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Analysis of green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor
ARTICLE IN PRESS
Physica B 391 (2007) 339–343 www.elsevier.com/locate/physb
Analysis of green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor B. Vengala Raoa, U. Rambabub, S. Buddhudua, a
Department of Physics, Sri Venkateswara University, Tirupati 517502, India b C-MET, IDA Phase-II, Cherlapally, Hyderabad 500051, India
Received 4 July 2006; received in revised form 15 October 2006; accepted 16 October 2006
Abstract This paper reports on the emission analysis of a green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor based on the measurements of excitation, emission and lifetimes. Besides this, we have also observed an intense green emission from this powder phosphor under an UV source. The emission transitions of (5D4-7F3,4,5,6) with lexci ¼ 257 nm have been measured. Particularly, the green emission transition (5D4-7F5) at 553 nm has been found to be more prominent and intense. Such green strong emission displaying powder phosphor will find applications in the development of coated screens in certain electronic systems. Apart from the emission analysis of this phosphor, XRD, SEM and FTIR studies have also been carried out in order to understand the structural details of it. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.55.Hx Keywords: Tb3+:Ca4GdO(BO3)3 phosphor; Luminescence
1. Introduction Luminescent materials are widely used in day-to-day life. Their best-known applications are CTV screen phosphors, projection TV phosphors, fluorescent lamps, scintillators, full color displays, X-ray storage, dosimetry of ionizing radiation screen intensifying phosphors, laser materials, etc. [1–10]. In recent times, the host matrix Ca4GdO(BO3)3 has drawn a great deal of attention as a potential optical material based on its stable crystalline structure, versatility, and chemical stability [11–16]. Rare-earth ions have widely been known as activators in different host matrices because of their high-efficient emission performance [17]. We have recently reported the results concerning emission spectra of Eu3+ in five different host matrices and identified that Ca4GdO(BO3)3 as an ideal host material [18]. Tb3+ phosphors are considered more important due to their efficiency in the display of sharp and intense green emission at 545 nm due to an electronic transition of 5D4-7F5 [19–22]. Literature reports show that several other host matrices containing terbium ions have earlier been studied Corresponding author. Tel.: +91 877 22616111; fax: +91 877 2249666.
E-mail address: [email protected] (S. Buddhudu). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.10.022
[23–30]. In continuation to our previous work on the red luminescent [18] Eu3+:Ca4GdO(BO3)3, in the present work we have undertaken the green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor for its systematic optical characterization.
2. Experimental Tb3+:Ca4REO(BO3)3 powder phosphor was prepared through a conventional solid-state reaction method. The chemical equation in this method is as follows: 8 CaCO3 þ 6 H3 BO3 þ ð1 x=2Þ Gd2 O3 þ x=2 Tb4 O7 ! 2 Ca4 Gd
ð1x=2Þ OðBO3 Þ3
: Tbx=2 þ 9 H2 O þ 8 CO2 þ x O2 ;
where x ¼ 0.2. The starting chemicals were analytical in grade such as CaCO3, H3BO3 and Gd2O3, Tb4O7 in 99.99% purity. Stoichiometric amounts of the starting materials were mixed and thoroughly powdered by using acetone in an agate mortar for a homogeneous mixing, and then it was collected into an alumina crucible for heating. The temperature was gradually raised from the room
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343
temperature to 900 1C where the mixture was kept for 10 h in an ambient atmosphere with an intermediate grinding.
1600
111
340
1000 -351
To understand the phase purity of this sample, the XRD pattern was measured at 2y ranging from 101 to 601 in step size of 0.02 and in time of 0.5 s, on a powder X-ray difractometer of Xpert PRO equipped with a Ni filter and Cu Ka (1.54 A˚) radiation at an applied voltage of 40 kV and with 40 mA as anode current, calibrated with Si. The morphology and average grain size of the powder phosphor were examined by means of a Philips XL30 ESEM. For an average grain size analysis, the samples were coated with a thin layer of gold by using JEOL fine coating ion sputter FC-1100 system, to prevent charging of the specimen. The SEM images of the samples with 20 KV and 25 pA beam current were recorded by using a 35 m camera attached to the high-resolution recording unit. The FTIR absorption spectrum of the sample was recorded on a Nicolet IR-200 spectrophotometer using KBr pellet technique in the wavenumber range of 400–4000 cm1. Both the excitation and emission spectra of the Ca4GdO(BO3)3:Tb3+ phosphor were measured on a Spex Fluorolog-2 Fluorimeter with a Datamax software to acquire the data with a Xe-arc lamp (150 W) as the excitation source. A Xe-flash lamp (25 W) with a phosphorimeter attachment was used to measure the lifetimes of the emission transitions of these powder phosphors.
