perspective yields (C&EN, July 29,1996, page 5). The Rice chemists use a laser to vaporize a graphite target inside a furnace heated to 1,200 °C. The target is 98.8% carbon, 0.6% nickel, and 0.6% cobalt. Smalley told the workshop that his group now is using the technique to make half a gram per hour of singlewalled nanotubes. Running the system 24 hours per day, they can produce 10 g of nanotubes per day. "We could form a company to sell gram amounts at $200 per gram," Smalley said. A fairly substantial body of theoretical between carbon electrodes, a method research and a growing amount of similar to that used to produce the first experimental data indicate that these nanotubes and related boron/nitrogen bulk quantities of fullerenes. Subsequently, researchers at NEC and analogs are likely to have remarkable at IBM Almaden Research Center in San properties. For one, they may be the Jose, Calif., independently showed that strongest materials ever produced. And addition of a small amount of transition- depending on their geometry, they will metal powder favors growth of single- behave electronically like a metal, a walled nanotubes. Unlike the multi- semiconductor, or an insulator. walled nanotubes, the single-walled spe"This is a seminal moment in the develcies are clearly single, giant molecules opment of carbon materials," Smalley told that are closely related to the fullerenes. the workshop. "There's a sense of inevitaLast year, Smalley, whose research focus bility about the nanotubes forming the has shifted in the past several years from bases of new technologies. I think this will fullerenes to nanotubes, and coworkers develop as a new branch of organic chemat Rice devised a technique that produc- istry. We have the real possibility of dees single-walled nanotubes in 70 to 90% veloping true, molecular electronic devices—any technology that in| volves getting an electron ™ from here to there stands a j§ good chance of being revo£ lutionized by nanotubes." I The challenge Smalley °~ posed to the workshop in Houston was to imagine Christmas morning 15 years from today. The Christmas presents arrayed under the tree for the assembled chemists and physicists would amount to an entire "Erector set" of nanotube technologies: Nanotubes of any length, functionality, and geometry you could imagine; a complete set of methods for manipulating them—picking them up, joining them together, adding chemical functionalities; and any quantity of nanotubes for any application at a few dollars a pound. Under those circumstances, Smalley asked the workshop, what nanotube technologies would be conceivable? The participants were somewhat hesitant to exSmalley: sense of inevitability about nanotubes trapolate from Smalley's hy-
NURTURING NANOTUBES
Remarkable properties of nanotubes suggest range of new technologies; workshop examines how to develop them Rudy M. Baum C&EN Washington
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ranslating basic research discoveries into useful technologies has never been an exact science, especially in the U.S. Some research moves rapidly from the lab to the marketplace, while other discoveries languish for lack of a champion. The fascinating species known as carbon nanotubes may or may not be rapidly assimilated into new technologies, but whatever their fate, it's not for lack of a champion. Richard E. Smalley, Gene & Norman Hackerman Professor of Chemistry and professor of physics at Rice University, Houston, and recipient of the 1996 Nobel Prize in Chemistry for the discovery of the fullerenes, believes that nanotubes have the potential to change humanity's future. And he's determined to nurture their development. One step in that nurturing process was a workshop held recently at Rice on the future of nanotubes. Organized by Smalley and sponsored by Rice and the Office of Naval Research, the goal of the workshop was to explore the potential technologies that might stem from nanotubes within 10 to 15 years if methods to synthesize them to order and to manipulate them at will can be developed. The workshop brought together about 100 academic and industrial scientists, research directors, and representatives of government agencies for three days of intensive bramstorming. Discovery of carbon nanotubes is generally credited to Sumio Iijima of NEC Fundamental Research Laboratories in Tsukuba, Japan. In 1991, Iijima published high-resolution electron micrographs showing what are now known as multiwalled nanotubes: long cylindrical structures with concentric walls and hemispherical end caps. Iijima's nanotubes had diameters on the order of 10 to 30 nm, and were created with an arc discharge
JUNE 30, 1997 C&EN 39
