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• Lead-based halide materials as a new generation of optoelectronic materials have attracted much attention in light emitting diodes (LEDs) [1–4], photodetectors [5–9], scintillators [10–14], and solar cells [15–19], due to their direct and adjustable bandgap, strong light absorption coefficient, and high photoluminescence quantum yield (PLQY). However, due to the physical and chemical instability and the toxicity of Pb²⁺, there are certain limitations in their applications, therefore, the study of lead-free halide materials has attracted much attention. A series of metal ions have been served to replace lead ions to avoid the drawbacks of lead-based halide materials. A possible non-toxic alternative is substitution of Pb²⁺ with the similar electronic configurations such as Sn²⁺, Ge²⁺ [20,21]. However, the main issue with Sn²⁺ or Ge²⁺ is that they are easily oxidized to tetravalence, thereby damaging the crystal structure and photoelectric properties of halide materials [22]. Zinc ions, as divalent ions, have the potential to replace lead ions due to their non-toxicity and stable valence [23].

  Self-trapped excitons (STEs) emission have gained much attention owing to large Stokes shift and broadband emission characteristics in metal halide materials [24–26]. In the soft lattice, when the electron-phonon interaction is strong enough, the electron-hole pairs of the excited state will cause elastic distortion in the lattice around them, thus forming the self-trapping. The Huang-Rhys factor (S), as a dimensionless constant, is usually used to describe the coupling strength of electrons and phonons in the lattice. It has been confirmed that the higher S value, the stronger electron-phonon coupling strength, and the greater possibility of STEs formation [27]. Excessive S can bring broadband emissions, but it will also cause the excitons in the excited state to emit phonons through non-radiative recombination and return to the ground state [28]. In other words, in order to achieve efficient emission of STEs, S should be an appropriate value, neither too large nor too small. However, the appropriate range of values for S has not been summarized.

  Ion doping is an important method to improve the luminescent performance of lead-free halide materials [29]. Ions with typical ns² electronic structure such as Sb³⁺, Te⁴⁺, Bi³⁺ are usually served as dopants to regulate the luminescence of halide materials because they can induce STEs emission in lead-free halide materials [30–35]. Due to electron-interaction and spin-orbit coupling matrix, nsnp excited configuration in ns² ions would split into four energy states ³P₀, ³P₁, ³P₂, and ¹P₁. Generally, the transitions ¹S₀ → ³P₀ and ¹S₀ → ³P₂ are forbidden. However, ¹S₀ → ³P₁ and ¹S₀ → ¹P₁ transitions are parity-allowed because of the spin–orbit coupling [25]. Therefore, doping ions with ns² outermost electronic configuration can provide new absorption and emission centers. Moreover, ns² electrons doped halide materials with broadband emission and large Stokes shift, which can effectively avoid self-absorption, are highly desirable for optical applications. Although single exciton emission has the characteristics of large Stokes shift and broadband emission, it still has some limitations, thus multiple exciton emissions have been considered. The multiple exciton emissions in single material have fundamental and practical implications in imaging, lighting applications and optoelectronic research [36,37]. For instance, the production of lighting devices such as white LEDs require multiple exciton emissions to boost color rendering performance. Consequently, it is necessary to dope a variety of ns² ions into lead-free halide materials to produce multiple exciton emissions and to study the excitation-acoustic coupling of various STEs.

  Herein, Cs₂ZnCl₄ SCs doped with Sb³⁺ and Te⁴⁺ have been fabricated by hydrothermal synthesis method. Taking advantage of the fact that ions with ns² electron structure can produce STEs emission, a series of anti-counterfeiting patterns have been prepared to achieve information encryption base on the varied luminescence characteristics of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs excited by selective excitation wavelengths. By analyzing the PL properties of the prepared zinc-based halides, we further studied the relationship between electron-phonon coupling of STEs and PLQYs, and summarized the semi-empirical range (25–40) of Huang-Rhys factors (S) for metal halides with excellent PL property based on the collected references.

