Thermally stimulated luminescence of CaS : Bi : Pd phosphors

Solid State Communications,

241—1246’ 1972,

Vol. 10, PP-i

Pergamori Press.

Printed in Great Britain

THERMALLY STIMULATED LUMINESCENCE OF CaS: Bi : Pd PHOSPHORS RD. Lawangar, CS. Shalgaonkar, S.H. Pawar and A.V. Narlikar Materials Research Laboratory, Department of Physics, Shivaji University, Koihapur, India

(Received 25 February 1972 by S. Amelinckx)

CaS phosphors co-activated with Bi~3and Pd~2 impurities in varying concentrations have been prepared and their thermoluminescence has been systematically studied. The general features of the curves are discussed and the activation energies have been estimated in three different ways. The escape frequency factor is determined using the theoretical model of Randall and Wilkins. Conclusions are drawn regarding the type of kinetics involved in the thermoluminescence process.

INTRODUCTION

0.05, 0.01, and 0.005 wt.% Bi and the Pd concen-

DESPITE considerable work that has been done on ZnS and CdS phosphors, very little attention has been paid to alkaline earth suiphide phosphors. These phosphors have gained significance since the discovery of their utility for sensitized luminescence and infrared stimulation. Recently, studies of CaS phosphors co-activated with Bi~3 and various rare earth elements have been reported by several workers. 1—3 The effect of ferromagnetic impurities on the behaviour of CaS : Bi phosphors has also been investigated.4’5 However, the type of kinetics involved in thermoluminescence (TL) process is not very clear. The present study was made on CaS : Bi : Pd phosphors. The object of the investigation was to measure the activation energy and escape frequency factor for samples containing varying concentrations of Bi~ and Pd~2, and to study how these impurities influence the general features of the glow curves. Further, an attempt has been made to determine the type of kinetics involved in the TL process.

EXPERIMENTAL PROCEDURE

tration was systematically varied for each series from 0 to 0.5 wt.%. The samples were excited for five minutes using an ultraviolet source which emitted predominently the 3650 A Hg doublet. The luminescence intensity was measured using a photomultiplier tube, RCA931A. The output of the photomultiplier was fed to a sensitive automatic plotter on which the glow curve was recorded. The TL measurements were carried out in the temperature range of 25—250°C at a linear heating rate of 0.54°C/sec. RESULTS AND DISCUSSION Figure 1 shows a set of 10 glow curves obtained for samples containing a fixed concentration of Bi and a varying concentration of Pd while Fig. 2 shows a set of 4 curves for samples with fixed concentration of Pd and a varying concentration of Bi. The compositions of the samples whose curves are shown in Figs. 1 and 2 are given in Table 1. All the curves exhibit two peaks in the temperature range studied. The curves are plotted after normalizing the maximum

CaS : Bi : Pd phosphors were prepared by the method of thermal reduction of purified gypsum as followed by Bhawalkar and Malhotra. 6 Four series of phosphors were prepared which contained 0 10,

intensity of the first peak to a value 100 and shifting the ordinates suitably to avoid overlapping and to render their comparison easier. The glow curves obtained for other samples containing 1241

1242

THERMALLY STIMULATED LUMINESCENCE OF CaS: Bi : Pd PHOSPHORS Vol. 10, No. 12

S 70 S69

I..

D

5 59

I‘V

I-

a

/

S67

Z

S 66

I-

z

563 S 64

$ 7

:

00

!

-—~

S61 100

S60

0

40

60

20

60

200

240

31

45

260

TEl1PERATUR~ IN C s58

_____________________________________ 40 $0 20 60 200 240 280

0

TEMPERATURE,

C

0

FIG. Glow curvesofforPdsamples containing a fixed 2.concentration and varying concen-

tration of Bi.

FIG. 1. Glow curves for samples containing a

fixed concentration of Bi and varying concentration of Pd. different concentrations of Bi and Pd were essentially very similar to those of Figs. 1 and 2 and they are, therefore, not illustrated. It is evident from Figs. 1 and 2 that glow curves are markedly affected by both Bi and Pd impurities. It may be seen from Fig. 1 that changing Pd concentration has the effect of altering the shape of the curve and changing the relative intensity of two peaks observed. Moreover, a change in the Pd concentration has relatively a greater effect on the intensity of the second peak. At higher concentrations of Pd the second peak becomes more predominerit, and also there is overlapping of two peaks. These results suggest that the deeper traps are affected most by Pd addition. At higher Pd concentrations this has the effect of populating the deeper traps in preference to the shallow ones. The change in Bi concentration has however the reverse effect (Fig. 2).

