Intercalation of Cu and Ru beneath the top graphene layer of graphite surfaces

STM experiments show that Cu forms encapsulated islands under the top graphene layers of graphite, as a result of vapor deposition of Cu on a sputtered graphite surface [JPCC 122, 4454]. Deposition at 800 K is optimal for formation of encapsulated multilayer Cu islands. Deposition below 600 K favors Cu clusters adsorbed on top of graphite, while deposition above 800 K favors single-layer intercalated Cu islands. To form Ru nanoislands below the top graphene layer requires deposition at 1000 K to 1180 K [Nanotechnology 29, 505601]. We present an extensive study of energetics using density functional theory to compare stabilities of a wide variety of configurations of atoms, clusters, and layers of Cu and Ru on/under the graphite surface. For Cu-graphite systems, the only configuration that is significantly more stable under the graphite surface than on top of it, is a single Cu atom. This analysis leads us to conclude that formation of encapsulated Cu islands is kinetically driven, rather than thermodynamically driven. On the contrary, the embedded Ru islands are thermodynamically favored over on-top clusters.