Multicolor luminescence of Cs₂KLuCl₆ for anti-counterfeiting and information encryption applications

Abstract

Sb³⁺-doped inorganic perovskites have attracted great attention for anti-counterfeiting and information encryption applications due to their excellent optical properties. We have synthesized a novel perovskite, Cs₂KLuCl₆, which exhibits weak cyan emission originating from the self-trapped excitons (STEs) of the host. Its photoluminescence quantum yield is enhanced from 2% to 68% by Sb³⁺ doping. The energy transfer channel from the STEs to Ho³⁺ was constructed, and the luminescent color was modulated from cyan to white with increasing Ho³⁺ doping concentration. The Sb³⁺/Ho³⁺ co-doped sample has two-color luminescence performance due to the introduction of new emission centers. Up-conversion green emission was obtained by introducing Yb³⁺/Er³⁺. Finally, the optical properties of the obtained samples were utilized to design an anti-counterfeit label and information encryption strategy.

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1. Introduction

The proliferation of counterfeiting is seriously jeopardizing socio-economic development, so the development of anti-counterfeiting and encryption technologies plays a crucial role in maintaining national economic security.^1–4 In recent decades, many anti-counterfeiting and encryption techniques have been developed, such as color patterns, bar codes and digital watermarks. However, these traditional anti-counterfeiting technologies are easily replicated and have insufficient anti-counterfeiting capabilities, so there is an urgent need to develop reliable anti-counterfeiting techniques to curb the proliferation of counterfeiting problems.^5,6 Luminescent materials activated under specific conditions are widely used in the field of anti-counterfeiting due to their advantages of flexible design, simple operation and high confidentiality,^7–9 for example, recognizing the difference between genuine and counterfeit banknotes under the irradiation of ultraviolet (UV) light. To date, many types of luminescent materials, such as metal halides,^10,11 quantum dots,¹² organic dyes^13,14 and rare-earth ion-doped materials,¹⁵ have been developed. Among them, rare-earth elements have abundant electronic energy levels, and luminescence ranges from the ultraviolet to the infrared, showing excellent anti-counterfeiting properties.^4,16 However, an unavoidable disadvantage is the weak absorption cross-section of rare-earth ions, resulting in weak luminescence intensity. In addition, the luminescent color of materials is usually the characteristic emission of rare-earth elements, which can be easily replicated by using materials emitting the same color, so the anti-counterfeiting performance can be greatly compromised.

Lead halide perovskites have become one of the most popular materials in the field of luminescence in recent years, and have demonstrated great potential for application in the field of anti-counterfeiting and information encryption.^17–19 For example, Chen et al. found that the luminescent color of CsPbBr₃ PNCs-dichloromethane gradually blue-shifted with the extension of UV treatment time, and uniquely, the color gradually recovered after water treatment, which was attributed to the differing migration behavior of the halide ions (Cl⁻ and Br⁻) in light and humid environments. Based on this dynamic luminescence process, which is difficult to imitate, an advanced anti-counterfeiting system has been designed.²⁰ However, the toxicity of lead has severely limited the commercialization of lead halide perovskites. Therefore, as next-generation materials to replace lead-based perovskites, double perovskites (DPs) have been attracting great attention in the scientific community.^21–23 In addition, since a wide variety of rare-earth ions can occupy the trivalent metal sites of DPs, more novel anti-counterfeiting materials are destined to be produced. For example, Wang et al. developed a novel rare-earth-based DP Cs₂NaHoCl₆ with enhanced down-conversion (DC) and up-conversion (UC) luminescence by Sb³⁺ and Yb³⁺ doping, respectively. Surprisingly, its UC luminescence produces changes visible to the naked eye with the increase of Yb³⁺ doping concentration or excitation power. Finally, they verified the advanced anti-counterfeiting properties of the Sb³⁺/Yb³⁺-doped samples.²⁴ Zhu et al. introduced Er³⁺ into Cs₂NaYbCl₆, which exhibited effective DC red emission under 380 nm excitation. Moreover, excellent UC yellow and green emission colors were exhibited under 980 and 1540 nm excitation, respectively. Finally, high-level anti-counterfeiting and information encryption applications were realized based on dual-mode multicolor fluorescence.²⁵

