Multicolor luminescence and afterglow from Cs₂NaScCl₆:Sb³⁺,Mn²⁺ crystals

Abstract

In contrast to single-band emitting phosphors, dual-band emitting phosphors provide a versatile tool for ratio-metric color tuning, enabling a wide color gamut and ensuing applications in information encryption and the anti-counterfeiting area. Here, a double perovskite, Cs₂NaScCl₆:xMn²⁺ single crystals, was grown by a hydrothermal method. Pristine Cs₂NaScCl₆ crystals exhibited a pure blue emission band originating from the self-trapped excitons. Upon doping with Mn²⁺, the crystals exhibited a dual-band emission profile, consisting of both STE and Mn²⁺ emissions. To this end, the ratio of two bands was manipulated with Mn²⁺-doping concentration, rendering a wide color gamut from blue to red. Interestingly, co-doping with Sb³⁺ ions significantly enhanced the overall photoluminescence, boosting the quantum yield from 8.8% to up to 62.4%. Such an enhancement is attributed to an efficient energy transfer process from the STE to Mn²⁺ based on an investigation of photoluminescence lifetime. Importantly, these crystals exhibited an intriguing afterglow after X-ray excitation, with a duration exceeding 3000 s. This remarkable phenomenon opens up many new possibilities for their application in the advanced anti-counterfeiting area.

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1. Introduction

Chloride-based perovskite crystals are a typical wide-bandgap semiconductor material with a high tolerance towards hetero-valence doping with various impurity ions, including cationic activators (Cu⁺, Mn²⁺, rare earth ions) and anionic substitution (Cl⁻, Br⁻, I⁻).^1–6 The doping not only introduces new luminescence centers but also creates abundant defect states.^7–12 Mn²⁺ ions exhibit visible light emission due to the ⁴T₁ → ⁶A₁ transition, which is highly sensitive to the crystal field environment. To this end, Mn²⁺ has a spectral tuning ability from green to red by adjusting the coordination number from 4 to 6.^13,14 In addition, the lifetime of Mn²⁺ emission is substantially long, extending to several milliseconds, which provides an ideal platform for time-gated imaging which is free of back-ground noise.¹⁵

Apart from the multicolor emission, the Mn²⁺ ions feature a multi-band excitation profile.¹⁶ The PL excitation spectra ranges from 300 nm to 550 nm, corresponding to a series of characteristic ⁶A₁(⁶S) transitions to ⁴T₁(⁴P), ⁴E_g(⁴D), ⁴T_2g(⁴D), ⁴A₁(⁴G), ⁴E(⁴G), ⁴T_2g(⁴G), and ⁴T_1g(⁴G) states. Such a discrete pattern allows a high freedom of excitation choice, providing added protection for anti-counterfeiting applications.^17,18 Besides, the excitation has been extended to the X-ray band recently. Xia et al. reported a phosphor of CsCdCl₃:Mn²⁺,R⁴⁺ (R = Ti, Zr, Hf and Sn) for X-ray storage, demonstrating an afterglow of 12 hours and a spatial resolution of 12.5 mm lp⁻¹ for X-ray imaging. Importantly, such a phosphor featuring a time-lapse manner is convenient for three-dimensional X-ray imaging of a curved body.¹⁹ Han et al. reported an X-ray excitable persistent luminescence by introducing Li⁺ into the Zn₂GeO₄:Mn²⁺ phosphor. Compared to single-mode encryption, orthogonal encryptions significantly enhanced the security of anti-counterfeiting as benefited from the independent readout in each mode.²⁰ Our group recently synthesized transparent perovskite crystals of Cs₂NaInCl₆:Mn²⁺, showing the potential of X-ray excitable afterglow for advanced three-dimensional information encryption.²¹ One drawback of the Mn²⁺-activated phosphor is the single emission band, which limits its anti-counterfeiting application in multicolor encryption.

