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Thermoluminescence characteristics of CaS doped with rare earth ions Ce and Sm
Current Applied Physics 11 (2011) 921e925
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thermoluminescence characteristics of CaS doped with rare earth ions Ce and Sm Geeta Sharma a, *, S.P. Lochab b, Nafa Singh a a b
Department of Physics, Kurukshetra University, Kurukshetra 136 119, India Inter University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2010 Received in revised form 19 August 2010 Accepted 27 December 2010 Available online 9 January 2011
CaS:Ce, Sm nanophosphors have been synthesized using solid state diffusion method with an aim to study the thermoluminescence characteristics of CaS:Ce,Sm system with varying concentrations of cerium and smarium in CaS host. The structural characterization of nanophosphors was done using XRD and TEM spectroscopy. We have investigated the effect of different concentrations of cerium and smarium in CaS host on the TL intensity. TL intensity is found to be maximum for CaS:Ce0.4Sm0.6. The effect of different heating rates on the TL glow curves recorded after a UV exposure of 560 mJ/cm2 has been investigated. Peak of the TL glow curve shifts toward higher temperature as the heating rate increases and the TL intensity decreases. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s has been studied and all the glow curves show a single peak around 440 K. With increasing dose the TL intensity increases linearly upto 560 mJ/cm2 of the dose and afterward saturation region is obtained upto 13.5 J/cm2. Beyond it a fall is recorded upto 80 J/cm2 of the exposure. Analysis of the TL glow curve and calculation of the trapping parameters has been done with the help of Glow curve deconvolution software. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Nanophosphors Luminescence
1. Introduction Thermoluminescence is a powerful tool for dosimeteric applications [1e3]. It is used for the study of traps and defect structure of the host lattice as TL is the emission of light from an insulator or a semiconductor after heating when it has absorbed energy from ionizing radiations such as gamma, UV, X-rays etc. Rare earth ions doped Alkaline Earth Sulphide hosts have remained the topic of interest for past many years because of their high luminescent yield and striking luminescent properties [4e7]. Research in nanomaterials has increased in recent years because of their different behavior (on account of quantum size effect and high surface to volume ratio) from their bulk counterparts, which fetch numerous applications to them [8]. Calcium Sulphide belongs to IIeVI group and crystallizes into NaCl type structure. Also known as Lenard phosphor, it is most versatile material for various applications. It is well known material for its luminescent behavior. CaS is a good phosphor for cathode ray tubes [9], TV screens, fluorescent lamps and thermo luminescent dosimeters [10,11]. Being a wide bandgap semiconductor CaS can accommodate large variety of dopants, and by just changing the host dopant combination one can tailor whole of the visible spectrum. Calcium Sulphide doped with rare earth
* Corresponding author. Tel.: þ91 9466471265. E-mail address: [email protected] (G. Sharma). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.020
ions such as cerium, smarium and europium finds applications for optical data storage and optoelectronic devices [12e14]. Recently we have reported the TL properties of Ce-doped CaS nanophosphors exposed to UV radiations [15]. It is well known that doping affects the luminescent properties of nanomaterials. Hence in continuation to our previous work we have investigated the TL behavior of CaS doped with cerium and smarium exposed to UV radiations. We have synthesized CaS:Ce, Sm nanoparticles using solid state diffusion method [16]. Paulose et al. have also synthesized Sm:Ce-doped CaS phosphors using similar method and studied their relaxation kinetics, but their phosphors were not single phased [17]. The synthesized nanophosphors have been characterized by X-ray diffraction. The morphology of the samples has been studied by using transmission electron microscopy. We have investigated the thermoluminescence characteristics of UV irradiated CaS:Ce, Sm nanoparticles. Effect of different heating rates on the glow curve has been studied and determination of the activation energy of traps has been carried out using glow curve deconvolution functions. 2. Experimental We have used solid state diffusion method [16] for synthesizing CaS:Ce, Sm for varying concentrations of cerium and smarium in the host lattice. Calcium sulphate, smarium nitrate, cerium nitrate, sodium thoisulphate, carbon powder and ethanol were the starting
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Fig. 3. TEM image of CaS:Ce0.4Sm0.6. Inset shows the SAED pattern of nanoparticles.
