Molecular Abacus

Zurich/Switzerland, November 13, 1996 -- Scientists at the IBM Research Division's Zurich laboratory have built an abacus with individual molecules as beads with a diameter of less than one nanometer, one millionth of a millimeter.

The world's smallest abacus will hardly be found at a trade fair in the Far East, where calculators of this simple kind are still used by dealers, because the "finger" required to move beads as tiny as individual molecules is the ultra fine tip of a scanning tunneling microscope (STM) - a needle of conical shape terminating in a single atom at the very tip. The STM also makes the result of a "calculation" visible when operated in imaging mode.

IBM scientists succeeded in forming 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 (see image).

"We have made significant progress in handling objects and creating functional units on the nanometer scale at room temperature", says James K. Gimzewski, leader of the nano science project at the Zurich Research Laboratory. "Our work demonstrates a further step in the new and fascinating field of 'nano-engineering', where solid-state physics and chemistry merge. We may be able to assemble more complex structures from the bottom up, as nature does, molecule by molecule, and thus break ground for entirely new fabrication technologies with a broad range of applications."

The beads used in the abacus experiment are the amazing soccer ball-like molecules formed by 60 carbon atoms (C60), the discoverers of which were recently announced as the recipients of the 1996 Nobel Prize in chemistry. These molecules are also known as buckminsterfullerenes or "bucky balls" , named after the American architect Buckminster Fuller, who invented the geodesic dome using the same pattern of hexagons and pentagons.

The researchers who have long been studying the properties and behavior of individual atoms and molecules and who have now realized the nano-abacus are Maria Teresa Cuberes, James K. Gimzewski, and Reto R. Schlittler at IBM's Zurich Research Laboratory. The effort is part of the "PRONANO" (processing on the nanometer scale) project sponsored by the Swiss Federal Office of Education tion and Science within the European Strategic Program for Research in Information Technology (ESPRIT) of the European Union. The scientific report on the subject has been published in Applied Physics Letters, Volume 69, Number 20 (p. 3016), November 11, 1996.

Sequence (movie) of STM images of the ten molecules that were moved precisely along a step one atom high on a copper surface, representing the numbers from zero to ten. The molecules are moved similar to the beads of an abacus.

Fig 1. The researchers (l-r) Teresa Cuberes, James Gimzewski, and Reto Schlittler, with a model of a molecular abacus and of a C60 molecule, which has the exact pattern of a soccer ball. An actual C60 molecule, of course, is much smaller. For comparison, if the molecule had the size of the model shown here, the STM tip with which it is moved would be larger than the Eiffel tower in Paris.

Fig 2. Molecular mechanics calculations resulted in this animation of a STM tip pushing a C60 molecule along asingle atomic step on copper. The step has a one atom kink in it. For details see H.Tang et al in Surface Science 386 (1997) 115-123.

Fig 3. Sequence of images that illustrate the positioning of individual molecules: a group of six molecules (1) is first disordered (2) by cruising through the group with the STM tip. Subsequently, one molecule after the other is pushed by the tip in a precisely controlled manner (2 - 5). This allows a ring to be formed (6) that would not normally be found in nature. Note that the process does not interfere with other molecules visible at the top of the images.

Fig 4. The four "legs" (yellow, at right) of the porphyrin-based molecule are easily identified by STM images. Computer simulation of the pushing process (left) revealed that the molecule would "walk" with slip-stick-like steps of its legs.