Green Synthesis And Luminous Properties Of Cs_xPb_yBr_z Perovskite Nanocrystals

All-inorganic perovskite nanocrystals（NCs）exhibit great application potentials in the fields of next-generation displays and solid-state lighting due to their excellent optical properties such as high photoluminescence quantum yield（PLQY）,tunable luminescence spectrum,narrow full width at half maximum（FWHM）.Supersaturated recrystallization method is a convenient method for synthesizing Cs Pb Br₃ NCs at room temperature.However,its large-scale application is hampered by the usual utilization of toxic solvents such as N-N dimethylformamide（DMF）and toluene to a certain extent.Meanwhile,perovskite NCs are susceptible to the exposure to water,oxygen,heat and light,resulting in the rapid degradation of device performance.Aiming at the current problems involving material preparation and stability for Cs Pb Br₃ NCs,this work developed a new green solvent system for preparing high-quality Cs Pb Br₃ NCs.The stability of the Cs Pb Br₃ NCs material was enhanced by strategies including in-situ loading into Eu-MOF mesoporous materials and in-situ encapsulation with low-dimensional perovskite phases.A microfluidic synthesis method with advantages in the continuous and large-scale preparation of high-quality perovskite NCs was proposed.The main results are summarized as follows:1.Based on the room temperature supersaturated recrystallization（RTSR）method,Cs Pb Br₃NCs were prepared with different precursor concentrations and precursor additions by using the acetic acid and toluene solvent systems.Then,the influences of antisolvent polarity on the morphology,crystallinity and optical properties of Cs Pb Br₃ NCs was investigated by introducing the polar green solvent ethyl acetate（EA）mixed with toluene to modulate the polarity of the antisolvent.It was found that the higher precursor concentrations or larger precursor additions resulted in the formation of Cs Pb₂Br₅ impurity phases,which were not conducive to the preparation of pure-phase Cs Pb Br₃ NCs.High quality Cs Pb Br₃ NCs with 85%PLQY were prepared at the precursor concentration of 0.4 mol/L and the precursor addition of 200 u L.It was initially demonstrated that the polarity of antisolvent is a key factor affecting the orientation growth of Cs Pb Br₃NCs,and the morphological transformation of Cs Pb Br₃ NCs from nanorods to nanowires was achieved by controlling the polarity of antisolvent.2.The new green solvent system was constructed by using low-toxic and different polarity green solvents such as ethyl acetate,propyl acetate and butyl acetate as anti-solvents to completely replace the toxic toluene,and one-dimensional Cs Pb Br₃ NCs with different morphologies were successfully synthesized.The length of the Cs Pb Br₃ NCs was continuously tunable in the range of14-316 nm,and the width in the range of 3-29 nm by adjusting the polarity of the anti-solvent and reaction time.This morphological or size tunability is mainly due to the combined effect of solvent polarity on precursor solubility and ligand adsorption equilibrium.Due to the strong quantum confinement effect,the obtained Cs Pb Br₃ NCs exhibited a wide photoluminescence emission（471–508 nm）and high PLQY up to 78%.Finally,cyan emission of Cs Pb Br₃ NCs（480 nm）is combined with blue,green,and red emissions from commercial powders for the fabrication of white light emitting diode（WLED）.The device displays a super high color rendering index of 96.5and excellent vision performance.3.High quality and highly stable Eu-MOF/Cs Pb Br₃ composites were prepared by using Eu-MOF to encapsulate Cs Pb Br₃ NCs based on the preparation of Cs Pb Br₃ NCs via the all-green solvent system.It was indicated that the crystal splitting growth mechanism dominates the evolution of Eu-MOF,and the synergistic effect of Al³⁺and Ac^-ions regulates the crystal splitting growth of Eu-MOF.Controlled preparation of three-dimensional hyperbranched structures of Eu-MOF with very large porosity can be achieved at room temperature by altering the amount of Al（Ac）₃ additive.It was demonstrated that acidic acetates were the key to obtaining 3D hyperbranched structures Eu-MOF.Then,Cs Pb Br₃ NCs were uniformly loaded inside the Eu-MOF structure by in-situ growth.Cs Pb Br₃ exhibits excellent optical properties and good stability due to the passivation and protection from the Eu-MOF structure.A ratiometric fluorescence probe for the detection of the organic solvent DMF was constructed by using the dual emission centres of Eu-MOF and Cs Pb Br₃,and the sensor exhibits excellent linearity and high sensitivity.4.Based on the RTSR method,a new technology for the simple and continuous synthesis of Cs_xPb_yBr_z-based perovskite（including Cs₄Pb Br₆,Cs Pb Br₃/Cs₄Pb Br₆,Cs Pb Br₃,Cs Pb Br₃/Cs Pb₂Br₅,Cs Pb₂Br₅）was developed by using a microfluidic system.Adjusting the flow rate of the precursor in the reaction is a convenient way to modify the concentration ratio of precursor and ligand,allowing real-time control of the ratio of caesium and lead sources in the system,thus enabling the transformation between different dimensional perovskite phases.It was found that Cs Pb Br₃/Cs₄Pb Br₆ and Cs Pb Br₃/Cs Pb₂Br₅ composite NCs demonstrate bright emissions and excellent stability in their solid form,where Cs Pb Br₃ NCs uniformly embedded in Cs₄Pb Br₆（or Cs Pb₂Br₅）matrix.The luminescence of the composites originates from the characteristic emission of Cs Pb Br₃ NCs,which is proved benefiting from energy transfer behavior.The highly stable Cs Pb Br₃/Cs₄Pb Br₆ NCs were used for the construction of WLED with a wide color gamut of 131%NTSC.