Atom geometry of nanostructured Fe films grown on c(2 x 2)-N/Cu(100) surface: An investigation by X-ray absorption spectroscopy with multishell analysis

We investigated the growth of Fe nanostructured films on c(2 x 2)-N/Cu(100) surface with Fe K-edge X-ray absorption fine structure (XAFS) in the near edge and in the extended energy region. The high photon flux of the incident X-rays allowed us to perform multishell analysis of the XAFS oscillations for Fe coverage Theta(Fe) < 1 ML. This data analysis yields a detailed investigation of the atom geometry and some insights in the film morphology. At Theta(N) < 0.5 ML (N saturation coverage) there is absence of contribution to XAFS from N atoms. First shell analysis of linearly polarized XAFS gives Fe-Fe (or Fe-Cu) bond length values varying between R-1 = 2.526 +/- 0.006 angstrom at the highest Fe coverage (3 ML) and R-1 = 2.58 +/- 0.01 angstrom at Theta(Fe) = 0.5 ML, Theta(N) = 0.3 ML, with incidence angle Theta = 35 degrees. These values are different from the case of bcc Fe (R = 2.48 angstrom); and compatible with fcc Fe (R-1 = 2.52 angstrom) and fcc Cu (R-1 = 2.55 angstrom). At the Fe lowest coverage (Theta(Fe) = 0.5 ML) the dependence of R, on the incidence angle indicates expansion of the outmost layer. Near edge spectra and multishell analysis can be well reproduced by fcc geometry with high degree of static disorder. At N saturation pre-coverage (Theta(N) = 0.5 ML) the XAFS analysis has to keep into account the Fc-N bonding. The results suggest two different adsorption sites: one with Fe in a fcc hollow site, surrounded by other metal atoms as nearest neighbours, and one resulting from an exchange with a Cu atom underneath the N layer. (c) 2006 Elsevier B.V. All rights reserved.