From the glow curve, the activation energy

E can be estimated in several ways. In the present investigation three methods were used. It is necessary that the peaks be isolated and this was done by the method suggested by Bettinali et al. ~ However due to excessive overlapping certain peaks could not be isolated and their activation energies thus could not be determined (See Table 1 and 2). The first method used to calculate E is the 6 where the intensity of thermoinitial rise method luminescence is expressed in the form I = F exp ( — E/kT) (1)

F being a function of number of completely filled traps and empty centres, which takes into account the transition probabilities involved. Assuming F to be constant9 in the initial part of the glow curve the equation (1) takes the form in I = — E/kT + const. (2)

Vol. 10, No. 12 THERMALLY STIMULATED LUMINESCENCE OF CaS : Bi : Pd PHOSPHORS

1243

Table 1. Activation energy and escape frequency factor of glow peaks for different samples of CaS : Bi : Pd phosphors, obtained by using Grossweiner’s method and Randall and Wilkins formula respectively. Sample No.

concentration

of Bi wt.%

Concentration of

Pdwt.%

Activation

Escape frequency 1) factor, S (sec. I- peak. II- peak.

energy E

I- peak

0,(eV) II- peak

S17

0.05

0.00075

0.56

0.66

7.22 x 106

S31

0.01

0.00075

0.51

0.61

7.76 x iO~ 4.99 x i04

S45

0.005

0.00075

0.38

—

—

S57

0.01

0.0

0.53

0.74

3.54 x 106

3.46

S58

0.1

0.0005

0.53

0.65

3.54 x 106

3.94 x i0~

S59

0.1

0.00075

0.51

—

S60

0.1

0.001

0.53

0.69

3.54 x 106

1.15

>

l0~

S61

0.1

0.0025

0.56

0.72

7.22 x 106

2.94

>

106

S62

0.1

0.005

0.51

0.73

7.01 x i0~ 2.46

v

10~

S63

0.1

0.0075

0.53

0.64

1.56 x 106

1.49 ~ i0~

S64

0.1

0.01

0.55

0.68

1.68 x 106

1.05 ~ 106

S65

0.1

0.025

0.50

—

2.55 x 10!

—

S66

0.1

0.05

0.53

—

1.75

S67

0.1

0.075

0.54

0.67

S68

0.1

0.1

0.54

—

S69

0.1

0.25

0.30

—

—

—

S70

0.1

0.5

0.35

—

—

—

A plot of in! against 1/T is linear having slope equal to F. Typical plots obtained for two samples are as in Fig. 3. The values of activation energies thus calculated for two peaks are denoted by E, and they are shown in Table 2. It is worth mentioning that the initial rise method is independent of the type of kinetics involved in the thermoluminescence process. This is due to the fact that here one is concerned with the initial part of the glow curve which is insensitive to the nature of kinetics. tO, l~

7.54 x 10!

—

4.47 ~ iU~

x

1.34

x

10~

—

106

1.97 x 106

<

—

6.19 ~ 10~

106

—

F

2

E 2 = (2/6) kTm2 (4) when retrapping is dominent. Here Tm is the glow peak temperature and 3 is half-width towards fall-off of the glow peak. The values of E1 and E2 thus calculated are listed in Table 2. For the case when recombination is via a conduction band the activation energies

for the first and second order kinetics are respectively given by,

2 (1 The second method dueoftothe Halperin and Braner, andfollowed it makesisuse symmetry of the glow peak about its maximum. 9, ¶2 We assume that the initial concentration of trapped electrons is equal to that of trapped holes. If the luminescence is due to the tunneling process, the activation energy E is given by

(3)

1 = is(1/3) kTm and when recombination important,

and

E3

E

—

2.58L~),

(5)

(1.72/T) kTm

2 (1 3A), (6) 4 = (2/T) kTm 2kTm,!E << 1 is a correction factor, —

where ~ = and T= w — 3, w being half-intensity width of the peak. The factor 6/w = ~ is called the symmetry factor and is characteristic of the type

1244

THERMALLY STIMULATED LUMINESCENCE OF CaS: Bi: Pd PHOSPHORS Vol. 10, No. 12

Table 2. Activation energies foT the glow peaks obtained by analyzing the glow curves of CaS : Bi Pd phosphors using initial rise method and symmetry of the glow peak. Sample No.

