[materials] angelw: a general method of shape controlled synthesis of perovskite nanocrystals

Halogenated perovskite nanocrystals are a new kind of semiconductor materials, which are widely used in various fields, such as displays, lasers, phototransistors and photovoltaic power generation due to their extraordinary optical and photoelectric properties Perovskite nanocrystals also have the advantages of easy synthesis, tunable surface chemistry, better photoluminescence quantum yield, quantum confinement effect and solution processability, which are very important for the application of most devices At present, the synthesis methods of high quality perovskite nanocrystals include ligand assisted precipitation (LARP), thermal injection, ultrasonic treatment, solvothermal, microwave and ball milling Among these methods, LARP method is based on adding bad solvent into the good solvent of perovskite precursor in the presence of ligand at room temperature to form nanocrystalline, and vice versa The size of perovskite precursors is determined by the size of their ligands The disadvantage of this method is that the use of polar solvents such as N, N-dimethylformamide will greatly affect the stability of nanocrystals; it may also lead to mixed nanocrystal morphology, such as mixed thickness of nano sheet dispersion, and lead to wide fluorescence spectrum In order to have a basic understanding of their structure property relationship and meet the needs of various technical applications, more and more attention has been paid to the simple synthesis of shape controlled perovskite nanocrystals The schematic diagram of shape controlled synthesis of perovskite nanocrystals (source: angel Chem Int ed.) was recently proposed by Dr Huang He and Dr lakshminarayana polavarapu, University of Munich, Germany A non-traditional ligand assisted precipitation method, which can synthesize perovskite nanocrystals without external stimulation (heat, microwave radiation, ultrasonic treatment, mechanical force or polar solvent), by simply mixing precursor ligand complex spontaneous crystallization The shape of perovskite nanocrystals can be changed from nano cube to nano sheet by changing the ratio of monovalent cations (formamidine cations (FA +) and CS +) to bivalent cations (Pb 2 +) precursor in the reaction medium Mechanism study shows that perovskite nanocrystals are formed by seed mediated growth, and this simple synthesis method is universal Relevant achievements were published in the international authoritative journal of chemistry, angel Chem Int ed (DOI: 10.1002 / anie 201906862) under the title of "spherical crystallization of perovskite nanocrystals in nonpolar organic solutions: a versatieapproach for their shape controlled synthesis" The synthesis, structure and morphology of perovskite nanocrystals (source: angelw Chem Int ed.) were synthesized by simply mixing two precursor complexes (monovalent cation ligand and lead halide ligand) in nonpolar solvent at room temperature The precursors are dissolved directly in nonpolar solvents by metal ion complexation of ligands, and crystallize spontaneously when the two precursors are mixed The precursor solution is very stable in a few months, and perovskite nanocrystals can be easily and rapidly prepared in large quantities in a few minutes before use Theoretically, this simple synthesis method can be easily applied to the synthesis of a series of ABX 3 Perovskite nanocrystals (a is monovalent cation, B is lead, X is halide ion) In the formation of perovskite nanocrystals, the precursor ligand transformation diagram (source: angelw Chem Int ed.) firstly, the author applied this method to the synthesis of formamidine lead iodide perovskite nanocrystals The precursor of formamidine oleate was injected into toluene solution containing PbI2 ligand complex under continuous stirring at room temperature The color of the reaction medium changes to orange After mixing the two precursors, the reaction medium immediately starts to fluoresce under ultraviolet light without heating or using any polar solvent The results show that the precursor ligand complex decomposes in the reaction medium and crystallizes spontaneously into formamidine lead iodide perovskite nanocrystals Then, the ratio of monovalent and bivalent precursors is reduced in the synthesis of perovskite nanocrystals by ultrasonic assisted, and the morphology of perovskite nanocrystals is transformed from 3D nano cube to 2D nano sheet In order to elucidate the growth mechanism of formamidine lead iodide perovskite nanocrystals when the two precursors are mixed, the author reacts in a cuvette and monitors the fluorescence spectra of nanocrystals formed in the reaction medium for a period of time The evolution of the fluorescence spectrum of the nanocube shows that the bimodal size distribution of the clusters formed in the early stage of the reaction disappears with the passage of time, because they are transformed into the nanocube by size focusing As the ligand solution acts as the coordination solvent, the current body ligand complexes do not crystallize when mixed in the ligand (oleamine / oleic acid) solution The energy difference between the precursor ligand complex coated with a compact ligand shell and the perovskite nanocrystals is so small that they spontaneously crystallize at room temperature immediately after they are mixed in an organic medium At the same time, the amount of formamidine cation precursor added to the reaction medium was reduced, which indicated that the anisotropic growth was advantageous under the lower ratio of monovalent and divalent cations Generally, reducing the reactant concentration will reduce the reaction rate, thus reducing the growth rate of nanocrystals; slow growth rate is conducive to the formation of anisotropic nanocrystals Finally, the author further confirmed that this method can be applied to the preparation of cspbi 3, csppbbr 3, fapbcl 3, fapbbr 3 and mapbbr 3, and showed the universality of this method Summary: Dr Huang He and Dr lakshminarayana polavarapu of the University of Munich have synthesized perovskite nanocrystals by spontaneous crystallization in nonpolar organic solvents at room temperature By changing the ratio between monovalent cations and Pb2 + precursor, the morphology of perovskite nanocrystals can be easily adjusted from 3D nano cube to 2D nano sheet; by changing the concentration and temperature of Pb2 + precursor, this simple method can be easily extended to obtain other perovskite nanocrystals (nanorods or nanowires) The method is suitable for all inorganic perovskite nanocrystals of organic-inorganic hybrid compounds and different halide compositions, and shows the versatility of the synthesis method This multi-functional and simple method not only opens up a new way for the synthesis of shape controllable perovskite nanocrystals with excellent scalability, but also expands the current understanding of the crystallization mechanism of perovskite nanocrystals in non-polar solvents.