EELS investigation of thin epitaxial NiO/Ag(001) films: surface states in the multilayer, monolayer and submonolayer range

In electron energy loss spectroscopy (EELS) at low primary energies, only a depth of about three to four layers is probed, and EELS spectra of NiO exhibit distinct loss structures, which arise from crystal field transitions at the Oh-coordinated Ni2+ sites in the bulk as well as at the C4v-coordinated surface sites. Besides the well-known surface state at E=0.6 eV, the loss structures at ΔE∼1 eV and ΔE=2.1 eV have been assumed to contain contributions from transitions, which take place at C4v sites. In order to extract the contributions of these surface states from those of the bulk, EELS spectra were recorded at NiO/Ag(001) films with NiO coverages from the submonolayer range up to thicknesses larger than the probing depth. It was found that in the case of ultrathin NiO films, i.e. if the number of Oh sites was strongly reduced, the only NiO-related loss structures that could be observed besides the plasmon of the Ag(001) substrate were the surface states at ΔE=0.6 eV and ΔE=2.1 eV. In contrast, the surface contributions of the 1.1 eV loss could not be observed, even for very low NiO coverages.