Ziyao
Hu
a,
Kun
Nie
*ab,
Xuyi
Wang
*c,
Xiuqiang
Duan
a,
Ranran
Zhou
a,
Mengyun
Wu
a,
Xiaoxue
Ma
*a,
Xiaodong
Zhang
a,
Luoxin
Wang
a,
Lefu
Mei
d and
Hua
Wang
a
aHubei Key Laboratory for New Textile Materials and Applications and State Key Laboratory of New Textile Materials & Advanced Processing Technology, School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, P. R. China. E-mail: knie@wtu.edu.cn; xxma@wtu.edu.cn
bKey Laboratory of Testing and Tracing of Rare Earth Products for State Market Regulation, Jiangxi University of Science and Technology, Ganzhou 341000, P. R. China
cChina Bluestar Chengrand Co., Ltd, High Tech Organic Fiber Key Laboratory of Sichuan Province, Chengdu 610042, P. R. China. E-mail: xuyiwang2015@163.com
dBeijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, P. R. China
First published on 6th February 2023
All-inorganic metal halide perovskites are widely studied because of their excellent photoelectric properties. However, due to the toxicity of CsPbX3 (X = Cl, Br, I) perovskites, it is difficult to apply them on a large scale. The lead-free nature and air stability make Cs2SnX6 (X = Cl, Br, I) perovskites possible candidates to replace CsPbX3 perovskites. Herein, we report the perovskite crystals (PCs) based on Te(IV)-doped Cs2SnCl6: Cs2Sn1−xTexCl6. Cs2Sn1−xTexCl6 PCs showed yellow emission under a 365 nm ultraviolet lamp. The photoluminescence quantum yield (PLQY) of Cs2Sn0.94Te0.06Cl6 PCs was 57.09%, which was proposed to be from the triplet Te(IV) ion 3P1 → 1S0 self-trapping excitons (STE) recombination. The perovskite crystals can be used to fabricate light-emitting diodes (LEDs). The fiber paper prepared from aramid chopped fibers (ACFs) and polyphenylene sulfide (PPS) fibers showed a bright yellow light under 365 nm ultraviolet light after being post-processed with Cs2Sn1−xTexCl6 PCs solution. The ACFs/PPS compound fiber paper modified with Cs2Sn1−xTexCl6 PCs maintained exceptional optical properties and could be stored in air for more than 4500 h. The fluorescence performance of the modified ACFs/PPS compound fiber paper could be applied to fluorescence anti-counterfeiting. The modification strategy and the applications in this work will provide a good choice for studying the optical performance of perovskites and broaden the application of ACFs/PPS compound fiber paper.
Doping refers to introducing impurities into intrinsic semiconductors to change their electrical properties.21 Existing studies have shown that doping has an important effect on improving the optical performance of perovskites.22 For instance, doping lanthanide into perovskite nanocrystals can greatly improve and expand the optical properties.23 Sb-doped Cs2SnCl6 shows a photoluminescence peak at 630 nm with orange-red emission, and Bi-doped Cs2SnCl6 can yield blue self-trapping excitons (STE) emission. However, the unequal valence states between Bi3+ and Sn4+ make a large number of anion sublattices easily appear in the double-doped Cs2SnCl6, which may reduce the photoluminescence quantum yield (PLQY).24 According to this view, tetravalent ion doping can significantly reduce the formation of extrinsic defects and promote optical properties. A lead-free perovskite Cs2Sn1−xTexCl6 can show a bright yellow light under the irradiation of 365 nm ultraviolet light and has high anti-water stability.25 A Te4+-doped Cs2SnCl6 vacancy-ordered perovskite variant shows green-yellow emission.26 However, the previous synthesis methods required some special ligands or Teflon liners. Therefore, it is urgent to synthesize Cs2Sn1−xTexCl6 using a more economical and convenient method. In this paper, Cs2Sn1−xTexCl6 perovskite crystals (PCs) with different doping ratios were synthesized via a more convenient hydrothermal method. A light-emitting diode (LED) lamp was fabricated by combining a 385 nm chip and Cs2Sn0.94Te0.06Cl6 PCs. The LED lamp could emit yellow light driven by a 600 mA current. Furthermore, we have modified fiber paper prepared from aramid chopped fibers (ACFs) and polyphenylene sulfide (PPS) fiber by post-processing it with the Cs2Sn1−xTexCl6 PCs solution. The modified ACFs/PPS compound fiber paper showed a bright yellow light under 365 nm ultraviolet light. The modified fiber paper could maintain exceptional optical properties even when soaked in a weak acid solution (pH = 5). The optical properties of the modified ACFs/PPS compound fiber paper can be applied in fluorescence anti-counterfeiting. The modification strategy and the application in this paper will provide a good choice for studying the optical properties of perovskites and broaden the application of ACFs/PPS compound fiber paper.
