The Unbearable Lightness of Conducting

TOKYO--Much as material scientists dream about it, convincing an insulator to conduct electricity is no easy feat. It typically requires doping the insulator with conductive impurities, a finicky process that works with only a limited number of materials. Now researchers have stumbled onto an easier approach that could be put to use in solar panels, liquid crystal displays (LCDs), and other applications where conventional opaque wiring affects transparency or the transmission of light.

The new conductor is a calcium-aluminum oxide known as C12A7. Like many ceramic oxides, including glass, C12A7 is optically transparent but electrically insulating. C12A7 crystals consist of a latticework of molecular cages, each defined by six positively charged calcium ions. Some of these cages contain so-called free oxygen ions. These negatively charged oxygen ions balance the positive charges of the calcium ions.

Partly by chance, a team led by Hideo Hosono and colleagues at an Exploratory Research for Advanced Technology (ERATO) project of the governmental Japan Science and Technology Corp. in Kawasaki discovered a process that could make C12A7 conduct electricity. Hosono heated C12A7 crystals at 1300°C for 2 hours in a hydrogen atmosphere, so that hydrogen ions replaced the free oxygen ions in the cages. Hydrogen ions are particularly photosensitive, and the team found that shining ultraviolet light on the annealed material transforms it from an insulator into a reasonably efficient conductor. Hosono theorizes that the UV light causes the caged hydrogen ions to emit an electron, which jumps to a nearby empty cage. The hydrogen atoms then combine to form stable H2 molecules, leaving the electrons free to migrate through the crystal as an electric current. In a further twist, the team reports in the 3 October issue of Nature, the crystals could be turned back into insulators. All the researchers had to do was heat them to above 320°C, then the H2 molecules split and recaptured the electrons.

Turning such a highly insulating material into a conducting material "is quite an accomplishment," says Thomas Mason, a materials scientist at Northwestern University in Evanston, Illinois. He adds that the process opens up the possibility of imprinting a microelectronic circuit on a thin, transparent film with a single flash of UV light through a patterned mask: The lines and points exposed to the light would become conducting wires and junctions, all separated by the insulating regions. Such circuits are currently used to control the pixels of liquid crystal displays; but forming them requires a time-consuming multistep photolithographic process. Hosono says that "invisible electronic circuits" could allow windows to double as solar panels and be put to use in other yet-to-be-imagined applications.

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