A superconducting ink that can be printed onto surfaces in a single-molecule-thick layer could prove useful for the building of circuits for quantum computers. The tungsten disulfide ink is more stable than other superconducting inks and it is simpler to make, which bodes well for future applications.
When a material is superconductive, electricity can pass through it with zero resistance, making it an extraordinarily efficient way to transmit energy. Superconductive materials also have special magnetic properties, but they tend to be difficult to make and they break down when exposed to air or to temperatures too far from absolute zero.
Xiaoyu Song and Leslie Schoop at Princeton University and their colleagues produced the tungsten disulfide ink using a process called chemical exfoliation. They started out with a material made of alternating layers of tungsten disulfide and potassium. “Imagine that you have a crepe cake – you have all these crepes stacked on top of each other and in between you have the cream filling. The tungsten disulfide is the crepe and the potassium is the filling,” says Song. When the layered material is placed into diluted sulphuric acid, it is similar to dunking a crepe cake in water: the potassium dissolves away, and only the thin layers of tungsten disulfide remain.
When the acid and remnants of potassium were rinsed away, the researchers were left with thin layers of tungsten suspended in water. This solution could then be printed onto a glass, plastic or silicon substrate, forming a layer of tungsten disulfide just one molecule thick.
The printed pattern remained stable at ambient conditions, with no protective container or coating, for at least 30 days. When it was frozen to temperatures below 7.3 kelvin (-266°C), even after being left in the open for a while, the ink became superconductive. “You could carry it around or install it at room temperature, and then you just have to freeze it,” says Schoop. “You’d need liquid helium, though – you couldn’t do it in your home freezer, unfortunately.”
This process is much simpler than those that have been used for other superconducting inks, which have required protective layers to keep them from degrading over time. That could make it easier to produce this ink industrially, although its temperature requirement blocks off some potential applications. “It could still be practical in things that are already cooled down, like in quantum computers or MRI machines where you already cool down your systems a lot,” says Schoop. In the future, the researchers hope that this method could be used to create inks that are superconductive at higher temperatures.