perspective chemistry professor at Harvard University, pothetical holiday gifts. discussed nanotube mechanics, conductiv It's hard to get scientists ity, and materials. Lieber and coworkers involved in flights of use atomic force microscopy (AFM) to fancy—it's their basic probe the mechanical properties of multinature to want to know walled carbon nanotubes. Theyfirstuse li all the specifications of thography to pin one end of a nanotube to whatever flying ma a substrate and use the AFM tip to measure chine they are boarding the lateral force necessary to deflect the before they take off. free end of the nanotube at varying That said, the workshop distances from the pinning point. They explored a wide range find that the nanotubes are, indeed, very of potential applications stiff, with a Young's modulus on the order of nanotubes and set out a variety of lines of Micrograph shows "rope" of single-walled carbon nanotubes. of 1.2 trillion pascal (terrapascal or TPa), about twice that of silicon carbide fibers. research needed before The Harvard chemists also find that, such applications can become reality. diameter and chirality." Tubes where η Nanotubes come in a variety of shapes equals m and where η minus m equals a unlike SiC fibers that eventually break un and sizes, but they all share a number of number divisible by three, Louie said, are der increasing strain, the nanotubes ap characteristics. Their basic structure con metallic. All other tubes are semiconduc pear to buckle with continued bending. sists of a graphene (graphitelike) sheet of tors with a range of band gaps, some Thus, the bend strength of carbon nano tubes (defined as the maximum strain be hexagonally bonded carbon rolled up into large enough to be insulating. a long, thin tube capped at each end with "Such structures allow the formation of fore breaking or buckling) is about half of a fullerene. Think nanometer-scale various metal-semiconductor, semiconductor- one-third that of silicon carbide fibers. chicken wire rolled into a tube, stitched semiconductor, and metal-metal junc While that has implications for the use of together flawlessly along the seam, and tions," Louie said, raising the possibility of the nanotubes in applications such as capped at each end with half a soccer ball. creating molecular-level electronic devices ceramic composites, the principal finding Nanotubes form from condensing carbon by bonding together different types of from lieber's work (and that of other vapor by mechanisms that share some fea nanotubes. Louie has also calculated the researchers who presented at the work tures with those that lead to mllerenes. electronic properties of nanotubes made shop) is the remarkable toughness of the The fundamental characteristic that dis from boron nitride (BN) and other com carbon nanotubes—they just don't break tinguishes nanotubes from one another is pounds. BN nanotubes are always wide under strain. Lieber described carbon the geometry of the bonds that make up band gap semiconductors, he told the nanotubes as "a uniquely tough, energythe tube. You can roll up a graphene sheet workshop. So it might be possible to wrap absorbing material." to form a tube at various angles with a metallic 10,10 carbon nanotube with a Or to take another example, Paul L. respect to the hexagonal lattice. One way BN nanotube to produce an insulated McEuen, a physics professor at UC Berke yields a symmetrically stepped pattern of nanowire. ley and a researcher at LBNL, reported on carbon-carbon bonds around the circum While theorists have been playing low-temperature transport measurements ference of the tube—the so-called arm with nanotubes for several years now, on ropes of single-walled nanotubes. The chair configuration characteristic of the constructing inside their computers hy Berkeley physicists deposit nanotubes on a nanotube most favored in the formation pothetical tubes with remarkable proper substrate, locate a rope of nanotubes with process, the 10,10 tube. Another way ties, experimentalists have only recently an AFM, and deposit metallic leads on top yields a regular zigzag pattern of bonds begun to grapple with the daunting task of portions of it. Work by McEuen and co around the circumference of the nano of obtaining accurate measurements of workers, and again by other researchers, tube. Many others result in a chiral twist the properties of nanotubes. suggests that nanotubes, in fact, behave as along the length of the nanotube. A nano For example, Charles M. Iieber, a genuine quantum wires. tube is characterized by a "roll-up vector" Workshop partici (n,rn) that specifies the orientation of the pants then dreamt up graphene sheet that hypotheticaUy gives some of the technologies rise to the tube. that may develop around nanotubes. As Smalley The workshop explored some of pointed out, continuous what is now known or speculated about fibers, ropes, and cables various nanotubes. Steven G. Louie, a spunfromnanotubes have physics professor at the University of potential applications in California, Berkeley, and a member of the all those areas currently Materials Sciences Division of Lawrence open to graphite fibers Berkeley National Laboratory (LBNL), for and high-strength fibers example, discussed the electronic prop Courtesy of Paul McEuen such as Kevlar: body erties of nanotubes and nanotube junc armor and helmets, air tions. "Calculations predict that the elec frames and rocket noz tronic properties of carbon nanotubes Ιμηα zles, structural support will be variable," Louie said. "For Rope of single-walled nanotubes (thin strand running and body panels for vehi example, they can be metallic or semicon horizontally) connected by metal contacts deposited cles, membranes for batducting, depending sensitively on tube lithographically for conductivity measurements. 40 JUNE 30, 1997 C&EN
