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Scientists Demonstrate New Way to Control
UPTON, NY — Using a low-temperature scanning tunneling microscope (STM) to selectively “tweak” the vibrations of individual molecules, scientists have demonstrated a new way to directly influence the outcome of chemical reactions. The ability to exert such control may one day allow scientists to eliminate unwanted byproducts or selectively produce end products with potential commercial value.
Zhen Song, now a research associate at the U.S. Department of Energy’s Brookhaven National Laboratory, used the technique to investigate the desorption of ammonia molecules from a copper surface while working with collaborators* at the Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, Germany. The work is described in the May 29, 2003, issue of Nature.
By detecting and controlling the tunneling electrons running between tip and sample, STM techniques enable scientists not only to measure the structure of materials on an atomic level, but also to manipulate molecules individually on the substrate.
“We selected a chemisorbed ammonia molecule on a copper surface under the microscope and used the tip of the STM to excite vibrations of the molecule,” said Song. “We found that the motion of the molecule can be controlled by tuning the parameters of the tunneling electrons: the electronic current and energy.”
Above a certain threshold energy, the tunneling electrons induced one mode of molecular vibration that resulted in a movement of the ammonia molecules to new positions on the copper surface. Below the threshold, the electrons induced a different mode of vibration that allowed the ammonia molecules to completely disassociate from the copper.
“We are able to select a particular reaction pathway by adjusting the electronic tunneling current and energy,” Song said.
This is the first example of using STM in mode-selective chemistry, a field that has previously been dominated by laser techniques. Using STM, the study of the reaction mechanism can be achieved with very low power irradiation, and the monitoring of the reaction is limited to a single molecule. This approach is complementary to the conventional laser techniques used in the study of mode-selective chemistry.
“It would be interesting to extend this methodology to more complex processes, for example, by searching for strategies of controlling and enhancing reactivity at surfaces through the discovery of new reaction pathways that are inaccessible via classical ‘thermal’ chemistry,” said Song.
* This research was a collaboration among scientists at the Fritz-Haber-Institut, Germany, the Consejo Superior de Investigaciones Científicas, Spain, the Universit Paul Sabatier, France, and Brookhaven National Laboratory.