Previous AFM studies have presented images of rutile TiO2(110) in various different contrasts, but have never resolved all surface species at the same time. The two main contrasts were explained in a very simple picture relying on a purely electrostatic tip-sample interaction where the tip basically can be treated as a point charge. According to this picture the AFM tip can have either a positive or a negative polarity and, thus, will image either the oxygen or the titanium atoms. Due to its simplicity, this model has been generalized to all kinds of tip models and nowadays serves as a commonly accepted standard explanation for contrast formation in NC-AFM from a purely electrostatic interaction.

Using NC-AFM at true atomic resolution, scientists at the University of Osnabrück have now succeeded in observing an "all-inclusive" contrast on the TiO2(110) surface. The "all-inclusive" mode reveals oxygen as well as titanium atoms and all characteristic defect species of that prototypical surface in the very same image.

Perfect match
Together with theoreticians from the Academy of Sciences of the Czech Republic and from Universidad Autónoma de Madrid they faced the challenge to unravel the physical mechanisms behind this phenomenon. An extended density-functional theory (DFT) study was carried out to model tip-sample interactions. To develop a resilient theory the researchers in Prague and Madrid paid special attention to find the most realistic and reliable tip-models for their calculations.

To one's surprise the all-inclusive contrast was found to be caused by a purely electrostatic tip-sample interaction. The calculations reveal an intricate redistribution of electronic charges on the tip apex that is far from a point-charge picture. The inclusion of the complex inhomogeneous charge redistribution around the apex atoms allowed the short-range electrostatic force between tip and sample to be evaluated accurately. The resulting simulated AFM image perfectly matches the experimental findings. The point-charge picture for explaining NC-AFM contrasts has, therefore, been seriously challenged by this study.

The researchers presented their work in the journal Nanotechnology.