Cesium lead halide perovskites, of the type CsPbX₃ (X=Cl, Br, I), are promising candidate materials for optoelectronics, solar devices and high-energy radiation detection [1-6]. Colloidal CsPbBr₃ nanocrystals with morphology of nanocubes [1], nanoplatelets [2-3] and nanowires [4] have been successfully synthesized in the last few years. Orthorhombic, tetragonal, and cubic phases were reported for bulk CsPbBr₃, the cubic phase being the high-temperature one [6]. The majority of CsPbBr₃ nanocrystals were reported to have cubic phase, e.g.  nanocubes [1] and nanoplatelets [2-3]. On the other hand, nanowires [4] and also nanocubes [5] exhibited orthorhombic phase, the latter ones despite their cubic shape. Here a study of colloidally grown CsPbBr₃ nanocubes (NCs), with edge length of about 40 nm, and large nanosheets (NSs) using high-resolution TEM (HRTEM) and selected area electron diffraction (SAED) shows that nanocrystals with both morphologies share the same orthorhombic phase (ICSD # 97851, see Figure 1). The observations were carried out by using an image Cs-corrected JEOL JEM-2200FS TEM (accelerating voltage = 200 kV). The lattice parameters, a’,  b’, c’, recalculated with respect to the ideal perovskite(a/ , b/ , c/2) for bulk orthorhombic CsPbBr₃ are very similar at room temperature (298 K) and further decreasing of temperature below the transition temperature 361 K increases the discrepancy between them. Here we performed HRTEM and SAED study for CsPbBr₃ NCs at various lower temperatures (Figure 2). The results have shown that the spacing among closely spaced peaks (298 K) increases at low temperature (153 K), indicating a larger discrepancy among the lattice parametes (see the arrows labelled in Figure 2). The orthorhombic CsPbBr₃ NCs and NSs exhibit significantly different facetting: the NCs are enclosed by {1-10}, {110} and {001} planes (Figure 3(a,b)). However, in the growth condition of NSs, the {001} planes are strongly passivated and the growth along [001] is inhibited. The growth in the plane of (001) leads to formation of large nanosheets confined in [001], and extended at the plane (001) and enclosed by {1-10} and {110} (Figure 3(c,d)).

References

[1] L. Protesescu et al., Nano Lett. 15, 3692 (2015)

[2] Y. Bekenstein et al., J. Am. Chem. Soc. 137, 16008 (2015)

[3] Q. Akkerman et al., J. Am. Chem. Soc. 138, 1010 (2016)

[4] D. Zhang et al., J. Am. Chem. Soc. 137, 9230 (2015)

[5] A. Swarnkar et al., Angew. Chem. Int. Ed. 54, 15424 (2015)

[6] M. Rodová et al., J. Therm. Anal. Calorim. 71, 667 (2003)

Acknowledgement: The research leading to these results has received funding from the European Union 7th Framework Programme under Grant Agreement No. 614897 (ERC Consolidator Grant “TRANSNANO”).

Figures:

Figure 1. (a, b) Low mag image of an ensemble of CsPbBr3 nanosheets and its SAED; (c, d) Low mag image of CsPbBr3 nanocubes and its SAED; (e) Azimuthal integration of the SAED in (b, d), in comparison with the powder XRD data for reference orthorhombic and cubic phases, some of the distinctive peaks for orthorhombic labelled.

Figure 2. Azimuthal integration of the SAED patterns taken for NCs at various temperatures and comparison with the powder XRD data for reference orthorhombic phase.

Figure 3. (a,b) HRTEM of a NC with zone axis [110]; (c) Low magnification image of a CsPbBr3 NS with zone axis [00-1]; (d) HRTEM of the NS in (c).

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