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Temperature dependence of Y2O2S:Tb cathodoluminescence
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0038-1098/88 $3.00 + .00 Pergamon Journals Ltd.
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State Communications, Vol.65,No.|, pp.35-36, 1988. in Great Britain.
TEMPERATURE DEPENDENCE
J.A.Kaliakatsos Physics *Physics (Received
OF Y 2 0 2 S : T b
CATHODOLUMINESCENCE
- G.E.Giakoumakis Department Department
- G.J.Papaioannou
- University - University
by S. A M E L I N C K X
of Athens of Ioannina
- Septemper
30,
1987)
The c a t h o d o l u m i n e s c e n c e of Y 2 0 2 S : T b is studied as a function of the t e m p e r a t u r e in the range 20OK to 300OK and as a f u n c t i o n of the e l e c t r o n b e a m v o l t a g e in the range 3kV to 20kV. The e f f i c i e n c y of this p h o s p h o r shows four well e x p r e s sed peaks in r e s p e c t to the t e m p e r a t u r e (at about 40OK, 105°K, 180OK and 290°K) and one peak in r e s p e c t to the e l e c t r o n b e a m v o l t a g e (at 13kV).
Photoluminescence and thermolumin e s c e n c e are w i d e l y u s e d for the study of the e n e r g y levels in p h o s p h o r s . A s we have shown in a p r e v i o u s p a p e r ] the study of the c a t h o d o l u m i n e s c e n c e as a f u n c t i o n of the t e m p e r a t u r e gives results s i m i l a r to the r e s u l t s of thermoluminescence with the a d d i t i o n a l advantage, that the peak p o s i t i o n s on the e f f i c i e n c y v e r s u s t e m p e r a t u r e plot do not d e p e n d on the w a r m i n g - u p rate. In this p a p e r we p r e s e n t our results for the c a t h o d o l u m i n e s c e n c e of Y 2 0 2 S : T b in the t e m p e r a t u r e range 20OK to 300°K. The m e a s u r e m e n t s have been carried out on 24 p h o s p h o r screens p r e p a r e d by sedimentation with surface densities ranging from 10mg/cm2 to 4 0 m g / c m 2. The p h o s p h o r m a t e r i a l was s u p p l i e d by Derby Luminescents Ltd.(polycrystalline grains of about 8~m average size, cod. no. YTO/70). The measurements have been c a r r i e d out in the r e f l e c t i o n mode configuration, w h i c h m e a n s that the light e m i t t e d f r o m the screens was m e a s u r e d from the e l e c t r o n gun side. The total error in m e a s u r i n g and e v a l u a t i n g the e f f i c i e n c y was b e t t e r than 6% w h i l e the error in the t e m p e r a t u r e e s t i m a t i o n was b e t t e r than 0.6OK. I n i t i a l l y we s t u d i e d the e f f i c i e n c y of the screen [n=light flux e m i t t e d from the s c r e e n / e l e c t r o n beam power] as a f u n c t i o n of the e l e c t r o n beam voltage for v a r i o u s c o n s t a n t temperatures (20 d i f f e r e n t values). As shown in fig. I this e f f i c i e n c y is not c o n s t a n t and it shows a peak at a b o u t 13kV. This b e h a v i o u r is typical for all the screens and all the t e m p e r a t u r e s , the peak p o s i t i o n r e m a i n i n g the same in all cases. The shape of the curves and the peak p o s i t i o n s can be e x p l a i n e d by taking into a c c o u n t the g r a n u l a r s t r u c t u r e of
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V(kV) Figure I: C a t h o d o l u m i n e s c e n c e of Y 2 0 2 S : T b screens versus voltage.
efficiency excitation
the phosphor, as we have d i s c u s s e d in a previous paper 2 concerning transimission mode c a t h o d o l u m i n e s c e n c e measurem e n t s on ZnCdS:Ag. After that we fixed the e l e c t r o n b e a m v o l t a g e at 13kV and we studied the dependence of the efficiency on the t e m p e r a t u r e . Fig. 2(a) shows a typical plot for the function n=f(T). The plots are the same for all the screens. It is c l e a r that there appear at least four well e x p r e s s e d peaks b e t w e e n 20°K to 300°K. Three of them -the ones c o r r e s p o n d i n g to the t e m p e r a t u r e s 105OK, 180OK and 290OK - are the same w i t h the peaks o b s e r v e d by Y a m a m o t o and Kano 3 during thermoluminescence experiments studing the 5D 4 ÷ 7F 5 t r a n s i t i o n s of Tb 3+ in Y 2 0 2 S : T b 3+. The first peak, w h i c h appears at about 40°K and which
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TEMPERATURE DEPENDENCE OF Y202S:Tb CATHODOLUMINESCENCE i
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T(°K) Figure 2: C a t h o d o l u m i n e s c e n c e e f f i c i e n c y of Y 2 0 2 S : T b versus t e m p e r a t u r e . C u r v e (a) : Total e m i s s i o n s p e c t r u m . C u r v e (b):Partial emissi o n s p e c t r u m l>530nm. is the h i g h e s t one has never before been r e p o r t e d in the literature . Of course in the work m e n t i o n e d above 3 it was not p o s s i b l e to be observed, b e c a u s e the
Vol. 65, NO.
