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Observation of Two Independent Energy Transfer Mechanisms in BaAl12O19: Ce3+0.06+Eu2+x phosphor
PERGAMON
Solid State Communications 120 (2001) 221±225
www.elsevier.com/locate/ssc
Observation of Two Independent Energy Transfer Mechanisms 31 in BaAl12O19: Ce0.06 1 Eux21 phosphor H.S. Jeon a,*, S.K. Kim b, H.L. Park c, G.C. Kim a, J.H. Bang c, M. Lee c a
School of Liberal Arts, Korea University of Technology and Education, Cheonan, 330-708, South Korea b Department of Physics, Ewha Women's University, Seoul 120-750, South Korea c Department of Physics, Yonsei University, Seoul 120-749, South Korea Received 28 June 2001; accepted 20 July 2001 by C.N.R. Rao
Abstract Two independent energy transfer mechanisms through electric multipole interactions between donor and acceptor ions have 31 been found in BaAl12O19 : Ce0.06 1 Eux21 phosphor. The ®rst energy transfer takes place between the Ce 31 ion and the Eu 21 ion which strongly depends on the concentration of Eu 21 ions. The second energy transfer occurs between the Ce 31 ion and the Ce 31-OMe complexes. Both energy transfer mechanisms were explained in terms of combined electric multipole interactions. q 2001 Published by Elsevier Science Ltd. PACS: 78.47.1p; 78.60.Hk Keywords: B. Chemical synthesis; E. Luminescence; E. Time-resolved optical spectroscopies
1. Introduction Energy transfer processes between donors and acceptors have wide applications to understand the physical or chemical characteristics in nature like in crystals [1±3], in molecular systems [4±6], in liquid crystals [7], furthermore it is a very useful mechanism in developing tuneable lasers [8] and a solar energy transfer [9] device and so on. Especially it contributes relevantly to the developing of the recent display industry. Energy transfers can be subdivided into radiative and non radiative energy transfer. For radiative transfers the pattern of donor ¯uorescence depends on the acceptor concentration, but the donor lifetime does not change with acceptor concentration.[10]. The nonradiative energy transfer arises through multipolar or exchange interactions. It changes the ¯uorescence lifetime of donor ions. The theory of nonradiative energy transfer has been discussed in detail by several authors like Forster, Dexter, Huber and others at al. [10±12] This research has examined non radiative energy transfers
* Corresponding author. E-mail address: [email protected] (H.S. Jeon).
31 between Ce 31 and Eu 21 ions in BaAl12O19 : Ce0.06 1 Eux21 31 phosphor. The observed emission band of Ce was 350 nm and the emission band of Eu 21 was 435 nm. Therefore the energy transfer from Ce 31 ion to the Eu 21 ion could occur. The time resolved ¯ourescence spectroscopy technique was used to investigate this phenomenum. The lifetime of the Ce 31 ion was reduced drastically according to the concentration of the Eu 21 ion. The measured data showed two kinds of decay patterns, one due to the energy transfer from Ce 31 to Eu 21 the other is from Ce 31 to Ce 31-OMe complexes created in the magnetoplumbite structural host materials. [13,14] The magnetoplumbite structure material (Ca, Sr, Ba, CeMg)Al12O19 has mirror planes which contain a cation (Ca 21, Sr 21, Ba 21). When the larger cation, Ce 31 substitutes at high concentrations for the original cation (Ca 21, Sr 21, Ba 21), the resulting structure is distorted and a part of the substituted site is replaced with oxygens forming OMe. The OMe stands for the oxygen in the metal sites. Consequently the Ce 31-OMe complex can perturbe the original Ce 31 energy level 5d ! ( 2F5/2, 2F7/2). which has been discussed previously. [15,16] This report discussed a new combined two independent energy transfer mechanism between donor and acceptor in 31 the BaAl12O19 : Ce0.06 1 Eux21 phosphor. One is described by
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00323-4
222
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225
2. Theoretical Background
BaAl12O19 : Ce0.06 + Eux
The nonradiative energy transfer process between donor and acceptor through the Coulomb interaction can be summarized as follows. [17,18] If only a donor-acceptor transfer exists, the donor decay rate is 3n 2
CL-Intensity (arb. units)
X=0.1
hP ti e
1
Where C is a parameter which contains the acceptor concentration. The exponent n shows the multiplier type of the Coulomb interaction between donor and acceptor (n 6; dipole±dipole, n 8; dipole±quadrupole, n 10; quadrupole±quadrupole´ ´´). t0 is the ¯uorescence lifetime of donor ions which can be changed in the nonradiative energy transfer processes.
