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Luminescence properties of a novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+
Journal of Luminescence 194 (2018) 682–685
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence properties of a novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+
MARK
⁎
Mingwen Wang , Wei Lin, Na Liu, Yaping Ye Department of Chemistry and Chemical Engineering, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, 100083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Long-lasting phosphor Persistent luminescence Afterglow Eu2+ Ho3+
A novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+ was synthesized by solid-state reaction. The phosphor was characterized by X-ray diffraction, photoluminescence spectra, afterglow decay curve and thermoluminescence spectrum. The photoluminescence is ascribed to the 5d-4f transitions of Eu2+ ions. There is no Ho3+ emission in the photoluminescence spectra, which means that Ho3+ is a coactivator. After irradiated with a UV light (365 nm), persistent luminescence can last more than 5626 s above 0.32 mcd/m2, which is 100 times than the sensitivity of human eyes in dark. Thermoluminescence curves indicate that only one trap depth in the phosphor, which is ascribed to the doping of Ho3+. The possible mechanism of persistent luminescence was discussed.
1. Introduction Persistent luminescence is an optical phenomenon where a material is excited by high energy radiation and emits visible luminescence for long time in room temperature after the excitation removed [1]. Materials with the characteristic, persistent are materials also called longlasting phosphors, which have been applied in the field of emergency lighting and display. Furthermore, their potential applications in ACLED and vivo bio imaging are reported in recent years [2–4]. Since Matsuzawa et al. discovered SrAl2O4: Eu2+, Dy3+ (green, 30 h) in 1996 [5], Rare earth long-lasting phosphors have been developed rapidly. The most efficient long-lasting phosphors are aluminates and silicates such as CaAl2O4: Eu2+, Nd3+ (blue, 5 h) [6], Sr4Al14O25: Eu2+, Dy3+ (blue, 15 h) [7], Sr2MgSi2O7: Eu2+, Dy3+ (blue, 10 h) [8], and Sr3MgSi2O8: Eu2+, Dy3+ (blue, 10 h) [9]. Yellow and red emitting long-lasting phosphors also have been reported, such as Sr3SiO5: Eu2+, Dy3+ (yellow, > 6 h) [10], Y2O2S: Eu3+, Ti4+, Mg2+ (red, 5 h) [11], and Ca2Si5N8: Eu2+, Tm3+ (red, 1 h) [12]. In recent years, phosphate long-lasting phosphors are being developed rapidly because of their good chemical stability comparing with aluminates and silicates. SrMg2P2O8: Eu2+, Tb3+ (blue, 1000 s) [13], BaMg2P2O8: Eu2+, Tb3+ (blue, 1000 s) [14], Ca6BaP4O17: Eu2+, Ho3+ (blue, 47 h) [15] have been reported. LiBaPO4: Eu2+ is a phosphor with good luminescence, chemical stability and higher thermal stability [16,17]. According to Dorenbos’ theory [18], Ho3+ is appropriate trap in Eu2+ doping phosphors including LiBaPO4. In this work, we successfully synthesized a novel ⁎
bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+ via solid-state reaction. We also investigated the defect properties by means of afterglow decay curve and thermoluminescence spectrum, which reveals that doping Ho3+ increases the afterglow time and lowers the trap depth. 2. Experimental 2.1. Synthesis The powder samples of LiBaPO4: Eu2+ and LiBaPO4: Eu2+, Ho3+ were prepared by high-temperature solid-state reaction. The raw materials were Eu2O3 (4N), Ho2O3 (4N), Li2CO3 (A.R.), BaCO3 (A.R.), and NH4H2PO4 (A.R.). The stoichiometric mixture of raw materials was homogeneously mixed in an agate mortar, and then an amount of ethanol was added, followed by further grinding for 15 min. Firstly, the obtained samples was heated to 750 °C for 5 h in air. After that, the sample was thoroughly mixed and heated at 1050 °C for 5 h in air. Finally, the sample was thoroughly mixed and heated at 1050 °C for 2 h in a thermal-carbon reducing atmosphere. After cooling to room temperature inside the Muffle furnace, the obtained samples were ground again in an agate mortar for next characterizations. 2.2. Measurement All of the phase structures of the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer
Corresponding author. E-mail address: [email protected] (M. Wang).
http://dx.doi.org/10.1016/j.jlumin.2017.09.035 Received 28 March 2017; Received in revised form 14 September 2017; Accepted 15 September 2017 Available online 18 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 682–685
M. Wang et al.
Fig. 1. XRD pattern of LiBa0.999PO4: 0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+.