In this paper, a novel rare-earth-based perovskite Cs₂KLuCl₆ (CKLC) was synthesized by a simple solvothermal method. Furthermore, Sb³⁺, Sb³⁺/Ho³⁺ and Er³⁺/Yb³⁺ were introduced. The photoluminescence (PL) intensity from the self-trapped excitons (STEs) of the host was greatly enhanced by doping of Sb³⁺, and the photoluminescence quantum yield (PLQY) was increased from 2% to 68%. Subsequently, the energy transfer (ET) channel of STEs → Ho³⁺ was constructed to break the weak absorption of Ho³⁺ while modulating the luminescent color. CKLC:0.3%Sb³⁺/25%Ho³⁺ emits warm white light under UV excitation and red light under 450 nm irradiation. Using its two-color luminescence, an anti-counterfeit label was prepared. To achieve more luminescence possibilities, we also introduced Yb³⁺/Er³⁺, confirming its UC luminescence mechanism as a three-photon process. Finally, the obtained samples above were used together for the application of information encryption.

2. Experimental section

2.1 Chemicals

All rare earth oxides (Lu₂O₃, Ho₂O₃, Er₂O₃ and Yb₂O₃, 99.99%), KCl (99.5%), HCl (37%) and CH₃CH₂OH (99.7%) were purchased from China Chuandong Chemical Reagents Company. CsCl (99.9%, Aladdin) and SbCl₃ (99%, Macklin) were also used for materials synthesis in this work. To form a 0.1 mol L⁻¹ SbCl₃ solution, 1.1406 g of SbCl₃ solid was dissolved in 50 mL of 12 M HCl. All chemicals were used without further purification.

2.2 Synthesis

For the preparation of pristine and ion-doped Cs₂KLuCl₆ crystals, 1 mmol CsCl, 0.4 mmol KCl and 0.25 mmol Lu₂O₃ were thoroughly ground and mixed, then loaded into a 30 mL Teflon liner, followed by the addition of 0.6 mL of 12 M HCl. SbCl₃ solution, Ho₂O₃, Er₂O₃ and Yb₂O₃ (while maintaining the total amount of M³⁺ as 0.5 mmol without change) were added into the above solution or solid to obtain Sb³⁺-doped, Sb³⁺/Ho³⁺ co-doped and Er³⁺/Yb³⁺ co-doped Cs₂KLuCl₆ crystals. The solution mixture was heated at 180 °C for 5 h in a stainless steel Parr autoclave and then it was cooled naturally to room temperature. The products were filtered out, washed with ethanol several times and dried at 70 °C. Samples were ground into powder from crystal particles for subsequent measurements.

2.3 Characterization

The phase and structure of samples were analyzed by powder X-ray diffraction performed on a Purkinje General Instrument MSAL-XD3 using Cu Kα radiation (λ = 0.15406 nm) under the voltage and current conditions of 36 kV and 20 mA, and at a scanning rate of 8° min⁻¹ in the 2θ range from 5° to 50°. The scanning electron microscopy (SEM) imaging, and the energy dispersive spectroscopy (EDS) and element mappings of the samples were conducted using a field emission scanning electron microscope (Sigma 360, ZEISS). The Raman spectra were collected on a Raman spectrometer (inVia 10400, Renishaw, UK) at 532 nm excitation. The PL, photoluminescence excitation (PLE) and time-resolved PL spectra were recorded using an Edinburgh FS5 fluorescence spectrophotometer. UC emission spectra were recorded using an Edinburgh FS5 fluorescence spectrometer combined with a 980 nm laser. PLQY measurement was carried out using an absolute PL quantum yield spectrometer (FLS1000). All characterization studies and identifications were performed at room temperature.

