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The behavior of Er3+ dopants during crystallization in oxyfluoride silicate glass ceramics
Journal of Alloys and Compounds 486 (2009) 261–264
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The behavior of Er3+ dopants during crystallization in oxyfluoride silicate glass ceramics Lejing Lin a,b , Guozhong Ren a,b,∗ , Minpeng Chen a,b , Yang Liu a,b a b
Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China Key Laboratory of Low Dimensional Materials & Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, China
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 28 May 2009 Accepted 28 May 2009 Available online 8 August 2009 Keywords: Nanocrystals X-ray diffraction Luminescence
a b s t r a c t In this paper, the changes of local environment of Er3+ during heat treatment process have been studied in 50SiO2 –45PbF2 –5PbO system. The samples were characterized by X-ray diffraction (XRD), absorption spectra, fluorescence decay curves and luminescence spectra. The experimental results indicate that PbF2 crystals were precipitated in the precursor sample. However, a significant fraction of Er3+ remains in the glassy phase. With increasing heat treatment time, the Er3+ in the glass matrix may enter into fluoride nanocrystals gradually. When the heat treatment time reached 15 min, most of the Er3+ ions were incorporated into the fluoride nanocrystals. Based on the experimental analysis, we considered that Er3+ not only act as nucleating agents for the precipitation of crystalline fluoride phase during nucleation process but also enter into nanocrystals during crystals growth process. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Optical properties of oxyfluoride transparent glass ceramic (TGC) doped with rare earth ions have attracted much attention in the design of optical device as lasers, fiber optics amplifiers and efficient upconverters [1–4]. TGC can be obtained when applied a precise heat treatment to the precursor glass. The most important advantage of TGC is that the optically active rare earth ions can incorporate into the fluoride nanocrystal phase with low phonon energy which is effective for upconversion [5–7]. Recently, some research on frequency upconversion has been focused on the glass ceramic that all rare earth ions are incorporated into fluoride crystalline phase after several hours heat treatment [7,8]. Crystal nucleation and growth are well known in phase separation process of TGC [9]. With satisfying suitable thermodynamic condition, the crystal nucleation will be arisen. So the crystal nucleation process is mainly dependent on the appropriate thermodynamic condition. The crystal growth process is strongly related to the heat treatment. Rare earth ions in the uniform glass matrix are incorporated into crystal phase during the whole heat treatment process. So, it is worthy to investigate the precipitation of rare earth ions during the nucleation and crystals growth processes. The fraction of the rare earth ions that is incorporated into the crystalline phase can be estimated from the spectroscopy [10]. Furthermore, the crystallization
∗ Corresponding author at: Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China. Tel.: +86 732 8292 113. E-mail address: [email protected] (G. Ren). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.154
and structural evolution of the samples can be verified by XRD analyses. Previous results [11] demonstrated that rare earth ions can act as nucleating agents for the precipitation of crystal fluoride phase during nucleation process. Therefore, the environmental configuration surrounding the Er3+ during crystals growth process in oxyfluoride silicate glass ceramics has been studied in this work, by means of the XRD, absorption spectra, and fluorescence decay curves for various heat treatment time. The results obtained show that Er3+ ions can enter into PbF2 crystals gradually during crystals growth process.
2. Experimental details The precursor sample (named S0) with the composition of 50SiO2 – 45PbF2 –5PbO–1ErF3 was prepared by well mixed high purity SiO2 , PbF2 , PbO and ErF3 as raw materials. The mixture was kept in the furnace at 1050 ◦ C for 15 min in a covered corundum crucible. The melt was poured into a copper plate and pressed by another copper plate. Then S0 was heat treated at 390 ◦ C for different times. For heat treatment time longer than 15 min there is very little change in the spectral properties of the samples. So the samples should have a gradual change when the heat treatment time was less than 15 min. Therefore, we selected the annealing time as follows: 1 min, 5 min, 10 min, 15 min and 4 h, named S0-1, S0-2, S0-3, S0-4 and S0-5, respectively. The densities were measured based on the Archimedes’ principle using deionized water as medium. XRD analyses were performed in a D/max-␥A System X-ray diffractometer using Cu K␣ radiation. The absorption spectra were recorded by a Shimadzu UV-3101PC spectrophotometer. The fluorescence decay curves of the 4 S3/2 level were obtained with a spectrometer (Jobin Yvon Triax 320) and a computer controlled digital oscilloscope when excited at 355 nm pulse laser. Upconversion spectra in the visible region were detected by a spectrophotometer (R500) under the excitation of 980 nm LD. All the measurements were carried out at room temperature.
