MIT researchers used hydrothermal synthesis -- a liquid-based method to grow submicroscopic wires -- to fab a functional light-emitting diode (LED) array made of zinc oxide nanowires in a microfluidic channel.
The LED was manufactured at the lab bench under fairly benign conditions at low process temperatures, which may open up new substrate material options like flexible polymers and plastics. Researchers using a syringe to push solution through a capillary tube one-tenth of a millimeter wide. Capital-intensive semiconductor manufacturing processes and facilities were unneccessary. The microfluidic structure used to build the nanowires also packaged the final LED device. Testing was carried out continuously through the manufacturing process.
Nanomaterial geometry is coupled with electrical and optical properties, notes Brian Chow PhD 2008, so MIT uses its system to control nanowire aspect ratios -- creating long thin wires, flat plates, etc. The researchers' goal was to determine the controlling factor for nanowire shape and structure, said Jaebum Joo PhD 2010, now a senior research scientist at Dow Chemical Co. They discovered that aspect ratios could be tuned via the zinc oxide's electrostatic properties in the growth solution. Ions of different compounds, when added to the solution, attach themselves electrostatically only to certain parts of the wire, which inhibits growth in that direction. Inhibition could be tuned by the specific properties of the compounds.
The manufacturing method could be ramped for large-scale production. Titanium dioxide and other materials may also be controllable in this manner, said Joo, enabling flexible displays, solar cells, and other end products. Zinc oxide could be used to make batteries, sensors and optical devices in addition to LEDs.
The MIT team is also investigating fabricating "spatially complex devices from the bottom up, out of biocompatible polymers" with hydrothermal synthesis, Joo adds, targeting life sciences and medical applications.
The method developed out of discussions between Joo (nanomaterials), Chow (applied chemistry), and Manu Prakash PhD 2008 now an assistant professor of bioengineering at Stanford University (applied physics) about better ways to manufacture electronic circuits than the front-end fab, back-end package, test linear methods in use today.
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 Award.