Abstract
Understanding and controlling defect formation during the assembly of nanoparticles is crucial for fabrication of self-assembled nanostructured materials with predictable properties. Here, time-resolved small-angle X-ray scattering was used to probe the temporal evolution of strain and lattice contraction during evaporation-induced self-assembly of oleate-capped iron oxide nanocubes in a levitating drop. We show that the evolution of the strain and structure of the growing mesocrystals is related to the formation of defects as the solvent evaporated and the assembly process progressed. Superlattice contraction during the mesocrystal growth stage is responsible for the rapidly increasing isotropic strain and the introduction of point defects. The crystal strain, quantified by the Williamson–Hall analysis, became more anisotropic due to the formation of stress-relieving dislocations as the mesocrystal growth was approaching completion. Understanding the formation of the transformation of defects in mesocrystals and superlattices could assist in the development of optimized assembly processes of nanoparticles with multifunctional properties.
Highlights
Understanding and controlling defect formation during the assembly of nanoparticles is crucial for fabrication of self-assembled nanostructured materials with predictable properties
The nanocube sizes and polydispersities were estimated by fitting a cubic model to the form factor, giving values of 6.8 ± 0.3 nm for NC068 and 9.1 ± 0.5 nm for NC091 (Figure S2), which corresponds very well to the edge lengths measured with transmission electron microscopy.[10,11]
The deviation from the cuboidal model at low scattering angles in the small-angle X-ray scattering (SAXS) patterns measured in the levitating drops at times exceeding 1180 and 800 s for the NC068 and NC091 dispersions, respectively, indicates that clusters were formed in the shrinking levitating drops (Figure 1c, red curves)
Summary
Understanding and controlling defect formation during the assembly of nanoparticles is crucial for fabrication of self-assembled nanostructured materials with predictable properties. Time-resolved small-angle X-ray scattering was used to probe the temporal evolution of strain and lattice contraction during evaporation-induced self-assembly of oleate-capped iron oxide nanocubes in a levitating drop. Ex situ electron microscopy has been used to probe various types of defects in CdSe nanorod liquid crystals grown at the interphase of a subphase,[21] in solution[22,23] or on a substrate[24] and in thin quasicrystalline and periodic, binary superlattices of spherical particles that were grown on a carbon substrate.[25,26] Mayence et al used 3D small-angle electron diffraction tomography to show stacking faults in spherical Pd nanoparticle superlattices and made analogies to dislocations in close-packed metals.[27] Time-resolved small-angle X-ray scattering (SAXS) and grazing-incidence SAXS (GISAXS)
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