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Fig. 2. Absorption spectra of Er3+ in S0; the inset shows the enlarged absorption spectra of S0 and S0-x (x = 1–4) in the range from 1400 to 1650 nm. Fig. 1. XRD patterns of S0 and S0-4. Table 2 Judd–Ofelt parameters ˝t (10−20 cm2 ) of Er3+ in S0 and S0-x (x = 1–4).
3. Results and discussion Fig. 1 shows the XRD patterns of S0 and S0-4. Several diffraction peaks corresponding to -PbF2 [12] crystals were clearly observed in the two samples, indicating that PbF2 crystals were precipitated in the precursor sample. The PbF2 crystals precipitated in S0 are probably owing to the excessive fluoride in the glass composition. The diameters of the crystals in the two samples were evaluated to be approximately 15 nm by the Scherrer formula [13]. Due to the much smaller size of precipitated PbF2 crystal than the wavelength of visible light, the two samples maintained excellent transparency. According to XRD patterns, the volume fraction of crystal phase can be approximately estimated by the ratio of the integrated area of the peaks and the total XRD patterns. The unit cell parameter (a) of the crystallites can be obtained from the XRD results. We also calculated the Er3+ concentration in the PbF2 nanocrystals based on the F− interstitial compensation mechanism [14]. All calculated data are listed in Table 1. The unit cell parameter is smaller than the unit cell parameter of pure PbF2 (a = 0.594 nm), indicating that the crystal was a solid solution of the type -PbF2 :Er3+ , in which the Pb2+ ions, with an ionic radius of 0.129 nm, were partially substituted by Er3+ ions, with an ionic radius of 0.100 nm. The crystallites are in fact formed of a solid solution Pb1−x Erx F2 . The volume fraction of crystalline phase increases slightly with the increase of the heat treatment time to 4 h. However, the increase of Er3+ concentration in the PbF2 nanocrystals is obvious with increasing the heat treatment time. Fig. 2 presents the optical absorption spectra of S0. The bands are assigned to the transition from the ground state 4 I15/2 to the indexed excited ones. The inset of Fig. 2 shows the enlarged absorption spectra of all the samples in the range from 1400 to 1650 nm. Although there were no obvious changes at the first few minutes with heat treatment, the main changes were observed when the heat treatment time was prolonged to 10 min. One can notice that the line shapes of S0-4 and S0-5 exhibit the Stark split with several weak components, evidencing the change of ligand field of Er3+ ions
Sample
˝2
˝4
˝6
S0 S0-1 S0-2 S0-3 S0-4 S0-5
2.21 2.17 2.10 1.92 1.38 1.14
1.19 1.25 1.06 1.12 0.99 0.72
1.00 0.95 0.98 1.05 0.87 1.21
due to the incorporation of the Er3+ into PbF2 nanocrystals. And the line shapes of S0, S0-1 and S0-2 are characteristic for environmental structure of Er3+ site amorphous. It should be pointed out that the line shapes of S0-3 are different from the others. The changes of the line shapes indicated that Er3+ in the glass matrix entered into PbF2 crystals gradually during heat treatment and most of the Er3+ had incorporated into crystals with the increase of heat treatment time to 15 min. Using the absorption bands, J–O parameters (˝2 , ˝4 and ˝6 ) can be calculated using J–O theory by a least squares fitting procedure [15,16]. The results are tabulated in Table 2. In general, ˝2 is found to be affected by the covalence between the rare earth ions and the ligand anions. The ˝2 decreases with the host changing from oxides to fluorides [17,18]. This is because fluoride environments around rare earth ions are more ionic than oxide, due to the large