Electronic waste is an increasingly serious problem, not least because it is hard to recycle the silicon-based components that make up the bulk of consumer electronic devices. Researchers at Duke University in the US have now taken a step towards remedying this by creating the first fully recyclable transistors made from carbon-based “inks” that can be printed on paper or another environmentally friendly substrate. While these devices are unlikely to replace their silicon cousins any time soon, they could find their way into specialist applications such as environmental sensors or biomedical sensing patches relatively quickly.
Electronics containing carbon-only components could be ideal for making printable, recyclable devices. Semiconducting carbon nanotubes (rolled up sheets of carbon) and conducting graphene (a sheet of carbon just one atom thick) are both good candidates for making such components. Another carbon-based compound, cellulose, has previously been employed as both a substrate and dielectric, and has the advantage of being both naturally biodegradable and the most abundant polymer on Earth.
All-carbon printed electronics, however, are few and far between because there aren’t many carbon-based dielectrics that can be processed in solution – a prerequisite for printing devices. While cellulose paper can work as a dielectric in high-power transformers, crystalline nanocellulose is generally used as a binder or in conjunction with another material that has a high dielectric constant, not as a standalone printable dielectric.
To make its all-carbon transistor, the Duke team led by Aaron Franklin put all these ingredients together for the first time, using crystalline nanocellulose extracted from wood fibres as a dielectric; carbon nanotubes as a semiconductor; and graphene as a conductor. After making inks from these three components, the researchers showed they could directly print them onto a paper substrate using a technique called aerosol jet printing at room temperature. By adding salt (sodium chloride) to the dielectric, they obtained devices with an on-current of 87 μA/mm and a subthreshold swing of 132 mV/dec, values that will allow the transistors to be employed in a wide variety of applications.
The team showed that their transistors could be recycled by submerging them in a series of ultrasonic baths and then centrifuging the resulting solution. They recovered the CNTs and graphene with over 95% efficiency, then reused the materials to reprint new transistors. The nanocellulose, which is naturally biodegradable, can also be recycled, as can the paper substrate.
Nanocellulose ink hits the spot
Franklin points out that nanocellulose has been used to make recyclable packaging for years. However, while its potential applications as an insulator in electronics were widely understood, nobody had figured out how to use it in a printable ink before.
Why carbon is an amazing material – part one
Demonstrating such a fully recyclable printed transistor is, he adds, a first step towards using the technology to make simple commercial devices. Two possibilities for early devices include environmental sensors (to measure energy consumption of a building, for example) or customized biosensing patches for monitoring medical conditions. Indeed, the researchers, who detail their work in Nature Electronics, have already made a fully printed, paper-based biosensor for sensing lactate from their transistor.
The researchers hope their results will increase interest in areas of materials and electronics research that focus on recyclability and novel fabrication techniques such as printing. “We now plan to further improve on the ink formulations and resultant performance of these printed electronics, reduce dependence on harsh chemicals in the process, and study a variety of applications these devices make possible,” Franklin tells Physics World.