New 2D material could have very large magnetic moment
Atom-thick layers of iron have been made in the tiny holes of a perforated piece of free-standing graphene. The work was done by an international team, which has also done calculations that suggest the new material has some potentially useful exotic properties, such as a large magnetic moment. However, the team believes the 2D structure becomes thermodynamically unstable when it is more than 12 atoms wide: a problem that would have to be overcome before the material could be put to work in practical applications such as magnetic data storage.
At first glance, a free-standing 2D metal seems impossible. This is because the bonding between atoms in a metal is mediated by conduction electrons, which are free to move in any direction. As a result, metals tend to have 3D crystal structures and no tendency to form planar sheets. This is unlike crystalline carbon, which is held together by highly directional covalent bonds that allow free-standing atom-thick sheets of graphene to exist. While single epitaxial layers of metal atoms can be created on a substrate, these are not true 2D materials because the atoms are bonded to the underlying structure.
In the new research, Mark Rümmeli and colleagues at the Leibniz Institute for Solid State and Materials Research in Dresden, Germany, and at institutes in Poland and Korea studied the behaviour of metal atoms at the edges of holes in graphene. They grew a sheet of graphene by chemical vapour deposition on a surface and detached it by etching the substrate with an iron-chloride solution. This left trace amounts of iron on the surface of the graphene. Irradiating the graphene with an electron beam created small holes and also encouraged the iron atoms to move around. The edge atoms of graphene are the most reactive because they contain dangling bonds; so when the mobile iron atoms encounter the edge of a hole, they bond to it. This continues with iron atoms bonding to the other iron atoms around the edge, until the hole is completely sealed with a 2D square lattice of iron.
The group's theoretical calculations show that the largest thermodynamically stable sheet would be about 12 atoms across – or just 3 nm – wide. The largest sheets observed in the experiment were only 10 atoms wide. Beyond this, the tendency of iron to form a 3D structure wins out over the bonding between the iron and carbon atoms at the edges. "The atoms usually form a tiny crystal that sticks to one of the edges," explains Rümmeli, who is now at the Institute for Basic Science in Korea.