classes_winter09_113AID28_Handout_5_Chem113A_W09 - RESEARCH...

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Molecule Cascades A. J. Heinrich,* C. P. Lutz,* J. A. Gupta, D. M. Eigler Carbon monoxide molecules were arranged in atomically precise configura- tions, which we call “molecule cascades,” where the motion of one molecule causes the subsequent motion of another, and so on in a cascade of motion similar to a row of toppling dominoes. Isotopically pure cascades were assem- bled on a copper (111) surface with a low-temperature scanning tunneling microscope. The hopping rate of carbon monoxide molecules in cascades was found to be independent of temperature below 6 kelvin and to exhibit a pronounced isotope effect, hallmarks of a quantum tunneling process. At higher temperatures, we observed a thermally activated hopping rate with an anom- alously low Arrhenius prefactor that we interpret as tunneling from excited vibrational states. We present a cascade-based computation scheme that has all of the devices and interconnects required for the one-time computation of an arbitrary logic function. Logic gates and other devices were implemented by engineered arrangements of molecules at the intersections of cascades. We demonstrate 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 of nanometer-scale structures, “molecule cas- cades,” that are both instructive (they enable detailed studies of adsorbate motion) and functional (they perform computation). The motion of single atoms and molecules on surfaces can be studied in a well-charac- terized environment with the STM ( 1 ). We investigated the hopping mechanism of CO molecules in molecule cascades through stud- ies of the hopping rate as a function of tem- perature, isotope, and local environment. We found that at temperatures below 6 K, the hopping motion of CO molecules in our cas- cades was due to quantum tunneling of the molecule between neighboring binding sites on the surface. The importance of quantum tunneling in hydrogen diffusion ( 2 ) was re- cently demonstrated in an STM study of hy- drogen atoms on a Cu(100) surface ( 3, 4 ). In contrast to the random walk of diffusion, the tunneling rate can be engineered in molecule cascades by controlling the hopping direction and interactions with neighboring molecules. At higher temperatures, we observed ther- mally activated hopping with an anomalously low Arrhenius prefactor, which we interpret as being due to tunneling of the CO molecule from a vibrationally excited state. Although the silicon transistor technology on which modern computation is based has shown rapid exponential improvement in speed 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 functional nanometer-scale structures has led to the ex- ploration of many alternative computation
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