Electronic properties of BaO on W(001)

In order to better understand the mechanisms responsible for the lowering of the work function that is observed when surface layers containing barium and oxygen are adsorbed on refractory metals such as tungsten, we have initiated a series of full-potential linearized augmented plane wave electronic structure calculations on well-defined model systems representing the Ba–O/W interface. We report here our initial results for a model in which c(2×2) overlayers of upright BaO molecules have been positioned on both sides of a five-layer W(001) film. Three independent self-consistent calculations were performed involving two different Ba–O separations and two different distances between planes of Ba and surface W atoms. We find that the work function of the clean five-layer tungsten slab (4.65 eV) is lowered by ∼1.8–2 eV by the adsorption of the Ba–O surface layer in this geometry, and that this result is relatively insensitive to the Ba–O separation in the overlayer. The most important factor determining the value of the work function seems to be the position of the barium plane above the W substrate. We find evidence of significant bonding between the d-like surface states of the tungsten substrate and both the Ba d and the oxygen 2p adsorbate levels. As a result, multiple dipoles are formed at the interface and the competition between these polarized charge distributions leads to a net lowering of the work function.