Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 147 (2015) 324–327
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Luminescence properties of Tb3+-doped borosilicate scintillating glass under UV excitation Chenggang Zuo a,⇑, Zhihua Zhou b, Ligang Zhu c, Anguo Xiao a, Yuandao Chen a, Xiangyang Zhang a, Yongbing Zhuang a, Xiaoyang Li a, Qizhi Ge d a
College of Chemistry and Chemical Engineering of Hunan University of Arts and Science, Changde 415000, China School of Chemistry and Chemical Engineering of Hunan University of Science and Technology, Xiangtan 411201, China College of Chemistry and Material of Yulin Normal University, Yulin 537000, China d College of Fu Rong of Hunan University of Arts and Science, Changde 415000, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+
Tb -doped Li2O–BaO–La2O3–Al2O3–
B2O3–SiO2 glasses were prepared. 3+ firstly increases and then decreases with decreasing B2O3/SiO2 ratio. 5 7 D4 ? FJ transitions are greatly enhanced with increasing concentration of Tb3+. Luminescence decay time of 2.13 ms is obtained.
Green emission of Tb
a r t i c l e
i n f o
Article history: Received 18 June 2014 Received in revised form 8 March 2015 Accepted 10 March 2015 Available online 1 April 2015 Keywords: Tb3+ Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses Luminescence Luminescence decay Scintillation
a b s t r a c t Transparent Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses doped with Tb3+ ion were prepared by high temperature melting method. Luminescence properties of Tb3+-doped borosilicate glasses have been investigated by transmission, excitation, emission and luminescence decay measurements. The transmission spectrum shows the glass has good transmittance in the visible region. Under the 236 nm UV excitation the intense green emission from 5D4 level is observed in Tb3+-doped borosilicate glass, comparable in intensity to the violet–blue emission starting from the 5D3 level. The green emission intensity of Tb3+ ion firstly increases and then decreases with the decreasing B2O3/SiO2 ratio in glass matrix. 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions of Tb3+ ion in borosilicate glass are greatly enhanced with increasing concentration of Tb3+ through the cross relaxation [Tb3+ (5D3) + Tb3+ (7F6) ? Tb3+ (5D4) + Tb3+ (7F0)] between two Tb3+ ions. Luminescence decay time of 2.13 ms is obtained for the emission transitions starting from 5D4 level in 2.5Li2O–20BaO–20La2O3–2.5Al2O3–20B2O3–35SiO2–0.5Tb4O7 glass. The results show that Tb3+doped borosilicate glasses would be potential candidates for scintillating material for static X-ray imaging. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Glasses doped with trivalent rare earth ions such as Ce3+, Tb3+ and Eu3+ are attractive scintillating materials because of its low ⇑ Corresponding author. Tel.: +86 0736 7186115. E-mail address: [email protected] (C. Zuo). http://dx.doi.org/10.1016/j.saa.2015.03.097 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
cost, large-volume production possibility and easy shaping of elements for applications in high energy physics, X-ray CT for industrial and medical imaging [1–4]. Among these trivalent rare earth ions, the most intense emission of Tb3+ ion is around 550 nm, which is convenient with silicon detectors and sensitive to human eyes [5]. Tb3+-doped silicate glass scintillators usually has higher luminescence than that of Eu3+- or Ce3+-activated [6], and
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Tb3+-doped glass is considerably slower in response than its Ce3+doped counterpart, but there is not the self-quenching phenomenon that occurs in Ce3+-doped glass when Ce3+ is oxidized to Ce4+ [7], Consequently, Tb3+-doped glass can be drawn into fiber easier than that of Ce3+-doped and it is more useful for radiography and nondestructive testing [5,6,8–10]. The advantages mentioned above make Tb3+-doped glass scintillator is a hot topic in the research on scintillating materials. In comparison with crystals, the energy transfer towards luminescence center efficiency is rather low in scintillating glass, which results in low light yield [11]. Many works have been devoted to enhance the luminescence intensity of Tb3+-doped scintillating glass by optimizing the composition of glass matrix [12–15] and conforming the suitable concentration of Tb3+ ion [16] and the sensitizer such as Gd2O3 or Ce2O3 [17–19]. In addition, glass ceramics were also prepared to improve the luminescence intensity of Tb3+ ion [20,21]. Great progress has been obtained in Tb3+-doped scintillating glass, however, based on our best knowledge, although borosilicate glass is often used as the matrix of luminescence materials, little attention has been paid to the Tb3+-doped borosilicate scintillating glass at present. Considering on the role of Li2O promoting luminescence of doped rare earth ions, BaO improving the X-ray absorption and fluorescence output, Al2O3 and B2O3 bettering glass structure, and La2O3 instead of BaO enhancing glass chemical stability, Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glass is chosen as the matrix doped with Tb3+ in our experiment. In this work, Tb3+-doped Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses were successfully fabricated by high temperature melting method. X-ray diffraction (XRD), optical transmission, and luminescent properties under UV have been investigated on the as-made glasses. Experimental The nominal compositions of Tb3+-doped Li2O–BaO–La2O3– Al2O3–B2O3–SiO2 glasses are listed in Table 1. Analytical reagent SiO2, H3BO3, Al2O3, BaCO3, Li2CO3 and 4N purity grade Tb4O7, La2O3 were used to prepare borosilicate glasses. The batches of the raw materials in a corundum crucible were melted in computer controlled electrical furnace at 1500 °C for 2 h in the normal atmosphere. And then the melts were poured onto a smooth surfaced preheated stainless steel plate. The resultant glasses were annealed in a muffle furnace at 500 °C for 2 h and cooled down slowly to the room temperature to remove the inner stress present in these glasses. The samples with a regular size of 20 mm 20 mm 2 mm were finally obtained after cutting, grinding and polishing. The obtained glasses are colorless and transparent, and intense green emission was visibly observed for the glass sample under excitation with 254 nm UV light from a mercury lamp, as shown in Fig. 1.
