Bustling research is producing sophisticated laboratory demonstrations, but commercialization of nanometer-sized devices remains a ways off
MITCH JACOBY, C&EN CHICAGO
At first there were only a few of them, but recently, their numbers have multiplied wildly. Newspaper headlines, magazine articles, journal papers, even television commercials now are loaded with those big "nano" words: nanometer, nanoscale, nanosecond, and nanotechnology, to name a few. And it seems that every week some organization is announcing yet another "nanoconference."
Why all the excitement? Because in recent years, as scientists have begun to get a handle on controlling matter at the nanometer scale, they've recognized that these skills can be used to make new materials with unique and useful properties--perhaps leading to a range of commercial applications in sensing, electronics, and other areas. Impressive laboratory demonstrations of nanoscale dexterity have been widely reported in the scientific and popular press, thereby drawing the attention of more scientists, companies, and technology investors.
Nanoelectronics--a developing field in which circuitry is composed of nanometer-sized electronic components--is one topic that's attracting a lot of interest in scientific and nonscientific corners alike. Researchers are driven to explore the field because further miniaturizing today's already small electronic circuits will lead to faster, more sophisticated, and more portable devices. Yet it's widely believed that in a decade or so, silicon-based circuitry will have been shrunk as small as is physically possible. And so the search is on for alternative materials from which nanoscale circuits can be constructed.
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|Switching of a catenane
||Switching of a rotaxane in a device setting
||Construction of a crossbar device
Carbon nanotubes and nanowires of materials such as gallium arsenide are being studied for just that purpose by several research groups. Other investigators, whose work is often labeled "molecular electronics," focus on the electrical behavior of individual molecules or small numbers of molecules. Scientists who work in these fields aim to design and build new types of computers in which the nanosized entities lie at the heart of logic circuits. Skeptics say that the proposed applications are exaggerated and that reports are full of "hype." But researchers around the globe are exploring and discovering nanometer-scale phenomena, and they are rapidly turning out large numbers of scientific papers.
"These are exciting and heady times," says Mark A. Ratner, surveying the combined topics of molecular and nanoelectronics. "There are wonderful experiments being reported these days," he says. "The excitement is justified." A nearly 30-year-old paper by the Northwestern University chemistry professor is often cited as the beginning of modern molecular electronics. In that theoretical study, Ratner and Arieh Aviram, who was at that time a graduate student, proposed that under certain conditions, individual organic molecules could function as p-n junction diodes. These simple electronic devices typically are formed at the interface between positive charge-carrying (p-type) and negative charge-carrying (n-type) semiconductors and can be used as rectifiers to convert AC current to DC current.
Ratner notes that few advances were made in molecular electronics until scanning probe microscopes became available in the late 1980s. The new tools gave the field a much-needed boost by making it possible to measure a whole range of phenomena, including the type Ratner had described in the 1970s. He adds that progress in other areas, for example, in methods for preparing self-assembled monolayers of molecules, was also instrumental in jump-starting nanoscale electronics. "Now the advances are coming thick and fast," he says.
"Thick and fast" aptly describes the flurry of research being carried out on carbon nanotubes. According to Phaedon Avouris, manager of nanoscale science and technology at IBM's T. J. Watson Research Center, Yorktown Heights, N.Y., the strawlike all-carbon structures with nanometer-sized diameters are endowed with unique properties that make them excellent candidates for nanoelectronic applications.
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|TINY WORLD Molecules that can be switched between conducting and nonconducting states--such as the UCLA-synthesized rotaxanes shown here--lie at the heart of some of today's lab demonstration circuits. In one state (right structure and upper right schematic), a cationic cyclophane ring (blue) encircles a tetrathiafulvalene (TTF) unit (green). Oxidizing the TTF unit causes electrostatic forces to drive the cyclophane ring to a dioxynaphthalene unit (red), switching the molecule to the other state (left).
COURTESY OF UCLA
NANOTUBES CAN BE metals or semiconductors, depending on their chirality, Avouris notes. And because of their strong chemical bonds and satisfied valencies, the materials boast high thermal, mechanical, and chemical stability. In addition, carbon nanotubes can be efficient conductors as a result of their tiny diameters, long lengths, and defect-free structures that make them ideal one-dimensional systems.
Just four years ago, Avouris' research group and, independently, scientists working with Cees Dekker, a professor of applied physics at Delft University of Technology, in the Netherlands, demonstrated that field-effect transistors (FETs) could be fashioned from carbon nanotubes. FETs are a type of switch in which a semiconducting channel bridges two electrodes designated "source" and "drain." Current flow between these electrodes is controlled by a third electrode known as a "gate." By applying a voltage to the gate, the semiconductor's state can be changed--reversibly--from insulating to conducting, thereby switching the transistor on or off. In conventional FETs, the bridging channel is made of silicon. In nanotube devices, the channel is a single carbon nanotube.
Today's sophisticated silicon-based integrated circuits owe much of their success to advances in FET technology. Manufacturers pack tens of millions of the tiny transistors into the postage-stamp-sized computer chips used in modern microprocessors. For that reason, the IBM research group and others have been working on improving the properties of nanotube FETs. Much of the work focuses on understanding and controlling electrical conduction through the tiny tubes.
One problem that plagues researchers looking to fashion circuit components from nanotubes is separating metallic tubes from the ones that are semiconducting. Common synthesis procedures produce spaghetti-like mixtures of nanotube ropes that are unusable for semiconductor applications because they contain both types of tubes.
But last year, Avouris and coworkers came up with a way to sort through the tangle. After fashioning electrodes around nanotube bundles via lithography methods, the team applied certain voltages to the gate electrodes to switch off the semiconducting tubes, converting them to insulators. Then, by applying high voltage to the circuit, the IBM group oxidized the metallic tubes, causing them to break down without affecting the semiconducting tubes. The group used the method to prepare FET arrays from single-walled nanotube ropes and to peel apart multiwalled nanotubes shell-by-shell (C&EN, April 30, 2001, page 13).
Just a few months later, the IBM group showed that elementary computing circuits known as logic gates--which typically are constructed from combinations of FETs--can be fashioned from a single nanotube bundle. Shortly thereafter, Delft's Dekker reported similar advances in constructing multi-FET nanotube logic circuits. Complex combinations of AND, OR, and NOT logic gates make up the core of digital processors.
To make NOT gates using nanotubes, Avouris and coworkers needed to come up with a procedure for preparing n-type nanotubes. NOT gates require both types of transistors. But the synthesis techniques available at that time, produced only p-type tubes. Hunting for a solution, Avouris and IBM coworkers Vincent Derycke, Richard Martel, and Joerg Appenzeller found that p-type nanotubes can be converted to n-type simply by annealing (heating) in vacuum. Armed with the new preparation trick, the team prepared separate n- and p-type transistors and fabricated a NOT gate.
Then they went a step further and fabricated a NOT gate using just one nanotube. For that feat, the team selectively converted a short length of a nanotube to n-type by doping it with potassium through a tiny window in a protective polymer coating while leaving the unexposed portion p-type. The single tube with p- and n-type segments was then used to construct an intramolecular NOT gate (C&EN, Sept. 3, 2001, page 9).
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