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).
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.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.