Zurich, Switzerland/Gothenburg, Sweden, July 23, 2004 ? Scientists of the IBM Zurich Research Laboratory and of Chalmers University of Technology, Gothenburg, have succeeded in manipulating and controlling the charge state of individual atoms. With this experiment, a new dimension of manipulation has been achieved. The ability to add or remove an electron charge to or from an individual atom is a decisive step in the development of future atomically small devices. This switching between different charge states of an individual atom enables, for example, the control of chemical reactivity, optical properties, or magnetic moment.
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 manipulation 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. This relaxation pattern creates an attractive potential for the additional charge on the Au adatom. 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 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, the creation of complex functionalized structures will require the ability to control not only the position and geometrical and mechanical properties of atoms, but also the electronic and chemical parameters at the atomic level." In 1990, Don Eigler of IBM's Almaden Research Center in San Jose, California, showed that, using an STM, atoms can be placed on top of a surface with atomic precision. Now, the next step has been achieved by manipulating the elements that make up 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. Possible applications of this phenomenon range from catalysis to nonvolatile memory devices 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. "Given the extremely high quantum yield of order unity, the switching of the charge state can probably also be operated by means of other techniques. In other words, the control of the charge state is not limited to the tunneling junction geometry," comments Gerhard Meyer.
The controlled deposition and removal of an electron charge onto and from an individual atom is a decisive step in the development of future atomically small devices. For instance, it could lead to a nonvolatile memory cell at the ultimate spatial limit. As Rolf Allenspach, manager of Zurich's research efforts related to the physics of nanoscale systems, puts it, "Practical atomic-scale memories would increase the amount of data that can be stored on a given area by at least ten thousand times". Another prospect is that material properties could be tailored at the atomic level.
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".
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