Contrast inversion in non-contact atomic force microscopy imaging of C60 molecules

Non-contact atomic force microscopy (NC-AFM) was applied to study C60 molecules on rutile TiO2(110). Depending on the tip-sample distance, distinctly different molecular contrasts are observed. Systematically decreasing the tip-sample distance results in contrast inversion that is obtained reproducibly on the C60 islands. This change in contrast can be related to frequency shift versus distance (df(z)) curves at different sample sites, unraveling crossing points in the df(z) curves in the attractive regime. We have performed simulations based on a simple Morse potential, which reproduce the experimental results. This combined experimental and simulation study provides insight into the mechanisms responsible for molecular contrast in NC-AFM imaging. Moreover, this work demonstrates the importance of distance-dependent measurements for unambiguously identifying molecular positions within a molecular island using NC-AFM.