200
151 241 400
001
110
400
130
600
240 -221 060 310 201 -151
800
331 -112
1200
Counts
3. Measurements
150
1400
0 10
20
40
30
50
60
2θ Fig. 2. XRD pattern of Ca4GdO(BO3)3:Tb3+ powder phosphor.
4. Results and discussion Under an UV lamp source at 254 nm, Tb3+:Ca4GdO(BO3)3 powder phosphor has displayed a strong green emission and is shown in Fig. 1 and Fig. 2 shows an XRD pattern of Tb3+:Ca4GdO(BO3)3 phosphor and the structure could be a monoclinic structured with a space group Cm. The XRD pattern was in good agreement with the literature reports [31–33]. A SEM micrograph of the
Fig. 1. Green luminescent Ca4GdO(BO3)3:Tb3+ powder phosphor under an UV source.
Fig. 3. SEM image of Ca4GdO(BO3)3:Tb3+ powder phosphor.
Tb3+:Ca4GdO(BO3)3 powder phosphor is shown in Fig. 3 and from which, morphologies and grain size for Tb3+:Ca4GdO(BO3)3 phosphor have been understood and found that there are agglomerated particles with an average grain size of 597 nm in the system. Based on the infrared spectrum, various B–O arrangements in Tb3+:Ca4GdO (BO3)3 powder phosphor has been understood. In the borate compounds containing (BO3)3 groups, the electronic delocalization in the planer borate anions has been predominant and which could induce NLO properties [34]. The FTIR spectral profile of Tb3+:Ca4GdO(BO3)3 powder phosphor has been shown in Fig. 4. The O–H stretching vibrations were found at 2970, 2874 and 1654 cm1. The absorption bands recorded around 1500 cm1 could be due to asymmetric stretching relaxation of the B–O bond of trigonal units. The B–O bond n3(E1) stretching vibrations of (BO3)3 units have been found at 1279, 1229 cm1 and the band at 950 cm1 indicates n1(A1) stretching vibration of (BO3)3. The IR bands that are observed at 795, 756, 718 cm1 could indicate B–O–B bending vibrations. The FTIR band at 617 cm1 has been attributed to the T1 (Ca)
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343
80.0
794.6 7617 4667 756.0 717.5
Exci.Int. (a.u)
1654.8
5 3+ 6--> K9(Tb ) 257 nm 7 5 3+ 12000 F6--> K7(Tb ) 267 nm 240 nm 10000
40.0
λemission=553 nm
8000 278nm
6000
7F
5 3+ 6--> H7(Tb )
4000 1228.6
1500.0
1000.0
200
500.0
1/cm Fig. 4. FTIR spectrum of Ca4GdO(BO3)3:Tb3+ powder phosphor.
2970 2874 1654 1500
Stretching vibrations of OH groups
1279 1229 950 795 756 718 617 467
5D
7 4--> F5
14000 12000
B–O asymmetric stretching relaxations of (BO3)3 n3(E1) stretching vibration of (BO3)3
553 nm
λexci=257 nm
543 nm
10000 8000 5D
6000
7F
4-->
6
483 nm
4000
584 nm
623 nm
2000
n1(A1) stretching vibration of (BO3)3 B–O–B bending vibrations
Ca (translatory lattice modes) Vibrational mode of Tb–O band
400
Fig. 5. Excitation spectrum of Ca4GdO(BO3)3:Tb3+ powder phosphor.
Emis.Int. (a.u)
Corresponding vibrational mode
350
Wavelength (nm)
Table 1 FTIR spectrum and energy level assignments for Tb3+:Ca4GdO(BO3)3 powder phosphor from 4000 to 400 cm1 FTIR peak energy (cm1)
300
250
7 4--> F3
2000.0
374 nm
5D
3000.0
5 3+ 6--> G6(Tb )
2000
1278.7
20.0 4000.0
7F
310 nm
5D -->7F 4 4
%T
60.0
7F
14000
2873.7 2970.0
341
400
450
500
550
600
650
Wavelength (nm) Fig. 6. Emission spectra of Ca4GdO(BO3)3:Tb3+ powder phosphor.
