Study On The Structure And Optical Properties Of All-Inorganic Lead Halide Perovskite Nanocrystals

Recently,all-inorganic cesium lead halide perovskites(CsPbX3,X=Cl,Br,and I)nanocrystals have emerged as a new type of semiconductor materials owing to their potential applicationsin the fields of solar cells,detectors and so on.At present,although the phase transition of CsPbX3 bulk single crystal has been thoroughly investigated,the research on the structure and phase transition of CsPbX3 nanocrystals(NCs)remains scarce,resulting in a limited understanding of their phase transition mechanisms Although previous studies have shown that defects can affect the phase transition of hybrid perovskites,it remains unclear as to whether structural defects can affect the phase transition and the phase transition can affect the luminescence properties of CsPbX3 NCs.In addition,doping,as a method for effectively regulating semiconductor properties,has been widely applied to the regulation of CsPbX3 NCs in recent years.However,an in-depth understanding of the local structures of doped ions and of the effect of structural defects on doping efficiency remains missing.Therefore,in this thesis we aim to study the following scientific issues:1)studying the phase transition behavior of CsPbX3 NCs,and clarifying the relationship between their defects,phase transitions,and optical properties;2)studying the local structure of doped ions,the effect of defects on doping efficiency,and the anomalous photophysical properties in doped CsPbX3 NCsFirstly,we found the reversible phase transition of CsPbC13 NCs by temperature-dependent synchrotron X-ray diffraction analysis.By comparing the phase transition process of highly defective CsPbC13 and high-quality CsPbCl3 NCs,coupled with Car-Parrinello molecular dynamics simulations,we clarified the important influence of defects on the phase transition of CsPbC13 NCs.Based on these,we further discovered that the photoluminescence irreversibility of CsPbC13 NCs is associated with the defect concentration.Combined with synchrotron X-ray absorption spectroscopy,we proposed that the reason for this irreversibility could be ascribed to the local structural reorganization in NCs caused by phase transition.Furthermore,we found that CsPbBr3 NCs also show reversible phase transitions and defect-related photoluminescence irreversibility.Our results offer an insightful understanding of the structural phase transition of all-inorganic perovskite NCs and underscores the important role of defects in affecting their structures and luminescent propertiesSecondly,we studied the local structure of Yb3+ions in Yb-doped CsPbCl3 NCs based on the analysis of X-ray absorption fine structure(EXAFS)spectra and confirmed that Yb3+occupied the Pb2+ site in NCs.In order to study the factors governing the doping efficiency,we proposed a one-pot synthesis strategy that is combined with a multi-step centrifugation process to prepare NCs with different sizes and defect concentrations.It is found that the doping concentration of Yb3+ ions in large-sized NCs is significantly higher than that of small-sized ones.Then,we further clarified the relationship between structural defects and doping efficiency using EXAFS and positron annihilation spectroscopy,which leads us to propose the mechanism of defect-assisted doping.At the same time,we found that the lanthanide-doped CsPbCl3 nanocrystals also follow the mechanism of defect-assisted doping.Combined with the systematic characterizations of photophyscical properties,we explained the anomalous changes of visible fluorescence lifetime and anti-thermal quenching phenomena in these doped NCs.Our results not only clarify the local structure of doped ions in CsPbX3 NCs,but also deepen the understanding of the factors that govern the doping efficiency of halide NCs.