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MIT Team Controls Size, Composition of Individual Nanowires

by Editor1 last modified March 04, 2012 - 11:22

A team of MIT researchers has found a way of precisely controlling the width and composition of nanowires as they grow, making it possible to grow complex structures designed for particular applications.

MIT Team Controls Size, Composition of Individual Nanowires

The team, led by MIT assistant professor of materials science and engineering Silvija Gradečak, showed they could control and vary both the size and composition of individual nanowires as they grew.

“We’re able to control both of these properties simultaneously,” Gradečak said. The team conducted their nanowire-growth experiments with indium nitride and indium gallium nitride, but the same technique could be applied to a variety of different materials, she added.

Nanowires offer quantum confinement effects that impact the behavior of electrons and phonons. This plays a significant role in the material’s behavior, which can affect how it conducts electricity and heat or interacts with light. Also, 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.

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’s team could control the size and composition of the seed particles and, therefore, the nanowires as they grew.

The team observed the nanowires with an electron microscope. Using a process called electron tomography, they were able to reconstruct the three-dimensional shape of individual nanoscale wires.

In a related study (published in the journal Nanoscale), the MIT team used another electron-microscopy 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, she says.

One likely application could be the development of low-cost LED bulb, according to the team. “For everyday applications, the high cost [of LEDs] is a barrier,” Grade?ak 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 silicon carbide substrates.

The individual wires form defect-free single crystals, reducing the energy lost due to flaws in the structure of conventional solar cells. And 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 — a combination that is hard to achieve in most materials, but is key to efficient thermoelectric generating systems.

Notably, these novel nanowires, which offer more control over the outcome, could be produced using tools already in use by the semiconductor industry, the MIT team added.

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.
The Nano Letters paper was co-authored by MIT graduate student Sam Crawford, Sung Keun Lim and researcher Georg Haberfehlner (CEA-Leti in Grenoble, France).

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