Enhanced yellow afterglow in Ca₂SnO₄:Dy³⁺ by co-doping Na⁺/K⁺

Abstract

A new yellow long afterglow luminescent material Ca₂SnO₄:Dy³⁺ was prepared by the solid state method at 1350 °C. The yellow LLP (long-lasting phosphorescent) material Ca₂SnO₄:Dy³⁺ was prepared by co-doping alkaline ions (K⁺ and Na⁺) and investigated using an X-ray diffractometer (XRD), photoluminescence spectrometer, afterglow meter and thermoluminescence spectrometer. The excitation and emission spectra showed an excitation peak at 350 nm and an emission peak at 573 nm. The optimized Dy³⁺ dopant concentration was found to be 1 mol%. For the improvement in the photoluminescence performance, we can co-dope it with alkaline ions (K⁺ and Na⁺), with the optimal concentration of the alkaline ions being 1 mol%. On the basis of the experimental results, we constructed a probable model for explaining the mechanism of LAG in detail.

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1. Introduction

Long afterglow (LAG) luminescence is an interesting optical process with the decay time extending to seconds, minutes, or even hours after the removal of the excitation source at room temperature. As a unique luminescence phenomenon, recently, LAG has been paid a lot of attention with practical applications in various fields, for example, safety lighting, dial-up displays, decoration, storage media, high-energy ray detection, optical fiber thermometers and live biological imaging.^1–5 In this phenomenon, energy is absorbed and stored under excitation (usually ultraviolet and visible light), and emission is visible for a long period even after the excitation stops.^6–9 Because of their exceptional properties in luminescence intensity, afterglow time, and chemical stability, a lot of blue and green afterglow phosphors of silicates and aluminates have been sold in the market, while yellow long-lasting phosphors remain relatively backward.^10,11 Some reported yellow-emitting long persistent phosphors are Sr₃SiO₅:Eu²⁺, Dy³⁺,¹² Ca₂Al₂Si₂O₇:Mn²⁺, Ce³⁺,¹³ Sr₃Al₂O₅Cl₂:Eu²⁺, Tm³⁺,¹⁴ Ca₂ZnSi₂O₇:Eu²⁺, Dy³⁺,¹⁵ Ca₂BO₃Cl:Eu²⁺, Dy³⁺,¹⁶ Ca₆BaP₄O₁₇:Eu²⁺, and Ho³⁺.¹⁷ However, they have short persistence time and low brightness. Hence, it is urgent and very desirable to explore new types of yellow LAG.

Recently, Shi et al.¹⁸ have reported a new yellow long-persistent luminescent Ca₂SnO₄:Dy³⁺ whose wideband emission peak is at 573 nm, where we can observe the afterglow for several minutes in the dark after removing the excitation source. Unfortunately, the brightness and duration are not enough for practical applications. In order to use yellow LAG phosphors in practical applications, it is necessary to further enhance their initial intensity and afterglow duration.

At the same time, alkali ions (Li⁺, K⁺, Na⁺, etc.) are well-known high-efficiency sensitizers, thereby enhancing the luminescence performance of the phosphors by charge compensation.¹⁹ Therefore, we believe that some intensive studies are still needed to investigate the photoluminescence behavior and afterglow performance of the Dy³⁺ mono-doped and alkali ion (Li⁺, K⁺, Na⁺) co-doped Ca₂SnO₄ phosphors. However, Li⁺ will volatilize when heated at a temperature higher than 1310 °C, and all test reagents may transition to a glassy state, which affects the experimental results.²⁰ To improve the properties of the yellow afterglow phosphors, in this study, we used alkali ions (K⁺ and Na⁺) for the improvement in the afterglow performance of the Ca₂SnO₄:Dy³⁺ phosphors. The Ca₂SnO₄ phosphors doped with Dy³⁺ and alkali metal ions (K⁺ and Na⁺) were obtained using the solid state reaction. We studied the luminescence properties and afterglow behavior of the phosphors. The results show that the basic ions K⁺ and Na⁺ can effectively improve the photoluminescence and afterglow luminescence of the phosphors.

