posted 01-24-2002 11:16 AM            

Silicon nanoparticles now come in family of sizes and fluorescent colors
CHAMPAIGN, Ill. — A process for creating silicon nanoparticles, developed at the University of Illinois, has now been shown to produce a family of discrete particle sizes useful for microelectronics, optoelectronics and biomedical applications.
As reported in the Jan. 21 issue of Applied Physics Letters, researchers demonstrated that the electrochemically etched particles come in particular sizes and fluoresce in distinct colors. The smallest four sizes are blue, green, yellow and red luminescent particles.

“The availability of specific particle size and emission in the red, green and blue range makes the particles useful for electronic displays and flash memories,” said Munir Nayfeh, a UI professor of physics and corresponding author of the APL paper. “The benign nature of silicon also makes the particles useful as ultra-bright fluorescent markers for tagging biologically sensitive materials.”

Current medical and biological fluorescent imaging is limited by the use of dye markers, which are not photostable, Nayfeh said. The dyes can break down under photoexcitation, room light or higher temperatures.

Not only are the new silicon particles photostable, they are also bright. The light from a single nanoparticle can be readily detected.

To convert bulk silicon into nanoparticles, Nayfeh and his colleagues use an electrochemical treatment that involves gradually immersing a silicon wafer into an etchant bath of hydrofluoric acid and hydrogen peroxide while applying an electrical current. The process erodes the surface layer of the material, leaving behind a delicate network of weakly interconnected nanostructures. The 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, which may be easily separated into the different size groups.

“The availability of different colored markers is very important for biomedical applications,” said Nayfeh, who also is a researcher at the UI’s Beckman Institute for Advanced Science and Technology. “By placing particles of different colors in strategic locations, you could study such phenomena as growth factors in cancer cells or how proteins fold.”

The silicon particles fluoresce when struck with ultraviolet light. They also can fluoresce when struck with two photons of infrared light – a technique that can non-invasively penetrate human tissue. In a separate paper, published in the Jan. 7 issue of Applied Physics Letters, the researchers also demonstrated laser oscillation in small aggregates of the silicon nanoparticles.

“At 6 microns in diameter, these clusters of particles are one of the smallest lasers in the world,” said Sahraoui Chaieb, a UI professor of theoretical and applied mechanics and a co-author of both papers. “This microlasing is an important step towards the realization of a laser on a chip, which could ultimately replace wires with optical interconnects.”

The emission was dominated by a deep-red color, said Chaieb, who also is a researcher at the Beckman Institute. The clusters are currently stimulated by green light from a mercury lamp. One of the researchers’ goals is to excite them instead with electricity.

posted 01-25-2002 04:07 PM            

Interestingly enough, it appears that some of this work was being carried out independently by a team from the Institute of Microstructure Physics and the Institute of Experimental Physics, Otto-von-Guericke University, both in Germany.

According to the abstract, "Phase separation and thermal crystallization of SiO/SiO2 superlattices results in ordered arranged silicon nanocrystals. The preparation method which is fully compatible with Si technologies enables independent control of particle size as well as of particle density and spatial position by using a constant stoichiometry of the layers. Transmission electron microscopy investigations confirm the size control in samples with an upper limit of the nanocrystal sizes of 3.8, 2.5, and 2.0 nm without decreasing the silicon nanocrystal density for smaller sizes. The nanocrystals show a strong luminescence intensity in the visible and near-infrared region."

To me, this sounds like the beginning of a cost-effective production methodology. Now it they can get the crystals to excite with electricity rather than light, we could see terahertz computers on our desktop before too long.!

I admit I'm not all that knowledgeable about solid-state physics, but from what I read, we could be in for some very exciting advances.

Thanks for posting the information!