They were always the best toys to find in the bottom of a cereal box as a child - the glowing decoder rings, the glowing plastic frogs, the glowing superballs. Why they do what they do, however, is a mental toy for adults.
The pale green light that children gleefully cup their hands to catch a glimpse of is an example of
phosphorescence. Everyone knows that phosphorescent materials, like those toys, glow after being exposed to light,
The answer lies in tiny defects of the material. In any other field, defects are considered a problem; in phosphorescent luminescence, however, nothing works without them. In a recent study by researchers at the University of Washington, different types of defects were introduced into a material to investigate which ones result in luminescent behavior... and which ones are just flaws. The UW group investigated the phosphorescent behavior of zinc sulfide, a semiconductor that phosphoresces when its crystal structure is contaminated by metal atoms, a condition referred to as being "doped" (unfortunately, it only glows under ultraviolet light, meaning that their research isn't going to be exciting any non-black-light-owning children any time soon).
Normally speaking, a semiconductor, like most other solids, with its bands of electrons separated by gaps in energy, wouldn't emit light unless it was heated up to extremely high temperatures. The light is caused by electrons being incited by the energy of the heat to jump up across those gaps, discovering that they don't quite belong there, and falling back to where they originated; when they fall, they release the energy that they absorbed as light. Semiconductors have smaller gaps between their bands than most other solids, and when their structures are doped, the result is flaws in the system of energy gaps. The staggered levels produced mean that an electron that has been jumped across the gap by UV light may fall back stepwise among the defect levels, producing light even at normal room temperatures.
The UW team examined the differences in phosphorescence depending on what metal was being used as the
dopant. Perhaps their task was easier said than done, considering that introducing the metals into the zinc sulfide
couldn't be done the way that marshmallow bits are mixed into a cereal; solutions of the metal ion had to be dried,
ground together carefully with zinc sulfide, and the resulting noxious powder heated to nearly 1000 degrees celsius
(we're talking twenty times as hot as a Phoenix summer day) to get the ions to diffuse through the zinc sulfide's
crystal structure. Care also had to be taken to not let the materials come into contact with iron or nickel, which
had been previously reported to completely obliterate any possible luminescence.
After some twitching noses from sulfur fumes, a few burned fingers, a great developed distaste for grinding grains of powder, and some waiting, the group was presented with a disappointing array of greyish, burnt-looking samples. When the trays were loaded into a UV light box, however, many of the samples crackled with color (such as the platinum-doped green that resembled the pale shade seen in those cereal box toys, or the uranium-doped robin-egg blue). After rinsing, even more of the samples showed the phosphorescent behavior, glowing and then gradually fading to a pale grey when removed from the light.
As black lights grow in popularity as a decoratory item, the novelty of watching white shirts turn purple wears thin, leading consumers towards products that glow the copper green-blue, or the tin brilliant green. As that demand grows, the UW team's findings can help create a wider variety of products, from decoder rings, to plastic frogs, and yes, even to superballs.
Jennifer Barrick is a University of Washington senior who wants to make Japan glow.