Atomically thick metal membranes

The success and promise of atomically thin carbon, in which carbon atoms are arranged in a honeycomb lattice, also known as graphene has triggered enormous enthusiasm for other two dimensional materials, for example, hexagonal boron nitride and molybdenum sulphide. These materials share a common structural aspect, namely, they are layered materials that one can think of as individual atomic planes that can be pulled away from their bulk 3D structure. This is because the layers are held together through so called van der Waals interactions which are relatively weak forces as compared to other bonding configurations such as covalent bonds. Once isolated these atomically thin layers maintain mechanical integrity (i.e. they are stable) under ambient conditions.

In the case of bulk metals, their crystalline structure is three dimensional, and is thus not a layered structure and moreover metallic atom bonds are relatively strong. These structural aspects of metals would seem to imply the existence of metal atoms as a freestanding 2D material is unlikely. The formation of 2D atomically thin metallic layers over other surfaces has previously been demonstrated, however in this case the metal atoms interact with the underlying substrate. On the other hand, metallic bonding is non-directional and this fact along with the excellent plasticity of metals at the nanoscale suggest atomically thin 2D freestanding membranes composed of metal atoms might just be possible. Indeed, this is what an international group of researchers based in Germany, Poland and South Korea have now demonstrated is possible using iron atoms. Aside from the demonstration that metal atoms can form freestanding 2D membranes there is significant interest in the potential of such 2D metal materials as they are expected to have exotic properties.

The international group of researchers from the Leibniz Institute Dresden (IFW), the Technische Universität Dresden, the Polish Academy of Sciences, Sungkyunkwan University and the Center for Integrated Nanostructure Physics, an Institute of Basic Science (Korea) used pores in mono-layer graphene to form free standing 2D iron (Fe) single atom thick membranes. To achieve this the researchers took advantage of the manner in which Fe atoms move across the surface of graphene when irradiated by electrons in a transmission electron microscope (TEM). As these atoms move across the surface if they encounter an open graphene edge they tend to get trapped there. The researchers were able to show, in situ, that large numbers of Fe atoms can get trapped in a pore and, moreover, configure themselves in an ordered manner to form a crystal with a square lattice. The spacing between atoms (lattice constant) was found on average to be 2.65±0.05Å which is significantly larger than that for the (200) Miller-index plane distance for the face centered cubic (FCC) phase or the (110) plane distance for BCC Fe.

This result was surprising, because usually lattices shrink when they have a lower coordination number, a process known as surface contraction. The researchers were able to show that the observed enlarged lattice spacing was due to strain which arises due to the lattice mismatch at the graphene edge and Fe membrane interface. Indeed, they could observe the lattice relax (contract) towards the center of the membranes. Supporting theoretical investigations by the researchers showed variations in the band structure of a 2D Fe membrane as compared to bulk Fe. The differences were due to some electron orbital's lying in plane and others being out of a plane, an effect that does not occur in 3D bulk Fe. The theoretical investigations also confirmed a result shown by previous theoretical calculations that 2D Fe membranes should have a significantly enhanced magnetic moment.

Future Potential

The demonstration of 2D Fe membranes is exciting because it shows that freestanding 2D materials that are not obtained from layered bulk materials can be achieved and that such 2D materials can be stable under ambient conditions. The technique developed by the researchers could pave the way for new 2D structures to be formed. These new 2D structures can be expected to have enhanced physical properties that could hold potential in a variety of applications. For example, the enhanced magnetic properties of atomically thin 2D Fe could make them attractive for magnetic recording media. They may also have interesting properties for photonic and electronic applications.