Practical Nanowire Devices

A way to align nanowires could lead to better sensors and flexible displays.

Researchers at Harvard University and the University of Hawaii have developed an easy way to align nanowires and carbon nanotubes over areas 100 times larger than is possible using existing methods. The researchers are also able to fabricate the nanowires on a number of different surfaces. The advance potentially paves the way to mass production of electronics devices based on these promising nanostructures.

Blowing bubbles: Pumping nitrogen gas into a mixture of nanowires and an epoxy polymer forms a large bubble with very thin sides. As the bubble inflates, the nanowires line up facing the same direction. A metal ring stabilizes the bubble until it’s large enough to come into contact with two silicon wafers. Once in contact, the thin film of nanowires transfers to the wafers. The nanowire-coated wafers can then serve as the base for electronic devices. Credit: Charles Lieber, Harvard

The technique, based on high-volume manufacturing methods used to produce plastic bags, could make it practical to employ nanowires and carbon nanotubes for controlling pixels on large, flexible displays and for accurately detecting multiple chemicals, viruses, and biomarkers for diseases. (See "Drugstore Cancer Tests.") The results were published online this week in the journal Nature Nanotechnology.

Researchers had previously developed small-scale prototype devices based on nanowires and carbon nanotubes. But moving beyond prototypes to commercial products requires a fast and easy way to arrange the tiny structures over large areas, says Charles Lieber, professor of chemistry at Harvard. "The lack of large-scale alignment and organization strategies has forced researchers to make small chips in a one-by-one process," he says. "This is the antithesis of economical manufacturing." Whereas previous methods could arrange nanowires over areas of only about a square centimeter, Lieber's new technique works on areas of hundreds of square centimeters, and it could be used to produce many chips at once. Or it could be used to make large arrays of transistors needed to control pixels on displays.

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The new technique involves blowing bubbles made of an epoxy polymer mixed with either nanowires or carbon nanotubes. The researchers pour the mixture onto a circular surface equipped with a small hole; the polymer-nanowire mixture forms a membrane over the surface. The researchers then force nitrogen gas through the hole, expanding the membrane until it forms a bubble about 25 centimeters wide and 50 centimeters tall. A metal ring stabilizes the bubble as it grows, with the polymer material stretching to become a 200-to-500-nanometer-thick film containing evenly spaced nanowires or carbon nanotubes lined up and facing in approximately the same direction. The researchers speculate that sheer forces caused by the growth of the bubble make the nanowires line up.

The resulting film can be transferred to a number of surfaces, including silicon and flexible plastic. To do this, the researchers position silicon wafers or other materials so that when the bubble inflates, the surface of the bubble presses against them.