Researchers in the US, Germany, Singapore and Spain have developed a new technique to obtain images of grain-boundary defects in graphene by analysing the behaviour of surface plasmons. Their study reveals that the defects act as electronic barriers and are responsible for the low electron mobility seen in some samples of graphene. The team also says that these barriers could find use as tuneable "plasmon reflectors" and "phase retarders" in plasmonic circuits of the future.

Graphene is a single atomic layer of carbon atoms that are arranged in a honeycomb lattice. It shows great promise for making electronic devices of the future thanks to its unique electronic and mechanical properties – which include extremely high electrical conductivity and exceptional strength.

A patchwork quilt

Defect-free graphene has the best mechanical and electronic properties but techniques for creating large, pristine graphene samples are limited by the emergence of grain-boundary defects. Much like the seams in a patchwork quilt, these defects form the boundaries between areas of perfect graphene. They are also notoriously difficult to characterize using conventional techniques such as transmission electron microscopy or optical microscopy.

The new nano-imaging technique developed by Dimitri Basov of the University of California at San Diego and colleagues was used to study graphene created by chemical vapour deposition (CVD) – a standard technique for making the material that suffers from grain-boundary problems.

Rippling across the surface

Surface plasmons are coherent wave-like oscillations of electrons that ripple across the surface of graphene and some other materials. In Basov's experiment the plasmons are created by a nanoscale antenna – the metallic probe of an atomic force microscope – that is placed near the graphene surface and excited by infrared light (see figure). The plasmon waves are reflected and scattered by the graphene grain boundaries, creating interference patterns.

"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 says. This discovery goes some way towards explaining why electrons travel slower in such graphene than in defect-free samples. On the other hand, grain boundaries might be exploited as plasmon reflectors and phase retarders – which are essential components for future graphene-based plasmonic circuits. Indeed, the team says that it is already looking at making such circuits by creating charge barriers in graphene that are 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 an optical 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."

Shrinking optics

Controlling plasmons in this way could be particularly useful for shrinking the size of optical devices. This is because light can interact with surface plasmons to create waves called surface plasmon polaritons (SPPs), which have much shorter wavelengths than the original light. As a result, devices controlling SPPs can be much smaller than their optical counterparts.

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," says Fei. "Such studies will help us better understand the exact relationship between structure and properties of these defects."

The research is reported in Nature Nanotechnology 10.1038/nnano.2013.197.