Seeing Moiré in Graphene

Moiré patterns appear when two or more periodic grids are overlaid slightly askew, which creates a new larger periodic pattern. Researchers from NIST and Georgia Tech imaged and interpreted the moiré patterns created by overlaid sheets of graphene to determine how the lattices of the individual sheets were stacked in relation to one another and to find subtle strains in the regions of bulges or wrinkles in the sheets. Credit: NIST View hi-resolution image

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
and www.mrs.org/s_mrs/bin.asp?CID=26616&DID=320529&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

Media Contact: Mark Esser, mark.esser@nist.gov, (301)
975-8735