e x c i t a t i o n of the samples h a d been done by UV r a d i a t i o n at a t e m p e r a t u r e higher than 40°K. Since it was not p o s s i b l e to carry out d e t a i l e d spectral d i s t r i b u t i o n measurements due to the low light level of the e m i s s i o n of the screens, we t r i e d to verify a p o s s i b l e c h a n g e in the e m i s sion s p e c t r u m of Y 2 0 2 S : T b by p l a c i n g a h i g h - p a s s filter (Ic=530nm) b e t w e e n the screen and the detecting photocathode and r e p e a t i n g all the measurements. Fig. 2(b) shows a typical plot of the n=f(T) function after the filtration. It is obvious that all the peaks are reduced, but e s p e c i a l l y the peak at 40OK is g r e a t l y r e d u c e d also in respect to the others being now lower than the one at 105°K. This fact indicates that there is a spectral distribution c h a n g e and proves c l e a r l y that the c o n t r i b u t i o n of the D 4 + 7Fj t r a n s i t i o n s to this peak is at least ver~ small. The e x i s t e n c e of this peak must be m a i n l y a t t r i b u t e d to the 5D 3 ÷ 7Fj transitions, w h i c h are fav o u r e d at low t e m p e r a t u r e s as it is also o b s e r v e d for Eu 3+ 4.
REFERENCES
I. J . A . K a l i a k a t s o s , P . C . E u t h y m i o u , C.D. Nomicos, G . E . G i a k o u m a k i s : A p p l . P h y s . , 31, 213 (1983). 2. G . E . G i a k o u m a k i s , C . D . N o m i c o s , Euthymiou: J.Appl. Phys., 51, (1980).
P.C. 4976
3. H . Y a m a m o t o , T.Kano: J.Electrochem. Soc., 126, 305 (1979). 4. W . H . F o u g e r , C . W . S t r u c k : J . C h e m . P h y s . , 52, 6364 (1970).
I
•
" e_~Printed
S
o
l
i
0038-1098/88 $3.00 + .00 Pergamon Journals Ltd.
d
State Communications, Vol.65,No.|, pp.35-36, 1988. in Great Britain.
TEMPERATURE DEPENDENCE
J.A.Kaliakatsos Physics *Physics (Received
OF Y 2 0 2 S : T b
CATHODOLUMINESCENCE
- G.E.Giakoumakis Department Department
- G.J.Papaioannou
- University - University
by S. A M E L I N C K X
of Athens of Ioannina
- Septemper
30,
1987)
The c a t h o d o l u m i n e s c e n c e of Y 2 0 2 S : T b is studied as a function of the t e m p e r a t u r e in the range 20OK to 300OK and as a f u n c t i o n of the e l e c t r o n b e a m v o l t a g e in the range 3kV to 20kV. The e f f i c i e n c y of this p h o s p h o r shows four well e x p r e s sed peaks in r e s p e c t to the t e m p e r a t u r e (at about 40OK, 105°K, 180OK and 290°K) and one peak in r e s p e c t to the e l e c t r o n b e a m v o l t a g e (at 13kV).
Photoluminescence and thermolumin e s c e n c e are w i d e l y u s e d for the study of the e n e r g y levels in p h o s p h o r s . A s we have shown in a p r e v i o u s p a p e r ] the study of the c a t h o d o l u m i n e s c e n c e as a f u n c t i o n of the t e m p e r a t u r e gives results s i m i l a r to the r e s u l t s of thermoluminescence with the a d d i t i o n a l advantage, that the peak p o s i t i o n s on the e f f i c i e n c y v e r s u s t e m p e r a t u r e plot do not d e p e n d on the w a r m i n g - u p rate. In this p a p e r we p r e s e n t our results for the c a t h o d o l u m i n e s c e n c e of Y 2 0 2 S : T b in the t e m p e r a t u r e range 20OK to 300°K. The m e a s u r e m e n t s have been carried out on 24 p h o s p h o r screens p r e p a r e d by sedimentation with surface densities ranging from 10mg/cm2 to 4 0 m g / c m 2. The p h o s p h o r m a t e r i a l was s u p p l i e d by Derby Luminescents Ltd.(polycrystalline grains of about 8~m average size, cod. no. YTO/70). The measurements have been c a r r i e d out in the r e f l e c t i o n mode configuration, w h i c h m e a n s that the light e m i t t e d f r o m the screens was m e a s u r e d from the e l e c t r o n gun side. The total error in m e a s u r i n g and e v a l u a t i n g the e f f i c i e n c y was b e t t e r than 6% w h i l e the error in the t e m p e r a t u r e e s t i m a t i o n was b e t t e r than 0.6OK. I n i t i a l l y we s t u d i e d the e f f i c i e n c y of the screen [n=light flux e m i t t e d from the s c r e e n / e l e c t r o n beam power] as a f u n c t i o n of the e l e c t r o n beam voltage for v a r i o u s c o n s t a n t temperatures (20 d i f f e r e n t values). As shown in fig. I this e f f i c i e n c y is not c o n s t a n t and it shows a peak at a b o u t 13kV. This b e h a v i o u r is typical for all the screens and all the t e m p e r a t u r e s , the peak p o s i t i o n r e m a i n i n g the same in all cases. The shape of the curves and the peak p o s i t i o n s can be e x p l a i n e d by taking into a c c o u n t the g r a n u l a r s t r u c t u r e of
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I
J
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l
I
15
20
V(kV) Figure I: C a t h o d o l u m i n e s c e n c e of Y 2 0 2 S : T b screens versus voltage.
