Ed Boyden - How to grow wires and tiny plates
Liquid processing method developed at
MIT can control the shapes of nanowires
and produce complete electronic devices.
David L. Chandler, MIT News Office
July 14, 2011
Researchers at MIT have found a way to grow
submicroscopic wires in water with great precision,
using a method that makes it possible to produce
entire electronic devices through a liquid-based
The team demonstrated the technique, called
hydrothermal synthesis, by producing a functional
light-emitting diode (LED) array made of zinc oxide
nanowires in a microfluidic channel. They were able
to do so on a lab bench under relatively benign
conditions: essentially using a syringe to push
solution through a capillary tube one-tenth of a
millimeter wide, without expensive semiconductor
manufacturing processes and facilities.
Unlike larger structures, with nanomaterials — those with dimensions measured in nanometers, or billionths of a
meter — differences in shape can lead to dramatic differences in behavior. "For nanostructures, there’s a
coupling between the geometry and the electrical and optical properties," explains Brian Chow PhD ’08, co-author
of a paper describing the results that was published July 10 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, says 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 says.
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," explains Jaebum Joo PhD '10, now a senior research scientist at Dow
Chemical Co., who was the lead author of the paper.
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 only to
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 says, 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, it 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
How to grow wires and tiny plates batteries, sensors and optical devices. And the processing method has "the potential for large-scale manufacturing," Joo says.
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 adds. 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 PhD '08, 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,"
They talked about the inefficiency of present methods, where electronic circuits are first built, then packaged, and
finally tested. They realized, he says, that "all these things could be done in one shot," and that's what they were
able to demonstrate 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 says. "The packaging is part of the way they’re synthesized."
Christopher Murray, University Professor of Chemistry and Materials Science and Engineering at the University
of Pennsylvania, calls this paper a "valuable contribution." Murray, who was not involved in this research, adds: "We are seeing a convergence right now, and it will really change our understanding of nanomaterials synthesis
and systems integration." This paper, he says, is "a very nice piece of work."
The research was carried out with Media Lab associate professors Edward Boyden and Joseph Jacobson, and
was funded by the MIT Center for Bits and Atoms, the MIT Media Lab, the Korea Foundation for Advanced
Studies, Samsung Electronics, the Harvard Society of Fellows, the Wallace H. Coulter Early Career Award, the
NARSAD Young Investigator Award, the National Science Foundation and the NIH Director’s New Innovator