Molding Atoms: Using a tiny template to make tinier structures

With the help of a molecular mold composed of exactly 188 atoms, researchers have been able to impose textures at an even smaller atomic scale on a metal surface.

STEP TRAINING. Seen from above, a molecular mold (gray) organizes top-layer copper atoms (yellow) into a two-atom-wide protrusion. Rosei et al./Science

The structures created–strips of copper exactly two atoms wide by seven long–will probably not prove of practical value. However, the accomplishment could lead to other molecular tools that yield more desirable products, says Federico Rosei of the University of Aarhus in Denmark.

Rosei and his colleagues at Aarhus and the National Center for Scientific Research in Toulouse, France, describe the molecule’s unexpected template behavior in the April 12 Science. The molecule–a 188-atom chunk of carbon and hydrogen–was invented several years ago, by the French members of the team, to conduct electrons one at a time.

Now, Rosei says, it appears that such molecules also might serve well as agents to create ultrathin metal wires between nanometer-scale electronic devices. Researchers have devised many molecular-scale electronic components, including transistors, made from nanowires and nanotubes (SN: 11/10/01, p. 294: Wiring teensy tubes, strands into circuits). However, wiring them together and connecting them to larger devices remain “big issues to be solved in order to advance this field” of molecular electronics, Rosei says.

Ultimately, he speculates, molecules of specific designs might also function as molds, or templates, for creating other nanometer-size structures–for instance, clusters of atoms called quantum dots (SN: 7/7/01, p. 7: Wee dots yield rainbow of molecule markers).

Shaped like a four-poster bed, the molecule conducts electricity through its mattress and is insulated from the underlying substrate by its legs. To study the electronic properties of such molecules, the Aarhus members of the collaboration made them evaporate from a crucible and redeposit on a copper surface. Once there, the molecules diffused to so-called step edges–regions where the surface abruptly leaps up by a single atom’s height or more.

When the researchers examined these surfaces, they were surprised to find a copper strip extending from the step edge wherever there was a molecular bed.

Rosei and his colleagues suspect that randomly wandering copper atoms roam beneath the four-poster and get trapped there. “We’ve noticed that the ability of a molecule to modify the surface depends on whether it has legs,” notes Rosei.

Ari Aviram of IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., says, “What’s beautiful here is that the atoms of the surface itself restructure themselves.”

“What [Rosei and his colleagues] see is something I never would have imagined happening,” comments Norman C. Bartelt of Sandia National Laboratories in Livermore, Calif. “They can make [the step edge] do something that it really doesn’t want to do.”

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