Enabling Collective Behaviour of Metal Halide Perovskite Nanocubes via Hierarchical Assembly into Superstructures

Metal halide Perovskite Nanocrystals (MH-PNCs) have recently regained popularity within the realm of semiconductor nanosystems, representing in some instances a preferrable alternative to the more “traditional” II-IV semiconductor Quantum Dots (QDs) because of their excellent optical properties, significatively lower toxicity, higher defect tolerance, combined with a composition-dependent tunable bandgap and extremely good carrier mobility. Among MH-PNCs, particular interest has been attracted by Cesium Lead Halides (CsPbX3), where the halide can be any combination of Cl, Br, I, in view of their reliable and monodisperse synthesis, strong luminescence and versatile tuning via halide exchange. In recent years, hierarchical assembly has been employed in order to couple MH-PNCs into ordered superstructures, with sizes in the micrometer range, displaying intriguing collective and “quantum” phenomena such as superradiant emission and exciton delocalisation. The fundamental photophysics underlying these superstructures, however, remains largely unclear. In this work, we report the assembly of orthorombic CsPbBr3 nanocrystals into micrometrical cubic superstructures. We investigated the effect of the ligand and assembly parameters on the morphology and behaviour of the resulting superstructures and we performed a thorough characterization of their photocycle down to the femtosecond time scale, including ultrafast measurements at the single superstructure level obtained via pump-probe microscopy. Our results help further clarify the complex photophysics of PNC superstructures, paving the way for future application in photonics and devices.