DOI: 10.1126/science.1076768, 1381 (2002);298 Science, et al.A. J. HeinrichMolecule Cascades This copy is for your personal, non-commercial use only. clicking here.colleagues, clients, or customers by , you can order high-quality copies for yourIf you wish to distribute this article to others here.following the guidelines can be obtained byPermission to republish or repurpose articles or portions of articles ): April 7, 2011 www.sciencemag.org (this infomation is current as ofThe following resources related to this article are available online at http://www.sciencemag.org/content/298/5597/1381.full.htmlversion of this article at: including high-resolution figures, can be found in the onlineUpdated information and services, http://www.sciencemag.org/content/298/5597/1381.full.html#ref-list-1, 9 of which can be accessed free:cites 32 articlesThis article 147 article(s) on the ISI Web of Sciencecited by This article has been http://www.sciencemag.org/content/298/5597/1381.full.html#related-urls8 articles hosted by HighWire Press; see:cited by This article has been http://www.sciencemag.org/cgi/collection/chemistryChemistrysubject collections:This article appears in the following registered trademark of AAAS. is aScience2002 by the American Association for the Advancement of Science; all rights reserved. The title CopyrightAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theScience on April 7, 2011www.sciencemag.orgDownloaded fromMolecule CascadesA. J. Heinrich,*† C. P. Lutz,* J. A. Gupta, D. M. EiglerCarbon monoxide molecules were arranged in atomically precise configura-tions, which we call “molecule cascades,” where the motion of one moleculecauses the subsequent motion of another, and so on in a cascade of motionsimilar to a row of toppling dominoes. Isotopically pure cascades were assem-bled on a copper (111) surface with a low-temperature scanning tunnelingmicroscope. The hopping rate of carbon monoxide molecules in cascades wasfound to be independent of temperature below 6 kelvin and to exhibit apronounced isotope effect, hallmarks of a quantum tunneling process. At highertemperatures, we observed a thermally activated hopping rate with an anom-alously low Arrhenius prefactor that we interpret as tunneling from excitedvibrational states. We present a cascade-based computation scheme that hasall of the devices and interconnects required for the one-time computation ofan arbitrary logic function. Logic gates and other devices were implemented byengineered arrangements of molecules at the intersections of cascades. Wedemonstrate a three-input sorter that uses several AND gates and OR gates,as well as the crossover and fan-out units needed to connect them.The scanning tunneling microscope (STM)can be used to build atomically precise struc-tures and investigate their physical and func-tional properties. Here we present a class ofnanometer-scale structures, “molecule cas-cades,” that are both instructive (they enabledetailed studies of adsorbate motion) andfunctional (they perform computation).The motion of single atoms and moleculeson surfaces can be studied in a well-charac-terized environment with the STM (1). Weinvestigated the hopping mechanism of COmolecules in molecule cascades through stud-ies of the hopping rate as a function of tem-perature, isotope, and local environment. Wefound that at temperatures below 6 K, thehopping motion of CO molecules in our cas-cades was due to quantum tunneling of themolecule between neighboring binding siteson the surface. The importance of quantumtunneling in hydrogen diffusion (2) was re-cently demonstrated in an STM study of hy-drogen atoms on a Cu(100) surface (3, 4 ). Incontrast to the random walk of diffusion, thetunneling rate can be engineered in moleculecascades by controlling the hopping directionand interactions with neighboring molecules.At higher temperatures, we observed ther-mally activated hopping with an anomalouslylow Arrhenius prefactor, which we interpretas being due to tunneling of the CO moleculefrom a vibrationally excited state.Although the silicon transistor technologyon which modern computation is based hasshown rapid exponential improvement inspeed and integration for more than four de-cades, it is widely expected that this improve-ment will slow as devices approach nanome-ter dimensions (5). The search for functionalnanometer-scale structures has led to the ex-ploration of many alternative computationschemes, most of which, like the silicon tran-sistor, are based on gating the flow of elec-trons. Such novel systems include quantumdots (6), organic molecules (7), carbon nano-tubes (8), nanowires (9), and the motion ofsingle atoms or molecules (10). However,alternative computation schemes that operateon different principles have also been pro-posed, among them electrons confined inquantum dot cellular automatons (11, 12),magnetic dot cellular automatons (13), andsolutions of interacting DNA molecules (14 ).Computation can also be achieved with pure-ly mechanical means (15), as exemplified bythe calculating engines of Babbage (16 ). Thetoppling of a row of standing dominoes canalso be used to perform mechanical compu-tation. Tipping a single domino causes manyothers to topple sequentially. Such a dominocascade can be used for the one-time trans-port of a single bit of information from onelocation to another, with the toppled and un-toppled states representing binary 0 and 1,respectively. Dominoes can be set in patternsthat perform logic operations. Here we dem-onstrate an analogous form of mechanicalcomputation on the nanometer length scalewith molecule cascades in which we imple-mented all of the logic gates and interconnec-tions required to perform the one-time calcu-lation of an arbitrary logic function.Apparatus and methods. The data pre-sented here were acquired with low-temper-ature ultrahigh-vacuum STMs: one fixed at 5K, and one that is variable in temperaturebetween 0.5 and 40 K. The variable-temper-ature STM uses a novel continuous-circula-tion3He refrigerator for temperatures from1.0 to 4.5 K. Below 1.0 K, the STM operatesin a single-shot mode with a hold time of 10hours. Helium-3 is liquefied without the needfor a pumped4He bath (a so-called “1 K pot”)through the use of the Joule-Thompson effectand counterflow heat exchange.