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New look for transparent nanowires

27 Jun 2007

Researchers may have found an alternative to indium tin oxide for applications in flat-panel displays, solar cells and LEDs.

As indium becomes more rare and expensive, researchers are looking for new metals for making optically transparent nanowires for applications in flat-panel displays, solar cells and light-emitting diodes. Now, a team from the University of Michigan has shown that nanowires can be fabricated from tin oxide doped with antimony, using an inexpensive, easily scalable technique. The wires, which are highly conducting, have resistivities of just 4.1 × 10–4 Ωcm and failure current densities up to 2.1 × 107 A/cm2.

"Previous studies on transparent conducting nanowires have either resulted in undoped samples with low conductivity or were based on indium tin oxide, a material with uncertain supply and soaring price due to the scarcity of indium," team leader Wei Lu told nanotechweb.org. "Our demonstration of transparent conducting nanowires using low-cost tin oxide (SnO2) through a simple, inexpensive fabrication method will help close the gap between novel laboratory test structures and large-scale, real-world applications."

Lu and colleagues made their nanowires using a catalyst-mediated, vapour–liquid–solid method. The researchers began by sputtering 10 nm of gold film on a silicon wafer. Next, they loaded this into a growth furnace with tin and antimony.

By heating the furnace to 900 °C, the gold film melts into gold nanoparticles. Meanwhile, the tin and antimony vaporize and react with the gold nanoparticles to form liquid alloy droplets. As the droplets become "supersaturated", tin and antimony precipitate, react with oxygen inside the furnace and finally form the nanowires.

"Our method is very simple and we envisage the production of kilograms of nanowires each time with a scaled-up version of the current set-up," explained Lu.

The nanowires are highly transparent in the visible range. For example, more than 77% of light can pass through a nanowire film 100 nm thick on top of a 0.5 mm thick glass slide. "Moreover, the nanowires are 'invisible' with an optical microscope – even those with a large diameter," said Lu. "The wires are only detectable in the special 'dark field' mode and even then you can see through them." The nanowires are also metal-like, that is, they are highly conducting and their resistance decreases with temperature.

Lu says that the nanowires could be used in organic solar cells, thin-film transistors, gas sensors and field emitters. Indeed, the Michigan team has already demonstrated a prototype field emitter with an ultralow turn-on voltage of 2 V. Some of these field-emitter applications may even be commercialized soon, reckons Lu.

The researchers are now working on the idea of using the transparent conducting nanowires as linkages in percolated thin-film networks. "For example, we have recently obtained much higher ethanol sensitivity in a mixed metal/semiconductor nanowire thin-film sensor compared with using semiconductor nanowires alone," explained Lu. Using the same material growth technique, the team is also developing high-performance, fully transparent thin-film transistor devices, in which a lower doping is used and the transparent nanowires behave like a semiconductor.

The work was reported in Appl. Phys. Lett.

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