Monday, June 13, 2005

"CHIPS: Silica, dye yield new kind of quantum dot"

An alternative to quantum dots-encapsulating organic dyes in a silica matrix-has been developed by researchers at Cornell University. The process, they said, could cut the cost of making optical computing devices, and render them chemically inert as well. The Cornell approach is a departure from the way most quantum dots are fabricated: nanoparticles being doped with heavy metals like cadmium-selenium. "We have encapsulated multiple organic dyes in the core of a nanoparticle," said Ulrich Wiesner, professor of materials science and engineering. "The core is then encapsulated into a pure silica shell for protection." The core-shell architecture can be used in applications ranging from flat-panel displays to medical imaging to sensor and optical lasers that emit a single photon at a time, he said. Wiesner's nanoparticles, which he calls "Cornell dots," are novel. They begin with a core 2.2 nanometers in diameter that contains a few colored dye molecules. The molecules are surrounded with 22.8 nm of silicon dioxide, resulting in quantum dots measuring 25 nm in diameter. This core-shell architecture, Wiesner said, makes his quantum dots as much as 30 times brighter than conventional fluorescent dyes. "The particles are very, very bright, because they act independently rather than quenching each other," he said. "Our dots are almost as bright as quantum dots." Wiesner collaborated with fellow Cornell professors Watt Webb and Barbara Baird. They were assisted by postdoctoral researcher Mamta Srivastava and graduate students Hooisweng Ow and Daniel Larson. The silicon dioxide-or silica-shell also prevents the dyes from fading, while allowing a variety of colors to be produced without changing the diameter of the core. The silicon dioxide-coated nanoparticles are also chemically inert, making them safer to manufacture and handle.