CONTRAST MECHANISMS IN PHOTOTHERMAL SCANNING-TUNNELING-MICROSCOPY

By irradiation of the tunneling junction of a scanning tunneling micro scope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal imag es were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsibl e for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from e ither the current variations caused by the non-zero reaction time of t he current control loop or from a geometry factor. In both cases the i mages reflect certain properties of the sample topography. On the othe r hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the ef fective barrier height.