Multilayer Diffraction Reveals That Colloidal Superlattices Approach the Structural Perfection of Single Crystals

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, 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 nanocrystal size, interparticle spacing, and their fluctuations for samples of self-assembled CsPbBr<sub>3</sub> and PbS nanomaterials. The average nanocrystal displacement of 0.32-1.4 Å in the studied superlattices provides a figure of merit for their structural perfection and approaches the atomic displacement parameters found in traditional crystals. The method requires a laboratory-grade diffractometer and an open-source fitting algorithm for data analysis, providing a competitive alternative to resource-intensive synchrotron experiments.

Observation of ordered organic capping ligands on semiconducting quantum dots via powder X-ray diffraction

“…As observed by Sandhyarani et al, the melting behavior of superlattices depends on these types of ordering interactions 12,31 . The use of GIWAXS to identify and evaluate this feature alongside SAXS, and grazing incidence small angle X-ray scattering (GISAXS) to characterize and analyze superlattices should yield further insight into the superlattice structures [13][14][15]43 . Altering the phase of the ligands could potentially manipulate the ligand ordering behavior to change distances between nanocrystals or modify superlattice assembly [43][44][45][46] .…”

“…The use of GIWAXS to identify and evaluate this feature alongside SAXS, and grazing incidence small angle X-ray scattering (GISAXS) to characterize and analyze superlattices should yield further insight into the superlattice structures [13][14][15]43 . Altering the phase of the ligands could potentially manipulate the ligand ordering behavior to change distances between nanocrystals or modify superlattice assembly [43][44][45][46] . We anticipate that these techniques can assist in optimizing future superlattices through employment of ligands with appropriate crystallization properties, and maintaining temperatures during superlattice formation that encourage certain ligand shell behavior.…”

Structure and Surface Passivation of Ultrathin Cesium Lead-Halide Nanoplatelets Revealed by Multilayer Diffraction

“…In this work, we investigate the structure of colloidal cesium lead-halide nanoplatelets along their thinnest dimension through the application of Multilayer Diffraction analysis to highly ordered nanoplatelet films. [24][25][26] The single-and mixed-halide Cs-Pb-X (X = Br or Cl-I) nanoplatelets were prepared by colloidal synthesis and self-assembled into thin films by slow solvent evaporation. The self-assembly process was exploited to create an extended periodicity along the thin direction of the nanoplatelets, producing a peculiar pattern of periodic peaks when the film is subject to a θ:2θ diffraction experiment.…”

“…The self-assembly process was exploited to create an extended periodicity along the thin direction of the nanoplatelets, producing a peculiar pattern of periodic peaks when the film is subject to a θ:2θ diffraction experiment. By applying the knowledge derived from the earlier work on lead-halide nanocrystal superlattices, 24,25 we developed an algorithm that enables the refinement of the nanoplatelets structure by means of a full-profile multiparametric fitting of their diffractogram. Despite relying only on a widely available lab-grade diffractometer, the amount of structural and compositional information that can be retrieved is noteworthy.…”

“…We also measured the anisotropic structural expansion of nanoplatelets with respect to bulk CsPbBr3, confirming a behavior reported for cesium lead-halide nanostructures in the past. 8,24,27 While a relatively simple structure and a well-documented relationship between thickness and optical properties eases the identification of the number lead-halide octahedra layers in the case of Cs-Pb-Br nanoplatelets, the presented method enables the investigation of more elusive materials as well. Here we report the example of Cs2PbCl2I2 Ruddlesden-Popper nanoplatelets, for which it is not possible to determine the number of layers from optical spectra.…”