Indirect energy transfer of Ce3+ → Eu2+ in CaAl12O19 phosphor

Solid State Communications,Vol. 102, No. 7, pp. 555-559, 1997 8 1997 Ekvier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/97 $17.00+.00

Pergamon

PII: Soo38-1098(!n)ooo36-7

INDIRECT

ENERGY TRANSFER

OF Ce3+ -

Eu2+ IN CaAl ,*O ,s PHOSPHOR

H.S. Jeon,” SK. Kim,LI S.C. Kim,b S.H. Park,b H.L. Parkbs* and S.-I. Mho’ “Department of Physics, Ewha Women’s University, Seoul 120-750, Korea department of Physics, Yonsei University, Seoul 120-749, Korea ‘Department of Chemistry, Ajou University, Suwon 442-749, Korea (Received

1 November

1996 by C.N.R. Rae)

Photoluminescence characteristics and the energy transfer between Ce3+ and Eu*+ in CaA112019 host lattice have been investigated. A series of concentrations of Ce 3’ ion with a fixed Eu2+ concentration in doubly doped CaAl ,*O ,s : Ce3+Eu2+ have been studied. The indirect energy transfer has been observed between Ce3+ and Eu*+ through the Ce3+-OMe complex in CaAl 12019 host lattice. The two nonradiative relaxation processes from the excited levels of Ce3+-OMe complex; one, to the lower emitting level of Ce3+-OMe complex themselves and the other, to the Eu*+ site are competing processes. 0 1997 Elsevier Science Ltd

2. EXPERIMENTAL

1. INTRODUCTION Energy transfer from donor to acceptor plays an important role in luminescence and solid state lasers. The luminescent properties of &Al12019 : Ce3+ and CaAl 120 is : Eu *+ have been studied for blue emitting phosphor applications [ 1,2]. The lowest absorption level of Eu*+ is located around 345 nm in CaA112019 : Eu*+ [3] and broad band emission of Ce3+ is peaking around 325 nm in CaAl 12019 : Ce3+ [l]. The energy transfer from Ce3+ to Eu2+ in CaF2 has been reported, where the emission wavelengths of Ce3+ are 325 nm and 345 nm and the absorption wavelength of Eu*+ is 340 nm [4]. There also exists a good overlap of the band in Ce3+ emission band and Eu*+ absorption CaAl120i9. One can thus anticipate the possibility of energy transfer from Ce3+ to Et?+ in CaAli201s host lattice when Ce3+ and Eu*+ ions are codoped. However, no previous studies on the energy transfer rom Ce3+ to Eu2+ in doubly doped CaAl120 1s : Ce 3+, Eu*+ has been made. In this paper, we present the evidence of indirect energy transfer from Ce3+ to Eu2+ in doubly doped CaAl ,*A1,s, through the first process of energy transfer from Ce3+ to Ce3+-OMe complex.

* To whom correspondence

The CaA112019, CaAl12019 : Ce3+ and CaAli2019 : Ce3+,Eu2+ have been synthesized through combustion synthesis process [5]. The starting materials are metal nitrates, i.e. Ca(N03)2-4H20, A1(N03)3.9H20, Ce(N03)3-4H20, and Eu(NO~)~-~H~O. 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. Urea works as a trigger in combustion process. Once the nitrate solution is heated to 55O”C, the sample temperature reaches at 1600°C within 30 s, due to the combustion process triggered by urea. The obtained phases have been checked through XRD technique. The dominant valence state of europium is Eu2+ in CaAl ,*O19. A trace of Eu3+ is also observed. The 254 nm line of Hg lamp has been used as an excitation source for the photoluminescence measurements. The photoluminescence spectra have been obtained at room temperature. In addition, we have measured the luminescence lifetime by employing 266 nm short pulse of Nd : YAG laser and a 500 MHz digital oscilloscope. 3. RESULTS

AND DISCUSSION

The XRD pattern of CaAl120 1s is depicted in Fig. 1. A single phase, CaA1120 is is obtained and its XRD tingetprint matches with the JCPDS file (Joint Committee

should be addressed. 555

556

ENERGY TRANSFER

30

35

40

OF Ce3+ -

45

50 2

8

Vol. 102, No. 7

Et?+ IN CaAlr20 is

55

60

65

70

wa

Fig. 1. XRD pattern of CaA112019. on Powder Diffraction Standards, No. 25-125). Photoluminescence spectra of Ce3+ doped CaAlr201s are shown in Fig. 2. An emission band centered at about 325 nm is observed, which is well explained in terms of the dipole allowed 5d - 4f transition. Also, the evolution of very broad emission band, which looks

‘..,

::

..

