Silicon cocktails: whipping up a glow.

Now that the initial novelty of teasing visible light from acid-etched wafers of porous silicon has started to subside, researchers are rapidly broadening the conditions under which this surprising glow appears. In the latest development, chemists have shown that microscopic particles of silicon suspended in a liquid also luminesce, giving off an orange-red light.

Michael J. Sailor and his collaborators at the University of California, San Diego, have developed what they call a "convenient" procedure for breaking porous silicon into tiny particles and generating luminescent suspensions in a variety of liquids. The particles retain their ability to give off light even when subsequently embedded in a flexible, transparent plastic film.

"This method of preparation opens up a new area of study for luminescent porous silicon and provides greater options for exploiting its properties," the researchers say. They detail their methods in the Jan. 3 SCIENCE.

The San Diego group prepared porous silicon by letting a hydrogen-fluoride solution electrochemically eat away parts of a wafer of pure, crystalline silicon. When illuminated by ultraviolet light, the resulting porous material glowed, emitting light visible to the naked eye as red-orange or orange-yellow.

"We observed visible luminescence immediately upon removal of the samples from the electrochemical etch," the researchers report.

The next step involved immersing the acid-treated wafer in a liquid such as toluene or methylene chloride, then placing the mixture in an ultrasonic cleaner for up to two hours. The cleaner's high-frequency sound waves caused the fragile silicon wafer to fragment into fine particles, which dispersed into the liquid and luminesced when exposed to ultraviolet light.

Electron microscope images revealed that the silicon particles present in the liquid had irregular shapes and that their diameters ranged from several microns to a fraction of a nanometer. These findings are consistent with the known structural properties of bulk porous silicon (SN: 8/31/91, p.135).

Sailor and his co-workers also made luminescent plastic films by adding polystyrene to a suspension of silicon particles in toluene, then pouring the mixture onto a glass slide and letting it harden in air. Although the film glowed with noticeably less intensity and its luminescence had shifted to shorter wavelengths than that of a silicon dispersion in toluene, the light remained visible to the naked eye.