Repulsive interaction and contrast inversion in noncontact atomic force microscopy imaging of adsorbates

To understand contrast formation in atomic resolution noncontact atomic force microscopy (NC-AFM), we investigate whether or not repulsive tip-sample interaction contributes to contrast formation. We relate attractive and repulsive interactions to contrast features depending on both oscillating amplitude and measured detuning. Simulations based on a Morse potential illustrate the mechanism behind contrast inversion due to repulsive interactions above an adsorbate on the surface. Experimental NC-AFM images of adsorbates on mica and TiO2 surfaces confirm our simulations. Furthermore, we discuss the influence of the topography feedback loop on contrast formation above adsorbates, which illustrates that data interpretation can become rather delicate for constant-detuning images.