Fig. 2. Excitation and emission spectra of LiBa1-xPO4: xEu2+. Inset: photoluminescence peaks intensity versus Eu concentration of LiBa1-xPO4: xEu2+.
with Ni-filtered Cu Kα radiation (1.5406 Å) at 40 kV and 40 mA. The photoluminescence excitation and emission spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Japan) using a Xenon lamp (150 W) as excitation source. Persistent luminescence was measured by Hitachi F-4500 fluorescence spectrophotometer turning off Xenon lamp after the samples were irradiated with a UV light (365 nm) for 5 min. Afterglow decay curve measurements were measured by OPT-2003Q long afterglow instrument (made by Beijing Normal University, China) after the samples were irradiated with a UV light (365 nm) for 5 min. The thermoluminescence (TL) spectrum was measured by BRGD 2000-D. Powder samples were first exposed for 5 min by UV light (365 nm), and then heated from room temperature to 250 °C with a heating rate of 1 °C/s. All measurements were carried out at room temperature except for the TL spectrum.
and R is the bond length. The greater the coordination number is, the bigger the radius is. The site of six-coordination is at stronger crystal field and excitation wavelength is red shifted. Eu2+ ions occupy ninecoordination sites firstly. With the increase of Eu2+ concentration, Eu2+ has to occupy the site of six-coordination which results in the red shift of excitation band. The best Eu2+ doping concentration is 0.01. More than 0.01, the concentration quenching will happen. Figs. 3 and 4 show the excitation and emission spectra of LiBa0.9992+ , xHo3+ and LiBa0.99-xPO4: xEu2+, 0.01Ho3+. With xPO4: 0.001Eu the increase of Ho3+ concentration, the photoluminescence intensity decrease gradually. However, it can be found that LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ has the best afterglow time. Increasing the doping concentration of Eu2+ could not improve the afterglow time, only enhance the photoluminescence intensity. Doping 0.001Eu2+, most of the Eu2+ occupies nine-coordination while little Eu2+ occupies six-coordination, which results that LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ has the best afterglow time. Chemical environment of Eu2+ plays an important role in increasing afterglow time. There is no Ho3+ emission in the photoluminescence spectra, which means that Ho3+ is a coactivator. Ho3+ was doping as trap centers into LiBaPO4: Eu2+ host. For most long-lasting phosphors, co-doping Eu2+ and trivalent rare earth ions can improve the photoluminescence intensity. In LiBaPO4: Eu2+, doping Ho3+ reduce the photoluminescence intensity. Reducing the intensity of photoluminescence can increase afterglow time in
3. Results and discussion 3.1. XRD phase The XRD patterns of the samples are given in Fig. 1. The XRD peaks of the LiBaPO4 sample are consistent with the standard data in the JCPDS card of PDF#14-0270 with hexagonal phase and space group P63. No extra phase is detected, indicating that co-doping Eu2+ and Ho3+ have no impact on the crystal structure. The material LiBaPO4 belongs to the stuffed tridymite structure and Ba ions are ninecoordinated and six-coordinated [17]. Based on the consideration of the ionic radii, Eu2+ (1.17 Å, CN = 6; 1.30 Å, CN = 9) is proposed to occupy Ba2+ (1.35 Å, CN = 6; 1.47 Å, CN = 9) sites in the LiBaPO4 host lattice. 3.2. Photoluminescence spectra Fig. 2 exhibits the excitation and emission spectra for LiBa1-xPO4: xEu2+ at room temperature. Under excitation at 356 nm, LiBa0.999PO4: 0.001Eu2+ emits a broad band peaking at 483 nm which is ascribed to the 5d-4f transitions of Eu2+ ions. Zhang et al. reported that LiBaPO4: Eu2+ has three emissions at 412 nm (CN = 9) disappearing at room temperature, 473 nm (CN = 9) and 520 nm (CN = 6) [17]. Different emission is due to the change of the crystal field splitting. According to the reports by Robertson et al. [19] and Jang et al. [20], crystal field splitting (Dq) can be determined of the following Eq. (1):
Dq =
1 2 r4 Ze 5 6 R
(1) Fig. 3. Excitation and emission spectra of LiBa0.999-xPO4: 0.001Eu2+, xHo3+. Inset: afterglow time versus Ho concentration of LiBa0.999-xPO4: 0.001Eu2+, xHo3+.
