Electron Transfer

The scanning tunneling microscope's (STM) ability to manipulate atomic-scale structures has been expanded to include changing the charge of a single atom on a surface.

With a voltage pulse from an STM tip, Jascha Repp, a physicist at the IBM Zurich Research Laboratory in Rüschlikon, Switzerland, and colleagues added an electron to a gold atom sitting on a thin insulating layer, creating an ion. They then removed the electron, returning the atom to its original neutral state [Science, 305, 493 (2004)]. They also were able to add charge to small clusters of gold atoms.

The work opens numerous research avenues, from studying charge states in single atoms and groups of atoms to controlling magnetic moments, and perhaps most provocatively, controlling catalytic activity.

The implications of the new work are "of great scientific interest," says Uzi Landman, physics professor at Georgia Institute of Technology, Atlanta, who has studied the catalytic activity of single palladium atoms.

Charge switching could also be harnessed for chemical sensors, notes Ulrich Heiz, physics professor at the University of Ulm, Germany. For example, small, negatively charged gold clusters adsorb O₂, whereas neutral or positively charged clusters don't. "A saturated chemical sensor could be perfectly cleaned after the action," Heiz says.

The IBM group's development of an ultrathin sodium chloride film only two or three atoms thick was key to the project's success, Repp says. STM is used with a metal substrate, but without an insulating layer on top, the metal would interfere with the structure of adsorbed atoms.

As Repp's group added and subtracted the electron, they were able to monitor the STM image of the atom as it changed from a simple blob to a sombrero-like structure, then back again.