There are many specialist inks in the world. Inks are made specifically for fountain pens, kids play with homemade invisible ink, and researchers create metallic ink to build circuits. Conductive ink has the benefit of being thinner than wire (especially if the ink is only one molecule thick), and it can be applied to flexible surfaces. The applications range from wearable electronics to supercomputing.
The latest research has been in making two-dimensional ink that is superconducting. Such inks could change the way that circuits are made for low-temperature environments like dilution fridges. Materials that are 2D but only semiconducting have been developed, as has a material that is 2D and superconducting but only stable when a protective coating is used. Those materials are complicated to make and produce little yield.
Xiaoyu Song; her PhD adviser, Leslie Schoop; and their colleagues were not satisfied with those inks. They focused on one material: tungsten disulfide. WS2 can take different lattice structures with different electronic properties. The material has been produced as an ink before but in the semiconducting structural phase. For a different atomic arrangement, known as 1T′, WS2 was predicted to be superconducting. But previous 1T′-WS2 samples prepared by mechanical exfoliation had not been proven to be superconducting.
To make an ink, researchers typically start with an ionic solid that contains the desired material. That solid is then submerged in an acid to remove the unwanted components. The resultant inks, however, often contain defects—monolayers in more than one atomic arrangement. Schoop and her colleagues realized a change in preparation methods was necessary. The first difference was in manufacturing temperature. They began with potassium tungsten disulfide (K0.5WS2), which yields much fewer defects in the final ink than other possible ionic solids. Unlike in previous attempts, they did not create that ionic solid at room temperature; instead, they used a furnace, heated to 850 °C. That resulted in ordered crystals with all the WS2 layers in the 1T′ structure.
To remove the potassium ions, K0.5WS2 was submerged in diluted acid and sonicated. Combining sonication with the chemical exfoliation preserved the 1T′ structure. Then, only monolayers of WS2 were left. That novel process produced a high yield of metallic ink at room temperature; the ink becomes superconductive below 7.3 K. Those layers of 1T′-WS2 are then put in a centrifuge that replaces the acid with water. The ink is suspended in water and can be stored for at least a month. Although it sounds commonplace, being stable in water—the cheapest possible solvent—at room temperature is a big leap forward for 2D superconductive ink.
When used, a single-molecule-thick layer of 1T′-WS2 ink can be printed onto some substrates, like silicon–silicon dioxide wafers and indium tin oxide–coated glass, and flexible substrates like silicone elastomer. Crucially, that ink has been shown to be stable at room temperatures for 30 days. Circuits can be assembled at room temperature and installed in freezing environments where superconductivity occurs. That versatility gives it a wide application range that includes integrated circuits and wearable devices. (X. Song et al., Sci. Adv. 9, eadd6167, 2023.)