Preparation And Spectral Properties Of Blue-violet Perovskite Nanocrystals

In recent years,all-inorganic perovskite nanocrystals,due to the excellent luminescence performance,high carrier mobility and simple preparation method,have shown broad application prospects in the optoelectronic fields such as solar cells,photodetectors,light-emitting diodes,and lasers.At present,red and green light-emitting diodes（LEDs）based on these materials have achieved external quantum efficiency（EQE）of more than 20%,while devices prepared based on blue-violet emission perovskite nanocrystals are seriously lagging behind because of their low defect tolerance and high ion migration.Therefore,it is of great significance to find ways to improve the luminous efficiency and stability of such materials.In this thesis,a series of blue-violet perovskite nanocrystals were obtained by direct synthesis and post-synthesis.In-depth research on the crystal structure and luminescence properties of the material has been carried out and the fluorescence quantum efficiency（PLQYs）and stability of these materials has been significantly improved,which can contribute to their further applications in optoelectronic devices.The specific research content is as follows:（1）The two-phase anion exchange method was proposed.The pre-synthesized Cs Pb X₃nanocrystals were exchanged using an aqueous solution of metal halide,and the influence of environmental temperature,treatment time,two-phase concentration,ligand and metal halide type on the morphology and optical properties of samples were studied.Due to the good solubility of Cd Cl₂in the water,it can provide continuous Cl^-ions,thereby ensuring full exchange in the high-concentration precursor;steady-state spectroscopy test and element characterization proved that the non-radiative recombination center caused by the Cl-vacancy or Pb⁰ was effectively passivated by rich Cl^-ions,water and Cd²⁺,and finally the Cs Pb Cl₃ nanocrystals with uniform size,good stability and PLQYs of nearly 100%were obtained.The two-phase anion exchange method provides a new strategy for obtaining blue-violet Cs Pb Cl₃ nanocrystals after synthesis,which can significantly improve the luminous efficiency and stability.This is helpful for further applications in the field of optoelectronics.（2）The ligand-assisted reprecipitation method was used to prepare Cs Cd Cl₃ nanocrystals,using cesium chloride and cadmium chloride as raw materials,DMF as the solvent,and adjusting the type and ratio of ligands to obtain Cs Cd Cl₃ nanocrystals with uniform size,blue-violet emission（luminous emission peak of 362 nm）and good stability.When Sb³⁺ions were used as the dopant,the sample appeared a broadband green emission of 500 nm.The X-ray powder diffraction（XRD）and ultraviolet absorption-emission spectroscopy（UV-PL）characterizations indicated that the latter’s emission may be from the self-trapped exciton transition of ³P_n-¹S₀（N=0,1,2）.This study not only has explored the doping mechanism of blue-violet emitting perovskite nanomaterials,but also provides a simple preparation strategy for the perovskite nanocrystals of new composition.（3）The ternary thermal injection method was used to prepare Cs₂Cd Cl₄ nanosheets,using cesium acetate as the cesium source,cadmium acetate as the cadmium source,and injecting the oleylamine chloride solution at a certain temperature to obtain the two-dimensional perovskite nanosheets with blue-violet emission（luminous emission peak of 446 nm）and uniform size;XRD and PL were used to characterize the effects of different experimental conditions（reaction temperature,reaction time and ligand chain length）on the growth process and luminous properties of nanosheets,which proved that properly prolonging the reaction time or choosing short-chain ligands can promote the growth of nanosheets on the crystal plane（001）and increase the luminous intensity.This research provides a simple synthesis route for the perovskite nanosheets of new composition.