Process yields semiconductor foam

Researchers from Wayne State University have made crystalline aerogels -- new semiconductor materials that are very porous, giving them very high surface areas.

Unlike conventional aerogels, the researchers' materials are crystalline. They are made from cadmium sulfide, zinc sulfide, lead sulfide and cadmium selenium.

The materials contains pores that average 15 to 45 nanometers, which gives the materials useful optical and electrical properties and also allows for rapid transport of gas. It has traditionally been difficult to prepare materials with pore sizes in this range.

The materials could eventually be used as chemical and biological sensors, solar cells and photocatalysts, according to the researchers. Photocatalysts trigger chemical reactions in the presence of light.

The researchers constructed the materials by first making semiconductor nanocrystals. They mixed the nanocrystals with an organic molecule that links the nanoparticles to form an air-filled gel. They dried the gel using highly compressed, or supercritical, carbon dioxide. The materials retain the electrical and optical properties of the nanocrystals rather than those of the bulk materials.

It will be 10 to 20 years before the researchers' materials can be used practically; the general assembly method could be applied using other materials sooner, however, according to the researchers. The work appeared in the January 21, 2005 issue of Science.