The persistent energy transfer of Eu²⁺ and Dy³⁺ and luminescence properties of a new cyan afterglow phosphor α-Ca₃(PO₄)₂:Eu²⁺,Dy³⁺

Abstract

A new cyan long-lasting phosphorescence (LLP) phosphor Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ was synthesized by a solid state reaction and its LLP properties were investigated for the first time. The Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphor exhibits two emission bands centered at 480 nm and 573 nm, which are ascribed to the 4f⁶5d¹–4f⁷ electronic transition of Eu²⁺ ions and the transition from ⁴F_9/2 to ⁶H_13/2 of Dy³⁺ ions, respectively. After removal of the excitation source, the bright cyan LLP could be observed for 5 h by the naked eye. Besides, the yellow LLP component originated from the persistent energy transfer (PET) from Eu²⁺ ions to Dy³⁺ ions. The incorporation of Dy³⁺ ions creates more appropriate energy traps and evidently enhances the LLP behavior of Ca₃(PO₄)₂:Eu²⁺ phosphor. A schematic model is constructed to convey trapping/detrapping processes and PET in the material.

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1. Introduction

In inorganic compounds, persistent luminescence is well-known for light emission decay controlled by crystal defects or impurity centers. This interesting optical phenomenon, which can exhibit light for several minutes or hours after the removal of an excitation source, occurs owing to some suitable defects in the forbidden band gap of the material.¹ Due to their energy efficiency and independence from external power sources, persistent phosphors are drawing considerable attention for their wide application in security signs, optical storage media and in vivo imaging.^2–7 Theoretically, we can obtain any color emitting persistent luminescence by mixing three primary color emitting persistent phosphors, for example, SrAl₂O₄:Eu²⁺,Dy³⁺ (green, >24 h),⁴ CaAl₂O₄:Eu²⁺,Nd³⁺ (blue, >5 h),⁸ Y₂O₂S:Eu³⁺,Ti⁴⁺,Mg²⁺ (orange-red, >5 h).⁷ Nevertheless, it is very difficult to ensure color purity throughout the decay time and most of the persistent phosphors can not be efficiently excited by the same excitation source. Thus, it is necessary to explore multi-color emitting persistent phosphors. Among these persistent phosphors, a cyan phosphor with a decent LLP performance is scarce.^9,10 Therefore, there is an increasing demand for new cyan LLP materials to meet the multicolor demand for practical utilization.

Phosphate-based phosphors have been widely explored as excellent luminescence materials due to stable physical and chemical properties, cheap starting materials and relatively mild preparation conditions.¹¹ Eu²⁺ ions, as one of the most widely used activators in phosphors, have been extensively studied. Eu²⁺ doped phosphates, such as red Ca₄(PO₄)₂O:Eu²⁺ phosphor,¹² yellow Ca₆BaP₄O₁₇:Eu²⁺ phosphor¹³ and blue Ba₅(PO₄)₃Cl:Eu²⁺ phosphor,¹⁴ have a broad emission band ranging from the blue region to the red region due to 4f–5d transitions being strongly influenced by the different strengths of the crystal field. These phosphate phosphors have been reported to possess excellent afterglow properties.^15–17

Phosphate phosphors with a tetrahedral rigid three-dimensional matrix have attracted more attention due to their excellent properties, such as large band gaps, moderate phonon energies and high chemical stability.¹⁸ Tricalcium phosphate, Ca₃(PO₄)₂, possesses at least four polymorphous modifications (namely, the β-, α-, α′-, and γ-phase), and the phase of the Ca₃(PO₄)₂ structure is strongly dependent on temperature and pressure.^19–23 Recently, Haipeng Ji et al. reported that an α-Ca₃(PO₄)₂:Eu²⁺ phosphor exhibits a bright visible cyan color emission under UV light excitation and discovered the phase is a monoclinic structure with the space group P21/a.²⁴ Yet, to the best of our knowledge, there are no reports about the persistent luminescence properties of α-Ca₃(PO₄)₂:Eu²⁺ and that of cyan color emitting persistent phosphors are lacking. This motivated us to explore the afterglow properties of this phosphor and we investigate the afterglow performances of α-Ca₃(PO₄)₂:Eu²⁺ and α-Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ in our present work.