3. Results and discussion

3.1 Phase structure

A solvothermal method was used to obtain the CKLC DP matrix and the corresponding ion-doped samples. The crystal adopts a cubic structure belonging to the space group Fm3̄m. Furthermore, the structure of CKLC is shown in Fig. 1a, in which K⁺ and Lu³⁺ form [KCl₆]⁵⁻ and [LuCl₆]³⁻ octahedra with six Cl⁻, respectively. These octahedra are arranged alternately, and Cs⁺ ions with larger radius are embedded in the octahedral void. The SEM image illustrates the formation of micron-sized particles, while the element mappings demonstrate that the sample consists of the elements Cs, K, Lu, and Cl, which are uniformly distributed in the crystal (Fig. 1b). Due to having the same valence state, coordination number (CN = 6) and similar ionic radius value as Lu³⁺, the introduced Sb³⁺, Ho³⁺, Er³⁺ and Yb³⁺ will occupy the lattice sites of Lu³⁺. Powder X-ray diffraction (PXRD) patterns show that the synthesized samples match the simulated values and do not feature impurity peaks (Fig. 1c). The peak shift observed in the experimental PXRD pattern of the CKLC matrix, compared to the simulated data, can be attributed to multiple factors, including differences in the instrument, tablet thickness and doping conditions. Importantly, this systematic deviation does not affect the sample's purity. Furthermore, compared to the Cs₂KLuCl₆ matrix materials, the diffraction peaks shift toward lower angle with increasing concentration of Ho³⁺, Er³⁺ and Yb³⁺, suggesting expansion of the crystal lattice, which confirms the successful incorporation of dopants into the Lu³⁺ site based on Bragg's law. It is noteworthy that the diffraction peaks of Cs₂KLuCl₆:Sb³⁺ hardly shift due to the low doping concentration of Sb³⁺. In addition, the SEM images, the element mapping results (Fig. S1–S3) and the EDS results (Table S1) of the ion-doped CKLC confirmed the material's particle size, chemical composition and homogeneous distribution of elements within the crystal. Specifically, the actual doping level is close to the nominal doping level (Table S2). Additionally, the Raman spectra (Fig. S4) of CKLC and CKLC:0.3%Sb³⁺ exhibit characteristic peaks similar to those of materials such as Cs₂NaHoCl₆ ²⁴ and Cs₂NaInCl₆.^26,27 These peaks originate from the vibrations of the [LuCl₆]³⁻ octahedra, indicating the formation of a double perovskite structure.

Fig. 1 (a) Crystal structure of the CKLC. (b) SEM image and element mappings of Cs, K, Lu and Cl elements in the CKLC crystal. (c) PXRD patterns of the undoped and ion-doped CKLC.

3.2 Optical properties of Cs₂KLuCl₆ and Cs₂KLuCl₆:Sb³⁺

We found the CKLC DP could emit faint light under UV irradiation. The PL spectrum indicates the host exhibits a broadband weak emission peaking at 498 nm with a large Stokes shift of 183 nm under 315 nm excitation (Fig. 2a). Furthermore, the PLE spectra monitored at different emission wavelengths have almost the same shape and features, proving the emission comes from the same excited state relaxation (Fig. 2b). To further study the origin of the PL emission of CKLC, we measured the time-resolved PL spectrum at room temperature. The emission at 498 nm displays slow decay with a long lifetime of 2.71 μs, excluding free exciton emission because its lifetime is generally on the ns scale (Fig. S5).²⁸ Therefore, we reasonably attribute the broadband weak emission with a large Stokes shift and long PL lifetime to the intrinsic host STEs of CKLC.^29,30

Fig. 2 (a) PL spectrum and (b) PLE spectra, monitored at different wavelengths, of CKLC. (c) PL spectra of CKLC with different Sb³⁺ concentrations under 315 nm excitation. (d) PLE spectra of CKLC and CKLC:0.3%Sb³⁺.

