Atom-thin iodine film makes its debut

Ever since graphene’s discovery 2 decades ago, the iconic 1-atom-thick sheet of carbon has inspired researchers to create a growing family of other 2D materials. Most are compounds, but almost 20 are composed of a single element—including silicon, boron and gold—and are collectively known as Xenes.

Researchers in China have now isolated and studied the first halogen Xene: iodinene (Matter 2025, DOI: 10.1016/j.matt.2025.102157). “We can confirm, at the atomic scale, that it is truly a monolayer,” says Xiaoxu Zhao of Peking University, part of the team behind the work. Other researchers had previously made nanosheets containing a few atomic layers of iodine stacked together and used them as an electrode material in a sodium-ion battery (Adv. Mater. 2020, DOI: 10.1002/adma.202004835).

Zhao’s team found that monolayer iodinene has a buckled, corrugated structure, similar to Xenes such as silicene. But while other Xenes are mostly semiconductors or insulators, calculations suggest that iodinene is metallic, potentially making it a useful addition to the family. Indeed, Zhao says that adding atoms of other elements to iodinene might even turn it into a superconductor.

The researchers took an unusual approach to making monolayer iodinene. Xenes are often formed by allowing a gas of the element to condense on a surface under high-vacuum conditions. But the researchers say that approach was not suitable for a volatile, reactive halogen.

Instead, they made a template for iodinene by mounting graphene onto a mesh grid used for holding electron microscopy samples. Then they dipped the templates in an aqueous solution of potassium iodide and iodine and heated them to roughly 60 °C for several hours. The slow evaporation of water caused iodine to crystallize on the templates and form a monolayer. Zhao hopes this simple method will now be used to make other Xenes.

“The growth method is very interesting because it’s based on chemical methods, which is not the conventional approach,” says 2D-materials researcher Carlo Grazianetti of Italy’s National Research Council (CNR) Institute for Microelectronics and Microsystems, who was not involved in the research. “This is very cheap, obtained at low temperature, and this is very promising for applications.”

The iodinene monolayer was not very stable: it degraded within a couple of days in ambient conditions. But Grazianetti thinks it should be possible to stabilize the monolayer by encapsulating it with another 2D material. And if iodinene is confirmed to be metallic, the simplicity of its production might make it a useful electrical contact to connect devices based on other 2D materials, he says.