Electrochemical Process Makes Silicon Nanoparticles

University of Illinois (UI, Urbana-Champaign) researchers have developed a process for converting bulk silicon into ultrasmall, nano-sized particles. The nanoparticles — about one billionth of a meter in diameter, containing about 30 silicon atoms — can be formed into colloids, crystals, films and collimated beams for unique applications in the electronics, optoelectronics and biomedical industries (Figure).

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

To create the nanoparticles, Nayfeh and his colleagues pulverize a silicon wafer using chemistry and electricity. "We use an electrochemical treatment that involves gradually immersing the wafer into an etchant bath while applying an electrical current," Nayfeh said. "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."

A method of producing silicon nanoparticles, each with about 30 silicon atoms, was developed. Shown is a layer of nanoparticles atop graphite. (Source: UI)

"The assembly of ultrasmall 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. •