news of the week
CHEMISTS AND THEIR NANOTUBES While U.K. researchers fitt single-watted tubes with ruthenium crystals...
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xford University chemists have devised a method for placing a metal inside single-walled carbon nanotubes. Their work could lead to ways to tailor the properties of the tiny tubes, by encapsulating nanoparticles such as quantum dots, for example. Using wet chemistry techniques, inorganic chemistry professor Malcolm L. H. Green, postdoctoral researchers Jeremy Sloan and Jens Hammer, and undergraduate student Marek Zwiefka-Sibley opened the tips of single-walled nanotubes and deposited single crystals of ruthenium metal inside [Chem. Commun., 1998, 347]. "These are the first filling demonstrations for single-walled nanotubes, and the work is thus original and important," comments Daniel T. Colbert, faculty fellow in the chemistry department at Rice University, Houston. "The filling of single-walled nanotubes is significant as a demonstration of the kinds of chemistry one can do with these more perfect single-walled fullerene nanotubes." Single-walled nanotubes offer several advantages over their multiwalled counterparts. Because they consist of only a single cylindrical graphitic layer, their physical dimensions are more uniform, they have far fewer defects, and they can transmit light. Another advantage: Because, "electromagnetic radiation can get in and out of single-walled carbon nanotubes," Green explains, "the properties of nanoparticlesized encapsulated material are potentially accessible. That is not possible when the nanoparticles are encapsulated in multiwalled tubes." For instance, Green suggests, it may be possible to fill single-walled nanotubes with quantum dots—nanosized crystals or particles with electronic and optical properties different from those of the bulk material. "This is a significant advance," says George C. Schatz, a chemistry professor involved in theoretical studies of quan4 FEBRUARY 16, 1998 C&EN
Green: single-walled nanotubes filled turn dot systems at Northwestern University, Evanston, 111. He explains that the optical spectra of quantum dots are dramatically blue-shifted. Quantum dots, therefore, have unique spectral signatures that may find a variety of applications—for example, in chemical sensors.
The Oxford researchers prepared the single-walled nanotubes by evaporating cobalt-doped graphite rods in an electricarc discharge chamber, a method that also yields multiwalled nanotubes. The singlewalled nanotubes are thought to grow on the cobalt particles, explains Green. Concentrated hydrochloric acid—a milder reagent than the concentrated nitric acid that is used to open multiwalled nanotubes—dissolves the cobalt at the tips, leaving the single-walled nanotubes open. The researchers encapsulate ruthenium inside the tubes by adding a saturated solution of rutheniumQDiï) chloride and reducing the metal in a stream of hydrogen. "The work is novel," observes David R. M. Walton, reader in chemistry at the School of Chemistry, Physics & Environmental Sciences of the University of Sussex, England. "What happens now depends very much on the ability of the Oxford team, and others, to scale up and improve the yields of singlewalled nanotubes." The Oxford researchers are currently attempting to repeat the experiment with ruthenium and other metals using a higher yield method for producing single-walled carbon nanotubes developed in France (C&EN, Aug. 25, 1997, page 26). "We are also investigating methods for increasing the yield of the encapsulated product and are also trying to establish the mechanism by which opening occurs," they state. Michael Freemantle
. . . IBM scientists bend them, shape them, any way they want to
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η a display of unprecedented dexteri ty, IBM scientists have demonstrated the ability to reshape, move, and cut—at will—individual multiwalled carbon nanotubes on a solid surface \J. Phys. Chem. B, 102, 910 (1998)]. The ability to manipulate the tubes with nanometer-scale agility may hasten the ongoing trend toward miniaturization of electronic devices. The study was conducted by postdoc toral researcher Tobias Hertel, research chemist Richard Martel, and Phaedon Avouris, manager of the nanometer-scale science and technology group at IBM's
T. J. Watson Research Center, Yorktown Heights, N.Y. Using the tip of an atomic force micro scope (AFM), the researchers carry out stepwise maneuvers to manipulate nano tubes—molding them into a variety of pat terns. In one example, the group rotated and reshaped a 0.9-um-long tube through a series of bending and straightening oper ations. In another example, the team used a longer nanotube to form the Greek letter theta (Θ). In yet another demonstrationone that the IBM scientists describe as "a purposeful manipulation"—the group transformed a single nanotube into a type
Using the tip of an atomic force microscope, scientists move and reshape a single carbon nanotube on a silicon surface. A surface landmark (arrow) indicates the tube's original position.
