X-ray absorption spectroscopy study of the Ni K edge in magnetron-sputtered nickel oxide thin films

Nickel oxide, NiOx, thin films were studied by means of x-ray absorption spectroscopy at the Ni K edge. Thin films were prepared by reactive de magnetron sputtering of a nickel target in an atmosphere that was an Ar-O-2 mixture. The results of the EXAFS analysis show that the thin films have nanocrystalline structure, which can be described within the non-reconstructed grain-boundary model. Average grain sizes of 21 +/- 3 Angstrom and 34 +/- 5 Angstrom were obtained for thin films deposited in sputter atmospheres with oxygen contents of 30% and 100%, respectively. The nanocrystalline structure of the thin films results in a reduction in the number of atomic neighbours in the crystal lattice coordination shells beyond the first one. Also, increases of the structural disorder and of the intragrain lattice parameter relative to those for crystalline NiO (c-NiO) were detected for the thin films. In the XANES region, the intensity of the pre-edge peak, attributed to the dipole-allowed transitions \1s3d(8+i)L(l)) --> \1s3d(8+i)L(i-1)), i = 1, 2 (where L denotes an O 2p hole and 1s stands for a Ni 1s core hole), was found to be smaller in the thin films than in c-NiO. This result was interpreted within the Zaanen-Sawatzky-Allen model as being due to an increase of the oxygen-nickel charge-transfer energy which is accompanied by a reduction of the extent of the 3d(8+i)L(i) (i = 1,2) ground-state configuration. The findings on the local atomic and electronic structures suggest that, compared to that for c-NiO, the ionicity of the nickel-oxygen bonding increases significantly in the thin films.