Researchers at the National
Institute of Standards and Technology (NIST) and the Georgia Institute of
Technology have demonstrated* that atomic scale moiré patterns, an interference
pattern that appears when two or more grids are overlaid slightly askew, can
be used to measure how sheets of graphene are stacked and reveal areas of strain.
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.
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.
This collaboration between NIST and the Georgia Institute of Technology 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 www.mrs.org/s_mrs/bin.asp?CID=8684&DID=320520&DOC=FILE.PDF
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.
* D. Miller, K. Kubista, G. Rutter, M. Ruan, W. de Heer, P. First and J. Stroscio.
Structural analysis of multilayer graphene via atomic moiré interferometry.
Physical Review B. 81. 125427. Published March 24, 2010. http://prb.aps.org/abstract/PRB/v81/i12/e125427