14000 a) τm= 2.02 ms (λemis= 553 nm) b) τm= 1.21 ms(λemis= 483 nm) c) τm= 1.08 ms (λemis= 584 nm)
12000 a Photon Counts
translatory lattice modes. The vibrational mode of Tb–O band has been found at 467 cm1. The above assignments have been made for Tb3+:Ca4GdO(BO3)3 phosphor and these results, thus, obtained are found to be quite clearly comparable with the results that are earlier reported in literature [35–38] and the assignment details are documented in Table 1. The excitation spectrum of Tb3+: Ca4GdO(BO3)3 powder phosphor is shown in Fig. 5 with strong excitation bands which could be attributed to the transition 4f8–4f7 5d1 (f–d transition) of Tb3+ in the range of 220–260 nm and besides this, there are two excitation bands between 265 and 290 nm, that are assigned to the transitions of 8S7/2-6IJ ¼ 15/2,13/2,11/2,17/2,9/ 3+ ion and which indicates that there exists an 2,7/2 of Gd efficient energy transfer from Gd3+ ion to Tb3+ ion [39–41]. Apart from this, in this range, Tb3+ ion has got certain absorption bands and these are assigned to the electronic transitions:7F6-5IJ ¼ 4,5,6,7,8, 7F6-5FJ ¼ 1,2,3,4,5, 7 F6-5HJ ¼ 3,4,5,6 [42,43]. A series of weak peaks in the range of 300–400 nm ascribe to the transitions between
10000
λexci=257 nm
8000 6000 b 4000 c 2000 0
2
6
4
8
10
Time (ms) Fig. 7. Green emission decay curves at different excitation wavelengths for Ca4GdO(BO3)3:Tb3+powder phosphor.
energy levels of the 4f8 configuration of Tb3+ ion [17,39,40,44]. The fluorescence of Tb3+ mainly originated the transitions from 5D3-7FJ and/or 5D4-7FJ (where
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343
342
:Radiative :Non- radiative 39x103cm-1
5
K9
5D
623 nm
584 nm
553 nm
483 nm
4
257 nm
Energy
5D 3
Cross relaxation 7F
0
7F
0
7F
7F
6
6 3+
Fig. 8. Emission mechanism in Ca4GdO(BO3)3:Tb
J ¼ 1,2,3,4,5,6, respectively). When the Tb3+: phosphors are excited by UV excitation of 257 nm, the Tb3+ ions (4f8) would be excited to the higher 4f7 5d1 levels and decay down to lower levels non radiatively to the 5D3 or/and 5D4 excited states. Due to a wide energy gap (i.e., at least 13,000 cm1) between these excited states and the 7FJ multiplet ground state, the relaxation process occurs radiatively that means emissions in the visible spectral region. The emission spectrum of Tb3+strongly depends on the Tb3+ content and at lower Tb3+ concentration, a blue emission attributed to the transitions 5D3-7FJ could be observed at the range of 350–470 nm [45,46]. This blue emission becomes disappears due to the Tb3+ concentration increase beyond a critical value where cross relaxation occurs [47]. The cross relaxation occurs which could be induced by a resonance between the excited states and the ground states of two Tb3+ ions as has been described earlier in literature through the following equation: [48,49]. 5
D3 ! 5 D4 ¼¼ 4 7 F6 ! 7 F0 and=or
5
D3 ! 7 F0 ¼¼ 4 7 F6 ! 5 D4:
Due to the cross relaxation occurrence, the absence of 5D3 emission in the PL spectrum of Ca4GdO(BO3)3:Tb3+ could be noticed and hence only the 5D4 emission with relatively a bright intensity has been found. The emission spectrum of Ca4GdO(BO3)3:Tb3+ phosphor has been shown in Fig. 6 with an excitation at 257 nm. The emission spectrum has revealed four emission bands of 5D4-7FJ (J ¼ 3, 4,5,6) with a prominent green emission band of 5D4-7F5 (553 nm) and which has been a magnetic-dipole transition which satisfies the selection rule of DJ ¼ 71. The other emission bands that are located at 483, 584, and 623 nm, could be attributed to the electronic transitions 5D4-7F6, 5 D4-7F4, 5D4-7F3 of Tb3+ [50,51]. Fig. 7 presents the decay curves of Ca4GdO(BO3)3:Tb3+ phosphor of the
powder phosphor.
