CHEMICAL & ENGINEERING
NEWS OF THE WEEK JUNE 7, 2Q04 - EDITED BY WILLIAM G. gqHl|jLg & JANET S. DODD
SELF-ASSEMBLY
I maintaining their integrity
TWISTED BY DESIGN Ταΐ^^ρ^ο™ mentary, Werner J. Blau and
Helical nanotubes show potential for -^ USe in molecular electronics
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SING A MOLECULE WITH I among the phenylene subjust 96 carbon atoms, restituents that anchor the T E G searchers inJapan have erechains, Aida says. And when the ated a new type of self-assembled helix curls tightly, it forms a tube. structure: helical, graphitelike Using this strategy, Aida and nanotubes up to 10 μιη long [SetFukushima's group was able to enee, 3 0 4 , 1481 (2004)}. W h e n prepare nanotubes in quantitative oxidized, the nanotubes have yield—a great advantage over the conductivity properties that sugsynthesis of other types ofgraphgest potential electronics appliitic nanotubes, according to Aida. cations for the new structures. The tubes exhibit remarkable uniThe work was carried out by formity, with inner and outer diUniversity of Tokyo chemistry ameters measuring 14 nm and 20 nm, respectively π-Stacking inprofessor Takuzo Aida, Takanori Fulaishima, and colleagues as part teractions occur along the length of the Aida Nanospace Project. of the tube between the small The researchers report that they graphitic building blocks, which first designed an asymmetrically are stacked into columns. The nanotubes are thermally stable, substituted hexa^/œrc-hexabenzocoronene (HBC) molecule as a building block for supramolecular self-assembly The platelike systemsofl3 fused benzene rings that make up the HBC moieties are akin to small graphite fragments. Consequently they tend to stack via7t-electronicinteractions. By adding hydrophobic dodecyl chains to one side of the HBC core and hydrophilic methylene glycol (TEG) chains to the other, the researchers imagined that, in a relatively polar solvent like tetrahydrofuran, the amphiphilic molecules would self-assemble into a two-dimensional bilayer ribbon. The researchers indeed found that the nonpolar dodecyl chains interdigitate, like sticky fingers, bringing the two HBC layers of the bilayer ribbon together. On the ribbon's surface, the polar TEG chains suppress the buildup of additional layers. This ribbon then curls into a helix, driven by steric repulsion I HTTP://WWW.CEN-ONLINE.ORG
^derjFiemingofTnnity College, Dublin, write that the new route "demonstrates that precise control of intermolecular and environmental forces can lead to graphitic nanotubes with defined dimensions, he licity, and electronic properties—exacdy as one frequently needs in molecular electronics and most other applications of carbon nanotubes." Motivated by the nanotubes'potential in molecular electronics, the researchers investigated the electroconductive properties of the structures. As prepared, the tubes are insulating. But when oxidized, their conductivity increases with decreasing ternperature—indicating that the oxidized tube is semiconducting. I This conductivity is interesting, the researchers say "because it is realized by a long-range intermolecular electronic communication through graphitelike molecI ular arrays."— BETHANY HALF0RD TWISTER Schematic illustration (below) and transmission electron micrograph (left) of the supramolecular helix and nanotube. Polar solvents—like the THF/water mixture that this TEM sample was prepared in—can prevent the helix from curling tightly into the tube structure.
C&EN
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7,2004
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