal is attributed to a change in the atomic structure of the tip and is described by a transition between the two potential wells of a two-level system localized near the tip apex. The case of two monatomic steps imaged in succession is also investigated in detail. The results indicate that the two-level system associated with the reversal of the tip polarity involves the movement of a very limited number of ions on the tip.