| Lead halide perovskite（Cs Pb X3,X=Cl,Br,and I）nanocrystals（NCs）have been received great attention in the fields of tnext-generation high-resolution displays and lighting,solar cells and biological imaging for their outstanding properties including facile preparation,high photoluminescence quantum yield（PLQY）and wide tunable emission with exceptional color purity.However,the soft ionic nature of NCs confers their preference for ionic surface ligands（oleic acid/oleic amine）that exhibit highly dynamic binding and the surface ligands are easily detached from the surface during the purification and storage period.This leads to decrease of PLQY and colloidal stability.In addition,the formation of tight bonding between the surface of perovskite NCs and ligands usually requires a high temperature condition.This thesis seeks for surface ligands that can modify Cs Pb Br3 NCs at room temperature.According to covalent bonding theory,the alkyl carboxylic acid and alkyl amine groups may coordinate with lead and bromine respectively,passivating the surface defects of the nanocrystal.Bidentate carboxylic acid ligands,hybrid amine ligands and multidentate Boc-D-glutamic acid ligands containing both carboxyl and amino groups are selected and used to modify Cs Pb Br3 NCs.The thesis explores the mechanism of interaction between various ligands and the surface of NCs,analyzes and evaluates the effect of ligand modification on improving the luminescence efficiency and stability of nanocrystals.In addition,in-situ growth of silica modified Cs Pb Br3 NCs is also investigated.The main researches of the thesis are as follows:（1）Bidentate carboxylic acid modified Cs Pb Br3 nanocrystals with enhanced luminescence and stability.This work proposed the synthesis of stable Cs Pb Br3 NCs by introducing a novel double-terminated ligand,4,4’-Azobis（4-cyanovalericacid）at room temperature.Theoretical calculations show that the bidentate acid ligand can anchor the NCs surface with a high binding energy of 2.47 e V,which is much larger than the 1.15 e V achieved by oleic acid modified NCs surface.Thus,the chelation effect ascribed to the bidentate acid ligand enables stabilization of Cs Pb Br3 NCs accordingly.Experiments also confirm that the PLQY of the Cs Pb Br3 NCs sample modified with bidentate acid ligand increases to 72%,which is higher than that of the NCs sample modified by oleic acid（38%）.Benefiting from the bidentate acid ligand firmly anchored to NCs surfaces,the Cs Pb Br3 NCs film sample exhibits higher water stability and it can remain stable for 720 min in water.In comparison,the pristine Cs Pb Br3 NCs sample shows luminescence quenching after 120 min.The resistance of the modified sample to ethanol,light and heat is also improved.（2）Regulated the luminescence and stability of Cs Pb Br3 nanocrystals by hybrid ligand.A hybrid ligand（DDAB/Zn Br2）passivation strategy was adopted to reconstruct the ligand chemistry of Cs Pb Br3 NCs.Experimental results show that the hybrid ligand can detach the original long-chain ligands from the perovskite surface and exchange the loose ligand,anchoring to the nanocrystal surface tightly.The proton transfer process which results in the instability would be suppressed effectively.Moreover,the hybrid ligand can supply additional Br to passivate the surface trap.The Cs Pb Br3 NCs treated with hybrid ligand can achieve a high PLQY of 95%.The luminescence intensity of original Cs Pb Br3 NCs decrease more than 60%after 14 days,while the treated Cs Pb Br3 NCs could maintain 85%of the initial intensity.The surface modified Cs Pb Br3 NCs sample also shows improved resistance to UV irradiation and thermal treatment.（3）Multidentate ligand treatment enables enhanced photoluminescence and stability of Cs Pb Br3 nanocrystals.A multidentate ligand of Boc-D-glutamic acid was explored to modify the Cs Pb Br3 NCs,achieving nearly 100%PLQY.Analysis show that this is due to the surface defect passivation and an efficient ligand exchange which achieves robust affinity and surface ligand compensation.The modified NCs sample show a 10%PL intensity reduction after 60 days storage at ambient conditions in contrast to 80%PL intensity decrease of the pristine NCs.Also,no obvious phase transformation occurs after 120 days in air while this was observed in untreated NCs sample after 14 days,demonstrating excellent structural stability of the modified sample.The resistance to various external stimuli such as polar solvents,thermal treatment or UV irradiation is improved.In addition,the as-fabricated white light-emitting diode（WLED）exhibits a high luminous efficiency of 93.5 lm/W,which can be used as phosphors for solid-state lighting application.（4）Modification of Cs Pb Br3 nanocrystals via In-situ growth of Si O2 layer and luminescence efficiency and stability investigations.Stability enhancement of perovskite nanocrystals with conventional Si O2 coating method suffered from long reaction times and complicated preparation procedure.In this research,a facile room temperature one-pot approach was developed to fabricate Cs Pb Br3/Si O2 NCs,which involves addition of Tetramethoxysilane（TMOS）in the reaction system in advance.Due to the surface passivation,the PLQY of the Cs Pb Br3/Si O2 NCs increase to 90%（the pristine Cs Pb Br3 NCs sample has a value of 70%）.The Si O2 layer formed by TMOS hydrolysis and condensation reactions can also protect the Cs Pb Br3 NCs from the external environment stimuli.Compared to the pristine Cs Pb Br3 NCs,the air stability,structural stability,polar solvent resistance and thermal stability of the Cs Pb Br3/Si O2 NCs are enhanced evidently. |
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