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.