News | March 6, 2000

Electrochemical Process Makes Silicon Nanoparticles

A new way of making nanoparticles of silicon could open the door to exotic electronic devices and new research tools. The technology, developed by investigators at the University of Illinois, Urbana-Champaign, uses an electrochemical process and ultrasound to create tiny particles—each containing about 30 atoms and measuring about a billionth of a meter in diameter. The particles can be formed into colloids, crystals, films, and collimated beams for unique applications in the electronics, optoelectronics, and biomedical industries.

"These nanoparticles have many useful properties that are unlike those of bulk silicon, including being a source of stimulated emission," said Munir Nayfeh, a UI professor of physics and a researcher at the university's Beckman Institute for Advanced Science and Technology. Potential uses of the particles, he noted, include single-electron transistors, semiconductor lasers, and markers for biological materials.

To create the nanoparticles, Nayfeh and his colleagues begin with a silicon wafer, which they pulverize using a combination of chemistry and electricity. "We use an electrochemical treatment that involves gradually immersing the wafer into an etchant bath while applying an electrical current," Nayfeh explained. "This process erodes the surface layer of the material, leaving behind a delicate network of weakly interconnected nanostructures. The silicon wafer is then removed from the etchant and immersed briefly in an ultrasound bath."

Under the ultrasound treatment, the fragile nanostructure network crumbles into individual particles of different size groups, Nayfeh said. The slightly larger, heavier particles precipitate out, while the ultra-small particles remain in suspension, where they can be recovered. Because of their unique characteristics, the nanoparticles could be used in low-power electronics, nonvolatile floating-gate memories, and optical displays and interconnects.

"The assembly of ultra-small silicon nanoparticles on device-quality silicon crystals provides a direct method of integrating silicon superlattices into existing or future down-scaled microelectronics architecture," Nayfeh said. "This could lead to the construction of single-electron transistors and electric charge-based memory devices, optimized to work at high temperature."

The nanoparticles also could form the basis for novel semiconductor lasers. Nayfeh and his colleagues have demonstrated stimulated, directed emission from within the walls of a microcrystallite reconstructed from the nanoparticles. The emission was dominated by a deep-blue color. "This type of laser could possibly replace the wires used to communicate between components in a circuit," Nayfeh said. "The blue color might also be useful for underwater communications systems." The benign nature of silicon also makes the nanoparticles useful as fluorescent markers for tagging biologically sensitive materials. The light from a single nanoparticle can be readily detected.

Nayfeh will describe the new process for making silicon nanoparticles at the March meeting of the American Physical Society. A paper is scheduled to appear in the March issue of Applied Physics Letters. A patent has been applied for.

For more information: James E. Kloeppel, Physical Sciences Editor, University of Illinois at Urbana-Champaign. Tel: 217-333-1085. Fax: 217-244-0161. Email: uinews@uiuc.edu.

Edited by Gordon Graff
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