Graphene, a single atomic layer of sp2-hybridized carbon arranged in a honeycomb lattice, is a promising material for making molecular electronic devices of the future thanks to its unique electronic and mechanical properties that include extremely high electrical conductivity and exceptional strength. Defect-free graphene has the best mechanical and electronic properties but current techniques to synthesize graphene sheets large enough to use in applications invariably produce grain boundary defects. Such defects (which can be likened to the seams in patchwork quilts, made of pieces of fabric that have been sewn together) are notoriously difficult to characterize using conventional microscopy techniques such as transmission electron or optical microscopy that provide only limited information. These techniques often destroy the sample being studied too or only work under vacuum conditions.

The new nano-imaging technique developed by Dimitri Basov of the University of California at San Diego and colleagues works by analysing surface plasmon waves (coherent oscillations of electrons) induced on the graphene surface when the material is "perturbed" by a nanoscale antenna excited by infrared light. (The antenna is in fact the metallic probe of an atomic force microscope). The plasmon waves are reflected and scattered by the graphene grain boundaries, so creating plasmon interference.

"By recording and analysing these interference patterns, we can map grain boundaries for large-area CVD films and probe the electronic and optical properties of individual grain boundaries at the same time," explains team member Zhe Fei.

Charged line defects

The analyses show that grain boundaries in CVD-grown graphene are charged line defects that act as obstacles to both charge transport and plasmon propagation, he told nanotechweb.org. They go someway in explaining why electrons travel slower in such graphene than in the pristine material. Such barriers might be exploited as plasmon reflectors and phase retarders – essential components for future graphene-based plasmonic circuits. Indeed, the team says that it is already looking at making such circuits by creating charge electronic barriers in graphene similar in structure to grain boundaries.

Plasmon reflectors are used to change the path of plasmon waves in a material, in analogy to a mirror (or a beam splitter) in optics, explains Fei. Plasmon phase retarders are used to add phase delay to the plasmon waves, in analogy to a waveplate. "Our experiments indicate that the graphene electronic barriers themselves are plasmon reflectors and phase retarders and so can be used to reflect plasmon waves and also to add phase delay to the reflected waves."

The nano-imaging technique might also be used to analyse a variety of other materials in which plasmon waves exist, he adds. Such materials include metals, superconductors and topological insulators. It might even be extended to structures that support surface phonons waves (vibrations of the crystal lattice), such as dielectric materials, for example.

"The electronic properties of a grain boundary are largely related to its atomic structure so we will now be correlating our technique with an atomic-scale method such as scanning tunnelling microscopy, to study grain boundaries," said Fei. "Such studies will help us better understand the exact relationship between structure and properties of these defects."

The present work is reported in Nature Nanotechnology doi:10.1038/nnano.2013.197.

Further reading

Conductive AFM probes grain boundaries and wrinkles in graphene (Aug 2012)
Grain boundary defects affect graphene's strength (Aug 2012)
CVD graphene nanoribbons make good interconnects (Aug 2012)