Thermoluminescence studies of bismuth doped BaxCa1−xS nanostructures

Physica B 406 (2011) 177–180

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Thermoluminescence studies of bismuth doped BaxCa1  xS nanostructures Surender Singh a,n, S.P. Lochab c, Ravi Kumar b, Nafa Singh a a b c

Department of Physics, Kurukshetra University, Kurukshetra 136119, India Centre of Material Research, National Institute of Technology, Hamirpur (H.P.), India Inter University Accelerator Center, Aruna Asaf Ali Marg, Post Box 10502, New Delhi 110067, India

a r t i c l e in f o

abstract

Article history: Received 14 September 2010 Received in revised form 13 October 2010 Accepted 14 October 2010

Bismuth doped Ba1  xCaxS:Bi (x ¼ 0–1) nanocrystallities have been prepared by the solid state reaction method and characterized by XRD and TEM. X-ray diffraction analysis shows the formation of the compounds in cubic structure at room temperature. Only partial replacement of Ba is possible and we found that Ba0.5Ca0.5S:Bi could not be prepared due to the difference between ionic radii of barium and calcium. Thermoluminescence studies of these samples after exposure to UV radiation have been carried out. The TL glow curve of BaxCa1  xS:Bi has been found to be a simple structure with a single peak at 405, 428 and 503 K for x ¼ 1, 0.8 and 0, respectively. The kinetic parameters at various heating rates namely activation energy (E), order of kinetics (b) and frequency factor (s) of the Ba1  xCaxS:Bi (x ¼0.2) (0.4 mol%) sample have been determined using Chen’s method. The deconvolution of curve was done using the GCD function suggested by Kitis. The effect of different heating rates and different amount of dose has also been discussed. & 2010 Elsevier B.V. All rights reserved.

Keywords: Nanostructures X-ray diffraction Luminescence

1. Introduction Thermoluminescence (TL) is thermally stimulated emission of light from an insulator or semiconductor following the previous absorption of energy from ionizing radiations, such as g-rays, X-rays, b-rays, a-particles, energetic ions, etc. The intensity of light emitted by the phosphors by heating reflects the dose given to it. BaS and CaS, as the members of the group IIA–VI alkaline earth sulfides (AES), have been known for a long time as versatile and excellent phosphor host materials and have regained great attention in recent years due to their potential for applications in displays. Recently, researchers’ interest towards nanomaterials has increased because of the enhanced optical, electronical and structural properties of these materials. Various techniques are available for the preparations of nanophosphors. TLD nanophosphors are no exceptions and they have also found applications in TL dosimetry of high energy radiations for the measurements of high doses, where the conventional microcrystalline phosphors in powder are saturated [1–4]. These different properties can be understood on the basis of quantum size effect and large surface to volume ratio. This paper describes the TL response of Ba1  xCaxS:Bi (x ¼.2) in its nanostructure form to UV radiation. The TL glow curve of the BaS:Bi has been found to have a

n Corresponding author. Tel.: + 91 1744 238196/238410x2130, 2482; fax: + 91 1744 238277. E-mail addresses: [email protected], [email protected] (S. Singh).

0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.10.034

single peak structure with a single peak at 405 K [5] and the TL glow curve of CaS:Bi has been found to have a single peak structure at 503 K [6]. After the partial replacement of Ba ions by Ca ions, it causes a shift of the peak to 428 K. The samples have been characterized by XRD and TEM. The effect of different amount of doses and heating rates on glow curves has been discussed. Also the variation in intensities of Ba1  xCaxS:Bi (x¼ .2) with different amounts of doses has been studied.

2. Experimental In the present investigation sodium thiosulphate (AR grade) was used as a flux. Bismuth was used as an activator. Solution of this compound was preferred for uniform distribution. The ingredients, barium sulphate, carbon and flux, were taken according to the proportion. The detail of phosphors preparation can be found elsewhere [7]. The products were analyzed by X-ray diffraction using a Bruker Advance D8 X-ray diffractometer with Cu Ka radiation operating at 40 kV and 40 mA. The morphology and sizes of the phosphors were determined by TEM carried out on a H-7500 (Hitachi Ltd., Tokyo, Japan) operated at 120 kV. Diluted nanophosphors suspended in absolute ethanol were induced on carbon coated copper grid and were allowed to dry in air. TL glow curves were recorded at a heating rate of 2, 5 and 10 K s  1 on a Harahaw TLD reader (Model 3500) having a standard clear glass filter (a clean out/neutral density filter to neutralize the non-linear response of the PMT) taking 5 mg of sample each time.

