Perovskite nanocrystals (NCs) have become leading candidates for solution-processed optoelectronics applications. While substantial work has been published on 3-D perovskite phases, the NC form of the zero-dimensional (0-D) phase of this promising class of materials remains elusive. Here we report the synthesis of a new class of colloidal semiconductor NCs based on CsPbBr, the 0-D perovskite, enabled through the design of a novel low-temperature reverse microemulsion method with 85% reaction yield. These 0-D perovskite NCs exhibit high photoluminescence quantum yield (PLQY) in the colloidal form (PLQY: 65%), and, more importantly, in the form of thin film (PLQY: 54%). Notably, the latter is among the highest values reported so far for perovskite NCs in the solid form. Our work brings the 0-D phase of perovskite into the realm of colloidal NCs with appealingly high PLQY in the film form, which paves the way for their practical application in real devices.

Zero-dimensional (0D) perovskites have recently opened a new frontier in device engineering for light conversion technologies due to their unprecedented high photoluminescence quantum yield as solids. Although many experimental and theoretical efforts have been made to understand their optical behavior, the origin of their green emission is still opaque. Here, we develop a complete experimental and theoretical picture of point defects in Cs-Pb-Br perovskites and demonstrate that bromide vacancies (V) in prototype 0D perovskite CsPbBr have a low formation energy and a relevant defect level to contribute to the midgap radiative state. Moreover, the state-of-the-art characterizations including atomic-resolution electron imaging not only confirm the purity of the 0D phase of Br-deficient green-emissive CsPbBr nanocrystals (NCs) but also exclude the presence of CsPbBr NCs impurities. Our findings provide robust evidence for defect-induced green luminescence in 0D perovskite NCs, which helps extend the scope of the utility of these bulk 0D quantum materials in optoelectronic applications.

The rapid development of halide perovskite synthesis offers the opportunity to fabricate high-quality perovskite nanocrystals (NCs), whose structural uniformity can lead to assembled supra-structures with improved device performance and novel collective properties. Light is known to significantly affect the structure and properties of halide perovskites and plays a crucial role in the growth and assembly of their crystals. Nevertheless, the light-induced growth mechanisms of perovskite NCs are not yet clearly understood. In this work, we performed a systematic study of the visible-light-induced template-free synthesis of CsPbBr 3 nanowires (NWs) generated through self-assembly of cubic (in phase and close to cubic morphology) NCs. Using atomicresolution electron microscopy, we visualized the cubic-toorthorhombic phase transition in NCs and the interface between coalesced NCs. Remarkably, the images of the interface revealed the coexistence of CsBr and PbBr 2 surface terminations in halide perovskites. Our results shed light on the mechanism underlying the observed anisotropic assembly of halide perovskites and elucidate the vital role of light illumination during this process. More importantly, as an elegant and promising green-chemistry approach, light-induced self-assembly represents a rational method for designing perovskites.

Materials, synthesis methods, EDX, TGA measurement results, size distribution and Arrhenius plots of integrated PL intensity (PDF)■ AUTHOR INFORMATION

Flexible copper halide films of 400 cm2 area were fabricated with outstanding mechanical stability, excellent film uniformity, nearly 100% photoluminescence quantum yields, and resistance to water and heat. The re-absorption-free X-ray imaging scintillators engineered based on these films exhibit superior scintillation performance with a detection limit as low as 48.6 nGy/s and 17 lp/mm X-ray imaging resolution, representing the highest imaging resolution for powder-based screens.