Speaker
Mr
Janez Kranjski
(Vojvodina Kranjska)
Description
When electronegative atoms adsorb on an electropositive metal surface, charge transfer occurs and the adatoms become negatively charged. Due to this charge accumulation, repulsive lateral interactions are expected between them, and according to the classical method of images they can be treated as dipole-dipole interactions that scale as Θ3/2, where Θ is the surface coverage of adatoms. This dependence is typical for chemisorbed atomic oxygen on transition metal surfaces and it is depicted in Figure 1a for atomic oxygen on Cu(111). However, in the case of O on Al(111) and Al(100) surfaces the opposite occurs and the magnitude of binding energy increases with increasing coverage (see Figure 1b). Analysis of the electronic structure with DFT calculations reveals that the attractive interactions are a consequence of a simple electrostatic stabilization. Namely, at full monolayer coverage the O adatoms are located close to the surface and together with positively charged surface Al atoms form an electrostatically stable interlaced layer of anions and cations. This is evident from Figure 1c which shows the charge density difference for O on Al(111) (note the alteration of positive (blue) and negative (red) regions). We conclude that the attractive interactions between negatively charged O adatoms at high-coverage stem from an interplay between Coulombic interactions and geometric effects (height of the adatoms), i.e. there exists a critical adatom height below which the lateral interactions are attractive and above which they are repulsive. We propose that this picture is generally applicable for electronegative adatoms on metal surfaces provided that (i) the adsorption bonding is sufficiently ionic and (ii) the adatoms are sufficiently small to come close enough to the surface.
Primary author
Mr
Janez Kranjski
(Vojvodina Kranjska)
Co-author
Mr
Matic Poberznik
(Jozef Stefan Institute)