  Sb³⁺ and Te⁴⁺ co-doped Cs₂ZnCl₄ SCs were synthesized through a hydrothermal synthesis method using ZnCl₂, SbCl₃, TeCl₄, CsCl, and concentrated HCl as the ingredients. Cs₂ZnCl₄:x%Sb³⁺,y%Te⁴⁺ means the feeding ratios of Sb/Zn and Te/Zn are x% and y%, respectively. The detailed synthesis procedure is provided in experimental section. Cs₂ZnCl₄ SCs crystallize in orthorhombic space group Pnma [38]. According to Fig. 1a, Cs₂ZnCl₄ belongs to typical zero-dimensional (0D) structure that Zn²⁺ ions occupy the center position of [ZnCl₄]²⁻ tetrahedrons and the isolated [ZnCl₄]²⁻ tetrahedrons are completely separated by peripheral Cs⁺ cations. According to powder X-ray diffraction (PXRD) patterns, Cs₂ZnCl₄ SCs with different doping concentrations of Sb³⁺ and Te⁴⁺ ions match well with the standard pattern of Cs₂ZnCl₄ (ICSD card No. 10-0318) (Fig. 1b). As shown in Fig. 1c, Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs present transparent under the natural light and become red exposed to 302 nm UV light, then turn yellow under the 395 nm light irradiation. And the corresponding PL spectra and CIE chromaticity are shown in Fig. S1 (Supporting information). Sb³⁺ and Te⁴⁺ ions were found to be uniformly distributed in Cs₂ZnCl₄ SCs according to energy dispersive X-ray spectroscopy (EDS) mapping (Fig. 1d). X-ray photoelectron spectroscopy (XPS) reveals the valence states of Sb³⁺, Te⁴⁺ and Zn²⁺ in Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs (Fig. 1e, Figs. S2a and b in Supporting information). Inductively coupled plasma optical emission spectrometry (ICP-OES) (Tables S1 and S2 in Supporting information) revealed that the actual Te⁴⁺ and Sb³⁺ concentrations are lower than its feeding concentrations, because the valence states of Te⁴⁺ and Sb³⁺ ions are different from Zn²⁺.

  Figure 1

  

  Figure 1.  Structural and compositional characterization of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. (a) Crystal structure diagram of Cs₂ZnCl₄. (b) Comparison of PXRD patterns of Cs₂ZnCl₄ doped with different contents of Te⁴⁺ and Sb³⁺. (c) Optical microscopic images of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs under natural light (top), 302 nm UV light (middle), and 395 nm UV light (bottom). (d) EDS mappings of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. (e) XPS spectrum of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺.

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  According to Fig. S2c (Supporting information), ultraviolet–visible (UV–vis) absorption spectra revealed that Sb³⁺ and Te⁴⁺ doping could introduce additional absorption peaks in the 250–500 nm range. Figs. 2a and b show that photoluminescence excitation (PLE) and photoluminescence (PL) spectra of Cs₂ZnCl₄:30%Sb³⁺ SCs and Cs₂ZnCl₄:5%Te⁴⁺ SCs, suggesting that they both feature broadband emissions and large Stokes shift. It could be seen that the broad red emission of Cs₂ZnCl₄:Sb³⁺ SCs centered at 720 nm with full-width at half-maximum (FWHM) of 190 nm comes from STEs emission of Sb³⁺ and the yellow emission of Cs₂ZnCl₄:Te⁴⁺ SCs centered at 590 nm (FWHM of 119 nm) originates from STEs emission of Te⁴⁺. The PLE spectrum of Cs₂ZnCl₄:30%Sb³⁺ contains two peaks at 260 and 315 nm, the former corresponds to the excitation peak of Cs₂ZnCl₄ matrix and the latter results from the contribution of Sb³⁺ orbitals [39]. The PLE and PL spectra of pure Cs₂ZnCl₄ are given in Fig. S3 (Supporting information), and it is indicated that Sb³⁺ and Te⁴⁺ ions successfully enter Cs₂ZnCl₄ unit cells and replace Zn²⁺ ions to occupy the center of the tetrahedrons [39,40]. Temperature-dependent PL spectra of Cs₂ZnCl₄:30%Sb³⁺ and Cs₂ZnCl₄:5%Te⁴⁺ SCs are measured from 100 K to 270 K (Figs. 2c and d). The PL intensity of Cs₂ZnCl₄:30%Sb³⁺ decreased gradually with increasing temperature, together with the blueshift of the PL peak, owing to the stronger nonradiative relaxation and electron-phonon coupling of Sb³⁺ when the temperature become higher [41]. The emission intensity of Te⁴⁺ decreased as the temperature increasing, which is associated with vibrational relaxation. To reveal the influence of electron-phonon coupling and obtain the Huang-Rhys factor (S), we analyzed the FWHM according to Eq. 1:

