Phys. Rev. Lett. 113, 186102 (2014)

By attaching a CO molecule to the tip of an atomic force microscope (AFM), the instrument has recently been used to provide a number of remarkable insights. For example, it has been used to resolve individual atoms and bonds within an adsorbed organic molecule and to discriminate between different types of chemical bond. It has also been reported that the approach can image hydrogen bonds between adsorbed species. However, Peter Liljeroth, Ingmar Swart and colleagues now suggest such intermolecular contrast may in fact be an artefact.

The researchers — who are based at Aalto University School of Science and Utrecht University — examined bis(parar-pyridyl)acetylene (BPPA) molecules adsorbed on a gold surface. The molecules self-assemble into tetramers on the surface, stabilized by hydrogen bonds between the nitrogen and hydrogen atoms on the pyridine rings at the ends of the molecules. The formation of these structures also leads to nitrogen atoms on two opposing molecules being placed in close proximity without any bonds existing between them. AFM images of the tetramers, taken using a CO-terminated tip, showed apparent intermolecular bonds between regions where hydrogen bonds should exist, but also in the region between the two nitrogen atoms where they should not.

Liljeroth and colleagues suggest that the contrast observed is predominately the result of the CO molecule bending on the AFM tip, an effect that picks up the shape of the potential energy surface between two molecules. This explanation is also in line with recent theoretical work from Pavel Jelínek and colleagues (Phys. Rev. B 90, 085421; 2014).