Carbon monoxide, a pollutant and a poison, is diamond in the rough to researchers at Carbon Nanotechnologies Inc. In the company's Houston laboratories, the gas hisses along at high pressure into a hot aluminum-walled reactor, where it encounters a pinch of iron-based catalyst. As the CO molecules rip apart, the metal coaxes carbon atoms to join into hexagons, which fit together into sheets that finally roll into seamless cylinders called carbon nanotubes. They can contain millions of atoms and stretch almost as wide as the period at the end of this sentence, yet they remain single molecules.
And what a molecule. "A carbon nanotube is the most beautiful molecule there is," says physicist Cees Dekker of the Delft University of Technology in the Netherlands. He's not just talking about the perfectly rolled atomic matrix, unlike anything else in nature. He is smitten by the nanotubes' unique properties: Stronger than steel, they absorb radio waves and can emulate both copper and silicon. Dekker and several colleagues are learning how to manhandle these tubular molecules into fundamentally new kinds of electronic components. If their efforts pay off, the result could well be a technological leap that makes today's PCs look as quaint as mechanical adding machines.
People knew little about carbon nanotubes until 1991, when Sumio Iijima, a materials scientist at NEC Fundamental Research Laboratories in Tsukuba, Japan, isolated them from a sooty deposit that formed when he hit a piece of carbon graphite with an electric spark. A few months later, computer modelers showed that the simplest versions of the molecule— the so-called single-walled nanotubes— have a split personality. They can behave like conductive metal wire or like a semiconductor, which carries electricity only above a specific voltage, if twisted so their hexagons wrap around like stripes on a barber pole. "They're the only molecules I am aware of that can be metallic on the scale of one molecule," says Dekker. Conductive and semiconductive devices are exactly what is needed to create electronic circuitry. Only in this case, one material can take on both duties, and do them in a tiny space.
Several research teams, including ones led by Dekker, have begun to butt nanotubes against each other to create structures that behave like transistors, switches that open or close flows of electrons, and like diodes, gates that allow electrons to travel in one direction only. The experiments indicate nanotubes can perform many of the functions now carried out by silicon chips. "The history of electronics has been to go from an idea to the realization of prototypes to the realization of circuits," says Dekker. "We really are moving toward circuits and applications."
But engineers have had more than 40 years of experience turning silicon wafers into useful electronic gizmos. With carbon nanotubes, they are starting over. Two years ago, at the University of California at Berkeley, physicist Alex Zettl began throwing the pieces together just to see what happened. He and his colleagues have allowed billions of nanotubes to settle into a jumble of molecular-scale needles. Anywhere the tubes touch becomes a potential diode or transistor. The pileup, Zettl found, behaves like a randomly wired computer: The circuits activate when current is applied, but they perform no useful tasks. Not yet, that is. "The technology simply has too much potential to not figure out how to use it," Zettl says. He proposes making a "tube cube," a dense block of nanotube circuits, and letting a software program explore the circuitry to see what it can do.
Harvard University chemist Charles Lieber believes it will take a more regimented architecture to make molecular electronics perform. He and his colleagues have been fabricating crossed arrays, in which one set of parallel nanotubes is suspended at right angles to another set just below. Each cross point where the tubes meet can behave like a switch or a memory element. "In theory, one could store a terabit of memory in a square centimeter," Lieber says. With that kind of density, the contents of the Library of Congress could be stored in a computer the size of a sugar cube— a prospect that helped inspire former president Bill Clinton to request $497 million for a National Nanotechnology Initiative in the 2001 budget.
Molecular computer mavens could use the money, because they still face many potential stumbling blocks. At the moment, no one can control nanotubes with nearly the finesse required for complex circuits. One possible approach is to attach chemical tags at desired locations on the tubes, a process called selective functionalization. These tags would repel or attract and attach a menagerie of tiny electronic wires and semiconducting rods. Under the inexorable logic of chemistry, the parts would assemble into networks of diodes and transistors. Self-assembly is crucial because it would be impractical to build multimillion-molecule circuits by physically joining minute carbon tubes into precise architectures.
Self-assembly, however, is not a simple process. It requires chemical modification of specific sites among thousands of identical carbon atoms in a nanotube's length— a task that stretches the limits of chemists' abilities. "Selective functionalization of carbon nanotubes is not easy and may not be possible," cautions James Tour of Rice University, who built some of the first working molecular switches using more-traditional organic molecules.
Nanotubes might make their computing debut by augmenting silicon, not replacing it, says James Ellenbogen, a vocal champion of molecular electronics and head of the Nanosystems Group at the MITRE Corporation, a not-for-profit research organization in McLean, Virginia. Right now silicon chips contain pockets of unused real estate. Ellenbogen predicts the vacancies will soon be filled with molecular memory ensembles, perhaps made from nanotubes. Rather than retrieve data from a hard drive or CD-ROM, the chip could tap into immediately accessible high-density carbon data storage.
And there are other intriguing ideas for using nanotubes. Companies such as Samsung in South Korea and Lucent Technologies in New Jersey are aiming to exploit them as minuscule electron guns in superthin video displays. Carbon nanotubes of varying lengths can soak up a wide band of radio waves, so they might be the right stuff for the electromagnetic shielding of cell phones and laptop computers. Physicist Paul McEuen of the Lawrence Berkeley National Laboratory believes that carbon tubes could be ideal sensors to explore the chemical environment inside living cells. Nanotubes also have a strength-to-weight ratio 100 times greater than steel's, prompting some scientists to muse about utilizing them to build a 23,000-mile-high elevator into space. As Dekker says, "There are so many people getting into this, the rate of innovation is incredible."
Dramatic price drops may aid the progress. For years, engineers have been shelling out the equivalent of $30,000 an ounce for high-quality tubes. Richard Smalley of Rice University— who earned the 1996 Nobel prize for his discovery of spherical carbon molecules called fullerenes— and the other cofounders of Carbon Nanotechnologies think they can bring the cost down to a hundredth the going rate within two years.
At those prices, nanotubes might soon be a scientist's best friend.
Richard Smalley's home page at Rice University: cnst.rice.edu/reshome.html.