In this work, a hydrothermal method was used to synthesize Cs₂NaScCl₆:xMn²⁺ single crystals. A pure blue emission band of self-trapped excitons (STEs) was observed from pristine Cs₂NaScCl₆ crystals. After Mn²⁺ doping, the crystals exhibited a dual-band emission of both STE and Mn²⁺ emissions, which rendered a multicolor luminescence by adjusting their ratio. Interestingly, the introduction of Sb³⁺ ions significantly enhanced the dual-band emissions, leading to a boost in the photoluminescence quantum yield (PL QY) from 8.8% to 62.4%, where the energy transfer (ET) process from the self-trapping exciton to Mn²⁺ played a critical role. Importantly, the crystals showed an afterglow after X-ray excitation, which lasted for more than 3000 s.

2. Results and discussion

Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ single crystals were grown in a hydrothermal reactor where a programmable cooling procedure (∼3 °C h⁻¹) was applied to regulate the crystallization. The size of the as-prepared crystals was in the range from 1 to 5 mm, which was closely related to the cooling rate.^22,23 As shown in Fig. 1a, the powder X-ray diffraction (XRD) patterns were well indexed to the standard diffraction pattern of Cs₂NaScCl₆ (CCDC 2054287). It should be noted that 1%-Sb³⁺ doping did not introduce any remarkable shift in the XRD peaks. Further doping with Mn²⁺ beyond the 30 mol% nominal feeding concentration in the precursor, however, generated an impurity phase of NaCl. Next, inductively coupled plasma (ICP) measurements were performed to identify the actual dopant concentration in the crystals (Table S1†). The actual concentration of Mn was 2.11 mol%, which was much lower than the nominal feeding concentration (20 mol%), largely due to the hetero-valence doping effect between Mn²⁺ and Sc³⁺.²¹ Compared with the Na⁺ ion (1.02 Å) and Cs⁺ ion (1.67 Å), the doped Mn²⁺ ion (0.83 Å) has a ionic radius closer to the Sc³⁺ ion (0.75 Å). In this sense, Mn²⁺ prefers to substitute the Sc³⁺ site in the lattice.

Fig. 1 Structure and luminescence of Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals. (a) X-ray diffraction patterns showed a high phase purity below 20% Mn²⁺-doping. The asterisk (*) marks an impurity phase of NaCl. (b) Steady-state PL spectra show a boosted PL QY of up to 62.4% after co-doping with both Mn²⁺ and Sb³⁺. (c) Photographs of the crystals under daylight, 365 nm UV light and X-ray, respectively. The X-ray-induced afterglow are shown in photographs on the right panel.

Despite the trace-amount of Mn²⁺ or Sb³⁺, the optical properties of Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals were significantly modified. In particular, the doping of Sb³⁺ or Mn²⁺ separately into the Cs₂NaScCl₆ matrix significantly enhanced its photoluminescence. The ensuing PL QY increased from 3.2% to 23.8% after 1 mol% Sb³⁺ doping due to the distortion of the octahedron (Fig. 1b and Fig. S1†).^24,25 In comparison, the introduction of Mn²⁺ yielded not only a distinct red emission but also an enhanced PL QY from 3.2% to 8.8% (Fig. S2†). Interestingly, when Mn²⁺ and Sb³⁺ were co-doped, the PL QY showed a significant increase of up to 62.4%. Due to the emergence of the red peak, the crystals exhibited a wide color gamut upon Mn-doping (Fig. 1c). Besides ultraviolet light, X-ray was found to be another effective excitation source. It should be noted that the red component in the radioluminescence (RL) profile was higher than that in PL spectra (Fig. S3†).

Apart from RL, the doped crystals exhibited an intense afterglow after ceasing the X-ray excitation. The crystals without Mn²⁺ doping exhibited a strong blue emission under X-ray excitation. Once the X-ray source was turned off, the blue RL decayed rapidly, indicating a weak afterglow. In stark contrast, the Mn-doped crystals exhibited a strong red afterglow, suggesting the key role of Mn²⁺ ions in activating the afterglow traps in halide crystals.²⁶ Remarkably, the afterglow signal in darkness could be clearly captured using a commercial camera (Canon 90D) even after a 20 min delay (Fig. 1c).

