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Emission properties of Eu3+ ions in alkali tellurofluorophosphate glasses
ARTICLE IN PRESS
Physica B 403 (2008) 1690–1694 www.elsevier.com/locate/physb
Emission properties of Eu3+ ions in alkali tellurofluorophosphate glasses D. Uma Maheswaria, J. Suresh Kumara, L.R. Moorthya,, Kiwan Jangb, M. Jayasimhadric a
Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Changwon National University, Changwon, Kyungnam 641-773, South Korea c Photonic Glasses Laboratory, Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, South Korea b
Received 13 July 2007; received in revised form 13 September 2007; accepted 24 September 2007
Abstract The present work reports the spectroscopic properties of 1 mol% Eu2O3 in alkali tellurofluorophosphate glasses 50(NaPO3)6+10 TeO2+20 AlF3+19RF+1Eu2O3 (R ¼ Li, Na and K) by using absorption, emission and decay measurements. The phenomenological Judd–Ofelt (J–O) intensity parameters O2 and O4 were obtained from the emission intensities of 5D0-7F2 and 5D0-7F4, respectively by taking the magnetic dipole 5D0-7F1 emission band intensity as reference. The radiative parameters such as spontaneous emission probabilities (AR), lifetimes (t), branching ratios (b) for different excited states have been predicted theoretically. The branching ratios were also determined by the areas under the emission bands and are compared with those of the predicted values. The predicted and experimental lifetimes for the 5D0 level in alkali tellurofluorophosphate glasses were compared and discussed in detail. r 2007 Elsevier B.V. All rights reserved. Keywords: Eu3+; Glasses; J–O theory; Luminescence; Rare earths
1. Introduction The study of optical properties of Ln3+ ions in glasses depends on the chemical composition of the glass former and modifier, which elucidate fundamental information about energy-level positions, absorption and emission cross-sections, radiative and non-radiative decay rates and branching ratios. Lanthanide-doped glasses play a very important role in the development of lasers and fiber amplifiers for optical telecommunications [1,2]. Based on the absorption and emission bands of intraconfigurational f–f transitions, the energy level intensities of Ln3+ ions in several host media were described and estimated quantitatively by using the Judd–Ofelt theory [3,4]. In the present work, we reported the absorption, emission and decay properties of Eu3+ in alkali tellurofluorophosphate glasses, since the trivalent europium ion is a well-known spectroscopic probe [5]. Spectroscopic Corresponding author. Tel.: +91 877 2242766; fax: +91 877 2248499.
E-mail address: [email protected] (L.R. Moorthy). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.09.089
studies of Eu3+ ions have been reported in different hosts such as water [6], crystals [7,8] and glasses [9–12] in order to characterize these materials for optical device applications. Fluorophosphate glasses have been the subject of several spectroscopic investigations due to their potential use as laser host matrices [13,14]. Normally phosphate glasses are hygroscopic, but addition of fluoride increases the resistance to water. In addition, fluoride compounds, by virtue of their excellent physical and chemical properties, high stability with respect to short wavelength radiation and transparency in a wide spectral region, are attractive features in the field of quantum electronics. Peng and Izumatani [15] determined O2, O4 and O6 parameters by using intensities of emission levels 5 D0-7F0, 7F1, 7F2, 7F3 and 7F4, respectively. Van Deun et al. [12] have used the individual oscillator strengths of absorption bands of 7F0-5L6, 7F0-5D2, 7F6 and 7 F1-7F6 transitions to evaluate the phenomenological intensity parameters Ol with a requirement that the difference between the measured and calculated oscillator strengths be minimized. Fermi et al. [16] and Capobianco
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et al. [17] have considered both absorption intensities as well as the emission branching ratios and derived the Ol (l ¼ 2, 4 and 6) parameters. The object of the present study is to determine the J–O parameters from the emission spectra and also to compare the measured and predicted radiative lifetimes of 5D0 level in Eu3+-doped tellurofluorophosphate glasses. 2. Experimental The molar composition of europium-doped tellurofluorophosphate glasses investigated in the present work is as follows: LiTFP : 50ðNaPO3 Þ6 þ 10TeO2 þ 20AlF3 þ 19LiF þ 1Eu2 O3 NaTFP : 50ðNaPO3 Þ6 þ 10TeO2 þ 20AlF3 þ 19NaF þ 1Eu2 O3 KTFP : 50ðNaPO3 Þ6 þ 10TeO2 þ 20AlF3 þ 19KF þ 1Eu2 O3
The above glasses were prepared by the well-known melt quenching technique as described in our earlier publication [18]. The chemicals used in preparation of host glasses were of reagent grade or above and the dopant (Eu2O3) supplied by Sigma-Aldrich company was of 99.99% purity. The weighed quantities of the chemical ingredients were thoroughly mixed in an agate mortar and heated in an electric furnace at about 960 1C for 1 h in a silica crucible to obtain a homogeneously mixed melt. The melt was transferred into a preheated brass mould and annealed at 350 1C for 6 h to remove thermal strains and then polished to measure their physical and optical properties. The refractive indices (n) were determined using an Abbe refractometer at sodium’s wavelength (589.3 nm) with 1-bromonaphthalene as contact liquid. Densities were measured by the Archimedes method using xylene as an immersion liquid. Refractive indices and densities for LiTFP, NaTFP and KTFP glasses were found to be 1.591 and 2.6566 g/cm3, 1.593 and 2.6582 g/cm3 and 1.599 and 2.6434 g/cm3 respectively. Absorption spectra in the UV–VIS and NIR regions were measured using a Varian Cary 5E UV–VIS–NIR spectrophotometer. Emission spectra in the wavelength range 550–750 nm were recorded on a Hitachi F-3010 fluorescence spectrophotometer using the excitation wavelength of 395 nm. Lifetime measurements were done by exciting the glass samples at 395 nm line of Ar+ laser. Decay curves were obtained using a mechanical chopper with a multi channel scalar interfaced to a personal computer which records and averages the signals at room temperature.
