Abstract

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 the poor stability and tendency to aggregate in air， hindering their practical applications. To address these issues， this study analyzes the effects of various surface ligand modifications on the stability and optical properties of Cs₂AgInCl₆ quantum dots， aiming in improved stability and optical characteristics. The research methodologies and findings presented in this paper hold promise for the application 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-hexylcapric 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 spectrometry， 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 was investigated， along with fluorescence and stability analyses. Furthermore， the luminescence mechanism was elucidated through examinations of low-temperature fluorescence，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 quantum 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. Fluorescence 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 demonstrated superior performance compared to traditional Cs₂AgInCl₆-OA，showcasing a prolonged fluorescence even after continuous 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， influenced by surface defects and Jahn-Teller distortion of the ［AgCl₆］^5- octahedron under strong electron-phonon coupling. Radiative recombination of self-trapped excitons drove emission in the double perovskite Cs₂AgInCl₆. In summary， DA ligands contribute 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.

Keywords：quantum dots； lead-free double perovskite； surface ligand； stability

CLC No:TB44； TQ591                    Type Code:A