teries and fuel cells, chemical filters, cata lyst supports, hydrogen storage (both as an absorbent material and for use in fabri cating high-pressure vessels), litliium-ion batteries, and capacitor membranes. Some workshop participants were quite specific in their predictions. For example, J. Gerard Lavin, an expert on carbon fibers at DuPont, projected potential infrastructure applications—such as cable stays on bridges, wraps for earthquake proofing staictures, and tendons in tall buildings to prevent wind sway—requiring 500 million lb of nanotubes per year. Others speculated on a variety of electri cal, optical, and mechanical devices where nanotubes could play a role. University of Texas chemistry professor Allen J. Bard, for example, discussed the possibility of using nanotubes as nanoscale electrodes. Others discussed the use of nanotubes as AFM and scanning tunneling microscope (STM) tips, applications already being real ized in rudimentary devices. Berkeley's McEuen imagined "using a nanotube as a local electrode probe of individual cells." Others immediately suggested biological applications of nano tubes ranging from a nanoscale hypoder mic needle that could inject a few mole cules into a specific region of a cell to carrying out molecular surgery on nucle ic acids. "The biophysics community is moving rapidly to use nanotubes as probes to bridge the macroscopic/micro scopic interface," McEuen said. Getting from here to there will require a vast amount of research, some of which was also explored at the workshop. Obvi
ously, methods for producing long, contin uous strands of nanotubes of a desired ge ometry need to be invented. Smalley sug gested some approaches. James R. Heath, a chemistry professor at the University of California, Los Angeles, who as a graduate student with Smalley was a member of the team that discovered fullerenes, talked about self-assembly chemistry recently invented in his lab that might be applied to the problem. But beyond that large challenge, nanochemists and nanoengineers need an ana lytical chemistry of nanotubes that will ac curately tell them what they are working with. They need to be able to process nanotubes once they've been synthesized and characterized—methods to cut, sort, and manipulate nanotubes. They need a chemistry for derivatizing nanotubes, one likely derived from the chemistry of derivatizing fullerenes. Whether nanotubes are going to change the world isn't clear yet, but it is clear that they are a remarkable addition to the basic building blocks available to humans to shape into new materials. The scientists and engineers who gathered at Rice, while acknowledging the signifi cant hurdles in front of them, were enthusiastic about the potential of nano tubes in a wide variety of applications. Michael Capoccia, a scientist with Northrop Grumman in Chandler, Ariz., made a typ ical comment: "From a structural engineer ing point of view, this is the most excit ing material since graphite fibers." I came away from the Rice/Office of Naval Research workshop on nanotubes
with a couple of thoughts. On a sober note, it seems to me that humans, at the close of the 20th century, are drawing on ever more complex basic science to devel op their ever more powerful technologies. Nanotubes appear to be a prime example. But this trend is leading to a growing gap in the research and development continuum between basic research and development. Applied research bridges the gap, but, as the gap grows, it's placing increasingly severe strains on what has always been the weak est research link in the U.S. Most basic scientists at top U.S. univer sities have always viewed applied research with some disdain—it's just not what the best scientists do. And while U.S. industry certainly carries out first-rate applied research, the ever decreasing time horizon imposed on industry by Wall Street has cut significantly into the resources available for long-term applied research. The situation, particularly at universi ties and government funding agencies, needs to change if today's basic research, like that on nanotubes, is to be successful ly translated into tomorrow's technologies. And it should change because, as clearly demonstrated at the workshop on nano tubes, the challenges facing applied research ers are every bit as rich and rewarding as those posed by fundamental research. To day, cutting-edge applied research is, in fact, fundamental research. And applied research requires adequate resources and rewards if it is to thrive and provide us with the advanced technologies we need to meet the demands of an ever more complex world. ^
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JUNE 30, 1997 C&EN 41
