Printing flexible transistors using nano inks

30 October 2017

The first printed transistor made entirely from layered materials could pave the way for low-cost electronic devices to be fitted into billions of everyday objects.

Printed electronics are already used in flexible screens, ‘smart’ labels and sensors. But many manufacturers want to add electronics to the packaging of products from medicines to milk, to track their movements, for example, or to monitor the freshness of food. That requires much cheaper printable circuits that can be easily mass-produced. “They’re not trying to compete with the standard silicon-based electronics in your computer,” says Jonathan Coleman, a nanotechnology researcher at Trinity College Dublin. “It’s about printing low-performance circuits extremely cheaply.”

A transistor is a core circuit component that turns an electrical signal on or off, a bit like a switch. Printed transistors have been made from a range of carbon-based molecules such as polymers, but their capabilities are quite limited and they tend to be sensitive to air and moisture. Meanwhile, printed transistors based on nanoparticles and nanotubes offer better performance, but are more difficult to mass manufacture.

Coleman and colleagues hope to overcome these challenges by using materials that contain one atom-thick layers. These materials include graphene, whose chicken-wire arrangement of carbon atoms makes it strong, flexible, and an excellent electrical conductor. Coleman’s team had previously pioneered a simple method called liquid exfoliation to prepare inks from such layered materials.

The researchers have now used an inkjet printer, loaded with inks that contained chunks of these materials, to build a transistor on a flexible plastic sheet. They printed the transistor’s electrodes using graphene ink; then they added tungsten diselenide to make the semiconductor channel between two electrodes; and laid down boron nitride as an insulating layer. Each of these components was made from a porous jumble of chunks, each 330–380 nanometres long and 13–17 atomic layers thick. Then the team infused the stacks with a liquid salt containing positive and negative ions. This ionic liquid helped charge to flow across the device, allowing it to switch on and off more effectively.

Transistors are evaluated on properties such as the mobility of their electrical charges, and the difference in current between the ‘on’ and ‘off’ states. By these measures, the transistor performed reasonably well, although it did not match up to commercial printed transistors. However, Coleman points out that this is the first device of its kind, and its performance should improve significantly as the team refines the components and assembly process: “We’re just at the start of this.”

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References

1. Science 356, 69–73 (2017). doi: 10.1126/science.aal4062