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 exh ibit distinct loss structures, which arise from crystal field transitions a t the O-h-coordinated Ni2+ sites in the bulk as well as at the C-4v-coordin ated surface sites. Besides the well-known surface state at E=0.6 eV, the l oss structures at Delta E similar to 1 eV and Delta E=2.1 eV have been assu med to contain contributions from transitions, which take place at C-4v sit es. In order to extract the contributions of these surface states from thos e of the bulk, EELS spectra were recorded at NiO/Ag(001) films with NiO cov erages from the submonolayer range up to thicknesses larger than the probin g depth. It was found that in the case of ultrathin NiO films, i.e. if the number of O-h sites was strongly reduced, the only NiO-related loss structu res that could be observed besides the plasmon of the Ag(001) substrate wer e the surface states at Delta E=0.6 eV and Delta E=2.1 eV. In contrast, the surface contributions of the 1.1 eV loss could not be observed, even for v ery low NiO coverages. (C) 2000 Elsevier Science B.V. All rights reserved.