Fig. 1. Absorption spectra of Eu3+ in alkali tellurofluorophosphate glasses in UV–VIS and NIR regions.
Table 1 Observed absorption band positions (cm1) for Eu3+: RTFP glasses Energy level
LiTFP
NaTFP
KTFP
F0- L6 F0-5D2 7 F0-7F6 7 F1-7F6
25373 21499 4811 4522
25328 21462 4801 4513
25360 21592 4800 4509
7 7
5
those reported for Eu3+: glasses [11,17,19]. The observed four absorption bands have been divided into two groups according to the spectra recorded between 380–480 nm in UV–VIS and 1900–2300 nm in NIR regions. The wavenumbers of the absorption bands along with their assignments are presented in Table 1. The peak positions of absorption bands in all the three glasses did not vary, but differences were observed in their integrated absorption coefficients [20,21]. The absorption peaks such as 7 F0-5L6, 5D2 and 7F6, 7F1-7F6 were resolved into Lorentzian or Gaussian shapes to determine the position and area of each individual absorption band. The lineshape of a level with single Stark level in Lorentzian is due to homogeneous line broadening. For a level with two or more Stark levels, the resonance frequencies between a lower level and an upper level of each Stark level are slightly different from a central frequency of the peak. Therefore, an inhomogeneous line-broadening situation is possible, so that the lineshape becomes Gaussian [22]. 3.2. Evaluation of J–O parameters from the emission spectra
3. Results 3.1. Absorption spectra and energy levels Optical absorption spectra at room temperature shown in Fig. 1 for Eu3+-doped LiTFP, NaTFP and KTFP glasses in the UV–VIS and NIR regions, are similar to
In the case of Eu3+ ion, it is possible to determine the Judd–Ofelt [3,4] parameters (Ol, l=2, 4 and 6) based on the analysis of emission spectra [23,24]. The doubly reduced matrix elements ||Ul|| corresponding to the 5 D0-7Fl transitions of Eu3+ ion ||U2|| for l=2, ||U4|| for l=4 and ||U6|| for l=6 are the only non zero matrix
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elements. The emission intensity (I) of a given transition is proportional to the area (S) under the emission curve. I ¼ hnAR N / S,
(1)
where hn is the transition energy, AR is the radiative transition rate and N is the population of emitting level (5D0). From the emission spectra of Eu3+: RTFP shown in Fig. 2, we have determined the Judd–Ofelt intensity parameters O2 and O4 by using the 5D0-7F2 and 5 D0-7F4 emission transitions, respectively. The radiative transition rate AR is given by [25,26]. 64P4 n3 nðn2 þ 2Þ2 X 5 AR ¼ h D0 jjU l jj7 Fl i, (2) 3hð2J þ 1Þ 9 l where nðn2 þ 2Þ2 =9 is the Lorentz local field correction, n being the refractive index of the medium. For the evaluation of J–O parameters, the intensity of allowed magnetic dipole 5 D0-7F1 emission transition was taken as the reference, since the magnetic dipole transition rate (Amd) is almost invariant in any host matrix. The reduced matrix elements ||Ul|| used in Eq. (2) were taken from earlier work [27]. The intensity parameters O2 and O4 are obtained by using the following relation [23]. 5 5 D0 !7 F2;4 D0 !7 F2;4 AR 5 ¼S 5 , (3) D0 !7 F1 D0 !7 F1
Fig. 2. Emission spectra of Eu3+ ions in alkali tellurofluorophosphate glasses.
