Thin metal films are used as coatings or wrappings (aluminum foil, for example), components in optical and laboratory instruments, and as chemical sensors and catalysts. For a lot of high-tech applications, including high-density recording media and electronic devices, the thinner the film, the better. The ultimate thin film would be a sheet one atom thick — a monolayer.
Carbon monolayers, known as graphene, have been around for a few years, and researchers have been busy exploring the unusual properties of these ultra-thin carbon sheets. It turns out that structures in which the atoms that are linked side-to-side, but not up and down, conduct electricity very differently than their multi-layer counterparts.
Because of the way that carbon atoms connect with one another, it forms thin sheets very easily. Graphene is just an extreme form of graphite, which is made of carbon sheets that are a few hundreds to several thousands of atoms thick. Graphene was discovered when scientists applied strips of adhesive tape to pieces of graphite and carefully peeled up the top atomic layer.
Making thin layers of metal is a more difficult proposition. Metal atoms tend to form chemical bonds in all directions, forming solid chunks. Thin sheets are formed by pressing metal slabs between rollers or vaporizing the metal and spraying it onto a surface. Applying sticky tape to a piece of metal might pull a few individual atoms loose, but it’s not very effective for peeling up an intact layer.
Also, very thin metal films require some sort of support structure, called a substrate, to hold them intact. That means that only the top of the film is available for use. Until now, a free-standing metal monolayer seemed like an interesting theoretical exercise that would never be realized in the real world.
The Dresden group made their discovery while they were making graphene. They grew graphene layers from a carbon-containing vapor deposited onto a nickel–molybenum substrate. To peel the graphene up from the substrate, they used an etching solution made of iron chloride.
When the researchers examined their graphene under an electron microscope, they found that the etching solution had left behind a residue. Under the microscope’s electron beam, this residue was converted to metallic iron, which formed very small crystals and atom clusters on the surface. Some individual iron atoms were stuck to the edges of holes in the graphene film.What was truly surprising, however, was the appearance of pure iron crystalline layers, one atom thick, suspended across some of the holes in the graphene sheets. Although the widest of these layers was only about ten atoms across, the layers were arranged in an orderly fashion that were definitely monolayer versions of an iron crystal.
The researchers repeated their procedure and examined several iron-contaminated graphene sheets using their electron microscope. They analyzed the structure and characteristics of the iron films, to make sure that these truly were pure iron monolayers. Their observations agreed well with the theoretical properties derived from calculations.
They watched the samples for several minutes each to see how the microscope’s electron beam affected the formation of iron monolayers. They found that, under the beam, iron atoms on the graphene surface moved around and collected in small holes, where they formed crystalline monolayers within a few seconds. The monolayers were stable for several minutes under the electron beam. However, if they left the samples under the beam for longer than that, the monolayers began to collapse to form non-crystalline three-dimensional particles.
Now that they know how to make iron monolayers, the Dresden researchers and their colleagues can try this technique with other metals and see just how large they can make these metallic sheets.