X=0.05
X=0.01 X=0.005
3. Experimental details 31 1 Eux21 phosphors were synThe BaAl12O19: Ce0.06 thesized through the combustion method [19]. The starting materials were metal nitrates, i.e, Ba(NO3)3, Al(NO3)3´9H2O, Ce(NO3)3´4H2O, and Eu (NO3)3´6H2O. These metal nitrates have been dissolved in water and made into a nitrate solution. An appropriate amount of urea has been added to the nitrate solution to act as a fuel in the combustion process. Once the nitrate solution is heated to 5508C, the sample temperature reaches over 16008C within 30 sec due to the combustion process triggered by urea. The phases of the obtained powder samples have been checked through the XRD technique. The luminescence character of the sample materials were investigated by the home made CL-spectrometer with an
X=0
300
1 t t0 2C´ t0
350
400
450
500
550
600
Wavelength (nm) 31 Fig. 1. CL Spectra of BaAl12O19: Ce0.06 1 Eux21
the Ce 31 ! Eu 21 transfer and the other is Ce 31 ! Ce 31OMe transfer. Both processes take place via mixed electric multipole interactions. The results were veri®ed by the lifetime measurement of donor ions.
100000
3+
BaAl12O19 : Ce
2+
0.06
10000
+ Eu
X
fit by eq. (1) X=0
Intensity (arb.units)
1000
simple exp.
100 10
X=0.01
1 X=0.005
0.1 0.01
X=0.05
1E-3 1E-4
: measured : fitted
X=0.1
1E-5 0
20
40
60
80
100
Time (ns) 31 Fig. 2. Decay patterns and Fitting results of BaAl12O19: Ce0.06 1 Eux21 at the 350 nm emission band. (Solid line: measured; Dotted line: ®tted by simple exponential function; Dashed line: x 0, ®tted by Eq. (1), x 0.005±0.1, ®tted by Eq. (2))
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225
223
Table 1 31 1 Eux21 at 350 nm excitation. The lifetime and the parameter values of BaAl12O19: Ce0.06 Eu 21 concentration (x)
a
b
t1(ns)
t2(ns)
C1
C2
0 0.005 0.01 0.05 0.1
0 0.894 0.765 0.997 0.999
1 0.106 0.235 0.003 0.001
15.9 5.16 0.80 0.73
62.5 63.6 121 440 646
19.1 9.18 3.71 3.52
1.25 3.96 6.01 7.06 6.03
excitation voltage 10 kV. The pressure of the vacuum chamber was kept at 10 26 Torr. The lifetime of the donor excitation has been measured with the femtosecond laser system. The Nd:YVO 4 laser was used as a pump laser, the Ti: Sapphire laser was used as a femtosecond laser. The original wavelength of 760 nm has been tripled to 254 nm by the doubler and the tripler pulse selectors. The gained signals were analyzed in the TCSPC-system (Time Correlated Single Photon Counting System). The ®nal probability of the excitation was attained as a function of time.
