  In present study we exploited Langmuir technique to produce self-assembled
arrays composed of monodisperse iron oxide nanoparticles 10 nm and 20 nm in
diameter and of their binary mixture. A combination of in-situ X-ray
reflectometry and Grazing-incident small-Angle X-ray scattering was used to
obtain in-plane and out-of-plane structure of the arrays directly on the water
surface. Surface pressure isotherms and X-ray reflectometry analysis showed
that monodisperse 10 nm nanoparticles form a highly ordered monolayer, while 20
nm particles pack in three-dimensional clusters with a short-range
(nearest-neighbor) correlations between the particles. In a binary mixture of
10 nm and 20 nm nanoparticles composed in proportion 3:1 the self-assembly
process results in a structure where the monolayer of 10 nm particles is
perturbed by the larger particles. Non-trivial mixing causes an enlargement of
interparticle distance but keeps the symmetry of two-dimensional lattice of
smaller nanoparticles. Estimation of the acting interactions and micromagnetic
simulation suggest the optimal formation for monodisperse and binary ensembles.
