Abstract
Colloidal superlattices are fascinating materials made of ordered nanocrystals, yet they are rarely called “atomically precise”. That is unsurprising, given how challenging it is to quantify the degree of structural order in these materials. However, once that order crosses a certain threshold, the constructive interference of X-rays diffracted by the nanocrystals dominates the diffraction pattern, offering a wealth of structural information. By treating nanocrystals as scattering sources forming a self-probing interferometer, we developed a multilayer diffraction method that enabled the accurate determination of the nanocrystal size, interparticle spacing, and their fluctuations for samples of self-assembled CsPbBr3 and PbS nanomaterials. The multilayer diffraction method requires only a laboratory-grade diffractometer and an open-source fitting algorithm for data analysis. The average nanocrystal displacement of 0.33 to 1.43 Å in the studied superlattices provides a figure of merit for their structural perfection and approaches the atomic displacement parameters found in traditional crystals.
Highlights
Colloidal superlattices are fascinating materials made of ordered nanocrystals, yet they are rarely called “atomically precise”
The quantitative structural analysis of 3D superlattices through grazing-incidence diffraction requires a strong background in diffraction theory and algorithms tailored to the symmetry of a specific sample.[10,14−19] Transmission electron microscopy (TEM) and electron diffraction are arguably more user-friendly techniques that have a broader spread among research centers due to their versatility, but are limited to the analysis of thin superlattices deposited on grids.[20−22] All this considered, developing a technique for nanocrystal superlattice characterization that combines the power of X -ray diffraction (XRD) with instrumental accessibility and user-friendly data analysis is an important goal
In the preceding work on lead-halide perovskite nanocrystal superlattices, we reported some unusual modulations of the Bragg peaks, here called superlattice fringes, which were absent in patterns of randomly oriented particles and arose from the mesostructure periodicity.[31]
Summary
Colloidal superlattices are fascinating materials made of ordered nanocrystals, yet they are rarely called “atomically precise”. X -ray diffraction (XRD) is one of the most widespread approaches to the characterization of nanomaterials.[1−6] small- and wide-angle X-ray scattering (especially grazing-incidence techniques)[7−13] are used to study the structure of colloidal nanocrystal superlattices and assemblies. Experiments with these techniques are performed with dedicated benchtop instruments or at specific synchrotron beamlines, and they are bound to instrumental accessibility. Such an approach promises to be highly relevant for the assessment of collective properties, which are defined by the nanocrystals themselves and by their positioning with respect to their closest neighbors
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