Where Dq is a measure of the energy level separation, Z is the anion charge, e is the electron charge, r is the radius of the d wavefunction, 683
Journal of Luminescence 194 (2018) 682–685
M. Wang et al.
Table 1 Decay times for two exponential components of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+. A0
A1
τ1/s
A2
τ2/s
0.59256
36.15969
14.27094
6.16814
612.73868
Fig. 4. Excitation and emission spectra of LiBa0.99-xPO4: xEu2+, 0.01Ho3+. Inset: afterglow time versus Eu concentration of LiBa0.99-xPO4: xEu2+, 0.01Ho3+.
LiBaPO4: Eu2+. Similarly, Li2SrSiO4: Eu2+, Dy3+ with persistent luminescence has the lower photoluminescence compared with Li2SrSiO4: Eu2+ [21]. 3.3. Persistent luminescence Persistent luminescence spectra of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ measured at different time after exposed for 5 min by UV light (365 nm) was exhibited in Fig. 5. At first 30 min, the luminescence of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ was rapidly decreased. After 30 min, the luminescence began to gradually reduce. After exposed for 5 min by UV light (365 nm), persistent luminescence of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ can last more than 5626 s above 0.32 mcd/m2, which is 100 times than the sensitivity of human eyes in dark. Although 2 h later it can be seen that the luminescence is still alive, it drops below 0.32mcd/m2. It was found that they can be well fitted using a doubleexponential Eq. (2) as follows:
I = A1 exp(
−t −t ) + A2 exp( ) + A0 τ1 τ2
Fig. 6. CIE chromaticity diagram of LiBa0.999PO4: 0.001Eu2+ (0.16, 0.30) and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ (0.19, 0.33) excited at 356 nm. Inset: persistent luminescence LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ after exposed for 5 min by UV light (365 nm).
phosphor is chiefly determined by τ2 which results in slower afterglow decay. The color of both LiBa0.999PO4: 0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ excited at 356 nm was calculated using the chromaticity coordinate calculation method based on the CIE1931 system. The CIE chromaticity coordinate of LiBa0.999PO4: 0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ are (0.16, 0.30) and (0.19, 0.33) respectively. The colors both are bluish green as shown in Fig. 6.
(2)
Where I represents the phosphorescence intensity, A1 and A2 are constants, A0 is the final intensity, t is the time, and τ1 and τ2 are the decay times of the exponential components, respectively. All values of the sample are listed in Table 1. The persistent luminescence of long-lasting
3.4. Thermoluminescence properties Figs. 7 and 8 exhibit thermoluminescence curves of LiBa0.999PO4:
Fig. 5. Persistent luminescence spectra of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ measured at different times after exposed for 5 min by UV light (365 nm). Inset: Afterglow decay curves of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+.
Fig. 7. Thermoluminescence curves of LiBa0.999PO4: 0.001Eu2+ after exposed for 5 min by UV light (365 nm).
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Journal of Luminescence 194 (2018) 682–685
M. Wang et al.
process results in the formation of persistent luminescence. 4. Conclusion In summary, a novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+ was synthesized by solid-state reaction. Co-doping Eu2+, Ho3+ in LiBaPO4 achieved good persistent luminescence without changing the emission band. After irradiated with a UV light (365 nm), persistent luminescence can last more than 5626 s above 0.32 mcd/m2. A possible persistent luminescence mechanism is proposed via function of Ho.Ba electron traps and V′′Ba hole traps. This could be helpful for developing new phosphate long-lasting phosphors. Acknowledgements
Fig. 8. Thermoluminescence curves of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ measured at different times after exposed for 5 min by UV light (365 nm).
The work was supported by the National Natural Science Foundation of China (Grant no. 21376027) and Basic Theoretical Research Foundation of Institute of Metallurgical Engineering, University of Science and Technology Beijing (No. 39390009).
Table 2 Parameters of the thermoluminescence curves of sample LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ with different delay time.
