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What does an electron cloud really look like?

Chemists in Europe can now take pictures of single molecules that are so sharp you can not only perceive the individual atoms within the molecule, but also work out the electrons that bond the atoms together.

Jascha Repp from Germany’s University of Regensburg and his associates published their method in Physical Review Letters in August. They plan to use the technique to design more potent solar cells, a technology that analytically relies on electron flow to capture sunlight proficiently.

La Trobe University physicist David Hoxley says the technique is “pretty amazing”. “If you told chemists about this 20 years ago they would have given you 10,000 different reasons why you could not do it.”

In 2009, Zurich-based IBM researcher Leo Gross pushed atomic force microscopy (AFM) to a new limit when he was able to make out the individual atoms in a molecule. Astonishingly, his stunning images resembled the ball and stick pictures of molecules that we all learned at school. 

In 2009, IBM scientists stunned the world with their AFM image of pentacene (bottom), shown here next to a ball and stick sketch of the same molecule

His microscope functioned by way of a metal probe that “scans” across a molecule’s surface like a finger running across Braille. The probe had an extremely fine point, ending in a single carbon monoxide (CO) molecule.

The technique could resolve individual atoms – the balls in those ball and stick models. But what about the sticks? These bonds among the atoms are in fact clouds of negatively charged electrons. “We know what the molecule appear like – now we need to see where the charge is,” says co-author Pavel Jelinek from the Institute of Physics of the Academy of Sciences in the Czech Republic. Until now researchers have used theory to compute how these charged clouds should appear, but “theory is not exact – we can’t actually trust it,” he says.

To discover where electrons are situated across the molecule, the team applied a small electric charge to the CO-tipped probe. As the charged tip images a surface, it is dragged down by a negative charge, and pushed away by a positive one.

Mapping this ‘electrostatic force’ across the molecule should disclose where its electrons are. But as with human laws of attraction, the chemistry becomes complex when you get too close. To perceive the charge among atoms, you need to be so close that the probe’s tip breaches the atom’s electron cloud. And that's a problem as, at that short distance, van der Waals forces kick in. This ‘sticky’ force (which geckos use to cling to walls) twitches to tug on the AFM tip as it scans across the molecule. The van der Waals forces are comparatively weak, but are still sufficient to skew the signals sensed by the probe.

Repp, Jelinek and their team worked out how to disentangle the electrostatic forces they required to measure, from the van der Waals forces they didn’t. They realized the electrostatic attraction among tip and molecule would vary subject to the charge applied to the tip – but that the van der Waals attraction would be unaffected. So by scanning the molecule with one charge at the tip, then repeating the scan with a different charge, by applying a little maths they should be capable of disentangling the different forces. “It’s kind of like filtering,” Jelinek explained.

The team tested their technique on two sets of hydrocarbon molecules, which varied only in the number of carbon-fluorine bonds in the molecule. According to theory, fluorine is very good at drawing electrons in the direction of it – and that’s what the team witnessed. The electron clouds sensed by their electrified probe were focused about the fluorine atoms.

Top: Sketch of the two molecules (fluorine, blue; carbon, dark grey; mercury [Hg], light grey; hydrogen, white). Below: AFM charge-distribution map for the same two molecules, displaying the electron cloud (yellow) that forms around the fluorine atoms. Parts of the molecules are overlaid with models of the molecular structure as a guide for the eye.

These pictures still seem fuzzy. This is because electrons are so small that quantum mechanics is at play – the position of the electrons will always be blurred. “There’s a characteristic uncertainty in the quantum mechanics of these system,” clarifies Hoxley. “The pictures are fuzzy because of this uncertainty – not because of the lack of resolution.”

Jelinek now desires to test the technique on molecules in an excited electronic state – which is what occurs when photons of sunlight hit a photovoltaic cell. If you could see how electrons jump around in these compounds when they’re struck by light, you could design them to be more resourceful, Jelinek says.

Have Repp and Jelinek took the sharpest images of molecules we’ll ever see? “I don’t think this is the last word,” says Hoxley. “But we’re pushing the limits.”

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