by the University of Michigan
NN ARBOR, Mich.— In findings that took the experimenters three years to believe, University of Michigan engineers and their collaborators have demonstrated that light itself can twist ribbons of nanoparticles.
The results are published in the current edition of Science.
Matter readily bends and twists light. That’s the mechanism behind optical lenses and polarizing 3-D 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 scale, bending or twisting molecules with sizes of a few nanometers, it has not been observed causing such drastic mechanical twisting to large particles.
The nanoparticle ribbons in this study were between one and four micrometers long. A micrometer is one-mil- lionth of a meter.
“I didn’t believe it at the beginning,” Kotov said. “To be honest, it took us three and a half years to re- ally fi gure out how photons of light can lead to such a remarkable change in rigid structures a thousand times bigger than molecules.”
Kotov and his col- leagues had set out in this study to create “superchi- ral” particles—spirals of nano-scale mixed metals that could theoretically fo- cus visible light to specks smaller than its wavelength. Materials with this unique “negative refractive index could be capable of pro- ducing Klingon-like invis- ibility 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 rib- bons 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 un- der 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 fl at ribbons in the dark and then illuminate them, we see a gradual twisting, twisting that in- creases as we shine more light,” Kotov said. “This is very unusual in many ways.”
The light twists the rib- bons by causing a stronger repulsion between nanopar- ticles in it.
The twisted ribbon is a new shape in nanotechnol-ogy, Kotov said. Besides superchiral materials, he envisions clever applica- tions for the shape and the technique of creating it.
Sudhanshu Srivastava, a postdoctoral researcher in his lab, is trying to make the spirals rotate.
“He’s making very small propellers to move through fluid—nanoscale submarines, if you will,” Kotov said. “You often see this motif of twisted structures in mobility organs of bacteria and cells.
This newly-discovered twisting effect could also lead to microelectromechanical systems that are controlled by light. And it could be utilized in lithography, or microchip production.
Glotzer, and Aaron Santos, a postdoctoral researcher in her lab, performed computer simulations that helped Kotov and his team better understand how the ribbons form. The simulations showed that under certain circumstances, the complex combination of forces between the tetrahedrally- shaped nanoparticles could conspire to produce ribbons of just the width observed in the experiments. A tetrahedron is a pyramid-shaped, three-dimensional polyhedron.
“The precise balance of forces leading to the self-assembly of ribbons is very revealing,” Glotzer said. “It could be used to stabilize other nanostructures made of non-spherical particles. It’s all about how the particles want to pack themselves.”