Sb³⁺ with ns² lone pairs is commonly used as a less toxic and highly efficient dopant to form [SbCl₆]³⁻ octahedra to enhance the STE emission of DPs, thus its introduction in this study.^31,32 As expected, the luminescence intensity of CKLC can be greatly increased by introducing trace amounts of Sb³⁺ due to the increased density of self-trapped exciton states and CKLC:Sb³⁺ exhibits the same PL spectra as CKLC under 315 nm excitation (Fig. 2c).^26,27,33 The luminescence intensity of CKLC is maximized when the Sb³⁺ doping concentration is 0.3 mol%, and PLQY increases from 2% in CKLC to 68% in CKLC:0.3%Sb³⁺. The CKLC:0.3%Sb³⁺ perovskite demonstrates enhanced PLQY compared to PLQY values reported for CsCu₂I₃ (17.7%),³⁴ Cs₂ZrCl₆ nanocrystals (75.6%),³⁵ and CsPbBr₃ quantum dots (41.9%).³⁶ Subsequently, however, the PL intensity decreases when further increasing the Sb³⁺ concentration due to the concentration quenching effect (Fig. 2c).³⁷ The PLE spectra of CKLC and CKLC:Sb³⁺ further suggest Sb³⁺ doping can prompt the sensitization of STE states (Fig. 2d). In addition, the ¹S₀ → ³P₁ (A band) transition of Sb³⁺, which has broad excitation bands with two peaks at 316 and 335 nm, may be identified. The Sb³⁺ excitation bands at about 277 nm and 250 nm can be assigned to the vibration-induced ¹S₀ → ³P₂ (B band) and dipole-allowed ¹S₀ → ¹P₁ (C band) transitions, respectively.³⁸

3.3 Optical properties of Cs₂KLuCl₆:Sb³⁺/Ho³⁺

It is well known that an effective ET channel can break the low luminous intensity caused by the small absorption cross-section of rare-earth ions.³⁹ Considering that CKLC:0.3%Sb³⁺ has a large excitation band in the UV region and cyan is the complementary color of red, here we introduce Ho³⁺ to modulate the luminescent color and increase its luminescence intensity. As shown in Fig. 3a, when Ho³⁺ was introduced, the characteristic emission originating from Ho³⁺ was observed in addition to STE emission under 315 nm excitation. The spectral peaks at 490, 543 and 657 nm are attributed to the ⁵F₃, ⁵F₄/⁵S₂, ⁵F₅ → ⁵I₈ transitions of Ho³⁺, respectively. As expected, the emission of STEs originating from [SbCl₆]³⁻ octahedra decreases continuously with increasing Ho³⁺ content, which is caused by the ET from STEs to Ho³⁺, and interestingly the luminescent color also changes from cyan, first to white, and finally to red (Fig. 3b). Moreover, CKLC:0.3%Sb³⁺/25%Ho³⁺ emits warm white light with a color coordinate of (x = 0.3377, y = 0.3672) under the excitation of 315 nm. The inset in Fig. 3b is a picture of this perovskite under UV irradiation. Unlike our previously reported Sb³⁺/Ho³⁺ co-doped Cs₂NaGdCl₆,⁴⁰ in which the luminescent color can only be tuned from blue to red with increasing Ho³⁺ content. Meanwhile, the ET efficiency (η_ET) of CKLC:0.3%Sb³⁺/yHo³⁺ can be obtained from the following equation:
where τ_s and τ₀ are the lifetime values of STEs with or without the presence of Ho³⁺, respectively. As depicted in Fig. S6, the PL lifetime of STEs decreases with increasing Ho³⁺ content, which is consistent with the decreasing PL intensity of STEs.

Fig. 3 (a) PL spectra and (b) CIE color coordinates of CKLC:0.3%Sb³⁺/y%Ho³⁺. (c) PLE spectra of CKLC:0.3%Sb³⁺ and CKLC:0.3%Sb³⁺/25%Ho³⁺. (d) PL spectra of CKLC:x%Sb³⁺/25%Ho³⁺.

