A Sponge's Guide to Nano-Assembly

Technology Review
Jun 06, 2006

SYNOPSIS: A new way to create complex nanostructures will improve batteries and solar panels.

One of the ongoing goals of nanotechnology is to easily and inexpensively create high-performance materials structured at the nanoscale. And one of the most promising strategies is to attempt to mimic nature's remarkable ability to self-assemble complex shapes with nanoscale precision. Now researchers at the University of California, Santa Barbara (UCSB), using clues gleaned from marine sponges, have developed a method of synthesizing semiconducting materials with useful structures and novel electronic properties. The first applications could be ways to make materials for more powerful batteries and highly efficient solar cells at a lower price.

"We are accessing structures that in some cases had never been achieved before. And in some cases we're discovering electronic properties that had never been known before for that class of materials," says Daniel Morse, professor of molecular genetics and biochemistry at UCSB, who led the project. The method works with a wide variety of materials. So far, he says, the group has made "30 different kinds of oxides, hydroxides, and phosphates."

Today's solar cells and batteries are held back, in part, by their limited ability to transport electrical charge carriers, such as electrons and positive ions, in and out of active materials. One advance that could help is increasing the surface area of a material, while at the same time maintaining a thin-film structure that can easily be incorporated as an electrode layer in a device.

Morse and his colleagues began their research by studying the methods used by marine sponges to make intricate glass skeletons called spicules (see illustration). One type of sponge produces a cylinder that looks as if it were made of woven glass fibers, although it isn't woven at all, but assembled molecule by molecule to make the structure.

In particular, the researchers studied a type of sponge that makes tiny needles of glass. They found that the genes responsible for the glass structures encode for enzymes that serve as both a physical template for the structure and a catalyst for assembling molecular precursors into the desired material.

The scientists developed a synthesis method that uses the basic principles behind the natural assembly method: slow catalysis and the use of a physical template. They found they were able to assemble not only glass, but also a variety of semiconducting materials that could be useful in devices.

The method begins with a solution of molecular precursors. The researchers then expose the solution to ammonia vapor, which, as it slowly diffuses into the solution, acts as a catalyst. The physical template for the material is the surface of the solution. At this surface, where the vapor concentration is greatest, the material forms a thin film.

"At first the crystals form at the [surface], but with time they begin to project down into the solution like stalactites growing down from the roof of a cave," Morse says. "What you end up with is a nanostructured thin film of semiconductor with very high surface area because of all the projecting thin plates or needles that project down into the solution."

The method works at low temperatures, about room temperature, whereas conventional techniques for making semiconducting thin films require a high temperatures -- 400 degrees Celsius, Morse says. It also does not require oft-used harsh acids and bases. In addition to making the process cheaper and easier, the mild conditions could lead to devices that incorporate materials that would be impossible to use with conventional processes. Sometimes, for example, the materials that can be used in a device are limited by the high temperatures used to make the materials. "If you can make them all at room temperature, then you may be able to dope them with dopants that you normally couldn't use at high temperature," says Angela Belcher, materials science and engineering and biological engineering professor at MIT, who finds Morse's work "very exciting."

Ultimately, the payoff from Morse's work studying biological mechanisms may be more than novel thin films, says Aravinda Kini, U.S. Department of Energy materials science and engineering programs manager. Although the current process works only for thin films, further understanding of the catalysis and templating methods of sponges could one day make it possible to fabricate complex machine parts by piecing together molecules. "It's still a dream, but imagine the blade of an aircraft engine being assembled from the bottom up, without any defects, without any very expensive fabrication methods," he says. "That's what is possible. That's what people are dreaming about."