Research On Luminescence Properties And Stability Of Lead Halide Perovskite Quantum Dots

In recent years,lead halide perovskite materials(CsPb X₃,X=Cl,Br,I)have become one of the most competitive research topics in the fields of display,detector and photo-voltaic due to their high defect tolerance,carrier mobility and photo-electric conversion efficiency.Perovskite quantum dots are expected to be the materials for the next generation of flexible devices due to their near 100%quantum yield and high color purity.In addition,other phases derived from perovskite such as hexagonal Cs₄PbBr₆and tetragonal CsPb₂Br₅have been observed in various forms.Studies have shown that these perovskites have structural-related physical properties,which have greatly expanded their application in the fields of sensing,catalysis and photo-electricity.Recent studies indicate that CsPb₂Br₅plays an important role in improving the lifetime,stability and solar cell efficiency of CsPbBr₃.Cs₄PbBr₆also exhibits excellent fluorescence contrast and reversibility.This paper aims at changing ligand and solvent polarity to improve the morphology,composition and stability of quantum dots,the main research contents are as follows:1.CsPbBr₃/CsPb₂Br₅composite nanocrystals(NCs)was prepared with oleic acid and cetyl trimethyl ammonium bromide(CTAB).By changing the polarity of bad solvent,found that the morphology of NCs changed from quantum dots to nanosheets,phase structure from CsPbBr₃/CsPb₂Br₅into CsPb₂Br₅phase,emission spectra and Urbach energy calculated with the absorption showed obvious solvent polarity dependence,transient fluorescence lifetime also has the change.When the phase ratio of CsPbBr₃to CsPb₂Br₅is about 0.2,quantum dots(CsPbBr₃)were embedded in nanosheets(CsPb₂Br₅),and the luminescence performance is optimized.Nanocrystals resistant to polar solvents were prepared by this method,and the spectral regulation within the range of 450-600 nm was realized.2.CsPbBr₃quantum dots(QDs)were prepared by oleylamine(1)and oleic acid/oleylamine(2),respectively.The effects of ligand changes on QDs were studied.By comparison,it is found that there is no obvious difference between the two in crystal structure and luminescence performance,and the band gap is 2.34 e V.However,the(2)film has low brightness and poor stability in the air.The(1)film is kept in the air for three days without any change in brightness,but the fluorescence quenching occurs immediately in the water.In addition,CsPbBr₃@Si O₂QDs(3)was made by wrapping the shell on the surface of(1),the size of(3)was found to increase,the band gap was reduced to 2.30 e V,and the emission peak was slightly red-shifted.(3)films are stable in both air and water.The stability of(3)in air and water was improved by changing both ligand and enclosure,but the(3)film quality was reduced by the hydrolysis of ethyl orthosilicate.3.Cs₄PbBr₆microcrystals(MCs)were prepared without the ligand.For MCs obtained by regulating the polarity and proportion of the bad solvents,found that the size of MCs increases with the content of hydrobromic acid,and it is believed that hydrobromic acid causes the nucleation and growth of MCs.The low polarity reagent is not conducive to the uniform size of MCs,but the high polarity reagent will accelerate the nucleation-growth process to obtain larger and uniform of MCs.The crystal structure of MCs does not change significantly,but the hydrobromic acid content will affect the growth of the MCs along the(012)surface.The luminescence spectra shows that the luminescence intensity of MCs with small size is higher,and the ratio of diffraction peak intensity corresponds to the change of luminescence intensity.This method shows that the reagent polarity affects the growth law of the MCs and the size and growth orientation have important effects on the luminescence property.