electronegativity of F− compared to O2− . The results presented in Table 2 show that with the increase of heat treatment time the ˝2 parameter decreases gradually, which suggests that the ligand field of Er3+ is more ionic. It is probably owing to the incorporation of Er3+ into PbF2 crystals gradually during the heat treatment process. The Er3+ ions in the PbF2 crystals are coordinated by F− ions, and more ionic bond environment of the Er3+ can be expected, which will lead to the decrease of ˝2 . From the absorption spectra and J–O parameters, there are few changes can be observed from S0, S0-1 and S0-2. These results show that only little fraction of rare earth ions entered into crystalline phase, i.e. most rare earth ions are still within the glass phase after heat treatment for 5 min. It should
Table 1 Lattice parameter, Er3+ concentration in PbF2 crystalline and the volume fraction of crystalline phase. Sample
Volume fraction of crystalline phase (%)
Lattice parameter (nm)
Er3+ concentration (mol%)
S0 S0-4
14 17
5.91 5.86
7.74 21.94
L. Lin et al. / Journal of Alloys and Compounds 486 (2009) 261–264
263
Fig. 4. Simplified energy level diagram of Er3+ . Fig. 3.
4
S3/2 → 4 I15/2 fluorescence decay curves of S0 and S0-3.
be taken into account that the heat treatment time starts when the samples put in a furnace, including the increasing temperature time which cannot be measured accurately. With the increase of the heat treatment time to 10 min, it is found that most important changes are obtained in S0-3. These changes are correlated with more Er3+ in the glass matrix incorporated to the crystals. Furthermore, the notable decrease of ˝2 for S0-4 and S0-5 indicated that most of Er3+ ions were incorporated into -PbF2 crystalline phase after heat treatment for 15 min. The luminescence decay measurements have been performed in S0 and the samples with different heat treatment time. The experimental decay curves of 4 S3/2 level of S0 and S0-4 are presented in Fig. 3. The decay curves show a highly nonexponential behavior indicating that the contribution of cross-relaxation processes [19] to the 4 S3/2 level is important in all the samples. The nonexponential experimental decay lifetime exp can be deduced by the following formula: exp =
I(t)t dt
(1)
I(t) dt
where R is the distance between donor and acceptor centers, n is the polarization-averaged index of refraction at energy v (cm−1 ), f is the oscillator strengths of both energy transfer centers, and g is the integrated overlap of the normalized donor emission and acceptor absorption spectral features. The superscript k = 6, 8 and 10 for electricdipole–dipole, dipole–quadrupole and quadrupole–quadrupole contributions, respectively. The contribution of R to the energy transfer probability is remarkable for each transition. The main energy transfer pathways between the rare earth ions are presented in Fig. 4. Among the processes, energy transfer (ET) is strongly dependent on the concentration of Er3+ . The shorter distances between earth ions in crystals favor ion-ion interactions, resulting in efficient ET to shorten the lifetime [21]. The shorter lifetimes found in S0-4 and S0-5 could be understood considering the reduction of distances between Er3+ when most of them have been incorporated into crystal phase after heat treatment for 15 min. The results are consistent with that obtained from absorption spectra. Fig. 5 shows the upconversion luminescence spectra of S0, S02 and S0-4 excited by 980 nm. The emission bands of the Er3+ ions can be assigned to 2 H11/2 –4 I15/2 (520 nm), 4 S3/2 –4 I15/2 (545 nm) and 4F 4 9/2 – I15/2 (660 nm) transitions, respectively.
