Synthesis Of Metal Ion-Alloyed Lead-Free Double Perovskite Nanocrystals And Regulation Of Their Luminescent Properties

In recent years,lead-based halide perovskite nanocrystals have garnered widespread attention in the field of optoelectronics due to their excellent optoelectronic properties and ease of solution processing.However,the toxicity of lead and issues with stability have severely limited their commercial applications.Therefore,the development of lead-free halide perovskite nanocrystals with excellent optoelectronic performance,non-toxicity,and stability has become particularly urgent and important.Lead-free double perovskite nanocrystals have garnered widespread attention due to their non-toxicity,stability,and tunable composition.However,these lead-free double perovskite nanocrystals often exhibit indirect bandgaps or direct bandgaps characterized by parity-forbidden transitions,leading to non-ideal luminescent properties,primarily reflected in low fluorescence intensity and low photoluminescence quantum yield（PLQY）.To address these issues,this thesis selected two types of lead-free double perovskite nanocrystals with different bandgap characteristics:Cs₂AgBiX₆（X=Cl^-,Br^-）with indirect bandgaps and Cs₂AgInCl₆ with direct bandgaps,as research subjects.By employing Na⁺alloying,Bi³⁺alloying,and Na⁺/Bi³⁺co-alloying strategies,the composition of these nanocrystals was regulated,and their luminescent performance was optimized.At the same time,this thesis also explored in detail the reasons for the enhancement of luminescent performance and proposed potential luminescent mechanisms.The main research contents are as follows:The main research contents are as follows:1.For Cs₂AgBiBr₆ double perovskite nanocrystals with indirect bandgaps,we proposed a strategy using alkali metal Na⁺alloying to optimize their luminescent performance.Monodisperse,uniformly sized,and cubic Cs₂AgBiBr₆ nanocrystals,as well as Cs₂AgBiBr₆ double perovskite nanocrystals alloyed with different Na⁺concentrations,were successfully prepared using a modified hot-injection method.Using characterization methods such as XRD and XPS,it was confirmed that Na⁺occupied the Ag⁺positions in Cs₂AgBiBr₆.By controlling the Na⁺alloying concentration,the PLQY of Cs₂AgBiBr₆ double perovskite nanocrystals was increased by an order of magnitude,and the exciton lifetime was extended by 9.2times.Furthermore,temperature-dependent fluorescence spectra and density functional theory elucidated the main reasons for the enhanced luminescent performance,namely the increase in exciton binding energy,the enhancement of longitudinal optical phonon energy,the enhancement of electron-phonon coupling effects,and the change in local position symmetry caused by Na⁺introduction,thereby breaking the forbidden transitions of Bi³⁺.2.To explore the effect of Na⁺alloying on the luminescent performance of double perovskite nanocrystals under different halogen environments,we prepared unalloyed and Na⁺-alloyed Cs₂AgBiCl₆ double perovskite nanocrystals with an indirect bandgap using a modified hot-injection method.The results showed that the Na⁺occupied the position of Ag⁺in the Cs₂AgBiCl₆ lattice.By adjusting the concentration of Na⁺alloying,the PLQY of Cs₂AgBiCl₆ double perovskite nanocrystals was significantly increased by up to 5 times,indicating that Na⁺alloying effectively enhances the luminescent performance of double perovskite nanocrystals under different halogen environments.Furthermore,through in-depth analysis of temperature-dependent fluorescence spectra and density functional theory revealed that the enhancement of luminescent performance was mainly attributed to the increase in exciton binding energy,the enhancement of longitudinal optical phonon energy,the strengthening of electron-phonon coupling effects,and the synergistic effect of breaking Bi³⁺forbidden transitions caused by Na⁺alloying-induced changes in local positional symmetry.3.In order to obtain double perovskite nanocrystals with higher luminescent performance,we selected Cs₂AgInCl₆ double perovskite nanocrystals with direct bandgaps.Employing high-temperature hot injection methods,we prepared Cs₂AgInCl₆ and Cs₂AgInCl₆ double perovskite nanocrystals alloyed with different Bi³⁺concentrations.The research results indicated that Bi³⁺occupies the position of In³⁺in Cs₂AgInCl_6。By adjusting the alloying concentration of Bi³⁺,the PLQY of Cs₂AgInCl₆ double perovskite nanocrystals was increased significantly from 3.9%to31.6%,and the exciton lifetime was extended by 4.2 times.At the same time,by utilizing temperature-dependent fluorescence spectra and density functional theory,it was revealed that the significant improvement in luminescent performance is mainly attributed to the breaking of parity forbidden transitions induced by Bi³⁺alloying,the increase in exciton binding energy,and the reduction in electron-phonon coupling effects.Furthermore,the Bi³⁺-alloyed Cs₂AgInCl₆ double perovskite nanocrystals demonstrated excellent stability.4.Building upon the research on Bi:Cs₂AgInCl₆ double perovskite nanocrystals,we further utilized alkali metal Na⁺alloying to optimize their luminescent performance.Bi:Cs₂AgInCl₆ double perovskite nanocrystals alloyed with different Na⁺concentrations were prepared using a high-temperature hot injection method.Using XRD,HRTEM,and XPS techniques confirmed that Na⁺replaced Ag⁺in Bi:Cs₂AgInCl₆.By systematically adjusting the Na⁺alloying concentration,the PLQY of Bi:Cs₂AgInCl₆ double perovskite nanocrystals was significantly increased to 56.6%.Moreover,through temperature-dependent fluorescence spectra and density functional theory analysis,the analysis revealed that the remarkable enhancement in luminescence performance primarily resulted from the synergistic effects of multiple factors:Na⁺alloying induced changes in local positional symmetry,leading to the disruption of local lattice symmetry,increased exciton binding energy,and enhanced electron-phonon coupling.Furthermore,the Na⁺-alloyed Bi:Cs₂AgInCl₆ double perovskite nanocrystals exhibited excellent stability.