,..-.. ,_.’

pi

;”

‘\

. ... -..._ x = 0.02 m “.., ‘.\ . . t.-\.. 0.005 lr ---..-_____.._.. -.-___ x =_.._. - ..___ .A-...-

. . . . . .._.

__

. . . .

. .

. ..__..

I

1

I

I

300

350

400

450

Wavelength (nm) Fig. 2. Photoluminescence

spectra of CaAl1~Ors : Ce:+.

like a background, on the low energy side of 325 nm Ce3+ emission is observed when the concentration of Ce3+ in CaAlr2019 is increased. This low energy broad emission band is shifted toward the lower energy with increasing Ce3+ concentration. The origin of this peak can be explained in terms of the structure of CaAlr2Ors. The structure of CaAl r201s was first reported in 1938 [6]. Kato and Saalfeld [7] refined the crystal structure of CaAl r2019 and confirmed its isotypism with PbFer201s which has the magnetoplumbite structure. The magnetoplumbite structure can be viewed as two spine1 blocks separated by a larger containing large cation, one small cation and three oxygens, along the c-axis. In the case of CaAl ,~O,Q, the large cation is calcium and the small cation is aluminium. Thus the sequence along the c-axis is represented as A-C-B, where the A and B are the spine1 blocks. The C represents the layer containing cations and oxygens. When the large cation substitutes at high concentrations for the calcium in CaAl i20 rs, the resulting structure is distorted and a part of the substituted site is replaced with neighboring oxygens forming OMe [ 1,8]. The OMe stands for the oxygen in the metal site. Thus, the Ce3+-OMe complex is also formed. This has been observed in (La0,9sCe0,02)0.s~l I1.90019.4 [l]. In addition, the emitting Ce3+ ions are known to be able to transfer effectively their excitation energy to Ce3+-OHe complex [l]. Hence, the emission of Ce’+OMe complex is the origin of low energy broad emission band in Fig. 2. The photoluminescence spectra of doubly doped CaAlr20 19: CeZ+,Eu?j&s are shown in Fig. 3. The emission

ENERGY TRANSFER

Vol. 102, No. I

: :

OF Ce3+ -

Eu*+ IN &Al120

rQ

-. CaAl,20,9 : Ce:+ + Eu*+~,,-,~~,,, ’ \ ‘, k, ‘, #’ .

300

350

400

500

450

550

LifeTime (ns)

Wavelength (nm) Fig. 3. Photoluminescence Ce3+ 0.005. X 7Eu*+

spectra

of

CaA11201Q

band of Eu*+ peaking at 425 nm is in the same energy range as the low energy broad emission band of Ce3+OMe complex. The intensity of 425 nm Eu*+ emission band is markedly enhanced when the Ce3+ concentration is increased in doubly doped CaAl I*0 ,Q: Ce~+,Eu,&s, where the concentration of Eu*+ is fixed. It may be misleading to draw a conclusion about energy transfer only based on the concentration dependence luminescence of intensity, especially when the energy transfer seems to be efficient as seen in Fig. 3. The energy transfer between Ce3+ and Eu2+ in CaAl I*0 19: Cei+,Eu*+o.oo5has been monitored by measuring the luminescence decay transients. The lifetimes of 325 nm emission band of Ce3+ at various concentrations in CaAlr201Q : Ce:+,Eu&s are measured and shown in Fig. 4. One can clearly see that the lifetime of Ce3+ site is independent of Ce3+ concentration in the case of fixed concentration of Eu*+ in codoped CaAl r20 IQphosphor. The luminescence decay transients of 325 nm emission of Ce3+ are fitted with a single exponential, Z(t) = IO e - ‘lr,

