Atomic force microscopy: first image of molecule showing chemical bonds

Pentacene: Image IBM and Science












Atomic force microscopy (AFM), measures the attractive force between atoms in the probe and the target. The image is created by bumping the probe over the atoms of the molecule – much in the way we might feel our way around in a dark bedroom. Another method Scanning tunneling microscopy STM uses such a probe to measure the charge density associated with individual atoms.

Both methods build up a picture of a target’s surface and should be suitable for imaging individual molecules. But they have not been able to approach the detail of TEM.

Atomic force microscope was used to image the molecule in unprecedented resolution, published for the first time in New Scientist issue of 28 August 2009.

The portrait of pentacene, an organic molecule consisting of five benzene rings, shows off the chemical bonds between the carbon and hydrogen atoms. It may seem a somewhat surprising first, since atoms have been imaged for decades.

The earliest pictures of individual atoms were captured in the 1970s by blasting a target – typically a chunk of metal – with a beam of electrons, a technique known as transmission electron microscopy (TEM)…. But strange though it might seem, imaging larger molecules at the same level of detail has not been possible – atoms are robust enough to withstand existing tools, but the structures of molecules are not.

Rather than relying on an optical system to produce pictures, atomic force microscopes use a probe that narrows to an atomic-scale tip, and measures the forces of attraction between the tip and the molecule’s components. Lead researcher Leo Gross was able to get the shot of pentacene because he stuck a molecule of carbon monoxide (CO) on the tip of the probe.

Though, this is not the first-ever image of a single molecule, that has been possible for quite some time. This IS, however, the first ever AFM image in which the chemical structure of a molecule is visible (you can see where the atoms are). There are many STM images with equally good resolution, but they map electron density of states so you get images of orbitals, whereas this is a map of where chemical bonds would form (where the sticks would attach in a ball and stick model). At higher temperatures, molecules wiggle around more so it’s harder to image them with such clarity. The sensor has to be focused in order to raster scan the surface (it’s like reading braille rather than taking a picture).






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One response to “Atomic force microscopy: first image of molecule showing chemical bonds

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