Thermoluminescence response and trap parameters determination of gamma exposed Ce doped SrS nanostructures

Journal of Alloys and Compounds 490 (2010) L33–L36

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Letter

Thermoluminescence response and trap parameters determination of gamma exposed Ce doped SrS nanostructures Ankush Vij a,∗ , S.P. Lochab b , Ravi kumar b , Nafa Singh a a b

Department of Physics, Kurukshetra University, Kurukshetra 136119, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India

a r t i c l e

i n f o

Article history: Received 18 September 2009 Received in revised form 8 October 2009 Accepted 9 October 2009 Available online 20 October 2009 Keywords: Nanostructures Thermoluminescence Glow curve Kinetic analysis Trapping parameters

a b s t r a c t Thermoluminescence of Ce doped SrS nanostructures exposed to Co-60 gamma radiations (0.1 Gy to 7 kGy) has been investigated. TL glow curves for gamma doses in the range of 0.1–200 Gy, consist of a dominant peak at 386 K with a very weak peak at 538 K. At higher doses (1–7 kGy), the peak at 386 K shifts to the higher temperature of 421 K, while the other peak at 538 K becomes more intense. This anomalous shifting of the first peak from 386 K to 421 K has been explained in the framework of Chen’s peak shape method. Kinetic analysis of the experimental TL glow curve has been carried out using glow curve deconvolution (GCD) functions to determine the trapping parameters. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Thermoluminescence (TL) is an important and powerful technique used for the dosimetry of the ionizing radiations. TL is the thermally stimulated emission of light from an insulator or semiconductor following the previous absorption of energy from ionizing radiations such as gamma rays, UV rays, X-rays, charged particles, etc. Rare earth ions have aroused much interest due to their high luminescence efficiency and after glow characteristics in different host materials [1–5]. With the advent of nanotechnology, phosphors in their nano dimensions i.e. nanophosphors have received much attention due to their modified luminescence properties as compared to that of their bulk counterparts [6–8]. Recently, we have reported the thermoluminescence of Ce doped SrS nanostructures exposed to UV radiations [9]. It is known that TL response of any material is different for different ionizing radiations or ions [10–11], so it is important to know about the nature of these centres or traps. Therefore, in continuation to our previous work, this report aims at the systematic thermoluminescence study of the cerium doped SrS nanostructures exposed to Co-60 gamma rays at room temperature. We have optimised the Ce concentration in SrS and studied the TL response of SrS:Ce nanostructures at low and high doses of gamma radiations. Moreover, the information of kinetic parameters is a pre-requisite for studying the dosimetric

∗ Corresponding author. E-mail addresses: vij [email protected], [email protected] (A. Vij). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.075

properties for any phosphor. For this we have made kinetic analysis of the experimental TL glow curves using Chen’s peak shape method [12] and glow curve deconvolution (GCD) functions based on Kiti’s general order equation [13] to determine the kinetic parameters namely activation energy (E), order of kinetics (b) and frequency factor (s). The effect of different heating rates on TL glow curve has also been investigated. 2. Experimental Solid state diffusion method has been used for synthesizing cerium doped strontium sulphide nanostructures in the presence of sodium thiosulfate. We have already reported the details for synthesis of these nanostructures [14]. The synthesized samples were irradiated with different doses of gamma rays in the range 0.1 Gy–7 kGy from a Co-60 source at room temperature. Prior to gamma exposure, the samples were annealed at 673 K for 15 min and then quenched on a metallic plate at room temperature to erase any residual information. After the desired exposure, TL glow curves were recorded on a Harshaw TLD Reader (Model 3500) having a neutral density filter, taking 5 mg of the sample each time in a nitrogen atmosphere at different heating rates of 2 K/s, 5 K/s and 10 K/s.

