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TINY WORLD Molecules that can be switched between conducting and nonconducting states—such as the UCLA-synthesized [2]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).
NANOSCALE ELECTRONICS Bustling research is producing sophisticated laboratory demonstrations, but commercialization of nanometer-sized devices remains a ways off M I T C H J A C O B Y , C & E N CHICAGO
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cently 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
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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 nanometersized electronic components—is one topic that's attracting a lot of interest in sciHTTP://PUBS.ACS.ORG/CEN
entific and nonscientifîc 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. 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 t h a t 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-yearold 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 (η-type) semiconductors and can be used as rectifiers to convert AC current to D C current. Ratner notes that few advances were made in molecular electronics until scan ning probe microscopes became available in the late 1980s. The new tools gave the field a much-needed boost by making it possi ble to measure a whole range of phenomena, in cluding the type Ratner had described in the 1970s. He adds that progress in other areas, for example, in methods for preparing self-assem bled monolayers of mol ecules, was also instru mental in jump-starting nanoscale electronics. "Now the advances are com ing thick and fast," he says. "Thick and fast" aptly describes the flur ry of research being carried out on carbon nanotubes. According to Phaedon Avouris, manager of nanoscale science and tech nology at IBM's T. J. Watson Research
Skeptics say that the proposed applications are exaggerated and that reports are full of "hype."
Ratner^ Center, librktown Heights, N. Y, the straw like all-carbon structures with nanometersized diameters are endowed with unique properties that make them excellent can didates for nanoelectronic applications. NANOTUBES CAN BE metals or semicon ductors, 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 work ing with Cees Dekker, a professor of ap plied physics at Delft University of Tech nology, in the Netherlands, demonstrated that field-effect transistors (FETs) could
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COVER STORY be fashioned from carbon nanotubes. FETs are a type of switch in which a semicon ducting 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 semi conductor's state can be changed—reversibly—from insulating to conducting, thereby switching the transistor on or off. In conventional FETs, the bridging chan nel is made of silicon. In nanotube devices, the channel is a single carbon nanotube. Today's sophisticated silicon-based in tegrated circuits owe much of their suc cess to advances in FET technology Man ufacturers pack tens of millions of the tiny transistors into the postage-stampsized computer chips used in modern mi croprocessors. For that reason, the IBM research group and others have been working on improving the properties of nanotube FETs. Much of the work fo cuses 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 semicon ducting. Common synthesis procedures produce spaghetti-like mixtures of nano tube ropes that are unusable for semicon
ductor 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 meth ods, the team applied certain voltages to the gate electrodes to switch off the semi conducting tubes, converting them to in sulators. 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 pre pare FET arrays from single-walled nano tube 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 cir cuits known as logic gates—which typi cally 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 log ic circuits. Complex combinations of AND, OR, and N O T logic gates make up the core of digital processors. To make N O T gates using nanotubes, Avouris and coworkers needed to come up with a procedure for preparing n-type nanotubes. NOTgates require both types
MAGNITUDE A series of micrographs in which each image is magnified roughly 10 times more than the previous one (left to right) reveals details of a new HP Labs molecule-based 64-bit prototype memory and logic chip. Using a newly patented procedure, researchers patterned a silicon wafer with 625 memory devices tone shown at top right). At the center of each memory circuit, an 8 x 8 array of crossed metal nanowire electrodes (bottom row) sandwiches roughly 1,000 rotaxane molecules forming 64 bits of nonvolatile memory.
