Open and Closed Edges of Graphene Layers

Edge structures of thermally treated graphite have been studied by means of atomically resolved high-resolution TEM. The method for the determination of a monolayer or more than one layer graphene sheets is established. A series of tilting experiments proves that the zigzag and armchair edges are mostly closed between adjacent graphene layers, and the number of dangling bonds is therefore minimized. Surprisingly bilayer graphene often exhibits $AA$ stacking and is very hard to distinguish from a single graphene layer. Open edge structures with carbon dangling bonds can be found only in a local area where the closed (folding) edge is partially broken.

Figure 1

Schematic presentations for closed, open, and partially open edge of a bilayer graphene.

Figure 2

(a) HR-TEM image of the graphite powders before heat treatment shows wavy edge structures, (b) HR-TEM image of the pyrolytic graphite powders after heat treatment shows the straight edge lines. Each edge line corresponds to a closed edge of bilayer graphene (c). The inserted Fourier transform diffractogram of the HR-TEM image indicates the 0.106 nm [(200) spots] resolution was achieved.

Figure 3

(a) HR-TEM image showing the closed bilayer graphene. The zigzag and armchair directions are marked with white and black arrowheads. A simulated image (b) using a model with the $AA$ stacking of the folded monolayer graphene (c). Each intersecting point between the armchair and the zigzag areas needs a pentagon-heptagon pair.

Figure 4

HR-TEM image of partially open edges for armchair (a–d) and zigzag edges (e)–(h). (B) An isolated hexagon extruding from the armchair edge where the closed edge is partially broken (a), (c) A simulated image based on a model described in (d). (e) An open edge structure of the zigzag edge. Two carbon atoms are debonded and protruded from the closed edge (f). (g) A simulated image based on the Klein model (h). The scale $\mathrm{\text{bar}}=1\mathrm{nm}$.