Fig. 1. XRD pattern of CaS:Ce0.4Sm0.6.
source. For recording TL, samples were exposed to UV radiations from a UVGL-58 handheld UV lamp operating at 230 V-50 Hz and emitting 253 nm. Prior to UV exposure, the samples were annealed at 400 C for 10 min and then quenched on a metallic plate at room temperature to erase any residual information. TL glow curves were recorded on a Harshaw TLD reader (Model 3500) fitted with 931B photomultiplier tube (PMT) by taking 5 mg of the sample each time. 3. Results and discussion 3.1. XRD and TEM The phase purity of the formed nanoparticles was checked using X-ray diffraction. Fig. 1 shows the XRD pattern of CaS:Ce0.4Sm0.6 nanophosphors. The average crystallite size was estimated from the FWHM of the most intense XRD peak using Debye Scherrer formula and it comes out w53 nm. The obtained XRD pattern is in good agreement with the standard data available in JCPDS Card number 77-2011 and shows cubic crystalline phase of the host lattice. Fig. 2 shows the XRD spectra for different concentrations of cerium and Fig. 2. The XRD spectra for different concentrations of cerium and smarium in the host lattice.
materials. The calculated quantities of starting materials were taken and mixed thoroughly with the help of an agate pestle and mortar. The charge was placed in a clean graphite crucible and a thin layer of carbon powder was spread over it. This crucible was covered with another similar crucible. This whole arrangement was placed in a muffle furnace and the charge was fired at 950 C for 2 h. After 2 h the charge was taken out and rapidly crushed while red hot with the help of a pestle and mortar. The details of the nanoparticle preparation are reported elsewhere [16]. The samples were characterized by X-ray powder diffraction (XRD) using an in-situ XRD set up (Bruker AXS) having a 3 kW X-ray Table 1 Calculated structural parameters for CaS:Ce,Sm system with varying concentrations of cerium and smarium using XRD results. Sample specification
2q(Degree)
Lattice constant(Ǻ)
Particle size (nm)
CaS:Ce0.4Sm0.6 CaS:Ce0.6Sm0.4 CaS:Ce0.8Sm0.2
31.384 31.379 31.445
5.696 5.697 5.685
53 52 50
Fig. 4. TL glow curves with varying cerium and smarium concentrations in CaS:Ce, Sm system for a UV exposure of 6.5 J/cm2.
G. Sharma et al. / Current Applied Physics 11 (2011) 921e925
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Fig. 5. TL glow curve of CaS:Ce0.4Sm0.6 at different heating rates recorded after a UV exposure of 560 mJ/cm2 .
Fig. 7. TL intensities of the UV irradiated CaS:Ce0.4Sm0.6 plotted against exposure time.
smarium in the host lattice. It is observed from Fig. 2 that there is a slight variation in the peak position and lattice constant with different concentrations of cerium and smarium. This indicates that the doping concentration affects the lattice parameters. Table 1 summarizes the values of diffraction angle (2q), particle sizes and lattice constants for CaS:Ce0.4Sm0.6, CaS:Ce0.6Sm0.4 and CaS:Ce0.8Sm0.2 nanophosphors. Fig. 3 shows the TEM image of CaS:Ce0.4Sm0.6. TEM micrograph shows nanoparticles having nearly spherical shape with an average diameter of 45e50 nm. The inset of Fig. 3 shows the SAED pattern of the nanoparticles formed. SAED pattern shows the closed ring structure which confirms the cubic crystalline phase of the sample. The XRD result is in close agreement with the TEM result.
