Light can bend rigid structures, at least on a microscopic scale, researchers report. Light-driven twisting of tiny ribbons could someday be used for negative refractive index materials, in microelectromechanical systems (MEMS), or for lithography on a scale between the macro- and the microworlds. “Twisting changes the physical dimensions of the ribbons, such as their length,” said lead investigator Nicholas A. Kotov, a chemical engineering professor at the University of Michigan. “This gives a possibility for changing the topology of the surface and a lithographic pattern similar to the Braille font.” Exposure to light twists nanoribbons and bunches them together. Courtesy of Nicholas Kotov, University of Michigan. Previously, the largest objects known to be physically affected by light were molecules. The new research showed light twisting ribbons thousands of times bigger. According to Kotov, the discovery was accidental. The researchers were examining the self-assembly of nanoparticles, with the idea that this might be a way to produce metamaterials with a negative refractive index or other unusual properties. Tangling To this end, they put cadmium telluride nanoparticles, which have an emission maximum of 550 nm, in water. They selected these particles as a research vehicle in part because they have permanent dipoles, or pairs of equally strong charges. They created the nanoparticles in such a way as to have a higher than average dipole and chemical reactivity, both of which affect interparticle forces. The slow oxidation of the tellurium ions led to the formation of cadmium sulfide/cadmium telluride nanocrystals. Within three days, the orange color of the original solution turned a dark green, indicating the completion of nanoparticle self-assembly. The resulting structures, the researchers found when they examined them using electron microscopes, were helical ribbons. While typically from 0.8 to 2 µm long, some were as long as 8 μm. Atomic force microscopy showed that the ribbons were only 10 to 12 nm thick and composed of three to four nanoparticle layers stacked atop one another. When the group started to unravel why the twisting occurred, they found something unexpected. Self-assembly that took place in the dark resulted in long, flat ribbons, not twisted ones. When those straight ribbons were exposed to ambient light, however, they formed dog-bone shapes and other complex structures. With enough exposure, they curled into either a right- or left-handed helix, with an equal chance of transforming into one or the other. No matter the handedness of the helix, increasing the intensity of the light increased the twist. This result was very surprising, Kotov said. It took several years, extensive experiments and painstaking computer simulations by his collaborator Sharon Glotzer to understand how light can twist such a relatively large rigid structure. Unravelling? The key is the effect of the photons on the forces acting between the nanoparticles. During the initial phase of the self-assembly, the attraction between nanoparticles is weak, and the results are spherically shaped aggregates. Light increases the dipole interaction between the nanoparticles, effectively boosting the repulsion between them. Relieving this strain leads to the intermediate shapes and, finally, to a twisted ribbon. At the moment, the induced twist is permanent. However, it should be possible to create dynamic constructs, given the right combination of nanoparticles and the appropriate media. Indeed, Kotov reported that one of his group’s research objectives now is the creation of structures that twist and untwist with light. That could then lead to MEMS devices that turn like propellers, do other mechanical work or act like motors under the right lighting conditions. This physical change also could be used in lithography. Another important point is that this work shows that particles can form structures as complex and intricate as those formed by biomolecules, something that could be of research interest. “Some proteins can form these helical structures, and we are investigating this analogy,” Kotov said. Kotov’s group included investigators from the University of Leeds in the UK, Chungju National University and Pusan National University in South Korea, Argonne National Laboratory in Argonne, Ill., and Jiangnan University in China. They reported on their work in the March 12, 2010, issue of Science.