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Pumping Out Nanowires
Atlanta - Mar 5, 2002 Using molten gallium as a catalyst, researchers at the Georgia Institute of Technology have simultaneously grown hundreds of thousands of silica nanowires from each micron-scale catalyst droplet. Bundles of the highly aligned and closely packed nanowires form unusual structures resembling cones, cherries, carrots and comets. Use of gallium catalysts could facilitate high-volume production of silica (SiO2) nanowires, improving the vapor-liquid-solid (VLS) process now used to make the structures. The gallium catalysts also produce nanowires that spontaneously divide into branching structures that could have potential applications as optical splitters in nanometer-scale photonic systems. The National Science Foundation-sponsored work was reported in the February 2002 issue of the Journal of the American Chemical Society. "These nanowires demonstrate many amazing growth phenomena unlike any previously observed through a conventional VLS growth process," explained Zhong Lin Wang, director of the Georgia Tech Center for Nanoscience and Nanotechnology, and a professor of materials science and engineering. "These silica nanowires could have applications ranging from optics to surface coatings. It's my hope that they can be useful as small-scale optical fibers useful for splitting a signal." The ability to grow large bundles of aligned nanowires from a single catalyst could help lower production costs, potentially opening up new applications for the structures. In standard VLS synthesis techniques used for producing silica nanowires, each wire grows from a single particle of gold, cobalt, nickel or other high melting point metal. Vapor-phase silicon evaporated from a wafer inside a high-temperature furnace condenses on the surface of the molten catalyst, where it combines with oxygen and crystallizes to form silica nanowires. In the standard process, the size of the catalytic particle controls the diameter of the nanowire grown from it. Because the wires grow individually from the catalyst particles, they produce tangled masses of varying lengths. But by using droplets of gallium 5 to 50 microns in diameter -- thousands of times larger than standard catalytic particles -- Wang and collaborators Zheng Wei Pan, Zu Rong Dai and Chris Ma grow hundreds of thousands of nanowires from a single catalyst. The nanowires attached to each droplet grow to approximately the same length, and remain well ordered, aligned to form hollow macro-scale structures that resemble snowy-white cones, carrots, cherries or comets. "The importance of this process is that it will allow us to grow many aligned wires rather than tangles of wires," said Wang. "The uniformity of the wires produced in this way could be very useful." Some of the nanowires spontaneously split into branches, often more than once, with the diameters of the branches equal. "This is very different from any existing types of nanowires," said Wang. "This branching ability makes it possible to produce junctions that may have applications to light propagation."
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