Detailed investigations of luminescence and defect properties were carried out by using photoluminescence excitation (PLE) and emission (PL) spectra, persistent luminescence emission, decay curves, and thermoluminescence (TL) spectra. Although the exact underlying mechanism for persistent luminescence is still an open question, it is generally accepted that the depth and concentration of traps in the band gap of the host determine the afterglow duration and brightness and TL glow curves are frequently measured to analyze the detailed information of the traps.^25–27 We also propose a model of persistent luminescence and persistent energy transfer (PET) between Eu²⁺ ions and Dy³⁺ ions in the Ca₃(PO₄)₂ phosphor.

2. Experimental

Ca₃(PO₄)₂:1% Eu²⁺, Ca₃(PO₄)₂:1% Dy³⁺ and Ca₃(PO₄)₂:1% Eu²⁺,1% Dy³⁺ were synthesized by a high-temperature solid-state reaction technique with CaCO₃ (A.R.), NH₄H₂PO₄ (A.R.), Eu₂O₃ (99.99%) and Dy₂O₃ (99.99%) as starting materials. The stoichiometric raw materials were homogeneously mixed and thoroughly ground in an agate mortar with appropriate ethanol addition for about 30 min. Then, the mixtures were transferred to alumina crucibles to calcine at 1480 °C for 4 h under a reducing atmosphere (N₂ : H₂ = 95 : 5) in an electric tube furnace. After calcination, the samples were furnace-cooled to room temperature and ground again to obtain the sample products in the form of fine powders.

The phase purity of the synthesized phosphor samples was 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 of 10° to 80°. The excitation and emission spectra were measured by an FLS-920T fluorescence spectrophotometer with Xe 900 (450 W xenon arc lamp) as the excitation source and the scanning step was 1 nm. Afterglow decay curves were measured with a PR305 long afterglow instrument after the samples were irradiated with ultraviolet light (254 nm) for 10 min. The TL curves were measured with an FJ-427A TL meter (Beijing Nuclear Instrument Factory) with a heating rate of 1 K s⁻¹ in the temperature range of 20 to 400 °C. Before measurement, 0.0010 g samples, pressed in pellets, were exposed to radiation for 1 min by 254 nm light. All measurements were carried out at room temperature except for the TL curves.

3. Results and discussion

3.1. Phase identification

Fig. 1a shows the XRD patterns of Ca₃(PO₄)₂, Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ samples. Clearly, all the diffraction peaks of the as-prepared Ca₃(PO₄)₂ compounds accompanied by different doping rare earth ions could be exactly indexed and match well with the pattern stimulated from the refined crystallographic data of α-Ca₃(PO₄)₂, indicating that all samples are identified as a single phase. Fig. 1b and c show the observed (crosses), calculated (solid line), and the position of Bragg reflections of the calculated pattern (short vertical lines) for the Rietveld refinement of Ca₃(PO₄)₂ and Ca₃(PO₄)₂:Eu²⁺ phosphors. The Rietveld refinement results indicate that Ca₃(PO₄)₂ crystallizes as a monoclinic structure with the space group P21/a. For the Ca₃(PO₄)₂ crystal, the lattice parameters were determined to be a = 12.8565(5) Å, b = 27.3555(7) Å, c = 15.2232(4) Å, β = 126.3164(17)°, and V = 4313.98(23) ų, and the reliability parameters of the refinement were R_wp = 12.31%, R_p = 9.67%, and χ² = 2.250. Meanwhile, the lattice parameters of Ca₃(PO₄)₂:Eu²⁺ became a = 12.8610(12) Å, b = 27.3570(26) Å, c = 15.2243(15) Å, β = 126.3330(13)°, and V = 4315.12(100) ų and the refinement finally converged to R_wp = 12.40%, R_p = 9.54% and χ² = 2.136, which are shown in Table 1. We can see that the crystal cell parameters of Ca₃(PO₄)₂:Eu²⁺ show an obvious increase as Ca²⁺ ions were substituted by larger Eu²⁺ ions in the Ca₃(PO₄)₂ host, and this led to the expansion of the lattice. This result also can be validated by the fact that all the diffraction peaks of Ca₃(PO₄)₂:Eu²⁺ slightly move to a lower 2θ value referring to the position of Ca₃(PO₄)₂ in Fig. 1a. In contrast, the diffraction peaks of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ offset to higher 2θ values due to the Dy³⁺ ions with smaller ionic radii. The crystal structure of Ca₃(PO₄)₂, using Rietveld refinement, is shown in Fig. 1d. The crystal structure of α-Ca₃(PO₄)₂ is extremely complex and there are 18 nonequivalent crystallographic Ca²⁺ sites, 48 oxygen sites and 12 phosphorus sites.²⁴

