̽��ֱ�� of Cambridge - Guohua Hu

Coffee stains inspire optimal printing technique for electronics

Anonymous — Wed, 12 Aug 2020 18:00:00 +0000

Have you ever spilled your coffee on your desk? You may then have observed one of the most puzzling phenomena of fluid mechanics – the coffee ring effect. This effect has hindered the industrial deployment of functional inks with graphene, 2D materials, and nanoparticles because it makes printed electronic devices behave irregularly.

Now, after studying this process for years, a team of researchers have created a new family of inks that overcomes this problem, enabling the fabrication of new electronics such as sensors, light detectors, batteries and solar cells.

Coffee rings form because the liquid evaporates quicker at the edges, causing an accumulation of solid particles that results in the characteristic dark ring. Inks behave like coffee – particles in the ink accumulate around the edges creating irregular shapes and uneven surfaces, especially when printing on hard surfaces like silicon wafers or plastics.

Researchers, led by Tawfique Hasan from the Cambridge Graphene Centre of the ̽��ֱ�� of Cambridge, with Colin Bain from the Department of Chemistry of Durham ̽��ֱ��, and Meng Zhang from School of Electronic and Information Engineering of Beihang ̽��ֱ��, studied the physics of ink droplets combining particle tracking in high-speed micro-photography, fluid mechanics, and different combinations of solvents.

Their solution: alcohol, specifically a mixture of isopropyl alcohol and 2-butanol. Using these, ink particles tend to distribute evenly across the droplet, generating shapes with uniform thickness and properties. Their results are reported in the journal Science Advances.

“ ̽��ֱ��natural form of ink droplets is spherical – however, because of their composition, our ink droplets adopt pancake shapes,” said Hasan.

While drying, the new ink droplets deform smoothly across the surface, spreading particles consistently. Using this universal formulation, manufacturers could adopt inkjet printing as a cheap, easy-to-access strategy for the fabrication of electronic devices and sensors. ̽��ֱ��new inks also avoid the use of polymers or surfactants – commercial additives used to tackle the coffee ring effect, but at the same time thwart the electronic properties of graphene and other 2D materials.

Most importantly, the new methodology enables reproducibility and scalability – researchers managed to print 4500 nearly identical devices on a silicon wafer and plastic substrate. In particular, they printed gas sensors and photodetectors, both displaying very little variations in performance. Previously, printing a few hundred such devices was considered a success, even if they showed uneven behaviour.

“Understanding this fundamental behaviour of ink droplets has allowed us to find this ideal solution for inkjet printing all kinds of two-dimensional crystals,” said first author Guohua Hu. “Our formulation can be easily scaled up to print new electronic devices on silicon wafers, or plastics, and even in spray painting and wearables, already matching or exceeding the manufacturability requirements for printed devices.”

Beyond graphene, the team has optimised over a dozen ink formulations containing different materials. Some of them are graphene two-dimensional ‘cousins’ such as black phosphorus and boron nitride, others are more complex structures like heterostructures – ‘sandwiches’ of different 2D materials – and nanostructured materials. Researchers say their ink formulations can also print pure nanoparticles and organic molecules.This variety of materials could boost the manufacturing of electronic and photonic devices, as well as more efficient catalysts, solar cells, batteries and functional coatings.

̽��ֱ��team expects to see industrial applications of this technology very soon. Their first proofs of concept – printed sensors and photodetectors – have shown promising results in terms of sensitivity and consistency, exceeding the usual industry requirements. This should attract investors interested in printed and flexible electronics.

“Our technology could speed up the adoption of inexpensive, low-power, ultra-connected sensors for the internet of things,” said Hasan. “ ̽��ֱ��dream of smart cities will come true.”

̽��ֱ��research was funded by the EPSRC, InnovateUK and the Royal Society.

Reference:
G. Hu et al. ‘A general ink formulation of 2D crystals for wafer-scale inkjet printing.’ Science Advances (2020). DOI: 10.1126/sciadv.aba5029.

Using an alcohol mixture, researchers modified how ink droplets dry, enabling cheap industrial-scale printing of electronic devices at unprecedented scales.

̽��ֱ��natural form of ink droplets is spherical – however, because of their composition, our ink droplets behave like pancakes

Tawfique Hasan

Drying droplets: the red arrows showing the end of the particle trajetories

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New graphene based inks for high-speed manufacturing of printed electronics

sc604 — Mon, 19 Oct 2015 10:38:54 +0000

A low-cost, high-speed method for printing graphene inks using a conventional roll-to-roll printing process, like that used to print newspapers and crisp packets, could open up a wide range of practical applications, including inexpensive printed electronics, intelligent packaging and disposable sensors.

