ENGINEERING

YORKTOWN HEIGHTS, NY -- Researchers have created the first functional logic circuit within a single molecule, an achievement that could one day help to replace silicon in microchips. This is a significant step toward smaller, faster and less power-consuming computers, according to the researchers. The finding will be published in the August 26 Web edition of Nano Letters, a peer-reviewed journal of the American Chemical Society, the world’s largest scientific society. The researchers also will present their findings August 26 at the Society’s 222nd national meeting in Chicago. “We believe that carbon nanotubes are now the top candidate to replace silicon when current chip features just can’t be made any smaller, a physical barrier expected to occur in about 10 to 15 years,” said Phaedon Avouris, manager of Nanometer Scale Science and Technology at the IBM T.J. Watson Research Center in Yorktown Heights, N.Y. The new circuit works on a miniature scale, using a hollow carbon tube approximately 1.4 nanometers in diameter, or approximately 100,000 times thinner than a human hair. The researchers changed the nanotube’s electrical characteristics so that some sections would allow the flow of electrons (called n-type, or negative, sections), while other sections would allow the flow of electric current using positive entities on the nanotube called positive holes (also known as p-type, or positive, sections). The sections were turned into transistors that encode the “NOT” logic function along the length of the nanotube, Avouris said. The characteristics of the resulting circuit — its ability to propagate voltage, called gain — allows for more transistors to be placed along the tube to make more complex circuits. Both p- and n-type sections are needed to build a logic circuit. While working with their previously assembled p-type transistors, the IBM team discovered a very simple way of producing n-type transistors: simply heating a p-type transistor in a vacuum, Avouris said. In the future, the researchers plan to create more complex circuits and to further improve the performance of the individual transistors, he added. All information stored in a computer is made up of two digits — ones and zeros — which indicate, for example, whether a circuit is on (one) or off (zero). The circuit described in the research can switch the ones to zeros, and vice versa, according to Avouris. Changing from a one to a zero directs the computer to perform separate functions, telling it essentially to do one thing, not another. The circuit, therefore, is called a “NOT gate” in computer parlance, one of two fundamental combinatorial logic circuits that computers use to perform computations. Also known as a voltage inverter, the gate sends out the opposite voltage from the one it receives. Other logic circuits include the “AND” and “OR” gates, which perform other computations. The “NOT” gate, in combination with either the “AND” or the “OR” gates, can form all other logic circuits. All these functions are currently accomplished using silicon chips in modern computers. The research would help maintain Moore’s Law, a prediction that suggested computers typically double their capacity and number of circuits every two years. Computers will eventually reach a maximum capacity with silicon that cannot be overcome, forcing the need for new materials capable of adding smaller computer circuits to maintain the advancements in the future, Avouris said. — By Jonathan Lifland