Atomic Jockeys

Spin doctors may be the masters of manipulation, but scientists are catching up fast--at least with atoms. In the current issue of Physical Review Letters, scientists describe how to push, pull, and slide individual atoms along a sheet of copper. The technique might lead to fabrication of tiny objects atom by atom, or even incredibly fine microcircuitry.

When the scanning tunneling microscope (STM) was invented in 1982, researchers could image individual atoms. Like someone reading Braille, the STM scans a surface with a fine titanium tip that senses the flow of electrons to individual atoms and builds a three-dimensional map of an object's surface. But before long, scientists were using it to jostle atoms and molecules. Now, three German physicists from the Freie Universität Berlin have discovered that they can move the atoms in three ways: pulling, pushing, and sliding.

Atomic interactions between the tip and the sample are the key to jockeying individual atoms. For instance, a molecule like carbon monoxide normally wouldn't bond to a metal, so it is repelled by the tip. "If you move an atom with a repulsive force on a flat sheet, you lose it immediately," says team member Gerhard Meyer. To prevent the atom from skittering away, the scientists use a copper crystal that is corrugated like cardboard; the carbon monoxide molecule is nudged gently down one of the grooves.

Metals like copper and lead, on the other hand, can bond with the tip, so there is an attractive force between them; when the tip is moved, the atom follows like an obedient dog. Here, the strength of the metal-metal bond determines the motion. With a loose, springy bond, the atom falls into every crevice along the way, like a pet who stops at every fire hydrant until the owner tugs the leash. But if the bond is tight, the atom follows in lockstep, skimming over the crevices.

"What's cool about it is that it will allow you to set up experiments you couldn't do before," says Donald Eigler, a physicist at IBM's Almaden Research Center in San Jose, California. "It is putting in place the knowledge to build things at the atomic scale." For instance, Stephen Minne, an electrical engineer at Stanford University, believes that micromanipulation of atoms might help engineers strip hydrogen atoms off silicon wafers, leading to tiny integrated circuits. All the research needs is a little push.

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