Controllable Preparation And Properties Of Cs₄PbX₆ Perovskite Nanocrystals

Semiconductor nanocrystals,also known as quantum dots,due to their unique physical and chemical properties,set off a research boom in scientific research community.In recent years,the appearance of perovskite nanocrystals has opened up a whole new field of research in addition to traditional inorganic semiconductor nanocrystals.The organic-inorganic hybrid perovskite material has become a promising new material star in recent years due to its excellent performance in the application of photoelectric conversion,and its research heat has remained high.For the other branch of the perovskite material,pure inorganic perovskite CsPbX₃?X=Cl,Br,I?,with the realization of its colloidal synthesis,it was found that the luminescent properties of this perovskite nanocrystals are very worthy of attention,and scientists have also studied it with unprecedented enthusiasm.In addition to the most common CsPbX₃ component,the pure inorganic component perovskite has another component that is Cs₄PbX₆.The current reports on Cs₄PbX₆ are very limited,and there is still some controversy about its properties.Some scientists reported that Cs₄PbBr₆ single-crystals and powders emit strong and green luminescence,while some scientists claimed that the green luminescence from Cs₄PbBr₆ originates from minor nanoscale CsPbBr₃ impurities encapsulated in the Cs₄PbBr₆ bulk matrix.Thus,it is important to find out a method to synthesize the pure phase Cs₄PbX₆ NCs and investigate its properties.In this work,we used a easy-to-implement method for the synthesis of pure phase Cs₄PbX₆ NCs,which not only has synthetic methodological significance,but also helps to reveal the nature of Cs₄PbX₆ itself.This paper mainly expounds in the following two aspects:1.We obtained the pure phase Cs₄PbX₆ perovskite NCs with proper modification of the original CsPbX₃ colloidal synthesis method,and the resulting Cs₄PbX₆ NCs have a regular spherical or spheroidal morphology,good monodispersity and adjustable sizes from 8 to 40 nm.Subsequently,the optical properties of Cs₄PbX₆NCs were confirmed:?1?The absorption peaks of the first excitons of Cs₄PbCl₆ NCs,Cs₄PbBr₆ NCs and Cs₄Pb I₆ NCs are located at 287 nm,314 nm and 365 nm respectively,and the position of the absorption peaks of Cs₄PbX₆ NCs is independent of their sizes which is different from other NCs with quantum size effect.The main reason is that,in the crystal structure of Cs₄PbX₆ perovskite NCs,[PbX₆]^4-octahedron is completely decoupled,which makes the size effect of the nanocrystal is not obvious,ie,the particle size has no significant effect on its band structure;?2?Pure phase Cs₄PbX₆ NCs are not fluorescent at room temperature.In addition to successfully achieving the colloidal synthesis of the pure phase Cs₄PbX₆ perovskite NCs,and clarifying their optical properties,we have also made efforts to improve their stability.The experimental results show that if the surface ligands of Cs₄PbX₆ NCs are replaced by 1-dodecanethiol or tri-n-octylphosphine oxide?TOPO?,their stability can be greatly improved,especially the latter has a significant effect on the improvement of the stability of Cs₄PbX₆ NCs.2.Some researchers reported that there is a conversion relationship between the CsPbX₃ perovskite NCs and Cs₄PbX₆ perovskite NCs.When PbX₂ is introduced into the Cs₄PbX₆ NCs,Cs₄PbX₆ NCs can be converted to CsPbX₃ NCs.With the assistance of some auxiliary ligands,the CsPbX₃ NCs can be transformed into Cs₄PbX₆ NCs.However,the Cs₄PbX₆ NCs converted from CsPbX₃ NCs in the published reports were very poor in both monodispersity and stability.Our work shows that when didodecyldimethylammonium halide?DDAX?is used as an initiator to convert CsPbX₃ NCs to Cs₄PbX₆ NCs,the obtained Cs₄PbX₆ NCs have good monodispersity and regular morphology,and the sizes of the Cs₄PbX₆ NCs can be tuned by the reaction temperatures.It concludes that this transformation is governed by a two-step dissolution-recrystallization mechanism.Additionally,if we add PbX₂to the converted Cs₄PbX₆ NCs,Cs₄PbX₆ NCs can be transformed back into CsPbX₃NCs in consistent with the conversion of directly synthesized Cs₄PbX₆ NCs to CsPbX₃ NCs with the addition of PbX₂.We can realize the reversible transformation between CsPbX₃ NCs and Cs₄PbX₆ NCs.Different from the mechanism of CsPbX₃-to-Cs₄PbX₆?two-step dissolution-recrystallization mechanism?,the process of Cs₄PbX₆-to-CsPbX₃ is the transformation mechanism.In summary,Cs₄PbX₆ perovskite NCs are successfully synthesized with controllable size and good monodispersity,and their stability improved by using of TOPO as surface ligands.Furthermore,CsPbX₃ NCs can be converted to Cs₄PbX₆NCs with DDAX as an auxiliary ligand,and the monodispersity and stability of the resulting Cs₄PbX₆ NCs are greatly improved.The introduction of PbX₂ into Cs₄PbX₆can achieve the conversion from Cs₄PbX₆ back to CsPbX₃,thus opening up a whole new path for the preparation of large seized CsPbX₃ NCs.In the course of our research,we also found that the mechanisms of these two reversible conversion processes are quite different.