Individual Surface Atoms Identified

YOU ARE BLINDFOLDED and presented with a tray filled with "marbles" of three different materials—glass, styrofoam, and gelatin. Could you identify which marble is which simply by touch? Most people could do this quite easily.

The nanoscale analog of this task—identifying different atoms on a surface—is much more difficult, but it now has been accomplished for the first time at room temperature, thanks to the exquisite touch of the atomic force microscope (AFM).

Scientists use the AFM to image and manipulate atoms and structures on a variety of surfaces. For imaging, the surface is scanned with the microscope's sharp, vibrating tip (a microscopic inverted pyramid), which is attached to a flexible cantilever. The atom at the apex of the tip "senses" individual atoms on the underlying surface when it forms incipient chemical bonds with them. Because these chemical interactions subtly alter the tip's vibration frequency, they can be detected and mapped.

Physicist Óscar Custance at Osaka University's Graduate School of Engineering, in Japan, and his colleagues now have exploited the short-range chemical forces acting between the AFM tip and surface atoms to distinguish between silicon, tin, and lead atoms on an alloy surface (Nature 2007, 446, 64).

The trick is to first measure these forces precisely for each type of atom expected in the sample. The team finds that any given AFM tip will interact most strongly with silicon atoms and, relative to the tip-silicon interaction, the tip will interact 23% and 41% less strongly with tin and lead atoms, respectively. Thus, each type of atom has a distinct force "fingerprint" that can subsequently be used to identify it. Such discrimination between different atoms is impossible by simply imaging the atoms, the researchers point out.

Scientists previously have used microscopes like the AFM to identify chemical species on surfaces, but only at cryogenic temperatures, not room temperature.

Alexander Shluger, a physicist at the London Centre for Nanotechnology and University College London, says the work could have a major impact in surface science, catalysis, and other areas and will inspire other groups to try the same approach.