three emission bands 5D4-7F6,5,4 with an excitation wavelength at 257 nm, the decay curve of the other emission band 5D4-7F3 could not be measured due to its low intensity situation. However, for the remaining bands the decay curves have been plotted in evaluating the lifetimes. An energy level scheme has been given in Fig. 8 to explain the emission mechanism involved from the Ca4GdO(BO3)3:Tb3+ phosphor [50,51]. 5. Conclusion We have successfully developed a brightly green luminescent Ca4GdO(BO3)3:Tb3+ powder phosphor by means of a solid-state reaction technique. By using a UV source, an eye catching and very intense green emission from this phosphor has been observed. Optical analysis has been carried out from the measurement of emission spectra, decay curves of the emission bands of this phosphor alongside understanding their particle sizes and structural details based on the measurement of the features of SEM, profiles of XRD and FTIR. Optical analysis of Ca4GdO(BO3)3:Tb3+ powder phosphor validates the importance of this phosphor as a potential green luminescent optical materials for its applications as a coating material on screens of certain electronic systems. References [1] K. Madhukumar, K. Rajendrababu, K.C. Ajithprasad, J. James, T.S. Elias, V. Padmanabhan, C.M.K. Nair, Bull. Mater. Sci. 29 (2) (2006) 119. [2] N. Joffin, B. Caillier, A. Garcia, P. Guillot, J. Galy, A. Fernandes, R. Mauricot, J. Dexpert-Ghus, Opt. Mater. 28 (2006) 597. [3] L. Techen, I. Leisum, C. Shyangwang, S. Jinnchang, J. Lumin. 118 (2) (2006) 293. [4] A. Komeno, K. Uematsu, K. Toda, M. Sato, J. Alloys Compds. 408/412 (2006) 871.
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343 [5] H. Lai, B. Chen, W. Xu, Y. Xie, X. Wang, W. Di, Mater. Lett. 60 (11) (2006) 558. [6] C. Liu, G. Che, Phys. Stat. Sol. A 203 (3) (2006) 558. [7] J. Selling, G. Corradi, M. Secu, S. Schweizer, J. Phys. D 38 (2005) 8069. [8] A. Potdevin, G. Chadeyron, D. Boyer, B. Caillier, R. Mahiou, J. Phys.D 38 (2005) 3251. [9] A. Lavat, C. Graselli, M. Santiago, J. Pomarico, E. Caselli, Cryst. Res. Technol. 39 (10) (2004) 840. [10] A.G. Murillo, C.L. Urlacher, C. Dujardin, C. Pedrini, J. Mugnier, J. Sol–Gel Sci. Technol. 26 (2003) 957. [11] Y. Wang, K. Uheda, H. Takizawa, U. Mizomoto, T. Endo, J. Electrochem. Soc. 148 (8) (2001) G430. [12] M. Iwai, T. Kobayashi, H. Fraya, Y. Wari, T. Sasaki, Jpn. J. Appl. Phys. 36 (1997) L276. [13] G. Aka, A. Kahn-Harari, D. Vivien, J.M. Benitez, J. Solid State Inorg. Chem. 33 (1996) 727. [14] R. Norrestam, M. Nygren, J.O. Bovin, Chem. Mater. 4 (1992) 737. [15] G.J. Diriksen, G. Blasse, J. Alloys Compds. 191 (1993) 121. [16] Y. Zhang, X.L. Chen, J.K. Liang, T. Xn, J. Alloys Compds. 315 (2001) 198. [17] G. Blasse, B.C. Grabmaier, Luminiscent Materials, Springer, Newyork, 1994. [18] B.V. Rao, U. Rambabu, S. Buddhudu, Physica B 382 (2006) 86. [19] B. Yan, X.Z. Xiao, Opt. Mater. 28 (2006) 498. [20] E.T. Goldburt, B. Kulkarni, R.N. Bhargava, J. Taylor, M. Libera, J. Lumin. 72/74 (1997) 190. [21] H. Amekura, A. Eckau, R. Carius, C. Buchal, J. Appl. Phys. 84 (1998) 3867. [22] W.V. Schaik, S. Lizzo, W. Smit, G. Blasse, J. Electrochem. Soc. 140 (1993) 216. [23] Y.Y. Choi, K.S. Sohn, H.D. Park, S.Y. Choi, J. Mater. Res. 16 (2001) 881. [24] B. Yan, H.H. Huang, J. Mater. Sci. Lett. 39 (2004) 3529. [25] H.H. Huang, B. Yan, Inorg. Chem. Commun. 7 (2004) 595. [26] H.H. Huang, B. Yan, Inorg. Chem. Commun. 7 (2004) 919. [27] J.J. Zhang, J.W. Ning, X.J. Liu, X.J. Pan, L.P. Huang, Mater. Lett. 57 (2003) 3077.