where S is the area under each emission curve related to the 5 D0-7F1,2,4 transitions obtained from the emission spectra and AR is their respective radiative transition rate. Thus, the J–O parameters Ol (l ¼ 2, 4) can be individually calculated by the following ratios of the relative intensities of the emission bands: 5 D0 !7 F2;4 e2 n2;4 3 nðn2 þ 2Þ2 S 5 Ol jjUl jj2 , (4) ¼ D0 !7 F1 S md n1 9n3 where Smd is the magnetic dipole line strength for 5D0-7F1 transition, n1,2,4 are the frequencies of 5D0-7F1,2,4 transitions respectively. The calculated O2 and O4 parameters are presented in Table 2. Using these J–O parameters, various radiative parameters such as transition probability rates (AR), lifetimes (t), branching ratios (b) for different excited levels have been predicted and are given in Table 3. Since the emission transition 5D0-7F6 at about 810 nm has not been observed from the emission spectra, the authors could not determine the O6 parameter, hence its contribution to photophysical properties has not been taken into account. 4. Discussion 4.1. Electronic transitions The absorption spectra of Eu3+-doped alkali tellurofluorophosphate glasses shown in Fig. 1 reveal only four absorption bands which allow us to identify three excited states, namely 5L6, 5D2 and 7F6. It can be seen that the 7 F0-7F6 and 7F1-7F6 energy levels lying in the NIR region are well resolved in all three glasses. The absorption bands observed for these glasses are attributed to the transitions 7F0-5L6, 5D2, 7F6 and 7F1-7F6 manifolds centered approximately at 394, 465, 2078 and 2211 nm, respectively. The absorption band assigned to 7F0-5L6 transition is found to be more intense than the other transitions. The J–O parameters determined by using the emission 5 7 peak intensity ratios of S D ! F2 =5 D0 ! 7 F1 and 0 5 S D0 ! 7 F4 =5 D0 ! 7 F1 are compared in Table 2 with those reported for Eu3+ in different hosts [28–30]. The J–O parameters in all three glasses are found to be O24O4. The higher O2 values indicate the higher asymmetry and covalent nature around Eu3+ ions in the present host.
Table 2 A comparison of Judd–Ofelt parameters (Ol 1020 cm2) of Eu3+ in alkali tellurofluorophosphate glasses with different hosts Reported Ol
LiTFP (present)
NaTFP (present)
KTFP (present)
48InF3–24BaF2 7AlF3–20NaF–1EuF3 [28]
75 NaPO3–24CaF2–1EuF3 [29]
K7 (EuW10O35) [30]
O2 O4 O6
5.63 1.95 –
5.29 1.89 –
5.52 1.83 –
5.35 1.18 1.30
5.12 5.45 4.12
5.18 0.91 4.61
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Table 3 Emission band positions (lp), effective line widths (Dleff), radiative transition probabilities (AR), peak stimulated emission cross-sections (slp) 1022, experimental and calculated branching ratios (b), lifetimes (t) and quantum efficiencies (Z) for 5D0 level of Eu3+ : RTFP glasses lp (nm)
Dleff (nm)
AR (s1)
slp (cm2)
bexp
bcal
texp (ms)
tcal (ms)
Z (%)
D0- F0 7 F1 7 F2 7 F3 7 F4
582 598 619 658 705
4.63 11.67 11.84 8.92 9.26
0.00 57.34 209.07 0.00 36.22
0.00 3.29 13.59 0.00 5.07
0.006 0.184 0.683 0.019 0.114
0.000 0.190 0.691 0.000 0.120
2495
3304
76
5
D0-7F0 7 F1 7 F2 7 F3 7 F4
581 594 618 655 705
4.88 10.32 11.22 9.35 8.45
0.00 57.56 197.31 0.00 35.28
0.00 3.63 13.42 0.00 5.39
0.006 0.183 0.680 0.019 0.113
0.000 0.198 0.680 0.000 0.122
2401
3446
70
5
D0-7F0 7 F1 7 F2 7 F3 7 F4
582 593 618 656 702
5.31 10.86 11.60 11.84 10.18
0.00 58.21 208.60 0.00 34.51
0.00 4.27 13.62 0.00 4.27
0.009 0.188 0.674 0.020 0.108
0.000 0.193 0.692 0.000 0.115
2495
3318
75
Glass
Level
LiTFP
5
NaTFP
KTFP
7
The photoluminescence spectra obtained by exciting with 395 nm at room temperature shown in Fig. 2 correspond to the emissions observed from the excited 5 D0 level. The emission spectra of Eu3+: RTFP glasses show five emission lines corresponding to 5D0-7FJ group [J ¼ 0, 1, 2, 3, 4] at 582, 597, 618, 657 and 705 nm transitions, respectively. Among those transitions, the three transitions corresponding to 5D0-7FJ [J ¼ 1, 2 & 4] group are more intense and the other two 5D0-7F0 and 5 D0-7F3 transitions weaker. Similar observations have also reported by several authors [11,17,19]. 5.2. Radiative properties The relative areas under the emission peaks, known as experimental branching ratios for 5D0-7FJ transitions of Eu3+ presented in Table 3 are found to be in the order 5 D0-7F247F147F447F347F0. These experimental branching ratios are in good agreement with those predicted by J–O theory for 5D0-7FJ transitions. The other important radiative parameters such as effective linewidth, Dleff and peak stimulated emission cross section (slp) have also been determined and are presented in Table 3. For Eu3+: RTFP glasses, slp is found to be maximum for 5D0-7F2 transition. The values of slp 1022 cm2 for 5D0-7F2 transition of Eu3+: LiTFP, NaTFP and KTFP glasses are found to be 13.594, 13.415 and 13.621, respectively. The emission intensity of 5D0-7F2 transition is subjected to local symmetry as this transition is allowed by electric dipole nature. Similarly 5D0-7F1 transition is allowed by magnetic dipole and therefore its intensity is independent of local symmetry [31,32]. It should be noted that terminal level is the ground state 7F0, therefore only at low temperatures the upper Stark components are unoccupied and population inversion may be expected.