References
Delay time (min)
Tm (K)
ET (eV)
0 30 60 90 120
383 385 389 386 386
0.766 0.770 0.778 0.772 0.772
[1] K. Van den Eeckhout, P.F. Smet, D. Poelman, Persistent luminescence in Eu2+-doped compounds: a review, Materials 3 (2010) 2536–2566. [2] H. Lin, B. Wang, J. Xu, R. Zhang, H. Chen, Y. Yu, Y. Wang, Phosphor-in-glass for highpowered remote-type white AC-LED, ACS Appl. Mater. Interfaces 6 (2014) 21264–21269. [3] A. Abdukayum, J.T. Chen, Q. Zhao, X.P. Yan, Functional near infrared-emitting Cr3+/ Pr3+ co-doped zinc gallogermanate persistent luminescent nanoparticles with superlong afterglow for in vivo targeted bioimaging, J. Am. Chem. Soc. 135 (2013) 14125–14133. [4] T. Maldiney, A. Bessiere, J. Seguin, E. Teston, S.K. Sharma, B. Viana, A.J. Bos, P. Dorenbos, M. Bessodes, D. Gourier, D. Scherman, C. Richard, The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells, Nat. Mater. 13 (2014) 418–426. [5] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, A new long phosphorescent phosphor with high brightness, SrAl2O4:Eu2+, Dy3+, J. Electrochem. Soc. 143 (1996) 2670–2673. [6] T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, J. Niittykoski, Mechanisms of persistent luminescence in Eu2+, RE3+ doped alkaline earth aluminates, J. Lumin. 94 (2001) 59–63. [7] Y.H. Lin, Z.L. Tang, Z.T. Zhang, Preparation of long-afterglow Sr4Al14O25-based luminescent material and its optical properties, Mater. Lett. 51 (2001) 14–18. [8] Y.H. Lin, Z.L. Tang, Z.T. Zhang, X.X. Wang, J.Y. Zhang, Preparation of a new long afterglow blue-emitting Sr2MgSi2O7-based photoluminescent phosphor, J. Mater. Sci. Lett. 20 (2001) 1505–1506. [9] A.A. Sabbagh Alvani, F. Moztarzadeh, A.A. Sarabi, Effects of dopant concentrations on phosphorescence properties of Eu/Dy-doped Sr3MgSi2O8, J. Lumin. 114 (2005) 131–136. [10] X. Sun, J. Zhang, X. Zhang, Y. Luo, X.-J. Wang, Long lasting yellow phosphorescence and photostimulated luminescence in Sr3SiO5: Eu2+ and Sr3SiO5: Eu2+, Dy3+ phosphors, J. Phys. D: Appl. Phys. 41 (2008) 195414. [11] X. Wang, Z. Zhang, Z. Tang, Y. Lin, Characterization and properties of a red and orange Y2O2S-based long afterglow phosphor, Mater. Chem. Phys. 80 (2003) 1–5. [12] B. Lei, K. Machida, T. Horikawa, H. Hanzawa, N. Kijima, Y. Shimomura, H. Yamamoto, Reddish-orange long-lasting phosphorescence of Ca2Si5N8:Eu2+,Tm3+ phosphor, J. Electrochem. Soc. 157 (2010) J196–J201. [13] G. Ju, Y. Hu, L. Chen, X. Wang, L. Hung, Persistent luminescence properties of SrMg2(PO4)2:Eu2+,Tb3+, Appl. Phys. A 114 (2013) 867–874. [14] G. Ju, Y. Hu, L. Chen, Z. Yang, T. Wang, Y. Jin, S. Zhang, Effects of Ln3+ (Ln = Ce, Pr, Tb and Lu) doping on the persistent luminescence properties BaMg2(PO4)2:Eu2+ phosphor, Ceram. Int. 41 (2015) 14998–15004. [15] H. Guo, W. Chen, W. Zeng, G. Li, Y. Wang, Y. Li, Y. Li, X. Ding, Structure and luminescence properties of a novel yellow super long-lasting phosphate phosphor Ca6BaP4O17:Eu2+, Ho3+, J. Mater. Chem. C 3 (2015) 5844–5850. [16] Z.C. Wu, J. Liu, M.L. Gong, Q. Su, Optimization and temperature-dependent luminescence of LiBaPO4:Eu2+ phosphor for near-UV light-emitting Diodes, J. Electrochem. Soc. 156 (2009) H153–H156. [17] S. Zhang, Y. Nakai, T. Tsuboi, Y. Huang, H.J. Seo, Luminescence and microstructural features of Eu-activated LiBaPO4 phosphor, Chem. Mater. 23 (2011) 1216–1224. [18] P. Dorenbos, Mechanism of persistent luminescence in Eu2+ and Dy3+ codoped aluminate and silicate compounds, J. Electrochem. Soc. 152 (2005) H107–H110. [19] J.M. Robertson, M.W. Van Tol, W.H. Smits, J.P.H. Heynen, Colour shift of the Ce3+ emission in monocrystalline epitaxially grown garnet layers, Philips J. Res. 36 (1981) 15–30. [20] H.S. Jang, W.B. Im, D.C. Lee, D.Y. Jeon, S.S. Kim, Enhancement of red spectral emission intensity of Y3Al5O12:Ce3+ phosphor via Pr co-doping and Tb substitution for the application to white LEDs, J. Lumin. 126 (2007) 371–377. [21] S. Cheng, X. Xu, J. Han, J. Qiu, B. Zhang, Design, synthesis and characterization of a novel orange-yellow long-lasting phosphor: Li2SrSiO4:Eu2+, Dy3+, Powder Technol. 276 (2015) 129–133. [22] K. Van den Eeckhout, A.J.J. Bos, D. Poelman, P.F. Smet, Revealing trap depth distributions in persistent phosphors, Phys. Rev. B 87 (2013). [23] F. Urbach, Lumineszenz der alkalihalogenide: II. Messungmethoden, Sitzungsberichte Akad. Wissenshaften Wien 139 (1930) 363.