2. Experimental

All the powder samples were prepared by a high-temperature solid-state reaction. Analytical reagents CaCO₃, SnO₂, Na₂CO₃, K₂CO₃, and Dy₂O₃ (99.99%) were used as raw materials. The Dy³⁺ doping concentrations were varied from 0.5 to 5 mol% and the concentration of the co-doped M⁺ (M = Na, K) was adjusted from 0.2 to 1.5 mol%. In a typical process, raw materials with different stoichiometric amounts were ground together in an agate mortar for 4 h and then pressed into tablets. The tablets were heated up to 1350 °C and held at this temperature for 10 h. Then, the samples were quickly cooled to room temperature with a rate of 20 °C min⁻¹. The fired tablets were ground to obtain the phosphorous powders.

We analyzed the crystalline structure of the samples within the range of 10–70° by X-ray diffraction (XRD) with a TD3500 X-ray diffractometer (Bruker AXS, Karlsruhe, Germany). We obtained the excitation and emission spectra using Xe 900 (450 W xenon arc lamp) by an FLS-920T fluorescence spectrophotometer. The scanning step is 1 nm. We measured the thermoluminescence curve using an FJ-427-A1 thermoluminometer (Beijing Nuclear Instrument Factory) when the heating rate was 1 K s⁻¹. Before the measurement, we irradiated the sample with ultraviolet (350 nm) radiation for different time periods.

3. Results and discussions

3.1 Structural determination

X-ray diffraction (XRD) was used to detect the purity of all the prepared samples. In Fig. 1, we can see the typical XRD patterns of different concentrations of Dy³⁺ (0.5 mol%, 1 mol%, and 2 mol%) doped in Ca₂SnO₄ and Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% K⁺ and Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% Na⁺ phosphor samples. We have also shown the pattern for the PDF card no. 46-0112 in Fig. 1; after comparison with it, we can index all the observed peaks to the pure phase of Ca₂SnO₄. In this system, the Dy³⁺ ions were incorporated into the Ca₂SnO₄ lattice by substituting Ca²⁺ because of their similar ionic radii. All the diffraction peaks can be indexed to an orthorhombic structure according to the reference file PDF #46-0112.

Fig. 1 XRD patterns of (a) Ca₂SnO₄:0.5 mol% Dy³⁺, (b) Ca₂SnO₄:1 mol% Dy³⁺, (c) Ca₂SnO₄:2 mol% Dy³⁺, (d) Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% K⁺ and (e) Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% Na⁺ phosphors.

From Fig. 1a, we can see that in Ca₂SnO₄, the mono-doping of Dy³⁺ and the co-doping of K⁺ and Na⁺ do not result in the formation of impurities or secondary phase. The diffraction peaks of the lattice plane, which are magnified in Fig. 1b, clearly shift towards smaller angles after doping K⁺ and Na⁺. Moreover, the shift in the diffraction peak is more obvious when K⁺ is doped. This is expected since the larger K⁺ ions replace the smaller Ca²⁺ ions in the Ca₂SnO₄ structure, consequently affecting the luminescence of the phosphors (Fig. 2).

Fig. 2 The crystal structure of Ca₂SnO₄.

Ca₂SnO₄ crystallizes in the orthorhombic crystal system with the space group Pbam; the unit cell parameters are a = 0.5753 nm, b = 0.9701 nm, and c = 0.3266 nm.²¹ In the Ca₂SnO₄ crystal, the shared edges of the SnO₆ octahedron are connected into a one-dimensional chain. SnO₆ octahedron and 4 of 6 oxygens of the adjacent 2Sn⁴⁺ ions form Sn⁴⁺–O²⁻ –Sn^{4 +}. The other two O²⁻ ions are connected to the Sn⁴⁺ –O²⁻ bond at the end of Sn⁴⁺ ion.

SEM was carried out to confirm the morphology of the Ca_1.98Dy_0.02SnO₄ phosphors synthesized at 1350 °C by the solid state reaction in air. The morphological images are shown in Fig. 3. It can be clearly seen from Fig. 3a that the particles are of uniform size with a good degree of dispersion. From Fig. 3b, it is clear that the prepared particles are almost spherical with a dimension in the range of 15–20 μm.

Fig. 3 SEM images of Ca_1.98Dy_0.02SnO₄ phosphor (a) particle uniformity and dispersion, (b) particle shape and size.

