Bias dependent apparent height of an Al2O3 thin film on NiAl(110), and of supported Pd clusters

We have investigated by scanning tunneling microscopy the relationship betw een applied tunnel bias voltage, V-t, and apparent height of a thin Al2O3 f ilm and of Pd clusters supported by the thin film. The apparent height of t he film shows a clear bias dependence only in the V-t interval from +1 to 4 V, where the apparent height increases from zero to 3.5 Angstrom. A simpl e model is presented which qualitatively explains the observed dependence. The apparent height of the Pd clusters increases by 5 Angstrom in the bias interval from +2 to +4 V and is independent of V-t outside this range. The observed height change of the clusters cannot be readily explained. We disc uss some possible reasons for this. (C) 2001 Elsevier Science B.V. All righ ts reserved.