Imaging the effect of electron transfer in a single molecule

So far, scanning tunnelling microscopy, one of the most sophisticated tools for investigations of the atomistic world, has been known to work on electrically conducting samples only. We have developed a novel variant of scanning probe microscopy that facilitates the extension of tunnelling microscopy to insulators. This enables us to map the changes of the electronic orbitals of individual molecules, when charging them.
Imaging the effect of electron transfer in a single molecule

State of affairs – By allowing the direct imaging of individual atoms and molecules, scanning tunnelling microscopy largely contributed to our current understanding of the atomistic world. It is well known that this technique works only on conducting materials. Atomic force microscopy does not have this constraint, but it provides very different and rather complementary information. For example, if you want to learn about the electronic states and orbitals of a molecule, you cannot simply use atomic force instead of tunnelling microscopy.

In their natural environment, molecules occur in different charge states, being the basis of many chemical reactions. On a conducting support, a molecule cannot adapt its charge state: any extra charge would just disappear into the support. In view of this, the limitation of tunnelling microscopy to conducting materials is – in fact – rather severe.

The time was ripe – There was, however, a set of published works that together built the fundament for developing a novel probe-microscopy variant with regards to overcoming the limitation sketched above. One very important prerequisite to our work was that atomic force microscopy can resolve minute variations in electrostatic forces, being even able to detect a single charge. Although this had already been realized quite some time ago, the field was revived by recent experiments, in which the charge state of individual atoms and molecules was controlled and probed with single-electron sensitivity by means of force microscopy.

Combining the ingredients – With these studies in mind, we sat down to develop a novel technique enabling tunnelling microscopy on insulators. If we let just a single electron tunnel back and forth between the tip of the microscope and a single molecule, so the idea, this should not require a current flow through a support, which can then be an insulator. In other words, the current is purely alternating, and there is no direct current involved. Nevertheless, the underlying process is still tunnelling and hence, also the images the technique provides are comparable to those of tunnelling microscopy – but working on insulators.

The key is synchronization – We learned that aspects of the above had been previously realized. Yet, our technique has turned out to be a game changer because of other aspects, one of which is addressed in the following.

It was clear that the tiny alternating current arising from the single electron cannot be sensed directly as a current by amplification. Instead, we had to detect it as a tiny force. Obviously, a child on a swing can swing quite strongly, even though the recoil from bending its legs will be moderate. The trick is that the child synchronizes its little kicks with the cycles of the swing. This is also the basis for our technique: We drive the electron to tunnel back and forth deliberately by voltage pulses and are thereby able to synchronize the tunnelling exactly with our cantilever’s cycle.

Cartoon illustration of what has been done. We investigated the changes that electronic orbitals undergo upon charging a single molecule. © L. Patera & J. Repp

Making it work – With this novel technique, we have chosen to take maps of electronic orbitals of individual molecules, which before were achieved with conventional tunnelling microscopy. The qualitative advantage to any previous tunnelling microscopy work is that, thanks to the insulating support, we can now give the molecules a positive or negative charge at will, and we can map and compare the orbitals in different redox states. In this way, we were able to resolve for the first time the effect of electron transfer and polaron formation on the orbital structure of individual organic molecules with Angstrom resolution.

Change in the electronic orbital of a single molecule upon charging. Blue and white represent the experimentally measured electron distribution of the neutral (left-hand side) and charged (right-hand side) molecule superimposed to its chemical structure. One can see that upon removal of one electron (from left to right) the electronic orbital shrinks considerably in size. © L. Patera & J. Repp

We think that tunnelling microscopy on insulators will open the door for many possibilities in the atomistic world. For example, one will be able to study electronic defect states in oxides, which are important in catalytic processes, or one may access localized states in topological insulators.

Read more details in our publication
L. L. Patera, F. Queck, P. Scheuerer & J. Repp
"Mapping orbital changes upon electron transfer with tunnelling microscopy on insulators"

Nature 566, 245–248 (2019); DOI 10.1038/s41586-019-0910-3

This text has been written by Laerte Patera and Jascha Repp.