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Nanocrystals Light Up In Color

Photographic image of the luminescent silicon nanocrystals excited at 320 nm in hexane. (A) 1.5 nm diameter silicon nanocrystals and (B) 2.5 - 4.0 nm diameter nanocrystals. The smaller nanocrystals emit in the near-ultraviolet and appear deep blue, whereas the larger particles appear blue-green. Office of Public Affairs University of Texas-Austin Austin, Texas<
Austin - May 1, 2001
Solving a problem that has eluded scientists and engineers for more than a decade, two professors at The University of Texas at Austin College of Engineering have devised a method to make silicon shine. Their tiny, highly efficient, light-emitting spherical silicon crystals hold great promise for future applications ranging from laser technology to flat panel displays such as computer monitors and TV screens.

Dr. Brian Korgel, principal investigator on the project, said: "Bulk silicon does not emit visible light, but our structures do. Our crystals have the added advantage of being tunable, meaning that we can create a specific color of light by adjusting the crystal's size. Smaller crystals give off blue light -- larger ones green, or even red."

The research by Korgel and Dr. Keith Johnston will be reported in the April 25 issue of the Journal of the American Chemical Society. Korgel is an assistant professor in the department of chemical engineering. Johnston holds the Matthew Van Winkle Regents Professorship in Chemical Engineering.

Silicon is the plentiful and inexpensive material that is the basis of transistor technology and the cornerstone of the electronics industry. But under normal circumstances, it does not emit light and this means costly alternative semiconductors such as gallium arsenide are used for LEDs (light emitting diodes), lasers, sensors and related products.

"If you make silicon smaller, into nanostructures, you can force it to emit light in visible wavelengths," Korgel explained. "People have been struggling for about 12 years to come up with ways of doing that."

The spherical silicon crystals Korgel and Johnston have produced are called nanocrystals or quantum dots. The engineers have made crystals that emit the colors blue and green, and they say a crystal emitting red is not far down the road.

Nanotechnology refers to the manipulation of materials on an atomic or molecular scale to construct highly miniaturized mechanical devices and to produce materials that display entirely different properties in the nano-world than when they exist in their normally larger form. Silicon, for example, does not normally emit light, but in the nano-world, it can do so.

"When we put them into devices, the physics changes from classical physics, which everybody understands to quantum-mechanical rules, which make things different. It's a great challenge to the industry. But it also presents opportunities, because you can create devices that work on entirely different principles," Korgel said.

Today's designers are working toward production of components measured in nanometers, units approximately one four-thousandth the width of a human hair. "Say you want a smaller and smaller computer. The challenge of shrinking the computer comes down to the technology to shrink the materials. That's what we do," he explained.

Korgel and Johnston grew the minute and colorful spheres by a relatively simple, inexpensive process known as arrested precipitation. In a highly pressurized titanium chamber, they first heat a mixture of chain hydrocarbon "ligand" (octanol) and organic solvent (hexane), to a temperature of 500 degrees.

Next, they add pure silicon reagent, causing it to degrade to silicon atoms. Ordinarily, the same atoms would soon recombine to form large crystals. But the octanol chains bind to the silicon surfaces inhibiting crystal growth, Korgel said.

"You end up with these sort of fuzzy particles of silicon that don't stick to each other," he said. "What controls their size is how many ligands you have. If you have a lot, the crystals will stay small. If you don't have very many, they'll continue to grow into very large crystals."

Once the chemical reaction is complete, the nanocrystals are harvested by evaporating the solvent, and can be assembled into devices. Typically, the nanocrystals contain between 100 and 2,000 atoms.

While other researchers in the United States and abroad have produced their own silicon emitters using different techniques, none have achieved Korgel and Johnston's combination of high efficiency, emission of light in the visible spectrum and the ability to "tune" crystals to produce different colors.

Korgel said the new materials "test our fundamental understanding of quantum mechanics. You can use them to do experiments and answer questions like: How do the properties of silicon change as it grows from an atom to a bulk solid?"

The work was funded in part by the Welch Foundation, the Petroleum Research Fund, and through Korgel's National Science Foundation Faculty Early Career Development Award and a DuPont Young Professor Grant.

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MicroMachines Key To Maintaining Large Space Structures
Fayetteville - April 24, 2001
Tiny micro electro-mechanical systems dubbed MEMS may be the key to maintaining giant space-based structures, according to U of A researchers Steve Tung and Larry Roe. Although space-based solar collectors or antenna arrays can be many square miles in size, tiny MEMS devices can keep them oriented correctly to ensure their long-term operation.

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