In findings that took the experimenters three years to believe, U-M 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, says Nicholas Kotov, principal investigator on the project. Kotov is a professor in the Department of Chemical Engineering, Department of Biomedical Engineering and Department of Materials Science and Engineering.
While light has been known to affect matter on the molecular scale — bending or twisting molecules a few nanometers in size — it has not been observed causing such drastic mechanical twisting to larger particles. The nanoparticle ribbons in this study were between 1 and 4 micrometers long. A micrometer is one-millionth of a meter.
“I didn’t believe it at the beginning,” Kotov says. “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” particles — spirals 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, says Sharon Glotzer, a professor in the Department of Chemical Engineering and Department of Materials Science and Engineering who also was involved in the experiments. The twisted nanoparticle ribbons likely are 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 says. “This is very unusual in many ways.”
The light twists the ribbons by causing a stronger repulsion between nanoparticles in them.
The twisted ribbon is a new shape in nanotechnology, Kotov says. Besides superchiral materials, he envisions clever applications for the shape and the technique used to create 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 says. “You often see this motif of twisted structures in mobility organs of bacteria and cells.”
The nanoscale submarines could conceivably be used for drug-delivery and in microfluidic systems that mimic the body for experiments.
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.
The paper is titled Light-Controlled Self-Assembly of Semiconductor Nanoparticles into Twisted Ribbons.
