Abstract
Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. Here we report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices. This is demonstrated by studying CsPbBr3 nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. It is shown that a gradual contraction of the superlattices and subsequent coalescence of the nanocrystals occurs over several days of keeping such structures under vacuum. As a result, a narrow, low-energy emission peak is observed at 4 K with a concomitant shortening of the photoluminescence lifetime due to the energy transfer between nanocrystals. When exposed to air, self-assembled CsPbBr3 nanocrystals develop bulk-like CsPbBr3 particles on top of the superlattices. At 4 K, these particles produce a distribution of narrow, low-energy emission peaks with short lifetimes and excitation fluence-dependent, oscillatory decays. Overall, the aging of CsPbBr3 nanocrystal assemblies dramatically alters their emission properties and that should not be overlooked when studying collective optoelectronic phenomena nor confused with superfluorescence effects.
Highlights
Excitonic coupling, electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics
The starting shape-pure cesium oleate-capped CsPbBr3 nanocrystals were synthesized via benzoyl bromide injection into a mixture of cesium and lead oleates in the presence of didodecylamine, as detailed in Methods.[31]
The photoluminescence quantum yield of the as-synthesized nanocrystals was ∼30%; such a modest value is attributed to the postsynthetic washing step involving ethyl acetate as an antisolvent.[31]
Summary
Electronic coupling, and cooperative interactions in self-assembled lead halide perovskite nanocrystals were reported to give rise to a red-shifted collective emission peak with accelerated dynamics. We report that similar spectroscopic features could appear as a result of the nanocrystal reactivity within the self-assembled superlattices This is demonstrated by studying CsPbBr3 nanocrystal superlattices over time with room-temperature and cryogenic micro-photoluminescence spectroscopy, X-ray diffraction, and electron microscopy. Regardless of the synthetic methods and surface passivation of CsPbX3 nanocrystal building blocks, their dynamic chemistry makes nanocrystal assemblies metastable, as it has been documented in reports of coalescence and halide expulsion under external stimuli such as pressure,[33] heating,[34−36] or illumination.[37] Much less is known about the nanocrystal stability in superlattices under common sample handling conditions (e.g., under vacuum or in the air) and its effect on the optical properties of individual superlattices. The spectral dynamics of this new peak were probed experimentally by transient spectrally resolved PL spectroscopy (Figure 6) and explained by Förster resonance electronic excitation transfer (FRET) theory[38,39] (Figure 7)
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