The Preparation And Properties Of Lead-Free Perovskite Nanocrystals And Their Composites

In recent years,perovskite nanocrystals（PNCs）,such as CsPbX₃ and MAPbX₃（MA=CH₃NH₃,X=Cl,Br,I）have been uprising stars in optoelectronic field for their outstanding photoluminescence properties,including simple preparation,low cost,narrow emission,tunable bandgap,high photoluminescence quantum yield（PLQY）and high color purity.PNCs have already been applied in light-emitting devices（LED）,solar cells,laser,photoelectric detector,and etc.However,two serious problems of PNCs have hindered their applications and development.One is toxicity of Pb,the other is the intrinsic instability of lead halide perovskite that PNCs degraded easily when exposed to light,heat,oxygen and water.Hence,our work is about to design and develop a series of new lead-free PNCs while improving the stability of PNCs through surface passivation or surface coating.The main content and results of this paper are as follows:（1）A series of alkali-metal doped Cs₃Bi₂Cl₉ nanocrystals（NCs）were successfully synthesized through modified ligand-assisted reprecipitation method（LARP）by changing the reaction solvents,dispersants,reaction temperatures and doping ratios.Alkali-metal-doping has caused a slight red shift in the emission peak of Cs₃Bi₂Cl₉ NCs.Due to the incorporation of alkali metal,an mpurity phase was formed on the surface of Cs₃Bi₂Cl₉ NCs,which may lead to a passivation on the surface and enhancement of the fluorescence performance,photo-and moistrure-stability.However,with the increase of the proportion of Rb⁺,Rb⁺ions continuously replaced Cs⁺ions,while more and more impurity phases were generated.Surprisingly,the unknown impurity phase（rubidium bismuth chloride）displayed a better fluorescence performance than that of the Cs₃Bi₂Cl₉ NCs.（2）According to the above study,we successfully fabricated new bismuth-based perovskite A’₇Bi₃Cl₁₆（A’=Rb,K）single crystals through hydrothermal method.They exhibited homogeneous crystal structure which were trigonal with a space group of P3^<sub>1c.In the unit cell,it contained ideal octehedra[BiCl₆]^3-and edge-sharing dimers[Bi₂Cl₁₀]^4-,while alkali-metal ions were filled in the vacancy for valence balance.Besides,Rb₇Bi₃Cl₁₆ NCs and K₇Bi₃Cl₁₆NCs were also synthesized through modified LARP method,with average diameter of 1.85±0.6 nm and 1.95±1.1 nm,respectively.Under the excitation of 365 nm,Rb₇Bi₃Cl₁₆ NCs and K₇Bi₃Cl₁₆ NCs exhibited broad emission peaks located in 432 nm and 433 nm with PLQY of28.43%and 22.85%,respectively.In addition,Rb₇Bi₃Cl₁₆ NCs and K₇Bi₃Cl₁₆ NCs showed enhanced photo-stability and moisture-stability.And the high moisture-stability might originate from its unique crystal structure,composition and the BiOCl protective layer that formed by[BiCl₆]^3-octahedra as a sacrificial agent.（3）In order to improve the water resistence and biocompatibility of Rb₇Bi₃Cl₁₆ NCs,a new core-shell PNCs composite,Rb₇Bi₃Cl₁₆ NCs@SiO₂ was successfully prepared by coating with（3-aminopropyl）triethoxysilane（APTES）.Its excitation wavelength was 375 nm,emission peak was loated in 445 nm with FWHM of 92 nm and PLQY of 37.03%.Through a series of cytotoxicity experiments and laser confocal imaging experiments,it’s proved that Rb₇Bi₃Cl₁₆ NCs@SiO₂ not only had low toxicity(half maximal inhibitory concentration,IC₅₀>395μg/mL),but also exhibited good biocompatibility.Therefore,Rb₇Bi₃Cl₁₆ NCs@SiO₂ is potential candidate applied in biological fields such as fluorescence imaging and intelligent drug release vector.