  (1)

  Figure 2

  

  Figure 2.  Normalized PL and PLE spectra of as-prepared (a) Cs₂ZnCl₄:30%Sb³⁺ SCs and (b) Cs₂ZnCl₄:5%Te⁴⁺ SCs. Pseudo color mapping of temperature-dependent PL spectra of (c) Cs₂ZnCl₄:30%Sb³⁺ SCs upon excitation at 315 nm and (d) Cs₂ZnCl₄:5%Te⁴⁺ SCs upon excitation at 420 nm along with temperature range from 100 K to 270 K. FWHM of the PL spectra as a function of temperature for (e) Cs₂ZnCl₄:30%Sb³⁺ SCs and (f) Cs₂ZnCl₄:5%Te⁴⁺ SCs, data are fitted by Eq. 1.

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  where the ħω_phonon is the phonon energy, k_B is the Boltzmann constant. The Huang-Rhys factor (S) and the phonon energy (ħω_phonon) are 27.63 and 22.65 meV for Cs₂ZnCl₄:30%Sb³⁺, respectively (Fig. 2e). For Cs₂ZnCl₄:5%Te⁴⁺ SCs, S is 25.84 and ħω_phonon is 23.40 meV (Fig. 2f). Such large S values indicate the strong electron-phonon coupling of Te⁴⁺ and Sb³⁺ in Cs₂ZnCl₄, which is responsible for the broadband and large Stokes shift of Sb³⁺ (445 meV, 405 nm) and Te⁴⁺ (420 meV, 170 nm) emissions.

