Modelling of adsorption of iron impurities at the surface of graphite

Using full-potential linearized combination of muffin-tin orbitals total energy calculations we have modelled the site selection of an iron atom at the surface of graphite. We find that the iron atoms 'float' above the graphite surface where the slight charge puckering below the iron atom indicates a very weak interaction with the carbon surface. We tested this effect both at relatively low (5.5% at the surface) iron concentrations and at high surface iron concentrations, and there is relatively little change in this behaviour with concentration. The stable position of the low-concentration iron impurities is a little less than half a graphite interlayer distance above the surface, with the stable location being above an empty carbon-ring centre. The charge distribution within the carbon atomic plane shows the normal s and p types of bonding between carbon atoms, quite similar to the pure carbon monolayer system, indicating that the presence of iron does not change the carbon-carbon interaction significantly. The charge distribution does, however, show some structure between the iron and carbon atoms, suggesting weak bonding between the iron and carbon atoms. Our results also indicate that the iron atoms do not contribute a large portion of their charges to the interstitial region, other than the 'bridging' region between the iron atoms and the six neighbouring carbon atoms.