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[28] M. Yin, W. Zhang, L. Lou, S. Xia, J.L. Krupa, Physica B 254 (1998) 141. [29] C.H. Kam, S. Buddhudu, Mater. Lett. 54 (2002) 337. [30] H. Meyssamy, K. Riwotzki, A. Kornowski, S. Naused, M. Haase, Adv. Mater. 11 (1999) 840. [31] A.B. Iyukhin, B.F. Dzhurinkii, Russ. J. Inorg. Chem. 36 (1993) 847. [32] Y. Wang, T. Endo, E. Xie, D. He, B. Liu, Micro. Elec. J. 35 (2004) 357. [33] I.V. Kityk, A. Mefleh, Physica B 262 (1999) 170. [34] V. Krishnakumar, R. Nagalakshmi, Spectrochim. Acta A 60 (2004) 2733. [35] A. de Andres, F. Agullo-Rueda, S. Toboad, C. Cascales, J. Campa, G. Ruiz-Valero, I. Rasines, J. Alloys Compds. 250 (1997) 396. [36] G. Dominiak-Dzik, W. Ryba-Romnowski, S. Golab, L. Macalik, J. Hanuza, A. Pajazkowska, J. Mol. Struct. 555 (2000) 213. [37] G. Lakshminarayana, S. Buddhudu, Spectrochim. Acta A 62 (2005) 364. [38] G. Dominiak-Dzik, W. Ryba-Romnowski, S. Golab, M. Baba, A. Pajazkowska, J. Mol. Struct. 614 (2002) 195. [39] C.K. Lin, M.L. Pang, M. Yu, J. Lin, J. Lumin. 114 (2005) 299. [40] X. Zhang, J. Zhang, L. Liang, Q. Su, Mater. Res. Bull. 40 (2005) 281. [41] X. Xiao, B. Yan, J. Non-Cryst. Solids 351 (2005) 3634. [42] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4443. [43] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (10) (1968) 4447. [44] M. Buijs, J.P.M. Vanvliet, G. Blasse, J. Lumin. 35 (1986) 213. [45] A.M. Berdowski, M.J.J. Lammers, G. Blasse, J. Chem. Phys. 83 (1995) 475. [46] F.S. Kao, T.M. Chen, J. Lumin. 96 (2002) 261. [47] L.G. Vanvitert, L.F. Johson, J. Chem. Phys. 44 (1996) 3514. [48] G. Blasse, Rev. Inorg. Chem. 5 (1983) 319. [49] D.J. Robins, B. Cockayne, B. Lent, J.L. Glasper, Solid State Commun. 20 (1976) 673. [50] C.H. Kam, S. Buddhudu, Physica B 337 (2003) 237. [51] G. Wakefield, H.A. Keron, P.J. Dobson, J.L. Hutchison, J. Phys. Chem. Solids 60 (1999) 503.
Physica B 391 (2007) 339–343 www.elsevier.com/locate/physb
Analysis of green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor B. Vengala Raoa, U. Rambabub, S. Buddhudua, a
Department of Physics, Sri Venkateswara University, Tirupati 517502, India b C-MET, IDA Phase-II, Cherlapally, Hyderabad 500051, India
Received 4 July 2006; received in revised form 15 October 2006; accepted 16 October 2006
Abstract This paper reports on the emission analysis of a green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor based on the measurements of excitation, emission and lifetimes. Besides this, we have also observed an intense green emission from this powder phosphor under an UV source. The emission transitions of (5D4-7F3,4,5,6) with lexci ¼ 257 nm have been measured. Particularly, the green emission transition (5D4-7F5) at 553 nm has been found to be more prominent and intense. Such green strong emission displaying powder phosphor will find applications in the development of coated screens in certain electronic systems. Apart from the emission analysis of this phosphor, XRD, SEM and FTIR studies have also been carried out in order to understand the structural details of it. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.m; 78.55.Hx Keywords: Tb3+:Ca4GdO(BO3)3 phosphor; Luminescence
1. Introduction Luminescent materials are widely used in day-to-day life. Their best-known applications are CTV screen phosphors, projection TV phosphors, fluorescent lamps, scintillators, full color displays, X-ray storage, dosimetry of ionizing radiation screen intensifying phosphors, laser materials, etc. [1–10]. In recent times, the host matrix Ca4GdO(BO3)3 has drawn a great deal of attention as a potential optical material based on its stable crystalline structure, versatility, and chemical stability [11–16]. Rare-earth ions have widely been known as activators in different host matrices because of their high-efficient emission performance [17]. We have recently reported the results concerning emission spectra of Eu3+ in five different host matrices and identified that Ca4GdO(BO3)3 as an ideal host material [18]. Tb3+ phosphors are considered more important due to their efficiency in the display of sharp and intense green emission at 545 nm due to an electronic transition of 5D4-7F5 [19–22]. Literature reports show that several other host matrices containing terbium ions have earlier been studied Corresponding author. Tel.: +91 877 22616111; fax: +91 877 2249666.
