Nanospheres on a Silver Platter

Illinois researchers in the Department of Materials Science and Engineering--post-doctoral fellow LinLin Wang and professor Duane Johnson--and their collaborators, have made an important breakthrough in understanding how "Bucky balls" modify metal surfaces to create there own attachment points and self-assemble into perfect single layers, opening the way to their use in nanoscale electronic devices.

Ever since the 1985 discovery of C60 Buckminsterfullerene--with its perfect shape and high symmetry (60 atoms of carbon arranged in the pattern of a soccer ball, having 20 hexagons and 12 pentagons)--these "Bucky balls" have fascinated scientists, physicists, and chemists alike. Due to its high symmetry and conjugated bond structure, the electronic properties of C60 are very unusual, and there is a massive research effort toward integrating it into molecular-scale electronic devices.
 
Only in recent months have scientists put together the theoretical models with the experimental images of the surface to understand how the molecule forms bonds with a metal substrate, such as silver, which is commonly used as an electrode material. The arrangement of silver and carbon atoms at such an interface affect the strength and stability of the metal-molecule bond as well as its electronic properties. The silver electrons are usually too low in energy to have significant intermingling with electrons in organic molecules, and prevents organic molecules from forming strong bonds with silver surfaces. Hence, silver is commonly considered a relatively inert (noble) metal that only forms strong bonds with very corrosive atoms such as oxygen, sulfur, or chlorine.

However, C60 does form bonds with silver surfaces and this mystery has now been explained. It turns out that C60 digs a hole of exactly one atom in the silver surface and settles into the hole by binding to the six remaining atoms around the vacancy. This previously unimagined process has been discovery by performing quantum mechanical calculations for the C60 molecules on a silver surface and comparing to experimental images.
 
The theoretical calculation showed that the binding strength increases dramatically at such a hole, so much so that the C60 atom actually causes the hole to be created. Because the hole is beneath the large Bucky ball, it has been previously difficult to identify. Now, calculations show that this data does match this arrangement and even predicts that such self-adhering processes may be present between C60 and other noble metal surfaces, leading the way to their use in molecular-scale electronic devices.
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The work was described in a "Viewpoint in Physics" article in Nature Physics 2, 64 (2009), by Georg Held. The research was performed by H. I. Li, K. Pussi, K. J. Hanna, L.-L.Wang, D. D. Johnson, H.-P. Cheng, H. Shin, S. Curtarolo,W. Moritz, J. A. Smerdon, R. McGrath and R. D. Diehl, "Surface Geometry of C60 on Ag(111)," Phys. Rev. Lett. 103, 056101 (2009) - Published July 27, 2009.

Contact: Duane D. Johnson, Department of Materials Science & Engineering, 217/265-0319.

If you have any questions about the College of Engineering, or other story ideas, contact Rick Kubetz, Engineering Communications Office, 217/244-7716, editor.