Variation in the TL intensity as a function of varying cerium and smarium concentrations is shown in the inset of Fig. 4. TL intensity is found to be maximum for CaS:Ce0.4Sm0.6 system. All the glow curves have similar structure i.e. a single peak in 450 Ke464 K range. CaS:Ce0.2Sm0.8 and CaS:Ce0.4Sm0.6 show a single peak at 452 K each, CaS:Ce0.6Sm0.4 shows a peak at 464 K with a shoulder on the lower temperature side at 414 K and CaS:Ce0.8Sm0.2 shows a peak at 456 K. Similar glow curve structure for all CaS:Ce, Sm systems show that with the varying concentration of the dopant the defect structure of the phosphor is not much affected; only the intensity varies. We have investigated the effect of different heating rates on the TL glow curves. Fig. 5 shows the TL glow curve at different heating rates (5 K/s, 10 K/s, 15 K/s) recorded after a UV exposure of 560 mJ/ cm2. With increasing heating rate the glow peaks shifts toward higher temperature and the TL intensity reduces, which may be due to the well known phenomena of thermal quenching of TL due to increase in heating rates [18e21]. These results are also in agreement with our earlier results [15]. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s is shown in Fig. 6. All the glow
3.2. Thermoluminescence characteristics Higher emission intensity is an essential requirement for a good phosphor. Hence we have investigated the effect of different concentrations of cerium and smarium in CaS host on the TL intensity. Fig. 4 shows the TL glow curves with varying cerium and smarium concentrations in CaS host for a UV exposure of 6.5 J/cm2.
Fig. 6. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s.
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based on the First Order Kinetic model [28]. The first order kinetic equation is given by Equation (1)
! E T Tm T2 IðTÞ ¼ Im exp 1 þ kT Tm T2m # E T Tm ð1 DÞ Dm exp Tm kT "
ð1Þ
where, D ¼ 2 kT/E, Dm ¼ 2kTm/E. Fig. 8 shows the experimental glow curve for CaS:Ce0.4Sm0.6 nanoparticles (exposed to 280 mJ/cm2 of UV dose at a heating rate of 10 K/s), deconvoluted into four peaks (using glow curve deconvolution software). The trapping parameters obtained from the deconvoluted glow peaks are shown in Table 2. The goodness of fit is obtained by figure of merit (FOM) which is 0.019 indicating a good fit between experimental and theoretical peaks. The frequency factor (s) [29] was calculated from Equation (2): Fig. 8. The experimental glow curve for CaS:Ce0.4Sm0.6 nanoparticles (exposed to 280 mJ/cm2 of UV dose) deconvoluted into four peaks at a heating rate of 10 K/s.
curves show a peak around 440 K. TL intensities of the UV irradiated samples have also been plotted against UV dose in Fig. 7. TL intensity increases linearly upto 280 mJ/cm2 of the dose and afterward a small change in the TL intensity is noticed upto 13.5 J/cm2, hence it may be the region of saturation. Beyond it a fall is recorded (upto 80 J/cm2) in the TL intensity with further increase in the dose of the UV radiations. In case of UV irradiated phosphors the TL response mainly generates from the surface traps, since these radiations cannot penetrate deeper and hence will not induce lattice defects. The density of surface defects increases with increase in the UV exposure leading to increase in peak intensity. The fall in the TL intensity at higher doses has been reported earlier by several authors [22,23] and is usually a consequence of competition between radiative and non radiative centers or between different kinds trapping centers [24]. There is a slight variation in the peak position with variation of the dose. Some authors have also reported such shifts in the TL peak position with ion beam bombardment and attributed this effect to disorganization of the initial energy bands [25,26]. 