Energy transfer processes in yttrium oxide activated with europium

Journal of Luminescence 40&41 (1988) 651—652 North-Holland, Amsterdam

651

ENERGY TRANSFER PROCESSES IN YTTRIUM OXIDE ACTIVATED WITH EUROPIUM D.B.M. KLAASSEN, R.A.M. van HAM

and T.G,M. van RIJN

Philips Research Laboratories, P.O. Box 80000, 5600 JA Eindhoven, The Netherlands Energy transfer between excited states of europium in yttrium oxide Is studied with dye—laser spectroscopy. Combining the results with spectra under cathode—ray excitation, relative feeding rates for the excited states are obtained. Using these transfer and feeding rates the subline— arity of the luminescence as a function of excitation density can be explained. 6.

1. INTRODUCTION The efficiency of phosphors under cathode—ray

precipitated yttrium and europium oxalates

(CR) excitation decreases with increasing excitation density. For high density applica—

4. RESULTS The emission

tions (projection television), linearity is a major selection criterion. The sublinearity of 5D the main red 0(C2) emission of Y203:Eu 3+ 1 cannot be explained only. Feeding of the by 5D ground state depletion 0(C2) level via a level— 5D by—level cascade from higherfrom Eu3+ 3(C2) 2’3 and via energy the transfer levelsat a different crystallographic (S ions 4 6) site hinders the application of the method proposed by de Leeuw and ‘t Hooft5.

spectrum of Y

3+ j~ 2O3:Eu well —documented4’7. From the integrated decay curves of the 5D 0(S6) emission upon excitation in the same level, decay rate state conditions wastheobtained8 (seeunder fig. steady— 1). To determine the radiative lifetimes of and the 5D(C non—radiative 2’3 levels wetransfer measured:rates between the 2) total decay rates of these levels (see I) the fIg. 1), ii) the emission spectra, both upon selective excitation of each of these levels and as a function of CEu -

2. EXPERIMENTAL The experimental laser set—up consisted of a

The relative integrated intensities from the

dye laser pumped with a nitrogen laser The CR—spectra were measured at 20 kV In a demount—

excited levels of Y 3+ under stationary 203:Eu CR excitation were calculated from the emission

able tube (j 106 A/cm2). The sublinearity of the luminescence was measured at 20 kV In a SEM

spectra (see fig. 2). These relative Intensities are determined by the radiative lifetimes, the

j ranged from

transfer rates and the relative feeding rates

1o6 to iü1 A/cm2. All spectra were corrected for the spectral response of the detection sys—

under CR excitation From measurements on the sublinearity of the

tem

luminescence, the ratio for the current densi-

under stationary CR excitation;

ties

j 05 at which the efficiency of the

3, PHOSPHOR SAMPLES The phosphor powders

concen—

luminescence of two, was found to is be decreased 5D 5D by a factor 5D 0 : 1 : ~D2: 3 = 0.04

trations (CEu) varying between 0.001 and 50 m% were prepared in a solid state reaction directly

0.5 : 0.9 : 1 (for the emissions from the C2 site at CEu = 1 m%).

3+ with

Eu

from mixed Y2O3 (6N) and Eu2O3 as well as from 0022—2313/88/$0350 © Elsevier Science Publishers By. (North-Holland Physics Publishing Division)

652

D.BM. Klaassen et a!.

/

Yttrium oxide activated with europium

5D

for the 5D 2 ~

~

:1’

~

level. Including

~

,“7

10

(0 o 0)

-c

-~

5D 3(C2) and 0.05 ground—state

for the

0(S6)

depletion and 5D~states In the

excitation out of the excited model5, we calculated the following ratio for

-

10

~=

.-

io~

~/•--~

=

5D 5 (for the emissions from the C2 site at CEu 1 m%) EDO: ~ : 2 : ~D3= 0.05 : 0.5 : 0.6 1, which Is In reasonable agreement with the

experimental results.

10 102

101 100 101 Europium concentration (%)

102 101

FIGURE 1 ~ecay rate of the excited levels of Eu + as function of CE . Dashed lines Indicate the results from rnodei~ calculations.

5D 0(C2)

Total

>.

10°

~.

~

(I)

C

5D

.~

i0’~

~

10 io~ 5D io° 101 100 1o~ 3-Europium concentration• (%)-~.

-

0(S6)

5. CONCLUSIONS

levels was added to the corresponding5DJ>o(S6) 5D~(C experimental were fitted to for a decay model by All population the solving higher ofthe thedifferential excited levels. equations from the 2)data emissions. TheEmission total

io’ -

rates of the excited levels were described by a part independent of CEu and 5D a part proportional to a power of CEu . For the the part Independent of CEU was taken 0aslevels the radiative decay rate for these levels.

FIGURE 2 The relative integrated 3+ under stationary fromlines the CR excitation as a function intensities of CEu Dashed excited levels of Y2O3:Eu Indicate the results from model calculations.

The results from the fit can be summarized as follows: I) the radiative lifetimes are: 5D

0.96 ms for the

REFERENCES 1. A. Bril, Physica 15 (1949) 361.

5D

0(c2) 1(C2) , 0.69 ms for 5D , 1.1 ms for the 5D the 2(C2) , 3 ms for the 3(C2) and 6.9 ins 5D for the 0(S6) level, which Is In good 3. agreement with theory ii) only transfer to the next lower level oc— curs; In combination with I) this Implies

2. 3. 4. 5. 6.

that the total decay rates In figure 1 are almost equal to these transfer rates (except for the EDo). ill) the relative feeding rates excitation are: 0.29 for the 5D 5D for the

1(C2)

,

0.05 for the

7. under

CR

5D0(c2) , 0.25 2(C2) , 0.36

8.

3.0. Axe et al., 3. Chem. Phys. 40 (1964) 3066; A. Bril et al., Proc. I.C.L. (1966) 1689; M.J. Weber, Phys. Rev. 171 (1968) 283. H. Forest 116 (1969) et 474.al., J. Electrochem. Soc. D.M. de Leeuw et al. J. Lum. 28 (1983) 275. The authors are indebted to J.M.E. Baken and H.T. Hintzen (Philips Research Laboratories), to J.L. van Koesveld (Philips TCDS), and to the Philips Lighting Division for the phosphor samples. R.G. Pappalardo et al., J. Electrochem. Soc. 132 (1985) 721; R.B. Hunt Jr. et al., 3. Lum. 34 (1985) 133. B. di Bartolo in: Energy Transfer Processes in Condensed Matter (Plenum Press, New York 1984) Chap. 2.