  Based on the wide emission characteristics of STEs from Te⁴⁺ and Sb³⁺, we further synthesized Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. Compared with Cs₂ZnCl₄:Sb³⁺ and Cs₂ZnCl₄:Te⁴⁺ SCs, PL of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs are almost the same, however, PLE of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs come from the superposition of PLE of Cs₂ZnCl₄:Sb³⁺ and Cs₂ZnCl₄:Te⁴⁺ SCs (Figs. 3a and b). Firstly, we synthesized Sb³⁺ doped Cs₂ZnCl₄ SCs with Sb/Zn feeding ratio of 10%, 20%, 30%, 40% and 50% (Fig. S4 in Supporting information). The actual Sb³⁺ doping concentrations are shown in Table S1 (Supporting information). It could be seen that with increasing Sb³⁺ doping concentration, the emission of Cs₂ZnCl₄:Sb³⁺ first increases and then decreases owing to the concentration quenching effect, achieving a maximum of FWHM up to 192 nm when the feeding ratio of Sb/Zn is 30% (Fig. S5a in Supporting information). According to the PL decay curves of Cs₂ZnCl₄:Sb³⁺ SCs, the lifetime based on single exponential-fitting is calculated to be 12.61, 12.68, 13.07, 12.84 and 12.10 µs for Cs₂ZnCl₄:10%−50%Sb³⁺, respectively (Fig. S5b in Supporting information). In addition, the lifetimes of Cs₂ZnCl₄:Sb³⁺ SCs show firstly a slightly monotonic increase and then decrease with increasing Sb³⁺ concentration. This is related to the increased non-radiative transition [42]. From the optimal concentration of Sb³⁺ doping, we further doped Te⁴⁺ with different concentrations from 10% to 50% to form Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. From the PL spectra of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs (Fig. S6a in Supporting information), it was found that the emission intensity also increases and then decreases with increasing the feeding ratio, satisfying the concentration quenching effect, and the optimum doping concentration of Te⁴⁺ was determined to be 40%. Time-resolved photoluminescence decay experiments (Fig. 3c) show that the lifetimes measured at 590 nm are 47.48, 52.49, 52.53, 58.41 and 56.80 ns for 10%−50%Te⁴⁺ co-doped Cs₂ZnCl₄ SCs, respectively, which are comparable to the decay lifetimes of STEs emissions of reported low-dimensional metal halides [40,43]. It can be easily seen from Fig. 3c that under 375 nm excitation, the decay times of the inherent radiation for different Te⁴⁺ concentrations monitored at 590 nm also firstly increase and then decrease slightly with increasing Te⁴⁺ concentration, which is associated with the non-radiative transition [42]. Furthermore, the lifetimes of Cs₂ZnCl₄:30%Sb³⁺,y%Te⁴⁺ monitored at 720 nm have been collected in Fig. S6b (Supporting information) and there is almost no significant change with the change of Te⁴⁺ concentration. As a result, we believe that Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs are the appropriate choice for subsequent discussion of Sb³⁺ and Te⁴⁺ co-doped Cs₂ZnCl₄ SCs. From Fig. 3d, the PL emission band (λ_ex = 365 nm) could be fitted as fit peaks 1 and 2, corresponding to Sb³⁺ and Te⁴⁺ emission, respectively, indicating the formation of double broad peaks of emission under the same excitation. Based on previous reports, the origins of PL of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs are illustrated in Fig. 3e [44,45]. Upon different excitations, electrons are first excited from 5s states of Sb³⁺ or Te⁴⁺ ions to their 5p states and then trapped in [SbCl₄]⁻ or [TeCl₄] tetrahedrons, which sequentially induced triplet dopant STEs, respectively. For Te⁴⁺ ions, by considering the contribution of lattice vibration to the transition process and the transition selection law, the high-energy band of PLE could be attributed to the ¹S₀ → ³P₂ transition of Te⁴⁺ via spin–orbit coupling, which is a double peak due to the Jahn-Teller effect. And the low-energy band of PLE is attributed to the ¹S₀ → ³P₁ transition of Te⁴⁺ [46]. Besides, the PLQYs of Cs₂ZnCl₄:30%Sb³⁺ and Cs₂ZnCl₄:5%Te⁴⁺ are 15.2% and 1.3%, respectively (Fig. S7 in Supporting information), the PLQY of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ is 0.8% (Fig. S8a in Supporting information). According to Fig. 2 and Figs. S8b and c (Supporting information), S of Cs₂ZnCl₄:Sb³⁺, Cs₂ZnCl₄:Te⁴⁺ and Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ are determined to be 27.63, 25.84 and 17.13, respectively. It seems to be a correlation between PLQYs and S.

  Figure 3

  

  Figure 3.  Optical characterization of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (a) PLE (λ_em = 720 nm) and PL (λ_ex = 315 nm) spectra of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (b) PLE (λ_em = 590 nm) and PL (λ_ex = 420 nm) spectra of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (c) Decay curves of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs with different Te⁴⁺ doping concentrations. (d) PL spectrum and fitting curves of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs under 365 nm excitation. (e) Energy level diagram of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs showing the excitation and emission processes.

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  In metal halide materials, the electron-phonon interaction plays an important role in the optical and electrical properties. As a dimensionless parameter, the S is mainly used to describe the strength of electron-phonon interaction. Generally speaking, when S >> 1, the electron-phonon interaction is characterized as strong coupling. Conversely, when S << 1, the electron-phonon interaction is considered to be weak coupling. For the STEs formation, the excitons are subject to carrier-phonon interaction, relaxation processes and internal conversion, and lose the energy known as self-trapping energy (E_st). Owing to strong coupling between phonons and excitons leading to lattice distortion, STEs emissions are typically characterized by broadband emission with FWHM of more than 60 nm. The lattice distortion energy (E_d) and ground state energy (E_GS) increase as the energy of STEs decreases during lattice distortion. Therefore, the emission energy (E_PL) can be described with the equation E_PL = E_g − E_b − E_st − E_d, where E_g is bandgap energy and E_b is exciton binding energy. The well-known S-factor is used to assess the tendency of STEs formation. Both S and E_st describe the degree of stabilization of the self-trapped state with respect to the untrapped state, resulting in the Stokes shift. The coordinate difference (ΔQ) between STEs and free excitons (FEs) is directly proportionate to S, based on ΔQ = S ^1/2. According to this formula, it can be concluded that the larger S, the greater the Stokes shift of self-trapped exciton emission.

