Photoelectron spectroscopy study of K adsorption on ZnO(1 0 1 0)

Adsorption state of K on the ZnO(1 0 1 0) surface has been studied by low energy electron diffraction, X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy. The K overlayer is grown in a two-dimensional (2D) mode without forming any ordered structures at room temperature. K adsorption induces a substantial work function lowering by 2.0 eV as well as a 0.2-eV downward bending of the substrate bands, both of which are indicative of the charge transfer from the K adatoms to the substrate. Ionic adsorption is also supported from the K 2p XPS measurements. The ionicity diminishes with increasing K coverage, but adsorbed K species are not completely neutralized even at the completion of the monolayer, i.e. the K overlayer does not accompany a clear ion-to-metal transition with increasing coverage. The structural change in the K overlayer upon heating the K-saturated surface is also investigated, and it is found that c(2×2) and (3×1) structures are formed by adsorbed K at anneal temperatures of 950–1050 and 1100–1200 K, respectively.