First-principles study of bismuth films on the Ni(111) surface

A recent experiment suggested that bismuth forms hexagonal films on the Ni(111) surface, of heights three, five, and seven layers. A quantum size effect based on free electrons was proposed in explanation. To test this idea, we calculate the total energies of Bi on the Ni(111) surface using density functional theory. We find that hexagonal film stabilities disagree with the observed odd layer preferences, and the structures are mechanically destabilized by adding capping atoms which pucker the hexagonal layers. Furthermore, we find that rhombohedral films based on the bulk Bi structure are energetically more favorable than the proposed hexagonal films. These structures also favor odd numbers of layers, but owing to covalent chemical bonding rather than confinement of free electrons. Specifically, a strongly bound adsorbed surface monolayer forms, followed by bulk-like rhombohedral bilayers.