Fashioning see-through metal.

Fashioning see-through metal

Do chickens have lips? Do doorknobs yodel? Can metal be transparent? The respective answers are no, no and you wouldn't think so. But it's time to rethink that last one.

The possibility of designing transparent metal films became clear several years ago to scientists at AT&T Bell Laboratories and their colleagues elsewhere. While testing the efficiency of semiconductor devices called photoelectrochemical cells, which gather solar energy to drive chemical reactions, they noticed there was no decrease in the rate of the reaction, which yielded hydrogen in this case, even when the cells were partially covered with platinum catalyst particles--the sites of hydrogen generation. The catalytic metal, deposited on the semiconductor surface as tiny, poorly interconnected metallic islands, behaved as though transparent. Such transparent metal catalysts might improve the efficiency of photoelectro-chemical cells made with them, the researchers suggested. Another potential application of transparent metals, one chemist told SCIENCE NEWS, would be for "smart windows" made of see-through metal/polymer composites whose transparency could be varied at will.

Building on the earlier studies, Charles R. Martin and Michael J. Tierney of Texas A&M University in College Station found a way to make thin gold structures transparent to infrared radiation, and they expect to extend the transparency to visible wavelengths. The researchers use a ceramic membrane pocked with tiny, 200-nanometer pores as scaffolding for growing gold microcylinders in an electrochemical bath. They find that membranes with gold-plugged pores transmit about twice as much light as they would if the plugs were opaque. They reported this work in the April 20 JOURNAL OF PHYSICAL CHEMISTRY.

A theory known as the effective medium theory holds that an ensemble of metal microcylinders will be transparent to light of wavelengths much larger than the cylinders' diameter. An optical electric field induces a "screening charge" on the surface of the microcylinders, according to the theory. This prevents them from absorbing the light energy, which effectively gets "squeezed" into voids between cylinders. The result: More light gets through metal films with this type of micro-structure than with a more continuous sheet-like structure. To make gold transparent to shorter, visible wavelengths, the researchers need scaffolding with pores 10 times finer than presently available.