Writing with warm atoms.

The ultrasharp tip of a scanning tunneling microscope (STM) can interact with individual atoms on a surface. Scientists have used this capability to position atoms in microscopic patterns--such as letters of the alphabet--but only at temperatures near absolute zero. Now, John B. Pethica and his coworkers at the University of Oxford in England have demonstrated that they can do the same sort of atomic manipulation at room temperature. The researchers describe their technique in the April 13 NATURE.

In 1990, a team of researchers painstakingly wrote the acronym IBM by using an STM tip to pick up xenon atoms and then place them onto a surface (SN: 11/17/90, p. 310). However, they had to perform the experiment in a vacuum at a supercold temperature of 4 kelvins. At higher temperatures, atoms become dislodged and jump free unless they can be made to bind more tightly to a surface, and heat-induced jiggling of the STM tip reduces its accuracy.

Pethica and his colleagues overcame these difficulties by using bromine atoms, which form strong chemical bonds with a copper surface, and by herding the atoms along their desired path using only a controlled side-to-side vibration of the STM needle. The researchers took advantage of a tiny electric current that flows between the tip and the target atom. That current heated up the atom, temporarily breaking its bond to the surface, and propelled the bromine away from the tip.

"Atom positioning is clearly possible at elevated temperatures," the researchers conclude. Because the outcome depends on the type of atom being maneuvered, they speculate that other combinations of materials may react in different ways. Such selectivity might make it possible to manipulate large molecules or complex structures to build electronic devices on a nanometer scale.