Fig. 3. Fluorescence decay profiles for 5D0 state of Eu3+: RTFP glasses.
The predicted radiative lifetimes for 5D0-7F2 transition (tcal) in all the Eu3+:RTFP glasses are collected in Table 3 and are compared with the experimental values (texp) obtained from the fluorescence decay curves shown in Fig. 3. The texp for 5D0 excited level is found to be comparable to tcal evaluated from the J–O parameters obtained from the emission band intensities. 6. Conclusion The optical behavior of Eu3+ ions is studied in alkali tellurofluorophosphate glasses. The phenomenological O2 and O4 are determined from the areas under the individual emission lines. In all the three RTFP glasses, O2 value is greater than O4; thus, it can be concluded that the radiative transition probabilities are mainly dependent on the 5 D0-7F2 hypersensitive emission transition. In general,
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the change in the environment of the dopant may yield reliable radiative transition probabilities for the generation of better luminescent sources. The results obtained in this work also allow us to suppose that, although the Eu3+ ions are immersed in an amorphous matrix, some degree of local order persists in the vicinity of these ions. Moreover, it seems that there is a unique environmental site distribution in RTFP glasses. The results of these investigations are found to be relatively more reliable, since the predicted radiative properties of 5D0 level agree well with the experimental values. The difference between the predicted and experimental lifetimes may be due to Eu3+ ion–ion interactions. The high brightness of 5 D0-7F2 emission at 618 nm from these highly stable RTFP glasses may show potential use in optoelectronic luminescent display systems. Acknowledgments This work has been carried out with the financial support of the Second Stage of Brain Korea 21 Project Corps at the Department of Physics, Changwon National University, Changwon, South Korea. References [1] E.R. Taylor, B.N. Samson, D.W. Hewak, J.A. Medeiros Neto, D.N. Payne, S. Jordey, M. Naftaly, A. Jha, J. Non-Cryst. Solids 184 (1995) 61. [2] M.J. Weber, J. Non-Cryst. Solids 123 (1990) 208. [3] B.R. Judd, Phys. Rev. 127 (1962) 750. [4] G.S. Ofelt, Chem. Phys. 37 (1962) 511. [5] C. Gorller-Walrand, K. Binnemans, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 23, North-Holland, Amsterdam, 1996 p. 121 (Chapter 155). [6] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [7] W.F. Krupke, Phys. Rev. 145 (1966) 325. [8] J.L. Adam, W.A. Sibley, D.R. Grabbe, J. Lumin. 33 (1985) 391.
[9] M. Eyal, E. Greenberg, R. Reisfeld, N. Spector, Chem. Phys. Lett. 117 (1985) 108. [10] R.S. Quimby, P.A. Tick, N.F. Borrelli, L.K. Cornelius, J. Appl. Phys. 83 (1998) 1649. [11] M. Dejneka, E. Snitzer, R.E. Riman, J. Lumin. 65 (1995) 227. [12] R. Van Deun, K. Binnemans, C. Gorller-Walrand, J.L. Adam, J. Alloys Comp. 283 (1999) 59. [13] S.E. Stokowski, R.A. Saroyan, M.J. Weber 1981: Nd-Doped Laser Glass-Spectroscopic and Physical Properties M-095, Rev. 2, Vols. land 2. Lawrence Livermore National Laboratory, Livermore, CA. [14] R. Balda, J. Fernandez, A. de Pablos, J. Phys. Coll. IV C 4 (1994) 509. [15] B. Peng, T. Izumitani, Rev. Laser Eng. 22 (1994) 16. [16] F. Fermi, L. Tellini, G. Ingletto, A. Vinattieri, M. Bettinelli, Inorg. Chim. Acta 150 (1988) 141. [17] J.A. Capobianco, P.P. Proulx, M. Bettinelli, F. Nagrisolo, Phys. Rev. B 42 (1990) 5936. [18] M. Jayasimhadri, L.R. Moorthy, K. Kojima, K. Yamamoto, N. Wada, N. Wada, J. Phys. D: Appl. Phys. 39 (2006) 635. [19] Y. Nageno, H. Takebe, K. Morinaga, T. Izumitani, J. Non-Cryst. Solids 169 (1994) 