Synthesis, Luminescence Properties And Application Of Highly Stable Lead Halide Perovskite Nanocrystals

As one of the most promising materials in the 21st century,lead halide perovskite nanocrystals have attracted extensive attention from all over the world.Due to its excellent photoelectric properties such as narrow-band emission,adjustable bandgap,high quantum efficiency,and large absorption cross-section,lead halide perovskite nanocrystals have great application potential in lighting display,solar cells,photodetectors and so on.Although the research on lead halide perovskite nanocrystals has been developed vigorously in recent years,its practical application is limited due to its poor stability.Because lead halide perovskite nanocrystals are ionic crystals,and their intrinsic formation energy is low,so they are easy to decompose when stimulated by water,oxygen,light and other factors in the environment.Moreover,with the decomposition of nanocrystals,it will also lead to the leakage of Pb²⁺,which is very harmful to the environment and organisms.Therefore,improving the stability of lead halide perovskite nanocrystals is one of the most important challenges for enhancing their luminescence properties and expanding their application range.To solve this problem,researchers have developed various methods to prepare lead halide perovskite nanocrystals with high stability and high photoluminescence quantum efficiency.From the perspective of nanostructure and composition,commonly used strategies can be classified into the following two types:surface engineering and compositional engineering.On the one hand,the surface defects of perovskite nanocrystals are passivated and their stability is improved by wrapping the core by inert shell or adding surface ligands.On the other hand,the formation energy values of the nanocrystals can be elevated by doping ions,and then the stability can be improved.However,there is still a lack of systematic research on fine synthesis,morphology,stability and luminescence properties for specific applications.Based on this,the following studies were carried out in this thesis:(1)For the application of lead halide perovskite nanocrystals in the field of two-photon fluorescence bioimaging,Si O₂-coated CsPbBr₃ core-shell nanocrystals(CPB@SiO₂ NCs)with uniform morphology,monodisperse and green-emitting were synthesized by one-step method,and the thickness of SiO₂ shell was precisely controlled within the range of 15-60 nm.The effects of SiO₂ shell thickness on single-photon and two-photon luminescence properties of NCs were systematically studied by TEM,PL spectra,fluorescence lifetime,two-photon fluorescence spectra,and power dependence curves.The results show that SiO₂ shell can inhibit the leakage of Pb²⁺and improve the single-photon and two-photon luminescence performance,as well as stability of nanocrystals.Meanwhile,cytotoxicity experiment and two-photon cell imaging were used to explore the possibility of the core-shell nanocrystals as two-photon bioimaging probes.This one-step method can accurately control the thickness of Si O₂ shell,and the synthesized CPB@SiO₂ NCs has good biocompatibility,which has the potential of application in two-photon fluorescence bioimaging.(2)For the application of lead halide perovskite nanocrystals in the field of white LED lighting,pure phase Cs Pb Cl₃,pure phase Cs₄PbCl₆ and mixed-phase CsPbCl₃-Cs₄PbCl₆perovskite nanocrystals were successfully synthesized according to different reactant ratios,and Mn²⁺doped pure phase and mixed-phase perovskite nanocrystals with orange emission properties were synthesized by using MnCl₂·4H₂O as manganese source.The morphology,phase,and microstructure of the samples were analyzed by XRD,TEM,UV-vis,and XPS.Combined with PL spectra,fluorescence lifetime,and temperature-dependent PL spectra,the effects of Cs₄PbCl₆ content in the mixed-phase on the luminescence properties and stability of the synthesized nanocrystals were analyzed.The results show that Cs₄PbCl₆ in the mixed-phase can optimize the growth process of CsPbCl₃ NCs,reduce the formation of defects,improve the luminescence performance and stability of the system.The internal quantum efficiency of the mixed-phase sample is 3 times that of the pure CsPbCl₃ sample,and the energy transfer efficiency of the exciton to Mn²⁺is 1.4 times that of the pure CsPbCl₃ sample.Furthermore,a near-UV excited white LED device has been successfully assembled by the mixed-phase nanocrystals combined with appropriate blue and green commercial phosphors.The white LED device has excellent properties such as warm white light emission,low color temperature,high color rendering index,and light color stability,which confirms the application potential of Mn²⁺doped mixed-phase perovskite nanocrystals in the field of white LED.