Jul 19, 2012
TEM tracks dislocations in graphene
Researchers in the UK and Japan have succeeded in tracking dislocations in graphene – a sheet of carbon atoms just one atom thick – with unprecedented resolution using electron microscopy. The work may help scientists better understand plasticity in 2D structures and how dislocation motion affects the mechanical properties of this, and other technologically important materials.
The strength, and the way a material deforms under a load, is often related to how dislocations – defects such as an extra half plane of atoms – move through the material. Although dislocations in 3D samples have been studied using high-resolution transmission electron microscopy (TEM), it is significantly more challenging to examine 2D materials like graphene. This is because the high-energy electrons normally used for imaging in TEM rapidly destroy carbon-based nanomaterials, such as graphene. The accelerating voltage of electrons in TEM needs to be reduced to relatively low voltages of around 80 kV to limit damage.
The problem, however, is that running a TEM with such low-energy electrons results in an increase in spherical and chromatic aberrations that blur the resulting images and reduce their spatial resolution. Although newer electron microscopes contain in-built hardware that can correct for spherical aberrations, this resolution is still not high enough to allow scientists to locate the exact positions of individual atoms in graphene – because of the chromatic aberration effects.
Now, Jamie Warner and colleagues of the University of Oxford and co-workers at the Japan Electron Optics Laboratory in Tokyo have found a way to reduce chromatic aberration effects by passing the electrons through a monochromator and so improve spatial resolution. The technique reduces the energy spread of the electrons before they actually hit the sample.
“Our approach allows for sub-angstrom resolution at 80 kV and the ability to pinpoint the exact position of single carbon atoms within the graphene lattice,” explained Warner. “We used this enhanced resolution to study edge dislocations (a unique form of defect that distorts lattice structure) in graphene for the first time with true atomic resolution.”
The researchers were also able to measure carbon-carbon bond elongation and compression within a dislocation too, and map the strain caused by the dislocations using an image-processing technique known as geometrical phase analysis (GPA). “We found that we could not only map out the full strain tensor from the dislocations, but also study how the strain fields moved as dislocations shift positions, or ‘creep and climb’, within the lattice,” he told nanotechweb.org. Comparing these experimental strain maps with those predicted by theory show a good match with the so-called Foreman dislocation model, he adds.
The researchers say that the results, published in Science, provide a detailed map of how the atoms in dislocations are configured in graphene, and will help them better understand how plasticity emerges in the material. They are now studying single substitutional impurity atoms in graphene and how these induce strain within the lattice. “We have already discovered that only some defect structures are stable,” added Warner. “We can readily create highly disordered regions within graphene, but in many cases these can ‘unwind’ and revert back to a pristine lattice.”
The team is also busy compiling a “catalogue” of defect and impurity families in the carbon material, he reveals.
About the author
Belle Dumé is contributing editor at nanotechweb.org