Utilizing short-chain surfactants for enhancing the stability and fluorescence performance of Cs2AgInCl6 quantum dots
Objective Lead-free double perovskite Cs2AgInCl6 quantum dots,which are non-toxic and optically superior compared to toxic lead-based perovskites have garnered considerable attention.However,challenges arise from their poor stability and ten-dency to aggregate in air,hindering their practical applications.To address these issues,this study analyzed the effects of vari-ous surface ligand modifications on the stability and optical properties of Cs2AgInCl6 quantum dots,aiming to improve their sta-bility and optical characteristics.The research methodologies and findings presented in this paper hold promise for the applica-tion of double perovskite Cs2AgInCl6 quantum dots in optoelectronic devices.Methods Cs2AgInCl6 quantum dots were synthesized via a thermal injection method,employing oleic acid(OA)and 2-hexylcap-ric acid(DA)as surface ligands.The synthesized quantum dots underwent comprehensive characterization including powder diffraction,Fourier infrared spectrometry,transmission electron microscopy,transmission,reflection,and absorption spectrom-etry,stability assessment,and transient fluorescence spectrometry.This study investigated the influence of different surface ligand modifications on the purity,particle size,and morphology of Cs2AgInCl6 quantum dots,along with fluorescence and sta-bility analyses.Furthermore,the luminescence mechanism was elucidated through examinations of low-temperature fluores-cence,power-dependent fluorescence,and circular polarization luminescence properties.Results and Discussion This paper investigated the impact of different surface ligand modifications on the purity,particle size,and morphology of Cs2AgInCl6 quantum dots.X-ray diffraction(XRD)analysis revealed consistent results with ICSD 1927876,a bulk standard card,for both types of quantum dots,indicating no impurity peaks.Notably,Cs2AgInCl6-DA quan-tum dots exhibited enhanced crystallization compared to Cs2AgInCl6-OA quantum dots,as evidenced by the 1 468 cm-1 peak for Cs2AgInCl6-OA and the 1 465 cm-1 peak for Cs2AgInCl6-DA,indicating tensile vibration characteristics of the—COO—group.Both types of quantum dots demonstrated a cubic structure,good dispersion,and high crystallinity of the perovskite.Fluores-cence and stability analyses were conducted for Cs2AgInCl6 quantum dots.Cs2AgInCl6-DA quantum dots exhibited notably higher luminescence intensity compared to Cs2AgInCl6-OA quantum dots,with a photoluminescence quantum yield of 4.67％.After continuous storage at room temperature for 80 days,DA-modified Cs2AgInCl6 quantum dots retained 95.43％of the initial luminescence,while Cs2AgInCl6-OA quantum dots exhibited a photoluminescence quantum yield of 2.79％,with fluorescence intensity decreasing to 56.27％after the same duration.Moreover,the average lifetime of Cs2AgInCl6-OA and Cs2AgInCl6-DA quantum dots was determined to be 1.236 9 ns and 3.936 6 ns,respectively.The luminescence mechanism was discussed based on low-temperature fluorescence,power-dependent fluorescence,and circular polarization luminescence studies.Both types of quantum dots showed a decrease in emission intensity with increasing temperature,suggesting inhibition of non-radiative recombination at lower temperatures.Furthermore,the exciton binding energy was calculated to be 64.67 meV for Cs2AgInCl6-OA quantum dots and 78.88 meV for Cs2AgInCl6-DA.While the photoluminescence peaks remained consistent for both types of quantum dot,noticeable differences existed in luminous intensity.Additionally,a polarization value,defined as the ratio of the difference in left-handed and right-handed polarized photoluminescence intensities to their sum and normalized to the range[-1,1],was introduced as a measure of the degree of photoluminescence emission polarization.The polarization value,or DP value,was found to be 5.561％for Cs2AgInCl6-OA and 5.333％for Cs2AgInCl6-DA.Conclusion This study reported the successful synthesis of pure-phase Cs2AgInCl6-OA and Cs2AgInCl6-DA quantum dots through thermal injection.The hydroxyl group within the ligand bound to the surface of Cs2AgInCl6 quantum dots as—COO—groups,ensuring uniform coverage of both the long-branched OA ligand and short-branched DA ligand.Both Cs2AgInCl6-DA and Cs2AgInCl6-OA quantum dots exhibited a cubic structure with excellented dispersion and uniform size.Cs2AgInCl6-DA dem-onstrated superior performance compared to traditional Cs2AgInCl6-OA,showcasing a prolonged fluorescence even after continu-ous exposure to air for 80 days,demonstrating remarkable stability.DA ligands effectively reduced surface defects of quantum dots,suppressing non-radiative recombination and enhancing fluorescence lifetimes and photoluminescence quantum yield.The long-branched OA and short-branched DA ligands played a crucial role in morphology control and preventing agglomeration by modifying the surface of quantum dots.The observed transition of free excitons from the valence band maximum(VBM)to the conduction band minimum(CBM)highlighted the interplay between radiative and non-radiative recombination pathways,influ-enced by surface defects and Jahn-Teller distortion of the[AgCl6]5-octahedron under strong electron-phonon coupling.Radia-tive recombination of self-trapped excitons drove emission in the double perovskite Cs2AgInCl6.In summary,DA ligands contrib-ute to preserving the stability of Cs2AgInCl6quantum dots and facilitating effective radiation recombination,thereby offering potential for the photoelectric application of emerging lead-free double perovskite Cs2AgInCl6 quantum dots.
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