Contrast mechanisms in photothermal scanning tunneling microscopy

Abstract

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