0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ after exposed for 5 min by UV light (365 nm). The afterglow time is affected greatly by the depths of trap. After high energy radiation removed, the captured charge carriers in traps escape and move to luminescence center under thermal disturbance, which results in persistent luminescence. Low depths of trap only cause short duration period of persistent luminescence. Deep depths of trap are very difficult to release charge carriers at room temperature, which also results in poor persistent luminescence. Appropriate trap depths (0.6–1.2 eV) is helpful for achieving good persistent luminescence [22]. The depths of trap (ET) can be calculated by analyzing the TL peak using Eq. (3) given by Urbach method [23]:
ET =
Tm 500
(3)
Where ET is the trap depths, Tm is the peak temperature (in Kelvin). The trap depths of LiBa0.999PO4: 0.001Eu2+ are 0.632 eV and 0.834 eV. Although the trap depths are satisfied with the ideal range, the intensity is too weak to capture more carriers. The calculated results of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ with different delay time are presented in Table 2. The trap depths are 0.776 eV, 0.770 eV, 0.772 eV and 0.772 eV. With different delay time, the trap depths have little change. This suggests that only one trap in the phosphor, which is ascribed to the doping of Ho3+. Doping Ho3+ at LiBaPO4: Eu2+ host, creating two positive Ho.Ba electron traps and one negative V′′Ba hole trap for charge compensation. When Ho2O3 replaces two BaO, an extra oxygen and two positive charge were left. Extra oxygen will attract positive charge becoming hole trap. Two positive charge will attract negative charge becoming electron trap. It is described in Eq. (4) as follow: .
3Ba2 + + 2Ho3 + → V′′Ba + 2HoBa
(4)
After excited by UV light, electrons are excited to 5d level of Eu2+ and go through the conduction band (CB) with long time irradiation, and then stored in electron traps Ho.Ba . Holes are also stored in hole traps V′′Ba via valence band (VB). When UV light was removed, the captured charge carriers in traps recombined in Eu2+. The whole
685
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Luminescence properties of a novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+
MARK
⁎
Mingwen Wang , Wei Lin, Na Liu, Yaping Ye Department of Chemistry and Chemical Engineering, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, 100083, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Long-lasting phosphor Persistent luminescence Afterglow Eu2+ Ho3+
A novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+ was synthesized by solid-state reaction. The phosphor was characterized by X-ray diffraction, photoluminescence spectra, afterglow decay curve and thermoluminescence spectrum. The photoluminescence is ascribed to the 5d-4f transitions of Eu2+ ions. There is no Ho3+ emission in the photoluminescence spectra, which means that Ho3+ is a coactivator. After irradiated with a UV light (365 nm), persistent luminescence can last more than 5626 s above 0.32 mcd/m2, which is 100 times than the sensitivity of human eyes in dark. Thermoluminescence curves indicate that only one trap depth in the phosphor, which is ascribed to the doping of Ho3+. The possible mechanism of persistent luminescence was discussed.