The PLE spectrum of CKLC:0.3%Sb³⁺/25%Ho³⁺ was recorded to further study the mechanism of excitation. In addition to the excitation band of STEs (230–350 nm), characteristic excitation peaks of Ho³⁺ at 449 and 459 nm are observed under monitoring of 657 nm irradiation, which correspond to the ⁵I₈ → ⁵F₁ and ⁵I₈ → ⁵G₆ transitions, respectively (Fig. 3c). This suggests that Ho³⁺ can be excited indirectly and directly.⁴¹ These sharp excitation peaks are due to the f–f transitions of Ho³⁺ from the ground state to excited state. However, the excitation band at 315 nm is larger and broader than that at 449 nm and 459 nm, indicating indirect excitation of Ho³⁺ is more effective than direct excitation. A comparison of the PL spectra of CKLC:0.3%Sb³⁺/25%Ho³⁺ under excitation at 315 nm and 449 nm is given in Fig. 3d, further illustrating that the weak absorption cross-section of the f–f transition from Ho³⁺ can be overcome by ET, thus increasing the PL intensity of Ho³⁺. Notably, the luminescence intensity of CKLC:0.3%Sb³⁺/25%Ho³⁺ was much greater than that of CKLC:25%Ho³⁺ under 315 nm excitation, indicating the ET channel of dopant STEs to Ho³⁺ is far more effective than the ET channel of host STEs to Ho³⁺ (Fig. 3d).

Based on the above discussion, we depict the possible mechanism of ET and transitions of CKLC:Sb³⁺/Ho³⁺ in Fig. 4. Under UV light excitation, the electrons of CKLC:Sb³⁺/Ho³⁺ are excited to the conduction band of the CKLC host and the 5p state of Sb³⁺. Subsequently, due to the strong electron–phonon coupling, the excited state electrons are trapped in the [LuCl₆]³⁻ and [SbCl₆]³⁻ octahedra, respectively, thus forming STE emission. At the same time, due to the existence of an energy transfer channel between STEs and Ho³⁺, part of the absorbed energy is transferred to the excited state of Ho³⁺ by radiative or non-radiative mechanisms, followed by non-radiative relaxation to ⁵F_J (J = 3, 4, 5), and finally the characteristic emissions from Ho³⁺ are generated when returning to the ground state. Thus, double emissions are generated in this process and the corresponding luminescent color can be modulated by controlling the doping concentration of Ho³⁺. In addition, the electrons in the ⁵I₈ ground state of Ho³⁺ can be directly excited to the ⁵F₁ level. After non-radiative relaxation to the ⁵F₃, ⁵F₄/⁵S₂, and ⁵F₅ levels, the electrons can return to the ground state, while fluorescing blue, green, and red emission colors, respectively.

Fig. 4 Energy transfer and transition mechanism of CKLC:0.3%Sb³⁺/25%Ho³⁺. GS represents the ground state and FE is the free exciton state.

3.4 Up-conversion luminescence properties of Cs₂KLuCl₆:Yb³⁺/Er³⁺

In order to obtain more luminescence possibilities in this matrix, we continued to incorporate Er³⁺/Yb³⁺ ions into CKLC. Because the concentration of doped ions will affect the emission intensity of fluorescent materials, it is necessary to explore the optimal doping concentration of Er³⁺ and Yb³⁺, respectively. When fixing the concentration of Yb³⁺, the emission position is unchanged when the doping concentration of Er³⁺ is changed, and the emission peaks consist of strong green emission (²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2) colors and a weak red emission (⁴F_9/2 → ⁴I_15/2) color (Fig. 5a). The maximum intensity is reached at 10% Er³⁺ doping and then decreases due to a concentration quenching effect. Following the same method and fixing the Er³⁺ concentration, the optimum doping concentration of Yb³⁺ was determined to be 35% (Fig. 5b). The UC spectra were recorded for different pump powers to study the mechanism of UC PL. As shown in Fig. 5c, the PL intensity increases with increasing excitation power, based on the formula:

I ∝ Pⁿ

where I is the UC PL intensity at different excitation powers, P is the pump power and n is the number of photons that need to be absorbed. Thus, a plot of logarithm of I versus logarithm of P should yield a straight line with slope n. As presented in Fig. 5d, the slopes of UC emission are 3.02, 2.95 and 3.39 for excitation at 524, 557 and 660 nm, respectively. The values are all close to 3, suggesting that a three-photon absorption process is responsible for the green and red emission colors. Notably, the red-to-green fluorescence ratio (R/G ratio) gradually increases as the excitation power increases, and the color coordinate also changes from the green (A: 93 mW) to the yellow light region (B: 978 mW) (Fig. S7).

Fig. 5 (a) UC PL spectra of CKLC:x%Er³⁺/35%Yb³⁺. (b) UC PL spectra of CKLC:10%Er³⁺/y%Yb³⁺. (c) UC PL spectra with different excitation power density values and (d) the dependence of pump power as well as UC PL intensity of CKLC:10%Er³⁺/35%Yb³⁺.

In general, the UC emission proceeds through a variety of processes such as ground state absorption (GSA), excited state absorption (ESA), ET and cross-relaxation (CR),⁴²because the absorption cross-section of Yb³⁺ is much larger than that of Er³⁺, and the energy absorbed by Yb³⁺ can be transferred to Er³⁺.⁴³ Therefore, Yb³⁺ acts as a sensitizer for absorbing the 980 nm laser and Er³⁺ acts as an activator. The possible up-conversion emission schematic is depicted in Fig. 6.^44–46 After a 980 nm photon is absorbed through the GSA process, an electron of Yb³⁺ transfers from the ground state ²F_7/2 to the excited state ²F_5/2. In addition, a small number of electrons from Er³⁺ can also be excited from ⁴I_15/2 to ⁴I_11/2 through the GSA process, while most electrons of Er³⁺ are transferred to the level of ⁴I_11/2 by the ET of ²F_5/2(Yb³⁺) + ⁴I_15/2(Er³⁺) → ²F_7/2(Yb³⁺) + ⁴I_11/2(Er³⁺). For the green emission, the populated ⁴I_11/2 level is excited to the ⁴F_7/2 level though the continuous ET or ESA of a second 980 nm photon. Subsequently, through non-radiation, the populated ⁴F_7/2 level immediately decays to the lower ²H_11/2 and ⁴S_3/2 levels due to the small energy gap between them, and then is excited to ⁴G_11/2 by the sequential ET or ESA of a third photon. Finally, the populated ⁴G_11/2 level is non-radially relaxed to the lower ²H_11/2 and ⁴S_3/2 levels. Er³⁺ will produce green emission colors at 524 nm and 557 nm by the radiative transitions from the ²H_11/2 and ⁴S_3/2 levels to the ground state ⁴I_15/2. After two-photon absorption and a series of non-radiative relaxations to the green-light emitting state (²H_11/2, ⁴S_3/2), Er³⁺ continues to absorb the energy of a third photon from Yb³⁺, sometimes transferring to a very dense variety of states labeled ⁴G/²K. Subsequently, Er³⁺ relaxes within the ⁴G/²K manifold, and transfers to the level of ⁴F_9/2 by back energy transfer (BET) to Yb³⁺ of ⁴G/²K(Er³⁺) + ²F_7/2(Yb³⁺) → ⁴F_9/2(Er³⁺) + ²F_5/2(Yb³⁺). When electrons in the ⁴F_9/2 level return to the ground state ⁴I_15/2, red light emission is produced at 660 nm.⁴⁷

Fig. 6 UC energy transfer and transition mechanism diagram of CKLC:10%Er³⁺/35%Yb³⁺.

