Migration of gold atoms in graphene ribbons: Role of the edges

The migration of gold atoms attached to single vacancies near the edges of graphene ribbons is studied using density-functional theory calculations. The stable position for a single gold atom is found to be on top of a vacancy, as in an infinite graphene sheet. An energy of 5 eV is needed for the Au atom to move through the vacancy to the other side of the sheet, but the Au atom can migrate in lateral direction together with the vacancy, with a migration barrier of about 2.2 eV. The sites near the edges of the graphene layer are energetically more favorable for gold-atom-vacancy pairs than sites in the middle of extended graphene layers. The migration barriers for different pathways show that it is easier for the gold atom to move toward the edge where it can be captured. When the gold atom reaches the edge, it can migrate along the edge with an energy barrier of only 1.4 eV. Our results explain recent experimental observations [Y. Gan et al., Small 4, 587 (2008)] and provide information on the dynamics of metal atoms on substitutional sites in graphene as well as on their agglomeration at defects and at edges of graphene ribbons.

Figure 1

(Color online) Possible positions for a Au atom in (a) a single vacancy and (b) a double vacancy in a graphene sheet. The inset in (a) shows an electron-density slice through the Au atom (white means high electron density).

Figure 2

(Color online) The energy profile for the diffusion of the Au-vacancy pair.

Figure 3

Configurations of the Au-vacancy pair at different sites on the graphene ribbons with (a) zigzag, (b) armchair, and (c) reczag edges. The total energy of optimized structures of the graphene ribbon with Au at different sites is shown right with (solid squares) and without (open squares) hydrogen passivation. The energy of the Au metal in the middle of the ribbon is set to zero. The lines are drawn to guide the eyes.

Figure 4

(Color online) Optimized structures of the zigzag graphene ribbon with Au at A-F sites as shown in Fig. 3. In the right column the edges are passivated by hydrogen atoms.

Figure 5

(Color online) The migration path for the Au atom at the edge site, as found by the NEB method.