Surface X-ray diffraction analysis of Fe nanostructured films grown on c(2 x 2)-N/Cu(100)

We report the results of a Surface X-Ray Diffraction (SXRD) study of Fe nanostructured films deposited on c(2 x 2)-N/Cu(100) at room temperature (RT), with Fe coverage theta(Fe)= 0.5 ML and theta(Fe) = 1 ML. The c( 2 x 2)-N/Cu(100) surface is an example of self-organised system, that can be used for growth of arrays of metal nano-islands and organic molecules assemblies. We chose two different: values of N coverage, theta(N) = 0.3 ML and theta(N) = 0.5 ML, the second value corresponding to N saturation. We monitored the presence of surface diffraction peaks in hk scans and we performed Crystal Truncation Rods (CTR) analysis with ROD fitting programme. In the case of theta(N) = 0.5 ML, i.e. at saturation coverage, the CTR could be fitted with one surface domain with p4gm(2 x 2) symmetry. In the surface cell adopted, N atoms occupy fourfold hollow sites, with Fe (intermixed with Cu) giving rise to a "clock" reconstruction previously observed on iron nitride films obtained by co-deposition and annealing. This result is an indirect confirmation of N surface segregation on top of the Fe films, occurring during the growth at RT. When subsaturation N coverage (theta(N) = 0.3 ML) is used as a substrate for Fe deposition, the best results could be obtained with a model where two surface domains are present: the first one corresponds to a surface cell with Fe sitting in four-fold hollow sites on bare Cu areas, with possible interdiffusion in the second lattice. The second domain is assigned to growth of Fe on the N-covered square islands occurring once the bare Cu areas are fully covered. The SXRD analysis on N-covered surface domains shows that the mechanism of reconstruction and of N segregation on top layer is already active at RT for all N-coverage values.