Synthesis Of Transition Metals Doped All-inorganic Perovskite Nanocrystals And Their Stability/Optical Properties

With the development of society,the contradiction between mankind's increasing demand for energy and the current situation of declining energy has become more and more prominent.The energy problem has become the ultimate challenge faced by every country and even every human being in their survival and development.Developing new kinds of energy sources and energy-saving and environmentally friendly materials is an important way to solve this problem.As a new generation of semiconductor materials,lead halide perovskite not only has a long carrier diffusion length and low exciton binding energy,but also has the advantages of economic and simple preparation process,so they have quickly become the appealing photovoltaic materials.Recent years,their photon-to-electron conversion efficiency has increased from 3.8%to 23%,which is close to the single crystalline silicon.In addition,lead halide perovskite nanocrystals as a new kind of semiconductor quantum dots exhibit very excellent optical properties,such as high photoluminescence quantum yield,narrow full width at half maxima,and controllable emission wavelength between the visible light range.At present,the external quantum efficiency of perovskite light-emitting diodes has been increased to more than 10%,laying a solid foundation for its application in the field of lighting and display.In addition,lead halide perovskite nanocrystals also show good application prospects in the fields of biosensing,photodetectors,catalysis and so on.Compared with organic-inorganic hybrid perovskite nanocrystals,all-inorganic perovskite nanocrystals not only have better thermo stability,but also exhibit more excellent optical properties.However,because the perovskite nanocrystals have the inherent property of ionic compounds,they show sensitivity for polar solvent,humidity,ultraviolet radiation,thermal radiation and ion exchange,which can destroy their structures and loss their original optical performance.This defect greatly hindered the process of their practical application,so improving the stability of perovskite nanocrystals has always been the focus and difficulty in this field.Numerous scientists have made great efforts and achieved some results,but there is still a long way to go before its commercial application.Moreover,while maintaining or even improving the optical properties of perovskite nanocrystals,it is also important for people to further develop a simple,economical and environmentally friendly preparation process.Here,we firstly study the effect ts of surface ligands on the formation,stability and optical properties of perovskite nanocrystals.Then the preparation of perovskite nanocrystals was optimized by transition metal halide doping strategy,and their stability and optical properties were improved simultaneously.Finally,Ni:CsPbBr₃nanocrystals and their composites with SiO₂ were successfully prepared by an in situ method with water or ethanol as solvent.Ni:CsPbBr₃-SiO₂complex not only has excellent stability,but also shows excellent application value in cell imaging and light-emitting diodes.The main contents are as follows:(1)During the preparation of CsPbBr₃ nanocrystals by hot-injection method,the effects of oleic acid and oleylamine on the growth of CsPbBr₃ nanocrystals were investigated by adjusting their volume ratio and reaction time.The results showed that when oleic acid was in excess,the fluorescent cubic CsPbBr₃ nanocrystals tended to transform into non-fluorescent tetragonal CsPb₂Br₅ microsheets.This kind of phase transition could occur no matter at high temperature or low temperature,the higher temperature and more oleic acid would promote it.When oleylamine is in excess,CsPbBr₃ nanocrystals were generated at high temperature without phase transition.However,the cubic CsPbBr₃ nanocrystals would be decomposed into hexagonal Cs₄PbBr₆ nanocrystals without luminescence at room temperature,and more oleylamine also can promote this kind of phase transition.Futhermore,Cs₄PbBr₆nanocrystals can be reconverted to CsPbBr₃ nanocrystals after heating,and their fluorescence also can be recovery.These results indicate that organic acid-base ligands play a role like the double-edged sword,which are not only crucial for the formation of CsPbBr₃ nanocrystals,but also induce changes in their crystal structure and optical properties.Finally,the coordination reaction between oleylamine and lead ions is the key factor affecting the formation and surface passivation of perovskite nanocrystals.Moreover,thermodynamic equilibrium between bromine ion and lead ion were verified,which may be the main reason for the poor stability of perovskite nanocrystals.(2)Because lead halide was used as the source of both lead and halide ions in previous synthesized protocols,it is difficult to precisely regulate the the ratio of lead and halide ions.Therefore,we tried to use the transition metal halides as the source of halide ions to prepare perovskite nanocrystals,which are more likely to coordinate with olamine.The results show that this simple method can improve the crystallinity and photoluminescence quantum yield of perovskite nanocrystals while preserving the original emission wavelength and full width at half maxima.On one hand,the transition metal halides can provide a rich environment of halide ions,which can eliminate the crystal defects and disorder of perovskite nanocrystals.On the other hand,these transition metal ions are mainly doped on the surface of nanocrystals and have stronger bond with the organic ligands,which would improve the stability of nanocrystals for high temperature and humidity.Finally,we also proved that the transition metal chlorides,especially NiCl₂,can co-doped CsPbCl₃ nanocrystals with Mn²⁺,which can futher improve the photoluminescence quantum yield and stability of Mn:CsPbCl₃ nanocrystals.These results provide a new idea for optimizing the preparation and doping of perovskite nanocrystals.(3)Water or ethanol was used as solvents,acetic acid and ammonia water were used as acid-base ligands,nickel bromide was used as bromine source to prepare Ni:CsPbBr₃ fluorescent films with quantum yield of more than 50%by an green in situ method.Nickel bromide play a key role here,which not only have a good solubility in water,but also can react with acid-base ligands and the formed complex to limit the diameter of Ni:CsPbBr₃ nanocrystals to about 5nm.This simple method is economical and environmentally friendly,and it also can well solve the contradiction between the reaction system of perovskite nanocrystals and silica,which lays a good foundation for the macro preparation of their composites.Finally,we successfully prepared Ni:CsPbBr₃-SiO₂ nanocomplexes with high stability to water,ultraviolet light and ion exchange reactions,and their application values for cell imaging and light-emitting diodes have been demonstrated here.