Researchers at the University of Manchester in the UK have built photosensors and programmable logic memory devices with inkjet printers using biocompatible, water-based 2D crystal inks that are highly concentrated for improved electrical performance.
The Manchester research, described in the journal Nature Nanotechnology, represents a significant development over current ink formulations that are typically toxic and difficult to process. While this new formulation is easier to produce, biocompatibility may be its most attractive feature, opening up potential applications in medical devices.
The introduction and development of graphene and other 2D materials as alternatives to conductive polymers for inkjet printing formulations has been thought to represent a groundbreaking development in flexible electronics.
The ability to stack layers of these 2D materials has keyed their emergence in inkjet printing. Where these different layers meet are known as heterojunctions. The combination of multiple heterojunctions are known as heterostructures that are held together by van der Waal forces, which enable each layer to essentially snap into place like a LEGO brick. This stacking capability makes it easier to tailor the electronic properties of the heterostructures to create functional devices.
Typically these heterostructures are made from 2D materials produced by micro-mechanical exfoliation, a time consuming process in which layers of graphene are pulled away from graphite using tape.
“In our work, we use solution-processed 2D materials,” said Cinzia Casiraghi, a professor at the University of Manchester, and one of the co-authors of the research in an e-mail interview with IEEE Spectrum. “When this material is deposited on a surface, then a film—formed by millions of single and few-layers nanosheets all assembled together—is produced after evaporation of the solvent.”
In solution-based processes, bulk materials are dispersed into a solvent and split into individual pieces, or platelets. Water by itself can’t be used as a solvent since graphite is not soluble in water. As a result, with water you don’t get any formulation. To overcome this limitation, surfactants are added to water to improve both the exfoliation as well as the printability. However, the negative byproduct of the surfactants is that the resulting inks have a low concentration of 2D crystals and a high amount of residual surfactant, which is difficult to remove and degrades the electrical performance of the material.
“Water-based inks were already commercially available before our work, but they are based on the use of surfactants,” explained Casiraghi. “In our work, we first exfoliate the starting material in water, with the help of an exfoliating agent and then, after exfoliation, we modify the composition by adding tiny amount of additives to tune the properties of the ink to achieve optimum printability.”
While the Manchester researchers and others have produced heterostructures with a solution-based processes previously, such as spin coating and vacuum filtration, these approaches didn’t give a high level of control in the fabrication of the heterostructure. These latest inks offer the ability to fabricate 2D material heterostructures by inkjet printing only, according to Casiraghi. By using an inkjet printer it becomes possible to print where you want, to print any arbitrary sequence of 2D materials in the heterostructure and to make hundreds of devices on a substrate all at the same time.
“A heterostructure-based device must have well-controlled and sharp interfaces to work efficiently and reliably; those were difficult to achieve with existing inks,” she added. “Our inks also contain a binder, which helps minimize the mixing of different 2D materials at the heterojunctions, giving rise to controlled interfaces.”
The Manchester scientists believe that their ink is ready to be used in device fabrication.
“We are in discussion with several companies with the aim of exploiting the inks for commercial products,” added Casiraghi.
While more work is needed to optimize and integrate the devices in consumer products, the Manchester team is already looking to apply the technology for smart labels used in packaging.
At the same time, with the colleagues at the University of Pisa, the Manchester team is further developing the logic memory concept by including additional elements in the device, such as diodes and transistors, which would allow the device to perform more complex functions and to increase the size of the memory.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.