3.2 Photoluminescence characteristics

From Fig. 4, we can see the PLE as well as PL spectra of the prepared Dy³⁺-doped Ca₂SnO₄ phosphor. There is a very strong band peak in the PLE spectrum at 350 nm because of the charge transfer of Dy³⁺–O²⁻. When the optimum excitation wavelength is 350 nm, there are several strong peaks in the range of 300–400 nm and 450–700 nm. They consist of the main emission and Dy³⁺ 4f–4f characteristic transition emission lines near 402 nm as well as the transition of the ⁴F_9/2 excited states to the ⁶H_J/2 ground states, covering ⁴F_9/2 → ⁶H_15/2 (482 nm), ⁴F_9/2 → ⁶H_13/2 (573 nm) and ⁴F_9/2 → ⁶H_11/2 (672 nm) separately. We can find that the intensity of the magnetic dipole transition ⁴F_9/2 → ⁶H_15/2 around 482 nm is fairly lower than that of the electric dipole transition ⁴F_9/2 → ⁶H_13/2 around 573 nm, implying that the Dy³⁺ ions occupy the positions with no inversion symmetry in Ca₂SnO₄.^22,23 This is in good agreement with the upper structural analysis results.

Fig. 4 Excitation (λ_em = 573 nm) and emission spectra (λ_ex = 350 nm) of Ca₂SnO₄:1 mol% Dy³⁺phosphor.

To further investigate how the doping of alkaline ions affects the properties of the Ca₂SnO₄:1 mol% Dy³⁺ phosphor, the excitation and emission spectra of five phosphors with various concentrations of the alkali ions were tested.²⁴ Fig. 5a, for the Na⁺-doped phosphor, shows that the excitation and emission intensities increase as the Na⁺ doping concentration increases. The intensity reaches a maximum for 1% doping and then gradually decreases. Therefore, the optimal doping concentration of sodium ions is 1 mol%. Similarly, we can also observe the results for K⁺ co-doped Ca₂SnO₄:1 mol% Dy³⁺ phosphor in Fig. 5b, which shows that the excitation and emission intensities increase as the doped K⁺ concentration increases; also, it is determined that the K⁺ concentration is 1 mol% and the intensity is the largest.

Fig. 5 PL spectra of Ca₂SnO₄:1 mol% Dy³⁺ phosphors with (a) Na⁺ doping and (b) K⁺ doping, and (c) the difference in K⁺, Na⁺ excitation intensities at different concentrations at 350 nm.

The effect of K⁺ and Na⁺ doping on the PL intensity of the samples was further studied (Fig. 5c). Fig. 5c shows the difference in the excitation intensities of K⁺ and Na⁺ at different concentrations at 350 nm. Interestingly, the PL intensity of the Na⁺-doped phosphors is lower than that of the K⁺-doped phosphors, which indicates that the K⁺ ions are better for the luminescence of the developed phosphors. These results largely indicate that we can dope alkali ions to improve the PL characteristics of the developed phosphor.

According to the measured emission spectra, we calculated the CIE chromaticity coordinates of different Ca₂SnO₄:1 mol% Dy³⁺, Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% K⁺ and Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% Na⁺ phosphors, as shown in Fig. 6. We calculated the CIE chromaticity coordinates as x = 0.4535, y = 0.4022. The samples co-doped with Na⁺ and K⁺ are also labeled in this figure. It is not difficult to see that the co-doping of Na⁺ or K⁺ ions has no significant effect on the color of the developed phosphor. Therefore, the as-prepared yellow phosphors have wide application prospects in w-LEDs and display applications.

Fig. 6 The CIE 1931 chromaticity coordinates of the Ca₂SnO₄:1 mol% Dy³⁺, Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% K⁺ and Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% Na⁺ phosphors.

