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CARBON NANOTUBE COMPUTER CIRCUITS
Novel processing and microfabrication lead to first single-molecule logic gate
Researchers at IBM have used a single carbon nanotube bundle to construct elementary computing circuits known as logic gates. The work pushes the feasibility of using individual molecules to build future generations of sophisticated microelectronic devices to new heights.
The work was announced last week during the American Chemical Society meeting in Chicago, at a molecular electronics symposium sponsored by the Division of Physical Chemistry. Details of the work were simultaneously published online [Nano Letters, published Aug. 26 ASAP, http://pubs.acs.org/journals/nalefd/index.html].
"Carbon nanotubes are now the top candidate to replace silicon when current chip features just can't be made any smaller," remarked Phaedon Avouris, a coauthor of the study and manager of nanometer-scale science and technology at IBM's T. J. Watson Research Center, Yorktown Heights, N.Y.
||SIGNALING IBM researchers use a single carbon nanotube bundle (blue) to fashion a logic circuit on a substrate patterned with three gold electrodes (yellow). By selectively removing part of a protective polymer coat (PMMA), the group is able to form the needed p-type and n-type transistors within the single nanotube bundle.
Logic gates are basic building blocks of digital electronic systems. They produce electrical output signals based on the state of one or more input signals. Complex arrays of three types of logic gates--AND, OR, and NOT--lie at the heart of today's digital processors.
The IBM team, which includes postdoctoral researcher Vincent Derycke, research staff members Richard Martel and Joerg Appenzeller, and Avouris, used carbon nanotubes to construct NOT gates, also known as voltage inverters because the devices return output signals that are the opposite of their input signals. A 0 signal coming in will be a 1 going out, for example.
The principal challenge in constructing NOT gates out of nanotubes, Avouris explained, is that invariably, without special processing, transistors fashioned from nanotubes are p-type--that is, they conduct positive charge carriers (holes). But NOT gates require n-type transistors, the type that conduct negative charge carriers (electrons), as well as p-type.
Some researchers have demonstrated recently that doping nanotubes with electropositive elements such as potassium is a viable method for preparing n-type nanotube transistors. Now the IBM team has discovered another way to do it. Simply heating (annealing) p-type nanotube transistors in vacuum converts p-type into n-type, they reported.
With methods in hand for preparing both types of nanotube transistors, the IBM group demonstrated that the nanometer-wide carbon tubes can be assembled into elementary computer circuits.
In one case, the researchers deposited two p-type nanotube transistors onto a substrate that was wired up to serve as a NOT gate. Then they coated one of the nanotubes with a protective polymer and vacuum annealed the entire assembly, converting both nanotubes to the n-type.
The team then exposed the device to low-pressure oxygen, causing only the unprotected portion to revert to p-type. The result was a fully functioning voltage inverter fashioned out of separate p- and n-type nanotube transistors.
Carrying the work to the next level of integration, the IBM scientists deposited a single-walled carbon nanotube bundle on an oxidized silicon substrate that had been prepatterned with three gold electrodes. Then they coated the entire assembly with a protective polymer. Finally, the group doped a region of the nanotube with potassium through a small window in the polymer that had been opened using electron-beam lithography. The procedure yielded a voltage inverter fabricated from a single nanotube bundle.
At the ACS meeting, A. Paul Alivisatos, a chemistry professor at the University of California, Berkeley, described the work as "a very significant advance," noting that unlike earlier work in this area, the IBM study convincingly demonstrates that it is possible to construct single-molecule logic circuits.
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