Fig. 1 XRD patterns of Ca₃(PO₄)₂, Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ (a); Rietveld analysis plots of XRD patterns of Ca₃(PO₄)₂ (b); Rietveld analysis plots of XRD patterns of Ca₃(PO₄)₂:Eu²⁺ (c); the crystal structure of Ca₃(PO₄)₂ using Rietveld refinement (d).

Table 1 Rietveld refinement and crystal data of Ca₃(PO₄)₂ and Ca₃(PO₄)₂:Eu²⁺ phosphors

Sample	Ca3(PO4)2	Ca3(PO4)2:Eu^2+
Phase	α	α
Space group	P21/a	P21/a
a (Å)	12.8565(5)	12.8610(12)
b (Å)	27.3555(7)	27.3570(26)
c (Å)	15.2232(4)	15.2243(15)
β (°)	126.3164(17)	126.3330(13)
V (Å^3)	4313.98(23)	4315.12(100)
Rwp, %	12.31	12.40
Rp, %	9.67	9.54
χ^2	2.250	2.136

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3.2. Optical performance

In Fig. 2, the excitation and emission spectra of the Eu²⁺ single-, Dy³⁺ single-, Eu²⁺ and Dy³⁺ dual-doped Ca₃(PO₄)₂ samples are shown for comparison. As depicted in Fig. 2a, Ca₃(PO₄)₂:Eu²⁺ exhibits a broad emission band centered around 480 nm and the full-width at half-maximum (FWHM) value is as high as 100 nm. This band belongs to the 5d to 4f transition of Eu²⁺ ions and the high FWHM is evidence that several cation sites occupied by Eu²⁺ ions exist in the structure of α-Ca₃(PO₄)₂. On the other hand, as shown in Fig. 2b, the excitation spectrum of Ca₃(PO₄)₂:Dy³⁺ consists of a series of line peaks in the range of 250–500 nm, which are ascribed to the transitions from the ground state of ⁶H_15/2 to the various excited states of 4f⁹ electronic configurations of the Dy³⁺ ions. The emission spectrum exhibits three sharp peaks at 482 nm, 575 nm and 664 nm, corresponding to the ⁴F_9/2 to ⁶H_15/2, ⁶H_13/2 and ⁶H_11/2 transitions, respectively. Notably, the emission spectrum of Ca₃(PO₄)₂:Eu²⁺ and the excitation spectrum of Ca₃(PO₄)₂:Dy³⁺ have a significant spectral overlap, which demonstrates that the energy transfer process may occur from Eu²⁺ ions to Dy³⁺ ions according to the Dexter theory.²⁸ Fig. 2c illustrates the excitation and emission spectra of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺. It is found that the excitation spectrum by monitoring the Dy³⁺ ions emission (575 nm) is similar to that by monitoring the emission of the Eu²⁺ ions (480 nm), which implies the existence of energy transfer from Eu²⁺ ions to Dy³⁺ ions in Ca₃(PO₄)₂ systems.

Fig. 2 Excitation (left) and emission (right) spectra of Ca₃(PO₄)₂:Eu²⁺ (a); Ca₃(PO₄)₂:Dy³⁺ (b); Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphors (c).

