Contrast inversion in non-contact atomic force microscopy imaging of C-60 molecules

Abstract

Non-contact atomic force microscopy (NC-AFM) was applied to study C-60 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 C-60 islands. This change in contrast can be related to frequency shift versus distance (d f (z)) curves at different sample sites, unraveling crossing points in the d f (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.