Interface effects in the electronic structure of TiO2 deposited on MgO, Al2O3 and SiO2 substrates

We report the Ti 2p X-ray absorption (XAS) and resonant photoemission (RPES) spectra of one equivalent TiO2 monolayer grown on MgO, Al 2O3 and SiO2 substrates. The Ti 2p XAS spectra of these systems were compared to atomic multiplet calculations projected in different octahedral crystal fields. The comparison indicates that the crystal field splitting and the Ti-O hybridization decrease along the MgO, Al 2O3 and SiO2 series. The analysis of the RPES spectra provides the Ti 3d contributions to the valence band in these systems. These were compared to configuration interaction calculations of a TiO 6 cluster for different Ti 3d-O 2p hybridizations. The Ti 3d states in the valence band shift to lower binding energies along the MgO, Al 2O3 and SiO2 series. These effects are attributed to changes in the electronic structure at the interface, which, in turn, are related to the formation of cross-linking Ti-O-M (M = Mg, Al, and Si) bonds. © 2010 Elsevier B.V. All rights reserved.This work has been financially supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) through contracts FUNCOAT-CSD2008-00023, MAT2007-66719-C03-03 and MAT2008-01497. The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 226716.Peer Reviewe

Effects of grain refinement and disorder on the electronic properties of nanocrystalline NiO

Nanocrystalline NiO thin films have been grown on different substrates by RF magnetron sputtering with mixed O2-Ar plasma composition. The oxygen content of the plasma was varied between 0 and 100 %. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS). SEM results reveal a grain refinement when oxygen is added to the plasma. This effect can also be observed by XRD, where an analysis of the peak width confirms this decrease in the grain size. The analysis of EXAFS data shows that the presence of O2 in the plasma induces lattice disorder, as evidenced by the observed increase of the Debye-Waller factor. These microstructural changes modify the electronic structure of the NiO thin films. The spectral line shape in Ni 2p XPS spectra shows clear differences between samples grown with and without O2 in the plasma. These differences can be explained in terms of the observed structural changes. © 2014 Springer Science+Business Media New York.This work has been supported by the Spanish MICINN, under projects ENE2010-21198-C04-04, MAT2011-27573-C04-04, and CSD2008-0023.Peer Reviewe

Hexagonally-arranged-nanoporous and continuous NiO films with varying electrical conductivity

Nickel oxide (NiO) thin films have been prepared by magnetron sputtering, with different Ar/O2 ratios in the plasma, on several substrates, including hexagonally arranged nanoporous anodic alumina membranes (AAM). The obtained films exhibit columnar growth, which makes it possible to preserve the hexagonal order of the AAM substrates in the NiO thin films. X ray diffraction patterns show a polycrystalline structure with a crystallographic texture that depends on the plasma composition. Additionally, the NiO lattice parameter increases with the oxygen content of the plasma. The presence of oxygen during deposition is responsible for these structural changes, as well as for an oxygen enrichment in the NiO films, which leads to changes in their electrical properties. The electrical resistivity of the films decreases with the oxygen content of the plasma, which suggests p-type conductivity due to oxygen enrichment in the NiO lattice. Indeed, an analysis of the EXAFS oscillations at the Ni-K edge confirms the lattice expansion and a decrease of the Ni-Ni coordination number when the oxygen content of the plasma increases, which points towards an increasing presence of Ni vacancies for larger values of the O2/Ar ratio. © 2013 Elsevier B.V. All rights reserved.This work has been supported by the Spanish MICINN, under projects ENE2010-21198-C04-04, CSD2008-0023, and MAT2011-27573-C04-04.Peer Reviewe