nanowire ink

Nanowire inks allow electronic circuits to be directly printed on paper

Electronic circuits can now be printed on cheap paper or plastic by using conductive inks that are formed from suspended metal nanoparticles in liquids.

A team from Duke University in North Carolina has been developing the new technique, which allows customisable circuit patterns to be printed on just about any surface.

Printed electronics are already being used on a wide scale in devices such as the anti-theft radio frequency identification (RFID) tags you might find on the back of new DVDs.

But they currently have one major drawback in that they have to be heated to melt all the nanoparticles together into a single conductive wire for the circuits to work. This process makes it impossible to print circuits on inexpensive plastics or paper.

The researchers demonstrated how tweaking the shape of the nanoparticles in the ink eliminated the need for heat.

By comparing the conductivity of films made from different shapes of silver nanostructures (pictured), it was discovered that electrons zip through films made of silver nanowires much more fluidly than films made from other shapes, like nanospheres or microflakes.

The electrons flowed so easily through the nanowire films that they could function in printed circuits without the need to melt them all together.

“The nanowires had a 4,000 times higher conductivity than the more commonly used silver nanoparticles that you would find in printed antennas for RFID tags,” said Benjamin Wiley, assistant professor of chemistry at Duke.

“So if you use nanowires, then you don’t have to heat the printed circuits up to such high temperature and you can use cheaper plastics or paper.

“There is really nothing else I can think of besides these silver nanowires that you can just print and it’s simply conductive, without any post-processing.”

These types of printed electronics could have applications far beyond smart packaging; researchers envisage using the technology to make solar cells, printed displays, LEDS, touchscreens, amplifiers, batteries and even some implantable bio-electronic devices.

Silver has become a go-to material for making printed electronics, Wiley said, and a number of studies have recently appeared measuring the conductivity of films with different shapes of silver nanostructures.

However, experimental variations make direct comparisons between the shapes difficult, and few reports have linked the conductivity of the films to the total mass of silver used, an important factor when working with a costly material.

“We wanted to eliminate any extra materials from the inks and simply hone in on the amount of silver in the films and the contacts between the nanostructures as the only source of variability,” said Ian Stewart who also worked on the project.

The team say they weren’t surprised that the long nanowire films had the highest conductivity. Electrons usually flow easily through individual nanostructures but get stuck when they have to jump from one structure to the next, Wiley explained, and long nanowires greatly reduce the number of times the electrons have to make this ‘jump’.

But they were surprised at just how drastic the change was. “The resistivity of the long silver nanowire films is several orders of magnitude lower than silver nanoparticles and only 10 times greater than pure silver,” Stewart said.

The team is now experimenting with using aerosol jets to print silver nanowire inks in usable circuits. Wiley says they also want to explore whether silver-coated copper nanowires, which are significantly cheaper to produce than pure silver nanowires, will give the same effect.

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