Proceedings of MATSUS23 & Sustainable Technology Forum València (STECH23) (MATSUS23)
DOI: https://doi.org/10.29363/nanoge.matsus.2023.040
Publication date: 22nd December 2022
Lead-halide perovskite nanocrystals have been the subject of countless publications. Most of these works report XRD patterns featuring Bragg peaks with unusual shapes, which appear composed of two or more overlapping contributions. These however are too narrow for a nanocrystal, and the material’s structure does not account for their formation. The quest to understand these observations led us to the (re)discovery of Multilayer Diffraction, an intriguing collective interference effect known for epitaxial thin films but never investigated for colloidal nanocrystals before.[1]
Multilayer Diffraction occurs when X-rays are diffracted by nanocrystals packed with a periodic arrangement: here, the X-rays scattered by each particle interfere with those diffracted by neighbors, creating fringes of constructive interference.[2] Since the interfering radiation comes from nanocrystals, the fringes are visible only in correspondence with their Bragg peaks, thus explaining the unusual peak shape. Being a collective interference phenomenon, Multilayer Diffraction is highly sensitive to disorder, and requires precise stacking of nanocrystals to be observed. This condition is typical of self-assembled nanocrystal superlattices or stacks of nanoplatelets. Nevertheless, the Bragg peak split is routinely observed in a variety of samples, and not just in highly ordered superlattices. Why is that so?
This is only one of the many questions we will answer. Together, we will discuss why the peak split affects only some of the Bragg peaks, we will explore the influence of morphology (i.e., nanocrystals vs nanoplatelets) on Multilayer Diffraction,[3] and we will explain why this effect was not reported for popular materials like Au or CdSe, despite the extensive investigations on their superlattices. We will present models for the quantitative description of Multilayer Diffraction, which can be exploited to extract structural information hardly accessible from other techniques, first and foremost the degree of structural disorder in nanocrystal assemblies. Finally, we will demonstrate that Multilayer Diffraction is often observed yet unrecognized in a variety of nanomaterials beyond metal halide perovskites. Your sample might be next!
This research was funded by the Project IEMAP (Italian Energy Materials Acceleration Platform) within the Italian Research Program ENEA-MASE (Ministero dell'Ambiente e della Sicurezza Energetica) 2021-2024 "Mission Innovation" (agreement 21A033302 GU n. 133/5-6-2021).
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