Formation of carbon species on Ni(111): Structure and stability

The structures and stabilities of a set of carbon species on the Ni(111) surface, as well as the energy barriers of the initial stages of carbon growth, were computed by using density functional theory. It was found that the stability is on the order of graphitic monolayer > C in Ni bulk > carbon chain > small cluster > atomic carbon. The formation of adsorbed graphitic monolayer is most preferred on Ni(111) thermodynamically. The adsorbed carbon atoms exist at the coverage of 0.25 and 0.5 monolayer, and they reconstruct Ni(111) at the coverage of 0.75 and 1 monolayer. The reconstruction will be interesting for self-assembly of surface structures and needs experimental confirmation. The adsorbed C-2 and C-3 carbon clusters on Ni(111) have linear structures, in which the terminal carbons prefer occupying 3-fold hollow sites. In the stable adsorbed C-3 cluster, the middle carbon atom occupies a top site. The linear C-4 cluster and the branched C-4 cluster have very similar stability. Like the adsorbed graphitic monolayer, the branched C-4 cluster occupies similar surface sites, and this implies a simple carbon growth mechanism. On the basis of the computed energy barriers, the lateral transfer and clustering of carbon is favored kinetically, while the carbon atoms sinking into the Ni bulk have high energy barriers. During the adsorption processes, electrons transfer from the Ni surfaces to the adsorbed carbon species, while the spin densities of the adsorbed carbon species and the Ni surfaces decrease.