Inkjet printing of two-dimensional crystals will be crucial for the introduction of the next generation of printing electronics. Although the technology has made significant progress in recent years, a major challenge for printed electronic components on an industrial scale is to achieve even crystal distribution; uneven distribution can lead to defective devices. The culprit is a phenomenon known as the “coffee ring effect”
Coffee rings are the model you get when liquid evaporates and leaves behind a ring of pre-dissolved solids – a coffee base 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. Related phenomena are wine wine.) The coffee ring effect occurs when a liquid evaporates and solids that have been dissolved in the liquid (such as a coffee base or 2D crystals) form bell ring. It happens because evaporation happens faster at the end than in the center. Any remaining liquid flows out to the edge to fill the gaps, dragging these solids with it. Mixing in solvents (water or alcohol) reduces the effect, as long as the drops are very small. Large drops create more even spots.
In the same way, when a drop of watercolor paint dries, the pigment particles of the paint disintegrate outwards, towards the edge of the drop. So artists who work with watercolors also have to deal with the effect of a coffee ring if they do not want this accumulation of pigment at the edges. As we announced in 2018, adding alcohol to watercolor paint can prevent it. Alternatively, an artist can wet the paper before applying the paint. Instead of the drop stuck to the paper, the ink runs out. This allows the artist to play with different effects, such as generating unusual color gradients.
The shape of the drops is also a factor in the effect of the coffee ring. As we reported in 2011, researchers found that the effect could be denied if the particles were ellipsoidal instead of spherical. In this way, they form loosely packed structures that can withstand capillary flow as they are transported to the edge of the drop. When the drop has completely evaporated, these particles are distributed more evenly. The more elongated the particles, the more uniform the deposition, providing a way to control the distribution of the material.
In the case of inks for two-dimensional printing of electronic components, adding only the right type of alcohol to the ink mixture can affect the shape of the droplets and suppress the effect of a coffee ring, according to a recent study led by researchers at the University of Cambridge, Durham Beihang University and University. The researchers used 2D crystal flakes of graphene, bismuth telluride, tungsten disulfide, boron nitride and black phosphorus, among other materials. The crystals are then dispersed independently in isopropanol (IPA), plus isopropanol mixed with ethanol, T-butanol and 2-butanol, in different samples.
They found that the effect of the coffee ring still manifested for IPA, IPA / ethanol and IPA /T-butanol mixtures. However, the effect was suppressed in the IPA / 2-butanol mixture, giving the same thickness in the printed forms. The authors suggest that the thicker zones of the decline are richer in IPA and therefore have a lower surface tension. The addition of 2-butanol to the mixture causes Marangoni to 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 shape of the ink droplets is spherical – however, due to their composition, the ink droplets take on the shape of a pancake,” said co-author Taufik Hassan of the University of Cambridge.
The team has already printed gas sensors and photodetectors as evidence of a concept that exceeds current industry requirements, and is confident that the sheer variety of materials they can use in their inks could also enable more efficient printing. catalysts, solar cells, batteries and functional coatings. The ink mixture can even print nanoparticles and organic molecules. And the process is scalable: scientists have printed 4,500 nearly identical sensors and photodetectors. This makes it a promising method for cheap production of electronic devices on an industrial scale.
“Understanding this fundamental behavior of ink droplets has allowed us to find this ideal inkjet printing solution for all types of two-dimensional crystals,” said co-author Guohua Hu of the University of Cambridge and the Chinese University of Hong Kong. “Our wording can be easily increased to print new electronic devices on silicone plates or plastics and even when spray-painting and wearing that already match or exceed the performance requirements of printed devices.”
DOI: Science Advances, 2020. 10.1126 / sciadv.aba5029 (For DOI).