Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺: a yellow emitting long-lasting phosphor with high brightness and long afterglow duration

Abstract

A yellow emitting long-lasting phosphor based on a Ca₆BaP₄O₁₇ matrix was successfully synthesized by means of solid state method in a reducing atmosphere. The persistent phosphor was characterized in detail by X-ray powder diffraction (XRD), photoluminescence (PL), persistent luminescence and decay curves as well as thermoluminescence (TL) spectra. When irradiated by ultraviolet light in advance, Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ phosphor shows a yellow persistent luminescence dominating at ∼553 nm at room temperature due to the 5d–4f transitions of Eu²⁺ ions and the chromaticity coordinate was calculated to be (0.431, 0.545), which is in the yellow region. Moreover, investigation of TL curves reveals that the incorporation of Gd³⁺ ion creates more appropriate energy traps and increases the density of intrinsic traps leading to the enhancement of afterglow. Consequently, the initial long lasting phosphorescence (LLP) intensity of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ (0.21 cd m⁻²) is significantly enhanced by a factor of 2 compared with Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Ho³⁺ and the sustained phosphorescence also can last approximately 19 h above the recognizable intensity level (0.32 mcd m⁻²). In order to study the trapping process and explain the mechanism of long afterglow, a schematic model is also proposed and discussed in detail.

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1. Introduction

Long-lasting phosphorescence (LLP) phosphors are well-known as a kind of important optical functional materials which can absorb, store energy and then release visible or near-infrared light for several minutes or tens of hours even after the removal of the activated light source.^1–3 This phenomenon is due to thermally stimulated recombination of charge carriers (electrons and/or holes) which are released slowly from electrons and hole traps at room temperature.^4–7 LLP can be widely applied in various important fields including high-energy irradiation dosimeters, optical storage media, decoration, traffic signage, medical diagnostics, solar energy utilization, and bio-imaging all attributed to their outstanding properties such as energy saving, recycling and environmental protection.^8–11

In the middle of the 1990s, the first commercial application of persistent luminescence begun with ZnS:Cu,Co for marking watch dials.¹² Nevertheless, the drawback of sensitivity to moisture hampered the utility of this phosphor. In 1996, Matsuzawa et al.¹³ discovered the green persistent phosphors of SrAl₂O₄:Eu²⁺,Dy³⁺. Since then, research on different afterglow materials has been extensively conducted and has brought efficient and stable afterglow phosphors emitting mainly in the blue-green spectral range. These phosphors still play a dominant role in visible light scope owing to extremely long-persistent times and strong emitting intensities. Unfortunately, the efficient long-wavelength LLP emission from yellow to red is lacking, e.g. Ca₆BaP₄O₁₇:Eu,Ho (yellow, >47 h),¹⁴ Li₂SrSiO₄:Eu,Dy (orange-yellow, >15 h),¹⁵ Sr₃SiO₅:Eu,Dy (yellow, >6 h),¹⁶ and Ca₂Si₅N₈:Eu,Tm (red, >1 h),¹⁷ Thus, we need explore new LLP phosphors to extend the family of this kind of light-storing materials. In recent years, many novel LLP phosphors based on different hosts have been reported and their possible mechanisms were discussed.^18–23 But the step of developing new LLP materials with excellent performance is still slow because there is a lack of detail about the LLP mechanism. In fact, one of the researching focuses of the LLP material is how to fabricate the appropriate traps in the corresponding material and only the suitable host doped with the right activator could yield considerable phosphorescence.²⁴

Phosphate compounds have excellent thermal stabilities and ionic charge stabilities in the lattice.²⁵ Besides, the luminescence of Eu²⁺ activated phosphors usually results from the allowed electric dipole transition (4f⁷ to 4f⁶5d¹) and thus the efficient Eu²⁺ emission can be expected in a large number of hosts.²⁶ On the other hand, the incorporation of Gd³⁺ could improve the luminescence of rare earth ions. Accordingly, Gd³⁺ acts as the sensitizer of Eu²⁺, Sm³⁺, Tb³⁺ and Dy³⁺ in many compounds.^27–30 Recently, Naoyuki Komuro et al. discovered the phase of Ca₆BaP₄O₁₇ with a monoclinic structure in space group C2/m and a large band gap (5.79 eV).³¹ In a previous work, we found that the incorporation of Ho³⁺ ion into the Ca₆BaP₄O₁₇:Eu²⁺ system as an auxiliary activator can enhance its afterglow intensity and duration.¹⁴ In our present paper, it was found that codoping Gd³⁺ ion can significantly enhance initial LLP intensity (0.21 cd m⁻²) by a factor of 2 compared with Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Ho³⁺ and the LLP also can last approximately 19 h. The luminescence and phosphorescence properties of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ were systematically investigated by photoluminescence (PL) spectra, LLP emission spectra and decay curves and thermoluminescence (TL) glow curves. Also, the possible LLP mechanism of this phosphor was discussed in this paper.