The X-ray diffraction (XRD) profile was obtained on a DX-2700 0
X-ray powder diffractometer with Cu Ka (k = 1.5406 Å A) radiation using an applied voltage of 40 kV and 40 mA anode current. Ultraviolet–visible transmission spectrum was measured using a Shimadzu UV-2500PC spectrophotometer. Excitation and emission spectra were recorded on a Hitachi F-2500 fluorescence spectrophotometer. The photoluminescence decay curve was obtained with a FLS-920 single photon spectrometer using a nanosecond flash lamp as excitation source. All the measurements were performed at room temperature. Results and discussion The XRD profile of Tb3+-doped borosilicate glass S7 is shown in Fig. 2. The XRD pattern of glass S7 exhibits the amorphous humps originating from borosilicate glass. The absence of any sharp diffraction peaks suggests that no crystallization had occurred upon quenching and subsequent annealing of the borosilicate glass. The broad band observed in the range from 2h = 20° to 35° is characteristic of vitreous SiO2 and indicates the amorphous nature of the studied materials [22]. Fig. 3 shows the transmission spectrum of Tb3+-doped borosilicate glass S3. It shows that the glass has good transmittance in the visible spectrum region. The UV cut-off wavelength is around 300 nm. Two weak absorption bands exist about at 378 and 484 nm in Tb3+-doped glass, which are assigned to the transitions from the ground state 7F6 to the higher 5D states of Tb3+. Fig. 4 presents the excitation (kem = 546 nm) and emission (kex = 236 nm) spectra of Tb3+-doped borosilicate glass S3 at room temperature. The excitation spectrum recorded by monitoring the 5 D4 ? 7F5 (546 nm) emission exhibits the maximum about at 236 nm corresponding to the transitions from the lower ground level 7F6 of the 4f8 configuration to the higher spin–allowed 4f75d1 level of the Tb3+ ion, while the weak bands located at 319, 353, 378 and 484 nm are assigned to the transitions from the ground state 7F6 to the higher excited states 5H7, 5D2, 5D3 and 5 D4, respectively. Under 236 nm UV excitation, the intense luminescence of Tb3+ ion consists of a series of sharp peaks in the region 350–650 nm. The emission bands located at 381, 417, 439 and 460 nm correspond to the 5D3 ? 7FJ (J = 6, 5, 4 and 3) transitions, respectively. Another set of emission bands peaked at 490, 546, 588 and 624 nm belong to the 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions. The most intense emission band is the 5D4 ? 7F5 transition peaked at 546 nm, and its FWHM is of about 13 nm.
Table 1 Nominal composition of as-prepared glasses. Glass sample
S1 S2 S3 S4 S5 S6 S7 S8
Composition (mol %) Li2O
BaO
La2O3
Al2O3
B2O3
SiO2
Tb4O7
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
30 25 20 15 20 20 20 20
25 30 35 40 35 35 35 35
0.5 0.5 0.5 0.5 0.25 0.75 1 2
Fig. 1. Photograph of Tb3+-doped borosilicate glass (S3) under 254 nm UV excitation.
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4000 3500 236
I(a.u.)
Intensity(a.u.)
3000
546 λex=236nm
λem=546nm
2500 2000
490
1500 1000
381 417 353378 439 487 319 460
500 0
30
40 50 2θ (degree)
60
70
The emission spectra of Tb3+-doped borosilicate glasses with different ratio of B2O3/SiO2 with the excitation of 236 nm light are shown in Fig. 5. The emission bands located at 381, 417, 439, 460, 490, 546, 588 and 624 nm can be observed in all of Tb3+-doped borosilicate glasses with different ratio of B2O3/SiO2. However, it could be found that: with the decreasing ratio of B2O3/SiO2, the green emission intensity of Tb3+ ion in borosilicate glasses firstly increases till the strongest luminescence intensity with the B2O3/ SiO2 ratio of 20/35, and then decreases with further decreasing ratio of B2O3/SiO2. It is well known that the addition of alkaline earth metal oxide can provide free oxygen’s and help to form non-bridging oxygen’s in glass network, which effects on the structural and optical behavior of these glasses. In the present study, the presence of BaO provides free oxygen’s. When the content of BaO is relative large in borosilicate glass, a number of oxygen ions are available which could change some network unit trigonal [BO3] to tetrahedral [BO4] resulting in compacting the network. With the change of B2O3/SiO2 ratio from 30/25 to 20/35, the amounts of [SiO4] and the [BO3]/[BO4] ratio increase. The covalent degree of oxygen bonding with glass network former cations decreases in the order of B3+ and Si4+ [23]. Thus an increase in the negative charge that Tb3+ ion might receive from the coordinated oxygen atoms will develop, consequently weakening the electrostatic pull of the nucleus on the outermost electrons, and allowing an electron to be promoted more readily into the excited state level, resulting in an increase of the emission intensity of Tb3+ ion in borosilicate
100 484 80 378
T/%
60 40 20 0 300
350
400 450 Wavelength/nm
624
250 300 350 400 450 500 550 600 650 700 Wavelength(nm)
Fig. 4. Excitation and emission spectra of Tb3+-doped borosilicate glass S3.
