SHAO Shan¹，XU Fan¹，CAO Bingqiang²

（1. School of Physics and Physical Engineering， Qufu Normal University， Qufu 273165， China；2. School of Materials Science and Engineering， University of Jinan， Jinan 250022， China）

Abstract

Objective In contrast to toxic lead-based perovskites， non-toxic and optically superior lead-free double perovskite Cs₂AgInCl₆quantum dots have garnered significant 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 modi⁃fications on the stability and optical properties of Cs₂AgInCl₆ quantum dots， resulting 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 using a thermal injection method， with 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. 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 The impact of different surface ligand modifications on the purity， particle size， and morphology of Cs₂AgInCl₆ quantum dots was investigated. X-ray diffraction （XRD） analysis showed consistent results with the bulk standard card ICSD 1927876 for both types of quantum dots， indicating no impurity peaks. Cs₂AgInCl₆-DA quantum dots exhibited superior crystallization compared to Cs₂AgInCl₆-OA quantum dots， as evidenced by the 1 468cm-1peak 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 displayed 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 significantly 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 80 days of continuous standing. 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，indicating inhibition of non-radiative recombination at lower temperatures. The exciton binding energy was calculated to be 64. 67 meV for Cs₂AgInCl₆-OA quantum dots and78. 88 meV for Cs₂AgInCl₆-DA. Photoluminescence peaks remained consistent for both types of quantum dots，but there were noticeable differences 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 intro⁃duced as a measure of the degree of photoluminescence emission polarization. The polarization value，or DP value，was determined to be 5.561% for Cs₂AgInCl₆-OA and5.333% for Cs₂AgInCl₆-DA.

Conclusion Pure-phase Cs₂AgInCl₆-OA and Cs₂AgInCl₆-DA quantum dots were successfully synthesized via thermal injection.The hydroxyl group in the ligand binds to the surface of Cs₂AgInCl₆ quantum dots as—COO— groups，indicating uniform coverage of the long-branched OA ligand and short-branched DA ligand. Both Cs₂AgInCl₆-DA and Cs₂AgInCl₆-OA quantum dots exhibit a cubic structure with excellent dispersion and uniform size. Cs₂AgInCl₆-DA demonstrates superior performance compared to traditional Cs₂AgInCl₆-OA. When exposed to air for 80 days continuously，Cs₂AgInCl₆-DA maintains significant fluorescence，demonstrating remarkable stability. DA ligands effectively reduce surface defects of quantum dots and inhibit non-radiative recombination，resulting in longer fluorescence lifetimes and higher photoluminescence quantum yield. The longbranched OA and short-branched DA ligands control morphology and prevent agglomeration by modifying the surface of quantum dots. Free excitons transition from valence band maximum （VBM） to conduction band minimum （CBM）. Some free excitons undergo non-radiative recombination due to surface defects，while others are trapped by self-trapping states resulting from JahnTeller distortion of the ［AgCl₆］^5- octahedron under strong electron-phonon coupling. Radiative recombination of self-trapped excitons drives emission in the double perovskite Cs₂AgInCl₆. In summary，DA ligands contribute to maintaining the stability of Cs₂AgInCl₆ quantum dots and promoting effective radiation recombination，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

Get Citation:SHAO S，XU F，CAO B Q. Utilizing short-chain surfactants for enhancing the stability and fluorescence performance of Cs₂AgInCl₆ quantum dots［J］. China Powder Science and Technology，2024，30（3）：139−149.

Received:2023-11-25.Revised:2024-02-12,Online:2024-04-16

Funding Project:国家自然科学基金项目，编号：51872161；国家重点研发计划项目，编号：2022YFC3700801。

First Author:邵山（1998—），女，硕士生，研究方向为无铅双钙钛Cs₂AgInCl₆制备技术。E-mail：17562054912@163. com。

Corresponding Author:曹丙强（1978—），男，博士，教授，山东省泰山学者海外特聘专家，博士生导师，研究方向为半导体材料与器件。E-mail：mse_caobq@ujn.edu.cn。

DOI：10.13732/j.issn.1008-5548.2024.03.012

Serial No:1008-5548（2024）03-0139-11