3.3 Afterglow properties

We recorded the decay curves of Ca₂SnO₄:1 mol%Dy³⁺, Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% K⁺ and Ca₂SnO₄:1 mol% Dy³⁺, Na⁺ (Fig. 7) immediately after exposure to 350 nm irradiation for 5 minutes. Generally, a decay curve indicates two processes: a fast decay process and a slow decay process. We also measured the afterglow decay curve of the sample for comparison. From Fig. 7, we can see that at the beginning, the decay rate is high; then, we observe a slow decay process mainly because of the faster release rate of the charge carriers in shallow traps and the slower release rate of the charge carriers in deeper traps. It can also be seen from the figure that the initial afterglow intensity and duration of the co-doped phosphors are different from that for the rare earth element-doped phosphors. The afterglow emission and initial photoluminescence intensity of the original Ca₂SnO₄:1 mol% Dy³⁺ phosphors are lower than those of the co-doped phosphors. For a further investigation of the reason for the change in the afterglow performance, we fitted the afterglow curves of Ca₂SnO₄:1 mol% Dy³⁺, Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% K⁺ and Ca₂SnO₄:1 mol% Dy³⁺, 1 mol% Na⁺ phosphors with the following three attenuation equations:^18,25

I = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) + A₃ exp(−t/τ₃) (1)

Here, I and I₀ represent the phosphorescence intensities, A₁, A₂ and A₃ represent the constants, t represents the time and τ₁, τ₂, and τ₃ represent the decay times for the three exponential components.²⁶ The results obtained using Origin 2017 software can be seen in Table 1. We can see the parameters from Table 1: a three index decay model represents the decay process of these phosphors, and the τ value is an important factor for the evaluation of the afterglow characteristics of the phosphors. The attenuation time τ increases as the afterglow characteristics get better. Single-doped samples have lower brightness than co-doped samples, which signifies the existence of traps. It is confirmed that co-doping with alkaline ions results in better afterglow properties.

Fig. 7 Afterglow of Ca₂SnO₄:Dy³⁺, Ca₂SnO₄:Dy³⁺, K⁺ and Ca₂SnO₄:Dy³⁺, Na⁺ phosphors.

Table 1 Simulated results for the afterglow curves of Ca₂SnO₄:Dy³⁺, Ca₂SnO₄:Dy³⁺, K⁺ and Ca₂SnO₄:Dy³⁺, Na⁺ phosphors

Composition	I0	A1	τ1 (s)	A2	τ2 (s)	A3	τ3 (s)
Ca2SnO4:Dy^3+	959	1.4 × 10^7	0.8	1.2 × 10^6	3.9	7.9 × 10^4	27.0
Ca2SnO4:Dy^3+, K+	1658	1.7 × 10^7	0.9	1.5 × 10^6	4.0	9.6 × 10^4	27.0
Ca2SnO4:Dy^3+, Na+	1543	1.6 × 10^7	0.8	1.4 × 10^6	4.2	8.9 × 10^4	2.0

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It is also reported that the persistent time of the LAG phosphors is affected greatly by the depths, because of the slow release of the trapped charge carriers by thermal stimulation after UV irradiation, traps play an important role in the decay process.²⁷ Hysteresis materials that have an appropriate trap depth and large trap density usually have good properties. Hence, for the determination of the trap center, the thermally stimulated luminescence (TSL) of Dy³⁺ and alkali ion (K⁺, Na⁺) co-doped as well as Dy³⁺ single-doped Ca₂SnO₄ phosphors was determined, as shown in Fig. 7.

Once the phosphor is excited, electrons and holes will be produced. These electrons and holes will be captured and released step by step due to thermal disturbance. Then, light can be emitted by the electrons and holes through radiative transitions. In general, these traps can be characterized by thermoluminescence. For the verification of the trap change, we obtained the thermoluminescence (TL) curves (Fig. 8). We can get the trap depth through the following formula:^28,29

Here, I represents the intensity of the luminescence peak, s represents the frequency factor, n₀ represents the capture concentration of capture carriers, E represents the capture depth (activation energy), k represents the Boltzmann constant, β represents the heating rate (1 K s⁻¹ in this paper), and b represents the kinetic order or attempt-to-escape frequency, indicating the order of the capture process: b = 1 for direct recombination and b > 1 for remapping the carriers. On the basis of the above-mentioned kinetic equation, all the thermoluminescence curves can be well-fitted by the multi-peak method. Usually, each TL peak is a capture center.³⁰ We can observe at least two traps of different depths. Accordingly, we can see the cutting parameters of the samples from Table 2. It can be found that such suitable trap depths were obtained with TSL peaks at low temperature (around 300–400 K). As mentioned above, the obtained phosphors have better decay performance because of the suitable capture types and capture depth.