Fig. 2. XRD profile of Tb3+-doped borosilicate glass S7.
500
Fig. 3. Transmission spectrum of Tb3+-doped borosilicate glass S3.
S1 S2 S3 S4
3500 3000
Intensity(a.u.)
20
588
2500 2000
λex=236nm
1500 1000 500 0 300
400
500 600 Wavelength(nm)
700
Fig. 5. Emission spectra of Tb3+-doped borosilicate glasses with different ratio of B2O3/SiO2 with the excitation of 236 nm light.
glasses. With further decreasing ratio of B2O3/SiO2, the amounts of [BO3] and [BO4] decreases, and the negative charge that Tb3+ ion might receive from the coordinated oxygen atoms will decrease, resulting in a decrease of the emission intensity of Tb3+ ion. Fig. 6 shows the emission spectra of Tb3+-doped borosilicate glasses with different Tb3+ concentration under 236 nm UV excitation. The emission peaks located at 381, 417, 439, 460, 490, 546, 588 and 624 nm can be observed in all of Tb3+-doped borosilicate glasses with different Tb3+ concentration. Among them, the emission at 546 nm is the strongest. The intensity of 546 nm emission increases with the increase of Tb3+ concentration in the present work. Meanwhile 490, 588 and 624 nm emissions have same tendency with increasing concentration of Tb3+. However, the intensities of 381, 417, 439 and 460 nm emissions decrease monotonically with increasing concentration of Tb3+. The above behavior could attribute to energy transfer process between Tb3+ ions. Fig. 7 shows the energy level schemes of Tb3+ ions and visible emission transitions in Tb3+-doped borosilicate glasses. From the energy level of Tb3+ ion in Fig. 7, it can be concluded that there are two non-radiative processes from 5D3 to 5D4 states: a fast cross-relaxation due to the resonance energy transfer (RET) and a multi-phonons assisted non-radiative relaxation due to the low energy gap between 5D3 and 5D4 states [24]. Since the energy difference between 5D3 and 5D4 level is very close to that between 7F6 and 7F0 level, the cross relaxation [Tb3+(5D3) + Tb3+(7F6) ? Tb3+(5D4) + Tb3+(7F0)] between two Tb3+ ions could occur when Tb3+ ion is very close to another due to high concentration of
C. Zuo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 147 (2015) 324–327
4000
Intensity(a.u.)
3500
A1, t and s stands for the amplitude, time and decay time constant of the exponential component, respectively. The luminescence decay at 546 nm under UV excitation yields the decay time s = 2.13 ms.
S5 S3 S6 S7 S8
4500
λex=236nm
3000 Conclusions
2500
5
5
Transparent Tb3+-doped Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses were fabricated by high temperature melting method. The glass has good transmittance in the visible spectrum region. Tb3+-doped borosilicate glasses show intense green emission under UV excitation. The green emission intensity of Tb3+ ion firstly increases and then decreases with the decreasing B2O3/SiO2 ratio in borosilicate glass matrix. 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions of Tb3+ ion are greatly enhanced through the cross relaxation between two Tb3+ ions with increasing concentration of Tb3+. Luminescence decay time of 2.13 ms from 546 nm emission of 2.5Li2O–20BaO–20La2O3–2.5Al2O3–20B2O3–35SiO2–0.5Tb4O7 glass is obtained. The Tb3+-doped borosilicate glasses would be potential candidates for X-ray scintillating material suitable to X-ray detection for slow event.
5
5
Acknowledgement
2000 1500 1000 500 0 300
400
500 Wavelength(nm)
600
700
Fig. 6. Emission spectra of Tb3+-doped borosilicate glasses with different Tb3+ concentration under 236 nm UV excitation.
30 D3
D4
D4
10
RET
λ=490nm λ=546nm λ=588nm λ=624nm
λex=236nm
20 λ=381nm λ=417nm λ=439nm λ=460nm
Energy/(103 cm-1)
D3
7
7
F0
F0
7
0
3+
8 3+ Tb (B)
F6
Fig. 7. Energy level schemes of Tb3+ ions and visible emission transitions in Tb3+doped borosilicate glasses. Energy transfer process between Tb3+ ions is indicated.
