• Table of Contents • C&EN Classifieds • Today's Headlines • Cover Story • Editor's Page • Business • Government & Policy • Science & Technology • ACS News • Calendars • Books • Career & Employment • Special Reports • Nanotechnology • What's That Stuff?          Back Issues 2002 2001 2000 1999 1998  Safety Letters  Chemcyclopedia ACS Members can sign up to receive C&EN e-mail newsletter.   Join ACS			October 7, 2002 Volume 80, Number 40 CENEAR 80 40 p. 9 ISSN 0009-2347 MATERIALS SCIENCE INSULATOR GOES CONDUCTING Simple UV method converts metal oxides to transparent conductors MITCH JACOBY Researchers in japan have demonstrated a simple procedure in which an insulating calcium-aluminum oxide can be converted to a transparent electrical conductor by heating the material and then exposing it to UV light [Nature, 419, 462 (2002)]. Transparent conductors could be useful for a number of applications in optoelectronics. The scientists suggest that the new technique may also be used with other main-group metal oxides for patterning thin films with conducting wires or for preparing data storage materials. According to the research group, which includes Hideo Hosono, a professor of functional ceramics at Tokyo Institute of Technology, postdoctoral associate Katsuro Hayashi of Japan Science & Technology Corp., and coworkers, H2 ions are incorporated into angstrom-sized cages in 12CaO·7Al2O3 by heating the compound to 1,300 °C in a mixture of hydrogen and nitrogen. The team notes that the hydrogen anions are formed through reactions of O22 ions in the crystal cages and reactant gases and can be detected via NMR and mass spectrometry methods. IN THE HOLE Unique structural and electronic properties of 12CaO*7Al2O3 enable the oxide to accommodate H– ions and undergo UV-induced conversion from insulator to conductor. COURTESY OF TOKYO INSTITUTE OF TECHNOLOGY Following heat treatment, the group reports, the oxide crystals bearing H2 ions remain transparent and insulating--their room-temperature conductivity is less than 10210 siemens per cm. But upon irradiation with UV light, the crystals change from colorless to yellow-green (but still transparent) and their conductivity jumps to 0.3 S per cm. The light-induced changes persist after the radiation is terminated. The group proposes that UV radiation stimulates electron emission from H2 ions, resulting in neutral atoms that combine to form H2 and electrons that are weakly bound to the slightly positively charged cages--similar to crystal defects known as F+ centers. The electrons can hop to nearby cages, thereby migrating throughout the crystal. In related work, Hosono and coworkers recently reported that oxygen anions (O2 and O22) can be generated and accommodated in large concentrations in the Ca-Al oxide [J. Am. Chem. Soc., 124, 738 (2002)]. "Our work shows that materials that are believed to be impossible to convert to electrical conductors can be converted," Hosono says. He adds that the photosensitivity of H2 ions and the Ca-Al oxide crystal's subnanometer-sized features make it possible to sidestep chemistry textbook descriptions of classical electrical insulators and conductors. Top Chemical & Engineering News Copyright © 2002 American Chemical Society			Related Stories Nanoscale Electronics [C&EN, Sept. 30, 2002] How Oxidation Gets A Foot Up [C&EN, Sept. 23, 2002] Oxide-Ion Conductors: New Family Reveals Novel Design Cincept [C&EN, Apr. 24, 2000] E-mail this article to a friend Print this article E-mail the editor
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