3. Results and discussion The SrS:Ce nanostructures were found to have whisker like morphology with a diameter of 55–60 nm and a length of several nanometers. The structural characterisation of these nanostructures has been reported elsewhere [14]. High luminescence yield is a pre-requisite for any good TL phosphor. For this purpose an optimum incorporation of the lumi-

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Fig. 3. TL glow curves exposed in the gamma dose range of 1–7 kGy. Fig. 1. Concentration quenching curve for SrS:Ce nanostructures exposed to 1 Gy of gamma dose.

nescence centres into the host, local environment around the dopant and doping concentration plays a significant role. Cerium (0.5 mol%) has been found to be an optimized quantity for the maximum TL intensity (Fig. 1). High doping shows the well known concentration quenching of the luminescence due to an enhanced interaction between the centres and the disturbed lattice periodicity [15]. Effect of different doses of gamma radiations on the TL glow curve of SrS:Ce (0.5 mol%) nanostructures has been investigated. Surprisingly, the TL glow curves at lower doses and relative higher doses exhibit different TL behaviour. Fig. 2 shows the effect of lower doses of gamma radiations on the TL glow curve at a linear heating rate of 10 K/s. TL glow curve has a strong peak around 386 K with another very weak intensity peak at 538 K for the doses in the range of 0.1–200 Gy. In this dose range, TL intensity is maximum for a dose of 100 Gy and then decreases with increasing dose. This variation of TL intensity with dose has been shown in the inset of Fig. 2. TL glow curves for the samples exposed in the range of 1–7 kGy were surprising (Fig. 3). TL glow curve at higher doses comprises of TL peaks at 421 K and 538 K. TL intensity for both the peaks increases with the increasing dose in this range. The comparison between TL glow curves at low doses and high doses shows that glow peak at 386 K shifts to 421 K while the peak at 538 K also becomes intense for higher doses. The observed TL peaks around 421 K and 538 K at

Fig. 2. TL glow curves exposed in the gamma dose range 0.1–200 Gy. Inset shows the variation of TL intensity with the dose.

higher doses agrees with a previous report by Singh et al. [16] in which they have also reported two peaks for Ce doped SrS phosphors at 410 K and 548 K. This small difference in temperature for both the peaks may be due to different grain sizes, different heating rates and dose given to samples as they have not mentioned the heating rate and dose. Fig. 4 shows the variation of TL intensity for both the TL peaks in the higher gamma dose range. TL intensities for both the peaks increase with increasing doses but peak at 538 K shows almost a linear behaviour with dose which is also important from dosimetric point of view. This linear TL behaviour in nanocrystalline materials for a wide range of doses may be explained on the basis of track interaction model (TIM) and a high surface to volume ratio, reported elsewhere [8]. The experimental TL peak at 386 K in the lower dose regime is well isolated from the second peak at 538 K. For identifying the reason of anomalous shift in temperature (first peak) for higher gamma doses, we applied Chen’s peak shape method [12] to the isolated peak at 386 K and evaluated the peak parameters i.e. ω(FWHM), ı,  and g defined as; g =

ı , ω

ı = T2 − TM ,

and

ω = T2 − T1

(1)

Here, g is called as geometric form factor. The theoretical value of the form factor ranges between 0.42 and 0.52; the value close to 0.42 is for the first order and value close to 0.52 is for the second order. The form factor for the glow peak at 386 K was calculated to

Fig. 4. Variation in TL intensity of both the TL peaks with the dose in the range 1–7 kGy.

A. Vij et al. / Journal of Alloys and Compounds 490 (2010) L33–L36

Fig. 5. Effect of different heating rates on TL glow curve for a sample exposed to 2 kGy gamma dose.

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Fig. 6. Glow curve deconvolution of SrS:Ce (0.5 mol%) nanostructures exposed to 2 kGy gamma dose at a heating rate of 10 K/s.