5000
I(t) = 5812.11exp(-t/2127.03) + 29.42
4000
decay curve 1 exp fit
3000 λem=546nm 2000 1000 0
0
2000
This research was financially supported by the Research Foundation of Education Bureau of Hunan Province (Grant No. 12C0814), the Construct Program of the Key Discipline in Hunan Province (Applied Chemistry) and the Scientific Research Foundation for the Doctor of Hunan University of Arts and Science, China.
7
F6
Tb (A)
Counts
327
4000 6000 Time(μs)
8000
10000
Fig. 8. Luminescence decay curve of Tb3+-doped borosilicate glass S3 at room temperature (kem = 546 nm). The fitted function I(t) is given in the figure.
Tb3+ [24–26]. Thus the population of 5D3 level decreases and that of 5 D4 level increases. It results in the intensities of the emissions originated the transitions from 5D3 level to lower 7F levels decrease and the intensities of the emissions from 5D4 level to lower 7F levels increase. The luminescence decay of the Tb3+-doped borosilicate glass excited in the spin allowed 4f75d1 band of Tb3+ were measured at room temperature and is shown in Fig. 8. The decay curve can be well fitted with a single exponential equation, I(t) = A1exp(t/s) + y0, where I(t) is the luminescence intensity,
References [1] M.J. Webber, J. Lumin. 100 (2002) 35. [2] M. Nikl, J.A. Mares, E. Mihokova, K. Nitsch, N. Solovieva, V. Babin, A. Krasnikov, S. Zazubovich, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, S. Baccaro, Radiat. Meas. 33 (2001) 593. [3] G.P. Pazzi, P. Fabeni, C. Susini, M. Nikl, E. Mihokova, N. Solovieva, K. Nitsch, M. Martini, A. Vedda, S. Baccaro, A. Cevilia, V. Babin, Nucl. Instrum. Methods B 191 (2002) 366. [4] G.R. Chen, S. Baccaro, R. Giorgi, A. Cecilia, E. Mihokova, M. Nikl, J. Non-Cryst, Solids 326–327 (2003) 339. [5] G. Zanella, R. Zannoni, R. Dall’Igna, P. Polato, M. Bettinelli, Nucl. Inst. Methods Phys. Res. A 359 (1995) 547. [6] C. Bueno, R.A. Buchanan, SPIE 1327 (1990) 79. [7] G.B. Spector, T. McCollum, A.R. Spowart, Nucl. Instrum. Methods A 326 (1993) 526. [8] C. Bueno, M.D. Barker, B.A. Betz, R.C. Barry, R.A. Buchanan, SPIE 2455 (1990) 114. [9] S. Baccaro, R. Dall’Igna, P. Fabeni, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni, J. Lumin. 87–89 (2003) 673. [10] J.A. Mares, M. Nikl, K. Nitsch, N. Solovieva, A. Krasnikov, S. Zazubovich, J. Lumin. 94–95 (2001) 321. [11] S. Baccaro, A. Ceclia, A. Cemmi, G. Chen, E. Mihokova, N. Nikl, IEEE Trans. Nucl. Sci. 48 (2001) 360. [12] J. Fu, M. Kobayashi, J.M. Parker, J. Lumin. 128 (2008) 99. [13] O.V. Kaz’mina, A.N. Abiyaka, Y.A. Moskalev, Glass Ceram. 64 (2007) 75. [14] J.S. Cheng, P.J. Tian, W.H. Zheng, J. Xie, Z.X. Chen, J. Alloys Compd. 471 (2009) 470. [15] B.C. Jamalaiah, J.S. Kumar, A.M. Babu, T. Sasikala, L.R. Moorthy, Physica B 404 (2009) 2020. [16] A.D. Sontakke, K. Biswas, K. Annapurna, J. Lumin. 129 (2009) 1347. [17] C.K. Zu, J. Chen, H.F. Zhao, B. Han, Y.H. Liu, Y.H. Wang, J. Alloys Compd. 479 (2009) 294. [18] C.G. Zuo, A.X. Lu, G. Zhu, Mater. Sci. Eng., B 175 (2010) 229. [19] X. Sun, S.M. Huang, M. Gu, Q.C. Gao, X.S. Gong, Z.P. Ye, Physica B 405 (2010) 56. [20] X. Sun, S.M. Huang, Nucl. Instrum. Methods Phys. Res., Sect. A 621 (2010) 322. [21] Z. Pan, K. Jame, Y. Cui, A. Burger, N. Cherepy, S.A. Payne, R. Mu, S.H. Morgan, Nucl. Instrum. Methods Phys. Res., Sect. A 594 (2008) 215. [22] M. Nagai, K. Kobayashi, Y. Nakajima, Solid State Ionics 136–137 (2000) 249. [23] J. Bei, G. Qian, X. Liang, S. Yuan, Y. Yang, G. Chen, Mater. Res. Bull. 42 (2007) 1195. [24] T. Tsuboi, Eur. Phys. J. Appl. Phys. 26 (2004) 95. [25] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [26] D.B. He, C.L. Yu, J.M. Cheng, S.H. Li, L. Hu, J. Alloys Compd. 509 (2011) 1906.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Luminescence properties of Tb3+-doped borosilicate scintillating glass under UV excitation Chenggang Zuo a,⇑, Zhihua Zhou b, Ligang Zhu c, Anguo Xiao a, Yuandao Chen a, Xiangyang Zhang a, Yongbing Zhuang a, Xiaoyang Li a, Qizhi Ge d a
College of Chemistry and Chemical Engineering of Hunan University of Arts and Science, Changde 415000, China School of Chemistry and Chemical Engineering of Hunan University of Science and Technology, Xiangtan 411201, China College of Chemistry and Material of Yulin Normal University, Yulin 537000, China d College of Fu Rong of Hunan University of Arts and Science, Changde 415000, China b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
3+
Tb -doped Li2O–BaO–La2O3–Al2O3–
B2O3–SiO2 glasses were prepared. 3+ firstly increases and then decreases with decreasing B2O3/SiO2 ratio. 5 7 D4 ? FJ transitions are greatly enhanced with increasing concentration of Tb3+. Luminescence decay time of 2.13 ms is obtained.