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3. Results and discussion 3.1. XRD and TEM Fig. 1 shows XRD spectra of four compounds (BaxCa1  xS:Bi where x ¼0, 0.2, 0.8 and 1). The X-ray diffraction peaks for the compounds with x¼ 0 and 1 matched with JCPDS file no. 77-2011 and 75-0896, respectively. The compound with the value of x¼ 0.2 and 0.8 shows a strain from CaS and BaS, respectively. The average grain size of the particles (for the compounds with x ¼0.8) was calculated using Debye Scherer Equation [8]. d¼

0:89l b cos yB

where d is the average diameter of the nanoparticles, l the ˚ radiation, b (in radians) the full wavelength of Cu Ka (1.543 A) width at half maxima (FWHM) and yB the Bragg angle. The average particle size was found to be 38 nm. Fig. 2 shows the TEM images of particles of Ba0.8Ca0.2S:Bi (0.4 mol%).

4. Thermoluminescence studies 4.1. Effect of different heating rates and doses on the TL response of Ba0.8Ca0.2S:Bi (0.4 mol%) nanocrystalline phosphors The influence of different heating rates between 2 and 10 K s  1 on thermoluminescence response has been investigated in Ba0.8Ca0.2S:Bi nanocrystalline phosphors. It is found that with the increase in heating rate the peak intensity and area under the peak decrease, while peak temperature shifts towards higher side. The peak temperature (Tm) changes from 384 to 410 K as the heating rate changes from 2 to 10 K s  1 (Fig. 3). The decreasing luminescence sensitivity of Ba0.8Ca0.2S:Bi (0.4 mol%) phosphors as

Fig. 2. TEM images of Ba0.8Ca0.2S:Bi (0.4 mol%).

3000

(420)

(400)

(222)

TL Intensity (a.u.)

(420)

(400)

(222)

(220)

(200)

(111) (111)

(420)

(331)

(4oo)

(311)

(222)

(200)

1500 10 K/s 1000

C

0 350

400

500 450 Temperature (K)

550

600

30

35

40

45 2θ

50

(4oo)

(331) (420)

Fig. 3. Effect of different heating rates on TL response of Ba0.8Ca0.2S:Bi (0.4 mol%).

(220)

(311) (222)

(111)

5 K/s

500

(111)

Intensity (a.u.)

A

2000

(220)

(200)

25

2500

B

(220)

(200)

2 K/s

55

60

65

D

70

Fig. 1. XRD pattern of BaxCa1  xS:Bi (0.4 mol%) A, B, C and D with x ¼0, 0.2, 0.8 and 1, respectively.

a function of the increasing heating rate is a phenomenon frequently observed in the practice of TSL. It has been suggested that it is due to the thermal quenching effect [9]. In Fig. 4, the TL intensity is plotted as a function of exposure time where more or less linear response is observed for exposure time of 10 h. This linearity over a wide range of dose may be explained on the basis of track interaction model [10,11].

S. Singh et al. / Physica B 406 (2011) 177–180

179

suggested by Kitis et al. [17] for the first, second and general order kinetics glow peaks given in Eqs. (3)–(5), respectively. First order:

7.00E+008

6.00E+008

  2       E TTm T E TTm IðTÞ ¼ Im exp 1 þ  2 exp ð1DÞDm  Tm Tm kT kT Tm

TL Intensity

5.00E+008

ð3Þ Second order:

4.00E+008 IðTÞ ¼ 4Im exp



3.00E+008

E kT



TTm Tm

    2 T2 E TTm ð1DÞexp þ 1 þ Dm 2 kT Tm Tm

 

ð4Þ General order:

2.00E+008

    b=ðb1Þ E ðTTm Þ T2 E TTm IðTÞ ¼ Im bb=ðb1Þ exp ðb1Þð1DÞ 2 exp þ Zm kT Tm kT Tm Tm

1.00E+008 0

2

4 6 8 10 Irradiation Doses (in Hours)

ð5Þ

12

Fig. 4. Peak TL intensity of Ba0.8Ca0.2S: Bi (0.4 mol%) as a function of exposure time at the heating rate of 2 K s  1.

