Charles T. Troy
ALBUQUERQUE, N.M. -- A fresh insight into the nature of phosphors promises brighter, more colorful displays and, ultimately, a challenge to liquid crystals for dominance in the portable display market, say scientists at Sandia National Laboratories.
Liquid crystal displays tend to appear blank when viewed from angles other than straight on, placed in direct sunlight, subjected to rapid changes in temperature or accelerated rapidly. In addition, their batteries run down quickly because their entire screens must be backlighted and then blocked out in sections to provide images. By contrast, a phosphor field emission display -- traditionally used to create light in most television screens -- only energizes pixels that provide information.
The Sandia breakthrough came when scientists probed the mechanism by which a phosphor emits light. They found that the amount of green light emitted by zinc oxide does not depend upon the thickness of the crystal, but upon the density of a defect: oxygen atoms missing from their places in the crystal. Single electrons that remain in the vacant spaces emit green light when a mild electric current is introduced.
The surface rules
"Our work has shown for the first time that the electronic properties at a material's surface have a dominant effect on its luminescent efficiency," said Sandia scientist Bill Warren.
Generating light from phosphors requires large voltage drops of approximately 25 kV -- drops incompatible with battery-powered portable units. By activating the surface, however, Sandia scientists believe they can produce phosphors that operate at 0.5 kV. Less power can be applied because of newly developed microscopic structures shaped like tiny cones that deliver small amounts of low-voltage current to each red-blue-green pixel on a phosphor screen less than a millimeter away.
Sandia's arsenal of analytical tools were the key to the discovery, said physicist Carl Seager. These include photothermal deflection spectroscopy, unavailable commercially, which determines a powder's optical absorption by measuring the increase in heat of a liquid in contact with the powder. The increase changes the liquid's refractivity, which bends a laser beam passing through it.
Sandia also uses a variety of other spectroscopic techniques, including cathodoluminescence and electron spin resonance, which allow the observation of light-emitting centers in atomic detail.