Strain Solitons and Topological Defects in Bilayer Graphene

Alden, J.S., A.W. Tsen, P.Y. Huang, R. Hovden, L. Brown, J. Park, P.L. McEuen, et al., 2013

Image courtesy of Proc. Natl. Acad. Sci

Abstract

Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151–157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6–11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene.

Impact Statement

The authors used dark field and high resolution STEM to image bilayer graphene. Shear stress and strain between the two layers creates different atomic registry, or displacement, between each sheet, which can have a significant effect on the electronic properties of the material. The structure of the bilayer was probed with TEM and found that the displacement in each sheet created grains with well-defined boundaries with soliton-like behavior. Using the Fusion heating system, they observed the dynamic
behavior of the solitons at high temperatures. At temperatures of 800° C and below, the solitons did not change or move. However, when the temperature was increased to 1000° C, solitons became dynamic. The bilayer began to anneal, and the boundaries between grains became straighter and shorter. Movies of soliton movement at high temperatures were also taken.