Graphene films grown by vapor deposition tend to be polycrystalline due to the nucleation and growth of islands with different in-plane orientations. Here, using low-energy electron microscopy, we find that micron-sized graphene islands on Ir(111) rotate to a preferred orientation during thermal annealing. We observe three alignment mechanisms: the simultaneous growth of aligned domains and dissolution of rotated domains, i.e., "ripening"; domain-boundary motion within islands; and continuous lattice-rotation of entire domains. By measuring the relative growth velocity of domains during ripening, we estimate that the driving force for alignment is on the order of 0.1 meV per C atom and increases with rotation angle. A simple model of the orientation-dependent energy associated with the moir\'e corrugation of the graphene sheet due to local variations in the graphene-substrate interaction reproduces the results. This work suggests new strategies for improving the van der Waals epitaxy of 2D materials.
@article{arxiv.1502.07791,
title = {Real-time observation of epitaxial graphene domain reorientation},
author = {Paul C. Rogge and Konrad Thürmer and Michael E. Foster and Kevin F. McCarty and Oscar D. Dubon and Norman C. Bartelt},
journal= {arXiv preprint arXiv:1502.07791},
year = {2015}
}