The Controlled Addition And Removal Of An Electron Represents a Milestone In Atom-Scale Science

As reported in the July 23 issue of Science magazine, IBM scientists Jascha Repp and Gerhard Meyer placed and removed a single electron on an individual gold (Au) atom by positioning the tip of a low-temperature scanning tunneling microscope (STM) above the atom and applying a voltage pulse. This pulse does not affect the lateral position of the gold atom adsorbed on an ultrathin (only two atomic layers thick) insulating sodium chloride (NaCl) film on a metal substrate.

Most importantly, both charge states of the atom are stable, that is, an additional electron remains on it until it is removed by a voltage pulse of reversed sign. The stabilization of the different charge states is achieved by tiny changes in the positions of the atoms in the ionic film. Owing to the film's large ionic polarization, the Cl- ion underneath the gold moves downward, while the surrounding Na+ ions move upward. In the STM image, the new charge state of the gold atom appears as a circular trough around the atom.

Jascha Repp, who designed and carried out the experiment, explains: "A simple electron transfer with no lasting changes of ion-core positions would not be stable because the electron residing in an excited state on the manipulated Au atom would rapidly tunnel beneath the insulating layer into the metal of the substrate."

"Our discovery is an important step towards using a single atom or molecule as a basic building block for possible future atomic-scale technology," says Gerhard Meyer, who leads the STM-related research efforts at IBM's Zurich Research Laboratory. "In the nanoworld, creating complex functionalized structures will require that we control not only the position of atoms, but also the electronic and chemical parameters as well." In 1990, Don Eigler of IBM's Almaden Research Center in San Jose, California, showed that an STM can place atoms on top of a surface with atomic precision. Now, a new capability has been achieved by manipulating the electrons of an atom. Jascha Repp points out: "The chemical and physical properties of ions in general are qualitatively different from those of the corresponding neutral atoms. Therefore our findings will have an impact not only on physics but also on chemistry. This research is likely to aid the atom-scale study of such diverse phenomenon as chemical catalysis to quantum information technology."

To interpret the experimental findings, Fredrik Olsson and Mats Persson from Chalmers University used first-principles density functional theory calculations. In agreement with the experiments, the theoretical investigation also finds two different stable states for Au atoms: One is nearly neutral, the other is negatively charged by one electron. The simple physical mechanism responsible for the existence of different charge states suggests that this finding is a common phenomenon for adsorbates on polar insulating films supported by a metal substrate.

The collaboration between IBM and Chalmers University was conducted within the framework of the European Union (EU) network on "Atomic and Molecular Manipulation as a new Tool for Science and Technology".