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65646362616059585756555453525150494847464544434241403938373635343332313029282726252423222120191817161514131211109876www.sciencemag.org SCIENCE VOL 294 9 NOVEMBER 20011293frequently includes two or three watermolecules. Prompted by a Lewis acid-baserelationship with the metal ion, watermolecules often produce OH−, which thengoes on to attack other molecules in closeproximity to the metal (9). An example isZn2+in carbonic anhydrase where the re-sultant OH−attacks CO2. The coordinationof these molecules is more likely to resem-ble the molecular arrangement determinedfrom cluster studies than the time-aver-aged picture derived from ions in solution.References 1. S. Sugano,Microcluster Physics(Springer, Berlin,1991).2. P. Kebarle,Annu. Rev. Phys. Chem. 28, 445 (1977).3. M. Peschke, A. T. Blades, P. Kebarle,J. Phys. Chem. A102, 9978 (1998).4. R. R. Wright, N. R. Walker, S. Firth, A. J. Stace,J. Phys.Chem. A 105, 54 (2001).5. X.-B. Wang, X. Yang, J. B. Nicholas, L.-S. Wang,Science294, 1322 (2001).6. A. J. Stace,Phys. Chem. Chem. Phys. 3, 1935 (2001).7. J. Burgess,Ions in Solution (Horwood, Chichester, UK,1988), pp. 51–61.8. G. Markovich, S. Pollack, R. Giniger, O. Cheshnovsky,J.Chem. Phys. 101, 9344 (1994).9. J. J. R. Frausto da Silva, R. J. P. Williams,The BiologicalChemistry of the Elements (Clarendon, Oxford,1997), pp. 299–312.S CIENCE’ S C OMPASSThe rapid miniaturization of electronicsto the micrometer scale has been a keyforce driving scientific and economicprogress over the past 25 years. Nano-meter-scale electronics (nanoelectronics) isthe closely watchednext frontier (1–5).Two reports in thisissue describe dra-matic steps towardthe realization of electronic nanocomputers.Bachtold et al. (page 1317) demonstratelogic circuits constructed from individualcarbon nanotube molecules (6). Huang et al.(page 1313) have assembled logic circuitsfrom semiconductor nanowires (7).In recent years, researchers have reporteda variety of molecular-scale wires andswitches (8–21), including molecular-scaletransistors based on carbon nanotubes (8) andsemiconductor nanowires (9). However, thetwo reports in this issue are the first to ad-vance molecular-scale electronics fully fromthe single-device level to the circuit level.Both groups developed new methods to meettwo key device requirements that previouslyprevented the realization of transistor circuits.First, the component transistors must producesignal amplification or “power gain” with anoutput to input ratio much greater than 1.Second, each transistor must be controlled byits own local “gate” contact.Bachtold et al.’s study builds on thesame group’s earlier discovery that individ-ual semiconducting nanotubes adsorbed be-tween two metal contacts on a silicon sub-strate behave like the field-effect transistorsin today’s microcomputers (8). However,the controlling gate contact in that experi-ment consisted of the entire supporting sili-con chip. In such a layout, multiple nan-otube devices placed on a chip all must beswitched simultaneously. Furthermore, thepower gain was less than 1 because the sili-con oxide insulator between the gate con-tact and nanotube was relatively thick, pre-venting sufficient capacitive coupling be-tween the gate contact and nanotube.To construct nanotube circuits, the grouphas now used electron beam lithography topattern local aluminum gate contacts and ex-posed them to air to form very thin insulat-ing layers on the aluminum leads (6). Insula-tor thickness is reduced substantially, en-abling the new nanotube transistors to oper-ate independently with a gain ratio in excessof 10, a remarkable increase. By wiring nan-otube transistors together with gold intercon-nects made by lithography, the authors haveconstructed a range of logic circuits.Huang et al. also build on their earlierachievements in devices to achieve cir-cuits. Earlier this year, the group demon-strated diodes and bipolar transistors madefrom nanowires in a crossed geometry (9).In the present work, they assemble OR andAND logic circuits with only diodes, butto construct other circuits required the de-velopment of nanowirefield-effect transistors.The new nanowire tran-sistors are formed byplacing two nanowiresin a crossed geometryand using thermal heat-ing to generate an insu-lating oxide betweenthe nanowires. As withBachtold et al.’s nan-otube transistors, thenanowire transistors fea-ture local gate contactswith thin insulators andare thus easily integratedinto transistor circuits.With the exception ofthe contacts, Huang etal.’s nanowire circuits areassembled without “top-down” methods such aslithography. Instead, “bot-tom-up” parallel assemblytools such as microflu-idics are used. This fea-ture enables them to buildand test relatively large numbers of devicesand demonstrate readily reproducible behav-ior in them. Furthermore, Huang et al.’s cir-cuits incorporate at least one naturalnanometer-scale metric—the constant, smalldimension of the crossing points of thenanowires—suggesting that the entire cir-cuits might be shrunk in a straightforwardway to the nanometer scale. This capabilityis important given that the circuits in bothstudies are still micrometer-scale systems.The two reports use very differenttypes of nanometer-scale structures anddifferent techniques for assembly, thuspursuing different routes to building elec-tronic nanocomputers. In the variety andcomplexity of the circuits they havedemonstrated, both surpass two other im-portant results in nanoelectronics an-nounced very recently by Derycke et al.(10) and Schön et al. (11, 12). Derycke etal. demonstrated a NOT logic circuit orPERSPECTIVES: NANOTECHNOLOGYToward NanocomputersGreg Y. Tseng and James C. EllenbogenS S S SSSSS S SSiO2NNNNZnABDCNanowiresSmall organic moleculesBiomoleculesCarbon nanotubesApproaches to molecular-scale electronics. (A) Diodes and tran-sistors based on semiconductor nanowires are assembled with mi-crofluidics to form logic AND, OR, NOR, and XOR circuits and logicfunctions such as a half adder (7). (B) Carbon nanotube transistors(8) are connected by gold interconnects to construct logic circuitssuch as a NOT circuit, NOR circuit, static random access memory(RAM) cell, and ring oscillator (6). (C) Field-effect transistors basedon self-assembled monolayers of polyphenylene molecules are com-bined to create a NOT circuit (11,12). (D) Porphyrin molecules storedigital information as electrical charges like dynamic RAM


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