3.3. Analysis of the TL glow curve and calculation of the trapping parameters The TL glow curve of CaS:Ce0.4Sm0.6 is a single broad peak at 452 K, hence it seems to be due to the superimposition of other peaks. Using peak shape method, geometric form factor mg ¼ (T2Tm)/(T2T1), where temperatures T1 and T2 correspond to half of the intensities on either side of the maximum, was calculated. Theoretically the form factor which ranges from 0.37 to 0.56 is close to 0.42 for first order kinetics and 0.52 for second order kinetics [27]. The value of mg in the present case is found to be 0.42 which indicates that this glow curve involves first order kinetics. Hence for analysis of TL glow curve, it was deconvoluted using glow fit deconvolution software which is Table 2 Trapping parameters for CaS:Ce0.4Sm0.6 nanostructures exposed to 280 mJ/cm2 UV dose at a heating rate of 10 K/s. Peak number
Tm(K)
E (eV)
Frequency factor (s)
1 2 3 4
378 406 445.4 490.8
0.8193 0.5884 0.5215 0.5004
7.5 109 1.23 105 2.49 104 1.17 103
bE kT2m
¼ s exp E=kTm ½1 þ ðb 1ÞD
(2)
where, b is the heating rate, b is the order of kinetics, D ¼ 2kTm/E and other symbols have their usual meanings. 4. Conclusions CaS:Ce, Sm nanoparticles were synthesized via solid state diffusion method. Particle sizes was calculated from the FWHM of the most intense XRD (200) peak. TEM confirms the cubic crystalline phase of the nanoparticles. XRD results are in close agreement with the TEM results. Thermoluminescence Characteristics of CaS:Ce,Sm system have been investigated. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s has been studied and all the glow curves show single peak around 440 K. For analysis of TL glow curve, it was deconvoluted using glow fit deconvolution software, which is based on the First Order Kinetic model. Acknowledgement One of the authors GS is thankful to the Director, I.U.A.C., New Delhi for financial help in the form of a fellowship under the project UFUP-44305. References [1] A. Pandey, P.D. Sahare, J.S. Bakare, S.P. Lochab, F. Singh, D. Kanjilal, J. Phys. D Appl. Phys. 36 (2003) 2400. [2] A. Pandey, P.D. Sahare, Phys. Stat. Sol. (a) 199 (2003) 533. [3] A. Pandey, P.D. Sahare, Shahnawaz, D. Kanjilal, J. Phys. D Appl. Phys. 37 (2004) 842. [4] S. Xiaolin, H. Guangyan, D. Xinyong, X. Dong, Z. Guilan, T. Guoqing, C. Wenju, J. Phys. Chem. Solid. 62 (2001) 807. [5] Y. Kojima, T. Toyama, J. Alloy. Comp. 475 (2009) 524. [6] M. Suchea, S. Christoulakis, M. Androulidaki, E. Koudoumas, Mater. Sci. Eng. B 150 (2008) 130. [7] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R. Kumar, S.P. Lochab, V. Mishra, N. Singh, Opt. Mat. 32 (2009) 164. [8] G. Cao, Nanostructures & Nanomaterials e Synthesis, Properties & Applications. Imperial College Press, London, 2004. [9] G.L. Marwaha, N. Singh, V.K. Mathur, Radiat. Eff. 53 (1980) 25. [10] G.L. .Marwaha, N. Singh, V.K. Mathur, Mater. Res. Bull. 14 (1979) 1489. [11] G.L. Marwaha, N. Singh, J.S. Nagpal, V.K. Mathur, Radiat. Eff. 55 (1981) 85. [12] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [13] L.N. Lewis, Chem. Rev. 93 (1993) 2693. [14] S.S. Pitale, S.K. Sharma, R.N. Dubey, M.S. Qureshi, M.M. Malik, Opt. Mater. 32 (2010) 461. [15] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Chal. Lett. 6 (2009) 705. [16] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Radiat. Eff. Defect. Solid. 164 (2009) 763.
G. Sharma et al. / Current Applied Physics 11 (2011) 921e925 [17] P.I. Paulose, J. Joseph, M.K. Rudra Warrier, G. Jose, N.V. Unnikrishnan, J. Lumin. 127 (2007) 583. [18] S.P. Kathuria, C.M. Sunta, J. Phys. D Appl. Phys. 15 (1982) 497. [19] S.P. Kathuria, S.V. Moharil, J. Phys. D Appl. Phys. 16 (1983) 1331. [20] N. Vana, G. Ritzinger, Radiat. Prot. Dosimetry. 6 (1984) 29. [21] R.K. Gartia, S.J. Singh, P.S. Mazumdar, Phys. Stat. Sol. (a) 106 (1988) 291. [22] V. Pagonis, R. Chen, J.L. Lawless, Radiat. Meas. 42 (2007) 198e204. [23] J.R. Cameron, N. Suntharalingam, G.N. Kenney, Thermoluminescent Dosimetry. The University of Wisconsin Press, Madison, 1968.
