A representative arrangement of graphene flakes in ink-jet printed graphene between two contacts (green). Color gradient corresponds to variation of flake potentials. (Image courtesy of the researchers)

Researchers at the University of Nottingham have cracked the conundrum of how to use inks to 3D-print novel electronic devices with useful properties, such as an ability to convert light into electricity. Their study shows that it is possible to jet inks containing tiny flakes of 2D materials such as graphene, to build up and mesh together the different layers of these complex customized structures.

Using quantum mechanical modelling, the researchers also pinpointed how electrons move through the 2D material layers, to completely understand how devices can be modified in the future.

Often described as a ‘super material’, graphene was first created in 2004. It exhibits many unique properties including being stronger than steel, highly flexible, and the best conductor of electricity ever made. Two-dimensional materials like graphene are usually made by sequentially exfoliating a single layer of carbon atoms — arranged in a flat sheet — which are then used to produce bespoke structures. However, producing layers and combining them to make complex, sandwich-like materials has been difficult and has usually required painstaking deposition of the layers one at a time by hand.

“By linking together fundamental concepts in quantum physics with state-of-the art-engineering, we have shown how complex devices for controlling electricity and light can be made by printing layers of material that are just a few atoms thick but centimeters across. According to the laws of quantum mechanics, in which the electrons act as waves rather than particles, we found that electrons in 2D materials travel along complex trajectories between multiple flakes. It appears as if the electrons hop from one flake to another like a frog hopping between overlapping lily pads on the surface of a pond,” said Professor Mark Fromhold, Head of the School of Physics and Astronomy.

Since its discovery, there has been an exponential growth in the number of patents involving graphene. However, in order to fully exploit its potential, scalable manufacturing techniques need to be developed. This new research shows that additive manufacturing — 3D printing — using inks, in which tiny flakes of graphene (a few billionths of a meter across) are suspended, provides a promising solution. By combining advanced manufacturing techniques to make devices, along with sophisticated ways of measuring their properties, and quantum wave modelling, the team worked out exactly how inkjet‐printed graphene can successfully replace single layer graphene as a contact material for 2D metal semiconductors.

“While 2D layers and devices have been 3D printed before, this is the first time anyone has identified how electrons move through them and demonstrated potential uses for the combined, printed layers. Our results could lead to diverse applications for inkjet‐printed graphene‐polymer composites and a range of other 2D materials. The findings could be employed to make a new generation of functional optoelectronic devices; for example, large and efficient solar cells; wearable, flexible electronics that are powered by sunlight or the motion of the wearer; perhaps even printed computers,” said Dr. Lyudmila Turyanska.

The researchers used a wide range of characterization techniques including micro‐Raman spectroscopy (laser scanning), thermal gravity analysis, a novel 3D orbiSIMS instrument, and electrical measurements, to provide detailed structural and functional understanding of inkjet‐printed graphene polymers and the effects of heat treating (annealing) on performance.

The next steps for the research are to better control the deposition of the flakes by using polymers to influence the way they arrange and align and trying different inks with a range of flake sizes. The researchers also hope to develop more sophisticated computer simulations of the materials and the way they work together, developing ways of mass-manufacturing the devices they prototype.