where I(t) represents the fluorescence intensity at the time t. The calculated lifetimes are listed in Table 3. It can be observed that the luminescence decay lifetimes of the 4 S3/2 level are shorter for S0-4 and S0-5. This is an expected result, as cross-relaxation processes reduce the lifetime of the 4 S3/2 level and these processes are important for high Er3+ contents. Since rare earth ions are incorporated into crystals after heat crystallization, the ions are located closer to each other than those dispersed homogeneously in the samples. The increase of the Er3+ ions concentration inside the crystal phase will reduce the distance between Er3+ ions. The ion–ion interaction, i.e. energy transfer can efficiently occur if the distance between ions becomes small enough. In terms of Dexter’s calculation, the dependence of energy transfer probability (WET ) on interionic distance was described as [20] WET =
Const 2 n4 Rk
fD fA
gD ()gA () d
(2)
Table 3 The calculated lifetimes exp of 4 S3/2 level for S0 and S0-x (x = 1–4). Sample
S0
S0-1
S0-2
S0-3
S0-4
S0-5
exp (s)
41.0
38.2
38.4
34.2
30.4
29.0
Fig. 5. Upconversion luminescence spectra of the Er3+ in S0 and S0-x (x = 1–4).
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As can be observed, the upconversion luminescence intensity increased with increasing heat treatment time. It is well known that the upconversion luminescence of rare earth ions is usually baffled by the multi-phonon relaxation. The multi-phonon relaxation of excited rare earth ions through multi-phonon processes depends exponentially on E/¯hωP , where E is the energy gap from the next level below and h ¯ ωP is the energy of the maximum lattice vibration of the surrounding host lattice [22,23]. The smaller the lattice vibration energy of the host is, the lower the multi-phonon relaxation probability is [24]. The approximate frequencies of the highest energy lattice vibration in silicate oxide glass are 1100 cm−1 and that of PbF2 crystal is 258 cm−1 [25]. As it was revealed by the absorption spectra and the fluorescence decay curves that the Er3+ ions enter into PbF2 crystalline phase gradually with increasing heat treatment time. Therefore, the upconversion intensity increases significantly with the increase of heat treatment time. 4. Conclusions The changes of Er3+ ions surrounding environment with increasing heat treatment time were investigated in 50SiO2 –5PbO–45PbF2 system. As it was confirmed by the absorption spectra and luminescence decays studies, the Er3+ ions not only act as nucleating agents for the precipitation of crystalline fluoride phase during nucleation process, but also can enter into nanocrystals gradually during crystals growth process. More Er3+ ions in the glass matrix were incorporated into crystalline phases after heat treatment and hence greatly increase the upconversion luminescence intensity. Acknowledgements This work was supported by the National Scientific Foundation of China (Grant No. 50502031) and the Open Project Program of
Low Dimensional Materials & Application Technology (Xiangtan University), Ministry of Education, China (No. DWKF0809). References [1] H. Hayashi, S. Tanabe, T. Hanada, J. Appl. Phys. 89 (2001) 1041. [2] R. Balda, S. García-Revilla, J. Fernández, V. Seznec, V. Nazabal, X.H. Zhang, J.L. Adam, M. Allix, G. Matzen. Opt. Mater. 31 (2009) 760. [3] G. Lakshminarayana, J. Qiu, J. Alloys Compd. 476 (2009) 720. [4] S. González-Pérez, I.R. Martín, F. Rivera-López, F. Lahoz, J. Non-Cryst. Solids 353 (2007) 1951. [5] F. Laboz, I.R. Matin, J. Mendez-Ramos, J. Chem. Phys. 120 (2004) 1680. ˜ [6] S.F. León-Luis, J. Abreu-Afonso, J. Pena-Martínez, J. Méndez-Ramos, A.C. Yanes, J. del-Castillo, V.D. Rodríguez, J. Alloys Compd. 479 (2009) 557. [7] F. Liu, E. Ma, D. Chen, Y. Wang, Y. Yu, P. Huang, J. Alloys Compd. 467 (2009) 317. [8] Z.J. Hu, E. Ma, Y.S. Wang, D.Q. Chen, Mater. Chem. Phys. 100 (2006) 308. [9] I. Gutzow, J. Schmelzer, The Vitreous State, Springer, Berlin, 1995. [10] F. Goutaland, P. Jander, W.S. Brocklesby, G.J. Dai, Opt. Mater. 22 (2003) 383. [11] G. Dantelle, M. Mortier, D. Vivien, G. Patriarche, J. Mater. Res. 20 (2) (2005) 472. [12] Y. Kawamoto, R. Kanno, J. Qiu, J. Mater. Sci. 33 (1998) 63. [13] P. Debye, P. Scherrer, Physikalishe Zeitschrift, Nachr Akad Wiss Gott. Math. Nat. KI IIa 1916 (1916) 1. [14] M. Beggiora, I.M. Reaney, M.S. Islam, Appl. Phys. Lett. 83 (2003) 467. [15] B.R. Judd, Phys. Rev. 127 (1962) 750. [16] J.