The forces governing the contrast in submolecular resolution imaging of molecules with atomic force microscopy (AFM) have recently become a topic of intense debate. Here, we show that the electrostatic force is essential to understand the contrast in atomically resolved AFM images of polar molecules. Specifically, we image strongly polarized molecules with negatively and positively charged tips. A contrast inversion is observed above the polar groups. By taking into account the electrostatic forces between tip and molecule, the observed contrast differences can be reproduced using a molecular mechanics model. In addition, we analyze the height dependence of the various force components contributing to the high-resolution AFM contrast.

Debye Institute for Nanomaterials Science Utrecht University P O Box 80 000 3508 TA Utrecht The Netherlands

Institute of Physics Academy of Sciences of the Czech Republic Cukrovarnicka 10 1862 53 Prague Czech Republic

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Quantifying the evolution of atomic interaction of a complex surface with a functionalized atomic force microscopy tip

Mapping the electrostatic force field of single molecules from high-resolution scanning probe images