SCIENCE & ENGINEERING NEWS

Irvine, CALIF. — UC Irvine chemists have developed a method for preparing metal nanowires featuring the conductivity, strength and length necessary for use in microelectronic devices, such as diodes and transistors, designed for emerging nanotechnologies.

Reginald Penner, UCI professor of chemistry – with graduate student researchers Michael P. Zach and Kwok Ng – built these new nanowires from molybdenum, an element used to strengthen steel. The wires were created using a method called step-edge decoration, which Penner said is proving to be an efficient way to construct these custom wires in large numbers. Results of the project will be published in the Dec. 15, 2000 issue of Science.

“Metal nanowires may be important to the future of nanotechnology, but there simply hasn’t been a good general way to make them,” Penner said. “Our new method for growing metal nanowires may prove to be an effective method for making the long, uniform conductive wires needed for connectors in future generations of nanometer-scale electronics.”

In building these wires through the step-edge decoration process, the Penner group first electrochemically deposited molybdenum dioxide onto a piece of graphite. Rudimentary wires began growing when their molecules linked onto step edges – molecular defects on the graphite surface where the emerging wires could gain a stronger atomic hold. After the brittle molybdenum dioxide wires were formed, they were heated in hydrogen gas at 350 degrees to remove the oxygen, leaving only the molybdenum metal.

The resulting pure molybdenum wires were smaller in diameter but also stronger, more conductive and more flexible than those created in the first step of the process. The metal wires were then embedded in a polystyrene film and peeled off the graphite surface. The nanowires measured between 10 nanometers and a half-micron in diameter and up to 100 microns, or one-tenth of a millimeter, in length.

“By heating the molybdenum oxide in hydrogen, the wires we created were 1,000 times more conductive than the wires we electroplated in the first step of the process,” Penner said. “For the practical applications that we foresee for these nanowires, the electronic conductivity is obviously extremely important.”

In developing a method to build long, uniform nanowires, Penner said that the step-edge decoration method used on graphite is proving to be more practical than using template synthesis, which is another approach to building these wires used by researchers.

“The problem with templates is that you need a different one for every diameter of wire you want to grow,” Penner said. “Also, thick templates needed to prepare long nanowires are not readily available. Our technique allows you to grow wires of any diameter you want as well as very long wires with a piece of graphite, which is something you can’t do with existing templates.”

Penner and his research team plan to continue their research by developing electronic devices with these nanowires, such as sensors, diodes and transistors.

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