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Communications and External Relations
ORNL's colorful nanocrystals could lead to faster computers
OAK RIDGE, Tenn.,
Sep. 20, 1996
In the Middle Ages, makers of stained glass windows introduced small metal solids separated from a solution into molten glass to produce vibrant colors. Using an accelerator, researchers at the Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL) can implant gold ions in high concentrations in glass surface layers, creating a variety of striking colors. Unlike 12th-century glaziers, the researchers also can control the microscopic behavior of the resulting nanocrystals, or clusters of several hundred metallic atoms each.
By using ion implantation, ORNL scientists Woody White, John Budai, Steve Withrow, Jane Zhu, Ray Zuhr, Darrell Thomas, and Dale Hensley of the Solid State Division, have synthesized a wide range of metal and semiconductor nanocrystals embedded in a number of technologically important materials, including silica (one common type of glass), sapphire, and even crystalline silicon. The objective of this research is to understand, predict, and control the size, structure, optical and physical properties of nanocrystalline composites produced by high-dose ion implantation. Because many of these materials absorb and emit light in remarkable ways, they could be used in the development of faster, smarter computers and better flat-panel displays for advertising signs, automobile dashboards, EXIT signs, and computer and calculator screens.
"Scientific and technological interest in nanocrystals extends far beyond stained glass," White says. "We have shown that ion implantation can be used in a novel way to create nanocrystals embedded in a number of host materials. In our approach, high-dose ion implantation of the near-surface region creates a solid solution that is supersaturated with impurity ions. When the sample is heated, the highly concentrated impurity ions precipitate out, forming nanocrystals approximately 10 nanometers (billionths of a meter) in diameter. In many cases, such nanocrystals cannot be synthesized by conventional methods."
In addition to their striking colors, such ion-implanted samples have many other useful optical properties such as a refractive index that depends strongly on the intensity of light striking the material. Such a property would be useful for making ultrafast switches that will route signals in future light-based communications networks and all-optical computers (which are faster than electronic ones because light travels faster than electrons).
The ORNL researchers have demonstrated that elemental semiconductor nanocrystals of silicon and germanium can be synthesized in silica and sapphire. "We observed a strong red light emitted from silica samples containing silicon nanocrystals," Budai said. "We found that the color of the light, or its wavelength, can be tuned by changing nanocrystal size. Because size determines the wavelength of emitted light, full-color panel displays for computers may someday be made of appropriately sized semiconductor nanocrystals formed by ion implantation."
They also synthesized more complex compound semiconductors and alloy nanocrystals (e.g., silicon germanium, gallium arsenide, cadmium selenide, and gallium nitride) by implanting combinations of different ions into silica and sapphire hosts. By controlling implantation and annealing conditions, they discovered that it is possible to control orientation and, in some cases, the crystal structure of the nanocrystals.
The scientists also incorporated compound semiconductor nanocrystals such as gallium arsenide into a silicon matrix. The optical properties of this nanocrystalline composite are expected to be considerably different from those of current silicon-based devices, because energetic electrons can be directly converted into visible light in gallium arsenide, but not in silicon. It may be possible to produce buried continuous layers of gallium arsenide by extending ion implantation to higher doses. This approach could provide a way to combine silicon-integrated circuits with high-speed gallium arsenide layers to achieve fully integrated optoelectronic devices.
This research was supported by DOE, Office of Energy Research, Basic Energy Sciences. ORNL, one of the Department of Energy's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Research Corp.
(photo available upon request) .