4.2. Analysis of glow curve by GCD curve fitting and trapping parameters The determination of kinetic parameters has been an active area of research and various techniques have been developed over the time to derive these parameters from glow curve. However we apply the peak shape method to the whole experimental glow curve and evaluate the parameters which defines as

where I(T) is the TL intensity at temperature T (K), Im the maximum peak intensity, E the activation energy (eV) and k the Boltzmann’s constant (8.6  10  5 eV K  1), D ¼2kT/E, Dm ¼2 kTm/E and Zm ¼ 1+(b  1) Dm. Fig. 5 shows the experimental glow curve for Ba1  xCaxS:Bi (0.4 mol%) doped BaS nanostructures at heating rate of 2 K s  1, which has been deconvoluted into four peaks using GCD function. The position of respective peaks, trap parameters and order of kinetics at heating rate of 2 K s  1 is shown in Table 1.

2.50E+008

o ¼ T2 T1 d ¼ T2 Tm and t ¼ Tm T1

a

2.00E+008

TL Intensity (a.u.)

where Tm is the maximum temperature and T1 and T2 are low and high half-intensity temperature values of the glow curve at the ascending and descending side of the peak, respectively. We adopted a similar approach as earlier reported by Furetta et al. [12] for analyzing the TL glow curve in which they first did deconvolution based on Gaussian function and then analyzed the individual peak using Chen’s peak method:  2 kTm Ea ¼ ca ð1Þ ba ð2 kTm Þ

1.50E+008

1.00E+008

with a ¼ t, d, o; t ¼Tm  T1; d ¼T2–T1; o ¼T2–T1; Ct ¼ 1:51 þ 3:0 ðmg 0:42Þ;

5.00E+007

Cd ¼ 0:976 þ7:3 ðmg 0:42Þ;

Co ¼ 2:52 þ 10:2 ðmg 0:42Þ; bt ¼ 1:58 þ4:2 ðmg 0:42Þ bd ¼ 0; bo ¼ 1: Once E and b are known, s can be evaluated by Chen and Kirsh [13]:

bE 2 kTm

  ¼ s expðE=kTm Þ 1 þðb1ÞDm

0.00E+000 350

ð2Þ

These parameters can be modified during the best fit procedure using the GCCD program until the best fit is achieved, which can be confirmed by the figure of merit (FOM). Some authors have reported similar studies for evaluation of kinetic parameters using Chen’s peak method applied directly to the peaks, which were deconvoluted using the origin 6.1 software without using any GCD function [14]. In the present study we deconvoluted the TL glow curves based on Gaussian functions into four peaks, which were also confirmed by the thermal cleaning method [15]. The isolated peaks were analyzed by Chen’s peak method [16] to evaluate the peak parameters using Eqs. (1) and (2). The calculated parameters were then used as initial parameters for the GCD basic function

400 450 500 Temperature(K)

550

Fig. 5. Comparison between theoretical ( ) and experimental (–o–) fitted glow curve of Ba0.8Ca0.2S:Bi (0.4 mol%) exposed to gamma radiation. Deconvoluted single fitted glow curves 1,2,3, (y) are also shown.

Table 1 Kinetic parameters using the GCD function. Peak number

Tm (K)

Order of kinetics (b)

Activation energy E (eV)

Frequency factor (s)

First peak Second peak Third peak

383 425 470

2 (0.49) 2 (0.47) 2 (0.48)

0.48 0.53 0.30

3.55  105 2.97  105 1.45  104

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S. Singh et al. / Physica B 406 (2011) 177–180

5. Conclusion In this report we partially replace the Ba by Ca in BaS. We found that due to replacement, TL glow peak is shifted towards higher temperature. While this peak shifts towards lower temperature in CaS. Only partial replacement is possible because ionic radii of Ba and Ca are different. The kinetic parameters for this sample at heating rate of 2 K s  1 is calculated by Chen’s method and the theoretical peak obtained by the GCD function given by Kitis overlapped the experimental peak. The kinetic parameters namely activation energy (E), order of kinetics (b) and frequency factor (s) of Ba1  xCaxS:Bi (0.4 mol%) sample have been determined using Chen’s method.

References [1] S.P. Lochab, P.D. Sahare, R.S. Chauhan, Numan Salah A Pandey, J. Phys. D: Appl. Phys. 39 (2006) 1786.

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