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of transistors. But the synthesis techniques available at that time, produced only ptype tubes. Hunting for a solution, Avouris and IBM coworkers Vincent Derycke, Richard Martel, andjoerg Appenzeller found that p-type nanotubes can be con verted to η-type simply by annealing (heat ing) in vacuum. Armed with the new prepa ration trick, the team prepared separate n- and p-type transistors and fabricated a N O T gate. Then they went a step further and fab ricated a N O T gate using just one nano tube. For that feat, the team selectively converted a short length of a nanotube to η-type by doping it with potassium through a tiny window in a protective poly mer coating while leaving the unexposed portion p-type. The single tube with p- and η-type segments was then used to con struct an intramolecular N O T gate (C&EN, Sept. 3,2001, page 9). FURTHER ADVANCES have come from transistor design modifications. Earlier this year, for example, the IBM group report ed that their so-called top-gated nanotube FETs outperform state-of-the-art silicon FETs in terms of switching rate and the amount of current they can carry per width of conductor. One key difference between the latest design and earlier designs is that the silicon wafers that support the FETs do not function as gates. Instead, the gate is fabricated above the nanotube—allow ing all FETs in contact with the silicon wafer to be switched independently In ad dition, the new FET design benefits from switching voltages that are an order ofmag nitude lower than those needed to switch older FETs \Appl Phys. Lett, 80, 3817, (2002)}. Just recently the IBM team developed a catalyst-free procedure for preparing sin gle-walled carbon nanotubes. Conven tional methods for preparing single-walled HTTP://PUBS.ACS.ORG/CEN
tubes, such as discharge and vapor depo sition techniques, rely on particles of nick el and cobalt or other catalytic tran sition metals. A problem with those procedures is that the metal particles left be- o0 hind in the products disturb "*> the nanotubes' electrical prop erties, forcing scientists to take complex purification measures. But now Avouris and coworkers have * shown that single-walled nanotubes can be preparedfromSiC in the absence ofmet al catalysts at temperatures above 1,500 °C. The method produces nanotubes that are 1.2 to 1.6 nm wide and are aligned along the face of the support. Other researchers have reported techniques for growing vertically
are making fast progress in synthesis, de vice fabrication, and testing. Although Harvard University chemistry professor Charles M. Lieber has pub lished widely on carbon nanotubes, semiconducting nanowires fig ure prominently into his re search program. The tiny wires are versatile build ing blocks that can be used in a bottom-up approach to conSTOP METALING A catalyst-free synthesis procedure developed at IBM produces aligned single-walled carbon nanotubes (vertical lines) from SiC without residual metal impurities.
aligned products, which they dub nanotube forests. But nanotubes that grow horizon tally are amenable to standard vapor depo sition methods and may be connected in parallel to lower their electrical resistance [Nano Lett, published online, Sept. 24, http://dx.doi.org/10.1021.nl0256309}. Despite the recent advances, Avouris as serts that nanotube FETs remain "far from optimized." Improvements could be made by using thinner insulators with higher di electric constants, he suggests. "But what's really needed is a better understanding of the mechanism of electrical switching in nanotube FETs." Carbon nanotubes aren't the only game in town. Nanowires made of semiconduc tors such as silicon, gallium arsenide, and indium phosphide are being investigated as candidate materials for nanoelectronics. Thefieldis advancing rapidly as researchers
TEAM EFFORT Molecular electronics research at UCLA brings together organic synthesis and device measurements through the efforts of (standing, from left) Jeppesen, Stoddartp Heath, and Yi Luo, and (kneeling, from left) Tseng, Kristen Beverly, and Paul Celestre.