Green emission of Tb
a r t i c l e
i n f o
Article history: Received 18 June 2014 Received in revised form 8 March 2015 Accepted 10 March 2015 Available online 1 April 2015 Keywords: Tb3+ Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses Luminescence Luminescence decay Scintillation
a b s t r a c t Transparent Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses doped with Tb3+ ion were prepared by high temperature melting method. Luminescence properties of Tb3+-doped borosilicate glasses have been investigated by transmission, excitation, emission and luminescence decay measurements. The transmission spectrum shows the glass has good transmittance in the visible region. Under the 236 nm UV excitation the intense green emission from 5D4 level is observed in Tb3+-doped borosilicate glass, comparable in intensity to the violet–blue emission starting from the 5D3 level. The green emission intensity of Tb3+ ion firstly increases and then decreases with the decreasing B2O3/SiO2 ratio in glass matrix. 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions of Tb3+ ion in borosilicate glass are greatly enhanced with increasing concentration of Tb3+ through the cross relaxation [Tb3+ (5D3) + Tb3+ (7F6) ? Tb3+ (5D4) + Tb3+ (7F0)] between two Tb3+ ions. Luminescence decay time of 2.13 ms is obtained for the emission transitions starting from 5D4 level in 2.5Li2O–20BaO–20La2O3–2.5Al2O3–20B2O3–35SiO2–0.5Tb4O7 glass. The results show that Tb3+doped borosilicate glasses would be potential candidates for scintillating material for static X-ray imaging. Ó 2015 Elsevier B.V. All rights reserved.
Introduction Glasses doped with trivalent rare earth ions such as Ce3+, Tb3+ and Eu3+ are attractive scintillating materials because of its low ⇑ Corresponding author. Tel.: +86 0736 7186115. E-mail address: [email protected] (C. Zuo). http://dx.doi.org/10.1016/j.saa.2015.03.097 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.
cost, large-volume production possibility and easy shaping of elements for applications in high energy physics, X-ray CT for industrial and medical imaging [1–4]. Among these trivalent rare earth ions, the most intense emission of Tb3+ ion is around 550 nm, which is convenient with silicon detectors and sensitive to human eyes [5]. Tb3+-doped silicate glass scintillators usually has higher luminescence than that of Eu3+- or Ce3+-activated [6], and
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Tb3+-doped glass is considerably slower in response than its Ce3+doped counterpart, but there is not the self-quenching phenomenon that occurs in Ce3+-doped glass when Ce3+ is oxidized to Ce4+ [7], Consequently, Tb3+-doped glass can be drawn into fiber easier than that of Ce3+-doped and it is more useful for radiography and nondestructive testing [5,6,8–10]. The advantages mentioned above make Tb3+-doped glass scintillator is a hot topic in the research on scintillating materials. In comparison with crystals, the energy transfer towards luminescence center efficiency is rather low in scintillating glass, which results in low light yield [11]. Many works have been devoted to enhance the luminescence intensity of Tb3+-doped scintillating glass by optimizing the composition of glass matrix [12–15] and conforming the suitable concentration of Tb3+ ion [16] and the sensitizer such as Gd2O3 or Ce2O3 [17–19]. In addition, glass ceramics were also prepared to improve the luminescence intensity of Tb3+ ion [20,21]. Great progress has been obtained in Tb3+-doped scintillating glass, however, based on our best knowledge, although borosilicate glass is often used as the matrix of luminescence materials, little attention has been paid to the Tb3+-doped borosilicate scintillating glass at present. Considering on the role of Li2O promoting luminescence of doped rare earth ions, BaO improving the X-ray absorption and fluorescence output, Al2O3 and B2O3 bettering glass structure, and La2O3 instead of BaO enhancing glass chemical stability, Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glass is chosen as the matrix doped with Tb3+ in our experiment. In this work, Tb3+-doped Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses were successfully fabricated by high temperature melting method. X-ray diffraction (XRD), optical transmission, and luminescent properties under UV have been investigated on the as-made glasses. Experimental The nominal compositions of Tb3+-doped Li2O–BaO–La2O3– Al2O3–B2O3–SiO2 glasses are listed in Table 1. Analytical reagent SiO2, H3BO3, Al2O3, BaCO3, Li2CO3 and 4N purity grade Tb4O7, La2O3 were used to prepare borosilicate glasses. The batches of the raw materials in a corundum crucible were melted in computer controlled electrical furnace at 1500 °C for 2 h in the normal atmosphere. And then the melts were poured onto a smooth surfaced preheated stainless steel plate. The resultant glasses were annealed in a muffle furnace at 500 °C for 2 h and cooled down slowly to the room temperature to remove the inner stress present in these glasses. The samples with a regular size of 20 mm 20 mm 2 mm were finally obtained after cutting, grinding and polishing. The obtained glasses are colorless and transparent, and intense green emission was visibly observed for the glass sample under excitation with 254 nm UV light from a mercury lamp, as shown in Fig. 1.