To shed light on the energy transfer (ET) process between dopants, steady-state PL spectra of varied crystals were collected under 340 nm excitation. The PL spectra showed a dual-band profile, suggesting the presence of two luminescence centers. The blue emission (∼450 nm) was attributed to the STE emission of the host, while the red emission (∼620 nm) was attributed to the ⁴T₁ → ⁶A₁ transition of Mn²⁺ (Fig. 2a and Fig. S4†). With the constant Sb³⁺-doping concentration, the red band increased with an increasing Mn²⁺-content, indicating the possibility of stable energy transfer from the STE state to Mn²⁺ (Fig. 2b). In fact, the red component increased up to 55.4% of the total emission intensity in the heavy Mn²⁺-doping region (∼60 mol%). Concurrently, the PL QY was elevated to a maximum of 62.4% at 20 mol% of Mn²⁺ doping. To this end, the nonradiative recombination by the quenchers (Q1) was outperformed by the energy cascade from free exciton (FE) to the STE, and then to Mn²⁺. However, when the Mn²⁺ doping concentration exceeded 30 mol%, non-emitting red crystals (as shown by dashed circles in Fig. S5†) were observed, suggesting an uneven Mn-element distribution in the product. Due to the concentration quenching effect, a further increase of Mn²⁺ would lead to a decrease in the PL QY (Fig. 2b).

Fig. 2 Investigation of the ET process in Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals. (a) Steady-state PL spectra under 340 nm excitation of crystals with varied Mn²⁺-doping concentrations. The spectra were normalized at 450 nm, which showed a decrease in the PL ratio of the STE and an increase in the PL ratio of Mn²⁺ (b, black dot plot). (b, red dot plot) PL QYs for a series of the Sb³⁺and Mn²⁺ doped crystals. (c) Excitation–emission mapping of Cs₂NaScCl₆:1%Sb³⁺,40%Mn²⁺ crystals. (d) Photoluminescence decay curves for the STE emission (450 nm). (e and f) Dependence of the energy transfer rate (W_ET) and efficiency (η_ET) on the Mn²⁺-doping concentration. (g) Energy level diagram of Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals and the proposed luminescence mechanism.

To further confirm the ET process, the excitation–emission map was collected for Sb³⁺ and Mn²⁺ co-doped crystals (Fig. 2c). The map clearly showed two excitation centers at ∼280 nm and ∼340 nm, belonging to the characteristic transitions from ¹S₀ to ³P₂ and ³P₁ of Sb³⁺, respectively (Fig. S6†). Such transition was effective in exciting the blue band since Sb³⁺ intrinsically localized the STE, which echoed well with previous reports.^27,28 It is worth noting that although the weak blue emission from the host STE showed a complete overlap with Sb³⁺ activated emission, they are not from the same origin since their excitation spectra largely differed as shown in Fig. S6c.† Intriguingly, such an excitation was found to be equally effective to stimulate the red band from Mn²⁺, providing strong evidence for the energy transfer from the STE to Mn²⁺. To further verify the ET process, the lifetime of STE emission was monitored with increasing Mn²⁺-content (Fig. 2d and Fig. S7†). With the increase in Mn²⁺-doping concentration, the lifetime (τ_x, x is the Mn²⁺-doping concentration) decreased from 2.35 μs to 1.17 μs (Table S2†). Accordingly, the energy transfer rate (W_ET) changed from 22.0 × 10⁴ s⁻¹ to 42.9 × 10⁴ s⁻¹, and the energy transfer efficiency (η_ET) reached 50.2% (Fig. 2e and f). Based on the above result, a possible luminescence mechanism of Sb³⁺ and Mn²⁺ co-doped crystals is proposed in Fig. 2g. New electronic transition channels (³P₂ and ³P₁ of Sb³⁺) were introduced at 280 and 340 nm due to the doping of Sb³⁺ ions. These channels competed with the quenchers (Q1), whereby the energy of free excitons was effectively transferred to the STE emitters. In addition, there was an efficient ET process from STE to Mn²⁺ emitters, which outperformed the quenchers (Q2). This route not only enhanced the Mn²⁺ emission but also increased the overall PL QY.

