An international research team has shown for the first time that metal membranes one layer of atoms thick can be stable under ambient conditions.
The technique could open the door for new types of 2D structures to be formed. These new 2D structures could have enhanced physical properties that hold potential in a range of applications.
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.
The enormous promise of atomically thin carbon, also known as graphene, has sparked interest in other two dimensional materials. Hexagonal boron nitride and molybdenum sulphide are two such materials.
All these materials share the same structural feature- they are layered materials that can be imagined as individual atomic planes which can be pulled away from their bulk 3D structure. This is because the layers are held together through what are called van der Waals interactions.
These interactions are relatively weak forces compared to other bonding mechanisms like covalent bonds. Once they are isolated these atomically thin layers maintain mechanical integrity and are stable under ambient conditions.
Metallic 3D vs. 2D Structure
With bulk metals, their crystalline structure is three dimensional, instead of a layered structure. Not only that, but metallic atom bonds are relatively strong. These structural aspects of metals imply that the existence of metal atoms as a freestanding 2D material is improbable.
Formations of 2D atomically thin metallic layers has previously been demonstrated over other surfaces. But in those cases the metal atoms also interact with the underlying substrate.
Metallic bonding, conversely, is also non-directional. This fact, along with the outstanding plasticity of metals at the nano-scale suggest atomically thin 2D freestanding membranes composed of metal atoms might just be possible.
This possibility is exactly what an international group of researchers has shown, using iron atoms. Aside from the demonstration that metal atoms can form freestanding 2D membranes there is noteworthy interest in the potential of such 2D metal materials as they are expected to have exotic properties.
Free Standing 2D Iron Membrane
The team of researchers used pores in mono-layer graphene to form free standing 2D iron (Fe) single atom thick membranes.
To do this, researchers leveraged the way in which iron 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 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 appreciably bigger than that for the 200 Miller-index plane distance for the face centered cubic (FCC) phase or the (110) plane distance for BCC Fe.
Miller indices are a notation system in crystallography for planes in crystal lattices. The crystallographic directions are fictitious lines linking nodes (atoms, ions or molecules) of a crystal.
Shrinking Lattice Surfaces
This was was a surprising find. Lattices typically shrink when they have a lower coordination number, in 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.
Researchers were able to see the lattice 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.