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MOLECULES ALL IN A ROW New method fabricates nanostructures automatically and simultaneously
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ousing materials with reactive chemicals may not sound like a promising way to make precise nanostructures. But according to re searchers in Canada, that simple proce dure can be made to work admirably. In laboratories all over the world, scientists are working at developing nanometer-scale electronic devices, chemical sensors, and other hardware for tomorrow's microscopic machin ery. Many advances have already been made, but moving nanoscience from great ideas to useful products requires coming up with fast, reliable, and prac tical fabrication methods as well as procedures for wiring up miniature structures into functioning circuits. Scanning tunneling microscopes (STMs) and related tools have been used in the past few years as tiny twee zers with unparalleled dexterity to build circuit elements from individual atoms. STM manipulations can be extremely accurate spatially, but the atom-at-atime approach used with STMs is too slow to make large numbers of de vices quickly. Self-assembly techniques would be ideal for these types of small-scale applications. But lacking the ability to carefully control the shape and location of the group of at oms or molecules that coalesce automatically, researchers have not been able to take advantage of those spontaneous processes, for example, to place tiny conducting wires exactly where they are needed. Now, scientists at the Canadian Na tional Research Council's (CNRC) Steacie Institute for Molecular Sciences, Ottawa, have demonstrated a selfassembly procedure in which organic molecules gather in neat rows on a sili con surface in a self-directing chemical growth process leading to lines of mol ecules with regular and controllable nanometer-scale dimensions {Nature, 406,48(2000)]. 10 JULY 10, 2000 C&EN
In the new procedure, developed by CNRC staff scientist Gregory P. Lopinski and senior research officers Robert A. Wolkow and Danial D. M. Wayner, the ar rangement of atoms at the solid's surface di rects impinging organic molecules into forma tion. The surface struc ture controls the orien tation of the lines and the spacing between ad sorbed molecules. And because many materials of technological impor tance can be prepared with crystalline surfac es, the enormous num Wolkow ber of identical rows of surface atoms may lead to parallel fab rication of identical complex nano structures. "Self-directed growth of molecular nanostructures is one form of self-rep lication that is essential for the real ization of molecular devices," says Calvin F. Quate, a professor of electrical engineering at Stanford University. 'This
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Minimal specimen pretreatment is sufficient to allow styrene molecules (orange protrusions in STM image, top) to self-assemble into nanometer-scale lines on silicon. As indicated schematically (above), styrene attaches to silicon via C-C double bonds, leaving an array of closely spaced phenyl rings.
represents a distinct advance in nanoscale devices." Demonstrating the technique's abil ity to pattern a surface with ordered lines of styrene molecules, the CNRC scientists prepare an atomically per fect silicon specimen in which an ex posed (111) crystallographic face is coated with a layer of hydrogen atoms. Then the group uses an STM tip to se lectively remove a few hydrogen at oms, leaving behind reactive dangling bonds (unsaturated silicon valencies). Flowing styrene over the surface caus es the organic molecule to bond at a reactive silicon atom ξ and pluck a hydrogen * atom from an adjacent | position. That step pro?, duces another reactive 2 silicon center, setting έ off a chain reaction, the group proposes. "We've limited the need for arduous atomby-atom STM proce dures," Wolkow com ments, "and unleashed a spontaneous process with the potential to drive growth of identical n a n o s t r u c t u r e s auto matically and simultaneously." Mitch Jacoby
China Approves BASF Petrochemical Complex BASF's plan to build an integrated pet rochemical complex in Nanjing is on a fast track. Coinciding with a visit to Germany by Chinese Premier Zhu Rongji, BASF received approval on June 30 for the project's feasibility study. The firm had submitted the study to Beijing authorities less than two months earlier. Representatives of BASF and its state-owned partner China Petroleum & Petrochemical Co. (Sinopec) immedi ately signed a joint-venture agreement as Zhu and German Chancellor Gerhard Schroder looked on. Located in the industrial heartland of a country that is a major importer of basic chemicals and polymers, the BASF complex is a key example of the company's "verbund" strategy—build ing integrated sites that have produc tion synergies. BASF expects the $2.6 billion project to be completed in
