Credit: © 2008 AAAS

The ability to manipulate individual atoms and molecules represents the ultimate limit in 'top-down' approaches to nanotechnology. The most famous example of single-atom manipulation was demonstrated in 1990, when two IBM researchers used a scanning tunnelling microscope to spell out the name of their company with 35 xenon atoms on a nickel surface. Now Oscar Custance of the National Institute of Materials Science in Tsukuba, and co-workers1 elsewhere in Japan, Spain and the Czech Republic have demonstrated a new variation on this theme with an atomic force microscope (AFM).

The interaction between an AFM tip and a surface can either be attractive or repulsive. In the past, Custance and co-workers have exploited attractive tip–surface interactions to create nanostructures on a surface, which they demonstrated by using tin atoms to spell out Sn — the atomic symbol for this element — on a germanium surface. Now they have exploited repulsive interactions between a silicon AFM tip and a monolayer of tin atoms on a silicon substrate to pattern the surface.

Whereas the approach based on attractive interactions involved using the AFM to move the atoms laterally on the surface, this latest work involves swapping a silicon atom at the end of the tip with a tin atom immediately below it on the surface. The ability of this 'vertical interchange' approach to produce complex patterns on a surface is demonstrated, as readers might have guessed, by writing the symbol Si with silicon atoms.