288. [20] G. Jose, V. Thomas, G. Jose, P.I. Paulose, V.V. Unnikrishnan, J. Non-Cryst. Solids 319 (2003) 89. [21] J.A. Medeiros Neto, D.W. Hewak, H. Tate, J. Non-Cryst. Solids 183 (1995) 201. [22] Y.G. Choi, J. Heo, J. Non-Cryst. Solids 217 (1997) 199. [23] H. Ebendorff-Heide Priem, D. Ehrt, J. Non-Cryst. Solids 208 (1996) 205. [24] M.J. Weber, Relaxation processes for excited states of Eu3+ in La F3, in: H.M. Cross white, H.W. Moos (Eds.), Optical Properties of Ions in Crystals, Wiley, New York, 1967. [25] O.L. Malta, H.F. Brito, J.F.S. Menezes, F.R. Goncalves e silva, S. Alves Jr., F.S. Farias Jr., A.V.M. De Andrade, J. Lumin. 75 (1997) 255. [26] O.L. Malta, M.A. Couto dos Santos, L.C. Thompson, N.K. Ito, J. Lumin. 69 (1996) 77. [27] P. Babu, C.K. Jayasankar, Physica B 279 (2000) 262. [28] K. Annapurna, S. Buddudu, J. Phys. D 26 (1993) 302. [29] K. Binnemans, R. Van Deun, C. Gorller–Walrand, J.L. Adam, J. Phys.: Condens. Matter 10 (1998) 7231. [30] R.D. Peacock, Mol. Phys. 4 (1973) 817. [31] G. Blasse, A. Bril, W.C. Niewpoort, J. Phys. Chem. Solids 27 (1966) 1587. [32] P.K. Gallagher, C.R. Kurkjian, P.M. Beidenbaugh, Phys. Chem. Glasses 6 (1965) 95.
Physica B 403 (2008) 1690–1694 www.elsevier.com/locate/physb
Emission properties of Eu3+ ions in alkali tellurofluorophosphate glasses D. Uma Maheswaria, J. Suresh Kumara, L.R. Moorthya,, Kiwan Jangb, M. Jayasimhadric a
Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Department of Physics, Changwon National University, Changwon, Kyungnam 641-773, South Korea c Photonic Glasses Laboratory, Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, South Korea b
Received 13 July 2007; received in revised form 13 September 2007; accepted 24 September 2007
Abstract The present work reports the spectroscopic properties of 1 mol% Eu2O3 in alkali tellurofluorophosphate glasses 50(NaPO3)6+10 TeO2+20 AlF3+19RF+1Eu2O3 (R ¼ Li, Na and K) by using absorption, emission and decay measurements. The phenomenological Judd–Ofelt (J–O) intensity parameters O2 and O4 were obtained from the emission intensities of 5D0-7F2 and 5D0-7F4, respectively by taking the magnetic dipole 5D0-7F1 emission band intensity as reference. The radiative parameters such as spontaneous emission probabilities (AR), lifetimes (t), branching ratios (b) for different excited states have been predicted theoretically. The branching ratios were also determined by the areas under the emission bands and are compared with those of the predicted values. The predicted and experimental lifetimes for the 5D0 level in alkali tellurofluorophosphate glasses were compared and discussed in detail. r 2007 Elsevier B.V. All rights reserved. Keywords: Eu3+; Glasses; J–O theory; Luminescence; Rare earths
1. Introduction The study of optical properties of Ln3+ ions in glasses depends on the chemical composition of the glass former and modifier, which elucidate fundamental information about energy-level positions, absorption and emission cross-sections, radiative and non-radiative decay rates and branching ratios. Lanthanide-doped glasses play a very important role in the development of lasers and fiber amplifiers for optical telecommunications [1,2]. Based on the absorption and emission bands of intraconfigurational f–f transitions, the energy level intensities of Ln3+ ions in several host media were described and estimated quantitatively by using the Judd–Ofelt theory [3,4]. In the present work, we reported the absorption, emission and decay properties of Eu3+ in alkali tellurofluorophosphate glasses, since the trivalent europium ion is a well-known spectroscopic probe [5]. Spectroscopic Corresponding author. Tel.: +91 877 2242766; fax: +91 877 2248499.
