Foresight Update 33
page 3
A publication of the Foresight Institute
 Room-Temperature Molecular Transistor
Uses Single Carbon Nanotube
by Jim Lewis
One of the hottest topics in nanotechnology research the past year has been the electronic properties of fullerene nanotubes. Such work was a major topic at the Fifth Foresight Conference on Molecular Nanotechnology (see "Track One: Carbon Nanotubes, Probes, and More" in Update 31 and "Media Watch" in Update 32). The "Recent Progress" column in Update 32 also reported experimental verification by two research groups of the fact that carbon nanotubes can be either metallic or semiconducting, as predicted by theory. The May 7, 1998, issue of Nature announced a major advance in molecular electronics by one of these groups, Cees Dekker and his students at Delft University of Technology in The Netherlands [SJ Tans, ARM Verschueren & C Dekker, "Room-temperature transistor based on a single carbon nanotube" 393:49-52]. This report demonstrates in principle that a single molecule can function as a transistor in a room temperature device.
The authors name their device a TUBEFET (single carbon nanotube field-effect transistor). It consists of a single semiconducting, single-wall carbon nanotube (about 1 nm in diameter) draped over two platinum electrodes spaced 400 nm apart on a SiO2 layer covering a silicon substrate, which serves as the gate electrode. Thus, although the switch is molecular, the other elements of the device are fabricated using current microtechnology so that the device as a whole is comparable in size to current silicon transistors. In their earlier paper, the authors measured nanotube properties at
4 K, but in this paper, the work was done at room temperature. As in the earlier paper, finding individual nanotubes that show the proper behavior is still an issue. Of more than 20 nanotubes measured in the current work, only five were semiconducting. These five tubes each behaved as a transistor when voltage was applied to the silicon gate, switching between conducting and insulating states, with the conductance of the single molecular switch varying over at least six orders of magnitude.
A Nature editorial in the same issue (pp. 15-17) states "This experiment is a first step in developing electronic devices based on semiconducting nanotubes. Other advances are likely to follow soon ... [depending] on how rapidly the techniques for fabricating, doping and manipulating nanotubes develop." The advance was also reported in Science News [May 9, 1998, 153:294] and in Chemical & Engineering News [May 11, 1998, p. 6]. C&EN quotes Jie Han of NASA Ames on the significance of this work: "It offers a novel routine to fabricate nanoscale molecular electronic devices." C&EN also offers a caveat from James Tour of the University of South Carolina: "It is a step toward truly molecular-scale systems, although at this point, it is a hybrid of molecular and solid-state materials that needs to be greatly reduced to take advantage of a truly molecular system. If you look at the gain--0.35 versus 100,000 in current transistors--we still have a long way to go."
Since practical molecular computers are still more than a few years away, perhaps the greatest significance of the current work is the conclusion the authors reach in their final paragraph:
"In discussions of the fundamental limits of integrated circuit dimensions, warnings are often expressed that at some point radically new device structures should be used because of dominant quantum mechanical effects. Such new device concepts have been explored in the fields of mesoscopic physics and in molecular electronics. It is quite striking that it seems possible to describe qualitatively the ultra-small TUBEFET device, which is based on a single nanotube molecule, by the same semiclassical models that are used for devices in today's computer industry."
Media Watch 33
Nanotechnology arrived on the American Chemical Society scene at the ACS national meeting in Dallas in late March and early April. Chemical & Engineering News, a publication of the ACS, devoted major coverage to the topic in its April 20 issue. (See a report on the conference in this issue.) "For nanotechnology, small is beautiful," wrote C&EN staffer A. Maureen Rouhi. "Nanotechnology--the creation and use of materials and processes on the nanometer scale with atomic precision--promises a fantastic world populated by the likes of tiny machines navigating blood vessels to battle cancer cells or robotic proteins assembling materials atom by atom. A sense of how close--or distant--this future is to reality could be gleaned from the Physical Chemistry Division symposium on device applications of nanoscale materials...Researchers are forging ahead to understand the fundamental properties of materials on the nanometer scale and to create nano-materials with unique properties. However, nanoscale devices built with atomic precision are not yet at hand. Even so, some business already are using nano-materials, but not yet quite in the truly nanotechnological sense."
The story extensively discussed a presentation by University of South Carolina chemistry professor James M. Tour to create "a world of nonmetallic wires based on molecules with conjugated carbon-carbon bonds." Tour's presentation is covered in detail in the ACS article beginning on page 1 of this newsletter.
Foresight Senior Associate Jim Von Ehr also was quoted regarding the goals of Zyvex, his year-old startup company whose goal is the ability to "make precision machinery that can do chemistry one atom or molecule at a time." He is quoted, "Our goal is to give manufacturers better tools so that they can manufacture just about anything at the cost of only raw materials and energy."
Proton motors--molecular scale rotary motors that bacteria use to swim--are moving from academic journals into the general media. Salt Lake Tribune Science Editor Lee Siegel wrote a solid report on research into the field by University of Utah researcher David Blair, complete with an illustration of a conceptual proton motor several one-millionths of an inch in size.
"Blair said some nanotechnology experts have suggested microscopic motors might be used 'in very tiny submarines that would go into a person's body and administer some treatment or drug,'" Siegel wrote. He quotes biologists from Texas A&M and Harvard University to round out his story.
