The molecule cascade technique, in which individual molecules move across an atomic surface like toppling dominoes, enabled IBM scientists to construct working digital-logic elements 260,000 times smaller than those used in today's most advanced semiconductor chips.
The findings were published today on the Science Web site.
The technique works because carbon monoxide molecules can be arranged on a copper surface in an energetically metastable configuration that can be triggered to cascade into a lower energy configuration, similar to toppling dominoes. The metastability is due to the weak repulsion between carbon monoxide molecules placed only one lattice spacing apart.
IBM scientists compare the technique to placing tennis balls next to each other in an egg carton. Since the tennis balls are slightly larger than the lattice spacing of the carton, they push against each other and can't nestle down into the hollows of the carton as deeply as they could if they were more widely separated.
Just as placing three tennis balls in a row of an egg carton is unstable, Andreas Heinrich, a physicist at IBM's Almaden Research Center in San Jose, Calif. and his colleague Christopher Lutz discovered that a triad of carbon monoxide molecules arranged in a chevron-shaped pattern on the copper surface would spontaneously rearrange by the outward motion of the central molecule. They then designed ways to link pairs of molecules so the rearrangement of an initial chevron formed a new chevron, and so on, in a cascade of molecular motion.
With respect to the computing done by the circuit, think of a cascaded molecular array as a 1, and a non-cascaded molecular array as a 0 ? the bits that make up all logic computing. The logic AND and OR operations and other features needed for complex circuits are created by intersections of two cascades. Heinrich and Lutz designed molecular arrangements that acted as "crossovers" (allowing two cascade paths to cross over each other) and "fanouts" (splitting one cascade into two or more paths).
The most complex circuit built using the technique was a 12x17-nanometer three-input sorter. It is so small that 190 billion of the circuits could fit atop a standard pencil-top eraser 7 millimeters in diameter.
"This is a milestone in the quest for nanometer-scale computer circuitry," Heinrich said. "The molecule cascade is not only a novel way to do computation, but it is also the first time all of the components necessary for nanoscale computation have been constructed, connected and then made to compute. It is way smaller than any operating circuits made to date."
As a sign of how far this technology has to go before it can be used commercially, IBM noted that the molecule cascades are currently assembled by moving one molecule at a time using an ultra-high-vacuum, low-temperature scanning tunneling microscope. That means it takes several hours to set up the most complicated cascades, and because there is no reset mechanism, these molecule cascades can only perform a calculation once.