A silica menagerie

Scanning electron microscope (SEM) image shows a large group of silica (SiO2) nanowire bundles grown from gallium droplets, which are the dark spots atop each bundle. Composed of hundreds of thousands of nanowires, these structures were grown atop a silicon wafer.

Carrots, cones, cherries, and comets are the unlikely array of nanoscale products from a gallium-based synthetic process devised by US researchers. The miniature menagerie could help in efforts to develop high-volume production of aligned silica-based nanowires with optical properties.

	Close-up of one group of bundles from the image above. The group is about 400 microns across.

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.

The use of gallium catalysts could be a major improvement over the vapor-liquid-solid (VLS) process now used to make nanoscale structures, but has the added benefit of spontaneous division that leads to branching structures that could be used as optical splitters for photonic systems.

Transmission electron microscope (TEM) image shows individual silica nanowires grown on a silicon wafer. Average diameter is about 20 nanometers.

"These nanowires demonstrate many amazing growth phenomena unlike any previously observed through a conventional VLS growth process," explains materials scientist Zhong Lin "ZL" Wang, "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."

	Zhong Lin "ZL" Wang

The conventional VLS method of forming silica nanowires involves growing each wire from a single seed particle of gold, cobalt, nickel, or another high melting metal. Silicon is evaporated from a wafer by a high-temperature furnace and condenses on the surface of the molten catalyst where it combines with oxygen and crystallizes to form silica nanowires. The results depend on the initial size of the catalytic particle and a tangled mass of wires with different lengths is usually the product.

SEM image shows a structure produced by a bundle of silica nanowires grown on an alumina substrate. The cherry is about 20 microns in diameter.

Enormous droplets of gallium 5 to 50 micrometers across, i.e. thousands of times bigger than the standard catalytic particles, has allowed Wang and collaborators Zheng Wei Pan, Zu Rong Dai, and Chris Ma to grow hundreds of thousands of nanowires from a single catalyst. Gallium nitride powder is heated in argon gas to 1150 Celsius in a tube containing a silicon wafer. The powder decomposes releasing nitrogen gas and forming globules of molten gallium, which are carried by the argon and condense on the silicon, vaporizing it and allowing the silicon to react with oxygen in the tube, forming silica. The process takes about five hours to yield bundles of silica nanowires.

The wires usually grow to about the same length, and remain well ordered. "The importance of this process is that it will allow us to grow many aligned wires rather than tangles of wires," explains Wang. "The uniformity of the wires produced in this way could be very useful." The branching of some of the nanowires is also quite unique and might allow the production of junctions for light propagation in photonic applications; as switches for instance.