MIT Grew a One-Atom-Thick Superconductor Underneath a Sheet of Graphene So Air Couldn't Destroy It

Researchers at MIT and collaborating institutions have found a way to make an atomically thin superconductor that survives contact with open air — removing a bottleneck that has kept one of the most promising materials in quantum engineering stuck at the scale of microscopic flakes. The work was published in the journal Nature.

The material is niobium diselenide: a single closely packed layer of niobium atoms sandwiched between layers of selenium, about a nanometer thick. It has an unusually high kinetic inductance, meaning it can store a large amount of inductive energy in a very small area. That property is valuable because the conventional way to get large kinetic inductance in a quantum circuit is to string together an array of devices called Josephson junctions, which eat up space. A tiny patch of the right thin film could replace the array outright and shrink the circuit.

The catch has always been oxidation. "Typically, once we make the material and remove it from its inert environment, it immediately starts to oxidize and degrade, ultimately becoming damaged," said co-lead author Xudong Sheldon Zheng, a graduate student in MIT's Department of Electrical Engineering and Computer Science. The standard recipe deposits chemical precursors onto a silicon dioxide substrate and then lays a protective sheet of graphene or hexagonal boron nitride on top. But the superconductor begins corroding almost the moment it forms, before the protection arrives — and the capping step itself demands a strict inert environment and delicate handling.

The MIT team inverted the order. They put the graphene down on the silicon dioxide first, then introduced the precursors and grew the niobium diselenide in the gap between the two layers. Because graphene adheres only weakly to silicon dioxide, that gap is less than a nanometer high, and the crystal forms nowhere else. The substrate holds the precursors in place long enough for growth to begin; the graphene lets them slide around and spread into a continuous, uniform monolayer. The result is a perfectly smooth sheet more than an inch across — and it emerges already encapsulated, so it can be carried out into ordinary air without degrading.

"It took a long time for us to understand how the growth could happen underneath the graphene," Zheng said. "Through collaboration and discussion, we eventually uncovered the mechanism for growing the material at the interface, and this solves a lot of problems and allows us to simplify our fabrication steps."

Making the film was half the problem. Wiring it up was the other half. "It is challenging to make a good electrical connection between this very thin material, which is only about 1 nanometer in thickness, and our electrodes, which are a few hundred nanometers in thickness," said co-author Zaman. The team etched the side walls of the film inside a vacuum chamber, preserving a clean edge, and developed an oxidation-free method for peeling the graphene-superconductor stack off its growth substrate. Integrated into a conventional superconducting microwave circuit, the material held its superconducting properties and its high kinetic inductance.

"Emerging superconductors that are only a monolayer thick have a lot of potential," Zheng said. "Thanks to our new process, they are no longer materials that can only be made at a very small scale. There are now exciting opportunities for scientists to study these materials, use them in circuits, and explore their practical applications." Beyond shrinking quantum computing hardware, the researchers point to ultrasensitive quantum detectors for communications and cosmology as likely beneficiaries.