1. Introduction Persistent luminescence is an optical phenomenon where a material is excited by high energy radiation and emits visible luminescence for long time in room temperature after the excitation removed [1]. Materials with the characteristic, persistent are materials also called longlasting phosphors, which have been applied in the field of emergency lighting and display. Furthermore, their potential applications in ACLED and vivo bio imaging are reported in recent years [2–4]. Since Matsuzawa et al. discovered SrAl2O4: Eu2+, Dy3+ (green, 30 h) in 1996 [5], Rare earth long-lasting phosphors have been developed rapidly. The most efficient long-lasting phosphors are aluminates and silicates such as CaAl2O4: Eu2+, Nd3+ (blue, 5 h) [6], Sr4Al14O25: Eu2+, Dy3+ (blue, 15 h) [7], Sr2MgSi2O7: Eu2+, Dy3+ (blue, 10 h) [8], and Sr3MgSi2O8: Eu2+, Dy3+ (blue, 10 h) [9]. Yellow and red emitting long-lasting phosphors also have been reported, such as Sr3SiO5: Eu2+, Dy3+ (yellow, > 6 h) [10], Y2O2S: Eu3+, Ti4+, Mg2+ (red, 5 h) [11], and Ca2Si5N8: Eu2+, Tm3+ (red, 1 h) [12]. In recent years, phosphate long-lasting phosphors are being developed rapidly because of their good chemical stability comparing with aluminates and silicates. SrMg2P2O8: Eu2+, Tb3+ (blue, 1000 s) [13], BaMg2P2O8: Eu2+, Tb3+ (blue, 1000 s) [14], Ca6BaP4O17: Eu2+, Ho3+ (blue, 47 h) [15] have been reported. LiBaPO4: Eu2+ is a phosphor with good luminescence, chemical stability and higher thermal stability [16,17]. According to Dorenbos’ theory [18], Ho3+ is appropriate trap in Eu2+ doping phosphors including LiBaPO4. In this work, we successfully synthesized a novel ⁎
bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+ via solid-state reaction. We also investigated the defect properties by means of afterglow decay curve and thermoluminescence spectrum, which reveals that doping Ho3+ increases the afterglow time and lowers the trap depth. 2. Experimental 2.1. Synthesis The powder samples of LiBaPO4: Eu2+ and LiBaPO4: Eu2+, Ho3+ were prepared by high-temperature solid-state reaction. The raw materials were Eu2O3 (4N), Ho2O3 (4N), Li2CO3 (A.R.), BaCO3 (A.R.), and NH4H2PO4 (A.R.). The stoichiometric mixture of raw materials was homogeneously mixed in an agate mortar, and then an amount of ethanol was added, followed by further grinding for 15 min. Firstly, the obtained samples was heated to 750 °C for 5 h in air. After that, the sample was thoroughly mixed and heated at 1050 °C for 5 h in air. Finally, the sample was thoroughly mixed and heated at 1050 °C for 2 h in a thermal-carbon reducing atmosphere. After cooling to room temperature inside the Muffle furnace, the obtained samples were ground again in an agate mortar for next characterizations. 2.2. Measurement All of the phase structures of the samples were characterized by powder X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer
Corresponding author. E-mail address: [email protected] (M. Wang).
http://dx.doi.org/10.1016/j.jlumin.2017.09.035 Received 28 March 2017; Received in revised form 14 September 2017; Accepted 15 September 2017 Available online 18 September 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 194 (2018) 682–685
M. Wang et al.
Fig. 1. XRD pattern of LiBa0.999PO4: 0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+.
Fig. 2. Excitation and emission spectra of LiBa1-xPO4: xEu2+. Inset: photoluminescence peaks intensity versus Eu concentration of LiBa1-xPO4: xEu2+.
with Ni-filtered Cu Kα radiation (1.5406 Å) at 40 kV and 40 mA. The photoluminescence excitation and emission spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer (Hitachi, Japan) using a Xenon lamp (150 W) as excitation source. Persistent luminescence was measured by Hitachi F-4500 fluorescence spectrophotometer turning off Xenon lamp after the samples were irradiated with a UV light (365 nm) for 5 min. Afterglow decay curve measurements were measured by OPT-2003Q long afterglow instrument (made by Beijing Normal University, China) after the samples were irradiated with a UV light (365 nm) for 5 min. The thermoluminescence (TL) spectrum was measured by BRGD 2000-D. Powder samples were first exposed for 5 min by UV light (365 nm), and then heated from room temperature to 250 °C with a heating rate of 1 °C/s. All measurements were carried out at room temperature except for the TL spectrum.