4. Results and Discussion 31 The Cathode Luminescence Spectra of BaAl12O19: Ce0.06 21 1 Eux (x 0±0.1) are shown in Fig. 1. The peak position
cm
back transfer
40 X 10
3
-1
of Ce 31 in the BaAl12O19 host is at 350 nm and Eu 21 is at 435 nm. The decreasing intensity of Ce 31 ion emission band depending on the Eu 21 concentration signi®es the energy transfer possibility between Ce 31 and Eu 21. The overall decay patterns of the 350 nm emission band of Ce 31 ions 31 in BaAl12O19: Ce0.06 1 Eux21 are illustrated in Fig. 2. In general the time evolution of donor decay forms the non exponential curve when there exists energy transfer. While it establishes an simple exponential behaviour in the absence of any energy transfer. In Fig. 2 all four decay curves of these phosphors doped with acceptors (x 0.005±0.1 cases) show the non exponential curve which contain two parts, the transient part and the delayed part. However the ®rst decay curve(x 0 case) does not show the exact exponential shape although no acceptor was doped. Therefore the distinct shape of this curve was examined by
5d 6
4f 5d
(3)
35 30
(1) 3+
3+
25
Ce
Ce
(2)
15
2+
3+
Eu
Ce - OMe
20
10 5 2
0
2
3+
Ce - OMe
F7/2
2
F7/2
F5/2
2
F5/2
3+
Ce
8
S7/2
2+
Eu
31 Fig. 3. Schematic diagram of energy transfers in BaAl12O19: Ce0.06 1 Eux21. It shows (1)Ce 31 ! Eu 21 transfer, (2)Ce 31 ! Ce 31-OMe transfer, (3)Ce 31-OMe ! Ce 31 back transfer.
224
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225
the ®tting processes. The dotted and dashed lines of the ®rst decay curve in Fig. 2 show two different ®tting results. The dotted line is ®tted by the simple exponential function and the dashed line is ®tted by Eq. (1). This proves strongly there 31 exists an energy transfer in BaAl12O19: Ce0.06 even though external acceptor ions are absent. Also it implies the existence of substitute acceptors. The origin of this energy transfer can be explained due to the crystal structure of BaAl12O19 which has the magnetoplumbite structure. It has already been said that Ce 31-OMe complexes are formed since a part of the substitute site of the Ba 21 can also be replaced with oxygens when the larger cation Ce 31 was doped in BaAl12O19. Therefore the good ®tting result in Fig. 2 by Eq. (1) indicates the energy transfer between Ce 31 and Ce 31-OMe complexes through the electric dipole±dipole interaction. In which Ce 31-OMe complexes can play a role of substitute acceptors. Since the rest four curves (x 0.005± 0.1) in Fig. 2 have shown two extremely separate non exponential decay curves, it is worth explaining the illustrated energy transfers in two different realms independently. Firstly the transient part manifests the energy transfer between Ce 31 and Eu 21, secondly the delayed part reveals the energy transfer between Ce 31 and Ce 31-OMe complexes. The obtained ®tting curves in each part were estimated from three different kinds of Coulomb interaction in Eq. (1) i.e. electric dipole±dipole interaction (n 6), electric dipole±quadrupole interaction (n 8), electric quadrupole±quadrupole interaction (n 10). Therefore the methods of ®nding the overall ®tting results have nine possibilities and the ®tting function can be written as follows hP ti ae
2
n3
1 t t1 2C1 ´ t1
1
1 be
n3
1 t 2 t 2C2 ´ t 2 2
2
2
with the constraint a 1 b 1. Where P t is the total energy transfer probability of donor ion when the Ce 31 ion was excited at 350 nm. a, b presents the energy transfer rates of Ce 31 ! Eu 21 and Ce 31 ! Ce 31-OMe complexes respectively. Also t1, t2 means the ¯uorescence lifetimes and C1, C2 means the C parameter values in Eq. (1) in each case. The nine possible energy transfer processes can be obtained from (n1 6, 8, 10) £ (n2 6, 8, 10). The solid lines in Fig. 2 are the measured decay curves which are rearranged on the y-axis ± the intensities of each curve are shifted on the y-axis to see the ®tting results better ± and the dashed lines are the ®tting results with Eq. (1) and Eq. (2). All the possible combinations of n1, n2 values were tested. As a result n1 6, n2 8 case has given the best ®t. It means that the dominant energy transfer between Ce 31 and Eu 21 is due to the electric dipole±dipole interaction and the dominant energy transfer between Ce 31 and Ce 31-OMe occurs via electric dipole±quadrupole interaction. The obtained parameter values are exhibited in Table 1. In most of the cases the a value is increasing and b is decreasing, which means that the energy transfer rate of Ce 31 ! Eu 21 is increasing according to the Eu 21 concen-
tration. On the contrary the transfer rate of Ce 31 ! Ce 31OMe is decreasing. The decreasing t1 shows the clear evidence of the growing rate of the energy transfer Ce 31 ! Eu 21. However t2 is increasing which means the energy transfer rate of Ce 31 ! Ce 31-OMe is reduced. This interesting behaviour can be explained by the back energy transfer process. Since the energy level of an Ce 31-OMe complex has a very broad band shape in Fig. 3 there exist many possibilities of back transfer from Ce 31-OMe complexes to donor ions. Lastly the general tendency of the two parameter values C1, C2 have not yet been understood fully. Therefore futher studies with other energy transfer models are needed. 5. Conclusion 31 1 Eux21 phosphor includes two The BaAl12O19: Ce0.06 kinds of energy transfer processes. Each process occurs independently by the Coulomb interaction. The transient part of the process is explained by the normal energy transfer between Ce 31 and Eu 21 through electric multipole interaction. The delayed part of the process is created from the substitute acceptor named Ce 31-OMe complexes, which are formed in BaAl12O19 phosphor. Therefore the delayed process means the Ce 31 ! Ce 31-OMe energy transfer. In this case the back energy transfer from the Ce 31-OMe complex to Ce 31 contributes to increase the donor lifetime.