Fig. 8 TSL curves of Ca₂SnO₄:Dy³⁺, Ca₂SnO₄:Dy³, K⁺ and Ca₂SnO₄:Dy³, Na⁺ phosphors.

Table 2 TSL parameters from the curve fitting technique

Composition	Peak temp. (K)	Et (eV)	S	n0
Ca2SnO4:Dy^3+	320	0.478	1682, 466	1.16 × 10^6
Ca2SnO4:Dy^3+, K^+	353	0.582	3140, 233	1.58 × 10^6
Ca2SnO4:Dy^3+, Na^+	347	0.571	1774, 536	1.47 × 10^6

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From the fitting peaks, by using the equation E = 25kT_m, the trap depths related to the peaks located in the range of 350–375 K were calculated as 0.478 eV for the Ca₂SnO₄:Dy³⁺ phosphor, 0.582 eV for Ca₂SnO₄:Dy³⁺, K⁺ and 0.571 eV for Ca₂SnO₄:Dy³⁺, Na⁺.³¹ Traps of appropriate depths are reported to play an important role in capturing electrons and slowly transporting them to the light-emitting center to produce afterglow.²⁹ If the depth of the traps is very deep, it is difficult for the trapped electrons to escape from the traps and release photons, leading to the degradation of the LLP performance.^32,33 If the trap depth is shallow, the trapped electrons can easily go back to the ground state and emit light and thus, the life of LLP is short.³² The results show that the n₀ value increases obviously after the co-doping of K⁺ and Na⁺. This means that more free electrons can be trapped in more electron traps formed. Thus, we obtained brighter initial phosphorescence and a longer afterglow time. As reported before, Dy³⁺ acts as a trap supplier. In the trap, replacing Ca²⁺ will cause defects in the crystal. We can express the process as 3Ca = 2DyCa* + VCa. Two positive defects of electrons captured by DyCa* and a negative vacancy of VCa will be produced due to each substitution by two Dy³⁺ ions .^24,25 The doping of Dy³⁺ will produce deeper traps and determine the afterglow in the later stage. In addition, the introduction of basic ions M⁺ (K⁺, Na⁺) can deepen the traps and increase the trap density, thus capturing more electrons and producing longer and stronger afterglow. We can write the equation as follows:

Dy³⁺ + M⁺ + 2CaCa²⁺ = DyCa + MCa′ + 2Ca²⁺ (3)

Based on the experimental results, for a clearer study of the afterglow process, we can see the phosphorescence mechanism of Ca₂SnO₄:Dy³⁺ (Fig. 9) and it is described by the following key steps: (1) under UV irradiation, the electrons in the ground state of Dy³⁺ are excited to enter the conduction band (CB), due to which the holes remain in the ground state. (2) Some of the excited electrons are transferred to the excited levels of Dy³⁺ through relaxation; then, yellow photoluminescence of Dy³⁺ and the recombination of the electrons and holes occur. (3) In VB, electron traps can trap and store some excited electrons, while hole traps can capture and store holes. Therefore, part of the excitation energy is stored instead of returning to the ground state for photoluminescence.³⁴ (4) After UV irradiation is cut off, the electrons are released slowly from the traps under the action of thermal activation and transferred to high-energy Dy³⁺ and then, the electrons and holes combine, resulting in yellow light emission of the Dy³⁺ ions.

Fig. 9 A schematic graph of the phosphorescence mechanism for Ca₂SnO₄:1 mol% Dy³⁺; ○ represent hole; ● represent electron.

4. Conclusion

We obtained a yellow LLP phosphor Ca₂SnO₄:Dy³⁺ by the conventional solid state method at 1350 °C in air. XRD analysis showed that all the samples exhibited a pure phase. Under the excitation of UV light, a rare earth activator has the ability of bright self-activation and sensitization. The luminescence and afterglow properties improved by doping basic ions (K⁺, Na⁺). The best doping concentration of K⁺ and Na⁺ was 1 mol%. On this basis, a simplified hysteretic mechanism model was proposed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (61901270), the Shanghai Sailing Program (17YF1419200) and the Science and Technology Planning Project of Zhejiang Province, China (2018C01046).

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