Aqueous Synthesis, Phase Transformation And Optical Properties Study Of All-inorganic Lead Halide Perovskite Nanocrystals

Lead halide perovskites nanocrystal（LHP NCs）have shown potential application in many fields with excellent optical and electronic properties,and have gained increasing attention.However,due to their inherent ionic properties and dynamic organic ligand binding to the surface,they are vulnerable to degradation under light,heat,water and oxygen,seriously hindering their practical application.In addition,to avoid structural damage caused by polar solvents,the traditional preparation of LHP NCs can usually only be carried out in non-polar solvents,and long chain alkyl ligands are used for surface capping.These methods not only have strong toxicity,harsh conditions,and complex operating steps,but also the resulting LHP NCs can only be stably dispersed in non-polar organic solvents.Postsynthesis transformation is an interesting chemical conversion process that hasbeen proven to be a simple and effective method in the synthesis of colloidal nanocrystals.These transformations give access to novel structures and performance that are difficult to synthesize directly.The structure and surface instability of lead halide perovskite NCs position them as interesting candidates for studying chemical transformations.Currently,it has been used to regulate the band gap,passivate surface,optimize synthesis methods,and improve the performance of LHP NCs by postsynthesis anion exchange,ligand exchange,and phase transformation processes.Among them,the all-inorganic lead halide perovskite mainly involves the phase transformation between three crystal structures,namely CsPbBr₃,Cs₄PbBr₆and Cs Pb₂Br₅,which can be interconverted by different post-processing methods.The study of this process is helpful to promote the development of LHP NCs in synthesis methods,new structures and properties.In recent years,much attention has been paid to the conditions,influencing factors,and intermediates of phase transformation,which could be used to prepare LHP NCs with novel structure and performance.In this paper,based on the post synthesis phase transition process,we demonstrate an aqueous synthetic method allowing for the fabrication of luminescent and water-stable CsPbBr3 NCs and obtained colloidal nanocrystals with high water stability and excellent luminescence properties,and successfully applied in cell imaging.Moreover,we investigated the application of rare earth complexes in initiating phase transformation and assembly of LHP NCs.As a kind of functional luminescent ligand,rare earth complexes can be modified on the surface of lead halide perovskite,which is conducive to the development of new optical properties and applications.This thesis is mainly presented as the following four parts:Chapter 1: Firstly,we describe the composition,structure,optical properties and existent problems of lead halide perovskite NCs.Then the main synthesis methods of LHP NCs,especially the advantages and progress of aqueous phase synthesis,are emphasized.Finally,the research on stability improvement strategies and crystal phase transformation processes is also reviewed.Chapter 2: Here,we propose an aqueous phase synthesis strategy to prepare CsPbBr3 NCs with stable dispersion and efficient luminescence in weakly acidic water systems.This strategy uses water to trigger the postsynthetic transformation from Cs4 PbBr6 to CsPbBr3.Due to the synergistic effect of acidic amino acid and oleamines,CsPbBr3 NCs could be successfully synthesized and stably present in aqueous systemsis.The CsPbBr3 NCs obtained by this method have a PLQY of up to85%,a narrow full width of half maximum（16 nm）,and a long fluorescence lifetime（32.8 ns）.After being stored in water for 88 hours,their fluorescence emission can still maintain about 80% of the initial intensity,exhibiting extremely high water stability.Based on the excellent water dispersibility and stability of this materials,as well as the good biocompatibility of amino acids,we have successfully applied them to cell imaging.Chapter 3: We design and synthesize a rare earth complex which can bind to the surface of LHP.By adding the complexes to the presynthesized CsPbBr3 NCs toluene solution,the controllable phase transformation from CsPbBr3 to Cs4 PbBr6 is successfully triggered.At the same time,we observe oriented attachment and self-assembly processes between nanoparticles,which can be regulated by the addition of oleic acid and oleamine ligands,with rare earth complexes further modified on the surface of the materials.The obtained materials exhibit both the green fluorescence of LHP and the red fluorescence of europium complexes.Due to their different response characteristics to temperature changes,they can be used as thermal stimulus-responsive fluorescent materials for information storage and encryption.Chapter 4: Summaries and perspectives of this thesis.