The X-ray diffraction (XRD) profile was obtained on a DX-2700 0
X-ray powder diffractometer with Cu Ka (k = 1.5406 Å A) radiation using an applied voltage of 40 kV and 40 mA anode current. Ultraviolet–visible transmission spectrum was measured using a Shimadzu UV-2500PC spectrophotometer. Excitation and emission spectra were recorded on a Hitachi F-2500 fluorescence spectrophotometer. The photoluminescence decay curve was obtained with a FLS-920 single photon spectrometer using a nanosecond flash lamp as excitation source. All the measurements were performed at room temperature. Results and discussion The XRD profile of Tb3+-doped borosilicate glass S7 is shown in Fig. 2. The XRD pattern of glass S7 exhibits the amorphous humps originating from borosilicate glass. The absence of any sharp diffraction peaks suggests that no crystallization had occurred upon quenching and subsequent annealing of the borosilicate glass. The broad band observed in the range from 2h = 20° to 35° is characteristic of vitreous SiO2 and indicates the amorphous nature of the studied materials [22]. Fig. 3 shows the transmission spectrum of Tb3+-doped borosilicate glass S3. It shows that the glass has good transmittance in the visible spectrum region. The UV cut-off wavelength is around 300 nm. Two weak absorption bands exist about at 378 and 484 nm in Tb3+-doped glass, which are assigned to the transitions from the ground state 7F6 to the higher 5D states of Tb3+. Fig. 4 presents the excitation (kem = 546 nm) and emission (kex = 236 nm) spectra of Tb3+-doped borosilicate glass S3 at room temperature. The excitation spectrum recorded by monitoring the 5 D4 ? 7F5 (546 nm) emission exhibits the maximum about at 236 nm corresponding to the transitions from the lower ground level 7F6 of the 4f8 configuration to the higher spin–allowed 4f75d1 level of the Tb3+ ion, while the weak bands located at 319, 353, 378 and 484 nm are assigned to the transitions from the ground state 7F6 to the higher excited states 5H7, 5D2, 5D3 and 5 D4, respectively. Under 236 nm UV excitation, the intense luminescence of Tb3+ ion consists of a series of sharp peaks in the region 350–650 nm. The emission bands located at 381, 417, 439 and 460 nm correspond to the 5D3 ? 7FJ (J = 6, 5, 4 and 3) transitions, respectively. Another set of emission bands peaked at 490, 546, 588 and 624 nm belong to the 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions. The most intense emission band is the 5D4 ? 7F5 transition peaked at 546 nm, and its FWHM is of about 13 nm.
Table 1 Nominal composition of as-prepared glasses. Glass sample
S1 S2 S3 S4 S5 S6 S7 S8
Composition (mol %) Li2O
BaO
La2O3
Al2O3
B2O3
SiO2
Tb4O7
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
20 20 20 20 20 20 20 20
20 20 20 20 20 20 20 20
2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5
30 25 20 15 20 20 20 20
25 30 35 40 35 35 35 35
0.5 0.5 0.5 0.5 0.25 0.75 1 2
Fig. 1. Photograph of Tb3+-doped borosilicate glass (S3) under 254 nm UV excitation.
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4000 3500 236
I(a.u.)
Intensity(a.u.)
3000
546 λex=236nm
λem=546nm
2500 2000
490
1500 1000
381 417 353378 439 487 319 460
500 0
30
40 50 2θ (degree)
60
70
The emission spectra of Tb3+-doped borosilicate glasses with different ratio of B2O3/SiO2 with the excitation of 236 nm light are shown in Fig. 5. The emission bands located at 381, 417, 439, 460, 490, 546, 588 and 624 nm can be observed in all of Tb3+-doped borosilicate glasses with different ratio of B2O3/SiO2. However, it could be found that: with the decreasing ratio of B2O3/SiO2, the green emission intensity of Tb3+ ion in borosilicate glasses firstly increases till the strongest luminescence intensity with the B2O3/ SiO2 ratio of 20/35, and then decreases with further decreasing ratio of B2O3/SiO2. It is well known that the addition of alkaline earth metal oxide can provide free oxygen’s and help to form non-bridging oxygen’s in glass network, which effects on the structural and optical behavior of these glasses. In the present study, the presence of BaO provides free oxygen’s. When the content of BaO is relative large in borosilicate glass, a number of oxygen ions are available which could change some network unit trigonal [BO3] to tetrahedral [BO4] resulting in compacting the network. With the change of B2O3/SiO2 ratio from 30/25 to 20/35, the amounts of [SiO4] and the [BO3]/[BO4] ratio increase. The covalent degree of oxygen bonding with glass network former cations decreases in the order of B3+ and Si4+ [23]. Thus an increase in the negative charge that Tb3+ ion might receive from the coordinated oxygen atoms will develop, consequently weakening the electrostatic pull of the nucleus on the outermost electrons, and allowing an electron to be promoted more readily into the excited state level, resulting in an increase of the emission intensity of Tb3+ ion in borosilicate
100 484 80 378
T/%
60 40 20 0 300
350
400 450 Wavelength/nm
624
250 300 350 400 450 500 550 600 650 700 Wavelength(nm)
Fig. 4. Excitation and emission spectra of Tb3+-doped borosilicate glass S3.