Activation energy, (eV) I-peak

Il-peak

E,

F1

E2

F3

E4

E,

E1

F2

F3

E4

S17

0.58

0.26

0.52

0.57

0.64

0.67

0.37

0.75

0.67

0.74

S31 S45

0.49 0.60

0.25

0.51

0.58 0.39

0.38

0.77

0.60

0.66

—

0.52 0.36

0.60

—

—

—

—

S57

0.57

0.30

0.60

0.55

0.61

0.74

0.51

1.01

0.77

0.83

S58

0.56

0.29

0.58

0.55

0.61

0.65

0.44

0.88

0.66

0.73

—

—

S59

0.53

—

—

0.51

0.58

—

—

—

—

—

S60

0.51

0.25

0.51

0.55

0.61

0.70

0.38

0.76

0.70

0.78

S61

0.57

0.27

0.53

0.57

0.64

0.72

0.36

0.71

0.74

0.85

S62

0.50

0.21

0.42

S63 S64 S65 S66

0.54 0.56 0.48 0.55

0.27 0.28

0.53 0.57

0.51 0.54 0.56 0.50 0.54

0.57 0.61 0.62 0.55 0.60

0.77 0.64 0.67

0.35 0.36 0.34

0.70 0.72 0.68

0.75 0.64 0.70

0.83 0.71 0.76

—

—

—

—

—

—

—

—

—

—

S67 S68

0.54 0.57

0.55 0.55

0.61 0.61

0.70

J.39

0.79

0.68

0.77

—

—

—

—

—

S69 S70

0.68 0.46

0.24 0.32

0.25 0.34

—

—

—

—

—

—

—

—

—

—

—

—

—

—

0.27

0.54

—

—

—

—

—

—

of kinetics involved in the process. The values of ,u~ ~ e~ (1 + A) correspond to the first order kinetics while larger values to the second order kinetics. But many times the presence of weak shoulder at the high temperature side of a glow peak causes an apparent increase in the ~ values which may wrongly indicate the process to be of second-order. The values of F3 and F4 calculated using (5) and (6) are shown in Table

2. From the comparison of the F values determined by the methods described above, it is possible to predict the type of the kinetics involved in thermoluminescence. On comparison it may be seen that values of E3 agree most with F1 suggesting the kinetics involved to be of the first-order. This contention is supported by the photoluminescence studies carried out on these phosphors reported previously elsewhere. The values of j~ have been calculated and are found to be of the order of 0.6 which in turn

correspond to the second-order kinetics, con-

tradicting the above conclusion. However as discussed earlier any such cariclus ion drawn from the magnitude of ~i9 is likely to be erroneous. Hence one can conclude that the probable type of kinetics involved is first-order. However it may be noted from Table 2 that the values of E for the first peak, for samples

S45, S69 and S70, calculated using symmetry of the peaks are significantly lower than E~values obtained by the initial rise method. This may be ascribed to the extensive overlapping of two peaks found in these samples. The third method used to compute the activation energies is Grossweiner’s method. 14 For the first-order glow curve here E is given by E = 1 51 kTm T1/r (7) .

Where T1 corresponds to the temperature on the low temperature side of the glow peak at which

Vol. 10, No. 12

THERMALLY STIMULATED LUMENESCENCE OF CaS: Bi: Pd PHOSPHORS

1245 16

modified the above relation empirically giving

__________________________________

25

E

~-PEA)~

=

1.41 k~!~-~i! T

(8)

2.c

The activation energies evaluated using equation (8) are shown in Table 1 and they are in good agreement with the values calculated using the initial rise method.

.5

I0

05

The variation in the peak temperature Tm for two peaks was studied as a function of Pd-concentration, the Bi-concentrr.ions remaining constant.

567 S 57

0 22 —

23

24

25

26

27

28

~

20

The results for two concentrations of Bi are plotted in Fig. 4. It is interesting to note that when the

(3 0 -J

peak temperature of the first peak increases, that of the Il-peak decreases and vice-versa.