Developed by researchers at the ̽��ֱ�� of Cambridge in collaboration with Cambridge-based technology company Novalia, the method allows graphene and other electrically conducting materials to be added to conventional water-based inks and printed using typical commercial equipment, the first time that graphene has been used for printing on a large-scale commercial printing press at high speed.

Graphene is a two-dimensional sheet of carbon atoms, just one atom thick. Its flexibility, optical transparency and electrical conductivity make it suitable for a wide range of applications, including printed electronics. Although numerous laboratory prototypes have been demonstrated around the world, widespread commercial use of graphene is yet to be realised.

“We are pleased to be the first to bring graphene inks close to real-world manufacturing. There are lots of companies that have produced graphene inks, but none of them has done it on a scale close to this,” said Dr Tawfique Hasan of the Cambridge Graphene Centre (CGC), who developed the method. “Being able to produce conductive inks that could effortlessly be used for printing at a commercial scale at a very high speed will open up all kinds of different applications for graphene and other similar materials.”

“This method will allow us to put electronic systems into entirely unexpected shapes,” said Chris Jones of Novalia. “It’s an incredibly flexible enabling technology.”

Hasan’s method, developed at the ̽��ֱ��’s Nanoscience Centre, works by suspending tiny particles of graphene in a ‘carrier’ solvent mixture, which is added to conductive water-based ink formulations. ̽��ֱ��ratio of the ingredients can be adjusted to control the liquid’s properties, allowing the carrier solvent to be easily mixed into a conventional conductive water-based ink to significantly reduce the resistance. ̽��ֱ��same method works for materials other than graphene, including metallic, semiconducting and insulating nanoparticles.

Currently, printed conductive patterns use a combination of poorly conducting carbon with other materials, most commonly silver, which is expensive. Silver-based inks cost £1000 or more per kilogram, whereas this new graphene ink formulation would be 25 times cheaper. Additionally, silver is not recyclable, while graphene and other carbon materials can easily be recycled. ̽��ֱ��new method uses cheap, non-toxic and environmentally friendly solvents that can be dried quickly at room temperature, reducing energy costs for ink curing. Once dry, the ‘electric ink’ is also waterproof and adheres to its substrate extremely well.

̽��ֱ��graphene-based inks have been printed at a rate of more than 100 metres per minute, which is in line with commercial production rates for graphics printing, and far faster than earlier prototypes. Two years ago, Hasan and his colleagues produced a prototype of a transparent and flexible piano using graphene-based inks, which took between six and eight hours to make. Through the use of this new ink, more versatile devices on paper or plastic can be made at a rate of 300 per minute, at a very low cost. Novalia has also produced a printed DJ deck and an interactive poster, which functions as a drum kit using the same method.

Hasan and PhD students Guohua Hu, Richard Howe and Zongyin Yang of the Hybrid Nanomaterials Engineering group at CGC, in collaboration with Novalia, tested the method on a typical commercial printing press, which required no modifications in order to print with the graphene ink. In addition to the new applications the method will open up for graphene, it could also initiate entirely new business opportunities for commercial graphics printers, who could diversify into the electronics sector.

“ ̽��ֱ��UK, and the Cambridge area in particular, has always been strong in the printing sector, but mostly for graphics printing and packaging,” said Hasan, a Royal Academy of Engineering Research Fellow and a ̽��ֱ�� Lecturer in the Engineering Department. “We hope to use this strong local expertise to expand our functional ink platform. In addition to cheaper printable electronics, this technology opens up potential application areas such as smart packaging and disposable sensors, which to date have largely been inaccessible due to cost.”

In the short to medium term, the researchers hope to use their method to make printed, disposable biosensors, energy harvesters and RFID tags.

̽��ֱ��research was supported by grants from the Engineering and Physical Sciences Research Council's Impact Acceleration Account and a Royal Academy of Engineering Research Fellowship. ̽��ֱ��technology is being commercialised by Cambridge Enterprise, the ̽��ֱ��’s commercialisation arm.

A low-cost, high-speed method for printing electronics using graphene and other conductive materials could open up a wide range of commercial applications.

Being able to produce conductive inks that could effortlessly be used for printing at a commercial scale at a very high speed will open up all kinds of different applications for graphene and other similar materials

Tawfique Hasan

Roll-to-roll printing of graphene ink

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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