Healing a Topological Scar*

A novel defect type in two-dimensional systems is presented, which changes the local coordination number of an atom in an otherwise regular structure. While point-like by itself, such a 'coordination defect' has an unusually large number of involved atoms and a potentially dramatic influence on the growth of the system following its formation, due to its introduction of a mismatch between bond network topology and physical ring size. The potential growth pathways after the occurrence of such a defect in graphene are followed using molecular dynamics and first-principles calculations; the inherent conflict between the topological requirements and the actual chemical/physical structure that occurs as the system heals the defect can result in varied morphologies, including a runaway feedback that spawns one or more semi-infinite grain boundaries. Energy comparisons from first principles are used to evaluate the likelihood of this result under various conditions. The appearance of this defect type is predicted to have similar ramifications across a broad array of two-dimensional systems, potentially providing a new method of controlling grain boundary behavior and location.