‘8

.6

The escape frequency factor S is evaluated using Randall and Wilkins formula based on

‘.4

monomolecular kinetics S 67

f3F

12

k Tm2

1~0 29.6

30

304

302

312

~

7

31.6

32

=

17

viz.

Se—

E.kTrn

(9)

324

where /3 is the rate of heating. Knowing F, /3, and Tm, S can be readily evaluated. The values of F obtained from formula (5) are used in equation (9) to obtain values of S. The values of S thus

~

FIG. 3. Typical plots of Log I vs. 1/T.

calculated are listed in Table 1. S

OlWt~8i

0005

in order to confirm some of the above contentions, studies of ESR low temperature measurements of TSC, and TSL are needed which are currently in progress and they will be reported later.

Wt%Bi

277 2~PE.AK 227 2

express their thanks— toThe Dr.authors D.R. Bhawalkar Acknowledgements would likeoftothe

w177

4

127

University of Sagar for constant encouragement,

77

and to the Research Officer, Mr. D.S. Padalkar, for experimental assistance.

0.

4 0.

27

l~ PEAK

0 CoNCEp4TR~rl0N

0-I

o~Pa.Cwt7’)

FIG. 4. Variation of peak temperature of two peaks with concentrations of Pd for a fixed concentration of Bi. the intensity is one-half of peak value. Dussel and Bube IS and Chen ‘~ showed that (7) yields values which are about 7% higher. Chen has

00

1246

THERMALLY STIMULATED LUMINESCENCE OF CaS: Bi : Pd PHOSPHORS Vol. 10, No. 12 REFERENCES

1.

ROTHSCHILD S., Solid State Physics in Flectronjcs and Telecommunications, Vol. 4, p. 705 (edited by DESIRANT M. and MICHIELS J.L.)Academjc Press, London (1960).

2. 3.

MOR S.L. and BHAWALKAR D.R., indian J. pure appl. Phys., 8, 320 (1970). PAWAR S.H. LAWANGAR RD. SHALGAONKAR CS. and NARLIKAR AV., Phil. %lag., 24, 727 (1971).

4.

SHALGAONKAR C.S. PAWAR S.H. LAWANGAR RD. and NARLIKAR A.V., to be published.

5.

ZOPE J.K., Private communications.

6.

BHAWALKAR DR. and MALHOTRA BR., Indian J. pure appi. Phys., 7, 163 (1969).

7.

BETTINALI C. FERRARESSO G. and MANCONI J.W., J. Chern. Phys., 50,~957 (1969).

8.

GARLICK G.F.J. and GIBSON.A.F., Proc. Phys. Soc., 60, 575 (1948).

9.

HALPERIN A. BRANER A.A. BEN-ZVI A. and KRISTIANPOLLER N., Phys. Rev., 117, 416 (1960).

10.

ARBELL H. and HALPERIN A., Phvs. Rev., 117, 45(1960).

11.

BETTINALI C.FERRARESSO G. and PACELLI E., Z. Phys. Chern ..~eue Folge. 68, 167 (1969).

12.

HALPERIN A. and BRANER A., Phys. Rev. 117, 408 (1960).

13. 14.

LAWANGAR RD. and NARLIKAR A.~’.,Accepted for publication in inthan J. pure app1. Phvs., (1972). GROSSWEINER LI., J. app!. Phvs. 24, 1306 (1953)

15.

DUSSEL GA. and BUBE RH., Ph’

16.

CHEN R., J. app!. Phys. 40, 570 (1969)

17.

RANDALL j.T. and WILKINS M.H.F., Proc. R. Soc. (London), A184, 366 (1945).

1s. Rev. 155, 764 (1967).

Des échantillons de phosphore de CaS simultanémerit actives ~ l’aide d’impuretés de Bi~ et Pd’ ont ét~préparés en diverses concentrations. Leur thermoluminescence a été systématiquement étudiée. Les caractéristiques générales des courbes sont discutées et les energies d’activation ont été estimées de trois manières différentes. Le facteur de fréquence d’échappement est déterminé sur la base du rnodèle théorique de Randall et Wilkins. Des conclusions sont tirées a propos du type de cinétique impliqué dans le processus de thermoluminescence.