limits to how small they can be, and switches of tinier size could in time become objects of great interest. Already, at least two research topics have grown out of Eigler's work.

At Stanford University, Thomas Albrecht, a graduate student working with Calvin Quate, has used standard techniques of microfabrication to create the tunneling microscope on a chip. Such a device could serve as an Eigler-type switch. Albrecht's instruments do not scan but rely on a sharp tip that rides at the end of a cantilever arm 1000 microns in length that is fabricated from layered aluminum and piezoelectric zinc oxide. Some 200 such devices can fit on a 3-inch silicon wafer, a standard in the electronics industry. Furthermore, because they are so small, they are not affected by vibrations from the outside world.

Eigler himself envisions that such switches could confine a movable atom within a molecular lattice, with zeolites and fullerenes being possible choices. Then there is the matter of the fine conducting connections that would carry flows of current to and from the switch, and these conductors or leads must dissipate the heat that will arise within them due to electrical resistance. "Will it work or will it fry?" Eigler asks. The question then becomes one of investigating the conducting and heat dissipation properties of leads built to nanometer dimensions, on a scale of perhaps tens of angstroms. Here, too, the scanning tunneling microscope offers help.

Chris Lutz, a colleague of Eigler at IBM's Almaden center, has used this instrument to move atoms of platinum on a surface of platinum. To do this, he had to bring the tip particularly close to the atoms so as to apply enough force to drag them across the surface. This represents a rather demanding experiment because platinum atoms would tend to bind somewhat strongly to a surface of the same material. It opens the way to working with atoms of different metals, assembled atop an insulator to produce microscopic patches. In Eigler's words, "We have the possibility someday of being able to study how electrons move through such small structures, and that's a very exciting direction for us."

LIMITATIONS OF THE TECHNIQUE

These techniques offer unparalleled opportunities to study the behavior of atoms adsorbed on surfaces individually or in small groups. In particular, a variant of the basic instrument, the atomic force microscope, measures the forces applied in moving such atoms and thus gives quantitative information. However, little in this area is easy. Atoms



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