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Molecular abacus:  C60 on Copper

Computer designers have long sought the benefits of building computers with the tiniest features possible -- eventually even down to a few atoms or molecules. IBM researchers are moving ever closer to that world with pioneering work using a scanning tunnelling microscope, or STM.

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. 

Sticky Wickets
While Eigler works at exceedingly low temperatures so he can make particularly precise scientific measurements, scientists at IBM's Zurich Research Laboratory have succeeded in positioning a certain type of molecule at room temperature -- an environment where many atoms and molecules will just not stay put.

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that was slippery enough to be pushed around by the STM tip, but  sticky enough to remain in place after the tip was withdrawn.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.

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