Molecular distortion on a metal surface

Scientists in Germany and France have recently determined the precise structure of a large organic semiconductor molecule after it chemically binds to a metal surface. The organic–metallic interface is very important in science, especially in the fields of catalysis, biosensing, and molecular electronics, but large molecules are difficult to study because of their tortuous shapes and many internal degrees of freedom. The researchers began with a superclean silver surface in ultrahigh vacuum. When the planar molecule (called PTCDA) adsorbed onto the surface, it reacted chemically and became bound to the surface. Next, x rays from the European Synchrotron Radiation Facility in Grenoble, France, were used to create normal-incidence standing waves at the surface. Because atoms at the wave field’s antinodes exhibit more photoemission than those at nodes, the resulting atomic-scale ruler allowed the researchers to determine where the component parts of the molecule were relative to the nearby metal surface, and learn a bit about the bonds as well. Surprisingly, the normally flat molecule showed some bending, mostly because of the readiness of some oxygen atoms to unexpectedly bind to surface silver atoms. Another discovery was that the molecule forms two types of bonds with the surface. (A. Hauschild et al., Phys. Rev. Lett. 94, 036106, 2005.http://dx.doi.org/10.1103/PhysRevLett.94.036106 )