Enhancement of yellow persistent luminescence in Eu²⁺-doped β-Ba₃P₄O₁₃ phosphor by Ga³⁺ codoping

Abstract

Novel yellow emitting persistent phosphors β-Ba₃P₄O₁₃:Eu²⁺ and β-Ba₃P₄O₁₃:Eu²⁺,Ga³⁺ were successfully synthesized via solid-reaction method. Ga³⁺ was proved to be an effective codopant to enhance persistent luminescence in Eu²⁺-doped β-Ba₃P₄O₁₃ phosphors. Eu²⁺–Ga³⁺ codoped β-Ba₃P₄O₁₃ showed about 6 times longer persistence time than the Eu²⁺-singly-doped phosphors. The yellow long persistent luminescence (LPL) of β-Ba₃P₄O₁₃:Eu²⁺,Ga³⁺ can persist about 10 h. The thermoluminescence (TL) analyses showed that co-doping Ga³⁺ can improve more appropriate traps leading to the enhancement of the afterglow. These results suggested that Ga³⁺ may play a critical role in creating traps in β-Ba₃P₄O₁₃:Eu²⁺.

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Introduction

Long persistent phosphors, exhibiting an emission of light lasting for hours after the excitation stoppage, have attracted considerable attention for their wide application in security signs, optical storage media, radiation detection and in vivo bio-imaging.^1–4

Eu²⁺ is the most widely used activator for persistent phosphors because its 5d electron state is usually close to the conduction band of the host, which makes trapping of electrons possible.⁵ Codoping trivalent rare earth ions has been a general way to improve the LPL of Eu²⁺ doped compounds since the afterglow properties of SrAl₂O₄:Eu²⁺,Dy³⁺ (ref. 6) were vastly improved via co-doping Dy³⁺ in SrAl₂O₄:Eu²⁺. By our observation, Dy³⁺ or Nd³⁺ are good candidates for Eu²⁺ active aluminates and silicates,^7–11 while Tm³⁺ is often chosen for Eu²⁺active nitrides.^12,13 However, for Eu²⁺-activated phosphate persistent phosphors, the above-mentioned rare earth ions usually have little influence on their persistent luminescence properties.¹⁴ More importantly, finding an appropriate codopant does not only benefit the apparent optical properties, but also contributes to revealing some important information on their mechanism. Hence, there is a strong desire for exploring suitable codopants to improve the LPL properties of Eu²⁺-activated phosphates.

Ba₃P₄O₁₃ has two polymorphic forms with a temperature of transformation of 870° ± 10.^15,16 The low-temperature form is triclinic and the high-temperature form is orthorhombic, labeled as α and β, respectively. In this paper, we report a novel LPL of Eu²⁺ doped β-Ba₃P₄O₁₃. It is found that the incorporation of Ga³⁺ into β-Ba₃P₄O₁₃:Eu²⁺ could remarkably enhance its LPL properties. The mechanism for the origin and enhancement of LPL is also discussed briefly.

Experimental

The investigated samples in this work were synthesized through the high temperature solid-state method with BaCO₃ (A. R.), NH₄H₂PO₄ (A. R.), Eu₂O₃ (99.99%) and Ga₂O₃ (99.99%) as raw materials. Starting materials were homogeneously mixed in an agate mortar by adding a known amount of ethanol, and ground for 1 h. The mixture was then placed into an alumina crucible and sintered at 1173 K for 2 h under reducing atmosphere in an electric tube furnace. Finally, after calcination, the samples were cooled to room temperature in the furnace and ground again into powder for subsequent use.

All the phase structures of samples were characterized by powder X-ray diffraction using a Rigaku diffractometer with Ni-filtered Cu K_α radiation at a scanning step of 0.02° in the 2θ range from 10° to 80°. Excitation and emission spectra were obtained by a FLS-920T fluorescence spectrophotometer with Xe 900 (450 W xenon arc lamp) as the light source. The afterglow decay curves were recorded using a PR305 long afterglow instrument after the samples were irradiated by ultraviolet light (254 nm) for 10 min. The thermoluminescence curves were measured by a FJ-427A1 meter (Beijing Nuclear Instrument Factory) with a heating rate of 1 K s⁻¹. Before the measurement, the samples were irradiated by ultraviolet light (254 nm) for different time. All the data were measured at room temperature except for the thermoluminescence curves.

Results and discussion

In this work, the effect of Eu²⁺ and Ga³⁺ doping concentration on LPL properties are investigated. The optimum phosphor is β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ in which the persistent time achieves maximum. The optimum concentration is 0.01 mol in case of Eu²⁺ single doped β-Ba₃P₄O₁₃. Only the optimum phosphors β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ are discussed in this paper.

