Nature 469, 389–392 (2011)

Graphene — a one-atom-thick flat sheet of carbon — can be grown on the metre scale at present. However, such large-scale films are not single crystals, and the honeycomb lattice of the carbon atoms is disrupted by defects and grain boundaries. These grain boundaries are expected to influence the electronic and mechanical properties of the material, but analysing the grain structure of graphene is challenging, not least because there is a 100,000-fold difference between the size of the grains and the size of the atoms that make up the grain boundaries. David Muller and colleagues have now shown that transmission electron microscopy (TEM)-based techniques can be used to characterize both graphene grains and their grain boundaries.

The researchers — who are based at Cornell University, Oregon State University, Brigham Young University and the Kavli Institute at Cornell for Nanoscale Science — were able to image the location and determine the atomic number of every atom at a grain boundary with the help of aberration-corrected annular dark-field scanning TEM. The atomic-scale imaging revealed that different grains were principally connected through pairs of carbon pentagons and heptagons. Moreover, by using dark-field TEM — a diffraction-sensitive imaging technique — the team could quickly map the location, shape and lattice orientation of hundreds of grains and boundaries.

Muller and colleagues also correlated their analysis of graphene grain structure with scanning probe and transport measurements, which revealed that the presence of the grain boundaries significantly decreases the mechanical strength of the material but has only a limited effect on its electronic properties.