Researchers at the University of California, Berkeley, have created nanoscale particles that can self-assemble into various optical devices. By controlling how densely the tiny silver particles assemble themselves, the researchers can make several different kinds of devices, including photonic crystals. The self-assembling materials could be made cheaply and on a large scale. As a result, the silver nanoparticles could be used to make metamaterials, color-changing paints, components for optical computers, and ultrasensitive chemical sensors, among many other potential applications.

Led by Peidong Yang, a professor of chemistry at Berkeley, the researchers have demonstrated that they can use the nanoparticles to increase the sensitivity of arsenic detection by an order of magnitude. They also made a very robust kind of photonic crystal called a plasmonic crystal. These new structures are "similar to photonic crystals, but better," says Peter Nordlander, a professor of physics at Rice University, who was not involved in the work. Photonic crystals allow some wavelengths of light to pass while filtering out others. They're used commercially to coat lenses and mirrors and in optical fibers; they could also be used in optical computers.

The silver nanoparticles that make up Yang's structures are octahedra with sides of about 150 nanometers; they are very regular in shape and size. Crystal structures made up of these nanoparticles can be made when the particles are simply placed in a test tube filled with water and allowed to pack together. When the water evaporates, a crystal structure remains.

Yang says that the simplicity of his group's process is important. Most nanostructured materials are made from the top down using lithography, which makes them hard to manufacture cheaply and on a large scale. In contrast, Yang's particles are grown in solution. And most self-assembled structures are made up of relatively small particles, says Paul Braun, a professor of materials science and engineering at the University of Illinois, Urbana-Champagne. Larger particles like those used by Yang's group have better optical properties, he says. "This is the first paper demonstrating high-quality self-assembly of metal particles [of this size]," says Braun of Yang's work, which was published in Nano Letters.

When the silver nanoparticles are loosely packed, the structures behave like photonic crystals, allowing some wavelengths of light to propagate and stopping others. When the nanoparticles are densely packed, the structures take on entirely new optical properties, behaving as so-called plasmonic crystals. At the edges of the silver particles, surface energy waves called plasmons become concentrated. Just as photonic crystals allow some photons to pass while restricting others, the new crystals control the flow of the energy contained in light in the form of plasmons. Nordlander says that this phenomenon enables the Berkeley structures to interact with light much more strongly than traditional photonic crystals do. For this reason, he says, the structures should have even more applications than photonic crystals.