Fig. 2. XRD profile of Tb3+-doped borosilicate glass S7.
500
Fig. 3. Transmission spectrum of Tb3+-doped borosilicate glass S3.
S1 S2 S3 S4
3500 3000
Intensity(a.u.)
20
588
2500 2000
λex=236nm
1500 1000 500 0 300
400
500 600 Wavelength(nm)
700
Fig. 5. Emission spectra of Tb3+-doped borosilicate glasses with different ratio of B2O3/SiO2 with the excitation of 236 nm light.
glasses. With further decreasing ratio of B2O3/SiO2, the amounts of [BO3] and [BO4] decreases, and the negative charge that Tb3+ ion might receive from the coordinated oxygen atoms will decrease, resulting in a decrease of the emission intensity of Tb3+ ion. Fig. 6 shows the emission spectra of Tb3+-doped borosilicate glasses with different Tb3+ concentration under 236 nm UV excitation. The emission peaks located at 381, 417, 439, 460, 490, 546, 588 and 624 nm can be observed in all of Tb3+-doped borosilicate glasses with different Tb3+ concentration. Among them, the emission at 546 nm is the strongest. The intensity of 546 nm emission increases with the increase of Tb3+ concentration in the present work. Meanwhile 490, 588 and 624 nm emissions have same tendency with increasing concentration of Tb3+. However, the intensities of 381, 417, 439 and 460 nm emissions decrease monotonically with increasing concentration of Tb3+. The above behavior could attribute to energy transfer process between Tb3+ ions. Fig. 7 shows the energy level schemes of Tb3+ ions and visible emission transitions in Tb3+-doped borosilicate glasses. From the energy level of Tb3+ ion in Fig. 7, it can be concluded that there are two non-radiative processes from 5D3 to 5D4 states: a fast cross-relaxation due to the resonance energy transfer (RET) and a multi-phonons assisted non-radiative relaxation due to the low energy gap between 5D3 and 5D4 states [24]. Since the energy difference between 5D3 and 5D4 level is very close to that between 7F6 and 7F0 level, the cross relaxation [Tb3+(5D3) + Tb3+(7F6) ? Tb3+(5D4) + Tb3+(7F0)] between two Tb3+ ions could occur when Tb3+ ion is very close to another due to high concentration of
C. Zuo et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 147 (2015) 324–327
4000
Intensity(a.u.)
3500
A1, t and s stands for the amplitude, time and decay time constant of the exponential component, respectively. The luminescence decay at 546 nm under UV excitation yields the decay time s = 2.13 ms.
S5 S3 S6 S7 S8
4500
λex=236nm
3000 Conclusions
2500
5
5
Transparent Tb3+-doped Li2O–BaO–La2O3–Al2O3–B2O3–SiO2 glasses were fabricated by high temperature melting method. The glass has good transmittance in the visible spectrum region. Tb3+-doped borosilicate glasses show intense green emission under UV excitation. The green emission intensity of Tb3+ ion firstly increases and then decreases with the decreasing B2O3/SiO2 ratio in borosilicate glass matrix. 5D4 ? 7FJ (J = 6, 5, 4 and 3) transitions of Tb3+ ion are greatly enhanced through the cross relaxation between two Tb3+ ions with increasing concentration of Tb3+. Luminescence decay time of 2.13 ms from 546 nm emission of 2.5Li2O–20BaO–20La2O3–2.5Al2O3–20B2O3–35SiO2–0.5Tb4O7 glass is obtained. The Tb3+-doped borosilicate glasses would be potential candidates for X-ray scintillating material suitable to X-ray detection for slow event.
5
5
Acknowledgement
2000 1500 1000 500 0 300
400
500 Wavelength(nm)
600
700
Fig. 6. Emission spectra of Tb3+-doped borosilicate glasses with different Tb3+ concentration under 236 nm UV excitation.
30 D3
D4
D4
10
RET
λ=490nm λ=546nm λ=588nm λ=624nm
λex=236nm
20 λ=381nm λ=417nm λ=439nm λ=460nm
Energy/(103 cm-1)
D3
7
7
F0
F0
7
0
3+
8 3+ Tb (B)
F6
Fig. 7. Energy level schemes of Tb3+ ions and visible emission transitions in Tb3+doped borosilicate glasses. Energy transfer process between Tb3+ ions is indicated.