of minitransistor by bridging an insulating gap in a conductor. "It's astounding," says John E. Fischer, professor of materials science and engineering at the University of Pennsylvania. Fischer, who studies electrical transport and other properties of carbon nanotubes, points out that "other research groups have reported similar scanned probe experiments but not with this level of control and versatility." In addition to preparing elaborate shapes, notes Fischer, the IBM group has quantified the forces that bind nanotubes to surfaces and has made useful suggestions about how to use the new information to conduct further experiments and construct electrically active devices. Comparing measurements of the way nanotubes are compressed or deformed by various forces to theoretical results, the IBM researchers deduce that van der Waals forces—long-range, attractive interactions between atoms and moleculeshold the tubes tightly against substrates. In the case of a 100-À-diameter tube, for instance, the group determined the structure is effectively pinned down with a binding energy of about 0.8 eV per À. The group notes that nanotube-surface interactions are strong enough that nanotubes bend and become slightly squashed to conform to surface topography. "Although these distortions introduce an enormous amount of strain in the nano-
tubes," says Avouris, "it turns out that once you put them on a surface, the interaction with the substrate is strong enough that it freezes them in these strained configurations." The group plans to capitalize on the enhanced chemical reactivity of strained molecules, as well as its ability to prepare molecules in strained configurations, to investigate tip-induced chemistry, says Avouris. The IBM scientists also are investigating the effect of tube shape and alignment on electrical transport properties. Depending on their structures, nanotubes can behave like metals or semiconductors, notes Avouris. These characteristics can be further modified through externally induced structural changes. The group aims to use information gained from such studies to prepare nanotubes with customized shapes and electronic properties. In addition to contorting nanotubes into arbitrary shapes, the IBM researchers
are exploring how to cut the microscopic conduits—another important skill for device miniaturization. Most attempts at severing the tubes with an AFM tip cause the carbon strips to be dragged across the surface rather than cut, the group says. Although the group reports successfully cutting nanotubes in some instances, it appears that chemical interactions—which bind the tubes even more firmly to the surface—must be present for cutting to succeed. "The paper establishes a couple of the prerequisites for device manufacture," says Fischer. An interested manufacturer might ask: "Can I take a metallic tube segment and place it in contact with a semiconducting one? This work certainly suggests you can do that," notes Fischer. "But now can I weld them together and make a junction that performs properly? That part we still don't know." Mitch Jacoby
New rail merger set, woes from last one not abated
strategy," says CN President and Chief Executive Officer Paul M. Tellier, linking CN's east-west route across Canada with IC's north-south route from Chicago to New Orleans, including the chemicals-rich region from New Orleans to Baton Rouge. The chemicals sector is an important and growing segment of revenue for the railroad, says a CN spokesman, and the merger will extend the market for shippers on both sides of the border. IC's assistant vice president for chemicals and petroleum, Bob Cleator, says the merger will provide "seamless access" to new markets for customers. Currently, rail traffic for chemicals and related products accounts for about 15% of CN's $3.0 billion in annual revenues and 30% of IC's revenues of about $700 million. Approval by IC shareholders and regulatory agencies is required for the deal to proceed. The U.S.'s Surface Transportation Board (STB) anticipates it will have jurisdiction over the merger, which should not prove too contentious. The rail lines for CN and IC do not overlap and, on completion of the merger, there will be few, if any, shippers that will lose one of their rail transportation choices. The plight of shippers with no alternative rail providers is
The most recent chapter in the consolidation of North American railroads opened last week with the announcement that Canadian National Railway Co. (CN) plans to merge with Illinois Central Corp. (IC), creating the fifth largest railroad on the continent. This comes as the chemical industry is still grappling with rail congestion problems in the Gulf Coast region resulting from last year's merger of Union Pacific (UP) and Southern Pacific (SP) railroads. Under the terms of last week's $2.4 billion deal, Montreal-based CN will purchase the outstanding shares of IC for $39 each and assume its debt of $560 million. This combination will allow a "three-coast
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