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[24] J.L. Lawless, R. Chen, D. Lo, V. Pagonis, J. Phys. Condens. Matter 17 (2005) 737e753. [25] J. Gaikwad, S. Thomas, S. Kamble, P.B. Vidyasagar, A. Sarma, Nucl. Instrum. Methods Phys. Res. B 156 (1999) 231. [26] N. Salah, P.D. Sahare, J. Phys. D Appl. Phys. 39 (2006) 2684. [27] R. Chen, J. Electrochem. Soc. 116 (1969) 1254. [28] A.J.J. Bos, Radiat. Meas. 41 (2007) S45. [29] C. Furetta, Handbook of Thermoluminescence. World Scientific, Singapore, 2003.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Thermoluminescence characteristics of CaS doped with rare earth ions Ce and Sm Geeta Sharma a, *, S.P. Lochab b, Nafa Singh a a b
Department of Physics, Kurukshetra University, Kurukshetra 136 119, India Inter University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 May 2010 Received in revised form 19 August 2010 Accepted 27 December 2010 Available online 9 January 2011
CaS:Ce, Sm nanophosphors have been synthesized using solid state diffusion method with an aim to study the thermoluminescence characteristics of CaS:Ce,Sm system with varying concentrations of cerium and smarium in CaS host. The structural characterization of nanophosphors was done using XRD and TEM spectroscopy. We have investigated the effect of different concentrations of cerium and smarium in CaS host on the TL intensity. TL intensity is found to be maximum for CaS:Ce0.4Sm0.6. The effect of different heating rates on the TL glow curves recorded after a UV exposure of 560 mJ/cm2 has been investigated. Peak of the TL glow curve shifts toward higher temperature as the heating rate increases and the TL intensity decreases. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s has been studied and all the glow curves show a single peak around 440 K. With increasing dose the TL intensity increases linearly upto 560 mJ/cm2 of the dose and afterward saturation region is obtained upto 13.5 J/cm2. Beyond it a fall is recorded upto 80 J/cm2 of the exposure. Analysis of the TL glow curve and calculation of the trapping parameters has been done with the help of Glow curve deconvolution software. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Nanophosphors Luminescence
1. Introduction Thermoluminescence is a powerful tool for dosimeteric applications [1e3]. It is used for the study of traps and defect structure of the host lattice as TL is the emission of light from an insulator or a semiconductor after heating when it has absorbed energy from ionizing radiations such as gamma, UV, X-rays etc. Rare earth ions doped Alkaline Earth Sulphide hosts have remained the topic of interest for past many years because of their high luminescent yield and striking luminescent properties [4e7]. Research in nanomaterials has increased in recent years because of their different behavior (on account of quantum size effect and high surface to volume ratio) from their bulk counterparts, which fetch numerous applications to them [8]. Calcium Sulphide belongs to IIeVI group and crystallizes into NaCl type structure. Also known as Lenard phosphor, it is most versatile material for various applications. It is well known material for its luminescent behavior. CaS is a good phosphor for cathode ray tubes [9], TV screens, fluorescent lamps and thermo luminescent dosimeters [10,11]. Being a wide bandgap semiconductor CaS can accommodate large variety of dopants, and by just changing the host dopant combination one can tailor whole of the visible spectrum. Calcium Sulphide doped with rare earth
* Corresponding author. Tel.: þ91 9466471265. E-mail address: [email protected] (G. Sharma). 1567-1739/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.12.020
ions such as cerium, smarium and europium finds applications for optical data storage and optoelectronic devices [12e14]. Recently we have reported the TL properties of Ce-doped CaS nanophosphors exposed to UV radiations [15]. It is well known that doping affects the luminescent properties of nanomaterials. Hence in continuation to our previous work we have investigated the TL behavior of CaS doped with cerium and smarium exposed to UV radiations. We have synthesized CaS:Ce, Sm nanoparticles using solid state diffusion method [16]. Paulose et al. have also synthesized Sm:Ce-doped CaS phosphors using similar method and studied their relaxation kinetics, but their phosphors were not single phased [17]. The synthesized nanophosphors have been characterized by X-ray diffraction. The morphology of the samples has been studied by using transmission electron microscopy. We have investigated the thermoluminescence characteristics of UV irradiated CaS:Ce, Sm nanoparticles. Effect of different heating rates on the glow curve has been studied and determination of the activation energy of traps has been carried out using glow curve deconvolution functions. 2. Experimental We have used solid state diffusion method [16] for synthesizing CaS:Ce, Sm for varying concentrations of cerium and smarium in the host lattice. Calcium sulphate, smarium nitrate, cerium nitrate, sodium thoisulphate, carbon powder and ethanol were the starting
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Fig. 3. TEM image of CaS:Ce0.4Sm0.6. Inset shows the SAED pattern of nanoparticles.
