INDUSTRY

YORKTOWN HEIGHTS, NY -- IBM researchers today announced they have created and demonstrated the world's first logic-performing computer circuit within a single molecule, which may someday lead to a new class of smaller and faster computers that consume less power than today's machines. The IBM team made a "voltage inverter" -- one of the three fundamental logic circuits that are the basis for all of today's computers -- from a carbon nanotube, a tube-shaped molecule of carbon atoms that is 100,000 times thinner than a human hair. IBM scientists will present the achievement today at the 222nd National Meeting of the American Chemical Society being held in Chicago. This is the second major research breakthrough this year by IBM scientists using carbon nanotubes to make tiny electronic devices. In April, the same IBM team became the first to develop a ground breaking technique (Science, Vol. 292, Issue 5517, April 27, 2001) to produce arrays of carbon nanotube transistors, bypassing the need to meticulously separate metallic and semiconducting nanotubes. The team used these nanotube transistors to make the circuit revealed today. "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 Dr. Phaedon Avouris, lead scientist on the project and manager of nanoscale science, IBM Research. "Such 'beyond silicon' nanotube electronics may then lead to unimagined progress in computing miniaturization and power." Building a Computer Circuit "Inverter" Out of Carbon Nanotubes The IBM scientists used nanotubes to make a "voltage inverter" circuit, also known as a "NOT" gate . They encoded the entire inverter logic function along the length of a single carbon nanotube, forming the world's first intra-molecular -- or single-molecule -- logic circuit. In the binary digital world of zeros and ones, a voltage inverter changes a '1' into a '0', and a ' 0' into a '1' inside computer chips. The processors at the heart of today' s computers are basically vast and intricate combinations of the NOT gate, with two other basic functions, "AND" and "OR" gates, which perform other computations. Voltage inverters typically comprise two types of transistors with different electronic properties - "n-type" (in which electrons carry the electrical current) and "p-type" (in which electron-deficient regions called "holes" carry the electrical current). All previous carbon nanotube transistors have been p-type only. These transistors, while fine for scientific studies, are not sufficient to build logic-performing computer circuits. Scientists at IBM and elsewhere have been able to alter the properties of nanotube transistors by adding atoms of another element, such as potassium, to the carbon nanotube. However, Avouris' team recently discovered a new, simpler way to convert p-type nanotube transistors into n-type transistors. They found that they could simply heat p-type transistors in a vacuum, which turns them into n-type transistors and that they could reverse this process by exposing the transistors to air. The team also discovered that in addition to converting an entire nanotube from p-type to n-type, they could also selectively convert part of a single nanotube to n-type, leaving the remaining part of the single nanotube p-type. The researchers used this process to build the world's first single-molecule logic circuit. More importantly, the output signal from IBM's new nanotube circuit is stronger than the input. This phenomenon, called "gain," is essential for assembling gates and other circuit elements into useful microprocessors. Circuits with a gain less than one are ultimately useless -- the electrical signal becomes so faint that it cannot be detected. Since IBM's nanotube circuit has a gain of 1.6, Avouris is hopeful that even more complex circuits could be made along single nanotubes. The IBM team is now working to create these more complex circuits, which is the next step toward molecular computers. In addition, the team is working to further improve the performance of individual nanotube transistors, and further integrate them into more complex circuits. The report on this work "Carbon nanotube inter- and intra-molecular logic gates" by Vincent Derycke, Richard Martel, Joerg Appenzeller and Phaedon Avouris of IBM's T.J. Watson Research Center in Yorktown Heights, N.Y. 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 online version is available at http://pubs.acs.org/nano.