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Low-cost water-based inkjet printing for electronics

Technology News |
By Julien Happich

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.

Samples of water-based two-dimensional crystal inks.

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.

Schematic of an all-printed GrB/WS2/GrT
photodetector  on a Si/SiO2 substrate.

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.

An array of all-printed Gr/WS2/Gr photodetectors on PET

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’).

A printed read-only memory composed of a horizontal line (word line) 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’.

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.

Micro graph image of the fabricated device,
with the bias voltage (Vp) source and load resistors
(RL) added.
 

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.

 

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