Contrast inversion in non-contact Dynamic Scanning Force Microscopy: What is high and what is low?

The present work proves that when non-contact Dynamic Scanning Force Microscopy (DSFM) is performed in ambient conditions wrong height measurements of heterogeneous samples can be obtained. In some extreme cases even contrast inversion can be observed. Alkanethiol islands on Au (1 1 1) have been used as model system, where contrast inversion is observed with different DSFM modes and various data acquisition parameters. To understand this effect, spectroscopy measurements have been used to show that contrast inversion is really a consequence of the differences in the interaction measured between tip and sample on the bare Au substrate and on the alkanethiol islands. We propose that this interaction is mainly induced by liquid necks forming between tip and sample, which is much stronger on the hydrophilic Au substrate than on the hydrophobic alkanethiol islands.