Interface effects in theNi2px-ray photoelectron spectra of NiO thin films grown on oxide substrates

Abstract

We report the Ni 2p x-ray photoelectron spectra of NiO thin films grown on different oxide substrates, namely, SiO 2 , Al 2 O 3 , and MgO. The main line of the Ni 2p spectra is attributed to the bulk component, and the shoulder at 1.5 eV higher binding energies to the surface component. The spectra of the NiO thin films show strong differences with respect to that of bulk NiO. The energy separation between the main peak and the shoulder increases with the substrate covalence. This indicates the strong covalent interactions between the NiO thin films and the oxide substrates, and reflects changes in the bonding at the interface from a more ionic to a more covalent interaction. These conclusions are supported by cluster model calculations with a reduced O 2p-Ni 3d hybridization.

X-ray absorption study of the local structure at the NiO/oxide interfaces

“…Our results point out that for low coverages the local order around Ni extends only to the first two coordination shells. In addition, these results also suggest the presence of the substrate atoms at the interface, being consistent with the formation of Ni-O-M (M = Si, Al, Mg) cross-linking bonds at the NiO/oxide interface proposed from XPS experiments on the same systems (Preda et al, 2008).…”

“…Interesting trends in terms of covalency-ionicity of the substrates have already been observed in the study of the electronic structure of TiO 2 / oxide interfaces with synchrotron radiation spectroscopies (Soriano et al, 2000(Soriano et al, , 2011; Sá nchez-Agudo et al, 2001). We have also observed similar trends in NiO/oxide interfaces throughout the interpretation of the Ni 2p XPS spectra (Preda et al, 2008).…”

“…Previous results on NiO/ oxide interfaces studied by means of X-ray photoemission spectroscopy (XPS) have shown a strong interaction of the NiO layers with the most covalent substrates whereas this interaction is lower for ionic oxide substrates (Preda et al, 2008). In general, these results suggest the formation of crosslinking bonds Ni-O-M, where M stands for the cation of the oxide substrate.…”

“…MgO was deposited in situ on a p-doped Si (111) wafer by reactive evaporation of Mg at room temperature in an oxygen atmosphere (1 Â 10 À5 mbar). It was not possible to deduce the thickness of the MgO thin film from the XPS intensities since it was much larger than the inelastic mean free path of the photoelectrons; however, it has been estimated as several micrometres from the atomic force microscopy (AFM) images (Preda et al, 2008). Al 2 O 3 and MgO substrates were prepared in situ previously to NiO deposition; thus they could be checked by XPS to detect for possible contamination.…”

Surface structure of nickel oxide layers on a Rh(111) surface

“…The Ni 2p 3/2 spectrum has been deconvoluted into three different core-level components with binding energies (BE) of 854.3 eV, 855.4 eV and 861.3 eV, which are characteristic of bulk NiO [33]. More specifically, the lowest BE main line has been assigned to a c core level and the ligand band, respectively, whereas the shoulder at 855.4 eV has been ascribed to a non-local screening process from neighboring NiO 6 units [34] and/or surface effects [35]. The remaining emission structure at 861.3 eV is a charge-transfer satellite (indicated S in Fig.…”