Molecular abacus: C60 on Copper
The STM can be used not only for imaging surfaces with atomic resolutions, but also for positioning atoms and molecules on such surfaces. In an STM Gallery Don Eigler, an IBM Fellow, displays examples of his work -- images of various atoms arranged on different surfaces in both artful and scientifically illuminating patterns. Among other achievements, Eigler's team has shown how to visualize quantum behavior on a metal surface and also how individual magnetic atoms can disrupt a material's superconductivity over short distances.
The challenge was to find a molecule that was slippery enough to be pushed around by the STM tip, but sticky enough to remain in place after the tip was withdrawn. The chemical bonds within the molecule also had to resist being broken or altered as the molecule is pushed. The Zurich researchers focused on an organic molecule having a total of 173 atoms, including at its core a stable ring of atoms known as a porphyrin. Computer simulation revealed that when pushed by the STM tip, this molecule "walks" in uncorrelated steps and exhibits exactly the desired degree of stickiness.
The Zurich researchers have used this technique to build an abacus with individual molecules as beads with a diameter of less than one nanometer, one millionth of a millimeter. Using the STM, they form stable rows of ten molecules along steps just one atom high on a copper surface. These steps act as "rails", similar to the earliest form of the abacus, which had grooves instead of rods to keep the beads in line. Individual molecules were then approached by the STM tip and pushed back and forth in a precisely controlled way to count from 0 to 10.