A method for building tiny "drawbridges" could allow engineers to use standard electrical switching techniques to construct color displays from pairs of nanoparticles that scatter different colors of light. The chemical bridges can be created and eliminated simply by applying or reversing a voltage. Researchers from Rice University's Smalley-Curl Institute said it is the first method demonstrated to produce dramatic, reversible color changes for devices built from light-activated nanoparticles. Stained-glass makers have long employed the light-scattering properties of tiny gold nanoparticles to produce glass with rich red tones. Though metal nanoparticles scatter bright light, researchers have found it difficult to use them to produce dramatically different colors. An electron microscope image showing a dimer of silver-plated gold nanoparticles. A layer of silver connects the particles. Images courtesy of C. Byers/Rice University. The Rice team's method for color switching incorporates pairs of metal nanoparticles that absorb light energy and convert it into plasmons. Each plasmon scatters and absorbs a characteristic frequency of light, and even minor changes in the wavelike sloshing of a plasmon shifts that frequency. The greater the change in plasmonic frequency, the greater the difference between the colors observed. To demonstrate the method, professor Christy Landes and study lead author Chad Byers, a graduate student in her lab, anchored pairs of gold nanoparticles — or dimers — to a glass surface covered with indium tin oxide (ITO), the same conductor used in many smartphone screens. By sealing the particles in a chamber filled with a saltwater electrolyte and a silver electrode, Byers and Landes formed a device with a complete circuit. They then applied a small voltage to the ITO to electroplate silver onto the surface of the gold particles. In that process, the particles were first coated with a thin layer of silver chloride. By later applying a negative voltage, the researchers caused a conductive silver bridge to form. Reversing the voltage caused the bridge to withdraw. "Engineers hoping to make a display from optically active nanoparticles need to be able to switch the color," Landes said. "That type of switching has proven very difficult to achieve with nanoparticles. People have achieved moderate success using various plasmon-coupling schemes in particle assemblies. What we've shown though is variation of the coupling mechanism itself, which can be used to produce huge color changes both rapidly and reversibly." Gold nanoparticles are particularly attractive for display purposes because, depending upon their shape, they can produce a variety of colors, according to the researchers. They are also extremely stable, and even though gold is expensive, a small quantity can produce an extremely bright color. The team said the method provides plasmonics researchers with a valuable tool for precisely controlling the gaps between dimers and other multiparticle plasmonic configurations. "In an applied sense, gap control is important for the development of active plasmonic devices like switches and modulators, but it is also an important tool for basic scientists who are conducting curiosity-driven research in the emerging field of quantum plasmonics," said Rice professor and nanophotonics researcher Peter Nordlander. The research was published in Science Advances (doi: 10.1126/sciadv/1500988). An animation illustrating the markedly different colors of light that are scattered due to plasmonic shifts that occur when no metal bridges are present (left) and when they are (right).