Seeing Moire in Graphene

Wednesday, March 31, 2010

The ability to determine the rotational orientation of graphene sheets and map strain is useful for
understanding the electronic and transport properties of multiple layers of graphene, a one-atom
thick form of carbon with potentially revolutionary semiconducting properties. The research
appears in the journal, Physical Review B, in volume 81, issue 12.

In digital photography, moiré (pronounced mwar-ray) patterns occur because of errors in the
rendering process, which causes grid patterns to look wavy or distorted. Materials scientists have
been using microscopic moiré patterns to detect stresses such as wrinkles or bulges in a variety
of materials.

Researchers created graphene on the surface of a silicon carbide substrate at the Georgia Institute
of Technology by heating one side so that only carbon, in the form of multilayer sheets of
graphene, was left. Using a custom-built scanning tunneling microscope at NIST, the researchers
were able to peer through the topmost layers of graphene to the layers beneath. This process,
which the group dubbed "atomic moiré interferometry," enabled them to image the patterns
created by the stacked graphene layers, which in turn allowed the group to model how the
hexagonal lattices of the individual graphene layers were stacked in relation to one another.
Unlike other materials that tend to stretch out when they cool, graphene bunches up like a
wrinkled bed sheet. The researchers were able to map these stress fields by comparing the
relative distortion of the hexagons of carbon atoms that comprise the individual graphene layers.
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.

"There's an ideal atomic lattice spacing in graphene. Knowing the strain distribution can help us
in our efforts to create graphene with good electronic properties," said Phillip N. First, professor
in the School of Physics at Georgia Tech. "So far, it looks as if multi-layered graphene has
excellent conduction properties and may be useful for electronic applications."
This collaboration between Georgia Tech and NIST is part of a series of experiments aimed at
gaining a fundamental understanding of the properties of graphene. Other examples of the
group's work can been seen at and
Their article, "Structural analysis of multilayer graphene via atomic moiré interferometry" was
selected as an Editor's Highlight in Physical Review B for the month of March, 2010.

CoS in the News, Mark Esser and David Terraso(writers)