Acknowledgements This work was supported by the Korea Science and Engineering Foundation. (Contract No. 2000-1-114-004-3) References [1] J.R. Salcedo, A.E. Siegman, D.D. Dlott, M.D. Fayer, Phys. Rev. Lett. (1978) 41131. [2] R.D. Wieting, M.D. Fayer, D.D. Dlott, J. Chem. Phys. 69 (1978) 2752. [3] S.K. Lyo, T. Holstein, R. Orbach, Phys. Rev. B18 (1978) 1637. [4] D.M. Burland, V. Konzelmann, R.M. Macfarlane, J. Chem. Phys. (1977) 671926. [5] V.M. Kenkre, R.S. Knox, Phys. Rev. B9 (1974) 5279. [6] M. Grover, R. Silbey, J. Chem. Phys. (1970) 522099. [7] J. Klafter, J. Jortner, Chem. Phys. Lett. (1977) 49410. [8] P.Y. Lu, Z.X. Xu, R.R. Alfano, J.I. Gerster, Phys. Soc. (1989) 2297. [9] W.H. Weber, J. Lambe, Appl. Opt. (1976) 152299. [10] Xingren Liu, Xiaojun Wang, Zhongkai Wang, Phys. Rev. B 39 (15) (1989) 10633. [11] Th. Forster, Ann. Physik (1948) 255. [12] D.L. Dexter, J. Chem. Phys. (1953) 21836. [13] A.L.N. Stevels, J. Electrochem. Soc. (1978) 125588. [14] A.L.N. Stevels, J. Luminesc. (1979) 2099.
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225 [15] H.S. Jeon, S.K. Kim, S.C. Kim, S.H. Park, H.L. Park, S.I. Mho, Solid State Communications 102 (7) (1997) 555. [16] H. S. Jeon, Ph. D Thesis, 1999, p25. [17] B. Henderson, G.F. Imbusch, Optical Spectroscopy of Inorganic Solids, Clarendon, Oxford, 1989, p. 445.
225
[18] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer (1994) 91. [19] J.J. Kingsley, N. Manickam, K.C. Patil, Bull. Mater. Sci 13 (3) (1990) 179.
