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 the desired structures.
However, producing layers and combining them to make complex, sandwich-like materials is challenging. To fully exploit the potential of graphene, there is a need for scalable manufacturing techniques.
Researchers in the UK from the University of Nottingham believe the solution could be 3D-printing – additive manufacturing – using inks in which flakes of graphene are suspended. Their study shows it is possible to print such inks so they build up in layers a few atoms thick but centimetres across. These layers mesh together to create complex, customised structures (Adv. Funct. Mater., doi: 10.1002/adfm.202007478).
Using quantum mechanical modelling, the researchers also pinpointed how electrons move through the 2D material layers, giving them a better understanding of how such devices could be modified in future. ‘According to the laws of quantum mechanics, in which the electrons act as waves rather than particles, we found electrons in 2D materials travel along complex trajectories between multiple flakes,’ says one of the researchers, Mark Fromhold. ‘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.’
The team believes that, 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. They used inkjet-printed graphene to make a field effect transistor and Ohmic contacts on a phototransistor – a 2D metal chalcogenide, InSe. They claim this is the first time inkjet-printed graphene has successfully replaced single-layer graphene as a contact material for 2D metal chalcogenides. They think their results could lead to a range of applications including large, efficient solar cells; wearable, flexible electronics powered by sunlight or motion; and potentially printed computers.
‘While graphene flakes have been printed in a variety of ways before, understanding the underlying transport phenomena is paramount to turning the material into a useful device,’ comments Chris Spadaccini of the Lawrence Livermore National Laboratory, US. ‘This paper does an excellent job of laying this foundation and demonstrates a field effect transistor and Ohmic contact, first of their kind devices in this material to my knowledge, perhaps paving the way for the scale-up of 2D materials.’