5000
I(t) = 5812.11exp(-t/2127.03) + 29.42
4000
decay curve 1 exp fit
3000 λem=546nm 2000 1000 0
0
2000
This research was financially supported by the Research Foundation of Education Bureau of Hunan Province (Grant No. 12C0814), the Construct Program of the Key Discipline in Hunan Province (Applied Chemistry) and the Scientific Research Foundation for the Doctor of Hunan University of Arts and Science, China.
7
F6
Tb (A)
Counts
327
4000 6000 Time(μs)
8000
10000
Fig. 8. Luminescence decay curve of Tb3+-doped borosilicate glass S3 at room temperature (kem = 546 nm). The fitted function I(t) is given in the figure.
Tb3+ [24–26]. Thus the population of 5D3 level decreases and that of 5 D4 level increases. It results in the intensities of the emissions originated the transitions from 5D3 level to lower 7F levels decrease and the intensities of the emissions from 5D4 level to lower 7F levels increase. The luminescence decay of the Tb3+-doped borosilicate glass excited in the spin allowed 4f75d1 band of Tb3+ were measured at room temperature and is shown in Fig. 8. The decay curve can be well fitted with a single exponential equation, I(t) = A1exp(t/s) + y0, where I(t) is the luminescence intensity,
References [1] M.J. Webber, J. Lumin. 100 (2002) 35. [2] M. Nikl, J.A. Mares, E. Mihokova, K. Nitsch, N. Solovieva, V. Babin, A. Krasnikov, S. Zazubovich, M. Martini, A. Vedda, P. Fabeni, G.P. Pazzi, S. Baccaro, Radiat. Meas. 33 (2001) 593. [3] G.P. Pazzi, P. Fabeni, C. Susini, M. Nikl, E. Mihokova, N. Solovieva, K. Nitsch, M. Martini, A. Vedda, S. Baccaro, A. Cevilia, V. Babin, Nucl. Instrum. Methods B 191 (2002) 366. [4] G.R. Chen, S. Baccaro, R. Giorgi, A. Cecilia, E. Mihokova, M. Nikl, J. Non-Cryst, Solids 326–327 (2003) 339. [5] G. Zanella, R. Zannoni, R. Dall’Igna, P. Polato, M. Bettinelli, Nucl. Inst. Methods Phys. Res. A 359 (1995) 547. [6] C. Bueno, R.A. Buchanan, SPIE 1327 (1990) 79. [7] G.B. Spector, T. McCollum, A.R. Spowart, Nucl. Instrum. Methods A 326 (1993) 526. [8] C. Bueno, M.D. Barker, B.A. Betz, R.C. Barry, R.A. Buchanan, SPIE 2455 (1990) 114. [9] S. Baccaro, R. Dall’Igna, P. Fabeni, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni, J. Lumin. 87–89 (2003) 673. [10] J.A. Mares, M. Nikl, K. Nitsch, N. Solovieva, A. Krasnikov, S. Zazubovich, J. Lumin. 94–95 (2001) 321. [11] S. Baccaro, A. Ceclia, A. Cemmi, G. Chen, E. Mihokova, N. Nikl, IEEE Trans. Nucl. Sci. 48 (2001) 360. [12] J. Fu, M. Kobayashi, J.M. Parker, J. Lumin. 128 (2008) 99. [13] O.V. Kaz’mina, A.N. Abiyaka, Y.A. Moskalev, Glass Ceram. 64 (2007) 75. [14] J.S. Cheng, P.J. Tian, W.H. Zheng, J. Xie, Z.X. Chen, J. Alloys Compd. 471 (2009) 470. [15] B.C. Jamalaiah, J.S. Kumar, A.M. Babu, T. Sasikala, L.R. Moorthy, Physica B 404 (2009) 2020. [16] A.D. Sontakke, K. Biswas, K. Annapurna, J. Lumin. 129 (2009) 1347. [17] C.K. Zu, J. Chen, H.F. Zhao, B. Han, Y.H. Liu, Y.H. Wang, J. Alloys Compd. 479 (2009) 294. [18] C.G. Zuo, A.X. Lu, G. Zhu, Mater. Sci. Eng., B 175 (2010) 229. [19] X. Sun, S.M. Huang, M. Gu, Q.C. Gao, X.S. Gong, Z.P. Ye, Physica B 405 (2010) 56. [20] X. Sun, S.M. Huang, Nucl. Instrum. Methods Phys. Res., Sect. A 621 (2010) 322. [21] Z. Pan, K. Jame, Y. Cui, A. Burger, N. Cherepy, S.A. Payne, R. Mu, S.H. Morgan, Nucl. Instrum. Methods Phys. Res., Sect. A 594 (2008) 215. [22] M. Nagai, K. Kobayashi, Y. Nakajima, Solid State Ionics 136–137 (2000) 249. [23] J. Bei, G. Qian, X. Liang, S. Yuan, Y. Yang, G. Chen, Mater. Res. Bull. 42 (2007) 1195. [24] T. Tsuboi, Eur. Phys. J. Appl. Phys. 26 (2004) 95. [25] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag, Berlin, 1994. [26] D.B. He, C.L. Yu, J.M. Cheng, S.H. Li, L. Hu, J. Alloys Compd. 509 (2011) 1906.
