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CARBON NANOTUBE ACHILLES' HEEL Electronic properties ofnanotubes are found to be very sensitive to atmospheric oxygen
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or years, theorists have been saying that the electronic properties of a single-walled carbon nanotube (SWNT) depend on its diameter and chirality (the arrangement of carbon hexagons making up the tube wall). But evidence is mounting that the chemical environment of the nanotube can exert a crucial influence. The latest evidence comes from physics professor Alex Zettl's group at the University of California, Berkeley, and Lawrence Berkeley National Laboratory. Zettl and coworkers have found that the electronic properties of SWNTs are extremely sensitive to oxygen in the air [S«*»c*, 287,1801 (2000)]. The electronic characteristics of nanotubes "can be reversibly 'tuned' by surprisingly small concentrations of adsorbed gases," they write, and an apparently semiconducting nanotube can be converted into one that is apparently metallic (conducting) through such exposure. The results demonstrate that nanotubes could be used as sensitive chemical sensors for gases such as oxygen. But at the same time, they also indicate that many of the electronic properties that researchers measured and thought were intrinsic to nanotubes may instead be due to extrinsic factors such as gas exposure.
Zettl shows a model of a nanotube.
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The factors influencing nanotube behavior are "a lot more complicated than we thought," says graduate student Keith Bradley, a coauthor on the Zettl paper along with Philip G. Collins and Masa Ishigami. "Everyone's going to have to be more careful" when measuring electronic proper©1999 Keith Bradley
Zettl's group found that oxygen molecules (green" on artist's rendering) adsorbed on carbon nanotubes cause changes in electronic properties.
ties of nanotubes and interpreting the results, Bradley points out. And many previous findings may have to be reinterpreted, he adds. SWNTs have been touted for their immense promise as components of future nanocomputers and other nanoelectronic devices. But the fabrication of such nanotube-based devices may be complicated by their sensitivity to air, Bradley says. 'This sounds like C60 all over again," comments fullerene/nanotube researcher Robert C. Haddon, a professor of chemistry and physics at the University of Kentucky, Lexington. 'The resistivity of thin films of C60 increases substantially
on exposure to oxygen," he tells C&EN. If it weren't for that, he says, scientists might be closer to having C60 thin-film transistors. One of the interesting aspects of Zettl's results is that they reveal the opposite behavior for nanotubes: Exposure to oxygen decreases nanotube resistance by 10 to 15%, making them better conductors. The Berkeley results are consistent with a recent report from researchers at Stanford University who found that the conductance of a semiconducting SWNT can be dramatically increased or decreased by exposure to gaseous nitrogen dioxide or ammonia (C&EN, Jan. 31, page 7). In their own experiments, the Berkeley researchers exposed bulk SWNT samples alternately to vacuum and air, or to pure nitrogen and pure oxygen, and found that the resistance of the nanotubes switched between two levels in step with the changing environment. The researchers also found that switching the nanotubes' environment from oxygen to vacuum and then back to oxygen caused the thermoelectric power (TEP) to switch signs—from positive to negative and then back to positive. TEP is a measure of the voltage that is induced in a nanotube mat exposed to a temperature gradient. And the sign of TEP is positive or negative depending on whether the charge carriers are holes or electrons, respectively. Thus, switching the nanotubes' environment changed the type of conductor they were, "which is really striking," Bradley says. The TEP results suggest an explanation for why "pristine" SWNTs that are handled in air are observed to have holelike carriers: It's not an intrinsic property of the tubes, but rather is due to oxygen doping (adsorption). The nature of this adsorption—whether it is physical or chemical and where on the tube it occurs—remains unknown. Using scanning tunneling spectroscopy, Zettl and coworkers also examined many individual SWNTs that were first baked in an inert gas to cleanse them of adsorbates. They observed that giving some baked semiconducting nanotubes a shot of oxygen gave them the characteristics of metallic tubes. But baked metallic tubes remained metallic after this treatment. Curiously, all of these "intrinsically" metallic tubes the group observed were part of bun-
dies or "ropes." In fact, the researchers note, 'We have been unable to find an isolated SWNT that remains metallic upon inert-gas cleaning." That finding suggests that additional factors may influence the electronic properties of nanotubes. "I think that's a very important point to follow up on," Haddon remarks. Ron Dagani
TLC Used As Tool For Combinatorial Synthesis A process that combines synthesis, separation, analysis, and screening on a thin-layer chromatography (TLC) plate could be exploited for the preparation and biological screening of combinatorial libraries of compounds, according to chemists in Norway. The process was developed by a group led by research scientist Lorenzo Williams at SINTEF Applied Chemistry, a research institute located in Oslo and Trondheim. The chemists carried out the parallel synthesis of a number of sulfonamides by reaction of various arylpiperazines with sulfonyl halides and showed that the reaction can be optimized rapidly on a glass TLC plate coated with silica gel. They then demonstrated that the sulfonamides can be screened on the plate for antimicrobial activity against bacteria. 'The system provides a simple and cost-effective method for performing four processes sequentially on the same reaction medium without the need for undesirable chemical manipulations or further handling," Williams tells C&EN. "One can rapidly optimize the reaction by screening an array of reaction conditions. For example, our first library, consisting of 40 compounds, was made manually available for biological screening in 30 minutes, including time of application of reagents, reaction time, and final separation." The parallel syntheses are carried out in a series of spots on the baseline of the TLC plate. Microwave irradiation is used to accelerate the reactions. The plate is then eluted to separate products from by-products. "We can identify these discrete compounds by various analytical techniques and screen for microbial growth inhibition using agaroverlay methods on the plate," Williams explains. The team showed that, after the opti-
mum reaction parameters are identified, the reactions can be scaled up to several hundred milligrams by adsorbing the reagents on bulk silica gel followed by treatment with microwave irradiation. A paper describing part of the work— the use of TLC as a tool for the optimization of microwave-assisted synthesis of arylpiperazine derivatives—will be published next week [Chem. Commun., 2000,435]. Williams points out that only the "proof of concept" of the method has been disclosed so far. 'The process is currently patent pending," he says. 'We are considering the publication of a full paper on the combined technology later in the year." The strategy provides the opportunity to do combinatorial chemistry without specialized equipment, notes David R. Kelly, lecturer in organic chemistry,
Cardiff University, Wales. "This work is very neat," he says. "In essence, the plate acts as a large number of reaction vessels and provides a means of purifying the products. It is so simple that it could be run as an undergraduate experiment, and it is a case where one asks why it hasn't been tried before." TLC expert Fred Rabel, a marketing manager at EM Science, Gibbstown, N.J., remarks that if s exciting to find yet another use for TLC. 'With the microwave assist, the reaction is driven to completion in record time," he says. "What is also nice is that the stoichiometrics are done directly on the plate." Williams and his SINTEF coworkers are now investigating the automation of the method and the use of other media for the process. Michael Freemantle
Genzyme, Elan Make Biotech Acquisitions Biotech holding company Genzyme Once the deal closes, BioMatrix Corp. and Irish drugmaker Elan Corp. shareholders will hold 47% of Genzyme both fortified their market positions last Biosurgery, Genzyme Tissue shareweek with significant biotechnology holders will hold 27%, and Genzyme acquisitions. Surgical shareholders will hold 26%. The deals are expected to close by Genzyme, CamTHE bridge, Mass., t im* nwh e agreed to pur- m chase BioMatrix, genzyfrie Biomatrix
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