Preparation Of All Inorganic Perovskite Nanocrystals And Their Optical Properties And Stability

All-inorganic lead halide perovskite（Cs Pb X₃,X=Cl,Br,I）nanocrystals（NCs）are potential emitting materials for illumination and display applications,owning to the low preparation cost,high photoluminescence quantum yields（PLQYs）,narrow emission full-width at half-maximum（FWHM）,tunable PL emission across the entire visible spectral region,and high defect tolerance.Therefore,fully excavating its excellent optical quality and improving its stability have become the focus and frontier research topics.In this paper,focuses on the above two key issues,we firstly optimized the newly developed halogen-hot-injection method,and the PNCs with high PLQYs（～100%）and high luminous color purity with the FWHM as low as 9 nm were controllable synthesized by fine regulating the reaction conditions.Then,a series of Mn²⁺doped Cs Pb X₃（Mn:Cs Pb X₃）NCs were synthesized,and the halogen content related luminescence kinetics process in Mn:Cs Pb X₃PNCs was deeply disscussed.At last,in order to improve their water stability,the Cs Pb X₃and Mn:Cs Pb Cl₃NCs were further coated by polystyrene（PS）to form PNCs@PS films.The main research contents and conclusions are as follows:（1）Cs Pb X₃（X=Cl,Br）NCs were synthesized by halogen-hot-injection method,and the optical properties were optimized by fine control of key reaction conditions such as temperature and the amount of halogen precursor.The（PLQYs）of intrinsic Cs Pb Cl₃NCs can reach 64%,and the FWHM is as narrow as～9 nm,which is the narrowest one reported in literatures.The PLQYs of the as-prepared Cs Pb Br₃NCs is as high as～100%,and the FWHM is～18 nm,which is comparable to the optimal value reported in the literatures.The excellent optical properties can be attributed to the flexible volume of the halogen precursor,which enables the most effective passivation of the non-radiative recombination centrals（the halogen vacancy defects）of the NCs surface.（2）Thanks to the recently developed synthesis approach of“halogen-hot-injection”for Cs Pb X₃NCs,which can independently control halogen content to optimize the optical quality of pure Cs Pb X₃NCs.A series of Mn²⁺-doped Cs Pb Cl₃samples were synthesized with different oleylamine-Cl（OAm-Cl）contents based on a halogen-hot-injection strategy.It is found the Mn²⁺concentration of as-prepared samples fixed at 0.65%.The kinetic process of exciton luminescence was analyzed by comparing their optical properties.We find that the energy transfer process from exciton to Mn²⁺ions show a halogen concentration dependent efficiency,indicating that the non-radiative recombination of exciton to halogen vacancy defect states is competitive with the energy transfer to Mn²⁺.However,Mn²⁺emission shows single exponential decay and lifetime is～1.78ms,indicating that the crystal field environment around Mn²⁺ion is uniform with negligible defects.When the test temperature rises from 80K to room temperature,the intensity of Mn²⁺emission increases,indicating that sufficient OAm-Cl can well passivate Cl vacancy and make the non-radioluminescence center of the deep defects become shallow ones.By changing the Ratio of Cl/Br,Mn²⁺doped Cs Pb(Cl_xBr_1-x)₃NCs with PLQY up to 90%were obtained.（3）In order to improve the environmental stability of as-prepared PNCs,the PNCs were coated in hydrophobic and transparent PS to obtain PNCs@PS film.The stability test showed that the PLQYs and PL contours of Cs Pb Cl₃@PS film remained unchanged after 35 days in air.For Cs Pb Br₃@PS film,the PLQYs still remain 93.6%of its original value after 35 days in atmosphere,and PL contours remained basically unchanged.After soaking in water for 40days,its PLQYs could retain 69.4%of the original value.For Mn:Cs Pb Cl₃@PS films,the PLQYs decreased to 69%and 33%of the original value after 30 days in the air and in water,respectively.Compared with uncoated nanocrystals,PS coating can effectively improve the water stability of perovskite nanocrystals.