Water-based and inkjet printable inks.
Sketch of a programmable read-only memory (PROM) composed of horizontal (word lines) and vertical lines (bit lines) made of ink-jet graphene. A logic ‘1’ is stored at regular intersections of the word line and the bit line, while a logic ‘0’ is programmed by printing WS2 between the two. The sketch shows a 4-bit memory storing the word ‘1010’.
Nobel medal printed with water-based graphene ink on paper.Researchers have come up with a recipe for inks containing novel two-dimensional materials like graphene that could enable the printing of simple electronic devices [McManus et al., Nature Nanotechnology (2017), doi: 10.1038/nnano.2016.281].
Printing electronic devices cheaply and easily could open up applications in smart packaging or identification tags for food and drinks, pharmaceuticals, and consumer goods. But while promising two-dimensional materials like graphene can be processed using inkjet printing, current ink formulations usually require toxic solvents, have low concentration, or require time-consuming and expensive processing.
Now Cinzia Casiraghi and her at from the University of Manchester have managed to achieve water-based, biocompatible ink formulations for a variety of two-dimensional materials including graphene, MoS2, WS2, and hexagonal boron nitride (h-BN).
“We use water as a solvent and a small aromatic compound as an exfoliating agent during liquid-phase exfoliation,” says Casiraghi.
The researchers then used small amounts of other additives to tune the surface tension and viscosity of the ink to render the formulation printable. Finally, in order to be able to print distinct layers of different inks on top of each other, to build-up what Casiraghi describes as ‘LEGO stacked devices’, the team added a binder to minimize intermixing.
“We carefully engineered the composition of the inks to be able to fully print devices in vertical geometry, so-called heterostructures,” she explains.
In contrast to existing ink formulations, those developed by Casiraghi and her team are water-based and biocompatible. With their improved approach, the team printed arrays of graphene/WS2-based photodetectors on silicon, paper, and plastic.
Micah J. Green of Texas A&M University believes that the significance of the findings lies in the applicability of the approach to a range of nanosheet types.
“The problem is that the additives needed to disperse nanosheets as a colloid are not the same as those needed to make a printable ink,” he explains. “That’s the key issue here: the researchers use pyrenes as dispersants, which are largely – but not completely – removed after exfoliation. Then they use a completely separate set of additives and surfactants to modify the properties for printing.”
Crucially, says Green, this means that heterostructures can be printed directly. The team worked with colleagues from the Università di Pisa to show exactly this, fabricating logic memories solely with printed two-dimensional material inks for the first time.
The simplicity of the ink formulations and their compatibility with inkjet printing on flexible substrates makes the researchers’ approach attractive for applications where thinner, lighter, cheaper, and easy-to-integrate components are useful. As well as applications like smart packaging, labels, and anti-counterfeiting, the inkjet printable formations could be suitable for biomedical devices such as biosensors.
“The approach is very practical as it is based on a simple piezoelectric inkjet printer,” says Casiraghi. “But from an industrial point of view, it will be necessary to scale-up the ink production.”
Casiraghi and her team are now working on improving the memory logic devices and fabricating a range of different devices targeted on the packaging sector.
This article was originally published in Nano Today (2017), doi: 10.1016/j.nantod.2017.02.003.