MIT researchers developed a technique to grow wafer-scale samples of ultrathin superconductors underneath a layer of protective graphene, overcoming oxidation issues that previously degraded the fragile materials in open air.
The process targets niobium diselenide, an ultrathin material composed of a single layer of niobium atoms sandwiched between selenium atoms. While niobium diselenide possesses high kinetic inductance that enables efficient energy storage in small quantum circuits, it degrades rapidly when exposed to oxygen. Previous manufacturing methods relied on exfoliation to produce small flakes, or attempted to place protective layers on top of the superconductor after synthesis.
Substrate gap growth
Rather than applying protection after growth, the research team placed a single layer of graphene on a silicon dioxide substrate first. Weak adhesion between the graphene and substrate leaves a gap smaller than one nanometer. When researchers introduced chemical precursors, the superconducting material grew directly within this narrow space.
The silicon dioxide substrate traps the precursors long enough for crystals to form, while the graphene guides the material into a continuous layer larger than an inch across. Because the graphene encapsulates the niobium diselenide during synthesis, scientists can move the material into ambient air without triggering oxidation.
Circuit integration
To incorporate the material into devices, the team used an oxidation-free transfer technique to peel the structure from its growth substrate. Etching the side walls inside a vacuum chamber preserved smooth edges, allowing researchers to connect the one-nanometer-thick film to electrodes several hundred nanometers thick.
When integrated into a conventional superconducting microwave circuit, the material maintained its superconducting properties and retained high kinetic inductance. Co-lead author Xudong Sheldon Zheng noted that the method converts monolayer superconductors from small-scale laboratory materials into viable components for circuit design.
The team demonstrated that this synthesis strategy works for other monolayer quantum materials beyond niobium diselenide. Research funding came from several organizations, including the U.S. National Science Foundation, the U.S. Department of Energy, and the U.S. Army Research Office.
