Synthesis of Single-Layer h-BN Film on Cobalt Foil by Eliminating Nitrogen Intercalation*

The growth mechanism of single- and multi-layer h-BN on Co foil was studied by means of source control and substrate carburization in our MBE system. We found that the edge-atom state and film/substrate interface distance play the key role in the formation of either single-layer h-BN film or multi-layer h-BN flakes. When ammonia is used as molecular source, the edge of as grown h-BN flake is terminated with hydrogen atom, which enables the diffusion of N atoms and NH_x (x=1, 2) radicals into the interface of h-BN flake and substrate surface, leading to the multilayer growth from bottom. When nitrogen plasma is used as atomic source, however, the edge of as grown h-BN flake is bonded to substrate surface, thus, the gate for diffusion is closed, and upcoming nitrogen atoms are caught by active edge atoms, leading to the lateral growth. Nevertheless, we found that ad-layers can be formed in some areas where h-BN film grows across different Co grains and loses its epitaxial relationship to <111> surface, which results in large interface distance. By introducing certain amount of interstitial carbon into Co substrate, the interface distance is decreased so as to close the channel for diffusion, and eventually the growth of uniform single-layer h-BN film is achieved.