E-mail address: [email protected] (L.R. Moorthy). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.09.089
studies of Eu3+ ions have been reported in different hosts such as water [6], crystals [7,8] and glasses [9–12] in order to characterize these materials for optical device applications. Fluorophosphate glasses have been the subject of several spectroscopic investigations due to their potential use as laser host matrices [13,14]. Normally phosphate glasses are hygroscopic, but addition of fluoride increases the resistance to water. In addition, fluoride compounds, by virtue of their excellent physical and chemical properties, high stability with respect to short wavelength radiation and transparency in a wide spectral region, are attractive features in the field of quantum electronics. Peng and Izumatani [15] determined O2, O4 and O6 parameters by using intensities of emission levels 5 D0-7F0, 7F1, 7F2, 7F3 and 7F4, respectively. Van Deun et al. [12] have used the individual oscillator strengths of absorption bands of 7F0-5L6, 7F0-5D2, 7F6 and 7 F1-7F6 transitions to evaluate the phenomenological intensity parameters Ol with a requirement that the difference between the measured and calculated oscillator strengths be minimized. Fermi et al. [16] and Capobianco
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et al. [17] have considered both absorption intensities as well as the emission branching ratios and derived the Ol (l ¼ 2, 4 and 6) parameters. The object of the present study is to determine the J–O parameters from the emission spectra and also to compare the measured and predicted radiative lifetimes of 5D0 level in Eu3+-doped tellurofluorophosphate glasses. 2. Experimental The molar composition of europium-doped tellurofluorophosphate glasses investigated in the present work is as follows: LiTFP : 50ðNaPO3 Þ6 þ 10TeO2 þ 20AlF3 þ 19LiF þ 1Eu2 O3 NaTFP : 50ðNaPO3 Þ6 þ 10TeO2 þ 20AlF3 þ 19NaF þ 1Eu2 O3 KTFP : 50ðNaPO3 Þ6 þ 10TeO2 þ 20AlF3 þ 19KF þ 1Eu2 O3
The above glasses were prepared by the well-known melt quenching technique as described in our earlier publication [18]. The chemicals used in preparation of host glasses were of reagent grade or above and the dopant (Eu2O3) supplied by Sigma-Aldrich company was of 99.99% purity. The weighed quantities of the chemical ingredients were thoroughly mixed in an agate mortar and heated in an electric furnace at about 960 1C for 1 h in a silica crucible to obtain a homogeneously mixed melt. The melt was transferred into a preheated brass mould and annealed at 350 1C for 6 h to remove thermal strains and then polished to measure their physical and optical properties. The refractive indices (n) were determined using an Abbe refractometer at sodium’s wavelength (589.3 nm) with 1-bromonaphthalene as contact liquid. Densities were measured by the Archimedes method using xylene as an immersion liquid. Refractive indices and densities for LiTFP, NaTFP and KTFP glasses were found to be 1.591 and 2.6566 g/cm3, 1.593 and 2.6582 g/cm3 and 1.599 and 2.6434 g/cm3 respectively. Absorption spectra in the UV–VIS and NIR regions were measured using a Varian Cary 5E UV–VIS–NIR spectrophotometer. Emission spectra in the wavelength range 550–750 nm were recorded on a Hitachi F-3010 fluorescence spectrophotometer using the excitation wavelength of 395 nm. Lifetime measurements were done by exciting the glass samples at 395 nm line of Ar+ laser. Decay curves were obtained using a mechanical chopper with a multi channel scalar interfaced to a personal computer which records and averages the signals at room temperature.
Fig. 1. Absorption spectra of Eu3+ in alkali tellurofluorophosphate glasses in UV–VIS and NIR regions.
Table 1 Observed absorption band positions (cm1) for Eu3+: RTFP glasses Energy level
LiTFP
NaTFP
KTFP
F0- L6 F0-5D2 7 F0-7F6 7 F1-7F6
25373 21499 4811 4522
25328 21462 4801 4513
25360 21592 4800 4509
7 7
5
those reported for Eu3+: glasses [11,17,19]. The observed four absorption bands have been divided into two groups according to the spectra recorded between 380–480 nm in UV–VIS and 1900–2300 nm in NIR regions. The wavenumbers of the absorption bands along with their assignments are presented in Table 1. The peak positions of absorption bands in all the three glasses did not vary, but differences were observed in their integrated absorption coefficients [20,21]. The absorption peaks such as 7 F0-5L6, 5D2 and 7F6, 7F1-7F6 were resolved into Lorentzian or Gaussian shapes to determine the position and area of each individual absorption band. The lineshape of a level with single Stark level in Lorentzian is due to homogeneous line broadening. For a level with two or more Stark levels, the resonance frequencies between a lower level and an upper level of each Stark level are slightly different from a central frequency of the peak. Therefore, an inhomogeneous line-broadening situation is possible, so that the lineshape becomes Gaussian [22]. 3.2. Evaluation of J–O parameters from the emission spectra
3. Results 3.1. Absorption spectra and energy levels Optical absorption spectra at room temperature shown in Fig. 1 for Eu3+-doped LiTFP, NaTFP and KTFP glasses in the UV–VIS and NIR regions, are similar to
In the case of Eu3+ ion, it is possible to determine the Judd–Ofelt [3,4] parameters (Ol, l=2, 4 and 6) based on the analysis of emission spectra [23,24]. The doubly reduced matrix elements ||Ul|| corresponding to the 5 D0-7Fl transitions of Eu3+ ion ||U2|| for l=2, ||U4|| for l=4 and ||U6|| for l=6 are the only non zero matrix
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elements. The emission intensity (I) of a given transition is proportional to the area (S) under the emission curve. I ¼ hnAR N / S,
(1)
where hn is the transition energy, AR is the radiative transition rate and N is the population of emitting level (5D0). From the emission spectra of Eu3+: RTFP shown in Fig. 2, we have determined the Judd–Ofelt intensity parameters O2 and O4 by using the 5D0-7F2 and 5 D0-7F4 emission transitions, respectively. The radiative transition rate AR is given by [25,26]. 64P4 n3 nðn2 þ 2Þ2 X 5 AR ¼ h D0 jjU l jj7 Fl i, (2) 3hð2J þ 1Þ 9 l where nðn2 þ 2Þ2 =9 is the Lorentz local field correction, n being the refractive index of the medium. For the evaluation of J–O parameters, the intensity of allowed magnetic dipole 5 D0-7F1 emission transition was taken as the reference, since the magnetic dipole transition rate (Amd) is almost invariant in any host matrix. The reduced matrix elements ||Ul|| used in Eq. (2) were taken from earlier work [27]. The intensity parameters O2 and O4 are obtained by using the following relation [23]. 5 5 D0 !7 F2;4 D0 !7 F2;4 AR 5 ¼S 5 , (3) D0 !7 F1 D0 !7 F1
Fig. 2. Emission spectra of Eu3+ ions in alkali tellurofluorophosphate glasses.