and R is the bond length. The greater the coordination number is, the bigger the radius is. The site of six-coordination is at stronger crystal field and excitation wavelength is red shifted. Eu2+ ions occupy ninecoordination sites firstly. With the increase of Eu2+ concentration, Eu2+ has to occupy the site of six-coordination which results in the red shift of excitation band. The best Eu2+ doping concentration is 0.01. More than 0.01, the concentration quenching will happen. Figs. 3 and 4 show the excitation and emission spectra of LiBa0.9992+ , xHo3+ and LiBa0.99-xPO4: xEu2+, 0.01Ho3+. With xPO4: 0.001Eu the increase of Ho3+ concentration, the photoluminescence intensity decrease gradually. However, it can be found that LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ has the best afterglow time. Increasing the doping concentration of Eu2+ could not improve the afterglow time, only enhance the photoluminescence intensity. Doping 0.001Eu2+, most of the Eu2+ occupies nine-coordination while little Eu2+ occupies six-coordination, which results that LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ has the best afterglow time. Chemical environment of Eu2+ plays an important role in increasing afterglow time. There is no Ho3+ emission in the photoluminescence spectra, which means that Ho3+ is a coactivator. Ho3+ was doping as trap centers into LiBaPO4: Eu2+ host. For most long-lasting phosphors, co-doping Eu2+ and trivalent rare earth ions can improve the photoluminescence intensity. In LiBaPO4: Eu2+, doping Ho3+ reduce the photoluminescence intensity. Reducing the intensity of photoluminescence can increase afterglow time in
3. Results and discussion 3.1. XRD phase The XRD patterns of the samples are given in Fig. 1. The XRD peaks of the LiBaPO4 sample are consistent with the standard data in the JCPDS card of PDF#14-0270 with hexagonal phase and space group P63. No extra phase is detected, indicating that co-doping Eu2+ and Ho3+ have no impact on the crystal structure. The material LiBaPO4 belongs to the stuffed tridymite structure and Ba ions are ninecoordinated and six-coordinated [17]. Based on the consideration of the ionic radii, Eu2+ (1.17 Å, CN = 6; 1.30 Å, CN = 9) is proposed to occupy Ba2+ (1.35 Å, CN = 6; 1.47 Å, CN = 9) sites in the LiBaPO4 host lattice. 3.2. Photoluminescence spectra Fig. 2 exhibits the excitation and emission spectra for LiBa1-xPO4: xEu2+ at room temperature. Under excitation at 356 nm, LiBa0.999PO4: 0.001Eu2+ emits a broad band peaking at 483 nm which is ascribed to the 5d-4f transitions of Eu2+ ions. Zhang et al. reported that LiBaPO4: Eu2+ has three emissions at 412 nm (CN = 9) disappearing at room temperature, 473 nm (CN = 9) and 520 nm (CN = 6) [17]. Different emission is due to the change of the crystal field splitting. According to the reports by Robertson et al. [19] and Jang et al. [20], crystal field splitting (Dq) can be determined of the following Eq. (1):
Dq =
1 2 r4 Ze 5 6 R
(1) Fig. 3. Excitation and emission spectra of LiBa0.999-xPO4: 0.001Eu2+, xHo3+. Inset: afterglow time versus Ho concentration of LiBa0.999-xPO4: 0.001Eu2+, xHo3+.
Where Dq is a measure of the energy level separation, Z is the anion charge, e is the electron charge, r is the radius of the d wavefunction, 683
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Table 1 Decay times for two exponential components of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+. A0
A1
τ1/s
A2
τ2/s
0.59256
36.15969
14.27094
6.16814
612.73868
Fig. 4. Excitation and emission spectra of LiBa0.99-xPO4: xEu2+, 0.01Ho3+. Inset: afterglow time versus Eu concentration of LiBa0.99-xPO4: xEu2+, 0.01Ho3+.
LiBaPO4: Eu2+. Similarly, Li2SrSiO4: Eu2+, Dy3+ with persistent luminescence has the lower photoluminescence compared with Li2SrSiO4: Eu2+ [21]. 3.3. Persistent luminescence Persistent luminescence spectra of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ measured at different time after exposed for 5 min by UV light (365 nm) was exhibited in Fig. 5. At first 30 min, the luminescence of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ was rapidly decreased. After 30 min, the luminescence began to gradually reduce. After exposed for 5 min by UV light (365 nm), persistent luminescence of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ can last more than 5626 s above 0.32 mcd/m2, which is 100 times than the sensitivity of human eyes in dark. Although 2 h later it can be seen that the luminescence is still alive, it drops below 0.32mcd/m2. It was found that they can be well fitted using a doubleexponential Eq. (2) as follows:
I = A1 exp(
−t −t ) + A2 exp( ) + A0 τ1 τ2
Fig. 6. CIE chromaticity diagram of LiBa0.999PO4: 0.001Eu2+ (0.16, 0.30) and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ (0.19, 0.33) excited at 356 nm. Inset: persistent luminescence LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ after exposed for 5 min by UV light (365 nm).
phosphor is chiefly determined by τ2 which results in slower afterglow decay. The color of both LiBa0.999PO4: 0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ excited at 356 nm was calculated using the chromaticity coordinate calculation method based on the CIE1931 system. The CIE chromaticity coordinate of LiBa0.999PO4: 0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ are (0.16, 0.30) and (0.19, 0.33) respectively. The colors both are bluish green as shown in Fig. 6.
