Synthesis Optimization And Crystalline Phase-Controlled Transformation Of Cesium Lead Halide Nanocrystals

In recent years,cesium lead halide Cs-Pb-Br perovskite nanocrystals?NC_S?exhibit excellent properties,such as shape-,size-and emission-tunability,high photoluminescence quantum yields?PLQYs?,highly tolerance to defects,narrow emission line width and facile synthesis.They have been appealing interests and portrayed as promising candidate for optoelectronic applications such as light-emitting diodes,solar cells,photodetector and lasers,etc.However,instability of CsPbBr₃ still limits the further application in photoelectric filed.For example,it is easy to decompose with light and heat.Especially in some harsh environment such as in polar solvents,the crystal lattice structure is contorted and disordered by the effects of polar molecules,thus loses optical properties.In contrast,due to the special structure,the derivatives of CsPbBr₃ such as hexagonal Cs₄PbBr₆ and tetragonal CsPb₂Br₅ which possess great thermal and wet stability,which has attracted the interest of researchers.However,there are still lack of systematic investigations into crystalline phase transformation between CsPbBr₃ and its derivations.Those different types of phases not only show diverse properties that are potential for further application of perovskite NCs,but also play an essential role in synthesis of perovskite NCs,helping community understand the underlying regulating effects of precursors.Herein,this thesis provides three parts discussing optimization and crystalline phase-controlled transformation of cesium lead halide NCs as followed:?1?We introduced APTES to replace OAm,and a thin protective shell was formed on the surface of nanocrystals by controlling the hydrolysis degree of APTES.We tested the stability of NCs in ethanol polar solvent and found that APTES-based NCs showed not only better optical properties,but also significantly improved the stability compared with OAm-based nanocrystals.?2?We applied Cs₂CO₃,Pb?AC?₂ and TMSBr as independent precursors in order to precisely tune the molarity ratios of precursors.By increasing the content of Cs alone,the uniform and regular“0D”Cs₄PbBr₆ nanocrystals were synthesized,and the reverse transformation from“0D”Cs₄PbBr₆ nanocrystals to“3D”CsPbBr₃ cubic nanocrystals was induced.?3?We investigated the effect of reaction time and the amount of Pb on the crystal structure of NCs.We firstly synthesized CsPb₂Br₅ nanosheets?NSs?which are 60-nm thick and over 1?m long through increasing the molarity ratios of Pb and prolonging the reaction time.The influence of the purification process on the purity of the final NSs was also explored.We found that by increasing the purification cycle,it is helpful to remove the redundant CsPbBr₃ NCs,and finally obtain the pure CsPb₂Br₅ NSs with strong green fluorescence.