Solid State Communications 120 (2001) 221±225
www.elsevier.com/locate/ssc
Observation of Two Independent Energy Transfer Mechanisms 31 in BaAl12O19: Ce0.06 1 Eux21 phosphor H.S. Jeon a,*, S.K. Kim b, H.L. Park c, G.C. Kim a, J.H. Bang c, M. Lee c a
School of Liberal Arts, Korea University of Technology and Education, Cheonan, 330-708, South Korea b Department of Physics, Ewha Women's University, Seoul 120-750, South Korea c Department of Physics, Yonsei University, Seoul 120-749, South Korea Received 28 June 2001; accepted 20 July 2001 by C.N.R. Rao
Abstract Two independent energy transfer mechanisms through electric multipole interactions between donor and acceptor ions have 31 been found in BaAl12O19 : Ce0.06 1 Eux21 phosphor. The ®rst energy transfer takes place between the Ce 31 ion and the Eu 21 ion which strongly depends on the concentration of Eu 21 ions. The second energy transfer occurs between the Ce 31 ion and the Ce 31-OMe complexes. Both energy transfer mechanisms were explained in terms of combined electric multipole interactions. q 2001 Published by Elsevier Science Ltd. PACS: 78.47.1p; 78.60.Hk Keywords: B. Chemical synthesis; E. Luminescence; E. Time-resolved optical spectroscopies
1. Introduction Energy transfer processes between donors and acceptors have wide applications to understand the physical or chemical characteristics in nature like in crystals [1±3], in molecular systems [4±6], in liquid crystals [7], furthermore it is a very useful mechanism in developing tuneable lasers [8] and a solar energy transfer [9] device and so on. Especially it contributes relevantly to the developing of the recent display industry. Energy transfers can be subdivided into radiative and non radiative energy transfer. For radiative transfers the pattern of donor ¯uorescence depends on the acceptor concentration, but the donor lifetime does not change with acceptor concentration.[10]. The nonradiative energy transfer arises through multipolar or exchange interactions. It changes the ¯uorescence lifetime of donor ions. The theory of nonradiative energy transfer has been discussed in detail by several authors like Forster, Dexter, Huber and others at al. [10±12] This research has examined non radiative energy transfers
* Corresponding author. E-mail address: [email protected] (H.S. Jeon).
31 between Ce 31 and Eu 21 ions in BaAl12O19 : Ce0.06 1 Eux21 31 phosphor. The observed emission band of Ce was 350 nm and the emission band of Eu 21 was 435 nm. Therefore the energy transfer from Ce 31 ion to the Eu 21 ion could occur. The time resolved ¯ourescence spectroscopy technique was used to investigate this phenomenum. The lifetime of the Ce 31 ion was reduced drastically according to the concentration of the Eu 21 ion. The measured data showed two kinds of decay patterns, one due to the energy transfer from Ce 31 to Eu 21 the other is from Ce 31 to Ce 31-OMe complexes created in the magnetoplumbite structural host materials. [13,14] The magnetoplumbite structure material (Ca, Sr, Ba, CeMg)Al12O19 has mirror planes which contain a cation (Ca 21, Sr 21, Ba 21). When the larger cation, Ce 31 substitutes at high concentrations for the original cation (Ca 21, Sr 21, Ba 21), the resulting structure is distorted and a part of the substituted site is replaced with oxygens forming OMe. The OMe stands for the oxygen in the metal sites. Consequently the Ce 31-OMe complex can perturbe the original Ce 31 energy level 5d ! ( 2F5/2, 2F7/2). which has been discussed previously. [15,16] This report discussed a new combined two independent energy transfer mechanism between donor and acceptor in 31 the BaAl12O19 : Ce0.06 1 Eux21 phosphor. One is described by
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00323-4
222
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225
2. Theoretical Background
BaAl12O19 : Ce0.06 + Eux
The nonradiative energy transfer process between donor and acceptor through the Coulomb interaction can be summarized as follows. [17,18] If only a donor-acceptor transfer exists, the donor decay rate is 3n 2
CL-Intensity (arb. units)
X=0.1
hP ti e
1
Where C is a parameter which contains the acceptor concentration. The exponent n shows the multiplier type of the Coulomb interaction between donor and acceptor (n 6; dipole±dipole, n 8; dipole±quadrupole, n 10; quadrupole±quadrupole´ ´´). t0 is the ¯uorescence lifetime of donor ions which can be changed in the nonradiative energy transfer processes.
X=0.05
X=0.01 X=0.005
3. Experimental details 31 1 Eux21 phosphors were synThe BaAl12O19: Ce0.06 thesized through the combustion method [19]. The starting materials were metal nitrates, i.e, Ba(NO3)3, Al(NO3)3´9H2O, Ce(NO3)3´4H2O, and Eu (NO3)3´6H2O. These metal nitrates have been dissolved in water and made into a nitrate solution. An appropriate amount of urea has been added to the nitrate solution to act as a fuel in the combustion process. Once the nitrate solution is heated to 5508C, the sample temperature reaches over 16008C within 30 sec due to the combustion process triggered by urea. The phases of the obtained powder samples have been checked through the XRD technique. The luminescence character of the sample materials were investigated by the home made CL-spectrometer with an
X=0
300
1 t t0 2C´ t0
350
400
450
500
550
600
Wavelength (nm) 31 Fig. 1. CL Spectra of BaAl12O19: Ce0.06 1 Eux21
the Ce 31 ! Eu 21 transfer and the other is Ce 31 ! Ce 31OMe transfer. Both processes take place via mixed electric multipole interactions. The results were veri®ed by the lifetime measurement of donor ions.