E-mail address: [email protected] (S. Buddhudu). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.10.022
[23–30]. In continuation to our previous work on the red luminescent [18] Eu3+:Ca4GdO(BO3)3, in the present work we have undertaken the green luminescent Tb3+:Ca4GdO(BO3)3 powder phosphor for its systematic optical characterization.
2. Experimental Tb3+:Ca4REO(BO3)3 powder phosphor was prepared through a conventional solid-state reaction method. The chemical equation in this method is as follows: 8 CaCO3 þ 6 H3 BO3 þ ð1 x=2Þ Gd2 O3 þ x=2 Tb4 O7 ! 2 Ca4 Gd
ð1x=2Þ OðBO3 Þ3
: Tbx=2 þ 9 H2 O þ 8 CO2 þ x O2 ;
where x ¼ 0.2. The starting chemicals were analytical in grade such as CaCO3, H3BO3 and Gd2O3, Tb4O7 in 99.99% purity. Stoichiometric amounts of the starting materials were mixed and thoroughly powdered by using acetone in an agate mortar for a homogeneous mixing, and then it was collected into an alumina crucible for heating. The temperature was gradually raised from the room
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343
temperature to 900 1C where the mixture was kept for 10 h in an ambient atmosphere with an intermediate grinding.
1600
111
340
1000 -351
To understand the phase purity of this sample, the XRD pattern was measured at 2y ranging from 101 to 601 in step size of 0.02 and in time of 0.5 s, on a powder X-ray difractometer of Xpert PRO equipped with a Ni filter and Cu Ka (1.54 A˚) radiation at an applied voltage of 40 kV and with 40 mA as anode current, calibrated with Si. The morphology and average grain size of the powder phosphor were examined by means of a Philips XL30 ESEM. For an average grain size analysis, the samples were coated with a thin layer of gold by using JEOL fine coating ion sputter FC-1100 system, to prevent charging of the specimen. The SEM images of the samples with 20 KV and 25 pA beam current were recorded by using a 35 m camera attached to the high-resolution recording unit. The FTIR absorption spectrum of the sample was recorded on a Nicolet IR-200 spectrophotometer using KBr pellet technique in the wavenumber range of 400–4000 cm1. Both the excitation and emission spectra of the Ca4GdO(BO3)3:Tb3+ phosphor were measured on a Spex Fluorolog-2 Fluorimeter with a Datamax software to acquire the data with a Xe-arc lamp (150 W) as the excitation source. A Xe-flash lamp (25 W) with a phosphorimeter attachment was used to measure the lifetimes of the emission transitions of these powder phosphors.
200
151 241 400
001
110
400
130
600
240 -221 060 310 201 -151
800
331 -112
1200
Counts
3. Measurements
150
1400
0 10
20
40
30
50
60
2θ Fig. 2. XRD pattern of Ca4GdO(BO3)3:Tb3+ powder phosphor.
4. Results and discussion Under an UV lamp source at 254 nm, Tb3+:Ca4GdO(BO3)3 powder phosphor has displayed a strong green emission and is shown in Fig. 1 and Fig. 2 shows an XRD pattern of Tb3+:Ca4GdO(BO3)3 phosphor and the structure could be a monoclinic structured with a space group Cm. The XRD pattern was in good agreement with the literature reports [31–33]. A SEM micrograph of the
Fig. 1. Green luminescent Ca4GdO(BO3)3:Tb3+ powder phosphor under an UV source.
Fig. 3. SEM image of Ca4GdO(BO3)3:Tb3+ powder phosphor.
