What does an electron cloud really look like?

Chemists in Europe can now snap images of single molecules that are so sharp you can not only see the individual atoms within the molecule, but also make out the electrons that bond the atoms together.

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

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

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

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.  

His microscope worked by way of a metal probe that “scans” across a molecule’s surface like a finger running across Braille. The probe had an extraordinarily 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 between the atoms are actually clouds of negatively charged electrons. “We know what the molecule looks like – now we want 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 scientists have used theory to calculate how these charged clouds should look, but “theory is not precise – we can’t really trust it,” he says.

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

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

Repp, Jelinek and their team worked out how to disentangle the electrostatic forces they wanted to measure, from the van der Waals forces they didn’t. They realised the electrostatic attraction between tip and molecule would vary depending on 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 able to disentangle the different forces. “It’s kind of like filtering,” Jelinek explained.

The team tested their technique on two sets of hydrocarbon molecules, which differed only in the number of carbon-fluorine bonds in the molecule. According to theory, fluorine is very good at drawing electrons toward it – and that’s what the team saw. The electron clouds detected by their electrified probe were concentrated around the fluorine atoms.

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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, showing 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. – American Physical Society

These images still appear fuzzy. This is because electrons are so small that quantum mechanics is at play – the location of the electrons will always be blurred. “There’s an inherent uncertainty in the quantum mechanics of these system,” explains Hoxley. “The images are blurry because of this uncertainty – not because of the lack of resolution.”

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

Have Repp and Jelinek captured the sharpest pictures 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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