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.