Science 348, 308–311 (2015)

In recent years, the atomic force microscope (AFM) has been used to image organic molecules with submolecular resolution by attaching a carbon monoxide molecule to the end of the microscope's tip. This approach has also allowed, for example, different types of chemical bond to be discriminated and for reaction-induced changes in the bonds of single molecules to be followed. Now, Franz Giessibl and colleagues have shown that the technique can be used to image metallic atoms and clusters with subatomic resolution.

The researchers — who are based at the University of Regensburg and Ludwig-Maximilians-Universität München — first examined single copper and iron atoms adsorbed on copper surfaces. The atoms appear as toroidal structures, rather than as single protrusions as would be expected in a typical AFM image. This, they suggest, is due to the electronic structure of the atoms, and, in particular, is the result of electrostatic attraction in the centre of the atoms and Pauli repulsions at the edges. Moreover, the toroidal image depends on the bonding symmetry of the atom to the underlying surface structure. When the approach is used to image small clusters of iron atoms, the structures appear as connected tori with each atom discernible.