structing nanoelectronic circuits, Lieber notes, because the size, structure, and func tional properties of nanowires can be con trolled readily via synthesis procedures. In 1998, Lieber and coworkers de scribed a vapor-liquid-solid synthesis method in which laser light is used to ab late nanometer-sized metal clusters that serve as nucleation centers and catalysts for nanowire growth. The Harvard re searchers used the technique to prepare uniform single-crystalline nanowires of sil icon and germanium with diameters as small as 3 nm and lengths up to 30 μηι. Since that time, the procedure has been used to prepare nanowires with even small er diameters, and it's been extended to a wide variety of materials. Examples include III-V semiconductors such as GaAs and II-VI semiconductors such as ZnSe. Recently, a number of research groups
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COVER STORY boosted the complexity of materials that can be prepared by the cluster-nucleation method. The teams prepared modulat ed structures—nanowires composed of dissimilar segments. The two-tone ma terials are made by turning the supplies of reactants on and off during synthesis with pulsed lasers or by other methods. These "heterostructured" products open the door to new sophisticated applica tions, such as terahertz-frequency pho ton emitters. Lieber, graduate student Mark S. Gudiksen, and coworkers used modulation meth ods to prepare nanowires with 21 alter nating segments of GaAs and GaP. And they prepared Si and InP nanowires with modulated doping, such that the wires were endowed with alternating p- and ntype regions. Using similar methods, PeidongYing, an assistant chemistry profes sor at the University of California, Berkeley prepared Si-SiGe nanowires. And Lars Samuelson of Lund University, in Sweden, synthesized InAs-InP nanowires (C&EN, Feb. 11, page 7).
eluding light-emitting diodes (LEDs) and logic gates. Most of the papers in which the results are reported were published in just the past 18 months. In the chemical sensing study, Lieber, graduate students Yi Cui and Qingqiao Wei, and Harvard assistant chemistry pro fessor Hongkun Park found that by functionalizing boron-doped Si nanowires, they could prepare efficient sensors. Modifying the wires with an arninosilane compound, for example, produced pH sensors that are linear over a large dynamic range. And they ob served that biotin-modified wires can be used to detect picomolar lev els of streptavidin. In addition, the Harvard chemists found that the wires could be used to detect proteins reversibly in real time [Science, 293,1289(2001)}. By crossing an n-type TO EVALUATE the usefulness of the tiny and a p-type InP nano wires in nanoscale electronics, the Har wire and then attaching vard researchers fashioned the wires into electrodes to the wires lithographically, test circuits that were fabricated using li Lieber and coworkers made a nanometerthography methods and then studied the sized junction—at the point of overlap— circuits' electrical properties. According that behaved as a current rectifier and an to Lieber, Si nanowire FETs, for example, LED [Nature, 409,66 (2001)}. Based on perform remarkably well. The devices ex the study, Lieber says that more complex hibit large on/off ratios (>105) — a re devices can be constructed from semi quirement for effective switches —and conducting nanowires simply by crossing
Only a few years ago, many scientists sat through molecular electronics presentations thinking that the far-fetched nanocomputing goals would never be reached.
θMODULAR MOLECULES UCLA chemists use synthesis techniques to prepare multipart molecules with customized structures and properties for nanoelectronic devices. [2]Catenanes (left) and [2]rotaxanes (center and * right) can be switched on and off depending upon redox state, which determines where in the molecule a cyclophane ring (blue) resides.
charge-carrier mobilities that exceed those measured in conventional silicon devices. In addition to constructing FETs from nanowires, Lieber's team has built and test ed photodetectors, biological and chemi cal sensors, and more complex devices, in42
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ods, the Harvard group developed the needed procedure. Their idea is to flow nanowires from a suspension into tiny par allel channels in a mold, in a layer-by-layer fashion, while rotating the mold orienta tion—and hence, nanowire flow direc tion—between layers [Science, 291, 630 (2001)}. With the crossed-nanowire method for making p-n junctions firmly in hand, Lieber and coworkers were well on their way toward making computational logic circuits. Near the end of 2001, the group published a study in Sci ence showing that their procedures could be used to make arrays of nanowire FETs config ured as AND, OR, and NORlogic gates. Lieber stresses that in crossednanowire structures, such as the Harvard log ic gates, the width and length of the FETchannels are controlled by synthesis and assem bly—not lithography—unlike today's mi croelectronic components.
