MIT researchers say they can control the composition and structure of tiny indium nitride and indium gallium nitride wires as they grow

Nanowires, microscopic fibres that can be grown in the lab are a hot research topic, with a variety of potential applications including 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 optimised for specific applications.

Nanowires have been of great interest because structures with such tiny dimensions, typically just a few tens of nanometres in diameter, can have very different properties than the same materials have in bulk.

That’s partly because on such a minuscule scale, quantum confinement effects come in to play. The behaviour of electrons and phonons within the material contribute a considerable amount to its properties. This can affect how it conducts electricity and heat or interacts with light.

What's more, because nanowires have a significantly large amount of surface area in relation to their volume, they are particularly well-suited for use as sensors, points out Silvija Gradečak,who heads the research team at MIT. 



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

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 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 InN and InGaN, they say the same technique could be applied to a variety of different materials.

The scientists observed the nanowires using electron microscopy and made adjustments to the growth process based on the results. Electron tomography also enabled them to reconstruct the three-dimensional shape of individual nanoscale wires.

The team has also published work regarding the use of electron-microscopy 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, says Gradečak. Such control of nanowire geometry and composition could enable devices with better functionality than conventional thin-film devices made of the same materials, she says.

One likely application of the materials developed by Gradečak and her team is in LED light bulbs, which have far greater durability and are more energy-efficient than other lighting alternatives. The most important colours of light to produce from LEDs are in the blue and ultraviolet range; ZnO and GaN nanowires produced by the MIT group can potentially produce these colours very efficiently and at low cost, she says.

While LED light bulbs are available today, they are relatively expensive. “For everyday applications, the high cost is a barrier,” Gradečak says. One big 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 SiC substrates. The nanowire devices have the potential to be more efficient as well, she says.

Such nanowires could also 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. Also, 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.

Complex structures made of nanowires with varying diameters could also be useful in new thermoelectric devices to capture waste heat and turn it into useful electric power. By varying the composition and diameter of the wires along their length, it’s possible to produce wires that conduct electricity well but heat poorly. This combination is hard to achieve in most materials, but is key to efficient thermoelectric generating systems.

The nanowires can be produced using tools already in use by the semiconductor industry, so the devices should be relatively easy to gear up for mass production, the team says.

Zhong Lin Wang, the Regents’ Professor and Hightower Chair in Materials Science and Engineering at the Georgia Institute of Technology, says that being able to control the structure and composition of nanowires is vitally important for controlling their nanoscale properties. The fine-tuning in the growth behaviour of these materials opens up the possibilities for fabricating new optoelectronic devices that are likely to have superior performance.

More details of this work have been published in the paper, " Controlled Modulation of Diameter and Composition along Individual III–V Nitride Nanowires" by Sung Keun Lim et al, in Nanoletters, published online on 7 February 2012. DOI: 10.1021/nl300121p

The work was supported by the MIT Centre for Excitonics, the U.S. Department of Energy, the MIT-France MISTI program and the National Science Foundation.