(1)

where IO is a constant and the r is the lifetime of Ce3+ in CaA11201Q : Ce~+,Eu&,s. The lifetimes of Ce 3+ for various concentrations of Ce3+ in codoped CaAl I*0 19: Ce 3+ Eu *+ are tabulated in Table 1. The lifetime of em&ion at 325 nm of Ce3+ is about 12 ns. The observed lifetime of Ce3+ in CaAl ,201Q lies in the reasonable

Fig. 4. Concentration dependent CaAl I2019 : Ce:+,Eug+&,s.

lifetime

of Ce3+ in

range, considering that the reported lifetime of 350 nm emission of Ce3+ in (~~0.98~~0.02)0.86~~11.90~19.4 is 20 ns [l] and that the lifetime of 362 nm emission of Ce3+ in YA103 is 19 ns [9]. It is well documented that the decay pattern of the donor ions should be affected by the presence of acceptor ions [lo]. Our spectroscopic data revealed, however, that it is not the case for CaAl,20 ,Q: Cez+,Et$& This fact may suggest that the lifetime of donor Ce3+ sites is determined mainly by the acceptor Ce3+-OMe complex when Ca2+ is substituted by Ce3+ and that the acceptor Eu 2+ ion which also substitutes the Ca*+ site does not affect the lifetime of the donor sites. The donor Ce3+ sites are distributed in a way that is related to the Ce3+-OMe complex in the lattice of CaAl 120 19. In addition, unlike the non-exponential decay which is usually observed for donor-acceptor energy transfer occurs, a single exponential decay of 2+ Ce3+ in CaAl r201Q : Cej+,Euo.oos supports that the energy transfer from Ce 3+ to Eu *+ is an indirect one through the

Table 1. Lifetimes of 325 nm emission CaAl1201Q : Ce:+,Eu&,5 at 300 K Concentration 0.005 mol 0.02 mol 0.04 mol 0.06 mol

of Ce 3+

of Ce3+ in

Lifetime, 11.96 12.03 12.44 12.89

2 + + +

7 0.19 0.18 0.08 0.14

ns ns ns ns

558 1

:\ i\ : + , R ENERGY TRANSFER

CaA1,2019

i b q&i \ i $L, b

Cex3+

4,5.. ‘o-q i

On

Eu~+~,,,~~

-

Q

.

x:

00&O

OF Ce3+ -

ooom

--____

.. . . . . . . ..

004m 002m

_.-.-.-._.

o@J5m

Eu*+ IN CaAl~0

Vol. 102, No. 7

I9

emission at 425 nm of CaAl ,2019 : Cei+,Eui$-,s are shown in Fig. 5. The emission band maximum of Eu2+ at 425 nm is superimposed on the broad emission band of Ce3+-OMe complex. The decay transients of 425 nm emission show two competing relaxation processes from the excited Ce3+-OMe complex whose energy is transferred from the donor Ce3+ ion. The solid curves are from the least-square fit of the data with the sum of two exponential decay, I(t) = (y c - “‘o + fl e - “Q

0

100

200

300

400

500

600

Lifetime (ns) Fig. 5. Ce3+ concentration CaAl 12O,9 : CeZ+Eui&s.

dependent

lifetime of Eu2+ in

Ce3+-0 Me complex in CaAl ,*019 host lattice, i.e. Ce3+ - Ce3+-OMe complex - Eu*‘. The excited Ce3+ sites partially emit their energy radiatively and partially transfer the energy effectively to the Ce3+-OMe complex which is followed by successive transfer to the Eu2+ sites. The photoluminescence decay transients of the

(2)

where cy and 0 are the fractions of the nonradiative relaxation from the excited level of the Ce3+-OMe complex to the lower emitting level of Ce3+-OMe complexes and of the energy transfer from Ce3+-OM, complex to Eu2+ site, respectively and sum of them is unity. The 7, is the lifetime of Ce3+-OMe complex and 70 is the lifetime of Eu2+ in CaAl12019 : CeF,Eu$&. The lifetimes of Ce3+ -OMe complex and Eu2+ sites are presented in Table 2. Both of the lifetimes of Ce3+-OMe complex and Eu 2+ sites are lengthened as the concentration of Ce3+ in CaAl I20 19: Cel+,E&& increases when Ce3+ concentration is lower than that of Eu’+, while both of the lifetimes of Ce3+-OMylecomplex and Eu2+ sites are shortened when Ce3+ concentration is higher than that of Eu2+. This anomalous phenomenon cannot be explained by normal energy transfer processes and further studies are necessary. Another trend to note is that the fraction (0) of the energy transfer from Ce3+-OMe complex to Eu2+ sites increases while that ((II) of the nonradiative relaxation to the lower emitting level of Ce3+-OMvle complexes decreases up to 0.04 mol of Ce3+ doping, which is lower than that of EL?+ concentration. This behavior can be explained as follows: as the concentration

125nm

EL? Fig. 6. Schematic

diagram of the indirect energy transfer in C~UU,~O,~ : Cel+,Eu&.

