Low-cost water-based inkjet printing for electronics
Doing away with the often toxic and expensive solvents typically used to disperse the active materials in conductive or semiconducting inks, the simply formulated water-based inks (with polysaccharide xanthan gum as the binder to achieve the right level of viscosity) allow for the fabrication of thin-ﬁlm heterostructures. Different printing passes on paper simply create thin layers of the 2D materials (about 5nm thin per pass), which can be stacked into elaborate electronic circuits, operational without any post-processing or high temperature annealing.
A PhD candidate and the first author of the paper “Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures” published in Nature Nanotechnology, Daryl McManus writes that in the scalable manufacturing process they use, no solvent exchange, chemical treatment or harsh conditions are necessary, and the ink compositions have been optimized to achieve optimal ﬁlm deposition for multistack formation, without material re-dispersion at the interfaces (which would compromise device reproducibility and performance).
To demonstrate the viability of their 2D crystal inks, the researchers printed various electronic circuits made up of several materials layers stacked one upon the other through multiple printing passes.
They created an array of 16 photodetectors on a Si/SiO2 substrate, printing for each photodetector first a graphene line about 50nm thick for the bottom electrode (GrB), a tungsten disulphide (WS2) square about 100nm thin for the photoactive element, and then printing a top electrode out of graphene again, perpendicular to the bottom line (GrT). They tested the photodetectors with different laser powers and found that all of the 16 heterostructures exhibited the same I–V characteristic, in effect achieving a fabrication yield of 100% on Si/SiO and proving the repeatability of the layer stacking/printing process.
The researchers also printed an array of 20 GrB/WS2/GrT photodectors onto a PET ﬁlm across an area of 30x40mm, the devices were operational without any annealing and remained operational while the film was being flexed (during a bending test, the photocurrent was stable up to about 2% strain).
Led by Prof. Cinzia Casiraghi, the team also printed read-only memories made out of the two-dimensional crystals. Words were encoded into the memory device during fabrication, by laying horizontal (word lines) and vertical (bit lines) stripes of graphene either in contact (storing a logic ‘1’) or insulated by a layer of WS2 (forming a logic ‘0’).
The stored word can be read by applying 0.5V to the word line and reading out the voltage bias across pulldown resistors on the bit lines.
The researchers concluded their experiments with a series of cytotoxicity studies to evaluate the biocompatibility of these water-based inks, hinting that the inks would be innocuous enough to be used on a large scale in smart packaging applications and identiﬁcation tags.
In 2015, McManus has received the Eli and Britt Harari Graphene Enterprise Award with a £50k prize which the PhD student used to spin-off his own startup, Think2d Ltd registered in October the same year.
The company has yet to announce deliverables but it may seek the help of third parties to industrialize and mass-produce the water-based inks and possibly license its technology together with IP libraries of proven printable electronic designs.