O K-edge x-ray absorption study of ultrathin NiO epilayers deposited in situ on Ag(001)

In a recent contribution [Phys. Rev. Lett. 91, 046101 (2003)] we used polarization-dependent, Ni K edge, x-ray absorption spectroscopy (XAS) to probe the structure of ultrathin NiO epilayers deposited on Ag(001). In that experiment samples were measured ex-situ and a 5 ML-thick MgO cap was used to avoid the hydroxylation of the NiO film. In the present paper we report complementary O K edge XAS data on the same system; NiO epilayers, in the 3-50 ML thickness range, were grown in situ in the end station of the ALOISA beamline of the ELETTRA facility. A quantitative analysis of the data in the extended energy range has been performed using multiple scattering simulations. We found that, even in the ultrathin limit, the local structure of the film is rock-salt and we obtained a quantitative evaluation of the average in-plane and out-of-plane film strain as a function of the film thickness T. An in-plane compressive strain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film, being the in- and out-of-plane nearest neighbor distances equal to r(parallel to)=2.048+/-0.016 Angstrom and r(perpendicular to)=2.116+/-0.018 Angstrom. These values are in agreement with the expected behavior for a tetragonal distortion of the unit cell. The growth-induced strain is gradually released with increasing T: it is still appreciable for 10 ML but is completely relaxed at 50 ML. Any significant intermixing with the Ag substrate has been ruled out. Combining O and Ni K edge results we can conclude that NiO films grow on Ag(001) in the O-on-Ag configuration, with an interface distance d=2.28+/-0.08 Angstrom. This expansion of the interplanar distance is in agreement with recent ab initio simulations. A comparison with the similar MgO/Ag(001) system is also performed.