where S is the area under each emission curve related to the 5 D0-7F1,2,4 transitions obtained from the emission spectra and AR is their respective radiative transition rate. Thus, the J–O parameters Ol (l ¼ 2, 4) can be individually calculated by the following ratios of the relative intensities of the emission bands: 5 D0 !7 F2;4 e2 n2;4 3 nðn2 þ 2Þ2 S 5 Ol jjUl jj2 , (4) ¼ D0 !7 F1 S md n1 9n3 where Smd is the magnetic dipole line strength for 5D0-7F1 transition, n1,2,4 are the frequencies of 5D0-7F1,2,4 transitions respectively. The calculated O2 and O4 parameters are presented in Table 2. Using these J–O parameters, various radiative parameters such as transition probability rates (AR), lifetimes (t), branching ratios (b) for different excited levels have been predicted and are given in Table 3. Since the emission transition 5D0-7F6 at about 810 nm has not been observed from the emission spectra, the authors could not determine the O6 parameter, hence its contribution to photophysical properties has not been taken into account. 4. Discussion 4.1. Electronic transitions The absorption spectra of Eu3+-doped alkali tellurofluorophosphate glasses shown in Fig. 1 reveal only four absorption bands which allow us to identify three excited states, namely 5L6, 5D2 and 7F6. It can be seen that the 7 F0-7F6 and 7F1-7F6 energy levels lying in the NIR region are well resolved in all three glasses. The absorption bands observed for these glasses are attributed to the transitions 7F0-5L6, 5D2, 7F6 and 7F1-7F6 manifolds centered approximately at 394, 465, 2078 and 2211 nm, respectively. The absorption band assigned to 7F0-5L6 transition is found to be more intense than the other transitions. The J–O parameters determined by using the emission 5 7 peak intensity ratios of S D ! F2 =5 D0 ! 7 F1 and 0 5 S D0 ! 7 F4 =5 D0 ! 7 F1 are compared in Table 2 with those reported for Eu3+ in different hosts [28–30]. The J–O parameters in all three glasses are found to be O24O4. The higher O2 values indicate the higher asymmetry and covalent nature around Eu3+ ions in the present host.
Table 2 A comparison of Judd–Ofelt parameters (Ol 1020 cm2) of Eu3+ in alkali tellurofluorophosphate glasses with different hosts Reported Ol
LiTFP (present)
NaTFP (present)
KTFP (present)
48InF3–24BaF2 7AlF3–20NaF–1EuF3 [28]
75 NaPO3–24CaF2–1EuF3 [29]
K7 (EuW10O35) [30]
O2 O4 O6
5.63 1.95 –
5.29 1.89 –
5.52 1.83 –
5.35 1.18 1.30
5.12 5.45 4.12
5.18 0.91 4.61
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Table 3 Emission band positions (lp), effective line widths (Dleff), radiative transition probabilities (AR), peak stimulated emission cross-sections (slp) 1022, experimental and calculated branching ratios (b), lifetimes (t) and quantum efficiencies (Z) for 5D0 level of Eu3+ : RTFP glasses lp (nm)
Dleff (nm)
AR (s1)
slp (cm2)
bexp
bcal
texp (ms)
tcal (ms)
Z (%)
D0- F0 7 F1 7 F2 7 F3 7 F4
582 598 619 658 705
4.63 11.67 11.84 8.92 9.26
0.00 57.34 209.07 0.00 36.22
0.00 3.29 13.59 0.00 5.07
0.006 0.184 0.683 0.019 0.114
0.000 0.190 0.691 0.000 0.120
2495
3304
76
5
D0-7F0 7 F1 7 F2 7 F3 7 F4
581 594 618 655 705
4.88 10.32 11.22 9.35 8.45
0.00 57.56 197.31 0.00 35.28
0.00 3.63 13.42 0.00 5.39
0.006 0.183 0.680 0.019 0.113
0.000 0.198 0.680 0.000 0.122
2401
3446
70
5
D0-7F0 7 F1 7 F2 7 F3 7 F4
582 593 618 656 702
5.31 10.86 11.60 11.84 10.18
0.00 58.21 208.60 0.00 34.51
0.00 4.27 13.62 0.00 4.27
0.009 0.188 0.674 0.020 0.108
0.000 0.193 0.692 0.000 0.115
2495
3318
75
Glass
Level
LiTFP
5
NaTFP
KTFP
7
