Cheap, Self-Assembling Optics

Researchers have made new nano building blocks for optical computing and solar-cell coatings.

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.

Light tricks: This cuvette contains a solution of silver nanoparticles in the process of self-assembling into a so-called plasmonic crystal whose optical properties are highly dependent on the space between the particles. At the top, the nanoparticles are relatively far apart. At the bottom of the cuvette, the nanoparticles are densely packed.

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.

Tiny tiles: Silver octahedra whose sides measure about 150 nanometers across pack together when suspended in water. The optical properties of the resulting crystals are highly dependent on the space between the particles.

Braun says that one exciting application that’s possible because of the cheap self-assembly process is that the Berkeley materials could be used to make tunable coatings that change color depending on the spacing between the silver particles. The same technique could be used to make materials that can change how strongly they transmit certain wavelengths of light. These coatings might serve as camouflage for military vehicles, lens coatings that can vary their transmission, and coatings for more-efficient solar cells. Unlike organic dyes being developed for these purposes, says Braun, the silver nanoparticles will probably hold up better over time.

The Berkeley building blocks might also be used to make new metamaterials for cloaking and super-resolution imaging, says Nicholas Fang, a professor of mechanical science and engineering at the University of Illinois, Urbana-Champagne. Most metamaterials, whether designed for the purpose of concentrating light in new microscopes or deflecting light around objects for invisibility cloaks, have scalability problems. Yang’s building blocks, says Fang, “will help conquer the bigger challenges of manufacturing.”

One application that Yang has already demonstrated is the use of plasmonic crystals made up of his building blocks to enhance the sensitivity of a chemical-detection technique called Raman spectroscopy. Yang’s group tested groundwater known to be contaminated with arsenic and found that the crystals increased the sensitivity of detection from ten to one part per billion–the most sensitive detection of arsenic yet performed. Yang says that he hopes the crystals will be incorporated into cheap, portable chemical sensors for use in places in India and China, where the drinking water contains arsenic at unhealthy, but previously undetectable, levels.

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