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Nanodevices Make Fresh Strides Toward Reality

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Science  21 Nov 2003:
Vol. 302, Issue 5649, pp. 1310
DOI: 10.1126/science.302.5649.1310

Nanoscientists have proven adept at turning tiny specks of semiconductors and metals into devices such as diodes and transistors and have even wired them into working circuits. But researchers must still vault several other daunting hurdles to compete with today's highly complex computer chips. Among them: finding ways to construct complex circuits without the aid of photolithography, the standard chip-patterning technology that doesn't work at the scale of individual molecules, and steering electronic impulses from large-scale wires down to particular nanoscale devices. Now teams report progress on both fronts.

On page 1380, biophysicist Erez Braun and colleagues at the Technion-Israel Institute of Technology in Haifa report using a combination of proteins and DNA to direct the synthesis of a carbon nanotube-based transistor, a success that could pave the way for complex circuitry to essentially build itself. Meanwhile, in another paper on page 1377, a team led by Harvard University chemist Charles Lieber reports creating a scheme for feeding electrical impulses to specific locations in a nanocircuit, an essential step for carrying out complex computation.

Although critics have questioned the field's near-term potential to turn out products (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.”

Braun, together with physics colleague Uri Sivan, students Kinneret Keren and Rotem Berman, and technician Evgeny Buchstab, wanted to employ biomolecules to assemble a working transistor. Their goal was to use a straw-shaped molecule called a carbon nanotube to carry an electric current between two metal electrodes. They coated nanotubes with streptavidin, a protein that forms a lock-and-key bond with another molecule called biotin. They then used an intricate series of reactions to create a chain of other proteins—capped with biotins—and a short piece of DNA to lasso and lash the nanotubes along the central region of a long DNA strand glued atop a silicon surface. Then, through another pair of reactions, the Technion researchers capped the ends of the long DNA molecules with tiny gold metal pads, which served as electrodes to carry electrical current into and out of the carbon nanotube. Finally, the team used electron beam lithography to pattern wires atop the silicon to connect to the metal pads.

The result was more than a dozen working nanotransistors, each just a few hundred nanometers in length and a fraction of the size of conventional transistors. “It's a fantastic demonstration,” Dekker says. Braun says the devices are still works in progress: The connections need improvement, and the e-beam technique used to pattern the wires is too slow to be a viable manufacturing technology. But he says his team is already tackling the problems and is trying to scale up the self-assembly technique to make more-complex circuits.

Lieber's team, meanwhile, set out to address a very different issue: ensuring that information in the form of electrical impulses can be fed into specific transistors in a nanocircuit, an essential step for carrying out computation. 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 where two nanowires cross can serve as a transistor, in which an electric potential applied to an “input” wire triggers a corresponding electric pulse in 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.

Lieber, together with his students Zhaohui Zhong and Yi Cui, postdoc Deli Wang, and Marc Bockrath, a physicist at the California Institute of Technology in Pasadena, set out to make the firing more selective. They 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, using traditional photolithography to direct a light-induced chemical reaction to specific crossbar intersections, they steered impulses to their desired destinations (see figure). The technique, Lieber adds, should also enable them to connect nanosized crossbar circuits to large wires that carry electrical pulses on and off computer chips. Together with the self-assembly technique, these results may give nanoelectronics researchers just the boost they need to begin moving molecular electronics from a basic science to a technology.

Choosy.

Chemical reactions at selected junctions control where current flows in a nanocircuit.

CREDITS: ADAPTED FROM Z. ZHONG ET AL.
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