PHOTOLUMINESCENCE: Simple silicate displays photoluminescence

"Never lose a holy curiosity," said Einstein, and Will Green took good heed. Green, a doctoral student in chemistry working under professor Michael J. Sailor at the University of California (San Diego, CA) persisted in an otherwise-failed experiment and so discovered a photoluminescent material composed of silicate containing a small amount of carbon, or what is essentially beach sand mixed with a little driftwood.

The secret is in the mixing. The carbon atoms are chemically deposited inside the lattice of the silicon. They apparently cause weaknesses in the silicon-oxygen bonds near the surface, permitting them to break and reform, which induces the phenomenon of fluorescence.

Green was investigating electrochemiluminescence of porous silicon in a formic acid solution. The resulting material was photoluminescent but unstable, burning out in about half an hour. But after Sailor urged him to write up his results and move on, Green continued to explore, determining that, out of solution, a white scum on the surface of the material glowed much brighter than porous silicon when placed under a black light. His silicate phosphor glowed a bright white—with an emission maximum between 450 and 600 nm—with a quantum efficiency of more than 30% when measured under 365-nm light, far above the porous silicon quantum efficiency of about 5% at room temperature.¹

Sailor`s group then developed a sol-gel technique to make the material. Using a network of porous silicon, a reaction of tetramethoxysilane (TMOS) with organic carboxylic acids produced a glass that, when thermally treated between 200°C and 500°C, produced a whitish material that displayed photoluminescence under an ultraviolet (UV) light. The material is simple, environmentally friendly, and stable if heated.

A literature search found two previous experiments that had reported making the material, but no one had ever put it under a black light. Sailor, who described Green as a "Edisonian" scientist, said it reminded him of the discovery of the photoluminescence of porous silicon itself—although that material was first made in 1956, its photoluminescent properties were not discovered until 1990 by Lee Canham. "This is a discovery on a fundamental level," said Sailor. "You can take sand and put carbon in the lattice in the right way and you`ll get something quite luminescent—which is not what you would expect."

Augmenting traditional phosphors

The material is the most-efficient phosphor yet created that does not contain metal activators, according to the research group. More-common phosphors, such as those used to coat the inside of fluorescent light tubes, rely on toxic metals cadmium, which when excited with UV light causes the phosphors to glow. Such metals create problems with disposal—fluorescent light tube disposal is considered the second-largest source of mercury pollution entering the environment by the Environmental Protection Agency. (Mercury is a fill gas in fluorescent lights.)

While not as energy-efficient as the best phosphors available, the material might be applied to further coat the inside of fluorescent tubes, increasing their production of visible light. About 55% of the energy consumed by electric fluorescent lighting is lost as heat. Most phosphor in a fluorescent tube is excited with a wavelength of 254 nm, producing a photon of visible light with lower energy. But because mercury also has an excitation wavelength of 356 nm that is not used by traditional phosphors, these photons escape as heat. The new silicate material, which can be excited with the longer wavelength, could add to the fluorescence and increase energy efficiency by a few percent.

The material might be more useful in lighting liquid-crystal laptop displays. Such displays use small UV light tubes that illuminate a phosphor-coated sheet of glass. The phosphor glows and backlights the rear of the screen, and the liquid-crystal shutters open and close to form the image. The phosphor requires a binder so it will stick to the glass, which is customarily a silicate that is optically dead. Sailor`s group has demonstrated that its new silicate can function as a binder—one that would glow in and of itself and add a few percent to energy efficiencies. That`s not a big deal for overhead lighting but it is for laptop lighting, where lifetimes are limited by screen lifetimes—the phosphor can flake away—and, more immediately, battery lifetimes.

Because the material converts blue light to white light very efficiently, another potential application is as a white-light-emitting device (LED), putting it on the front face of a blue LED. Nichia Chemical Industries Ltd. (Anan, Japan) does this today, using a conventional white phosphor that contains cerium, an expensive metal. Biotechnology firms have also shown interest in the material, recognizing that it is probably nontoxic to cells. Sailor and his group are looking at using different carboxylic acids to impart different properties to the material. One type produces a water-soluble version that glows, others have a slightly different spectral distribution, and yet another is optically active, rotating plane-polarized light.

"It`s the kind of chemistry that`s simple, straightforward, and easy to do," said Sailor. "It seems so simple that somebody should have discovered it a long time ago. I`m still waiting to get a call from someone saying, `Ah, we discovered this in 1959, why didn`t you reference us?`"