To investigate the electron–phonon coupling effect, temperature-dependent PL spectra were recorded for a co-doped crystal (Fig. 3a and Fig. S8c†). Its STE emission was significantly robust against thermal quenching, which is quite different from that of the Mn²⁺-free crystal (Fig. S8a and b†). Instead, the Mn²⁺ emission was severely quenched at elevated temperatures, showing an identical thermal quenching profile to that of Sb³⁺-activated crystals. Indeed, by fitting the temperature-dependent PL intensity, the exciton binding energy (E_a) of STE and Mn²⁺ could be calculated to be 45.5 meV and 44.0 meV, respectively (Fig. S9†).²⁹ The similar E_a evidenced the clear energy-cascade route from the STE to Mn²⁺. To quantitatively understand the electron–phonon coupling strength, the full-width-at-half-maxima (FWHM) of emission peaks were fitted against the temperature (Fig. 3b). When the temperature increased from 93 K to 473 K, the FWHM of the STE emission increased from 360 meV to 510 meV, showing an evident thermal broadening. The Huang–Rhys factor, S, was obtained by fitting the following eqn (1):³⁰

where ħω is the phonon energy, k is the Boltzmann constant, and T is the temperature. The value of S for STE emission was about 12.8, suggesting that 12.8 phonons (43.4 meV, 350 cm⁻¹) were generated in every single radiative transition of STE emission.^31,32 To shed more light on the preferred vibrational modes, Raman spectra of the single crystal were collected under 532 nm laser excitation (Fig. 3c). Three characteristic peaks at 197 (T_2g), 260 (E_g), and 343 cm⁻¹ (A_1g) were ascribed to the breathing, asymmetric stretching, and symmetric stretching modes of the vibrational [ScCl₆]³⁻ octahedron, respectively.^33,34 Therefore, the STE transition was found to be preferable to couple with the symmetric stretching of octahedra. Concurrently, the Mn²⁺ emission was found to be favorable to couple with the asymmetric stretching vibrations (32.2 meV, 260 cm⁻¹). However, the coupling strength (S = 7.2) was weaker than that of the STE emission due to the partially shielded d–d transition.³⁵ Interestingly, the STE band showed a red shift upon heating, which was opposite to the Mn²⁺ band (Fig. S8d†). The red shift was attributed to the thermal expansion of the lattice. In contrast, the blue shift of the Mn²⁺ emission was caused by the participation of phonons. With the aid of phonons, electrons were promoted to a higher energy level with the increasing temperature, resulting in a blue shift in the emission band.

Fig. 3 Temperature-dependent emission of Cs₂NaScCl₆:1%Sb³⁺,60%Mn²⁺ crystals. (a) Pseudo color mapping of temperature-dependent PL spectra under 365 nm excitation of Mn-doped crystals. (b) The FWHM of the STE and Mn²⁺ emission is plotted as a function of temperature. (c) Raman spectrum of the Cs₂NaScCl₆:1%Sb³⁺,60%Mn²⁺ crystals.

Apart from the photoluminescence, the crystals exhibited a strong radioluminescence under X-ray excitation. The RL spectra showed a similar profile to the PL spectra, including both STE and Mn²⁺ emissions. However, their intensity ratio varied significantly with the Mn²⁺-content, which suggested a wider color gamut under X-ray excitation (Fig. 4a and Fig. S10†). Intriguingly, Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals continued to glow after the radiation ceased. In fact, the afterglow remained two orders of magnitude higher than the background noise after a 3000 s delay (Fig. 4b) for all samples with varied Mn²⁺ contents. The afterglow profile was identical to the RL spectra, consisting of both STE and Mn²⁺ bands. While the Mn²⁺ band was usually attributed to the ⁴T₁ → ⁶A₁ transition, the STE band was quite novel.³⁶