Fig. 3 (a) SEM image of Cs2SnCl6. (b–f) Elemental mapping images of Cs, Sn, Te, Cl in Cs2Sn0.94Te0.06Cl6. (g) SEM image of Cs2Sn0.94Te0.06Cl6. |
The PL and photoluminescence excitation (PLE) spectra of Cs2Sn1−xTexCl6 PCs are shown in Fig. 4a and b. The un-doped Cs2SnCl6 was hardly luminous, while the PL intensities of Cs2Sn1−xTexCl6 were significantly enhanced. It could be observed that the PL intensity of Cs2Sn1−xTexCl6 was the highest when x = 0.06 and then decreased with the increase in Te4+ doping. According to previous reports, this reduction in PL was attributed to the concentration quenching process where the excitation energy migrated to the quenching sites through the lattice.27 They calculated the critical distance (R) for [TeCl6]2−–[TeCl6]2− energy transfer. According to the formula R6 = (0.6 × 1028)(4.8 × 10−16f/E4)SO, the calculated R is almost three times the shortest Te–Te distance, which means that there is the possibility of energy transfer. f is the oscillator strength of the relevant 1S0→3P1 transition and is taken to be 10−2; E, the energy of maximum spectral overlap, is 2.5 eV; SO, the spectral overlap of emission and absorption, amounts to 0.1 eV−1. The peak position of the PL intensity showed a slight red shift with the increase in the amount of Te4+ doped. For halide perovskites with a soft lattice, the photogenerated electrons at the excited state levels tend to be coupled with lattice vibration, causing transient lattice distortion, changing nuclear coordinates, and producing local STE.28 Tetravalent Te, as a typical ion with the outer electronic configuration of 5s2, has five energy levels: ground state of 1S0 and singlet/triplet excited states of 1P1/3Pn (n = 0, 1, 2).29 Self-trapping excitons widely exist in perovskites.30 In some perovskite crystals, the electrons and holes generated by excitation will immediately be self-trapped because the self-trapped state is more stable. According to other reports, both a broad PL band and large Stokes shift are typical features of STE emission.31,32 Take Cs2Sn0.94Te0.06Cl6 for example, the PL band starts at 450 nm and ends at 700 nm. The excitation wavelength is 385 nm, and the emission wavelength is 553 nm. Based on these data, it can be reasonably inferred that the fluorescence property of Te-doped Cs2SnCl6 is due to STE emission. Fig. 4c and d are the PL and PLE spectra of Cs2Sn0.94Te0.06Cl6 PCs. It can be observed that the PL and PLE spectra almost maintain the same wave profile no matter how the excitation and emission change, which proves that the emission band is the luminescence characteristic of Cs2Sn0.94Te0.06Cl6 PCs, not the surface defects or the lattice defects of the samples. The test results verify the STE and ion luminescence characteristics of Cs2Sn1−xTexCl6. Fig. 4e and f are the two-dimensional (2D) and three-dimensional (3D) fluorescence spectra of Cs2Sn0.94Te0.06Cl6 PCs. It can be observed that the emission wavelength varies between 500 nm to 600 nm, while the excitation wavelength varies between 250 nm to 450 nm. The time-resolved (TRPL) decay curves (shown in Fig. 5a) of Cs2Sn1−xTexCl6 PCs can be well fitted with the bi-exponential eqn (1):33
I(t) = A1exp(−t/τ1) + A2exp(−t/τ2). | (1) |
Fig. 5 (a) TRPL decay curves of Cs2Sn1−xTexCl6 (x = 0.02, 0.06, 0.08) PCs. (b) Chromaticity coordinates (CIE 1931) of Cs2Sn1−xTexCl6 (x = 0.02, 0.04, 0.06, 0.08, 0.10) PCs. |
The average fluorescence lifetime can be calculated using eqn (2):21
τavg = (A1τ12 + A2τ22)/(A1τ1 + A2τ2). | (2) |
It can be calculated that the average fluorescence lifetimes of Cs2Sn1−xTexCl6 PCs are 2.06 μs (x = 0.02), 38.61 μs (x = 0.06) and 1.83 μs (x = 0.08). At the same time, combined with the PL spectra (Fig. 4(a)), the emission intensity of the sample is positively related to its lifetime. The chromaticity coordinate (CIE) values of un-doped Cs2SnCl6 PCs and Cs2Sn1−xTexCl6 can be calculated to be (0.23, 0.21), (0.38, 0.52), (0.39, 0.54), (0.39, 0.55), (0.39, 0.54), (0.38, 0.53), which appeared in the yellow area of the CIE (Fig. 5b). The color purity of Cs2Sn1−xTexCl6 PCs can be calculated from eqn (3):34
(3) |
Meanwhile, we modified the ACFs/PPS compound fiber paper by post-processing it with Cs2Sn0.94Te0.06Cl6 PCs solution. Fig. 6a shows that the modified ACFs/PPS compound fiber paper showed bright yellow light under UV light while the unmodified paper did not. Although stored in air for more than 4500 h, the modified fiber paper still showed a bright yellow light (Fig. 6b). Furthermore, the modified paper can still maintain luminescence characteristics when it was immersed in a weak acid solution (Fig. 6f). The PLQY of the modified fiber paper soaked in a weak acid solution was 17.63%. It could be observed that the fluorescence intensities of the modified fiber paper decreased with the increase in temperature (Fig. 6c). It was similar to the reduction in PL intensity due to the participation of phonons in nonradiative recombination at high temperatures. The thermal stability of the modified ACFs/PPS compound fiber paper can be verified by the thermogravimetric analysis (Fig. 6d). PL spectra of the ACFs/PPS composite fiber paper were measured at 50 °C and after falling back from 250 °C (shown in Fig. S4(a)†). The modified composite fiber paper could retain 82% of the original emission intensity when it fell from 250 to 50 °C. In addition, PL intensities of the ACFs/PPS composite fiber paper were measured at different temperatures. The intensity of PL decreased gradually with increasing temperature (shown in Fig. S4(b)†). However, after heating for 5 min, an enhancement in the PL intensity at some temperatures can be seen. Furthermore, the average intensity of PL can still maintain 98% of the original PL intensity when it falls from 100 to 40 °C. It can be observed that the sublimation temperature of the modified ACFs/PPS compound fiber paper is about 300 °C (Fig. 6c). Based on the above data, it can be concluded that the ACFs/PPS composite fiber paper has a certain degree of thermal stability. These test results have laid a good foundation for use of the modified ACFs/PPS compound fiber paper as a fluorescent anti-counterfeiting material.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr00301a |
This journal is © The Royal Society of Chemistry 2023 |