Atom hauler: molecular rig snags multi-atom loads.

A molecule with a knack for picking up and delivering atoms may prove a useful tool for atomic-scale construction.

Scientists in France and Germany who created and tested the molecule say that it and similar custom-made structures might aid tasks such as building molecular-scale circuitry, depositing arrays of atom clusters with special optical or magnetic properties, and cleaning up debris on nanoconstruction sites.

For more than a decade, researchers at laboratories equipped with scanning tunneling microscopes, or STMs, have pushed around individual atoms. On its own, however, the tiny, extremely sharp tip of an STM has trouble maneuvering more than one atom at a time, says Leo Gross, now at IBM Zurich Research Laboratory in Ruschlikon, Switzerland. "With four atoms, it is almost not possible," he notes.

Gross and his colleagues used an STM tip to shove the new molecule around on a copper surface. The molecule easily scarfed up as many as five loose copper atoms and then delivered the atom cluster to a desired location, the team reports in the December Nature Materials. The experiments, which took place at ultra-low temperatures and under high vacuum, were a collaboration between a group--including Gross and led by Francesca Moresco--at the Free University of Berlin and a group led by Christian Joachim of the Center for Materials Elaboration and Structural Studies in Toulouse, France.

The scientists designed and synthesized the molecule, known as hexa-t-butyl-hexaphenylbenzene (HB-HBP). It's a hexagon composed of carbon rings. Six tripodlike feet, each known as a t-butyl group, support the structure. This elevated ring structure loosens the copper atoms' bonds with the metal surface.

Whereas an STM tip can slide a single unladened HB-HBP molecule over a copper surface with ease, it must push harder with each atom the molecule takes on. Loaded with five copper atoms, the molecule requires much more force but will still move. Add a sixth atom, and it won't budge.

To release the cargo, the scientists use the STM tip to lift the HB-HBP molecule. The copper atoms stay behind in a clump.

Last year, other researchers reported using cagelike carbon molecules called fullerenes to take up and deposit one to four potassium atoms from a silver surface. The new work goes further, Joachim says, because the scientists designed HB-HBP expressly to carry out atom-hauling chores.

Moreover, comments physicist Gerald Dujardin of Universite Paris-Sud in Orsay, France, the finding "opens the way to use a molecule as a real machine" for nanofabrication. Instead of being pushed by an STM tip, such molecular machines could fuel their own movements with light or chemical energy, he says.