FIGURE 2.3 Surface of nickel as seen by a scanning tunneling microscope. Bumps correspond to individual atoms. (Courtesy of Donald Eigler, IBM.)

"It is quite remarkable that something like this, which you can build yourself, you don't need very fancy machines, just common stuff and a little bit of glue, fits in the palm of your hand, yet it can provide you with remarkable, unprecedented images of atoms on surfaces," says Don Eigler of the IBM Research Division's Almaden Research Center in California, another leader in this field. "It's a great set of eyes," he adds. In his hands it has also become a tool for manipulating atoms.

Eigler and his colleagues have operated their equipment under conditions of high vacuum and temperatures close to absolute zero. The vacuum protects their surfaces against contamination, allowing them to remain atomically clean. The low temperatures, in turn, encourage atoms to stay put rather than hopping around, as they would do if the equipment were warmer.

At the outset, Eigler faced the issue of identifying particular atoms. "They don't come with labels on them," he warns. "We start off with an atomically clean surface and we observe the mean number of defects and what kinds they are. Then we put down a lot of the atoms that we want to study, and we see that they all appear to be a particular kind. From that, we learn to identify what they look like."

An important set of experiments have involved xenon atoms on a surface of nickel. Xenon is an inert gas; it does not readily take part in chemical reactions. Still, that does not mean that its atoms lie loosely on the surface like ping-pong balls; they indeed experience forces, though these are weak. One of them is the Van der Waals force, which results when electrons surrounding an atom show greater density on one side than the other. Such shifts in charge density can result from interactions of atoms with their neighbors. A second force arises from overlap of the