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Small Particles Could Find Big Uses

Photonics Spectra
Jun 2000
Hank Hogan

At the University of Illinois, a researcher may be on the way to something big by starting small. Physics professor Munir Nayfeh has developed a technique for creating nanoparticles out of bulk silicon. When excited, the nanoparticles emit in the blue so strongly that the light is visible to the unaided eye in room light.

"This certainly opens new avenues for semiconductor lasers," Nayfeh said. For instance, it may be possible to integrate all optoelectronics onto a single silicon chip. The nanoparticles also can be formed into colloids, crystals, films and collimated beams for applications in electronics, optoelectronics and biomedicine.

Excited by a frequency-tripled Nd:YAG, a silicon nanoparticle colloid produces blue emission that can be seen by the naked eye in room light.

The nanoparticle production process begins with a silicon wafer, such as is used to manufacture integrated circuits. The wafer is immersed in an etchant bath and subjected to an electrical current. This causes the top layer to dissolve, forming a delicate network of nanostructures. Exposing the wafer to a brief ultrasound treatment shakes the nanostructures free.

A transmission electron microscope image shows a thin graphite grid coated with nanoparticles by immersion in the colloid.

The larger and heavier nanoparticles sink, while the smaller and lighter ones float. Extracting the nanoparticles from the liquid yields a few distinct and tightly controlled sizes. Meanwhile, the wafer can be sent through the whole process again to produce more nanostructures. Because it uses only chemicals,a tabletop device, the process is efficient and has a high throughput. The nanoparticles have a tight size distribution, with a variation within a group of less than 10 percent. This means that the nanoparticles tend to have the same optical and other properties. That uniformity makes practical application easier, and research is under way on how to scale up the production process.

The nanoparticles could also shed some light on basic materials science. Although produced from bulk silicon, the nanoparticles have some of the material characteristics of a gas. "We believe we may have a new phase: a transition between solid and gas," Nayfeh said.

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