Special Reports

For a decade now physicists have been imaging individual atoms with the aid of instruments such as the scanning tunnelling microscope. Now they are stringing those images together to make movies of atoms in motion.

'We're basically figuring out the building blocks of atomic motion,' says Ellen Williams of the University of Maryland, College Park. Williams and her colleagues are using three different imaging instruments to produce the videos. They described the techniques at the annual meeting of the American Physical Society held in Pittsburgh last month.

One instrument is the scanning tunnelling microscope, in which an electrically charged needle is moved across the surface of an object to be studied. The electrical current which passes from the probe to the surface is extremely sensitive to the separation between the probe and the surface. A computer uses data on the current to deduce the contours of the object.

The technique, invented by Gerd Binnig and Heinrich Rohrer who carried off the 1986 Nobel Prize for Physics, can produce highly detailed images which allow scientists to watch individual atoms. Unfortunately, producing movies from the images is a slow process because the probe has to scan painstakingly across the surface.

A second imaging technique is low-energy electron microscopy, in which a beam of electrons is reflected off an object and is then focused to produce an image of the object's surface. 'This is a lot more like the type of microscopy that you would do with a light microscope,' says Williams. This approach can generate real-time videos, with 30 images per second, but the images are less detailed than those from scanning tunnelling microscopy.

The third imaging technology is reflection electron microscopy, in which a beam of electrons is reflected off a surface at a shallow, grazing angle. This also produces detailed images, but from a smaller area of the sample, says Williams.

With the aid of the three techniques, physicists can monitor atomic movements. For instance, they can observe how pits and bumps form on a flat surface, or they can observe the ragged edge of a surface, where atoms are constantly attaching and detaching themselves. 'We can really start to see matter moving around on surfaces,' says Williams.

As an example, she screened a video image of a surface of silver atoms. When the surface was in a vacuum, it was pockmarked with a continually shifting pattern of pits and bumps. But when oxygen was pumped in, oxygen atoms clumped in straight lines on the silver surface with empty spaces between.

So far, no one understands why the oxygen atoms keep the region between them pristine. But once physicists understand such processes, they might be able to develop new tools for nanofabrication or for manufacturing atomic-sized structures for use in electronics. 'That's the hope,' says Williams.