PULLMAN, Wash. — A team of experimental and theoretical researchers from Washington State University and Pacific Northwest National Laboratory have observed that gold displays unexpected properties when it exists in extremely small particles. The current issue of Science magazine carries an article describing their research.
“As we divide and subdivide a piece of metal, its properties do not change dramatically until we reach the nanometer scale (1 billionth of a meter),” said team leader Lai-Sheng Wang, physics professor at WSU Tri-Cities and affiliate senior chief scientist at PNNL in Richland. “As a metal particle’s size approaches the nanometer dimension, all of its properties change. The properties not only differ from those of the bulk material, but also show strong dependence on the particle’s size and shape. For example, a 60-atom carbon cluster, the famous buckyball, has the shape of a soccer ball with properties very different from either graphite or diamond, the other known forms of pure carbon.”
For their studies on gold, the team created pyramid-shaped, 20-atom gold clusters by focusing an intense laser beam onto a gold target to generate small amounts of gold vapor. The gold vapor was then condensed into a soup of clusters of various sizes in a high-pressure helium gas chamber. Negatively charged clusters were then selected using a mass spectrometer.
“We observed experimentally that the 20-atom gold clusters exhibit large energy gaps,” said Wang, “this means that a large amount of energy is required to involve their electrons in chemical reactions. This large energy gap has two implications. First, the 20-atom gold clusters would be very inert chemically. Second, materials made of the little gold pyramids would be an insulator or a semiconductor, even though bulk gold is known to be the best electrical conductor. Such a material also would not be ‘gold’ colored. In addition, the chemical inertness of the 20-atom gold clusters suggests that they also may be a good catalyst if dispersed on a surface. We hope that the chemical inertness of the gold pyramids will allow chemists to use them as potential building blocks to assemble new materials.”
The pyramidal structure was deduced by comparing the measured photoelectron spectrum with theoretical calculations carried out by PNNL scientist Jun Li. The research was carried out at the Department of Energy’s William R. Wiley Environmental Molecular Sciences Laboratory, a user facility located at PNNL, by WSU graduate student Xi Li and postdoctoral fellow Hua-Jin Zhai. The calculations were performed by supercomputers at the Molecular Science Computing Facility in EMSL.
Wang receives funding for his work from the National Science Foundation. His recent work on atomic clusters has led to the discovery of the first penta-atomic planar carbon molecules and the discovery of aromaticity in all-metal molecules. Other ongoing work in his group has opened a new field of physical chemistry related to solution phase species and multiply charged anions. His research has been reported in the following publications: Nature, Science, Physical Review Letters, Nano Letters, Journal of the American Chemical Society, Angewandte Chemie International Edition, Chemical and Chemical Engineering News and Science News.
Educated in the Peoples Republic of China and at the University of California, Berkeley, Wang joined the physics faculty at WSU in 1993 and holds a joint position between WSU and PNNL, where he is associated with the Chemical Structure and Dynamics Department of the Chemical Sciences Division. He was named the WSU Westinghouse Distinguished Professor in Materials Science and Engineering and an Alfred P. Sloan Research Fellow in 1997. He received a National Science Foundation Career Award in 1996 and a National Science Foundation Creativity Award in 2001.