Methylammonium Lead Bromide Perovskite Nanocrystals:Preparation,and Characterization Of Their Optical Properties And Stabilities

Nano-materials have wide application prospects in optoelectronics,sensing,catalysis and other fields because of its unique physical and chemical properties.Specifically,methylammonium lead bromide perovskite nanocrystals(CH3NH3Pbr3 PNCs)have attracted much attention due to their tunable wavelength,narrow emission and high photoluminescence quantum yield,which opens up new possibilities for optoelectronic devices.However,nanocrystals(NCs)are often less stable because of their large surface-to-volume(S/V)ratio and high percentage of surface atoms,resulting in an increase in surface defects.Surface defects lead to exciton trapping states which affect the optical properties of perovskite nanocrystals.And the instability of the material to water,ultraviolet light,temperature,oxygen and other factors,as well as ion migration,may lead to crystal growth,thus reducing the photoluminescence quantum yield,which poses a major challenge for commercial applications in lighting and backlight display.Current research results prove that surface passivation of CH3NH3Pbr3 PNCs is crucial to achieve high stability and desired optical properties.CH3NH3Pbr3 PNCs were prepared with a ligand-assisted reprecipitation method by using different capping ligands,which can passivate the surface defects of perovskite to improve its optical properties and stability in order to obtain high-efficiency and stable perovskite light-emitting devices.The main contents of the research include:(1)The surface properties of nanocrystals are the key factors to determine their optical properties and stability.Melamine,a low-cost and short-chain triazine analogue,was used as a surface passivation capping ligand to prepare methylamine lead perovskite nanocrystals(CH3NH3Pbr3lMela PNCs)by ligand-assisted reprecipitation method.X-ray diffraction(XRD)and high resolution transmission electron microscopy(HRTEM)results show that the prepared perovskite nanocrystals are indeed cubic CH3NH3PbBr3 crystal structure,and the size distribution of CH3NH3PbBr3/Mela-0.04 PNCs is uniform.The UV-vis and PL spectra show that the absorption and fluorescence emission of CH3NH3PbBr3/Mela PNCs are adjustable:in the visible range.The fluorescence emission peak is narrower(?19 nm),and the relative photoluminescence quantum yield is greatly increased.The surface properties of CH3NH3PbBr3/Mela PNCs were analyzed by X-ray photoelectron spectroscopy(XPS)and Fourier transform infrared spectroscopy(FT-IR)to infer the ligand passivation mechanism.A synergistic effect between NH3+ and the electron-rich nitrogen atoms,likely contributes to enhance the PL by effectively passivating of the trap states of CH3NH3PbBr3/Mela PNCs.Furthermore,melamine-capped CH3NH3PbBr3/Mela PNCs show high stability in protic solvents as a result of the steric bulkiness of the triazine rings,owing to the planar structure together with hydrogen bonding of melamine,which prevents solvent molecules from reaching and reacting with the core of CH3NH3PbBr3/Mela PNCs.This study demonstrates a simple and effective approach for stabilizing CH3NH3PbBr3 for potential applications such as solar cells and light-emitting diodes(LED).(2)Compared with monodisperse nanocrystals,self-assembled nanocrystalline materials have intergranular connectivity,which has an important impact on charge transfer and transmission in device applications.Therefore,they can provide some unique properties and additional functions.Tridentate ligands(L-cysteine)with amino,carboxyl and thiol groups were used as surface passivation capping ligands to prepare methylamine lead perovskite nanocrystals(CH3NH3Pbr3/Cys PNCs)with excellent optical properties.According to UW-Vis and PL,the size and wavelength and color in the visible region of CH3NH3PbBr3/Cys PNCs can be adjusted with different concentration of L-cysteine.The relative photoluminescence quantum yield can reach 53.7%.Importantly,the introduction of functional ligands that are beneficial for the assembly of perovskite nanocrystals,has opened up new avenues for the fabrication of highly efficient optoelectronic and photovoltaic devices.Based on the optimal concentration of L-cysteine,Cubic self-assembly of methylamine lead perovskite(PNCS-Cys-60)was synthesized by liquid phase method.XRD and HRTEM results show that a.prolonged aging time to 60 hours eventually leads to generation of cubic the self-assembly of PNCS-Cys-60 with excellent luminescent properties composed of numerous pre-formed spherical CH3NH3PbBr3 PNCs.The results of UV-Vis and PL analysis showed that the absolute photoluminescence quantum yield increased from 2.4%to 16.6%and the fluorescence intensity increased by 7.5 times with prolonging aging time to 60 h.The relative fluorescence intensity of PNCs-Cys-60 remained above 73%for 2 h:in ethanol solvent,indicating that the optical properties and stability of PNCs-Cys-60 self-assembly were excellent.Time-resolved fluorescence analysis(TRPL)showed that the slow decay time(?2)of PNCs-Cys-60 perovskite self-assembly was 642 ns.XPS and FT-IR analysis shows that the synergistic effect and cross-linking effect of amino(-NH2),carboxyl(-cCOOH)and sulfrydryl(-srSH)groups in L-cysteine,which not only guide the self-assembly process by cross-linking but also protect the individual nanocrystals from aggregation due to strong binding interactions between the L-cysteine ligands and the nanocrystals.Furthermore,PNCs-Cys-60 supercrystals have been used in the fabrication of white light-emitting diodes(WLED)and green LED with excellent performance.The self-assembly process of CH3NH3PbBr3 PNCs initiated by L-cysteine crosslinking is promising for.rational design and controllable fabrication of functional perovskite-based materials for optoelectronics applications.