experimental TL glow curve for SrS:Ce nanostructures. be 0.56 which seems to be unrealistic, and indicates that it may be a superposition of more than one peak. Therefore, it is reasonable to assume that the glow curve at 386 K consists of more than one peak, and each peak shows different TL behaviour leading to an interplay of fall and rise of intensities of peaks within the main glow peak, which results into the shifting of the glow curve from 386 K to 423 K at higher gamma doses. This argument is also supported by the fact that the TL intensity of 386 K peak starts decreasing after a dose of 100 Gy as shown in Fig. 3. The effect of different heating rates on the TL glow curve has been investigated for the samples exposed to a dose of 2 kGy which is shown in Fig. 5. We noted that as we increase the heating rate from 2 K/s to 10 K/s, TL intensities decrease with a shift in the peak to a higher temperature which is in agreement with our previous report [9]. This may be assigned to the well known thermal quenching of TL due to an increase in heating rates. The kinetic parameters namely activation energy (E), order of kinetics (b) and frequency factor (s) for any experimental TL glow curve can be determined using various methods [17]. In the present study, we chose SrS:Ce nanostructures exposed to 2 kGy dose of gamma radiations to determine the trapping parameters. TL glow curve for SrS:Ce nanostructures exposed to a gamma dose of 2 kGy consists of two glow peaks at 422 K and 538 K with a shoulder at 355 K. Since glow peaks were not isolated from each other, therefore Chen’s peak shape method cannot be used to evaluate the trapping parameters. Therefore, we used glow curve deconvolution functions (GCD) for general order kinetics glow curves (given in Eq. (1)) suggested by Kiti’s et al. [13] to isolate each peak in the

I(T ) = Im bb/b−1 exp

 E T −T  m



kT

× (b − 1)(1 − )



Tm T2 2 Tm

 × exp

 E T −T  m kT

Tm

−b/b−1 + Zm

(2)

Here I(T) is the TL intensity at temperature T(K), Im the maximum peak intensity, b the frequency factor, E the activation energy (eV), k the Boltzmann’s constant,  = 2kT/E and Zm = 1 +(b − 1)m ). For a rough estimation of the peak parameters to be fitted in this equation, we deconvoluted the TL glow curve into three peaks using Origin 6.1 software and applied Chen’s peak shape method to the each deconvoluted peak to find the peak parameters. The geometric form factor defined in Eq. (1) was used to estimate the order of kinetics (b). We used the Chen’s correlation between order of kinetics (b) and geometric form factor (g ) to determine the general order of kinetics (other than first and second order). The activation energy was estimated from the following equation:



E˛ = C˛

2 kTm ˛



− b˛ (2kTm )

(3)

where C = 1.51 + 3.0(g − 0.42), Cı = 0.976 + 7.3(g − 0.42), Cω = 2.52 + 10.2(g − 0.42) and b = 1.58 + 4.2(g − 0.42), bı = 0, bω = 1 The calculated parameters E and b were used as initial parameters for fitting in Eq. (2) and theoretical peaks were generated. These parameters were modified till a theoretical glow curve was obtained by their convolution to overlap on the experimental TL

Table 1 Trapping parameters for SrS:Ce (0.5 mol%) nanostructures exposed to 2 kGy gamma dose at different heating rates. Heating rate

Peak number

Tmax (K)

Order of kinetics (b)

Activation energy (E in eV)

Frequency factor (s)

10 K/s

1 2 3

350 418 541

1.6 1.8 1.7

0.70 0.52 0.47

9.4 × 109 1.2 × 107 1.5 × 104

5 K/s

1 2 3

348 410 532

1.7 1.8 2

0.72 0.50 0.46

1.1 × 1010 4.1 × 106 1.4 × 104

2 K/s

1 2 3

328 395 517

1.7 1.8 1.9

0.61 0.48 0.45

5.6 × 107 8.5 × 105 2.9 × 104

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glow curve. Fig. 6 shows the experimental TL glow curve for SrS:Ce nanostructures exposed to 2 kGy dose of gamma, which has been deconvoluted into three peaks using GCD functions. The goodness of fitting was determined by calculating figure of merit (FOM) defined by

 TLExprimental − TLFit  FOM = TLFit

where TLExperimental and TLFit represent the experimental TL intensity data and the values of the fitting functions, respectively. The summation extends over all the available experimental data points. The resolution of fitting method can be refined by repeating the process of calculating the FOM for different values of E and finding the value of E that minimizes the value of FOM. In the present study FOM was calculated to be 0.021 which also confirms a very good agreement between theoretical and experimental glow curve. Once activation energy (E) and order of kinetics (b) were determined, the frequency factor (s) [18] was calculated from Eq. (4): ˇE 2 kTm

