Move over graphene! Scientists forge bismuthene and host of atoms-thick metals

Tiny anvil squeezes metal atoms into super-thin sheets with strange properties

Researchers have found a way of consistently making sheets of metal that are just atoms thick.Credit: Getty

Taking inspiration from ancient techniques, researchers have forged relatively large sheets of metal that are just atoms thick, enabling them to more readily study the weird properties of these materials in the laboratory. The method can be applied to any metal with a low melting point, and the team has used it to make 2D sheets of bismuth, gallium, indium, lead and tin. The feat was reported today in Nature¹.

Javier Sanchez-Yamagishi, a physicist who studies 2D materials at the University of California, Irvine, likens the accomplishment to the creation of graphene, an atomically thin layer of carbon. “This is just a starting point,” he says. “Now, other people can step in and start studying” the metal sheets’ properties.

The tiniest metal forge

The properties of 2D materials are very different from those of chunks of material with the same chemical formula. For example, graphene is mechanically tougher and more conductive of heat and electricity than is the graphite in pencil tips. Since the first studies of graphene in 2004², researchers have made many atomically thin materials. Most of them, including graphene, however, are generated by peeling sheets from layered crystalline materials. “Like a book, you can pick a single page up and make a 2D sheet,” says Guangyu Zhang, who studies nanomaterials at the Chinese Academy of Sciences in Beijing.

Metals don’t have this peelable layered structure, and most have been unstable when scientists have tried to shape them into ultrathin sheets. However, researchers have created small swathes of atoms-thick metals. Last year, for instance, a team reported a method³ that produces flakes of gold, called ‘goldene’, that are hundreds of nanometres wide. Other methods involve depositing vaporized metal atoms onto a surface to form thin sheets⁴. But studying these materials has been challenging because metal atoms bind tightly to whatever they are placed on, making it difficult to isolate their properties from those of the surface material. Two-dimensional metal sheets are also unstable: metal atoms tend to clump into nanoparticles, and when exposed to air, the ultrathin sheets oxidize, so researchers have only been able to study them inside vacuum chambers using a limited set of techniques.

Zhang was inspired to develop the latest technique by a video of copper forging. To make sheets of copper, the metal is heated in a furnace, then hammered and squeezed on a large anvil.

It took his team seven years to adapt the concept to work at the nanoscale. The biggest challenge was finding a sufficiently flat anvil to squeeze the metal layers. The researchers eventually selected sapphire, which is very hard, and they coated it in molybdenum disulfide (MoS₂), which is atomically flat.

Source: Adapted from Figure 1a of ref. 1.

To make an ultrathin metal sheet, the team heated a droplet of the metal between two sapphire anvils, which were pressed together as the metal cooled (see ‘Putting the squeeze on’). Because MoS₂ interacts more strongly with metal than sapphire, the researchers could pry the resulting sandwich of MoS₂–metal–MoS₂ sheets out of the apparatus like a panini from a press.