Relationship between surface dipole, work function and charge transfer: Some exceptions to an established rule

Local-density-functional calculations are used to study the change of work functions induced by a layer of adsorbates. We investigated and compared the work function of a monolayer of Mo, Ag, Au, Fe, Co, Ni, Nb, Li, N, and O on W(100), W(110), W(211), and W(111) surfaces. While many systems obey the commonly accepted rule that electronegative adsorbates increase the work function of the surface, we find some exceptions. For example, overlayers of Fe, Co, and Ni increase the work function of W(100), W(211), and W(111), but decrease the work function of the W(110) surface, although the charge transfer is the same in all orientations. We found that even a layer of oxygen can decrease the work function of W(100), although there are always electrons transferred from the W substrate to the oxygen adsorbates. In order to understand these results, we established the relationship between surface dipole density and work function within the framework of local-density formalism. It turns out that subtle details of the charge transfer can determine the sign and magnitude of surface dipole change, leading to a strong dependence on the orientation of the substrate, with the consequence that the work-function changes are not always governed by the sign and quantity of adsorbate induced charge transfer.