Focus: Watching Single Atoms Move
Using the scanning tunneling microscope (STM), researchers can manipulate and image individual atoms on surfaces with a level of precision that was impossible before the STM’s invention. They can study not only static structures, but also the dynamics of atoms, triggering motion by changing temperature or depositing “catalyzers” on the surface, for example. In the 6 April PRL, a European team demonstrates a new method for studying surface atom dynamics: removing one atom and watching the vacancy move as other atoms hop into the newly-created openings. They also speculate on the method’s value for possible atom-scale devices.
To image a germanium surface, investigators first bombard the crystalline wafer at high temperature with argon ions, a process that smooths the surface at the atomic scale by removing impurities and allowing extra germanium atoms to bind to the “dangling” bonds that remain above the surface. The “reconstructed” surface has a larger unit cell than that of a bare crystal face and on one specific crystal plane of germanium (known as “(111)”) is made entirely of “adatoms,” which are bound only to the layer below and not to one another.
Patrici Molinàs-Mata, currently of the Commission for Atomic Energy (CEA) in Saclay, France, and his colleagues show in their PRL paper that the unique surface reconstruction of the (111) face of germanium allows for a new type of dynamics experiment. After a single atom is removed from the surface, the neighboring atoms stay in place at room temperature, leaving a clear hole. An adjacent atom can then hop into the hole, leaving another vacancy, or it might sit for some time in a “metastable” site between two normal atomic positions. In other materials, at room temperature, a vacancy often moves only a short distance before being trapped by higher energy barriers of the more complex surface reconstruction, but in germanium the thermal hopping can move a vacancy long distances (hundreds of Angstroms) across the surface, in precise steps, even at room temperature.
After plucking out one atom, Molinàs and his colleagues counted the number of times they observed each of nine distinct arrangements of vacancies and atoms at metastable sites to estimate the probabilities and free energies of the configurations. The group notes, however, that they cannot determine the entropic contributions to the free energies without varying temperature. In fact, Molinàs says, varying temperature would allow him to take full advantage of the technique, by studying dynamic properties, such as the heights of energy barriers between atomic positions. But he sees this effort mainly as a demonstration of a new technique for measuring surface dynamics with a well-behaved system. He believes it also shows that “the STM can be used not only to build up atomic structures but also to prepare dynamic systems at the atomic scale.” Molinàs also speculates that, at low enough temperatures, the system could be exploited to construct devices, perhaps by creating lines of vacancies that would have distinct electrical properties.
Brian Swartzentruber, of Sandia National Laboratory, says the technique is useful because the imaging allows a complete view of the local neighborhood near the vacancy, and “the neighborhood is changing,” he says. He adds that the experiment is easier than others that require heating samples in the microscope. “It’s nice to have a system where stuff starts to happen at room temperature.”