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Researchers demo light twisting nanoparticle ribbons

Posted: 20 Apr 2010 ?? ?Print Version ?Bookmark and Share

Keywords:light twist nanoparticle? lithography? MEMS? nanotechnology?

After 72 hours of exposure to ambient light, strands of nanoparticles twisted and bunched together.

University of Michigan engineers and their collaborators have demonstrated that light can twist ribbons of nanoparticles.

Matter readily bends and twists light. That's the mechanism behind optical lenses and polarizing 3D movie glasses. But the opposite interaction has rarely been observed, said Nicholas Kotov, principal investigator on the project. Kotov is a professor in the departments of Chemical Engineering, Biomedical Engineering and Materials Science and Engineering.

While light has been known to affect matter on the molecular scalebending or twisting molecules a few nanometers in sizeit has not been observed causing such drastic mechanical twisting to larger particles. The nanoparticle ribbons in this study were between 1- and 4?m long.

"I didn't believe it at the beginning," Kotov said. "To be honest, it took us three and a half years to really figure out how photons of light can lead to such a remarkable change in rigid structures a thousand times bigger than molecules."

Kotov and his colleagues had set out in this study to create "superchiral" particlesspirals of nano-scale mixed metals that could theoretically focus visible light to specks smaller than its wavelength. Materials with this unique "negative refractive index" could be capable of producing Klingon-like invisibility cloaks, said Sharon Glotzer, a professor in the departments of Chemical Engineering and Materials Science and Engineering who was also involved in the experiments. The twisted nanoparticle ribbons are likely to lead to the superchiral materials, the professors say.

To begin the experiment, the researchers dispersed nanoparticles of cadmium telluride in a water-based solution. They checked on them intermittently with powerful microscopes. After about 24 hours under light, the nanoparticles had assembled themselves into flat ribbons. After 72 hours, they had twisted and bunched together in the process. But when the nanoparticles were left in the dark, distinct, long, straight ribbons formed.

"We discovered that if we make flat ribbons in the dark and then illuminate them, we see a gradual twisting, twisting that increases as we shine more light," Kotov said. "This is very unusual in many ways."

The light twists the ribbons by causing a stronger repulsion between nanoparticles in them.


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