Fig. 3 shows the persistent luminescence spectra of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ measured at different times after removal of the UV irradiation source. In the process of persistent emission, the profiles and peaks of the persistent emission spectra display no apparent changes compared with the emission spectrum of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ which indicates that the persistent luminescence arises from the 5d to 4f transition of Eu²⁺ ions and the ⁴F_9/2 to ⁶H_13/2 transition of Dy³⁺ ions. Obviously, Dy³⁺ ion phosphorescence originates from the persistent energy transfer (PET) of the Eu²⁺ ions as the Ca₃(PO₄)₂:Dy³⁺ phosphor presents no persistent luminescence. As it is seen from the sample photographs shown in Fig. 3a and b, the phosphor of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ emits bright cyan colors under UV light and also cyan persistent luminescence can be clearly observed after the removal of the UV excitation.

Fig. 3 Persistent emission spectra of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ measured at different times after the removal of the UV irradiation source. The inset shows the photos of photoluminescence (a) and persistent luminescence (b).

3.3. Decay characteristics

After the removal of the UV light source (254 nm), both Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphors show cyan persistent luminescence. The decay curves were measured, as shown in Fig. 4. Ca₃(PO₄)₂:Eu²⁺ has a weak persistent luminescence and only can be visible for more than 1 min by the naked eye. Whereas, the phosphorescence of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ can last more than 5 h above the recognizable intensity level (0.32 mcd m⁻²) and the initial intensity can reach about 0.36 cd m⁻². In general, the trend of the decay process contains a rapid-decaying process at first and then a slow-decaying one.^29,30 The decay curves of Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphors can be described by an exponential function as follows:

I(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) + A₀ (1)

where I(t) represents the persistent luminescence intensity at time t and A₀ is the background luminescence intensity, A₁ and A₂ are constants while τ₁ and τ₂ are the decay times for the rapid- and slow-decay components, respectively. The slow-decaying process determines the duration of the afterglow.³¹ The fitting results are listed in Table 2. As can be seen, τ₂ of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ is more than 50 times greater than that of Ca₃(PO₄)₂:Eu²⁺. This result explains why the afterglow duration of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ is longer than that of Ca₃(PO₄)₂:Eu²⁺. Fig. 4b presents the reciprocal afterglow intensity (I⁻¹) of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ as a function of time (t). The I⁻¹–t curve can be well fitted by a straight line. The linear dependence of I⁻¹ versus t indicates that the persistent luminescence in the Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphor is probably caused by one effective trap center or a series of distributed continuous traps and we will discuss this in depth according to the results shown in the TL spectra.

Fig. 4 Afterglow decay curves of Ca₃(PO₄)₂:Eu²⁺,Dy^3+. The insets show the afterglow decay curve of Ca₃(PO₄)₂:Eu²⁺ (a); and the reciprocal afterglow intensity (I⁻¹) of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ as a function of time (t) (b).

Table 2 Fitting results of the decay curves of Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphors

Phosphor	A0	A1	A2	τ1/s	τ2/s	Adj. R^2
Ca3(PO4)2:Eu^2+	0.00031	0.00163	0.00137	22.91553	2.16407	0.99805
Ca3(PO4)2:Eu^2+,Dy^3+	0.00345	0.27225	0.08287	11.75882	113.26171	0.99472

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3.4. Thermoluminescence characteristics

The persistent luminescence is governed by the thermal liberation of trapped carriers. Generally, TL measurements are a useful tool to investigate the information of the traps based on the thermal liberation of trapped carriers. The TL curves of Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ are presented in Fig. 5. The solely Eu²⁺ doped sample has three TL peaks at T₁ = 315 K, T₂ = 365 K, and T₃ = 412 K, while the TL peaks of the Eu²⁺ and Dy³⁺ co-doped sample are located at T′₁ = 320 K, T′₂ = 340 K, and T′₃ = 395 K. Ordinarily, the effective TL peak in the range of 320–400 K is suitable for a room temperature persistent phosphor to free the trapped charge carriers slowly by thermal energy.^32,33 The T₃ and T′₃ glow peaks situated at deep trap areas do not contribute to the persistent luminescence observed at room temperature. On the other hand, the concentration of shallow traps (T′₁ and T′₂) is increased dramatically in the Eu²⁺ and Dy³⁺ co-doped sample. This result indicates that doping with Dy³⁺ ions largely produces lots of defects, which accounts for the LLP at room temperature and is responsible for the enhancement of the initial LLP intensity and duration.

