A new liquid processing method that can control the shapes of nanowires and produce complete electronic devices has been developed at MIT. The technique, called hydrothermal synthesis, works by producing a functional LED array made of zinc oxide nanowires in a microfluidic channel. Researchers use a syringe to push solution through a capillary tube one-tenth of a millimeter wide, without expensive semiconductor manufacturing processes or facilities. They said they were able to do this on a lab bench under relatively benign conditions. Unlike larger structures, nanomaterials exhibit dramatic differences in behavior that vary according to their shapes. Zinc oxide nanostructures are directly synthesized in parallel microfluidic channels (held by the metal frame) by flowing reactants through the tubing. The microfluidic structure not only creates the device but also becomes the final packaged functional LED device itself. (Image: Jaebum Joo) “For nanostructures, there’s a coupling between the geometry and the electrical and optical properties,” said Brian Chow, co-author of the paper that was published in the journal Nature Materials. “Being able to rationally tune the geometry is very powerful because you can, in turn, tune the functional properties.” The system Chow and his colleagues developed can precisely control the aspect ratio (the ratio of length to width) of the nanowires to produce anything from flat plates to long, thin wires. There are other ways of making such nanowires, said Chow, who did this work as a postdoc at MIT. “People have demonstrated good control over the morphology of wires by other means, particularly at much higher temperatures or in organic solvents. But to be able to do this in water under these low-temperature conditions is attractive” because it may make it easier to manufacture such devices on flexible polymers and plastics, he said. Control over the shapes of the wires has until now been largely a trial-and-error process. “We were trying to find out what is the controlling factor,” said lead author Jaebum Joo, now a senior research scientist at Dow Chemical Co. The key turns out to be the electrostatic properties of the zinc oxide material as it grows from a solution, they found. The ions of different compounds, when added to the solution, attach themselves electrostatically to only certain parts of the wire — just to the sides, or just to the ends — inhibiting the wire’s growth in those directions. The amount of inhibition depends on the specific properties of the added compounds. While this work was done with zinc oxide nanowires — a promising material that is being widely studied by researchers — the MIT scientists believe the method they developed for controlling the shape of the wires “can be expanded to different material systems,” Joo said, perhaps including titanium dioxide, which is being investigated for devices such as solar cells. Because the benign assembly conditions allow the material to be grown on plastic surfaces, he says, the process might enable the development of flexible display panels, for example. But there are also many potential applications using the zinc oxide material itself, including the production of batteries, sensors and optical devices. And the processing method has “the potential for large-scale manufacturing,” Jo said. The team also hopes to be able to use the method to make “spatially complex devices from the bottom up, out of biocompatible polymers,” Joo said. These could be used, for example, to make tiny devices that could be implanted in the brain to provide high-resolution, long-term sensing and stimulation. Manu Prakash, now an assistant professor of bioengineering at Stanford University, says this was a very interdisciplinary project that emerged when he (studying applied physics), Joo (studying nanomaterials) and Chow (in applied chemistry) were close friends in graduate school and began discussing better ways to manufacture electronic circuits. “We began talking about how our different fields affected this one problem,” Prakash said. They talked about the inefficiency of present methods, consisting of building electronic circuits first, packaging them next and, finally, testing them. They realized, he said, that “all these things could be done in one shot,” and that’s what they demonstrated in the work described in this paper: The microfluidic device used for processing became the final packaging of the device, and testing was carried out continuously through the manufacturing process. “It’s a bottom-up way of thinking about it,” Prakash said. “The packaging is part of the way they’re synthesized.” For more information, visit: www.mit.edu