Taking Superatoms to a New Level

Materials made of clusters of elemental atoms called "superatoms" could one day be used as inexpensive catalysts for chemical processing, environmental remediation, or in new sources of energy, according to recent work by professor of chemistry and physics A. Welford Castleman and his team.

Graphical displays of energy peaks were similar between a nickel atom and a titanium-monoxide molecule
Left: Graphical displays of energy peaks were similar between a nickel atom and a titanium-monoxide molecule. Right: Bright spots in the images, which correspond to the energy of the electrons emitted during their removal from the atoms' outer shells, appeared to be simiarl between a nickel atom (right, top) and a titanium-monoxide molecule (right, bottom). Credit: Castleman lab, Penn State

Superatoms, which share their free electrons to fill out their available orbitals, mimic certain properties of single atoms on the periodic table. The researchers discovered that the electronic signatures of various superatoms were similar to those of specific single atoms, and that the atoms they resembled could be predicted by looking at the periodic table.

"For example," Castleman said, "by looking at the periodic table, you can predict that titanium monoxide will be a superatom of nickel. Simply start at titanium, which has four outer-shell electrons, and move six elements to the right, because atomic oxygen possesses six outer-shell electrons. The element you end up on is nickel, whose 10 outer-shell electrons make it isoelectronic with the 10 outer-shell electron molecule resulting from the combination of titanium and oxygen. We thought this finding must be a curious coincidence, so we tried it with other atoms and we found that a pattern emerged."

Using a technique called photoelectron imaging spectroscopy, the researchers measured the energy required to remove electrons from atoms and molecules while taking snapshots of the electron detachment event. They found that the binding energies of the superatoms were the same as those of their corresponding atoms. In addition to the nickel/titanium monoxide pair, the team also discovered the zirconium monoxide/palladium and tungsten carbide/platinum pairs. Replacing expensive palladium or platinum by their cheap counterparts could offer dramatic savings, in addition to the scientific value of the research.

The findings were reported in the 28 December online issue of the Proceedings of the National Academy of Sciences. The researchers are A. Welford Castleman, Eberly Distinguished Chair in Science and Evan Pugh Professor in the Departments of Chemistry and Physics, Samuel Peppernick, a former Penn State graduate student who now is a postdoctoral researcher at the Pacific Northwest National Laboratory, and Dasitha Gunaratne, a Penn State graduate student.

This snapshot is a shortened version of the news release from the Eberly College of Science http://www.science.psu.edu/news-and-events/2009-news/Castleman12-2009.htm.

This article was featured in Focus on Materials - Winter 2010.