Synethesis And Photoluminescence Of CsPbX₃ Nanocrystals With High Stability

Peculiar properties of lead halide perovskites,including high photoluminescence quantum yield(PLQY),size-tunable band gap,narrow emission peak width,and long charge carrier diffusion length,render them potential applications in light-emitting diodes,solar cells and photodetectors.However,the poor stability of perovskite nanocrystals makes them prone to agglomeration,degradation and phase transition under environmental conditions(polar solvents,light,heat,etc.),which restricts the further development of lead halide perovskites.Therefore,it is still urgent for scientists to improve the stability of lead halide perovskite nanocrystals.In this paper,the stability of CsPbX₃(X=Cl,Br,I)nanocrystals has been improved by controlling the crystal growth,surface modification and encapsulation.The temperature-dependent photoluminescence of these nanocrystals has been investigated in details.On this basis,the potential application of CsPbX₃ nanocrystals in multiple fluorescence anti-counterfeiting has been explored via combining with rare-earth upconversion nanocrystals.The main contents can be summarized as follows:(1)Synethesis and temperature-dependent photoluminescence of orthorhombic(?-)CsPbX₃ nanocrystals with high stability.At room temperature,cubic(?-)CsPbI₃ easily transforms from the cubic phase to the non-perovskite phase,resulting in severe luminescence quenching.On this account,we choose 3-aminopropyltriethoxysilane(APTES)with short chain as the surface ligand to prepare?-CsPbI₃ nanocrystals with different sizes.The stability of these?-CsPbI₃ nanocrystals upon exposure to polar solvent and heat has been investigated.The morphology and crystal phase of the nanocrystals before and after treatment have been characterized by XRD and High-Resolution TEM.These?-CsPbI₃ nanocrystals present significantly higher stability than?-CsPbI₃ nanocrystals in both polar solvents and heating process(up to 375 K).The?-CsPbI₃ nanocrystals still maintains more than 80%of their initial PL intensities after dispersing in ethanol containing solvent for 24 h or after heating/cooling cycles.While?-CsPbI₃ nanocrystals quickly turn into nonperovskite phase after being exposed to ethanol or heated to 375 K.Low-temperature steady-state/transient fluorescence spectra demonstrate that?-CsPbI₃ nanocrystals have size-dependent exciton/electron-phonon coupling interactions.(2)Synthesis of CsPbX₃@Oxide core-shell nanoparticles through bridging ligands.Oxide coating is a common method to improve the stability of semiconductor nanoparticles.It is still challenging to develop a universial method for preparing CsPbX₃@Oxide core-shell nanocrystals due to the different stability of CsPbX₃nanocrystals.As a surface capping ligand for preparing highly stable CsPbX₃ nanocrystals,APTES is also a typical bridging ligand for binding nanoparticles with oxide surface.Monodisperse CsPbI₃@Si O₂ core-shell nanocrystals are prepared by controlling the hydrolysis and cross-linking of APTES and tetramethoxysilane(TMOS).We find that hydrolysis control of TMOS is important for successful coating of Si O₂ shell via varying ammonia concentration,ethanol dosage and temperature.This method is not only suitable for preparation of CsPbX₃@Si O₂(X=Cl,Br,I)core-shell nanocrystals,but also suitable for preparing CsPbI₃@Ta₂O₅ and CsPbI₃@Zr O₂core-shell nanocrystals by using tantalum ethoxide and zirconium propoxide as precursors,demonstrating the universality of this coating method.Upon exposure to water,ultraviolet light or heated,CsPbI₃@Oxide core-shell nanocrystals show better stability than uncoated CsPbI₃ nanocrystals,indicating that the oxide shell can effectively protect the nanocrystals,and improve the stability of CsPbI₃ nanocrystals.(3)Photoluminescence performance of CsPbX₃/Rare-earth doped upconversion nanocrystal composites.Based on the above work,Na YF₄:Tm@Na Yb F₄@Na YF₄(CSS:Tm)nanocrystals with high upconversion luminescent(UCL)intensity and efficiency are used to sensitize the highly stable CsPbX₃ nanocrystals,and the near-infrared-triggered photon upconversion of CsPbX₃ nanocrystals is realized.The full-spectrum luminescence of CsPbX₃ nanocrystals in visible region triggered by near-infrared light is realized by changing halogen composition and concentration of CsPbX₃ nanocrystals.PL decays show that radiative energy transfer is predominant,and the lifetimes of CsPbX₃ nanocrystals are thus greatly prolonged.More importantly,the photoluminescence intensity of Cs Pb(Br/I)₃nanocrystals increases faster than that of CSS:Tm as the temperature decreases,the emission of the composite consequently changes from dark blue to purple red.Moreover,the UCL of CSS:Tm nanocrystals is power-dependent,and the emission color of the composite also changes with excitation power in a certain range.Therefore,the emission color can be adjusted via temperature,excitation power and excitation wavelength.The composite has real-time and dynamic multi-color modulation characteristics,which can be used in high-quality and high-security multimodal anti-counterfeiting.