Bias dependent apparent height of an Al2O3 thin film on NiAl(1 1 0), and of supported Pd clusters

We have investigated by scanning tunneling microscopy the relationship between applied tunnel bias voltage, Vt, and apparent height of a thin Al2O3 film and of Pd clusters supported by the thin film. The apparent height of the film shows a clear bias dependence only in the Vt interval from +1 to +4 V, where the apparent height increases from zero to 3.5 إ. A simple model is presented which qualitatively explains the observed dependence. The apparent height of the Pd clusters increases by 5 إ in the bias interval from +2 to +4 V and is independent of Vt outside this range. The observed height change of the clusters cannot be readily explained. We discuss some possible reasons for this.