Hoping to one day shrink computer chips to vanishingly small proportions, researchers have been creating diodes and transistors on the scale of nanometers. They've even wired them into working circuits. Now, in the 21 November issue of Science, two teams report advances that could help propel nanocircuits from the realm of basic research toward a real-world technology.
Biophysicist Erez Braun and colleagues at the Technion-Israel Institute of Technology in Haifa describe a transistor assembled by biomolecules. In a series of intricate reactions, the team used a combination of DNA and proteins to direct the construction of tiny transistors--each just a few hundred nanometers in length--in which carbon nanotubes carried an electric current between two tiny gold metal pads that served as electrodes.
Meanwhile, a team led by Harvard University chemist Charles Lieber has created a scheme for feeding electrical impulses to specific locations in a nanocircuit, an essential step for carrying out complex computations. The experiment builds on years of work at Lieber's lab to construct circuits from an array of nanowires patterned in a "crossbar" array resembling the lines of a ticktacktoe board. Previous work had shown that each intersection of nanowires can serve as a transistor, in which an electric voltage applied to an "input" wire triggers an electric pulse to flow down a perpendicular "output" wire. But there's a hitch: Because a single input wire crosses several output wires, it can trigger multiple pulses simultaneously--not what is wanted for computations.
To make the firing more selective, Lieber's team designed a grid of nanowires that produced inactive transistors at each nanowire crossing, unless the wires at the junction were activated by a specific chemical reaction. Then, they used traditional lithography to direct a chemical reaction to specific crossbar intersections. That activated selected transistors and enabled them to steer impulses to their desired destinations.
Although critics have questioned nanoscientists' potential to turn out products soon (Science, 24 October, p.556), Cees Dekker, a biophysicist and molecular electronics expert at Delft University of Technology in the Netherlands, says the new studies underscore that basic research in molecular electronics remains vibrant. "Both papers together show the field is progressing. There are strategies to move towards connected networks [of devices]. That's the direction the field should take."