Preparation And Optical Characteristics Of Fluorescent Lead Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals(NCs)with the structure of ABX₃ has many excellent optical properties including adjustable emission wavelength,ultra-wide spectral absorption range,extremely narrow fluorescence emission spectrum,high carrier mobility with long fluorescence lifetime etc.They have been broadly used to fabricate optoelectronic devices,such as solar cell,laser and LEDs.However,their applications are limited greatly due to their inherent instability.It is still a great challenge to improve their stability.In this thesis,we focused on the synthesis of the high stable lead halide perovskite nanocrystals and their optic characterization,mainly including the following three parts.Firstly,we developed a new method for the synthesis of lead halide perovskite nanocrystals mediated by various amines,which Cs Pb Br₃ nanocrystals(NCs)were prepared in dimethyl sulfoxide(DMSO)and hydrobromic acid aqueous solution in the presence of amines.The composites mainly content Cs Pb Br₃ and Cs₄Pb Br₆,which the Cs Pb Br₃ NCs were wrapped inside the non-luminous Cs₄Pb Br₆.Amines regulate efficiently the morphologies of the composites with improving the stability and optical properties of Cs Pb Br₃NCs.Secondly,we developed a novel method for the synthesis of lead halide perovskite nanocrystals in the present of 2-methylimidazole(2-MIM).Pb Br₂ reacted with Cs Br or MABr(MA=CH₃NH₃⁺)to form MPb Br₃@Pb Br OH(M=MA,Cs)in water,and the reaction process without needing other solvents.During the reaction,Pb Br OH was rapidly formed and encapsulated MPb Br₃ NCs inside it,which worked as a good protector on MPb Br₃ NCs and made MPb Br₃ NCs have high stability to water,high temperature and ultraviolet light.In addition,the low content of internal MPb Br₃ NCs were dispersed in the Pb Br OH matrix,preventing the agglomeration of NCs in the Pb Br OH matrix.Density functional theory(DFT)calculations showed that the positive energy consumption of water molecules entering the Pb Br OH lattice,which was much higher than that of MPb Br₃.The surface Pb Br OH acted as a water-proof layer to prevent water entering the interior and damaging the perovskite nanocrystals.Thirdly,the effects of other imidazoles to the reaction of MBr with Pb Br₂ were investigated in detailed.Luminescent MPb Br₃@Pb Br OH NCs were obtained as well.The optical properties of MPb Br₃ NCs in the presence of 2-ethylimidazole,2-propylimidazole or 2-ethyl-4-methylimidazole showed the better luminescence properties than imidazole and 1-methylimidazole,indicating that imidazole compounds substituted on the second position of imidazole have the better effect for the preparation of lead halide perovskite fluorescent nanocrystals.