Quantum onions rarely blink.

What is a quantum dot that doesn't blink like a quantum dot? The answer: a quantum onion.

That's what All Mews of the University of Mainz in Germany calls his invention, a ball of orderly arranged semiconductor atoms in nested layers.

The optical properties of such onions could shed light on how electric charges become trapped in regions of semiconductors, Mews says. He also speculates that using the luminescent onions as labels for molecules might improve the accuracy of research studies, diagnostic tests, and some commercial processes.

Quantum dots, crystalline clusters of up to several thousand atoms, confine their charge-carrying electrons and the electrons' positively charged counterparts, called holes, to volumes so small that the quantum mechanical nature of those charge carriers is altered (SN: 11/23/96, p. 327).

An onion is a quantum dot with a more complex structure. Mews starts with a core of cadmium sulfide, adds a shell of mercury sulfide, then finishes with an outer layer of cadmium sulfide. The mercury sulfide layer can be as little as one atom thick.

Compared with research on regular quantum dots, "we went one step more," Mews says. With the extra layer, the researchers added an impurity that can trap charges.

Scientists have begun to investigate swapping quantum dots for dye molecules as markers that respond to laser light (SN: 10/24/98, p. 271). In the past few years, however, researchers have discovered that glowing quantum dots and dye molecules both spontaneously blink off for a tenth of a second to 10 seconds. The blinking happens more often if the laser light is intense.

By contrast, quantum onions rarely blink, Mews reports. He suspects that their layering deters charges from visiting the onion's outermost surface, where they somehow are responsible for blinking.