2. Experimental

A series of persistent phosphors of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025 which are denoted as samples S1–S6, respectively) were synthesized by the solid-state reaction technique using CaCO₃ (A.R.), BaCO₃ (99.99%), NH₄H₂PO₄ (A.R.), Gd₂O₃ (99.99%) and Eu₂O₃ (99.99%) as starting materials. The required amounts of starting materials were homogeneously mixed and thoroughly ground in an agate mortar by adding an appropriate amount of ethanol, followed by further grinding for 30 min. Subsequently, the mixture was transferred into alumina crucibles and sintered at 1280 °C for 10 h in a weak reductive atmosphere (5%H₂–95%N₂ mixed flowing gas) in a tubular oven. After annealing, the samples were cooled to room temperature naturally and ground again to obtain final sample products in the form of fine powder.

The phase and purity identification of the synthesized phosphor samples were investigated by a Rigaku D/Max-2400 X-ray diffractometer (XRD) using a Rigaku diffractometer with Ni filtered Cu Kα radiation at scanning steps of 0.02° in the 2θ range from 10° to 80°. The excitation and emission spectra were measured by a FLS-920T fluorescence spectrophotometer equipped with Xe 900 (450 W xenon arc lamp) as the excitation source and the scanning step was 1 nm. The decay curves were recorded using a PR305 long afterglow instrument after the samples were irradiated with ultraviolet light for 15 min. The TL curves were detected using a FJ-427A TL meter (Beijing Nuclear Instrument Factory) with a heating rate of 1 K s⁻¹ in the temperature range from 20 to 400 °C. Prior to the TL measurements, about 0.0001 g samples pressed in pellets were exposed to radiation for 2 min by UV lights. All measurements except for the TL curves were carried out at room temperature.

3. Results and discussion

3.1. Phase identification and crystal structure analysis

A novel persistent phosphor material of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ was synthesized by a conventional solid-state method. The Ca₆BaP₄O₁₇ sample was refined using the Maud refinement program by the Rietveld method.^32,33 The value of the profile R factor (R_wp) = 10.68%, and the profile R factor (R_p) = 8.91%, indicating that the structural date are reliable. The crystal structure of Ca₆BaP₄O₁₇ and the cationic sites made with Balls–Sticks are shown in Fig. 1. The Ca1 site is surrounded by eight oxygen atoms with an average bond length of 2.51 Å, and the Ca2 site by seven oxygen atoms with an average bond length of 2.42 Å, the Ba site is coordinated by twelve oxygen atoms with an average bond length of 3.04 Å. The ionic radii of Ba²⁺ is 1.61 Å for twelve coordinated ions, and the ionic radii of Ca²⁺ for seven and eight coordinated ions is 1.06 Å and 1.12 Å, respectively. Therefore, for the consideration of ionic radii matching, the doping Eu²⁺ (r = 1.20 Å, CN = 7; r = 1.25 Å, CN = 8) and Gd³⁺ (r = 1.20 Å, CN = 7; r = 1.25 Å, CN = 8) ions tend to substitute for the two different Ca²⁺ sites rather than the Ba²⁺ sites in the Ca₆BaP₄O₁₇ matrix.

Fig. 1 Crystal structure of Ca₆BaP₄O₁₇:0.02Eu²⁺ and the cationic sites made with Balls–Sticks.

Fig. 2 shows the XRD of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ phosphors. The typical XRD patterns of samples (S1–S6) are in good agreement with refine results indicating that all samples are of a single phase. It can be clearly seen that no impurity peak was detected in the compositions, and results clearly suggest that the Eu²⁺ and Gd³⁺ ions are successfully incorporated in the host lattice without inducing significant changes in the crystal structure. Besides, all the peaks of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ (S2–S6) slightly move to a higher 2θ value referring to the position of Ca₆BaP₄O₁₇ and Ca₆BaP₄O₁₇:Eu²⁺ (S1). This finding can be explained by the fact that Gd³⁺ ion with smaller ionic radii substitutes for Ca²⁺ leading to a decrease in the lattice constants of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺.

Fig. 2 XRD patterns of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025).

