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The geometry of atomic bonds

The geometry of atomic bonds

In 2012, a team from IBM’s research labs in Zurich managed to reveal the individual bonds that hold a molecule together. The bond order and length of individual carbon-carbon bonds in C60, often known as a buckyball because of its football form, and two planar polycyclic aromatic hydrocarbons (PAHs), which resemble microscopic flakes of graphene, were photographed by employing a method known as atomic force microscopy, or AFM.

We found two different contrast mechanisms to distinguish bonds. The first one is based on small differences in the force measured above the bonds. We expected this kind of contrast but it was a challenge to resolve. The second contrast mechanism really came as a surprise: bonds appeared with different lengths in AFM measurements. With the help of ab initio calculations we found that the tilting of the carbon monoxide molecule at the tip apex is the cause of this contrast.

Leo Gross

Dr. Leo Gross setting the equipment for the molecular close-up © IBM
A nanographene molecule, exhibiting bonds of different length, seen for the first time under a ‘camera’ composed of a single molecule © IBM
A nanographene molecule undergoes the same studio treatment, this time showing an irregular structure within © IBM

The scientists had previously succeeded in seeing a molecule’s chemical structure, but not the subtle differences in the bonds. Bond order discrimination is close to the technique’s current resolution limit, and additional effects frequently obscure the bond order contrast. As a result, the scientists had to choose and create molecules that had no perturbing background effects.

The photos reveal the molecule’s inner workings, with the darker areas representing the densest regions of the atom and the lighter spots representing the lightest. This information indicates what type of bonds they are, as well as how many electrons pairs of atoms share and what is happening chemically within the molecules.

Bond-Order Discrimination by Atomic Force Microscopy, Leo Gross, Fabian Mohn, Nikolaj Moll, Bruno Schuler, Alejandro Criado, Enrique Guitián, Diego Peña, André Gourdon and Gerhard Meyer

Published: September 2012
DOI: 10.1126/science.1225621

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