Investigation On Luminescence Regulation And Stability Of Metal Halide Perovskite Nanocrystals

All inorganic lead halide perovskite（Cs Pb X₃,X=Cl,Br,I）nanomaterials show broad application prospects in lighting,display,fluorescent probe and other aspects,owing to the advantages of large absorption cross section,narrow emission spectral line,high photoluminescence quantum yields,tunable emission wavelength,etc.,which become a research focus in the field of photoelectric materials and devices.However,lead halide perovskite is easy to suffer from agglomeration,phase transition and degradation under exposure to water,oxygen,heat and light radiation,leading to luminescence peak deviation,broadening or fluorescence quenching.In addition,halogen ion exchange is easy to occur in the mixing of different components of perovskite nanomaterials due to their ionic properties,which affects the luminescent quality.The poor stability of lead halide perovskite seriously hinder the practical application in photoelectric devices.Therefore it is a very important and practical research topic to improve the luminescent stability and properties of lead halide perovskite nanomaterials.The improvement of luminescent stability of lead halide perovskite is focused on in this paper.In normal the stability improvement strategies include inorganic porous material coating,ligand passivation,organic packaging and multiple strategies are used to improve the stability of perovskite materials.The microscopic structure,luminescent properties and stability improvement mechanism of perovskite composite materials are studied systematically.Besides,the optical properties of lead-free double perovskite nanomaterials are also studied in view of the lead toxicity of lead halide perovskite.The main research contents are listed as follows:（1）CsPbBr₃nanocrystals（NCs）are grown on flower-like TiO₂microspheres by means of an in situ adsorption approach.The as synthesized TiO₂-CsPbBr₃composites show better water and thermal stability than pure CsPbBr₃NCs.The TiO₂matrix plays a good role in protecting the embedded CsPbBr₃NCs against water degradation and thermal decomposition.In addition,the method is universal.Cs Pb X₃NCs composed of different halides are successfully anchored into TiO₂matrix to construct TiO₂-Cs Pb X₃composites with tunable luminescence colors and stable hybrid structures,halogen anion exchange can be effectively inhibited.（2）CsPbBr₃NCs are successfully encapsulated into hollow YF₃:Eu³⁺porous nanospheres by hot injection method,and multi-color emitting CsPbBr₃+YF₃:Eu³⁺monodisperse nanocomposites are obtained.The emission spectrum of the composites is composed of blue,green and red bands,which are attributed to the STE emission of YF₃matrix,exciton emission of CsPbBr₃and Eu³⁺f-f transition emission,resulting in white light emission from a single matrix.Cs Pb X₃composed of different halides can be also combined with YF₃:Eu³⁺,leading to tunable luminescence color.According to the stability results,the CsPbBr₃+YF₃:Eu³⁺nanocomposites exhibit better water,light and thermal stability than pristine CsPbBr₃NCs.The composite strategy with YF₃:Eu³⁺porous spheres can not only improve the stability of Cs Pb X₃NCs,but also bring rare earth ion luminescence characteristics to Cs Pb X₃NCs.This study provides a feasible technical path to solve the difficulty of rare earth ions entering into Cs Pb X₃NCs lattice and to make full use of the luminescence advantages of rare earth ions.（3）CTAB-F solution is used to passivate CsPbBr₃NCs,and the phase structure,crystal grain morphology and fluorescence properties of the passivated CsPbBr₃NCs have been studied.Benefited from the dual passivation of CTA⁺long alkyl chain and F^-,CsPbBr₃NCs show both significantly improved PLQY and better thermal stability and water resistance.The encapsulation of polystyrene dense network is adopted to prevent the erosion of water on CsPbBr₃NCs and further improve its humidity stability.It is proved that the combination of passivation and cladding strategy can improve the stability more comprehensively and effectively.（4）Bi³⁺,Sm³⁺and Mn²⁺ions are successfully incorporated into Cs₂Ag_0.4Na_0.6In Cl₆lead-free double perovskite crystals by the precipitation method.The effects of these ions on the structure and optical properties of the double perovskite crystals are investigated.Bi³⁺doping can effectively adjust the optical band gap of double perovskite,and significantly improve the luminescence efficiency,the best PLQY reaches 97.33%.After doping Sm³⁺in Cs₂Ag_0.4Na_0.6In Cl₆,both the broad peak from STE of the double perovskite host and the characteristic luminescence peaks of⁴G_5/2-⁶H_J（J=5/2,7/2,9/2）transition of Sm³⁺appear in the PL spectra.In addition,Mn²⁺doped samples exhibit dual emission centered at 538 nm and 618 nm,resulting from the STE emission of the double perovskite host and the⁴T₁→⁶A₁level transition of Mn²⁺,respectively.The PLQY increases by 100%after Mn²⁺doping.These results confirm that metal,transition metal and rare earth metal ions can effectively modulate the energy level structure,band gap,luminous efficiency and luminous color of double perovskite crystals.The as prepared ions doped Cs₂Ag_0.4Na_0.6In Cl₆double perovskite phosphor shows great potential for photoelectric applications.