  The Huang-Rhys factor (S) also affects PL emission via the Franck-Condon factor. And the Franck-Condon factor is calculated by the recurrence method [47]. Assuming that the excited and ground states have similar phonon frequencies, the Franck-Condon factor (F, at zero temperature) could be simplified as

  (2)

  where χ_m is the excited state harmonic phonon wavefunctions and χ_n is the ground state harmonic phonon wavefunctions. The photoluminescence peak appears at n ≈ S, thus

  (3)

  which is a monotonic decreasing function of S. Since the PL intensity is proportionate to S, the larger S, the smaller the radiative rate and the lower the emission efficiency. Therefore, the S could be potentially used as the key for the design of efficient STEs emission. To obtain effective emission of STEs, the Huang-Rhys factor should be intermediate. When the S is overly large, there is strong phonon assisted non-radiative recombination, however, the too small S can disrupt the stability of the STEs state and weaken broadband emission. The S and PLQYs of various metal halide materials are collected in Table S3 (Supporting information) to get more specific semi-empirical range of S. In order to be more intuitive, a scatter diagram with S as x axis and PLQY as y axis has been provided in Fig. S9 (Supporting information). When S of metal halide materials is below 25 or above 40, the PLQYs of halide materials we collected become lower. In other words, S of metal halide luminescent materials with excellent PLQY is between 25 and 40. The PLQY of Cs₂ZnCl₄:30%Sb³⁺ with S in such semi-empirical range increases by order of magnitude compared with that of Cs₂ZnCl₄:5%Te⁴⁺ and Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺. The semi-empirical range of S might be used as a reference for finding materials with high PLQYs. It is worth noting that the extent of PLQY is not entirely dependent on S, it is also closely related to factors such as substrate material, type and concentration of doping ion, volume defects. The PLQY of the synthesized Cs₂ZnCl₄:30%Sb³⁺ is not high enough, possibly due to the volume defects produced during the rapid cooling process in the synthesis process, thus reducing the luminescence.

  In addition to the ultra-wide emission of Sb³⁺ and Te⁴⁺ STEs, Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs also have excellent environmental stability. The thermogravimetric analysis (Fig. S10a in Supporting information) shows that Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs have exceptional thermal stability because it does not decompose until 600 ℃, which is fully compatible with the use of the material under normal conditions. Moreover, as shown in the unchanged PXRD patterns after five months of exposure to the ambient atmosphere (Fig. S10b in Supporting information), Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs overcome the humidity sensitivity of lead halide materials. After five months, the luminescence intensity of the SCs remained at a high level, with 80.4% of the initial PL intensity for Te⁴⁺ and 89.5% for Sb³⁺ (Fig. S11 in Supporting information). The above results demonstrate the excellent stability of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs, which gives them further potential for practical applications.

  The multi-exciton emissions of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs enable dynamic color emissions by precisely adjusting the excitation wavelength (Figs. 4a and b). For instance, 315 nm light mainly excites the STEs of Sb³⁺, resulting in red emission, and 420 nm excitation light mostly excites the STEs of Te⁴⁺ to produce yellow emission (Fig. 4c). Since the excitation spectra of Sb³⁺ STEs and the Te⁴⁺ STEs partially intersect, Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs can achieve continuous adjustable emission from red to yellow by changing the excitation wavelength from 315 nm to 420 nm. The color coordinates of yellow emission at 590 nm is (0.5054, 0.4789) and the color coordinates of red emission at 716 nm is (0.5634, 0.4214). The excitation-dependent emission of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs offers a possibility for the encryption of information. The encryption and decryption process are presented in Fig. 4d and a series of anti-counterfeiting patterns are produced by using the powder ground of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs and Cs₂ZnCl₄:Sb³⁺ (Fig. 4e). As a proof-of-concept, Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ and Cs₂ZnCl₄:Sb³⁺ powder were selectively filled to obtain different pixelated patterns (3 × 5 dot matrix). The pattern composed of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ and Cs₂ZnCl₄:Sb³⁺ powder appeared uniformly red under 302 nm excitation. However, under the excitation of 395 nm lamp, cryptographic information emerged as orange emission owing to chromatic luminescence switching of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs and displayed the numbers “2,3,4,8” separately. These performances indicate the SCs have great potential for anti-counterfeiting application.

