Registering visitors to metal surfaces.

The place a foreign atom or molecule occupies on a surface can have an important effect on chemical processes. Now, researchers have a new tool for determining whether such a visitor sits on top of the substrate atoms or in the crevices between them.

Gerhard Meyer and his colleagues at the Free University of Berlin use the needle tip of a scanning tunneling microscope to position a copper (Cu) atom alongside several carbon monoxide (CO) molecules on a copper surface (upper image). The copper atom settles on the surface in a well-understood way, enabling the researchers to determine its position accurately. The scientists then use the array of surface atoms to define a reference grid and precisely locate the CO molecules.

The position of a single CO molecule, in turn, establishes a reference for locating other atoms or molecules. The lower illustration shows how a lead (Pb) atom and an ethylene molecule (C2H4) might nestle among the orderly surface rows of copper. The darkest shaded spheres signify the deepest copper atoms. The researchers describe their technique in the Sept. 2 Physical Review Letters.