Low dose STEM imaging and in situ TEM of metal halide perovskites

Abstract text (incl. figure legends and references)

Metal halide perovskites (MHP) are promising semiconductors for the next generation of optoelectronic devices. Due to their tunable bandgap, they fulfil the requirement for various applications, including solar cells, light-emitting diodes, lasers and photodetectors. Unfortunately, MHPs still lack of long-term stability. Degradation occurs due to their phase instability and under environmental triggers, such as moisture and oxygen. In order to be able to extend the life time of MHPs a fundamental understanding of degradation mechanisms is required. To investigate these mechanisms we apply in situ HRSTEM under oxygen and moisture. Another challenge for TEM investigations of MHPs is their high electron beam sensitivity. Therefore, we developed low-dose protocols using integrated differential phase contrast (iDPC)1, see Fig.1b. Hence, we are able to investigate the MHP without degradation due to Pb-cluster formation or amorphization and quantitatively interpret the STEM images using statistical parameter estimation theory. For heterostructures, consisting of perovskite nano cubes and a chalcohalide, we applied the described STEM techniques to study the interface nature (Fig. 1c,d)2. Interestingly, the cationic subnetwork of Pb and Cs is maintained through the interface, which results in an epitaxial relationship. We also found that the perovskite can be used as disposable epitaxial template for the phase-selective synthesis of lead sulfochloride nanocrystals. Furthermore, in order to study perovskites with organic cations as well as the exchange of cations or halides, we utilize 4D STEM data sets to retrieve the phase information via integration center of mass (iCOM) reconstructions. This enables us to study the organic cations, which would not be possible using HAADF imaging. The developed methods provide important insights into the structure of the perovskites and enable in situ TEM investigations, which are essential to improve the properties and increase their chemical stability.

Figure 1. High resolution STEM imaging of metal halide perovskites. (a) HAADF image of a CsPbCl3 nanocrystal (NC). (b) Corresponding iDPC image of the NC in panel (a) revealing the Cl atomic columns. (c) High-resolution HAADF STEM image of a single Pb4S3Cl2/CsPbCl3 heterostructure. At the top is a model of the Pb4S3Cl2/CsPbCl3 epitaxial interface superimposed on a close-up view of panel (c), highlighting the continuity of the Pb2+/Cs+ cationic subnetwork. Atom color code: Cs = cyan; Pb = orange/white; S = yellow; Cl = green. (d) Column intensity map of the Pb-containing columns in the perovskite phase and of the Pb columns in the Cs2PbX4-like subnetwork of the Pb4S3Cl2 domain. The color in the intensity map correlates with the total intensity scattered from the corresponding atomic column (red = higher intensity; blue = lower intensity). (c,d) Reprinted from Ref. [2].

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