Atom-by-Atom Observation of Grain Boundary Migration in Graphene

• Published in: Nano letters 2012-06, Vol.12 (6), p.3168-3173
• Main Authors: Kurasch, Simon, Kotakoski, Jani, Lehtinen, Ossi, Skákalová, Viera, Smet, Jurgen, Krill, Carl E, Krasheninnikov, Arkady V, Kaiser, Ute
• Format: Article
• Language: English
• Subjects: Boundaries; Computer Simulation; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Curvature; Diffusion; Diffusion in nanoscale solids; Diffusion in solids; Exact sciences and technology; Fullerenes and related materials; diamonds, graphite; Grains; Graphene; Graphite - chemistry; Materials science; Mathematical analysis; Migration; Models, Chemical; Models, Molecular; Nanostructure; Nanostructures - chemistry; Nanostructures - ultrastructure; Particle Size; Physics; Specific materials; Three dimensional; Transport properties of condensed matter (nonelectronic)
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/nl301141g

Grain boundary (GB) migration in polycrystalline solids is a materials science manifestation of survival of the fittest, with adjacent grains competing to add atoms to their outer surfaces at each other’s expense. This process is thermodynamically favored when it lowers the total GB area in the sample, thereby reducing the excess free energy contributed by the boundaries. In this picture, a curved boundary is expected to migrate toward its center of curvature with a velocity proportional to the local radius of boundary curvature (R). Investigating the underlying mechanism of boundary migration in a 3D material, however, has been reserved for computer simulation or analytical theory, as capturing the dynamics of individual atoms in the core region of a GB is well beyond the spatial and temporal resolution limits of current characterization techniques. Here, we similarly overcome the conventional experimental limits by investigating a 2D material, polycrystalline graphene, in an aberration-corrected transmission electron microscope, exploiting the energy of the imaging electrons to stimulate individual bond rotations in the GB core region. The resulting morphological changes are followed in situ, atom-by-atom, revealing configurational fluctuations that take on a time-averaged preferential direction only in the presence of significant boundary curvature, as confirmed by Monte Carlo simulations. Remarkably, in the extreme case of a small graphene grain enclosed within a larger one, we follow its shrinkage to the point of complete disappearance.