The Studies On The Controllable Luminescence Behavior And Mechanism Of CsPbBr₃ Quantum Dots

Fully-inorganic cesium lead trihalide(CsPbX₃,X=Cl_xBr_yI_1-x-y,0≤x,y≤1)perovskite colloidal quantum dots（QDs）or nanocrystals（NCs）attracted much attention in recent years as highly promising photovoltaic and optoelectronic material,such as high absorption coefficient,the narrow full-width at half maximum（FWHM）,high photoluminescence quantum yield（PLQY）,controllable composition and size,tunable emission spectrum,photoluminescence and electroluminescence dual characteristics.These excellent properties have made CsPbX₃ QDs become one of the most promising optoelectronic materials,and widely used in solar cells,light-emitting diodes,photodetectors,lasers and other fields.In recent five years,although the CsPbX₃ QDs have developed rapidly,the halide perovskite materials are sensitive to polar solvents,resulting in accelerated degradation of CsPbX₃ QDs upon exposure to such solvents.Such ionic nature of CsPbX₃ QDs leads the ligands to be highly loose and easy to completely desorb once attacked by any polar solvents.Hence,it is difficult to retain the colloidal integrity during the surface treatment.When the highly polar solvents（e.g.,water,ethanol,acetonitrile,and methanol）are added to purify the CsPbX₃ QDs,the ionic bonds in CsPbX₃ QDs can be completely destroyed resulting in a poor PLQY,ink stability and structural integrity.In addition,when the low polar solvents（e.g.,octane,hexane,and toluene）are added to purify the CsPbX₃ QDs,the bonds between the ligands and the surface atoms of CsPbX₃ QDs can not be broken and it is different to remove the surface ligands.Hence,a novel surface treatment method was successfully developed by dispersing the CsPbX₃ QDs in nopolar/polar mixture solvents with different volume ratios.In this method the ligand density was sequentially decreased and the PLQY,ink stability and monodispersity of CsPbX₃ QDs can be still preserved.In this study,we have investigated the effect of surface ligand density on the exciton decay pathways and their contribution in dynamical traces of single perovskite CsPbBr₃ QD to study emission behavior and optical properties,by employing single molecule fluorescence lifetime imaging microscopy（FLIM）and time-tagged,time-resolved,and time-correlated single photon counting（TTTR-TCSPC）techniques.The surface ligand density of the perovskite CsPbBr₃ QDs could be regulated by using toluene/ethyl acetate mixed solvents with different volume ratios.The lifetime investigation and analysis of single perovskite CsPbBr₃ QD indicate that short-lived band-edge emission and long-lived trap-state emission are competitive to affect optical properties.And the contribution of trap-state emission gradually increases with the decreasing surface ligand density.These results will provide insights in realizing the full potential of these fully-inorganic CsPbX₃ QDs in photovoltaic and optoelectronic applications.The specific work arrangements are as follows:1.The FLIM system used in this study is homebuilt and optimized,which can achieve single-molecule fluorescence intensity imaging,record fluorescence intensity trajectory,and test fluorescence lifetime.2.The fully-inorganic perovskite CsPbBr₃ QDs were prepared by the hot-injection method.And they were synthesized in 1-octadecene using oleic acid and oleylamine as surface ligands.The as-prepared CsPbBr₃ QDs had high absorption ratios in UV-vis spectra region,narrow emission spectra of 19 nm,bandgap of 2.37 eV,and high quantum yield of 92.69%.Meanwhile the perovskite CsPbBr₃ QDs are mono-dispersed with a cubic shape and an average size of 10.2 nm which is close to the Bohr diameter of⁷ nm.3.The CsPbBr₃ QDs were treated with different volume ratios of the toluene/ethyl acetate mixed solvents（the volume ratios of ethyl acetate in the mixed solvents are 0%,10%,20%,...,90%,100%,11groups）,and the basic characterizations were conducted for these 11 groups of samples.With the proportion of ethyl acetate in the mixed solvents increasing from 0%to 50%,the quantum dots still had good crystallinity and monodispersity,the fluorescence emission peak of CsPbBr₃ QDs was unchanged,while the intensity was gradually decreased,in the meanwhile the absolute fluorescence quantum yield was gradually decreased from 92.69%to 6.89%.When the proportion of ethyl acetate exceeded 50%in the toluene/ethyl acetate mixed solvents,the quantum dots were aggregated into large-sized particles,while the quantum dots still exhibited evident exciton absorption behaviors.The average lifetime of single CsPbBr₃QD was increased from 4.06 ns to 12.54 ns and then decreased to 7.20 ns.When the volume ratios of ethyl acetate were 90%and 100%,their first absorption band edge had a slight red shift.The decreased surface ligand density of the CsPbBr₃ QDs was checked by the Fourier transform infrared spectroscopy（FTIR）study.It was confirmed that the volume ratio of ethyl acetate in the toluene/ethyl acetate mixed solvents could control the quantity of surface organic ligands in the CsPbBr₃ QDs.As perovskite QDs have a″soft?and predominantly ionic lattice,their optical and electronic properties are relevant with surface states,which are found to play a crucial role in the PLQY and the exciton dynamics of the CsPbBr₃ QDs.At the single-particle level,the lifetime investigation and analysis of single CsPbBr₃ perovskite QD indicate that short-lived band-edge emission and long-lived trap-state emission are competitive to affect optical properties.And the contribution of trap-state emission gradually increases with decreasing surface ligand density.Therefore,the toluene/ethyl acetate mixed solvents with different volume ratios in our experiments can control the content（or density）of the surface-insulating organic ligands of CsPbBr₃ QDs to regulate the photoluminescence behavior of CsPbBr₃ QDs.We have used solution-processed method to control the surface ligand density of CsPbBr₃ QDs with the toluene/ethyl acetate mixed solvents with different volume ratios.These results showed that the 1:1volume ratio of the toluene/ethyl acetate was optimal to obtain a suitable surface ligand density and PLQY,ink stability and monodispersion.To shed light on the nature of surface ligand density and its effect on carrier recombination,we had systematically investigated the exciton decay dynamics of the CsPbBr₃ QDs via surface ligand density control at single-dot level.The systematic single-dot measurements and analysis on single QD provide an in-depth understanding of the nature of the surface ligand density and the trap states,their roles in PLQY,and a guide to manipulate the recombination dynamics in these systems.We demonstrate that there are two ways of band-edge emission and surface trap-state emission for exciton to release energy.The short-lived band-edge emission and long-lived trap-state emission are competitive to affect optical properties.This mild solution-processed method and the results at single-particle level provide new effective approach and information for improving the optoelectronic properties of CsPbBr₃QDs.