The ability to determine the rotational orientation of graphene sheets and show strain can help us understand the electronic and transport properties of multiple layers of graphene, the form of carbon of one-atom thickness that has shown great semiconducting properties.
The team, from the National Institute of Standards and Technology (NIST) and the Georgia Institute of Technology, have created graphene on the surface of a silicon carbide substrate by heating only on one side, leaving carbon in the form of multilayer sheets of graphene.
Published in the journal Physical Review B [Miller et al., Physical Review B (2010) doi: 10.1103/PhysRevB.81.125427], the study used a scanning tunneling microscope to search through the top layers of graphene to inspect those underneath. They process, labelled “atomic moiré interferometry”, allowed them to model how the hexagonal lattices of the individual graphene layers were stacked in relation to one another.
The scientists chose to study multilayer epitaxial graphene because of its potential for industrial applications, and its exciting physical characteristics. Moiré patterns had been previously been observed in graphene, but they then realized their usefulness when used macroscopically.
Using this method, moiré patterns on multilayer graphene and graphite are observed as a result of a modulation in the electronic structure due to the interlayer atomic alignment. The moiré pattern can provide information about three or four layers.
Significantly, the study has shown how the techniques of optical strain measurement can be applied to “atomic moiré interferometry”, as moiré patterns show distortion when one lattice is stretched with respect to the other. If one graphene sheet in multilayer epitaxial graphene has slightly more strain than the next, the moiré pattern will also appear stretched. Their technique is so sensitive that it is able to detect strains in the graphene layers causing as little as a 0.1 percent change in atom spacing.
The strain and rotation angles are key to the research, especially as the structural properties have a big influence on the electronic properties of the material. The moiré pattern has the ability to amplify the distortion due to a small strain, allowing the measurement to gain significantly better resolution.
Moiré patterns have been used for many years in optics applications to measure strain, but the team now hope to extend their measurements to map local strain fields and identify the biggest sources of strain, which will allow them to develop even higher quality graphene films.