3.2. Photoluminescence and phosphorescence properties of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025)

Fig. 3 displays both the excitation and emission spectra of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ samples. The monitored emission wavelength for the excitation spectra is 553 nm and the excitation spectra of the corresponding emission wavelength is 370 nm. Monitored at 370 nm, the emission spectra show an unresolved broad band predominating at 553 nm from 480 to 730 nm, which can be ascribed to the electric-dipole-allowed 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions. The Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ phosphors exhibit intense yellow luminescence. In addition, the typical emission of Gd³⁺ is not observed, indicating that Gd³⁺ ions are not serving as a luminescence center in Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ but may play the role of a trapping center. When the content of Gd³⁺ ions increased to 0.01 mol, their codoping markedly enhanced the PL intensity of the phosphor. It is noted that the PL intensity is reduced when the concentration of Gd³⁺ ions is 0.015 mol and then the PL intensity gradually increases until the concentration of Gd³⁺ ions reaches 0.025 mol. The unusual quenching concentrations may be related to the occupancy sequence of two Ca²⁺ sites for Eu²⁺ ions, the defects and lattice distortion by codoping different concentrations of Gd³⁺ ions.

Fig. 3 Emission (λ_ex = 370 nm) and excitation (λ_em = 553 nm) spectra of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025); (inset) photograph of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ under UV irradiation.

An interesting result of the present work is that we have observed yellow-emitting afterglow phosphorescence in Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025) phosphors even after switching off the excitation source. Fig. 4 shows the phosphorescence spectra of the Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ sample after the excitation source is stopped at different periods of time (t = 90, 180, 300, 600, 1200 s). The phosphorescence spectra still reveal one broad band located at 553 nm, which is similar to its emission spectrum. In addition, the typical emission of Gd³⁺ is not observed. Therefore, it is justified to say that its phosphorescence also can be derived from Eu²⁺ in the above-mentioned two crystallographic Ca sites. Calculation from phosphorescence spectrum using chromaticity coordinate calculation method based on the CIE1931 (Commission International de I'Eclairage France) system reveals that the phosphorescence chromaticity coordinates of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ are (0.431, 0.545) and the LLP color is yellow.

Fig. 4 Phosphorescence spectra of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ measured at different periods of time after the removal of the UV irradiation source; (inset) CIE 1931 Chromaticity Diagram of the Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ phosphor.

3.3. LLP decay curves of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025)

Fig. 5 depicts the LLP decay curves of the Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ phosphors recorded immediately after removing the UV irradiation source. It can be seen that all the LLP decay curves of codoped phosphors consist of a rapid decay process in the beginning dominating an intense LLP intensity and later a slow decay process responsible for longer afterglow duration. Whereas, the LLP decay curve of Ca₆BaP₄O₁₇:0.02Eu²⁺ resembles a vertical line and the phosphorescence can be visible only for 1 h by naked eyes. Compared with other samples, the Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ phosphor obviously exhibits the best phosphorescence properties. Interestingly, the PL intensity of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ phosphor is not the strongest compared with that of other reference samples. After the excitation source was switched off, the yellow phosphorescence of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ was observed for 19 h by naked eyes and the initial intensity can reach about 0.21 cd m⁻². Since the eye sensitivity is about 100 times better than 0.32 mcd m⁻², we can in fact observe a much longer LLP in complete darkness.³⁴

Fig. 5 LLP decay curves of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025); (inset) the delay time as a function of the Gd³⁺ contents.

3.4. Thermoluminescence characteristics

It is well known that trapping centers play a crucial role in photo-energy storage in persistent, photostimulated and thermo-stimulated phosphors.^35,36 Generally speaking, the corresponding information about the trapping level can be obtained by TL measurements, the shallower the trap depth, the lower the temperature of the TL peak.³⁷Fig. 6 presents the TL glow curves of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025). It can be seen that the TL glow curve of Ca₆BaP₄O₁₇:0.02Eu²⁺ consists of three very weak peaks located at about 307 K, 343 K and 397 K. This result reveals that there are at least three types of traps with different depths in the Ca₆BaP₄O₁₇:0.02Eu²⁺ material. According to Clabau et al.,³⁷ Eu²⁺ can exist in an oxidized state Eu³⁺ so that the electron excited to a 5d orbital can potentially be trapped by anion vacancies. Therefore, the traps involved in LLP of the Ca₆BaP₄O₁₇:0.02Eu²⁺ material could be due to oxygen vacancies or clusters which are created during the synthesis process at a high temperature and in a reducing atmosphere.^38,39 Furthermore, the weak TL signal suggests that the concentration of carriers captured at the intrinsic traps is very low. However, we can see that there emerges a new peak at around 330 K and the TL intensity of P2 (343 K) and P3 (397 K) are greatly enhanced for the co-doped samples, which means that the doping of Gd³⁺ ions largely improves the trap levels and creates new traps in the samples of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺. Due to the nonequivalent substitution, an excess of positive charge in the host must be compensated. The only possible way to fulfill the charge compensation is that two Gd³⁺ ions replace three Ca²⁺ ions to balance the charge of the phosphor, which will form two positive defects and one negative defect. The positive defects could form electron-traps while negative defects act as hole-traps. Thus, the new traps can be caused by and defects. The TL intensity of P1 (330 K) and P3 (397 K) peaks as a function of the Gd³⁺ contents are showed in the inset of Fig. 6. It is found that the TL intensity of two peaks increases simultaneously and then decreases when the Gd³⁺ contents is more than 0.015 mol due to the concentration quenching effect.⁴⁰ Thus, taking the analyses of decay and TL properties into account together, it can be concluded that the Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ sample shows the best persistent luminescence performance.

