MICROSCOPES capable of revealing the astonishing beauty of an atom can hardly be called blunt instruments. But to date, these tools have either been too destructive or offered disappointing resolution. Now researchers at IBM have come up with a delicate method which has provided unparalleled details of the structure of a molecule.
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 iterations 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 hydrogen atom. But while this method works for atoms in a lattice or thin layer, the electron bombardment destroys more fragile individual molecules.
Other techniques use a tiny stylus-like scanning probe. This can be used to measure either the effect of quantum tunnelling of electrons between the probe and the surface of the target, called scanning tunnelling microscopy (STM), or the attractive force between atoms in the probe and the target, called atomic force microscopy (AFM). These methods are suitable for individual molecules but have not been able to approach the detail ofTEM.
Leo Gross and 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. Although the molecule is highly fragile, the researchers were able to capture the details of the hexagonal carbon rings and deduce the position of the surrounding hydrogen atoms (Science, DOl: 10.1126/science.1176210). The team first fixed a single carbon monoxide molecule to the end of the probe. This allowed them to invoke a quantum mechanical effect called the Pauli exclusion principle, which says that 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 repulsive force operated between them. The image was created by bumping the probe over the atoms in the molecule – much in the way we might navigate around in a dark bedroom. The researchers measured the amount of repulsive force the probe encountered at each point, and used this to 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 2007, his team used AFM to distinguish individual atoms on a silicon surface . "This is the highest resolution I have ever seen," he says of the IBM image. The IBM researchers believe their technique may open the door to super-powerful computers made from components built out of precisely positioned atoms and molecules. The work may also provide insights into the actions of catalysts in chemical reactions, potentially allowing researchers to understand what is happening at the atomic level, says Gross. Mac Gregor Campbell.
Source of Information : NewScientist(2009-09-05)