At the moment, most electronic devices are made from metal and complementary metal-oxide-semiconductors (CMOS). However, such materials cannot be easily used to make transparent and flexible electronics. 2D materials, which include graphene (a sheet of carbon just one atom thick), have come into their own here since they were discovered more than a decade ago. As well as being optically transparent, they are mechanically strong. And that is not all: because 2D crystals possess out-of-plane van der Waals interactions they can be readily combined into one multilayer stack whose electronic properties can be precisely controlled.

Now, a team of researchers led by Cinzia Casiraghi has come up with a new way to inkjet print several 2D materials, including graphene, MoS2, WS2 and hexagonal boron nitride, to make functional electronic devices.

“This is a multidisciplinary work that combines ink-formulation optimization, biological studies, electrical engineering and materials science,” explains Casiraghi. “We have carefully engineered the composition of the 2D inks to be able to fully print devices in vertical geometries, also known as ‘heterostructures’, in which thin films of different materials are deposited on top of one another as in a LEGO stack. These inks are water-based and are biocompatible, unlike previous formulations.”

By carefully selecting the sequence of the 2D materials in the stack, the researchers succeeded in producing fully-printed arrays of photodetectors on silicon, paper and plastic. They used the same approach to print logic memories made solely from 2D materials.

“We also studied the biocompatibility of the 2D material inks,” adds Casiraghi. “Until now, there had only been a few studies on the cytotoxicity of these formulations and our new analysis is of fundamental importance in view of these materials being used in different consumer products in the future.”

The Manchester team began by preparing a water-based 2D material dispersion using the traditional “assisted liquid-phase exfoliation” technique. In this process, the bulk material splits into individual platelets and disperses in a solvent with the help of an exfoliating agent,” explains Casiraghi. “You then add a small amount of additive to optimize the ink’s properties. Eventually a binder will also be added to this dispersion to minimize re-mixing of the different 2D materials at the interface of the LEGO multi-stack.”

The structures produced have many good properties, she tells “First, they are water-based, and water is a friendly, low-cost, low boiling point solvent. They are also highly concentrated, which reduces printing time, since more material is printed per unit area. They are optimized for inkjet printing too and no pre-treatment or heating of the substrate is required. This means that we can also print them on temperature-sensitive materials like paper and plastic.”

According to the researchers, the inks could be used, in principle, to build any type of device, regardless of how complex it is – for example heterostructures made up of 20–50 different layers with any combination of 2D materials available. “At the moment we are able to print arrays of heterostructures over large areas, in which each heterostructure can be designed ad hoc to sense or store a particular type of signal or information,” says Casiraghi.

The team, reporting its work in Nature Nanotechnology doi:10.1038/nnano.2016.281, is now busy looking into applications for the packaging sector (smart labels, for instance). “At the same time, and in collaboration with colleagues from the University of Pisa in Italy, we are further developing the logic memory concept,” adds Casiraghi. “Including additional elements in the devices, such as diodes and transistors, would allow us to perform more complex functions and increase the size of the memory.”