Thanks to specialised microscopes, we have long been able to see the beauty of single atoms. 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. Now researchers at IBM have come up with a way to do it.
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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).
Later refinements of this technique, such as the TEAM project at the Lawrence Berkeley National Laboratory in California achieved resolutions of less than the radius of a single hydrogen atom. But while this method works for atoms in a lattice or thin layer, the electron bombardment destroys the arrangement of atoms in molecules.
Other techniques use a tiny stylus-like scanning probe to explore the atom-scale world. One method uses such a probe to measure the charge density associated with individual atoms – a technique called scanning tunnelling microscopy (STM).
Another, called 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.
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.
Leo Gross and his colleagues at IBM in Zurich, Switzerland, modified the AFM technique to make the most detailed image yet of pentacene, an organic molecule consisting of five benzene rings (see picture).
The molecule is very fragile, but the researchers were able to capture the details of the hexagonal carbon rings and deduce the positions of the surrounding hydrogen atoms.
One key breakthrough was finding a way to stop the microscope's tip from sticking to the fragile pentacene molecule because of attraction due to electrostatic and van der Waals forces – van der Waals is a weak force that operates only at an intermolecular level.
The team achieved this by fixing a single carbon monoxide molecule to the end of the probe so that only one atom of relatively inactive oxygen came into contact with the pentacene.
Although van der Waals force attracted the tip to its target, a quantum-mechanical effect called the Pauli exclusion principle pushed back. This happens because electrons in the same quantum state cannot approach each other too closely. As the electrons around the pentacene and carbon monoxide molecules are in the same state, a small repulsive force operates between them.
The researchers measured the repulsive force the probe encountered at each point, and from this they could construct a "force map" of the molecule. The level of detail available depends on the size of the probe: the smaller the tip, the better the picture.
The image is "astonishing", says Oscar Custance of Japan's National Institute for Materials Science in Tsukuba. In 2007, his team used AFM to distinguish individual atoms on a silicon surface, but he acknowledges that the IBM team has surpassed this achievement. "This is the highest resolution I have ever seen," he says.
The IBM researchers believe their technique may open the door to super-powerful computers whose components are built with precisely positioned atoms and molecules. The work may also provide insights into the actions of catalysts in reactions, allowing researchers to understand what is happening at the atomic level, says Gross.
Journal reference: Science, DOI: 10.1126/science.1176210