3.5 Anti-counterfeiting and information encryption application

Due to the ET from STEs to Ho³⁺, it can be inferred from the excitation spectrum in Fig. 3c that CKLC:0.3%Sb³⁺/25%Ho³⁺ emits warm white light under a wide range of excitation wavelengths in the UV region (240–350 nm). As a verification process, the emission spectra of the sample were recorded at four wavelengths (Fig. 7a). At 450 nm excitation, red light is emitted due to the direct excitation of Ho³⁺. Since two-color luminescence was obtained in a single sample, we filled the “SWU” mold with CKLC:0.3%Sb³⁺/25%Ho³⁺. By simply changing excitation wavelengths, the objective of two-color anti-counterfeiting can be achieved (Fig. 7b). The two-color anti-counterfeiting strategy leverages the complexity of the internal energy-level structures within CKLC:0.3%Sb³⁺/25%Ho³⁺. The obtained materials respond to excitation light of varying energies by emitting at distinct wavelengths, thereby realizing high-level anti-counterfeiting through this dynamic color-changing characteristic. Specifically, the “SWU” mold with CKLC:0.3%Sb³⁺/25%Ho³⁺ appears white under natural light and ultraviolet light, and red under blue light (450 nm). In addition, use of optical encryption technology has attracted wide attention as a new encryption method in recent years. Therefore, we coated digital molds with CKLC:0.3%Sb³⁺, CKLC:0.3%Sb³⁺/25%Ho³⁺ and CKLC:10%Er³⁺/35%Yb³⁺, respectively, achieving a consistent appearance for each doped material. Under natural light as well as UV light we can only see the false information “8888” and “6398”. Only when excited at 450 and 980 nm is the genuine information of “1750” and “711” shown, thus achieving the function of information encryption (Fig. 7c). Furthermore, we measured the PL spectra of the as-synthesized samples on the day of preparation and again after 20 days (Fig. S8). The emission intensity of the samples after 20 days decreased to 0.87 (CKLC), 0.80 (CKLC:0.3%Sb³⁺), 0.71 (CKLC:10%Er³⁺/35%Yb³⁺), and 0.87 (CKLC:0.3%Sb³⁺/25% Ho³⁺) of their initial values, respectively (Fig. S9). These results indicate that CKLC and its doped derivatives exhibit good stability, which is beneficial for the material's application in the field of anti-counterfeiting and information encryption.

Fig. 7 (a) PL spectra and (b) digital photographs of CKLC:0.3%Sb³⁺/25%Ho³⁺ under different excitation wavelengths. (c) Digital pictures of digital molds coated with CKLC samples under different excitation wavelengths.

4. Conclusion

In conclusion, we have synthesized a novel DP CKLC with weak cyan emission due to STEs. Sb³⁺ doping greatly enhances the intensity of STE emission and increases the PLQY from 2% to 68%. In the Sb³⁺/Ho³⁺ co-doped system, red emission originating from the f–f transition of Ho³⁺ was also observed. It was demonstrated by steady-state and transient PL spectra that an ET channel exists between STEs and Ho³⁺. The ET from dopant STEs to Ho³⁺ was more efficient than that from host STEs to Ho³⁺, overcoming the weak emission due to the weak absorption cross-section of the f–f transition of Ho³⁺. Moreover, the emission color of the co-doped samples changed from cyan to white and finally to red as the Ho³⁺ concentration increased. In addition to indirect excitation, Ho³⁺ can be directly excited, and thus there is excitation wavelength-dependent luminescence in the co-doped sample. To obtain more luminescence possibilities, we also introduced Er³⁺/Yb³⁺ and demonstrated that its luminescence mechanism features a three-photon absorption process. Finally, the ion-doped samples were used together to design an anti-counterfeit label and information encryption strategy.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

All relevant data are within the article and its supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5dt02031j.

Acknowledgements

This project was financially sponsored by the National Natural Science Foundation of China (NSFC52172154).

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