Blair may be accused of E. coli abuse, however. "To measure torque generated by bacterial motors, Blair puts bacteria in a blender to chop off most of the length of each flagellum, leaving a stub. Then he uses antibodies to glue the stub to [a] glass slide. With the stub of a propeller tied down, the bacterium spins at the same speed its motor rotates," Siegel wrote. Other researchers have found that flagella are made of 20 types of protein, and are controlled and operated by another 30 types. One protein has been identified as most involved in motor rotation, and two others make each of the motor's eight stators. Blair has found evidence that protons attached to specific sites in each stator, changing the shape of the stator, which somehow makes the rotor turn, Siegel reported. Researchers haven't yet discovered how, Siegal reported. (Update 32 reported on Japanese research into proton motors.)
Discovery Magazine reported in January on work by researchers at the National Renewable Energy Laboratory in Golden, CO, who have managed to use carbon nanotubes as miniature storage tanks for hydrogen gas. "This seemingly esoteric stunt may have a very practical payoff: it could provide the means to safely store clean-burning hydrogen gas in a new generation of non-polluting vehicles. Says materials scientist Michael Heben, who worked on the project: 'This points to the possibility of high-density hydrogen storage at room temperatures,'" the magazine reported. The Colorado researchers found that when hydrogen gas flows over carbon nanotubes, the tubes soak up the gas like sponges. They estimate that a gas tank-size block of carbon tubes filled with hydrogen could power a typical car for 200 miles.
Self-Assembling Molecules have made it all the way to the business press. Business Week magazine reported in its April 6 issue on work by two University of Rochester researchers on "the largest synthetic structures ever made by self-assembly." Reporting on work published March 20 in Science, the business publication used an inappropriate analogy for self assembly--"much as eggs and sperm self-assemble into human beings"--but otherwise described the work in a straightforward manner. "The molecules form a variety of shapes, including spheres, cylinders, rings and disks--all of which have a fluorescent glow. The researchers say the structures could be utilized as drug-delivery vehicles, adhesives, lasers, sensors, and for other industrial applications," the magazine said.
Another self-assembly advance was described in Discovery in January, writing about a "layer cake of mushrooms" at the molecular level created by University of Illinois chemist Samuel Stupp. His work involved molecules called rodcoils, short chains of carbon with one end a rigid rod while the other is a floppy ribbon. These rodcoils self-assemble into thick bundles with the floppy ends forming a spongy dome over the bundle. The bundles stack themselves into "trillions of mushrooms nested in neat layers," the magazine reported. "Stupp believes this may be a step toward using self-assembling molecules to perform complex tasks," the article said. "He has already begun testing the mushrooms as a spray-on deicing chemical for aircraft. The stem end of the mushroom would stick to the airplane, while the other would prevent the formation of ice crystals."
Focus, a British publication providing popular coverage of science and technology (somewhat akin to Discover in the U.S.) introduced its readers to the possibilities of nanotechnology in an article entitled "Nanotechnology...the Diamond Age." Writer Emma Bayley covered all the key points: nature as Proof of Concept, self-replication, the desirability of diamondoid materials because of the strength of carbon-carbon bonds. And she understood the implications of nanotechnology well enough to declare, "even sci-fi movies have made a mess of trying to depict life in a Diamond Age."
The story relies on good authorities; it quotes Foresight Institute Executive Director Chris Peterson, Xerox Corp.'s Ralph Merkle, and a reference to Engines of Creation. The story carries a sidebar with a timeline prediction of impacts of nanotechnology, cumulating in 2030 with "biodegradable medical devices that are small enough to fit inside a cell. There is no limit to the molecular or structural defects that can be repaired nor the viruses or bacteria that can be destroyed. We can live indefinitely."
Electronic Engineering Times, a weekly publication with a circulation of more than 150,000, surveyed nanotechnology-related Web sites in its March 30 issue, starting at Foresight's web site, and referring to other sites in the field, including Mitre Corp. www.mitre.org/research/nanotech, mostly devoted to electronics-related nanofabrication; a number of university sites, IBM's gallery of atomic-scale images www.almaden.ibm.com/vis/stm/gallery.html, and scenarios of nanotechnology futures www.wired.com/wired/scenarios/museum.html. However, the overview ignored such basic and content-rich sites as Ralph Merkle's meta-site http://sandbox.xerox.com/nano/ and the NASA Ames site http://science.nas.nasa.gov/Groups/Nanotechnology/. Fortunately, links to those and other sites are available from within the Foresight Institute's pages.
The Futurist, a newsletter published by the consulting firm of Coates & Jarratt, Inc., takes the view that "Nanotechnology is moving from science fiction to science fact." In its June-July 1998 issue, the newsletter opined that "although ...most practical objects are still decades away, increasingly complex experiments show progress toward fabricating objects at the molecular level." The article discusses advances in carbon nanotube research, though it focuses upon the material strength of nanotubes rather than their electrical properties, which are drawing increasing research attention. The story concludes, "More progress reports will come in November at the next Foresight Conference, an annual symposium primarily sponsored by the Foresight Institute, a think tank based in Palo Alto, CA."
From Foresight Update 33, originally
published 30 May 1998.
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