Site-Selective Adsorption of C Atoms on Al(111) Surfaces

Although carbon is a common contaminant of Al surfaces, neither its favorable adsorption sites nor its bonding and electronic properties are known or understood to date. Here we report on density-functional theory calculations for this system which identify two nearly degenerate adsorption sites: (i) a ``fcc-type'' sixfold-coordinated site halfway between the first and second Al layers and (ii) a ``hcp-type'' fourfold-coordinated site slightly below the surface layer. We argue that only the C adatoms at the hcp-type sites have been detected in recent scanning tunneling microscopy experiments, but predict that the fcc-type C adatoms should exist in thermal equilibrium as well.