  Figure 4

  

  Figure 4.  Dynamic color delivery in Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. (a) Contour plot of excitation-dependent PL of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (b) PL spectra of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs under different excitation wavelengths. (c) The corresponding CIE chromaticity coordinates of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs under 302 nm (bottom) and 395 nm (top) excitation. (d) Schematic diagram of information encryption. (e) Pattern encryption and decryption process under natural light (top), 302 nm UV light (middle) and 395 nm light (bottom).

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  In summary, we synthesized Zn-based halides doped with Sb³⁺ and Te⁴⁺, and investigated the STEs emissions. Furthermore, we fabricated the anti-counterfeit patterns based on the continuously tunable emissions of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs to achieve the effect of information encryption. The relationship between electron-phonon coupling of STEs and PLQYs was studied. Based on the collected references, the semi-empirical range of the Huang-Rhys factors for metal halide materials with excellent PL property has been summarized, which could provide the reference for designing the high-performance metal halide materials.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the financial aid from the National Natural Science Foundation of China (No. 22271273), and International Partnership Program of Chinese Academy of Sciences (No. 121522KYSB20190022).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109800.

  
  
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  Figure 1  Structural and compositional characterization of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. (a) Crystal structure diagram of Cs₂ZnCl₄. (b) Comparison of PXRD patterns of Cs₂ZnCl₄ doped with different contents of Te⁴⁺ and Sb³⁺. (c) Optical microscopic images of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs under natural light (top), 302 nm UV light (middle), and 395 nm UV light (bottom). (d) EDS mappings of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. (e) XPS spectrum of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺.

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  Figure 2  Normalized PL and PLE spectra of as-prepared (a) Cs₂ZnCl₄:30%Sb³⁺ SCs and (b) Cs₂ZnCl₄:5%Te⁴⁺ SCs. Pseudo color mapping of temperature-dependent PL spectra of (c) Cs₂ZnCl₄:30%Sb³⁺ SCs upon excitation at 315 nm and (d) Cs₂ZnCl₄:5%Te⁴⁺ SCs upon excitation at 420 nm along with temperature range from 100 K to 270 K. FWHM of the PL spectra as a function of temperature for (e) Cs₂ZnCl₄:30%Sb³⁺ SCs and (f) Cs₂ZnCl₄:5%Te⁴⁺ SCs, data are fitted by Eq. 1.

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  Figure 3  Optical characterization of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (a) PLE (λ_em = 720 nm) and PL (λ_ex = 315 nm) spectra of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (b) PLE (λ_em = 590 nm) and PL (λ_ex = 420 nm) spectra of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (c) Decay curves of Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs with different Te⁴⁺ doping concentrations. (d) PL spectrum and fitting curves of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs under 365 nm excitation. (e) Energy level diagram of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs showing the excitation and emission processes.

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  Figure 4  Dynamic color delivery in Cs₂ZnCl₄:Sb³⁺,Te⁴⁺ SCs. (a) Contour plot of excitation-dependent PL of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs. (b) PL spectra of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs under different excitation wavelengths. (c) The corresponding CIE chromaticity coordinates of Cs₂ZnCl₄:30%Sb³⁺,40%Te⁴⁺ SCs under 302 nm (bottom) and 395 nm (top) excitation. (d) Schematic diagram of information encryption. (e) Pattern encryption and decryption process under natural light (top), 302 nm UV light (middle) and 395 nm light (bottom).

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