Mechanism of high-resolution STM/AFM imaging with functionalized tips

TLDR

High‑resolution AFM/STM with functionalized tips is established, yet the imaging mechanism remains poorly understood. The study introduces a numerical STM/AFM model that incorporates probe relaxation caused by tip‑sample interactions. The model simulates probe relaxation under tip‑sample forces to explain imaging contrast. The model reproduces experimental contrasts and shows that probe relaxation into local potential minima causes sharp submolecular resolution, while apparent intermolecular bonds are merely potential ridges, not true hydrogen bonds.

Abstract

High-resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) imaging with functionalized tips is well established, but a detailed understanding of the imaging mechanism is still missing. We present a numerical STM/AFM model, which takes into account the relaxation of the probe due to the tip-sample interaction. We demonstrate that the model is able to reproduce very well not only the experimental intra- and intermolecular contrasts, but also their evolution upon tip approach. At close distances, the simulations unveil a significant probe particle relaxation towards local minima of the interaction potential. This effect is responsible for the sharp submolecular resolution observed in AFM/STM experiments. In addition, we demonstrate that sharp apparent intermolecular bonds should not be interpreted as true hydrogen bonds, in the sense of representing areas of increased electron density. Instead, they represent the ridge between two minima of the potential energy landscape due to neighboring atoms.