Tb3+:Ca4GdO(BO3)3 powder phosphor is shown in Fig. 3 and from which, morphologies and grain size for Tb3+:Ca4GdO(BO3)3 phosphor have been understood and found that there are agglomerated particles with an average grain size of 597 nm in the system. Based on the infrared spectrum, various B–O arrangements in Tb3+:Ca4GdO (BO3)3 powder phosphor has been understood. In the borate compounds containing (BO3)3 groups, the electronic delocalization in the planer borate anions has been predominant and which could induce NLO properties [34]. The FTIR spectral profile of Tb3+:Ca4GdO(BO3)3 powder phosphor has been shown in Fig. 4. The O–H stretching vibrations were found at 2970, 2874 and 1654 cm1. The absorption bands recorded around 1500 cm1 could be due to asymmetric stretching relaxation of the B–O bond of trigonal units. The B–O bond n3(E1) stretching vibrations of (BO3)3 units have been found at 1279, 1229 cm1 and the band at 950 cm1 indicates n1(A1) stretching vibration of (BO3)3. The IR bands that are observed at 795, 756, 718 cm1 could indicate B–O–B bending vibrations. The FTIR band at 617 cm1 has been attributed to the T1 (Ca)
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343
80.0
794.6 7617 4667 756.0 717.5
Exci.Int. (a.u)
1654.8
5 3+ 6--> K9(Tb ) 257 nm 7 5 3+ 12000 F6--> K7(Tb ) 267 nm 240 nm 10000
40.0
λemission=553 nm
8000 278nm
6000
7F
5 3+ 6--> H7(Tb )
4000 1228.6
1500.0
1000.0
200
500.0
1/cm Fig. 4. FTIR spectrum of Ca4GdO(BO3)3:Tb3+ powder phosphor.
2970 2874 1654 1500
Stretching vibrations of OH groups
1279 1229 950 795 756 718 617 467
5D
7 4--> F5
14000 12000
B–O asymmetric stretching relaxations of (BO3)3 n3(E1) stretching vibration of (BO3)3
553 nm
λexci=257 nm
543 nm
10000 8000 5D
6000
7F
4-->
6
483 nm
4000
584 nm
623 nm
2000
n1(A1) stretching vibration of (BO3)3 B–O–B bending vibrations
Ca (translatory lattice modes) Vibrational mode of Tb–O band
400
Fig. 5. Excitation spectrum of Ca4GdO(BO3)3:Tb3+ powder phosphor.
Emis.Int. (a.u)
Corresponding vibrational mode
350
Wavelength (nm)
Table 1 FTIR spectrum and energy level assignments for Tb3+:Ca4GdO(BO3)3 powder phosphor from 4000 to 400 cm1 FTIR peak energy (cm1)
300
250
7 4--> F3
2000.0
374 nm
5D
3000.0
5 3+ 6--> G6(Tb )
2000
1278.7
20.0 4000.0
7F
310 nm
5D -->7F 4 4
%T
60.0
7F
14000
2873.7 2970.0
341
400
450
500
550
600
650
Wavelength (nm) Fig. 6. Emission spectra of Ca4GdO(BO3)3:Tb3+ powder phosphor.
14000 a) τm= 2.02 ms (λemis= 553 nm) b) τm= 1.21 ms(λemis= 483 nm) c) τm= 1.08 ms (λemis= 584 nm)
12000 a Photon Counts
translatory lattice modes. The vibrational mode of Tb–O band has been found at 467 cm1. The above assignments have been made for Tb3+:Ca4GdO(BO3)3 phosphor and these results, thus, obtained are found to be quite clearly comparable with the results that are earlier reported in literature [35–38] and the assignment details are documented in Table 1. The excitation spectrum of Tb3+: Ca4GdO(BO3)3 powder phosphor is shown in Fig. 5 with strong excitation bands which could be attributed to the transition 4f8–4f7 5d1 (f–d transition) of Tb3+ in the range of 220–260 nm and besides this, there are two excitation bands between 265 and 290 nm, that are assigned to the transitions of 8S7/2-6IJ ¼ 15/2,13/2,11/2,17/2,9/ 3+ ion and which indicates that there exists an 2,7/2 of Gd efficient energy transfer from Gd3+ ion to Tb3+ ion [39–41]. Apart from this, in this range, Tb3+ ion has got certain absorption bands and these are assigned to the electronic transitions:7F6-5IJ ¼ 4,5,6,7,8, 7F6-5FJ ¼ 1,2,3,4,5, 7 F6-5HJ ¼ 3,4,5,6 [42,43]. A series of weak peaks in the range of 300–400 nm ascribe to the transitions between
10000
λexci=257 nm
8000 6000 b 4000 c 2000 0
2
6
4
8
10
Time (ms) Fig. 7. Green emission decay curves at different excitation wavelengths for Ca4GdO(BO3)3:Tb3+powder phosphor.