100000
3+
BaAl12O19 : Ce
2+
0.06
10000
+ Eu
X
fit by eq. (1) X=0
Intensity (arb.units)
1000
simple exp.
100 10
X=0.01
1 X=0.005
0.1 0.01
X=0.05
1E-3 1E-4
: measured : fitted
X=0.1
1E-5 0
20
40
60
80
100
Time (ns) 31 Fig. 2. Decay patterns and Fitting results of BaAl12O19: Ce0.06 1 Eux21 at the 350 nm emission band. (Solid line: measured; Dotted line: ®tted by simple exponential function; Dashed line: x 0, ®tted by Eq. (1), x 0.005±0.1, ®tted by Eq. (2))
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225
223
Table 1 31 1 Eux21 at 350 nm excitation. The lifetime and the parameter values of BaAl12O19: Ce0.06 Eu 21 concentration (x)
a
b
t1(ns)
t2(ns)
C1
C2
0 0.005 0.01 0.05 0.1
0 0.894 0.765 0.997 0.999
1 0.106 0.235 0.003 0.001
15.9 5.16 0.80 0.73
62.5 63.6 121 440 646
19.1 9.18 3.71 3.52
1.25 3.96 6.01 7.06 6.03
excitation voltage 10 kV. The pressure of the vacuum chamber was kept at 10 26 Torr. The lifetime of the donor excitation has been measured with the femtosecond laser system. The Nd:YVO 4 laser was used as a pump laser, the Ti: Sapphire laser was used as a femtosecond laser. The original wavelength of 760 nm has been tripled to 254 nm by the doubler and the tripler pulse selectors. The gained signals were analyzed in the TCSPC-system (Time Correlated Single Photon Counting System). The ®nal probability of the excitation was attained as a function of time.
4. Results and Discussion 31 The Cathode Luminescence Spectra of BaAl12O19: Ce0.06 21 1 Eux (x 0±0.1) are shown in Fig. 1. The peak position
cm
back transfer
40 X 10
3
-1
of Ce 31 in the BaAl12O19 host is at 350 nm and Eu 21 is at 435 nm. The decreasing intensity of Ce 31 ion emission band depending on the Eu 21 concentration signi®es the energy transfer possibility between Ce 31 and Eu 21. The overall decay patterns of the 350 nm emission band of Ce 31 ions 31 in BaAl12O19: Ce0.06 1 Eux21 are illustrated in Fig. 2. In general the time evolution of donor decay forms the non exponential curve when there exists energy transfer. While it establishes an simple exponential behaviour in the absence of any energy transfer. In Fig. 2 all four decay curves of these phosphors doped with acceptors (x 0.005±0.1 cases) show the non exponential curve which contain two parts, the transient part and the delayed part. However the ®rst decay curve(x 0 case) does not show the exact exponential shape although no acceptor was doped. Therefore the distinct shape of this curve was examined by
5d 6
4f 5d
(3)
35 30
(1) 3+
3+
25
Ce
Ce
(2)
15
2+
3+
Eu
Ce - OMe
20
10 5 2
0
2
3+
Ce - OMe
F7/2
2
F7/2
F5/2
2
F5/2
3+
Ce
8
S7/2
2+
Eu
31 Fig. 3. Schematic diagram of energy transfers in BaAl12O19: Ce0.06 1 Eux21. It shows (1)Ce 31 ! Eu 21 transfer, (2)Ce 31 ! Ce 31-OMe transfer, (3)Ce 31-OMe ! Ce 31 back transfer.