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [17] H.E. Heidepriem, D. Ehrt, M. Bettinelli, A. Speghini, J. Non-Cryst. Solids 240 (1998) 66. [18] X. Zou, T. Izumitani, J. Non-Cryst. Solids 162 (1993) 68. [19] M. Abril, J. Méndez-Ramos, I.R. Martín, U.R. Rodríguez-Mendoza, V. Lavín, A. Delgado-Torres, V.D. Rodríguez, J. Appl. Phys. 95 (2004) 5271. [20] P.L. Dexter, J. Chem. Phys. 21 (1963) 876. [21] D.Q. Chen, Y.S. Wang, l.Y. Yu, E. Ma, Mater. Chem. Phys. 101 (2007) 464. [22] M.J. Weber, Phys. Rev. 171 (1968) 283. [23] R. Reisfeld, L. Boehm, Y. Eckstein, N. Lieblich, J. Lumin. 10 (1975) 193. [24] X.P. Fan, J. Wang, X.S. Qiao, M.Q. Wang, J.-L. Adam, X.H. Zhang, J. Phys. Chem. B 110 (2006) 5950. [25] C.B. Layne, W.H. Lowdermilk, M. Weber, Phys. Rev. B 16 (1977) 10.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The behavior of Er3+ dopants during crystallization in oxyfluoride silicate glass ceramics Lejing Lin a,b , Guozhong Ren a,b,∗ , Minpeng Chen a,b , Yang Liu a,b a b
Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China Key Laboratory of Low Dimensional Materials & Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, China
a r t i c l e
i n f o
Article history: Received 9 February 2009 Received in revised form 28 May 2009 Accepted 28 May 2009 Available online 8 August 2009 Keywords: Nanocrystals X-ray diffraction Luminescence
a b s t r a c t In this paper, the changes of local environment of Er3+ during heat treatment process have been studied in 50SiO2 –45PbF2 –5PbO system. The samples were characterized by X-ray diffraction (XRD), absorption spectra, fluorescence decay curves and luminescence spectra. The experimental results indicate that PbF2 crystals were precipitated in the precursor sample. However, a significant fraction of Er3+ remains in the glassy phase. With increasing heat treatment time, the Er3+ in the glass matrix may enter into fluoride nanocrystals gradually. When the heat treatment time reached 15 min, most of the Er3+ ions were incorporated into the fluoride nanocrystals. Based on the experimental analysis, we considered that Er3+ not only act as nucleating agents for the precipitation of crystalline fluoride phase during nucleation process but also enter into nanocrystals during crystals growth process. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Optical properties of oxyfluoride transparent glass ceramic (TGC) doped with rare earth ions have attracted much attention in the design of optical device as lasers, fiber optics amplifiers and efficient upconverters [1–4]. TGC can be obtained when applied a precise heat treatment to the precursor glass. The most important advantage of TGC is that the optically active rare earth ions can incorporate into the fluoride nanocrystal phase with low phonon energy which is effective for upconversion [5–7]. Recently, some research on frequency upconversion has been focused on the glass ceramic that all rare earth ions are incorporated into fluoride crystalline phase after several hours heat treatment [7,8]. Crystal nucleation and growth are well known in phase separation process of TGC [9]. With satisfying suitable thermodynamic condition, the crystal nucleation will be arisen. So the crystal nucleation process is mainly dependent on the appropriate thermodynamic condition. The crystal growth process is strongly related to the heat treatment. Rare earth ions in the uniform glass matrix are incorporated into crystal phase during the whole heat treatment process. So, it is worthy to investigate the precipitation of rare earth ions during the nucleation and crystals growth processes. The fraction of the rare earth ions that is incorporated into the crystalline phase can be estimated from the spectroscopy [10]. Furthermore, the crystallization
∗ Corresponding author at: Institute of Modern Physics, Xiangtan University, Xiangtan 411105, China. Tel.: +86 732 8292 113. E-mail address: [email protected] (G. Ren). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.154
and structural evolution of the samples can be verified by XRD analyses. Previous results [11] demonstrated that rare earth ions can act as nucleating agents for the precipitation of crystal fluoride phase during nucleation process. Therefore, the environmental configuration surrounding the Er3+ during crystals growth process in oxyfluoride silicate glass ceramics has been studied in this work, by means of the XRD, absorption spectra, and fluorescence decay curves for various heat treatment time. The results obtained show that Er3+ ions can enter into PbF2 crystals gradually during crystals growth process.
