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he practical consequences of covalent chemistry on the electronic properties of single-walled carbon nanotubes are highlighted in two newly published papers. REACTIVITY Selective chemistry can be used to separate metallic nanotubes from a mixture. ADAPTED FROM SCIENCE © 2003 Robert C. Haddon and coworkers at the University of California, Riverside, have shown that metallic nanotubes functionalized with dichlorocarbene take on semiconducting properties [Science, 301, 1501 (2003)]. The effect is due to a change in the hybridization of carbon atoms in the nanotube wall from sp2 to sp3. The change causes the electronic band gap to widen to that of a semiconductor. "Any control of nanotube property is useful" because nanotubes are being explored for various applications based on their electrical conductivity, notes Michael S. Strano, assistant professor of chemical and biomolecular engineering at the University of Illinois, Urbana-Champaign. Indeed, Strano, chemistry professors James M. Tour and Richard E. Smalley at Rice University, and coworkers have developed selective covalent chemistry that differentiates between metallic and semiconducting single-walled carbon nanotubes [Science, 301, 1519 (2003)]. The chemistry is reduction of a diazonium salt by a carbon nanotube to form a carbon-carbon bond. "The first step involves the nanotube donating an electron to the reagent," Strano says. "Valence electrons in metallic nanotubes are more reactive than those in semiconducting ones, so they donate first." And in a mixture of metallic and semiconducting nanotubes, the metallic ones react completely before any of the semiconducting ones are touched. Heating removes the new covalent bonds and restores the original nanotube. Thus, the chemistry can be used to physically separate metallic nanotubes and to recover them in the pristine state. Work is under way to demonstrate this application, Strano says. |
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