224
H.S. Jeon et al. / Solid State Communications 120 (2001) 221±225
the ®tting processes. The dotted and dashed lines of the ®rst decay curve in Fig. 2 show two different ®tting results. The dotted line is ®tted by the simple exponential function and the dashed line is ®tted by Eq. (1). This proves strongly there 31 exists an energy transfer in BaAl12O19: Ce0.06 even though external acceptor ions are absent. Also it implies the existence of substitute acceptors. The origin of this energy transfer can be explained due to the crystal structure of BaAl12O19 which has the magnetoplumbite structure. It has already been said that Ce 31-OMe complexes are formed since a part of the substitute site of the Ba 21 can also be replaced with oxygens when the larger cation Ce 31 was doped in BaAl12O19. Therefore the good ®tting result in Fig. 2 by Eq. (1) indicates the energy transfer between Ce 31 and Ce 31-OMe complexes through the electric dipole±dipole interaction. In which Ce 31-OMe complexes can play a role of substitute acceptors. Since the rest four curves (x 0.005± 0.1) in Fig. 2 have shown two extremely separate non exponential decay curves, it is worth explaining the illustrated energy transfers in two different realms independently. Firstly the transient part manifests the energy transfer between Ce 31 and Eu 21, secondly the delayed part reveals the energy transfer between Ce 31 and Ce 31-OMe complexes. The obtained ®tting curves in each part were estimated from three different kinds of Coulomb interaction in Eq. (1) i.e. electric dipole±dipole interaction (n 6), electric dipole±quadrupole interaction (n 8), electric quadrupole±quadrupole interaction (n 10). Therefore the methods of ®nding the overall ®tting results have nine possibilities and the ®tting function can be written as follows hP ti ae
2
n3
1 t t1 2C1 ´ t1
1
1 be
n3
1 t 2 t 2C2 ´ t 2 2
2
2
with the constraint a 1 b 1. Where P t is the total energy transfer probability of donor ion when the Ce 31 ion was excited at 350 nm. a, b presents the energy transfer rates of Ce 31 ! Eu 21 and Ce 31 ! Ce 31-OMe complexes respectively. Also t1, t2 means the ¯uorescence lifetimes and C1, C2 means the C parameter values in Eq. (1) in each case. The nine possible energy transfer processes can be obtained from (n1 6, 8, 10) £ (n2 6, 8, 10). The solid lines in Fig. 2 are the measured decay curves which are rearranged on the y-axis ± the intensities of each curve are shifted on the y-axis to see the ®tting results better ± and the dashed lines are the ®tting results with Eq. (1) and Eq. (2). All the possible combinations of n1, n2 values were tested. As a result n1 6, n2 8 case has given the best ®t. It means that the dominant energy transfer between Ce 31 and Eu 21 is due to the electric dipole±dipole interaction and the dominant energy transfer between Ce 31 and Ce 31-OMe occurs via electric dipole±quadrupole interaction. The obtained parameter values are exhibited in Table 1. In most of the cases the a value is increasing and b is decreasing, which means that the energy transfer rate of Ce 31 ! Eu 21 is increasing according to the Eu 21 concen-
tration. On the contrary the transfer rate of Ce 31 ! Ce 31OMe is decreasing. The decreasing t1 shows the clear evidence of the growing rate of the energy transfer Ce 31 ! Eu 21. However t2 is increasing which means the energy transfer rate of Ce 31 ! Ce 31-OMe is reduced. This interesting behaviour can be explained by the back energy transfer process. Since the energy level of an Ce 31-OMe complex has a very broad band shape in Fig. 3 there exist many possibilities of back transfer from Ce 31-OMe complexes to donor ions. Lastly the general tendency of the two parameter values C1, C2 have not yet been understood fully. Therefore futher studies with other energy transfer models are needed. 5. Conclusion 31 1 Eux21 phosphor includes two The BaAl12O19: Ce0.06 kinds of energy transfer processes. Each process occurs independently by the Coulomb interaction. The transient part of the process is explained by the normal energy transfer between Ce 31 and Eu 21 through electric multipole interaction. The delayed part of the process is created from the substitute acceptor named Ce 31-OMe complexes, which are formed in BaAl12O19 phosphor. Therefore the delayed process means the Ce 31 ! Ce 31-OMe energy transfer. In this case the back energy transfer from the Ce 31-OMe complex to Ce 31 contributes to increase the donor lifetime.
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