2. Experimental details The precursor sample (named S0) with the composition of 50SiO2 – 45PbF2 –5PbO–1ErF3 was prepared by well mixed high purity SiO2 , PbF2 , PbO and ErF3 as raw materials. The mixture was kept in the furnace at 1050 ◦ C for 15 min in a covered corundum crucible. The melt was poured into a copper plate and pressed by another copper plate. Then S0 was heat treated at 390 ◦ C for different times. For heat treatment time longer than 15 min there is very little change in the spectral properties of the samples. So the samples should have a gradual change when the heat treatment time was less than 15 min. Therefore, we selected the annealing time as follows: 1 min, 5 min, 10 min, 15 min and 4 h, named S0-1, S0-2, S0-3, S0-4 and S0-5, respectively. The densities were measured based on the Archimedes’ principle using deionized water as medium. XRD analyses were performed in a D/max-␥A System X-ray diffractometer using Cu K␣ radiation. The absorption spectra were recorded by a Shimadzu UV-3101PC spectrophotometer. The fluorescence decay curves of the 4 S3/2 level were obtained with a spectrometer (Jobin Yvon Triax 320) and a computer controlled digital oscilloscope when excited at 355 nm pulse laser. Upconversion spectra in the visible region were detected by a spectrophotometer (R500) under the excitation of 980 nm LD. All the measurements were carried out at room temperature.
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L. Lin et al. / Journal of Alloys and Compounds 486 (2009) 261–264
Fig. 2. Absorption spectra of Er3+ in S0; the inset shows the enlarged absorption spectra of S0 and S0-x (x = 1–4) in the range from 1400 to 1650 nm. Fig. 1. XRD patterns of S0 and S0-4. Table 2 Judd–Ofelt parameters ˝t (10−20 cm2 ) of Er3+ in S0 and S0-x (x = 1–4).
3. Results and discussion Fig. 1 shows the XRD patterns of S0 and S0-4. Several diffraction peaks corresponding to -PbF2 [12] crystals were clearly observed in the two samples, indicating that PbF2 crystals were precipitated in the precursor sample. The PbF2 crystals precipitated in S0 are probably owing to the excessive fluoride in the glass composition. The diameters of the crystals in the two samples were evaluated to be approximately 15 nm by the Scherrer formula [13]. Due to the much smaller size of precipitated PbF2 crystal than the wavelength of visible light, the two samples maintained excellent transparency. According to XRD patterns, the volume fraction of crystal phase can be approximately estimated by the ratio of the integrated area of the peaks and the total XRD patterns. The unit cell parameter (a) of the crystallites can be obtained from the XRD results. We also calculated the Er3+ concentration in the PbF2 nanocrystals based on the F− interstitial compensation mechanism [14]. All calculated data are listed in Table 1. The unit cell parameter is smaller than the unit cell parameter of pure PbF2 (a = 0.594 nm), indicating that the crystal was a solid solution of the type -PbF2 :Er3+ , in which the Pb2+ ions, with an ionic radius of 0.129 nm, were partially substituted by Er3+ ions, with an ionic radius of 0.100 nm. The crystallites are in fact formed of a