Credit: © 2010 APS / C.WEISS et al.

Scanning tunnelling microscopes (STMs) provide fascinating and useful insight into the atomic-scale physical and electronic structure of surfaces. But, although these devices are readily able to resolve isolated atoms on a surface, their ability to image the details of complex chemical structures is limited: conventional STM techniques principally probe the electronic states around the Fermi energy of a sample, whereas most of the information about its chemical structure is contained in the deeper orbital states of its constituent atoms.

Christian Weiss and colleagues, however, have obtained detailed images of the local electron density — and thereby of the bonding structure of complex organic molecules — by adsorbing deuterium molecules onto the end of an STM tip (http://arxiv.org/abs/1006.0835; 2010).

In previous work (New J. Phys. 10, 053012; 2008), the researchers found that when they collected images of platelet-like organic molecules using an STM tip doused with liquid hydrogen immediately before imaging, it markedly increased the contrast of the images produced (pictured, middle panel), compared with those collected with a bare tip (top panel). More remarkable, however, was the fact that the shapes that emerged in these images were strikingly similar to textbook drawings of the chemical structure of the molecules being imaged (bottom panel). Yet it wasn't clear what the technique was measuring.

To better understand the mechanism that produces this contrast, Weiss et al. have repeated the experiment with deuterium molecules, used it to analyse the much simpler structure of a dimer of gold atoms adsorbed on a surface, and performed ab initio calculations to try to determine what exactly the tunnelling current is probing.

The experimental images obtained using deuterium were much the same as when using hydrogen. The magnitude of the tunnelling current suggests that the STM tip was covered with a single monolayer of D2 molecules, and the sharpness of the features in the images obtained (of the order of 0.05 nm) implies that this current flows predominantly through a single molecule at the apex of the tip.

Their analysis suggests that the observed contrast represents the short-range repulsive force that acts on the D2 molecule as a consequence of the Pauli exclusion principle. This force is determined by the total electron density in the sample, thereby enabling the distribution of core electrons and, in turn, the overall chemical structure of the surface to be mapped.