Coffee stains inspire optimal printing technique for electronics
A single protein may determine how much scarring occurs

Date:
August 12, 2020
Source:
University of Cambridge
Summary:
Using an alcohol mixture, researchers modified how ink droplets dry,
enabling cheap industrial-scale printing of electronic devices at
unprecedented scales.



FULL STORY ==========================================================================
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 University of Cambridge, with Colin Bain from the Department of Chemistry of Durham University, and Meng Zhang from School of Electronic
and Information Engineering of Beihang University, 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.

"The 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. The
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.

The 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. "The
dream of smart cities will come true."

========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
story is licensed under a Creative_Commons_License. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Guohua Hu, Lisong Yang, Zongyin Yang, Yubo Wang, Xinxin Jin,
Jie Dai,
Qing Wu, Shouhu Liu, Xiaoxi Zhu, Xiaoshan Wang, Tien-Chun Wu,
Richard C.

T. Howe, Tom Albrow-Owen, Leonard W. T. Ng, Qing Yang, Luigi G.

Occhipinti, Robert I. Woodward, Edmund J. R. Kelleher, Zhipei Sun,
Xiao Huang, Meng Zhang, Colin D. Bain, Tawfique Hasan. A general ink
formulation of 2D crystals for wafer-scale inkjet printing. Science
Advances, 2020; 6 (33): eaba5029 DOI: 10.1126/sciadv.aba5029 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200812144141.htm

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