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them and forming tic-tac-toe patterns. But taking advantage of the crossednanowire idea requires developing a meth od for preparing the patterns efficiently and reproducibly By combining surface patterning tech niques and microfluidic alignment meth
THESE FABRICATION concepts make it possible "to envision a straightforward path to nanometer-scale electronics of the future," Lieber says. His vision, which is supported by experiments conducted by his group in the past several months, includes a nanoscale logic device known as a decoder that can address individual elements in crossed-wire arrays. Using a handful of molecules —or even just one of them—to make nano scale switches or other devices is an idea that's gathered momentum in recent times. In 1999, researchers working with chemistry professors James R. Heath and J. Fraser Stoddart of the University of California, Los Angeles, demonstrat ed that simple circuits —fuses —could be made from a layer of [2}rotaxane mol ecules. Then a team headed by Rice Uni versity chemistry professor James M. Tour and Yale University electrical en gineer Mark A. Reed reported success in making reversible switches using ben zene thiol molecules. Other groups have also been investi gating the possibilities of single-mole cule systems. For example, a team at UC Berkeley including Park (now at Har vard), Paul L. McEuen (now at Cornell), and A. Paul Alivisatos, trapped a single HTTP://PUBS.ACS.ORG/CEN
C 6 0 molecule in a tiny gap between a pair of electrodes to study electronic conduction mechanisms [Nature, 4 0 7 , 57 (2000)]. And just recently a single molecule of a transition metal-organic complex—containing either one Co atom or two V atoms —was turned into a singlemolecule transistor in independent studies by Park and McEuen (C&EN, June 17, page 4). Before molecular devices can be wired up and tested, organic chemists need to synthesize the molecules that drive them. In the U C L A collaboration, Stoddart's group prepares t h e compounds and Heath's group turns them into circuit components. To make the molecules switchable, Stoddart's group designs t h e m so that one segment of a molecule can move (rotate or
translate) relative to another segment. Stoddart notes that, contrary to remarks made by many researchers, the molecules are not difficult to prepare nowadays. But it took some years to build up the necessary expertise in molecular recognition concepts, TT-donorArT-acceptor interactions, and other subjects. He adds that his group has synthesized "hundreds, if not thousands" of catenane and rotaxane analogs since 1989. In 2000, the UCLA team demonstrated a reusable switch based on a [2}catenane —a molecule with a pair of interlocked rings. The switching state of the redox-active molecule was shown to change reversibly depending on an applied potential. Electrostatic repulsion drives rotation of one ring relative to the other between open and closed states. LOOKING TO IMPROVE device perfor mance by tailoring the molecules, Jan O. Jeppesen, a visiting graduate student in Stoddart's lab, and postdoctoral associate Hsian-Rong Tseng prepared various rotaxane analogs of the catenane that brought success in 2000. The series of amphiphilic bistable [2]rotaxanes are dumbbell-shaped molecules capped with large stoppers that are threaded through a cyclophane ring. As with the [2]catenanes, the ring's position relative to the other piece of the molecule is controlled by redox interactions. Heath and coworkers constructed circuits with the new molecules using a layer-by-layer deposition method to fabricate tic-tac-toe structures with a few thousand
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molecules situated at each intersection. The team used this fabrication method to prepare 64-bit random-access memory circuits [ChemPhysChem, 3, 519 (2002)}. And Hewlett-Packard Labs announced earlier this month that R. Stanley Williams, Yong Chen, and coworkers have also succeeded in using the UCLA [2}rotaxane molecules to fabricate a 64-bit memory circuit. Only a few years ago, many scientists sat through molecular electronics presentations thinking that the far-fetched nanocomputing goals would never be reached. But the flurry of demonstration results in just the past two years appears to have decreased the number of nonbelievers. Yet some naysayers remain. Just recently, a California professor was furious about a C&EN story on molecular electronics and wrote to the editor, exclaiming, "In reality, there is no such topic!" The professor has a few surprises in store. In reality, molecular and nanoelectronics are bustling fields. Large numbers of commercial products may not be available in the near future, but as the vibrant research activity reveals, they're coming. •
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