Clever geometry is the basis of a new material that is said to be ideal for secure data encryption and dense optical information storage [Adv. Mater., 16, 516 (2004)].
The material consists of a lattice of onionlike spheres in which the particle core and its layers each contain a different dye. The material can hold four or more pieces of information in one spot—not just two as in binary optical data storage. And it opens a door to high-density three-dimensional optical data storage.
“The approach is really simple,” says lead researcher Eugenia Kumacheva, associate professor of chemistry at the University of Toronto, who worked with postdocs Ilya Gourevich and Hung H. Pham and microscopist James E. N. Jonkman. They start with colored colloids—polymeric nanospheres labeled with a dye—for example, an ultraviolet dye. Then they envelop the nanosphere, what Kumacheva calls the core, with a shell of another polymer labeled with a dye that has a spectrum entirely distinct from the first—say, a visible dye. Any number of dye-polymer shells can be added. The last shell then becomes the matrix that holds the layered particles.
What this means, Kumacheva says, is that when the embedded nanospheres are lit with UV light, the UV dye shows up. Visible light brings out a different hue. With two dyes, “we have four different ways to write and then read on a single spot,” Kumacheva says: no dye, dye one, dye two, and both dyes together. Three dyes offer eight (23) variations, and so on.
The system, says Younan Xia, associate professor of chemistry at the University of Washington, “can be easily extended into multishelled colloids to include as many types of dyes as one would like to have.” For example, Kumacheva has added a near-infrared dye to a third polymer.
Other materials scientists have tried a variety of methods to place one or more dyes in the same spot. Kumacheva’s advance is in her successful creation of core-shell particles embedded in a matrix. She judiciously chose for the inner core a polymer, poly(methyl methacrylate), that would remain solid while a shell of poly[(methyl methacrylate)-co-(butyl methacrylate)] formed around it. “So when we start heating the arrays of particles, the shell softens, flows, and forms a matrix while the cores remain intact.”
The particles and matrix can be configured for high-density 3-D data storage. They can also be considered “intriguing building blocks for fabricating photonic crystals,” Xia says. And, Kumacheva notes, the technology is well suited for creating identification cards nearly impossible to fake. Canada and the U.S., she says, have an interest in passports and immigration cards that cannot be counterfeited. She imagines a card that shows a picture under visible light, a signature under UV illumination, and a fingerprint under near-IR.