energy levels of the 4f8 configuration of Tb3+ ion [17,39,40,44]. The fluorescence of Tb3+ mainly originated the transitions from 5D3-7FJ and/or 5D4-7FJ (where
ARTICLE IN PRESS B. Vengala Rao et al. / Physica B 391 (2007) 339–343
342
:Radiative :Non- radiative 39x103cm-1
5
K9
5D
623 nm
584 nm
553 nm
483 nm
4
257 nm
Energy
5D 3
Cross relaxation 7F
0
7F
0
7F
7F
6
6 3+
Fig. 8. Emission mechanism in Ca4GdO(BO3)3:Tb
J ¼ 1,2,3,4,5,6, respectively). When the Tb3+: phosphors are excited by UV excitation of 257 nm, the Tb3+ ions (4f8) would be excited to the higher 4f7 5d1 levels and decay down to lower levels non radiatively to the 5D3 or/and 5D4 excited states. Due to a wide energy gap (i.e., at least 13,000 cm1) between these excited states and the 7FJ multiplet ground state, the relaxation process occurs radiatively that means emissions in the visible spectral region. The emission spectrum of Tb3+strongly depends on the Tb3+ content and at lower Tb3+ concentration, a blue emission attributed to the transitions 5D3-7FJ could be observed at the range of 350–470 nm [45,46]. This blue emission becomes disappears due to the Tb3+ concentration increase beyond a critical value where cross relaxation occurs [47]. The cross relaxation occurs which could be induced by a resonance between the excited states and the ground states of two Tb3+ ions as has been described earlier in literature through the following equation: [48,49]. 5
D3 ! 5 D4 ¼¼ 4 7 F6 ! 7 F0 and=or
5
D3 ! 7 F0 ¼¼ 4 7 F6 ! 5 D4:
Due to the cross relaxation occurrence, the absence of 5D3 emission in the PL spectrum of Ca4GdO(BO3)3:Tb3+ could be noticed and hence only the 5D4 emission with relatively a bright intensity has been found. The emission spectrum of Ca4GdO(BO3)3:Tb3+ phosphor has been shown in Fig. 6 with an excitation at 257 nm. The emission spectrum has revealed four emission bands of 5D4-7FJ (J ¼ 3, 4,5,6) with a prominent green emission band of 5D4-7F5 (553 nm) and which has been a magnetic-dipole transition which satisfies the selection rule of DJ ¼ 71. The other emission bands that are located at 483, 584, and 623 nm, could be attributed to the electronic transitions 5D4-7F6, 5 D4-7F4, 5D4-7F3 of Tb3+ [50,51]. Fig. 7 presents the decay curves of Ca4GdO(BO3)3:Tb3+ phosphor of the
powder phosphor.
three emission bands 5D4-7F6,5,4 with an excitation wavelength at 257 nm, the decay curve of the other emission band 5D4-7F3 could not be measured due to its low intensity situation. However, for the remaining bands the decay curves have been plotted in evaluating the lifetimes. An energy level scheme has been given in Fig. 8 to explain the emission mechanism involved from the Ca4GdO(BO3)3:Tb3+ phosphor [50,51]. 5. Conclusion We have successfully developed a brightly green luminescent Ca4GdO(BO3)3:Tb3+ powder phosphor by means of a solid-state reaction technique. By using a UV source, an eye catching and very intense green emission from this phosphor has been observed. Optical analysis has been carried out from the measurement of emission spectra, decay curves of the emission bands of this phosphor alongside understanding their particle sizes and structural details based on the measurement of the features of SEM, profiles of XRD and FTIR. Optical analysis of Ca4GdO(BO3)3:Tb3+ powder phosphor validates the importance of this phosphor as a potential green luminescent optical materials for its applications as a coating material on screens of certain electronic systems. References [1] K. Madhukumar, K. Rajendrababu, K.C. Ajithprasad, J. James, T.S. Elias, V. Padmanabhan, C.M.K. Nair, Bull. Mater. Sci. 29 (2) (2006) 119. [2] N. Joffin, B. Caillier, A. Garcia, P. Guillot, J. Galy, A. Fernandes, R. Mauricot, J. Dexpert-Ghus, Opt. Mater. 28 (2006) 597. [3] L. Techen, I. Leisum, C. Shyangwang, S. Jinnchang, J. Lumin. 118 (2) (2006) 293. [4] A. Komeno, K. Uematsu, K. Toda, M. Sato, J. Alloys Compds. 408/412 (2006) 871.
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