First-principles study for stability and binding mechanism of graphene/Ni(111) interface: Role of vdW interaction.

The interaction between graphene and Ni(111) surface has been investigated systematically by density functional theory calculations, in which two different functionals PBE and optB88-vdW are used. PBE calculation indicates no binding between graphene and Ni(111) surface, while optB88-vdW, which is evidenced to consider van der Waals interaction reasonably, predicts the correct binding picture. The accurate potential energy surfaces suggest that top-fcc, bridge-top, and top-hcp are possible stable structures of graphene on Ni(111) surface, which are also found to have very close energies, in agreement with coexistence of different phases found experimentally. Different from PBE, the optB88-vdW functional predicts that top-fcc is the most stable configuration, following by bridge-top and then top-hcp, which is consistent with the surface distribution given by a statistical analysis of high-resolution scanning tunneling microscopy (STM) images. The Dirac points are destroyed in chemisorbed phases of all stable structures. Further analysis indicates that strong hybridization between Ni-3d and C-2p orbitals and asymmetry induced by substrate are responsible for the gap opening at K point. The detailed binding mechanisms have been analysed using differential charge density and the STM images.