Developing flexible and printable electronics on paper with graphene and layered materials

Thanks to their unique and valuable properties, graphene and layered materials like hexagonal boron nitride and molybdenum disulfide are opening the door to new possibilities the realm of printable electronics, pioneered within the Graphene Flagship. Indeed, as printed materials become thinner, lighter and more flexible, the printable electronics industry continues to grow – in fact, it is set to be worth over $20 billion by 2025.

We spoke to Gianluca Fiori, Professor of Electronics at the University of Pisa, Italy, and Deputy Leader of the Graphene Flagship's Electronic Devices Work Package, about graphene's role in the design of innovative electronic devices and circuits. Fiori is working on new device architectures that can be printed on paper and other low-cost materials, in collaboration with Graphene Flagship partners the University of Manchester, UK, and Vienna University of Technology, Austria.

What brought you to the emerging field of flexible electronics?

I am an electrical engineer with a background in device simulations. I worked on electrical and noise characterisation of nanoscale devices during my PhD. I have experience investigating the main mechanisms at play in novel devices, as well as assessing their potential performance against the requirements of industry. Recently, I obtained an ERC Consolidator Grant for flexible electronics with graphene and layered materials.

Why do you think that graphene and layered materials could be the enabling technology for flexible printed electronics?

They have intrinsic characteristics that make them ideal for printed and flexible electronic applications. Water-based biocompatible inks made from layered materials can be combined to form heterostructures, which are fundamental to fabricating electronic devices.

Being extremely thin, they can bend up to a very large degree of strain while preserving their excellent electrical properties. Although this field of research is still in its infancy, devices with performances comparable to those of mainstream organic semiconductors have already been demonstrated, which really showcases the potential of these new technologies.

Is paper a suitable substrate for electronic devices? Can you tell us about some potential applications?

Paper is flexible, cheap and recyclable. It is readily available, and I believe it is the best substrate for flexible and foldable electronics. The applications of printable electronics on paper are extremely broad.

For example, think of smart paper devices attached to patches and diapers, which can be used for biometric readings, and smart packaging that monitors the supply chain and provides product information. The Internet of Things (IoT) will drive the spread of printable electronics even further.

What have you worked on recently?

A fully printable electronic circuit on paper needs various active and passive building blocks. Transistors are an example of an active circuit component. Examples of passive components include resistors, capacitors and inductors. Since 2018, we have collaborated with Graphene Flagship partner Cinzia Casiraghi's group, at the University of Manchester, to investigate passive components for electronics such as capacitors.

At the same time, we also tested a graphene-based transistor on paper. Its electrodes were printed with silver ink, while the channel and dielectric layer were printed with ink made of graphene and hexagonal boron nitride, respectively.

This year, my team together with colleagues at Graphene Flagship partners the University of Manchester, UK, Vienna Institute of Technology, Austria and and IIT, Italy, reported a molybdenum disulfide transistor printed on paper, with a performance comparable to the best devices obtained with more traditional techniques. 

What are the challenges of producing paper transistors? 

We inkjet-print most elements of the transistor, such as the contacts, dielectric layer, gates and connections. However, printing the semiconductor channel with good electric properties is challenging. We tackled this problem by transferring strips of molybdenum disulfide grown with chemical vapour deposition (CVD) on top of the paper substrate.

What's your next challenge?

The results we obtained so far are really promising, and we are now putting a lot of work into improving the device's speed so that we can exploit this technology in complex electronic circuits.

What's your vision?

In the short and medium term, we will most likely interface printed flexible devices with rigid circuits to obtain flexible and wearable systems with a certain degree of functionalities including sensing, computing and information transmission. In the long term, I believe that we will move towards really complex circuits fully printed on flexible substrates: it will take time, but it is within reach. 

Beyond paper electronics, have you been involved in other interesting projects?

Recently we took part in a project led by Thomas Mueller at Graphene Flagship partner Vienna University of Technology, investigating the use of layered material semiconductors in analogue electronics. We simulated one of the most important integrated circuits for analogue electronics, the operational amplifier, which was fabricated and characterised by Mueller's group.

References

Torrisi, Felice, et al. "Inkjet-printed graphene electronics." ACS nano 6.4 (2012): 2992-3006.

MarketsandMarkets™. "Printed Electronics Market with COVID-19 Impact Analysis by Printing Technology (Screen, Inkjet, Gravure), Application (Displays, Sensors, Batteries), Material (Inks, Substrates), End-Use Industry, and Geography - Global Forecast to 2025." (2020)

McManus, Daryl, et al. "Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures." Nature Nanotechnology 12.4 (2017): 343-350.

Worsley, Robyn, et al. "All-2D material inkjet-printed capacitors: toward fully printed integrated circuits." ACS nano 13.1 (2018): 54-60.

Conti, Silvia, et al. "Low-voltage 2D materials-based printed field-effect transistors for integrated digital and analog electronics on paper." Nature communications 11.1 (2020): 1-9.

Polyushkin, Dmitry K., et al. "Analogue two-dimensional semiconductor electronics." Nature Electronics 3.8 (2020): 486-491.