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Buckling of grain boundaries in graphene

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The morphology and local electronic properties of graphene, a two-dimensional cristal made out of carbon, have been measured by scanning tunneling microscopy. These results will allow a better understanding of the electrical properties of polycristalline graphene.

Graphene is a recently discovered material consisting of a single atomic layer of carbon. Its excellent electrical properties make it a strong candidate for many applications. However, industrial samples are polycrystalline and therefore have many boundaries between crystalline domains (grain boundaries) that alter these properties. The study of the properties of these defects is essential to the development of this material. This is now done through a European collaboration between the University Paris Diderot, the Ecole Polytechnique Fédérale de Lausanne and the University of Namur. Surprisingly, while graphene is a two-dimensional crystal, grain boundaries have a tendency to buckle when the angular disorientation between the cristalline domains is low. For larger disorientation, researchers observe flat and regular patterns that show potentially interesting electronic properties for applications.
The experiment conducted in the laboratory Materials and Quantum Phenomena has been possible thanks to the use of a scanning tunneling microscope at low temperature and high-quality samples of graphene on silicon carbide, made in Namur. Through a systematic analysis of a large number of images of grain boundaries resolved at the atomic level, researchers were able to show that these defects undergo a buckling transition for a disorientation angle lower than 19 °. Calculations made in Lausanne using the density functional theory confirm this observation and explain this phenomenon by large localized stresses in the grain boundaries, which are partially relaxed by buckling. For larger angles of disorientation, flat periodic grains composed of a succession of carbon atoms in pentagonal and heptagonal sites were measured for the first time and fully characterized. Calculations show that this boundary is a filter that selects electrons of graphene in direction and speed. Controling such defects could lead to new types of electronic devices made out of graphene.

Figure :

3D STM image of several graphene cristalline grains separated by grain boundaries. They can be either flat (center) or buckled (top) as function of the disorientation angle between grains.

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Grain Boundaries in Graphene on SiC(000-1) Substrate. Yann Tison, Jérôme Lagoute, Vincent Repain, Cyril Chacon, Yann Girard, Frederic, Joucken, Robert Sporken, Fernando Gargiulo, Oleg V. Yazyev, and Sylvie Rousset, Nanoletters 14, 6382 (2014).