= s exp

 −E  kTm

[1 + (b − 1)m ]

(4)

where m = 2kTm /E, ˇ is the linear heating rate and k is Boltzmann’s constant. Table 1 shows the calculated trapping parameters for the deconvoluted peaks of the SrS:Ce nanostructures exposed to 2 kGy dose of gamma at different heating rates i.e. 10 K/s, 5 K/s and 2 K/s. 4. Conclusions Thermoluminescence of SrS:Ce nanostructures exposed to Co60 gamma radiations have been investigated and it is found that TL glow curves for samples exposed to doses in the range of 0.1–200 Gy comprise of a main peak around 386 K with a very weak peak at 538 K, while for the samples exposed to the doses in the range of 1–7 kGy, TL peak is at 421 K and the second peak at 538 K also becomes intense. This anomalous shifting of first TL peak has been explained on the basis of Chen’s peak shape method. The trap parameters were determined for the samples exposed to 2 kGy gamma dose

using glow curve deconvolution (GCD) functions at different heating rates. Acknowledgements We are thankful to the Director, Inter University Accelerator Centre (IUAC), New Delhi for providing the experimental facilities. The authors acknowledge the help by Miss Shaila Bahl during gamma irradiation and TL experiments. One of the authors, A. Vij is extremely thankful to IUAC, New Delhi for providing financial support for this work under the UFUP project. References [1] J. Holsa, T. Aitasalo, H. jungner, M. Lastusaari, J. Niittykoski, G. Spanno, J. Alloys Compd. 374 (2004) 56–59. [2] N. Kodama, Y. Tanni, M. Yamaga, J. Lumin. 87–89 (2000) 1076–1078. [3] Y. Kojima, T. Toyama, J. Alloys Compd. 475 (2009) 524–528. [4] L.H. Jiang, Y.L. Zhang, C.Y. Li, J.Q. Hao, Q. Su, J. Alloys Compd. 482 (2009) 313–316. [5] H. Wu, Y. Hu, Y. Wang, B. Zeng, Z. Mou, L. Deng, W. Xie, J. Alloys Compd. (2009), doi.10.1016/j.jallcom.2009.07.002. [6] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [7] A. Pandey, P.D. Sahare, J.S. Bakare, S.P. Lochab, F. Singh, D. Kanjilal, J. Phys. D: Appl. Phys. 36 (2003) 2400–2406. [8] S.P. Lochab, P.D. Sahare, R.S. Chauhan, N. Salah, R. Ranju, A. Pandey, J. Phys. D: Appl. Phys. 40 (2007) 1343. [9] A. Vij, S.P. Lochab, S. Singh, R. Kumar, N. Singh, J. Alloys Compd. 486 (2009) 554–558. [10] V. Kumar, R. Kumar, S.P. Lochab, N. Singh, J. Phys. D: Appl. Phys. 39 (2006) 5137. [11] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R. Kumar, S.P. Lochab, V. Mishra, N. Singh, Opt. Mater. (2009), doi:10.1016/j.optmat.2009.06.018. [12] R. Chen, Y. Kirish, Analysis of Thermally stimulated Processes, Pergamon, New York, 1981. [13] G. Kittis, J.M. Gomez-Ross, J.W.N. Tuyn, J. Phys. D: Appl. Phys. 31 (1998) 2636. [14] A. Vij, S. Singh, R. Kumar, S.P. Lochab, V.V.S. Kumar, N. Singh, J. Phys. D: Appl. Phys. 42 (2009) 105103. [15] B. Huttl, U. Troppenz, K.O. Velthaus, C.R. Ronda, R.H. Mauch, J. Appl. Phys. 78 (1995) 12. [16] V. Singh, T.K. Gundu Rao, J.-J. Miao, J.-J. Zhu, Radiat. Eff. Defects Solids 160 (7) (2005) 265–274. [17] R. Chen, S.W.S. Mckeever, Theory of Thermoluminescence and Related Phenomenon, World Scientific Press, Singapore, 1997. [18] C. Furetta, Handbook of Thermoluminescence, World Scientific, Singapore, 2003.