Periodic grain boundaries formed by thermal reconstruction of polycrystalline graphene film.

Grain boundaries consisting of dislocation cores arranged in a periodic manner have well-defined structures and peculiar properties and can be potentially applied as conducting circuits, plasmon reflectors and phase retarders. Pentagon-heptagon (5-7) pairs or pentagon-octagon-pentagon (5-8-5) carbon rings are known to exist in graphene grain boundaries. However, there are few systematic experimental studies on the formation, structure and distribution of periodic grain boundaries in graphene. Herein, scanning tunneling microscopy (STM) was applied to study periodic grain boundaries in monolayer graphene grown on a weakly interacting Cu(111) crystal. The periodic grain boundaries are formed after the thermal reconstruction of aperiodic boundaries, their structures agree well with the prediction of the coincident-site-lattice (CSL) theory. Periodic grain boundaries in quasi-freestanding graphene give sharp local density of states (LDOS) peaks in the tunneling spectra as opposed to the broad peaks of the aperiodic boundaries. This suggests that grain boundaries with high structural quality can introduce well-defined electronic states in graphene and modify its electronic properties.