Fig. 1 shows the typical X-ray diffraction (XRD) patterns of β-Ba₃P₄O₁₃, β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺. The results reveal that a predominant phase of β-Ba₃P₄O₁₃ (JCPDS no. 12-0689) is presented in all powder samples. No phase transformation or impurity is observed, clearly implying that the doping Eu²⁺ and Ga³⁺ does not cause any significant change in host structure.

Fig. 1 XRD patterns of β-Ba₃P₄O₁₃, β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺.

The PLE and PL spectra of β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ are depicted in Fig. 2a. The PLE spectrum shows a strong broad band from 250 nm to 450 nm, which is attributed to the 4f⁷ → 4f⁶5d¹ transition of the Eu²⁺ ions. The PL of the β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ phosphor shows broad asymmetric emission band from 400 nm to 800 nm, which is attributed to the 4f⁶5d¹ → 4f⁷ transition of the Eu²⁺ ions. The spectral shapes are almost identical in the two samples. With regard to the relative intensity, β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ shows about 4 times higher intensity than the β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ in the PL and PLE spectra. The improvement of photoluminescence efficiency by Ga³⁺ codopants can be associated with size mismatches between the Ga³⁺ and the host cation.^17–19 The charge mismatch resulted in the creation of the charge compensating Ba vacancy. The Ba vacancy situated around the Eu²⁺ lowered the local site-symmetry of the Eu²⁺. This possibly gave rise to a stronger effect on the enhancement of the luminescence. By Gaussian deconvolution (Fig. 2b), the emission spectra β-Ba₃P₄O₁₃:Eu²⁺,Ga³⁺ phosphor can be decomposed into two Gaussian profiles with peaks centered at 558 nm (2.22 eV) and 610 nm (2.03 eV), respectively. Based on the consideration of effective ionic radii with different coordination numbers, the doping rare-earth ions, Eu²⁺ are proposed to occupy the regular Ba²⁺ sites rather than P⁵⁺ sites in the β-Ba₃P₄O₁₃ host lattice. The two peaks can be identified as the different emission environments of the Ba²⁺ ions being occupied by Eu²⁺ ions.

Fig. 2 (a) PLE and PL spectra of β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺. (b) The deconvoluted Gaussian components of Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺.

The LPL decay curve of β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ are presented in Fig. 3a. It can be seen that the persistent time of β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ is only 1.5 h. An encouraging result of the present work is that by co-doping Ga³⁺, the persistent time increases largely. The persistent time of the β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ is nearly 10 h at a recognizable intensity level (≧0.32 mcd m⁻²). The decay curves and fitting curves of β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ phosphor are presented inset in Fig. 4a. It is clearly exhibits that the decay process consists of a fast decay process and a slow decay part which are well-fitted into a biexponential function as follows:

here, I(t) is the LPL intensities at time t. I₁ is final intensity. τ₁ and τ₂ are the decay time, A₁ and A₂ are the constants, and t is time. The fitting parameters also are listed in Table 1. It can be obvious that the decay processes of β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ are possessed a biexponential decay character. After removal the excitation, the intensity of LPL decreases rapidly and then very slowly. It is clearly observed from Table 1 that t and τ are increased by codoping Ga³⁺, resulting in the improvement of afterglow properties. Persistent luminescence spectra of β-Ba₃P₄O₁₃:Eu²⁺,Ga³⁺, measured with an interval 30 s after the end of excitation, are presented in Fig. 3b. They also reveal one broad band which are similar to their emission spectrum. The profiles of the persistent luminescence spectra do not change with decay time, indicating that the yellow LPL of β-Ba₃P₄O₁₃:Eu²⁺,Ga³⁺ also originates from the two kinds of Eu²⁺ ion emission centers. The inset of Fig. 3b is the corresponding CIE chromaticity diagram of the β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ at 30 s after the removal of the excitation source. Point with chromaticity coordination of (0.53, 0.46) in Fig. 3b locates in the region of yellow color, indicating that the persistent luminescence color of β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ is yellow.

Fig. 3 (a) LPL decay curve of two samples. (b) Persistent luminescence spectra of Ba₃P₄O₁₃:Eu²⁺,Ga³⁺ and corresponding CIE chromaticity diagram.

Fig. 4 TL curves (a) and deconvolution of TL glow curve (b and c) of Ba_2.99P₄O₁₃:Eu_0.01²⁺ and Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺.

