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The Next Big Thing:
Making Silicon Nanowire, Nanotubes and Nanospheres

Silicon and its oxide – silica – find widespread industrial applications and form the backbone of modern microelectronics. Work by Jim Gole, Mark White and collaborators is opening potential new silicon uses in nanocatalysis, nanosensors and nanoelectronics – while blazing a nanoscale trail for materials such as tin oxide and gallium nitride.

Georgia Tech researchers produce high-quality nanostructures from silicon and silica, including bundles of silica fiber (left) and silica nanospheres 30 nanometeres in diameter (right).

Using a simple and flexible high-temperature synthesis technique, the Georgia Tech researchers produce nanowires, nanotubes, nanospheres, "nanodiskettes" and nanofiber arrays from silicon, silica, silicon carbide and tin oxide. Operating at 1,300 to 1,500 degrees Celsius, the vapor-liquid-solid (VLS) process generates structures that are largely defect-free.

"We have developed a simple and generalized technique to make these nanostructures," explains Gole, a professor in the School of Physics. "We can make virtually any structure we want by putting the right materials together in the right spatial configuration under the right conditions."

Researchers vary the gas mixture, pressure, flow rate and starting materials to produce:

• Nanowires with 20-nanometer cores of crystalline silicon surrounded by 10-nanometer sheaths of silica.

• Silica nanospheres in uniform sizes from 8 to 45 nanometers. By coating the nanospheres with copper, White – a professor in the School of Chemical Engineering – produces catalytic structures that convert ethanol to aldehyde more efficiently than conventional catalysts. The nanospheres could replace silica powders now used for catalytic supports.

• Silica nanotubes that grow from silicon substrates. The nanotubes have outer diameters of 70 nanometers and walls 20 nanometers thick, leaving 30-nanometer channels that may have applications in high-molecular-weight hydrocarbon catalysis.

• Nanofiber arrays that resemble brushes.

• Biaxial fibers composed of silicon and silicon carbide with potential electronic uses.

• Tin oxide nanowires, nanotubes and disc-shaped structures dubbed "nanodiskettes." Beyond sensor applications, the nanotubes may be useful as chemical reactors.

The group has also succeeded in placing tin oxide nanocrystallites onto silica nanospheres.

The VLS synthesis process uses double concentric alumina tubes heated in a furnace. Precursor materials, such as a 50-50 mixture of silicon and silica, are placed in a crucible within the tubes, and pressurized argon gas is flowed through. Nanostructures form in a cooled region at one end of the tubes.

The researchers will soon lower their process temperature to accommodate production of gallium nitride structures.

For more information, contact James Gole, School of Physics, Georgia Tech, Atlanta, GA 30332-0430. (Telephone: 404-894-4029) (E-mail: james.gole@physics.gatech.edu); or Zhong Wang, School of Materials, Science and Engineering, Georgia Tech, Atlanta, GA 30332-0245. (Telephone: 404-894-8008) (E-mail: zhong.wang@mse.gatech.edu)

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Last updated: July 14, 2001