Lead halide perovskites are the most promising semiconductor material of modern times. This class of materials is currently investigated for its use in applications from solar-cells and photo-detectors (in particular for hard radiation) to light emitting devices such as LEDs and LCD displays [1]. Nanocrystals of cesium lead bromide (CsPbBr₃) have been found to emit single photons with long coherence times [2], that can be optically coupled to give rise to super-fluorescence [3]. This effect was thus far observed in ordered perovskite nanocrystal assemblies. The intrinsic optical characteristics of these nanocrystals and their relation to the nanocrystal habit are, however, not well studied due to a lack of monodisperse ensembles with tunable crystal habit. We present a size selection procedure which enables the isolation of monodisperse ensembles of CsPbBr₃ nanocrystals in a size range between 4.1 and 13.3 nm [4]. We analyzed their crystal habit by small angle X-Ray scattering and were able to quantum mechanically model the change of the first and second excitonic transition as well as their radiative lifetime as a function of the nanocrystal size and to predict the effect of the nanocrystal shape. We, additionally, demonstrate the self-assembly of these nanocrystals into super-crystals and their super-fluorescence. Having access to more sizes than previously the effect of the nanocrystal size on the super-fluorescence characteristics could be investigated. This shows that these monodisperse ensembles are a valid reference system for the further investigation of the coupling mechanism giving rise to the super-fluorescence behavior.

[1] Kovalenko, M. V.; Protesescu, L.; Bodnarchuk, M. I., Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 2017, 358, 745-750.
[2] Utzat, H.; Sun, W.; Kaplan, A. E. K.; Krieg, F.; Ginterseder, M.; Spokoyny, B.; Klein, N. D.; Shulenberger, K. E.; Perkinson, C. F.; Kovalenko, M. V.; Bawendi, M. G., Coherent single-photonemission from colloidal lead halide perovskite quantum dots. Science 2019, 363, 1068–1072
[3] Raino, G.; Becker, M. A.; Bodnarchuk, M. I.; Mahrt, R. F.; Kovalenko, M. V.; Stoferle, T., Superfluorescence from lead halide perovskite quantum dot superlattices. Nature 2018, 563, 671-675.
[4] Franziska Krieg et al. submitted