Fig. 5 The TL curves of Ca₃(PO₄)₂:Eu²⁺ and Ca₃(PO₄)₂:Eu²⁺,Dy³⁺.

In order to obtain more trap distribution information responsible for the whole decay process, TL glow curves were measured, where samples are excited by UV light for 10 min and placed in a dark environment for several hours. As depicted in Fig. 6, the TL intensity of T′₁ (320 K) disappeared after 30 minutes because the charge carriers escaped from the shallow traps much faster than those in deep ones. At the same time, it is found that the deep traps at 340 K (T′₂) play a major role and their TL intensities decrease relatively slowly with time. The trap depth E can be estimated from the TL glow curves using an approximate equation as follows,^34,35

where E is the trap depth, and T_m is the temperature of the TL glow peaks (in Kelvin). The calculated trap depths are plotted as a function of decay time, as shown in the inset of Fig. 6. As the decay time increases up to 1 h, the trap depth increases quickly from ∼0.65 eV to ∼0.69 eV. And then the trap depth gradually deepens to ∼0.71 eV along with the decay time. This result indicates the presence of a continuous trap with ∼0.71 eV depth distribution in Ca₃(PO₄)₂:Eu²⁺,Dy³⁺. When the decay time is over 5 h, the trap depth remains almost unchanged at ∼0.74 eV. This constant trap depth is too deep to release charge carriers at room temperature. Therefore, there are two types of traps in Ca₃(PO₄)₂:Eu²⁺,Dy³⁺, i.e., two discrete trap depth distributions (T′₁ and T′₃) and a wide range of continuous trap depth distribution.

Fig. 6 The TL glow curves of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺which is excited by UV light for 1 min and placed in a dark environment for several hours.

3.5. Afterglow mechanism

On the basis of the above results and discussion, a schematic model to interpret the persistent luminescence and persistent energy transfer from Eu²⁺ to Dy³⁺ ions in Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ is proposed and illustrated in Fig. 7. Under UV light (254 nm) irradiation, the electrons of the valence band are excited into the conduction band and holes are left in the valence band. Subsequently, some of the electrons undergo a nonradiative transfer from the conduction band to the 4f⁶5d¹ level of the Eu²⁺ ions and the ⁴F_9/2 level of the Dy³⁺ ions. These excited electrons jump to the ground levels and recombine with holes leading to the characteristic emission of Eu²⁺ and Dy³⁺ ions. On the other hand, the residual minority of excited electrons are trapped by shallow or deep electron traps through the conduction band. After the stoppage of irradiation, the trapped electrons are detrapped from electrons traps with the assistance of thermal activation energy and transfer back to the excited states of Eu²⁺ ions and then the direct recombination of electrons and holes leads to persistent luminescence of Eu²⁺ and Dy³⁺ ions. Herein, we presume that Dy³⁺ ion phosphorescence is not directly from the traps but originates from the PET of Eu²⁺ ions due to the solely Dy³⁺ doped phosphor Ca₃(PO₄)₂:Dy³⁺ demonstrating no afterglow phenomenon.

Fig. 7 A schematic model to interpret the trapping/detrapping processes and persistent energy transfer.

4. Conclusions

In summary, a novel cyan emitting LLP phosphor of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ was successfully synthesized by a traditional high-temperature solid-state method. XRD refinement demonstrates the high phase purity and monoclinic structure of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺. The Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ phosphor has bright cyan LLP and the LLP can last more than 5 h at a recognizable intensity level. The mechanisms of the photoluminescence and afterglow phenomenon were proposed based on the experimental results. Dy³⁺ ions not only act as luminescent centers, but also introduce foreign trap centers. The persistent luminescence of Dy³⁺ ions originates from the PET of Eu²⁺ ions. The TL glow curves with different delay times after ceasing the UV irradiation provide the trap distribution information and have demonstrated that the continuous trap distribution of Ca₃(PO₄)₂:Eu²⁺,Dy³⁺ shows an obvious TL peak at 340 K.

Acknowledgements

This work is supported by the National Natural Science Funds of China (Grant No. 51372105) and the Fundamental Research Funds for the Central Universities (Grant No. lzujbky-2016-235). The authors are thankful for the support from the Gansu Province Development and Reform Commission.

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