solid solution Pb1−x Erx F2 . The volume fraction of crystalline phase increases slightly with the increase of the heat treatment time to 4 h. However, the increase of Er3+ concentration in the PbF2 nanocrystals is obvious with increasing the heat treatment time. Fig. 2 presents the optical absorption spectra of S0. The bands are assigned to the transition from the ground state 4 I15/2 to the indexed excited ones. The inset of Fig. 2 shows the enlarged absorption spectra of all the samples in the range from 1400 to 1650 nm. Although there were no obvious changes at the first few minutes with heat treatment, the main changes were observed when the heat treatment time was prolonged to 10 min. One can notice that the line shapes of S0-4 and S0-5 exhibit the Stark split with several weak components, evidencing the change of ligand field of Er3+ ions
Sample
˝2
˝4
˝6
S0 S0-1 S0-2 S0-3 S0-4 S0-5
2.21 2.17 2.10 1.92 1.38 1.14
1.19 1.25 1.06 1.12 0.99 0.72
1.00 0.95 0.98 1.05 0.87 1.21
due to the incorporation of the Er3+ into PbF2 nanocrystals. And the line shapes of S0, S0-1 and S0-2 are characteristic for environmental structure of Er3+ site amorphous. It should be pointed out that the line shapes of S0-3 are different from the others. The changes of the line shapes indicated that Er3+ in the glass matrix entered into PbF2 crystals gradually during heat treatment and most of the Er3+ had incorporated into crystals with the increase of heat treatment time to 15 min. Using the absorption bands, J–O parameters (˝2 , ˝4 and ˝6 ) can be calculated using J–O theory by a least squares fitting procedure [15,16]. The results are tabulated in Table 2. In general, ˝2 is found to be affected by the covalence between the rare earth ions and the ligand anions. The ˝2 decreases with the host changing from oxides to fluorides [17,18]. This is because fluoride environments around rare earth ions are more ionic than oxide, due to the large electronegativity of F− compared to O2− . The results presented in Table 2 show that with the increase of heat treatment time the ˝2 parameter decreases gradually, which suggests that the ligand field of Er3+ is more ionic. It is probably owing to the incorporation of Er3+ into PbF2 crystals gradually during the heat treatment process. The Er3+ ions in the PbF2 crystals are coordinated by F− ions, and more ionic bond environment of the Er3+ can be expected, which will lead to the decrease of ˝2 . From the absorption spectra and J–O parameters, there are few changes can be observed from S0, S0-1 and S0-2. These results show that only little fraction of rare earth ions entered into crystalline phase, i.e. most rare earth ions are still within the glass phase after heat treatment for 5 min. It should
Table 1 Lattice parameter, Er3+ concentration in PbF2 crystalline and the volume fraction of crystalline phase. Sample
Volume fraction of crystalline phase (%)
Lattice parameter (nm)
Er3+ concentration (mol%)
S0 S0-4
14 17
5.91 5.86
7.74 21.94
L. Lin et al. / Journal of Alloys and Compounds 486 (2009) 261–264
263
Fig. 4. Simplified energy level diagram of Er3+ . Fig. 3.
4
S3/2 → 4 I15/2 fluorescence decay curves of S0 and S0-3.
