mmm i mm SCIENCE
SUPERCONDUCTING NANOTUBES Small diameter is the key to high superconducting temperature
TINY TUBES Superconducting nanotubes inside zeolite pores (inset), against backdrop of zeolite crystals.
C
ARBON NANOTUBES MAY BE
made only of carbon, but their unusual shape and size give them a host of intriguing
electronic properties, the most recently discovered one being superconductivity Hong Kong University of Science & Technology physicists Ping Sheng, Ning Wang, Zi-Kang Tang, and colleagues now have convincing experimental evidence that nanotubes with exceptionally small diameters exhibit superconductivity at relatively high temperatures {Science, 292, 2462(2001)]. The Hong Kong researchers used a technique they developed
INNOVATION
Cleaning Glass>: A Thing Of Tl he Past?
A
particle widely used as a white, opaque pigment in paint—titanium dioxide—is at the heart of a new system to make selfcleaning clear window glass. British glassmaker Pilkington says it is the first to make a glass that could put professional window washers out of business. All the glass needs is sun and rain to stay clean. Now in production at the company's Ottawa, III., plant 80 miles southwest of Chicago, the glass should be available in new windows later this year. Pilkington Glass's Kevin D. Sanderson, an
SEEING CLEARLY Water sheets off glass coated with Ti0 2 (left) while leaving streaks on regular glass.
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inorganic chemist with a Ph.D. from Imperial College of Science, Technology & Medicine at London University, says the glass cleans itself by two modes. First, the Ti0 2 on its surface acts as a catalyst in the presence of ultraviolet light to reduce organic dust and grime to water and carbon dioxide. Second, because Ti02 reduces surface tension, rainwater "sheets down the surface" and washes dirt away.
Sanderson says he led a five-year effort of about 20 chemists, chemical engineers, production engineers, and physicists who are exploring new coatings technology to make the new Pilkington Activ glass. What they came up with was "a proprietary mixture of chemical precursor materials and a proprietary chemical vapor deposition process" to lay down a 500- to 600-A film < on the surface of the glass 2 while it is still in its mol< ten state at about 700 °C. g The proprietary mixture 1 flows onto the glass as a o gaseous stream, reacts, | and binds to the glass, ^ forming Ti02. £ Others are working on % similar technology. PPG s says it will introduce a self-cleaning glass soon.—MARC REISCH
to synthesize single-walled nanotubes inside the channels of zeolite crystals (C&EN, Nov. 6, 2000, page 9). The tubes, which have diameters of only 4 A, appear to become superconducting at about 15 K. These latest results come on the heels of another recent report, from French and Russian researchers, that small bundles of single-walled nanotubes superconduct, albeit at a low temperature of 0.55 K [Phys. Rev. Lett., 86,2416(2001)}. T h e Hong Kong group's results agree with predictions made by University of California, Berkeley, physics professors Marvin L. Cohen and Steven G. Louie. Their calculations indicate that the smaller the tube diameter, the higher the superconducting temperature. This is due to the greater curvature of the tube, which increases the interaction between electrons and lattice vibrations known as phonons— a property essential for superconductivity It's to this extreme curvature that some scientists attribute the superconductivity of nanotubes' fullerene cousins. Alkali metaldoped fullerenes superconduct at temperatures up to 40 K, and electron hole-doped fullerene crystals superconduct at 52 K (C&EN, Dec. 4,2000, page 12). Mildred S. Dresselhaus, a physics professor at MIT, calls the new research "noteworthy," observing that the authors may have evidence of so-called onedimensional superconductivity This has been an elusive phenomenon that may have numerous exotic properties because electrons are confined to one dimension. "This may be the best example" of 1-D superconductivity, Cohen notes. The group will now try doping the nanotubes to see if they can increase the superconducting temperature even further. — ELIZABETH WILSON HTTP://PUBS.AC5.ORG/CEN