Fig. 6 The TL glow curves of Ca₆BaP₄O₁₇:0.02Eu²⁺,xGd³⁺ (0 ≤ x ≤ 0.025); (inset) the TL intensity of P1 (340 K) and P3 (382 K) peaks as a function of the Gd³⁺ contents.

The fading of TL is due partly to the bleaching of the trapped carriers by thermal energy. The TL glow curves with different delay time after ceasing the UV irradiation may also provide important information on the trap distribution in materials.³⁹ The effective TL peak is situated slightly above room temperature (320–400 K), which is a temperature leading to better LLP properties.^41,42 So, in our present work, the TL peaks at 330 K (P1), 343 K (P2) and 397 K (P3) account for the LLP in room temperature. As shown in Fig. 7, the TL intensity of P1 (330 K) due to the shallow traps decreases very quickly and approximately has disappeared after 30 minutes because that the charge carriers escape from shallower traps much faster than that in deep ones. At the same time, it is found that the deep traps at 343 K (P2) and 397 K (P3) play a major role and their TL intensities relatively decrease slowly with time as showed in the inset of Fig. 7. This result indicates that the shallow traps corresponding to the P1 (340 K) peak should mainly be responsible for an intense LLP intensity and a rapid decay process in the beginning, while the deep traps at 343 K (P2) and 397 K (P3) lead to darker initial phosphorescence and longer afterglow duration of the Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ material.

Fig. 7 The TL glow curves of Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ sample after UV irradiation for 2 min and then delaying for a different period of time.

3.5. Afterglow mechanism

Based on the above results, a simple model of Ca₆BaP₄O₁₇:Eu²⁺,Gd³⁺ LLP mechanism is proposed and illustrated in Fig. 8. Under ultraviolet-light excitation, the ground-state electrons of Eu²⁺ ions are promoted to the 5d electron state (process 1), which is partly overlapped with the conduction band. The excited electrons are subsequently captured by both shallow electron traps (P1) and deep electron trap oxygen vacancies or clusters (P2 and P3) through the conduction band (process 2) and thus the excitation energy is efficiently stored. After a sufficient illumination time, all of the appropriate traps are filled. Subsequently, the electrons are released from traps in the temperature order under the action of thermal activation at an appropriate temperature and then transferred to the emission centers (process 3), followed by the recombination and finally resulting in the LLP.

Fig. 8 Schematic diagram of the afterglow process.

4. Conclusions

By introducing Gd³⁺ ions into Eu²⁺ doped calcium barium phosphate Ca₆BaP₄O₁₇, we first obtain a novel LLP phosphor Ca₆BaP₄O₁₇: Eu²⁺,Gd³⁺ by the conventional solid state method. When the contents of Gd³⁺ ions increased to 0.015 mol, its codoping markedly enhanced the PL and phosphorescence intensity. After the UV light excitation source was switched off, the bright yellow LLP was observed in Ca₆BaP₄O₁₇: Eu²⁺,Gd³⁺ phosphor and the LLP can last near 19 h at a recognizable intensity level. The analysis of TL glow curves reveals that the shallow traps (330 K) are mainly responsible for an intense LLP intensity and a rapid decay process, while the deep traps (343 K and 397 K) lead to darker initial phosphorescence and longer afterglow duration of the Ca₆BaP₄O₁₇:0.02Eu²⁺,0.015Gd³⁺ material.

Acknowledgements

This work was supported by a Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120211130003) 35 and the National Natural Science Funds of China (Grant no. 51372105).

Notes and references

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