efficiency excitation
the phosphor, as we have d i s c u s s e d in a previous paper 2 concerning transimission mode c a t h o d o l u m i n e s c e n c e measurem e n t s on ZnCdS:Ag. After that we fixed the e l e c t r o n b e a m v o l t a g e at 13kV and we studied the dependence of the efficiency on the t e m p e r a t u r e . Fig. 2(a) shows a typical plot for the function n=f(T). The plots are the same for all the screens. It is c l e a r that there appear at least four well e x p r e s s e d peaks b e t w e e n 20°K to 300°K. Three of them -the ones c o r r e s p o n d i n g to the t e m p e r a t u r e s 105OK, 180OK and 290OK - are the same w i t h the peaks o b s e r v e d by Y a m a m o t o and Kano 3 during thermoluminescence experiments studing the 5D 4 ÷ 7F 5 t r a n s i t i o n s of Tb 3+ in Y 2 0 2 S : T b 3+. The first peak, w h i c h appears at about 40°K and which
35
36
TEMPERATURE DEPENDENCE OF Y202S:Tb CATHODOLUMINESCENCE i
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T(°K) Figure 2: C a t h o d o l u m i n e s c e n c e e f f i c i e n c y of Y 2 0 2 S : T b versus t e m p e r a t u r e . C u r v e (a) : Total e m i s s i o n s p e c t r u m . C u r v e (b):Partial emissi o n s p e c t r u m l>530nm. is the h i g h e s t one has never before been r e p o r t e d in the literature . Of course in the work m e n t i o n e d above 3 it was not p o s s i b l e to be observed, b e c a u s e the
Vol. 65, NO.
e x c i t a t i o n of the samples h a d been done by UV r a d i a t i o n at a t e m p e r a t u r e higher than 40°K. Since it was not p o s s i b l e to carry out d e t a i l e d spectral d i s t r i b u t i o n measurements due to the low light level of the e m i s s i o n of the screens, we t r i e d to verify a p o s s i b l e c h a n g e in the e m i s sion s p e c t r u m of Y 2 0 2 S : T b by p l a c i n g a h i g h - p a s s filter (Ic=530nm) b e t w e e n the screen and the detecting photocathode and r e p e a t i n g all the measurements. Fig. 2(b) shows a typical plot of the n=f(T) function after the filtration. It is obvious that all the peaks are reduced, but e s p e c i a l l y the peak at 40OK is g r e a t l y r e d u c e d also in respect to the others being now lower than the one at 105°K. This fact indicates that there is a spectral distribution c h a n g e and proves c l e a r l y that the c o n t r i b u t i o n of the D 4 + 7Fj t r a n s i t i o n s to this peak is at least ver~ small. The e x i s t e n c e of this peak must be m a i n l y a t t r i b u t e d to the 5D 3 ÷ 7Fj transitions, w h i c h are fav o u r e d at low t e m p e r a t u r e s as it is also o b s e r v e d for Eu 3+ 4.
REFERENCES
I. J . A . K a l i a k a t s o s , P . C . E u t h y m i o u , C.D. Nomicos, G . E . G i a k o u m a k i s : A p p l . P h y s . , 31, 213 (1983). 2. G . E . G i a k o u m a k i s , C . D . N o m i c o s , Euthymiou: J.Appl. Phys., 51, (1980).
P.C. 4976
3. H . Y a m a m o t o , T.Kano: J.Electrochem. Soc., 126, 305 (1979). 4. W . H . F o u g e r , C . W . S t r u c k : J . C h e m . P h y s . , 52, 6364 (1970).
I