February 22, 2012 — Massachusetts Institute of Technology (MIT) researchers used gases to precisely control nanowires’ width and composition as they grow, which could yield complex structures optimally designed for particular applications, like LED substrates or solar panels.
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Nanowires can have very different properties than the same materials in bulk, because of quantum confinement effects, based on the behavior of electrons and phonons within the material. Nanowires may conduct electricity and heat or interact with light in new and useful ways. The high surface-area-to-volume ratio makes nanowires especially useful in sensing applications versus bulk materials.
The team, led by MIT assistant professor of materials science and engineering Silvija Gradečak, controlled and varied the size and composition of individual wires as they grew from metal seed particles. They adjusted the gases used in growing the nanowires, which affected the size and composition of the seed particles, simultaneously. 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.
These initial experiments used indium nitride and indium gallium nitride (InGaN), semiconductors used to manufacture light-emitting diodes (LEDs) among other devices; the technique could be applied to various materials.
The team used electron microscopy to observe nanowire growth, making adjustments to the growth process based on what they learned about the growth patterns. Electron tomography measurements were used to reconstruct the three-dimensional shape of individual nanoscale wires.
The nanowire geometry and composition were so precisely structured that they could enable new semiconductor devices with better functionality than conventional thin-film transistors, Gradečak says. Applications such as blue and ultraviolet LEDs could be produced with zinc oxide (ZnO) and gallium nitride (GaN) nanowires grown to produce these colors very efficiently and at lower cost than sapphire or silicon carbide used today. Other applications are solar-energy panels, with nanowires tuned to specific wavelengths of light; or new thermoelectric devices to capture waste heat and turn it into electric power, where the wires could be grown to conduct electricity well but heat poorly.
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 (http://pubs.acs.org/doi/abs/10.1021/nl300121p).
In addition to Gradečak, the Nano Letters paper was co-authored by MIT graduate student Sam Crawford, Sung Keun Lim PhD ’11 and researcher Georg Haberfehlner of the research and technology organization CEA-Leti in Grenoble, France. The Nanoscale paper was co-authored by MIT graduate student Xiang Zhou, Megan Brewster PhD ’11 and postdoc Ming-Yen Lu. 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.
In a related study recently published in the journal Nanoscale, the team also used a unique electron-microscopy technique called cathodoluminescence to observe what wavelengths of light are emitted from different regions of individual nanowires. (http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr11706a)
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Courtesy of David L. Chandler, MIT News Office.