(2)
Where I represents the phosphorescence intensity, A1 and A2 are constants, A0 is the final intensity, t is the time, and τ1 and τ2 are the decay times of the exponential components, respectively. All values of the sample are listed in Table 1. The persistent luminescence of long-lasting
3.4. Thermoluminescence properties Figs. 7 and 8 exhibit thermoluminescence curves of LiBa0.999PO4:
Fig. 5. Persistent luminescence spectra of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ measured at different times after exposed for 5 min by UV light (365 nm). Inset: Afterglow decay curves of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+.
Fig. 7. Thermoluminescence curves of LiBa0.999PO4: 0.001Eu2+ after exposed for 5 min by UV light (365 nm).
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process results in the formation of persistent luminescence. 4. Conclusion In summary, a novel bluish green long-lasting phosphor LiBaPO4: Eu2+, Ho3+ was synthesized by solid-state reaction. Co-doping Eu2+, Ho3+ in LiBaPO4 achieved good persistent luminescence without changing the emission band. After irradiated with a UV light (365 nm), persistent luminescence can last more than 5626 s above 0.32 mcd/m2. A possible persistent luminescence mechanism is proposed via function of Ho.Ba electron traps and V′′Ba hole traps. This could be helpful for developing new phosphate long-lasting phosphors. Acknowledgements
Fig. 8. Thermoluminescence curves of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ measured at different times after exposed for 5 min by UV light (365 nm).
The work was supported by the National Natural Science Foundation of China (Grant no. 21376027) and Basic Theoretical Research Foundation of Institute of Metallurgical Engineering, University of Science and Technology Beijing (No. 39390009).
Table 2 Parameters of the thermoluminescence curves of sample LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ with different delay time.
References
Delay time (min)
Tm (K)
ET (eV)
0 30 60 90 120
383 385 389 386 386
0.766 0.770 0.778 0.772 0.772
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0.001Eu2+ and LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ after exposed for 5 min by UV light (365 nm). The afterglow time is affected greatly by the depths of trap. After high energy radiation removed, the captured charge carriers in traps escape and move to luminescence center under thermal disturbance, which results in persistent luminescence. Low depths of trap only cause short duration period of persistent luminescence. Deep depths of trap are very difficult to release charge carriers at room temperature, which also results in poor persistent luminescence. Appropriate trap depths (0.6–1.2 eV) is helpful for achieving good persistent luminescence [22]. The depths of trap (ET) can be calculated by analyzing the TL peak using Eq. (3) given by Urbach method [23]:
ET =
Tm 500
(3)
Where ET is the trap depths, Tm is the peak temperature (in Kelvin). The trap depths of LiBa0.999PO4: 0.001Eu2+ are 0.632 eV and 0.834 eV. Although the trap depths are satisfied with the ideal range, the intensity is too weak to capture more carriers. The calculated results of LiBa0.989PO4: 0.001Eu2+, 0.01Ho3+ with different delay time are presented in Table 2. The trap depths are 0.776 eV, 0.770 eV, 0.772 eV and 0.772 eV. With different delay time, the trap depths have little change. This suggests that only one trap in the phosphor, which is ascribed to the doping of Ho3+. Doping Ho3+ at LiBaPO4: Eu2+ host, creating two positive Ho.Ba electron traps and one negative V′′Ba hole trap for charge compensation. When Ho2O3 replaces two BaO, an extra oxygen and two positive charge were left. Extra oxygen will attract positive charge becoming hole trap. Two positive charge will attract negative charge becoming electron trap. It is described in Eq. (4) as follow: .
3Ba2 + + 2Ho3 + → V′′Ba + 2HoBa
(4)
After excited by UV light, electrons are excited to 5d level of Eu2+ and go through the conduction band (CB) with long time irradiation, and then stored in electron traps Ho.Ba . Holes are also stored in hole traps V′′Ba via valence band (VB). When UV light was removed, the captured charge carriers in traps recombined in Eu2+. The whole
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