Vol. 102, No. 7

ENERGY TRANSFER OF Ce3+ - Eu2+ IN CaAl iZO19

5.59

Table 2. Lifetimes of 425 nm emission of Eu2+ and Ce3+-OM, complexes in CaAl ,zO,~ : Ce:+,Eu$& at 300 K Concentration of Ce3+

Lifetime, 7,

ar

0.005 mol 0.02 mol 0.04 mol 0.06 mol

12.53 ? 0.0464 ns 28.6 + 0.966 ns 41.5 + 2.589 ns 35.6 + 0.774 ns

0.79 0.65 0.52 0.59

? + + ?

0.016 0.012 0.016 0.006

Lifetime, r.

P

440.8 600.6 677.7 568.3

0.21 0.35 0.48 0.41

+ + 2 2

12.683 ns 9.775 ns 13.663 ns 5.117 ns

Ifr0.0045 + 0.0045 + 0.0085 * 0.0033

of dopant Ce3+ increases in the CaAlr20 19host lattice, Jeon, S.K. Kim) were also supported in part by the the concentration of Ce3+-OMvle complex also increases KOSEF, the Center for Theoretical Physics (SNU) and which makes the indirect energy transfer from Ce3+-OMe the Basic Science Research Institute Program, Ministry complex to Eu*+ sites more efficient. When the concen- of Education Project No. BSRI-96-2427. tration of Ce3+ ions exceeds that of EL?’ ions in CaA112019: Cel+,Eu$&, the excess concentration of REFERENCES Ce3+-OMe complexes compared to that of the acceptor Eu2+ causes the increase of the fraction (Q) of the 1. Stevels, A.L.N., J. Electrochem. Sot.: Solid-State relaxation rate to the lower emitting level of Ce3+-OMe Science and Technology, 125, 1978, 588. 2. Verstegen, J.M.P.J. and Stevels, A.L.N., complexes, which is clearly seen in Table 2. J. Luminesc., 9, 1974, 406. In summary, we report the first observation of the 3. Stevels, A.L.N. and Schrama de Pauw, A.D.M., indirect energy transfer from Ce3+ to Eu*+ through J. Electrochem. Sot.: Solid-State Science and Ce3+-OMe complex in CaAl I*0 19: Ce?,Eui&s phosTechnology, 123, 1976, 691. phor. This process is based upon the formation of 4. Caldino, G.V., Delacruz, C., Muhoz, H.B. and Ce3+-OMe complex close to Ce3+ site in this host lattice Rubio, J., Solid State Commun., 69, 1990, 347. and on the fact the energy transfer from Ce3+ to this 5. Kingsley, J.J., Manickam, N. and Patil, K.C., Bull. Mat. Sci., 13, 1990, 179. Ce3+-0Me complex is very efficient. The nonradiative relaxation to the lower emitting level of Ce3+-OMMe 6. Adelskjold, V., Arkiv Kemi Mineral Geol, 124, 1938, 1. complexes themselves and the energy transfer from 7. Kato, K. and Saalfeld, Neues Jahrb Mineral Ce3+-OMe complex to Eu*+ sites are competing proAbhard, 109, 1968, 192. cesses, which can be clearly seen from the luminescence 8. Stevels, A.L.N., J. Luminesc., 20, 1979, 99. transients. 9. Lyo, Li-Ji and Hamilton, D.S., J. Luminesc., M&49,

work was supported in part by Korea Research Foundation in 1996. Two of us (H.S.

Acknowledgements-This

1991, 251.

10. Caldino, V.G., Munoz, A.F. and Rubio, J.O., J. Phys.: Condens. Mutt., 5, 1993, 2195.