Inkjet printing of two-dimensional crystals will be crucial for ushering in the next generation of printed electronics. While the technology has made a lot of progress in recent years, a major challenge to industrial-scale printed electronic components is achieving uniform distribution of the crystals; uneven distribution can result in faulty devices. The culprit is a phenomenon known as the “coffee ring effect.” Now scientists have created a new family of inks that can suppress the effect, according to a new paper in the journal Science Advances.
Coffee rings are the pattern you get when a liquid evaporates and leaves behind a ring of previously dissolved solids—coffee grounds in the case of your morning cup of joe, 2D crystals in the case of inkjet printing of electrical components. (You can also see the effect with single-malt scotch. A related phenomena is wine tears.) The coffee ring effect occurs when a single liquid evaporates and the solids that had been dissolved in the liquid (like coffee grounds or 2D crystals) form a telltale ring. It happens because the evaporation occurs faster at the edge than at the center. Any remaining liquid flows outward to the edge to fill in the gaps, dragging those solids with it. Mixing in solvents (water or alcohol) reduces the effect, as long as the drops are very small. Large drops produce more uniform stains.
Similarly, when a drop of watercolor paint dries, the pigment particles of color break outward, toward the rim of the drop. So artists who work with watercolors also have to deal with the coffee ring effect if they don’t want that accumulation of pigment at the edges to happen. As we reported in 2018, adding alcohol to the watercolor paint can prevent it. Alternatively, an artist may wet the paper before applying the paint. Instead of the drop remaining pinned to the paper, the ink runs off. This allows the artist to play with various effects, such as generating unusual color gradients.
The shape of the droplets is also a factor in the coffee ring effect. As we reported back in 2011, researchers found that the effect can be negated if the particles are ellipsoidal instead of spherical. That way, they form loosely packed structures that can resist the capillary flow as they are transported to the edge of the drop. When the drop has completely evaporated, these particles are more evenly distributed. The more elongated the particles, the more uniform the deposition, providing a way to control the distribution of material.
In the case of inks for 2D printing of electronic components, adding just the right kind of alcohol to the ink mixture can influence droplet shape and suppress the coffee ring effect, according to this latest study, led by researchers from the University of Cambridge, Durham University, and Beihang University. The researchers used 2D crystal flakes of graphene, bismuth telluride, tungsten disulfide, boron nitride, and black phosphorus, among other materials. The crystals were then dispersed in isopropanol (IPA) alone, plus isopropanol mixed with ethanol, t-butanol, and 2-butanol, in different samples.
They found that the coffee ring effect still occurred for the IPA, IPA/ethanol, and IPA/t-butanol mixtures. But the effect was suppressed in the IPA/2-butanol mixture, producing uniform thickness in the printed shapes. The authors suggest that thicker areas of the drop are richer in IPA and therefore have a lower surface tension. Adding 2-butanol into the mix results in a Marangoni flow from thicker to thinner areas, creating a natural negative feedback mechanism to suppress the capillary flows that lead to the coffee ring effect and spread the droplet out like a pancake.
“The natural form of ink droplets is spherical—however, because of their composition, our ink droplets adopt pancake shapes,” said co-author Tawfique Hasan of the University of Cambridge.
The team has already successfully printed gas sensors and photodetectors as proof of concept that exceed current industry requirements, and they are confident that the sheer variety of materials they can use in their inks could also enable the printing of more efficient catalysts, solar cells, batteries, and functional coatings. The ink mixtures can even print nanoparticles and organic molecules. And the process is scalable: the scientists printed 4,500 almost identical sensors and photodetectors. That makes it a promising method for cheap, industrial-scale fabrication of electronic devices.
“Understanding this fundamental behavior of ink droplets has allowed us to find this ideal solution for inkjet printing all kinds of two-dimensional crystals,” said co-author Guohua Hu of the University of Cambridge and the Chinese University of Hong Kong. “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.”