Perovskite nanocrystals under the Transmission Electron Microscope: towards reliable data

Perovskite nanocrystals (NCs) is a big group of materials, which attracts a lot of attention of the researchers due to their high commercialization potential in the optoelectronic field. Colloidal synthesis of NCs is a flexible and convenient approach for tuning the obtained properties, therefore making perovskite NCs appealing for applications in LED, lasers, solar cells, etc. However, there are still challenges to overcome, such as poor phase stability and the high toxicity of the lead in the composition, which triggers the ongoing studies. Accordingly, to ensure progress in this direction, it is essential to develop reliable methods of characterization. Comparably insufficient attention in the perovskite community is given to the appropriate practices of transmission electron microscopy (TEM) when in the reality the one is facing the challenges during the sample preparation, measurement, and interpretation of the results.

In fact, the major drawbacks of the perovskite NCs are also reflected in the TEM measurements. Low phase stability leads to the fast degradation in the ambient condition during the storage and the sample preparation. The Pb2+ species (which are typically in a crystal structure of the top-performing NCs) can be reduced to Pb0 by irradiation with high-energy electrons, driving the deterioration of the original NCs and complicating the analysis [1]. But arguably one of the most affecting ones is the presence of the organic ligands (fatty amines) in the samples. Acting as the solvent, reducing, and coordinating agent, its utilization is necessary in the colloidal NCs synthesis. Moreover, in the ready-made samples, the ligands form a so-called shell, which coats the NCs. And while this shell serves as a protective layer, shielding the NCs from the detrimental factors, it strongly affects the quality and the reliability of the TEM images. Typically employed ligands contain hydrocarbon chains, causing contamination due to the combustion of hydrocarbons, which results in the formation of dark areas during the imaging [2]. Accordingly, the removal layer of the ligands is challenging since it will mean accelerated degradation and might cause irreversible changes to the NCs during the process.

Here we summarize our experience from the TEM study on a series of the perovskite NCs including well-studied and new compositions. We provide some hints for collecting reliable TEM data. The results of our work will hopefully assist the community in overcoming the current challenges of the TEM measurements of the perovskite NCs and will be valuable for the other studies performed for similar materials.
[1] Z. Dang et al., “In Situ Transmission Electron Microscopy Study of Electron Beam-Induced Transformations in Colloidal Cesium Lead Halide Perovskite Nanocrystals,” ACS Nano, vol. 11, no. 2, pp. 2124–2132, 2017, doi: 10.1021/acsnano.6b08324.
[2] Ayache, Jeanne, et al. Sample preparation handbook for transmission electron microscopy: techniques. Vol. 2. Springer Science & Business Media, 2010.