Inkjet printing of two-dimensional crystals will be essential for the use of the next generation of printed electronics. While technology has made great strides in recent years, a major challenge for industrially printed electronics components is to achieve uniform crystal distribution; uneven distribution can result in faulty equipment. The culprit is a phenomenon known as the “coffee ring effect”;. Scientists have now 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 grounds in the case of your joe breakfast cup, 2D crystals in the case of inkjet printing of electrical components. (You can also see the crumbling effect with a malt. A related phenomenon is summer tears.) The coffee ring effect occurs when a single liquid evaporates and solutions that were dissolved in the juice (like coffee grounds or 2D crystals) a story ring is formed. Occurs because evaporation occurs faster at the edge than at the center. Liquiddo remaining liquid is poured out on the edge to fill the gaps, pulling them solid with it. Mixing in solvents (water or alcohol) reduces the effect, as long as the droplets are very small. Large dots produce more uniform spots.
Similarly, when a drop of watercolor paint dries, the dye pigment particles explode outward, toward the edge of the spot. So artists working with watercolors also have to deal with the effect of the coffee ring if they do not want the pigment accumulation to happen at the edges. As we reported in 2018, adding alcohol to watercolor paint can prevent it. Alternatively, an artist can wet the paper before applying the paint. Instead of the remaining dot stuck to the paper, the ink runs out. This allows the artist to play with various effects, such as generating unusual color gradients.
The shape of the dots is also a factor in the effect of the coffee ring. As we reported in 2011, the researchers found that the effect could be denied if the particles are ellipsoid instead of spherical. In that way, they form loose packed structures, which can resist capillary flow as they are transported to the edge of the point. When the drop has completely evaporated, these particles are distributed more evenly. The longer the particles, the more uniform the deposition, providing a way to control the distribution of the material.
In the case of inks for 2D printing of electronic components, adding the right type of alcohol to the ink mixture can affect the dot shape and suppress the effect of the coffee ring, according to this latest study, led by University researchers of Cambridge, Durham University, and the University of Beihang. The researchers used 2D crystal flakes, graphene, telluride bismuth, tungsten disulfide, boron nitride and black phosphorus, among other materials. The crystals were 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 IPA, IPA / ethanol and IPA /t-butanol mixtures. But the effect was suppressed in the IPA / 2-butanol mixture, producing uniform thickness in printed forms. The authors suggest that thicker fall areas are richer in IPA and therefore have a lower surface tension. The addition of 2-butanol to the mixture results in a leak of Marangonium from the thicker areas to the thinner ones, creating a natural negative feedback mechanism to suppress the capillary flows that lead to the coffee ring effect and spread the droplet as a discount.
“The natural shape of the ink dots is spherical – however, due to their composition, our ink dots adopt pancake shapes,” said co-author Tawfique Hasan of Cambridge University.
The team has already successfully printed gas sensors and photodetectors as proof of concept that exceed current industry demands, and they are confident that the large variety of materials they can use in their inks can also enable the printing of more catalysts. efficient, solar cells, batteries, and functional gear. Ink mixtures can even crush nanoparticles and organic molecules. And the process is scalable: scientists printed almost 4,500 almost identical sensors and photodetectors. This makes it a promising method for cheap, industrial-scale electronics manufacturing.
“Understanding this basic ink dot behavior has allowed us to find this ideal solution for printing ink on all types 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 to print new electronic devices in silicone or plastic strips, and even in spray painting and coated wear, that match or exceed the productivity requirements for printed equipment.”
DOI: Advances in Science, 2020. 10.1126 / sciadv.aba5029 (About DOI).