Table 1 Constants (A) and decay times (τ) of Eu²⁺ and Eu²⁺,Ga³⁺ doped sample

Sample	A1	A2	τ1	τ2
Ba2.99P4O13:Eu0.01^2+	0.0073	0.0049	91	981
Ba2.97P4O13:Eu0.01^2+,Ga0.02^3+	0.031	0.027	110	1477

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The significant role of traps has been recognized in the field of LPL and the persistent luminescence is governed by the slow liberation of trapped charge carriers by thermal stimulation.^20–22

In order to characterize the traps, TL measurements are performed on β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ and Ga³⁺ codoped sample β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ and their TL curves are illustrated in Fig. 4a. It is obvious that the shape of TL curve of Ga³⁺ sample is very similar to that of Eu²⁺ single doped sample, which indicated that the codopant Ga³⁺ does not import new trap into the phosphor. Compared with Eu²⁺ single doped sample, the TL intensity of Ga³⁺ codoped sample increases largely, suggesting the concentration of trap increases by codoping Ga³⁺. The deconvolution of TL glow curves of β-Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺ and β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ are shown in Fig. 4b and c, respectively. It is obvious that there are two traps in both single doped and co-doped samples. By codoping Ga³⁺ in the sample, the intensity of T1 is increased largely, whereas the T2 is nearly invariable. As indicated above, the persistent time of co-doped sample is prolonged nearly six times compared with that of the Eu²⁺ single-doped sample. Therefore, the T1 may be responsible for the yellow LPL. In order to further verify assumption, the trap densities (n₀) and trap depths (E_t) are calculated by using Chen's equation.^23,24

E_t = [2.52 + 10.2 × (μ_g − 0.42)] × (k_BT_m²)/ω − 2k_BT_m

n₀ = ωI_m/{β × [2.52 + 10.2 × (μ_g − 0.42)]}

where β is the heating rate, T_m is the temperature of the TL peak, ω is the FWHM, ω = δ + τ, τ is the low temperature half-width and δ is the high temperature half-width, the asymmetry parameter μ_g = δ/δ + τ, k_B is the Boltzmann constant, I_m is the intensity of TL peak. The results are listed in Table 2. With the codoping of Ga³⁺, the trap density largely increases from 7.20 × 10⁶ cm⁻³ to 1.38 × 10⁷ cm⁻³, whereas trap depth is little changed, suggesting that codoping Ga³⁺ does not import new trap into the phosphor, only increasing the trap concentration. When Eu²⁺ is introduced into β-Ba₃P₄O₁₃ host lattice and substitutes for Ba²⁺, lattice disorder is strengthened. As sintered in reducing atmosphere, the two positive charges is easy to form.^5,25 With the codoping of Ga³⁺, the lattice distortion is further strengthened because the ion radius of Ba²⁺ and Ga³⁺ is much difference. Hence, more could be generated, resulting in the enhancement of TL band. Thus, the TL band can be attributed to the generation of positive charge defect which can serve as electron traps in Eu²⁺ single-doped and Ga³⁺-codoped samples. Under UV excitation, electrons are promoted from the occupied 4f levels of Eu²⁺ to conduction band. The excited electrons are subsequently captured by through the conduction band. Trapped electrons can exist for a long time at room temperature even after the irradiation light source is removed. Under thermal agitation, stored electrons can be released back to the conduction band, then electrons transfer to the 5d levels of Eu²⁺, which leads to the 4f⁶5d¹–4f⁷ (8S_7/2) yellow LPL.

Table 2 TL parameters of Ba_2.99P₄O₁₃:Eu_0.01²⁺ and Ba_2.97P₄O₁₃:Eu_0.01²⁺,Ga_0.02³⁺

Sample	Tm (K)	Et (eV)	n0 (cm^−3)
Eu^2+,Ga^3+	335	0.78	1.38 × 10^7
Eu^2+	332	0.76	7.20 × 10^6

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Conclusion

A new yellow long-lasting phosphor β-Ba₃P₄O₁₃:Eu²⁺,Ga³⁺ is synthesized by a solid state reaction. The Eu²⁺ substituting the Ba sites show an asymmetric emission band centered at 597 nm. The persistent time of β-Ba_2.99P₄O₁₃:Eu_0.01²⁺ is only 1.5 h. Codoping non-rare earth Ga³⁺ can increase the intensity T1, resulting in the improvement of afterglow properties. The duration of Eu²⁺,Ga³⁺ co-doped β-Ba₃P₄O₁₃ is about 10 h at a recognizable intensity level (≧0.32 mcd m⁻²). This investigation provides a new and efficient non-rare codopant for Eu²⁺-activated phosphate persistent phosphors, which contribute to the mechanism of codopants.

Acknowledgements

This work was supported by 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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