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Future electronics might be based on carbon nanotubes

08 April 2015

Researchers find a way to purify arrays of single-walled carbon nanotubes, possibly providing a step toward post-silicon circuits and devices.

Thermal gradients associated with mild heating of a metallic carbon nanotube induces thermocapillary flows in a thin organic overcoat. The result is an open trench with the tube at the base (image: J.Rogers/UIUC)

The exceptional properties of carbon nanotubes have tantalized researchers for years, particularly as they could serve as a successors to silicon in laying the logic for smaller, faster and cheaper electronic devices. Like silicon, they can be semiconducting in nature and they can undergo fast and highly controllable electrical switching.

But a big barrier to building useful electronics with carbon nanotubes has always been the fact that when they're arrayed into films, a certain portion of them will act more like metals than semiconductors - a flaw that limits their potential in terms of electronic devices.

According to Professor John Rogers of the University of Illinois-Urbana Champaign (UIUC), the purity needs to exceed 99.999 percent - meaning even one 'bad' carbon nanotube in 100,000 is enough to kill an electronic device. "If you have lower purity than that," he said, "that class of materials will not work for semiconducting circuits."

Now Rogers and a team of researchers have shown how to strip out the metallic carbon nanotubes from arrays using a relatively simple, scalable procedure that does not require expensive equipment. Their work is described in the Journal of Applied Physics.

Though it has been a persistent problem for the last 10-15 years, the challenge of making uniform, aligned arrays of carbon nanotubes packed with good densities on thin films has largely been solved by different groups of scientists in recent years. But the purity problem remains.

There were some methods of purification that were easy to do but fell far short of the level of purification necessary to make useful electronic components. Very recent approaches offer the right level of purification but rely on expensive equipment, putting the process out of reach of most researchers.

However, the UIUC researchers were able to deposit a thin coating of organic material directly on top of a sheet of arrayed nanotubes in contact with a sheet of metal. They then applied current across the sheet, which allowed the current to flow through the nanotubes that were metal conductors, but not the bulk of the tubes, which were semiconducting.

The current heated the metal nanotubes fractionally, and just enough to create a 'thermal capillary flow' that opened up a trench in the organic topcoat above them. Unprotected, the metallic tubes could then be etched away using a standard benchtop instrument, and the organic topcoat simply washed away.

This left an electronic wafer coated with semiconducting nanotubes free of metallic contaminants, and the team tested the result by building arrays of transistors. "You end up with a device that can switch on and off as expected, based on purely semiconducting character," Rogers adds.

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