be taken into account that the heat treatment time starts when the samples put in a furnace, including the increasing temperature time which cannot be measured accurately. With the increase of the heat treatment time to 10 min, it is found that most important changes are obtained in S0-3. These changes are correlated with more Er3+ in the glass matrix incorporated to the crystals. Furthermore, the notable decrease of ˝2 for S0-4 and S0-5 indicated that most of Er3+ ions were incorporated into -PbF2 crystalline phase after heat treatment for 15 min. The luminescence decay measurements have been performed in S0 and the samples with different heat treatment time. The experimental decay curves of 4 S3/2 level of S0 and S0-4 are presented in Fig. 3. The decay curves show a highly nonexponential behavior indicating that the contribution of cross-relaxation processes [19] to the 4 S3/2 level is important in all the samples. The nonexponential experimental decay lifetime exp can be deduced by the following formula: exp =
I(t)t dt
(1)
I(t) dt
where R is the distance between donor and acceptor centers, n is the polarization-averaged index of refraction at energy v (cm−1 ), f is the oscillator strengths of both energy transfer centers, and g is the integrated overlap of the normalized donor emission and acceptor absorption spectral features. The superscript k = 6, 8 and 10 for electricdipole–dipole, dipole–quadrupole and quadrupole–quadrupole contributions, respectively. The contribution of R to the energy transfer probability is remarkable for each transition. The main energy transfer pathways between the rare earth ions are presented in Fig. 4. Among the processes, energy transfer (ET) is strongly dependent on the concentration of Er3+ . The shorter distances between earth ions in crystals favor ion-ion interactions, resulting in efficient ET to shorten the lifetime [21]. The shorter lifetimes found in S0-4 and S0-5 could be understood considering the reduction of distances between Er3+ when most of them have been incorporated into crystal phase after heat treatment for 15 min. The results are consistent with that obtained from absorption spectra. Fig. 5 shows the upconversion luminescence spectra of S0, S02 and S0-4 excited by 980 nm. The emission bands of the Er3+ ions can be assigned to 2 H11/2 –4 I15/2 (520 nm), 4 S3/2 –4 I15/2 (545 nm) and 4F 4 9/2 – I15/2 (660 nm) transitions, respectively.
where I(t) represents the fluorescence intensity at the time t. The calculated lifetimes are listed in Table 3. It can be observed that the luminescence decay lifetimes of the 4 S3/2 level are shorter for S0-4 and S0-5. This is an expected result, as cross-relaxation processes reduce the lifetime of the 4 S3/2 level and these processes are important for high Er3+ contents. Since rare earth ions are incorporated into crystals after heat crystallization, the ions are located closer to each other than those dispersed homogeneously in the samples. The increase of the Er3+ ions concentration inside the crystal phase will reduce the distance between Er3+ ions. The ion–ion interaction, i.e. energy transfer can efficiently occur if the distance between ions becomes small enough. In terms of Dexter’s calculation, the dependence of energy transfer probability (WET ) on interionic distance was described as [20] WET =
Const 2 n4 Rk
fD fA
gD ()gA () d
(2)
Table 3 The calculated lifetimes exp of 4 S3/2 level for S0 and S0-x (x = 1–4). Sample
S0
S0-1
S0-2
S0-3
S0-4
S0-5
exp (s)
41.0
38.2
38.4
34.2
30.4
29.0
Fig. 5. Upconversion luminescence spectra of the Er3+ in S0 and S0-x (x = 1–4).
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As can be observed, the upconversion luminescence intensity increased with increasing heat treatment time. It is well known that the upconversion luminescence of rare earth ions is usually baffled by the multi-phonon relaxation. The multi-phonon relaxation of excited rare earth ions through multi-phonon processes depends exponentially on E/¯hωP , where E is the energy gap from the next level below and h ¯ ωP is the energy of the maximum lattice vibration of the surrounding host lattice [22,23]. The smaller the lattice vibration energy of the host is, the lower the multi-phonon relaxation probability is [24]. The approximate frequencies of the highest energy lattice vibration in silicate oxide glass are 1100 cm−1 and that of PbF2 crystal is 258 cm−1 [25]. As it was revealed by the absorption spectra and the fluorescence decay curves that the Er3+ ions enter into PbF2 crystalline phase gradually with increasing heat treatment time. Therefore, the upconversion intensity increases significantly with the increase of heat treatment time. 4. Conclusions The changes of Er3+ ions surrounding environment with increasing heat treatment time were investigated in 50SiO2 –5PbO–45PbF2 system. As it was confirmed by the absorption spectra and luminescence decays studies, the Er3+ ions not only act as nucleating agents for the precipitation of crystalline fluoride phase during nucleation process, but also can enter into nanocrystals gradually during crystals growth process. More Er3+ ions in the glass matrix were incorporated into crystalline phases after heat treatment and hence greatly increase the upconversion luminescence intensity. Acknowledgements This work was supported by the National Scientific Foundation of China (Grant No. 50502031) and the Open Project Program of
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