San Jose, CA, USA - 24 Oct 2002: IBM researchers have built and operated the world's smallest working computer circuits using an innovative new approach in which individual molecules move across an atomic surface like toppling dominoes.
The new "molecule cascade" technique enabled the IBM scientists to make working digital-logic elements some 260,000 times smaller than those used in today's most advanced semiconductor chips.
The circuits were made by creating a precise pattern of carbon monoxide molecules on a copper surface. Moving a single molecule initiates a cascade of molecule motions, just as toppling a single domino can cause a large pattern to fall in sequence. The scientists then designed and created tiny structures that demonstrated the fundamental digital-logic OR and AND functions, data storage and retrieval, and the "wiring" necessary to connect them into functioning computing circuitry.
The most complex circuit they built -- a 12 x 17-nanometer three-input sorter -- is so small that 190 billion could fit atop a standard pencil-top eraser 7mm (about 1/4-inch) in diameter. A nanometer is a billionth of a meter; the length of five to 10 atoms in a line.
"This is a milestone in the quest for nanometer-scale computer circuitry," said Andreas Heinrich, a physicist at IBM's Almaden Research Center in San Jose, Calif., and one of the lead authors of the research article published in today's online edition of Science Magazine, Science Express. "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."
"Molecule cascades show how we are learning to harness the properties of very small structures," added IBM Fellow Don Eigler. "I was amazed at how rapidly we progressed from initial discovery to design and operation of functional circuitry."
This molecule cascade and the quantum mirage that Eigler and colleagues discovered two years ago are intriguing examples of novel nanoscale science and information-processing approaches that also yield new insights in the properties and interactions of atoms, molecules and surfaces.
Heinrich, Eigler and colleagues Christopher Lutz and Jay Gupta are continuing their exploratory research to find additional nanometer-scale computing systems based on the cascade mechanism.
IBM's molecule cascade 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, just as with toppling dominoes. The metastability is due to the weak repulsion between carbon monoxide molecules placed only one lattice spacing apart.
This situation is analogous 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, Heinrich and Lutz learned 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.
What enables computation is that each cascade carries a single bit of information. By analogy, a toppled domino can be thought of as a logical "1," and a untoppled domino can be thought of as a logical "0." Similarly, a cascaded or non-cascaded molecular array can represent a logical "1" or "0," respectively.
The logic AND and OR operations and other features needed for complex circuits are created by cleverly designed 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).
These molecule cascades are currently assembled by moving one molecule at a time using an ultra-high-vacuum, low-temperature scanning tunneling microscope (STM). It takes several hours to set up the most complicated cascades. Since there is no reset mechanism, these molecule cascades can only perform a calculation once. While these initial cascades rely on the motion of a molecule, Eigler envisions that it should be possible to make nanometer-scale cascades using other fundamental interactions, such as electron spin. Such cascades may also be resettable, allowing repeated calculations, similar to ordinary computer circuitry.
Other features of molecule cascades include:
Energy: An intriguing aspect of molecule cascades is their minuscule energy consumption. The three-input sorter is estimated to expend only 1 electron-volt of energy -- 100,000 times less than the equivalent semiconductor circuit.
Temperature: IBM's initial cascades were created and operated a 4-10 degrees above absolute zero. In their paper, the scientists show how cascades operate faster at higher temperatures.
Precision construction: IBM's molecule cascades were created by positioning carbon monoxide molecules one at a time, a lengthy process. Other cascade mechanisms may not need to be built so precisely.
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