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Wednesday, 22 February 2012

A new twist on nanowires: Controlling the composition and structure of these tiny wires as they grow

MIT News
Feb 22, 2012

Nanowires fabricated using the new techniques developed by Gradečak and her team can have varying widths, profiles, and composition along their lengths, as illustrated here, where different colors are used to indicate compositional variations. Image courtesy of the Gradečak laboratory

Nanowires — microscopic fibers that can be “grown” in the lab — are a hot research topic today, with a variety of potential applications including light-emitting diodes (LEDs) and sensors. Now, a team of MIT researchers has found a way of precisely controlling the width and composition of these tiny strands as they grow, making it possible to grow complex structures that are optimally designed for particular applications.

The results are described in a new paper authored by MIT assistant professor of materials science and engineering Silvija Gradečak and her team, published in the journal Nano Letters.

Nanowires have been of great interest because structures with such tiny dimensions — typically just a few tens of nanometers, or billionths of a meter, in diameter — can have very different properties than the same materials have in their larger form. That’s in part because at such minuscule scales, quantum confinement effects — based on the behavior of electrons and phonons within the material — begin to play a significant role in the material’s behavior, which can affect how it conducts electricity and heat or interacts with light.

In addition, because nanowires have an especially large amount of surface area in relation to their volume, they are particularly well-suited for use as sensors, Gradečak says.

Her team was able to control and vary both the size and composition of individual wires as they grew. Nanowires are grown by using “seed” particles, metal nanoparticles that determine the size and composition of the nanowire. By adjusting the amount of gases used in growing the nanowires, Gradečak and her team were able to control the size and composition of the seed particles and, therefore, the nanowires as they grew. “We’re able to control both of these properties simultaneously,” she says. While the researchers carried out their nanowire-growth experiments with indium nitride and indium gallium nitride, they say the same technique could be applied to a variety of different materials.
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