Wednesday, November 01, 2006

Silicon and Sun

In his lab facing the Pacific Ocean, Daniel Morse is learning new ways to build complex semiconductor devices for cheaper, more efficient solar cells. He has an unlikely teacher: sea sponges.

By Kevin Bullis

Daniel Morse holds a species of marine sponge commonly known as Venus's flower basket. (Credit: Gregg Segal)

In his beachfront office overlooking the Santa Barbara channel, Daniel Morse carefully unwraps one of his prized specimens. An intricate latticework of gleaming glass fibers, it looks like a piece of abstract art or a detailed architectural model of a skyscraper. But it's actually the skeleton of one of the most primitive multicellular organisms still in existence--a species of marine sponge commonly known as Venus's flower basket. Morse, a molecu­lar biologist at the University of California, Santa Barbara, wants to know how such a simple creature can assemble such a complicated structure. And then he wants to put that knowledge to work, making exotic structures of his own.

The lowly sponge has come up with a remarkable solution to a problem that has puzzled the world's top chemists and materials scientists for decades: how to get simple inorganic materials, such as silicon, to assemble themselves into complex nano- and microstructures. Currently, making a microscale device--say, a transistor for a microchip--means physically carving it out of a slab of silicon; it is an expensive and demanding process. But nature has much simpler ways to make equally complex microstructures using nothing but chemistry--mixing together compounds in just the right combination. The sponge's method is particularly elegant. Sitting on the seabed thousands of meters below the surface of the western Pacific, the sponge extracts silicic acid from the surrounding seawater. It converts the acid into silicon dioxide--silica--which, in a remarkable feat of biological engineering, it then assembles into a precise, three-dimensional structure that is reproduced in exact detail by every member of its species.

What makes the sponges' accomplishment so impressive, says Morse, is that it doesn't require the toxic chemicals and high temperatures necessary for human manufacture of complex inorganic structures. The sponge, he says, can assemble intricate structures far more efficiently than engineers working with the same semiconductor materials.

This primitive creature and a number of other marine organisms have become an inspiration for researchers who hope to find simpler and cheaper ways to build inorganic structures, such as semiconductor devices, for use in computer microchips, advanced materials, and solar cells. The goal is to make silicon and other inorganics self-assemble into working electronics in the same way that the sponge assembles silica into complex shapes (see "Others in Bio-Inspired Materials,"). Energy-intensive, billion-dollar semiconductor fabrication facilities might then be replaced by vats of reacting compounds. But while practical industrial processes are still some way off, scientists are coming to understand how sponges and other sea creatures perform their microengineering miracles.

Morse and his team, for instance, are already using biological tricks learned from the sponge to make new forms of semiconductors with intriguing electronic properties, including the ability to convert light into electricity--properties that could be useful in making cheaper, more efficient solar cells. His group, says Morse, is building "structures that had never been achieved before."