The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.
After the exploratory studies of the 1980s, anomalous small-angle X-ray scattering (ASAXS) is now a mature technique to disentangle complex subjects in materials science: this is illustrated through selected examples.
Different unmixed states of a Cu-Ni-Fe alloy have been studied by anomalous small-angle x-ray scattering (ASAXS). The data were obtained at different x-ray energies near the Fe and Ni absorption edges: ASAXS intensities increase by a factor of 3, varying with the atomic contrast (with respect to Cu). Various methods for the determination of partial structure factors (PSF) have been tested and results are found reliable if we use the intensity data in the proximity of at least two edges. The homoatomic PSF curves have similar shapes with a maximum I which increases and shifts towards lower q values as the power laws I -t ' and q -t, and the scaling behavior is obeyed. These features are very similar to the ones observed during late stages of aging of concentrated binary systems. On the other hand, there is evidence that the phase separation of Cu-Ni-Feo l& at 500'C cannot be modeled by a two-phase system, and different possible partitioning of Cu, Ni, and Fe are discussed.
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