Nanowires — microscopic fibers that can be “grown” in a lab — have a variety of potential applications, including LEDs and sensors. 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.

Nanowires fabricated using the MIT techniques can have varying widths, profiles, and composition along their lengths. Different colors are used to indicate compositional variations. (Image: the Gradeˆcak laboratory)

Structures like nanowires with such tiny dimensions — a few tens of nanometers in diameter — can have very different properties than the same materials have in their larger form. That’s in part because at such small 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. Nanowires also have an especially large amount of surface area in relation to their volume, so they are particularly suited for use as sensors.

The 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, the team was able to control the size and composition of the seed particles and, therefore, the nanowires as they grew. Although the nanowire-growth experiments were conducted with indium nitride and indium gallium nitride, the same technique could be applied to different materials.

The team observed the nanowires with electron microscopy, making adjustments to the growth process based on what they learned about the growth patterns. Using electron tomography, they reconstructed the 3D shape of individual nanoscale wires. The researchers also used a unique electronmicroscopy technique called cathodoluminescence to observe what wavelengths of light are emitted from different regions of individual nanowires.

Precisely structured nanowires could facilitate a new generation of semiconductor devices. Such control of nanowire geometry and composition could enable devices with better functionality than conventional thin-film devices made of the same materials.

One likely application of the materials is LED light bulbs. The most important colors of light to produce from LEDs are in the blue and ultraviolet range. The zinc oxide and gallium nitride nanowires MIT developed can potentially produce these colors very efficiently and at low cost.

While LED light bulbs are available today, they are relatively expensive. An advantage of this new approach is that it could enable the use of much less expensive substrate materials — a major part of the cost of such devices, which today typically use sapphire or silicon carbide substrates. The nanowire devices have the potential to be more efficient as well.

The nanowires could find applications in solar-energy collectors for lower-cost solar panels. Being able to control the shape and composition of the wires as they grow could make it possible to produce very efficient collectors. The individual wires form defect-free single crystals, reducing the energy lost due to flaws in the structure of conventional solar cells. By controlling the exact dimensions of the nanowires, it’s possible to control which wavelengths of light they are “tuned” to, either for producing light in an LED or for collecting light in a solar panel.

The nanowires can be produced using tools already in use by the semiconductor industry, so the devices should be relatively easy to mass-produce.

This work was done by MIT assistant professor of materials science and engineering Silvija Gradecˆak, MIT graduate students Sam Crawford and Xiang Zhou, Dr. Sung Keun Lim, Dr. Megan Brewster, post-doc Ming-Yen Lu, and Georg Haberfehlner of CEA-Leti, Grenoble, France. For more information, Click Here .

Lighting Technology Magazine

This article first appeared in the May, 2012 issue of Lighting Technology Magazine.

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