Fig. 4 Radioluminescence (RL) afterglow of Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals. (a) X-ray excited radioluminescence spectra of crystals with varied Mn²⁺-doping concentrations. (b) Afterglow decay curves of crystals with varied Mn²⁺-doping concentrations. (c) Time-resolved afterglow spectra were similar to the RL spectrum, but the STE emission decayed faster than the Mn²⁺ emission. (d) TL curves were collected at different charging temperatures (298–373 K) at a heating rate of 2.5 K s⁻¹. The trap depth of TL centers was calculated and displayed in (e). (f) Schematic diagram of the luminescence mechanism of Cs₂NaScCl₆:1%Sb³⁺,5%Mn²⁺ crystals.

In order to understand the afterglow properties, the distribution of active traps was probed through a thermoluminescence experiment. It should be noted that the active traps refer to those where carriers can escape under thermal vibration and recombine to produce afterglow photons. The thermoluminescence curves were recorded via a thermal cleaning method (Fig. 4d). Typically, the TL curves were obtained with a heating rate of 2.5 K s⁻¹ after X-ray charging for 1 min at different temperatures (298–373 K). The empirical formula E = T_m/500 was used to estimate the trap depth. The trap depth remained essentially identical with the increase of charging temperature, indicating only one active trap in the TL process (Fig. 4e).³⁷ Concurrently, the shape of TL curve remained identical while the intensity saturated after 5 min of charging (Fig. S11†). Based on the above discussion, we proposed a possible afterglow mechanism as shown in Fig. 4f. The incorporation of Mn²⁺ as impurity ions into the crystals significantly increased the number of active traps, whereby more excited electrons were trapped upon excitation. The electrons were stimulated by phonons and de-trapped to the conduction band as FEs. Thereafter, the energy of FE transferred to both STE and Mn²⁺, where the radiative transition generated the afterglow emission.

The multicolor feature of the crystals provided a facile tool for advanced anti-counterfeiting applications. To this end, we prepared three crystals using varied doping strategies, exhibiting blue, red, and pink under X-ray excitation. The crystals were first ground into powders and then mixed with epoxy resin. After filling, drying, demolding and cutting, anti-counterfeiting patterns with different Mn²⁺ contents were obtained (Fig. 5a). Due to the different Mn²⁺ contents, they not only showed a multicolor tint under excitation but also exhibited an extended afterglow after ceasing the excitation. Importantly, the red afterglow could recover through thermal stimulation, which provided added protection to the encrypted information (Fig. 5b).

Fig. 5 The anti-counterfeiting pattern of multicolor Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ phosphors. (a) Fabrication processes of the anti-counterfeiting pattern. (b) Radioluminescence photographs of the pattern, showing a dynamic luminescence as time evolves.

3. Conclusion

In this work, Cs₂NaScCl₆:1%Sb³⁺,xMn²⁺ crystals were synthesized by a hydrothermal method. Multicolor luminescence and ensuing afterglow were activated by doping with Mn²⁺ ions. Importantly, further doping with Sb³⁺ ions significantly boosted the PL QY up to 62.4% through quencher passivation, as evidenced from time-resolved spectroscopy. The multicolor luminescence and dynamic afterglow enabled a sophisticated encoding technique, opening many possibilities to advanced applications including information encryption and anti-counterfeiting.

Data availability

Data for this article, including Fig. 1a, b, Fig. 2a–e, Fig. 3a, b, and Fig. 4a–e† are available at Science Data Bank at https://doi.org/10.57760/sciencedb.11546.

Conflicts of interest

All authors declare that they have no competing interests.

Acknowledgements

This work is financially supported by the Natural Science Foundation of Shandong Province (No. ZR2023JQ026) and the 20-Iterm Project in Universities of Jinan city (No. 202228091).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02019g

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