The photoluminescence spectra obtained by exciting with 395 nm at room temperature shown in Fig. 2 correspond to the emissions observed from the excited 5 D0 level. The emission spectra of Eu3+: RTFP glasses show five emission lines corresponding to 5D0-7FJ group [J ¼ 0, 1, 2, 3, 4] at 582, 597, 618, 657 and 705 nm transitions, respectively. Among those transitions, the three transitions corresponding to 5D0-7FJ [J ¼ 1, 2 & 4] group are more intense and the other two 5D0-7F0 and 5 D0-7F3 transitions weaker. Similar observations have also reported by several authors [11,17,19]. 5.2. Radiative properties The relative areas under the emission peaks, known as experimental branching ratios for 5D0-7FJ transitions of Eu3+ presented in Table 3 are found to be in the order 5 D0-7F247F147F447F347F0. These experimental branching ratios are in good agreement with those predicted by J–O theory for 5D0-7FJ transitions. The other important radiative parameters such as effective linewidth, Dleff and peak stimulated emission cross section (slp) have also been determined and are presented in Table 3. For Eu3+: RTFP glasses, slp is found to be maximum for 5D0-7F2 transition. The values of slp 1022 cm2 for 5D0-7F2 transition of Eu3+: LiTFP, NaTFP and KTFP glasses are found to be 13.594, 13.415 and 13.621, respectively. The emission intensity of 5D0-7F2 transition is subjected to local symmetry as this transition is allowed by electric dipole nature. Similarly 5D0-7F1 transition is allowed by magnetic dipole and therefore its intensity is independent of local symmetry [31,32]. It should be noted that terminal level is the ground state 7F0, therefore only at low temperatures the upper Stark components are unoccupied and population inversion may be expected.
Fig. 3. Fluorescence decay profiles for 5D0 state of Eu3+: RTFP glasses.
The predicted radiative lifetimes for 5D0-7F2 transition (tcal) in all the Eu3+:RTFP glasses are collected in Table 3 and are compared with the experimental values (texp) obtained from the fluorescence decay curves shown in Fig. 3. The texp for 5D0 excited level is found to be comparable to tcal evaluated from the J–O parameters obtained from the emission band intensities. 6. Conclusion The optical behavior of Eu3+ ions is studied in alkali tellurofluorophosphate glasses. The phenomenological O2 and O4 are determined from the areas under the individual emission lines. In all the three RTFP glasses, O2 value is greater than O4; thus, it can be concluded that the radiative transition probabilities are mainly dependent on the 5 D0-7F2 hypersensitive emission transition. In general,
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the change in the environment of the dopant may yield reliable radiative transition probabilities for the generation of better luminescent sources. The results obtained in this work also allow us to suppose that, although the Eu3+ ions are immersed in an amorphous matrix, some degree of local order persists in the vicinity of these ions. Moreover, it seems that there is a unique environmental site distribution in RTFP glasses. The results of these investigations are found to be relatively more reliable, since the predicted radiative properties of 5D0 level agree well with the experimental values. The difference between the predicted and experimental lifetimes may be due to Eu3+ ion–ion interactions. The high brightness of 5 D0-7F2 emission at 618 nm from these highly stable RTFP glasses may show potential use in optoelectronic luminescent display systems. Acknowledgments This work has been carried out with the financial support of the Second Stage of Brain Korea 21 Project Corps at the Department of Physics, Changwon National University, Changwon, South Korea. References [1] E.R. Taylor, B.N. Samson, D.W. Hewak, J.A. Medeiros Neto, D.N. Payne, S. Jordey, M. Naftaly, A. Jha, J. Non-Cryst. Solids 184 (1995) 61. [2] M.J. Weber, J. Non-Cryst. Solids 123 (1990) 208. [3] B.R. Judd, Phys. Rev. 127 (1962) 750. [4] G.S. Ofelt, Chem. Phys. 37 (1962) 511. [5] C. Gorller-Walrand, K. Binnemans, in: K.A. Gschneidner Jr., L. Eyring (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Vol. 23, North-Holland, Amsterdam, 1996 p. 121 (Chapter 155). [6] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [7] W.F. Krupke, Phys. Rev. 145 (1966) 325. [8] J.L. Adam, W.A. Sibley, D.R. Grabbe, J. Lumin. 33 (1985) 391.
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