FOR IMMEDIATE RELEASE
September 24, 1996
ALBUQUERQUE, NM -- A fundamental insight achieved into crystalline materials called phosphors may help industry increase the amount and quality of colored light they give off and lead to the replacement of liquid crystal portable display screens, say scientists at Sandia National Laboratories, a U.S. Department of Energy national security laboratory.
Sandia's foray into phosphor luminescence should help create improved portable computer display screens for future foot soldiers, tank commanders, supersonic airplane pilots, and -- through licensing agreements -- civilian laptop and portable TV users.
Liquid crystal displays have the annoying and, in critical situations, dangerous tendency to go blank if looked at 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 the entire screen is backlit and then blocked out in sections to provide images.
A phosphor field emission display -- traditionally used to create light in most television screens -- only energizes pixels that provide information.
The insight came when Sandia scientists tried to understand 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.
"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. "Now we're changing the chemistry of the surface to achieve the greatest luminescence."
A special research and development agreement with Motorola's Product Development Department is expected to go into effect in late September, brokered by the federally funded, New Mexico-based nonprofit development corporation AMMPEC. [AMMPEC stands for Advanced Materials and Manufacturing Processes for Economic Competitiveness.] Sandia will help improve phosphors for Motorola's flat panel field emission displays. The work was funded in March for a three-year span by a grant of $1 million from the Defense Advanced Research Projects Agency.
Aspects of the work have been published this year in Applied Physics Letters (January 15) and the Journal of Applied Physics (May 15). A third paper accepted for publication in the Journal of Luminescence is in the Sept. issue.
Zinc oxide -- better known in cream form as a sun block and baby's skin soother -- was chosen because of its simple, two-component lattice. While most phosphors comprise three, four, or even five elements in complex lattices, Sandia scientists believe that development of other phosphors will benefit from the knowledge gained by studying zinc oxide. Other phosphors include those that emit blue and red light, the primary colors which combine with green to form to form full-color TV or computer images.
"Zinc oxide does not quite have the right chromaticity -- it doesn't look naturally green, like green grass," said Sandia scientist David Tallant. "But the color can be balanced. It also can be used in monochrome displays. We used that material to tune up a method to study other phosphors.
"Industry is going ahead with phosphor flat panel field emission displays because it believes it can work out the problems. In a few years, it'll build something much better, but it can make flat panel displays work now, and the company that prospers in the long run will be the one that gets a usable product out on the market as soon as possible and continues to improve it," Tallant said.
Previous methods of generating light from phosphors, like those used in most television sets, require large voltage drops across bulky cathode ray tubes to blitz relatively large volumes of phosphor. The voltage required -- approximately 25 kilovolts -- is incompatible with battery-powered portable units, and the efficiency of light generated by that method declines rapidly when incoming energy drops below 5 kilovolts.
By activating the phosphor surface, Sandia scientists believe they can produce phosphors that operate at 0.5 kilovolt. The scientists now use a few thousand volts to better benchmark efficiencies at lower voltage. Less power can readily be applied because new technology has 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.
"The portable display-building community wants a device that operates at low voltage, a material whose surface dominates its properties, and an understanding of how surface defects that generate light can be used to improve device performance," said Tallant.
"At low voltages, the surface properties of phosphors dominate their light emissions, and surface engineering becomes a key element in improving device performance," said Sandia scientist and University of New Mexico post-doctoral staff member Karel Vanheusden.
"There are huge Department of Defense needs for flat panel displays for aircraft, and field emission displays will outperform liquid crystal displays in a number of applications," said Warren.
Sandia's unique analytic capabilities were used to obtain a fundamental linkage between luminescence and specific defects or dopants, said Sandia physicist Carl Seager.
The tools include photothermal deflection spectroscopy, unavailable commercially, which measures optical absorption in a powder by measuring the increase in heat of a liquid in contact with the powder. The heat increase causes a change in the liquid’s refractivity. That change bends a laser beam passing through it.
"The beam bending is similar in principle to the bending of light in desert air above hot asphalt, the phenomenon which gives rise to mirages," said Seager.
The amount of bending, when related to the amount of heating, measures changes in temperature to one ten-thousandths of a degree Celsius, and can be calibrated to reveal the amount of light initially absorbed by the phosphor powder. The technique is further refined by measuring the amount of light absorbed at particular wavelengths -- "invaluable knowledge in assessing the chemical and electronic properties of these phosphors," said Seager.
While light is usually measured by the amount that passes through materials, so much light is dispersed by powders that accurate absorption measurements are difficult to perform.
Sandia also uses a variety of other spectroscopy techniques, including cathodoluminescence -- the observation of light emitted from powders during bombardment by electrons at a variety of voltages -- and electron spin resonance, which allows the observation of light-emitting centers in atomic detail.
Sandia National Laboratories is a multiprogram national laboratory operated by a subsidiary of Lockheed Martin Corporation for the DOE. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has broad-based research and development programs contributing to national defense, energy and environmental technologies, and economic competitiveness.
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Last modified: June 12, 2001
Sandia National Laboratories is operated by Lockheed Martin Corp. for the U.S. Department of Energy.