Colloidal Preparation And Growth Machanism Of Cesium Lead Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals have received enormous attentions in recent years for their extensive opportunities as a potential material in optoelectronic applications.These tiny nanocrystals having size and composition variable color tuned illuminations are typically prepared in reaction flask and followed exciting reaction chemistry.For all inorganic CsPbX3(X = Cl,Br,I)perovskites,varying the reaction temperature,tuning the chain length of amine ligands,using mixture of ligands and introducing specific impurities,the size,shape and phase of these nanocrystals were tuned.However,in comparison to decades long studied chalcogenide nanocrystals,the physical insights of understanding the crystal growth mechanism for these perovskite nanocrystals are limited.For phase stability and retaining the size as well the emission intensity,reactions are mostly ice cooled soon after the injection of most reactive precursors and this restricts the study of long range successive growth of these nanocrystals.The fate of nanocrystals with progress of reactions under different reaction conditions is still unclear.The study of reaction kinetics,the shape evolutions with progress of reaction,the facet deformation leading to different shapes,the wide range of size tunability without compromising the monodispersity,the impact of growth ripening on prolonged annealing,the monomer concentration variations with reaction time etc.which are associated with colloidal nanocrystals growth mechanism are still not widely explored and needs further investigations.In addition,the different composition of CsxPbXy nanocrystals can be prepared by adjusting the ratio of reaction precursors,but the conversion relationship between them needs to be further explored.The main results obtained are as follows:(1)The synthesis methods of CsPbX3 perovskite nanocrystals were optimized.By reducing the reaction temperature and reaction rate,the effective control of the reaction system was realized and helped much for understanding the reaction system.Single-sized CsPbBr3 perovskite nanocrystals were successfully synthesized by controlling the concentration of oleylamine ligands at room temperature and then the corresponding CsPbX3 perovskite nanocrystals were obtained by anion exchange method.The emission width of single-sized CsPbX3 perovskite nanocrystals are only 10?24 nm and according to the NTSC TV color standard,which allow a wider gamut of pure colors covering up to 130%of the NTSC color standard.(2)We have found that the residual precursors and excess organic ligands are the main reasons for the uncontrollable synthesis of CsPbX3 perovskite nanocrystals at high temperature through the exploration of the reaction system.Therefore,we pioneered a "two-step" method to prepare high-quality CsPbBr3 perovskite nanocrystals with controllable size and morphology.First,CsPbBr3 perovskite nanoclusters were prepared at room temperature and purified.Then the ultrathin nanowires,the controllable thickness of nanoplatelets,the adjustable size of nanocubes and the polyhedron shaped CsPbBr3 perovskite nanocrystals were obtained by hot-inject the purified nanoclusters as the reaction source at different temperatures.Finally,CsPbX3 perovskite nanocrystals with different components can be obtained by anion exchange method.In addition,the CsPbBr3 perovskite nanocrystals prepared by the "two-step" method has a small size distribution,and an ordered self-assembly phenomenon occured in the mother solution to form the corresponding superlattice.(3)We modified the "two-step" method to study the gro,wth mechanism of CsPbBr3 perovskite nanocrystals in a single reaction by step-rising the reaction temperature.Combining both optical and microscopic observations,it could be further stated that the growth was a step-growth of a unit cell(?0.6 nm)even though there involved a shape change.As the optical tunability observed here remained one of the widest tunable systems carried out in a single reaction,the band positions were further correlated with confinement dimensions and respective bandgaps.The evolution process of different shape CsPbBr3 perovskite nanocrystals was clearly seen by TEM images,and its growth mechanism was discussed in details.(4)The "two-step" method was systematically studied.A certain amount of oleic acid,oleylamine,CsOA and PbBr2 solution were added into the reaction system respectively,the results of the reaction were analyzed and the conversion relationship between different composition Cs,PbBry nanocrystals was given.It was found that the reaction system was not controllable when oleic acid and oleylamine were added.The size,morphology and monodispersity of the obtained CsPbBr3 nanocrystals were very poor.However,when the appropriate amount of CsOA was added to the reaction system,the Cs4PbBr6 nanocrystals were obtained,which size could be controlled by changing the reaction temperature.In addition,after adding a certain amount of PbBr2 to the Cs4PbBr6 nanocrystals,it was found that all of the Cs4PbBr6 nanocrystals would be converted into CsPbBr3 nanocrystals.