Fig. 1. XRD pattern of CaS:Ce0.4Sm0.6.
source. For recording TL, samples were exposed to UV radiations from a UVGL-58 handheld UV lamp operating at 230 V-50 Hz and emitting 253 nm. Prior to UV exposure, the samples were annealed at 400 C for 10 min and then quenched on a metallic plate at room temperature to erase any residual information. TL glow curves were recorded on a Harshaw TLD reader (Model 3500) fitted with 931B photomultiplier tube (PMT) by taking 5 mg of the sample each time. 3. Results and discussion 3.1. XRD and TEM The phase purity of the formed nanoparticles was checked using X-ray diffraction. Fig. 1 shows the XRD pattern of CaS:Ce0.4Sm0.6 nanophosphors. The average crystallite size was estimated from the FWHM of the most intense XRD peak using Debye Scherrer formula and it comes out w53 nm. The obtained XRD pattern is in good agreement with the standard data available in JCPDS Card number 77-2011 and shows cubic crystalline phase of the host lattice. Fig. 2 shows the XRD spectra for different concentrations of cerium and Fig. 2. The XRD spectra for different concentrations of cerium and smarium in the host lattice.
materials. The calculated quantities of starting materials were taken and mixed thoroughly with the help of an agate pestle and mortar. The charge was placed in a clean graphite crucible and a thin layer of carbon powder was spread over it. This crucible was covered with another similar crucible. This whole arrangement was placed in a muffle furnace and the charge was fired at 950 C for 2 h. After 2 h the charge was taken out and rapidly crushed while red hot with the help of a pestle and mortar. The details of the nanoparticle preparation are reported elsewhere [16]. The samples were characterized by X-ray powder diffraction (XRD) using an in-situ XRD set up (Bruker AXS) having a 3 kW X-ray Table 1 Calculated structural parameters for CaS:Ce,Sm system with varying concentrations of cerium and smarium using XRD results. Sample specification
2q(Degree)
Lattice constant(Ǻ)
Particle size (nm)
CaS:Ce0.4Sm0.6 CaS:Ce0.6Sm0.4 CaS:Ce0.8Sm0.2
31.384 31.379 31.445
5.696 5.697 5.685
53 52 50
Fig. 4. TL glow curves with varying cerium and smarium concentrations in CaS:Ce, Sm system for a UV exposure of 6.5 J/cm2.
G. Sharma et al. / Current Applied Physics 11 (2011) 921e925
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Fig. 5. TL glow curve of CaS:Ce0.4Sm0.6 at different heating rates recorded after a UV exposure of 560 mJ/cm2 .
Fig. 7. TL intensities of the UV irradiated CaS:Ce0.4Sm0.6 plotted against exposure time.
smarium in the host lattice. It is observed from Fig. 2 that there is a slight variation in the peak position and lattice constant with different concentrations of cerium and smarium. This indicates that the doping concentration affects the lattice parameters. Table 1 summarizes the values of diffraction angle (2q), particle sizes and lattice constants for CaS:Ce0.4Sm0.6, CaS:Ce0.6Sm0.4 and CaS:Ce0.8Sm0.2 nanophosphors. Fig. 3 shows the TEM image of CaS:Ce0.4Sm0.6. TEM micrograph shows nanoparticles having nearly spherical shape with an average diameter of 45e50 nm. The inset of Fig. 3 shows the SAED pattern of the nanoparticles formed. SAED pattern shows the closed ring structure which confirms the cubic crystalline phase of the sample. The XRD result is in close agreement with the TEM result.
Variation in the TL intensity as a function of varying cerium and smarium concentrations is shown in the inset of Fig. 4. TL intensity is found to be maximum for CaS:Ce0.4Sm0.6 system. All the glow curves have similar structure i.e. a single peak in 450 Ke464 K range. CaS:Ce0.2Sm0.8 and CaS:Ce0.4Sm0.6 show a single peak at 452 K each, CaS:Ce0.6Sm0.4 shows a peak at 464 K with a shoulder on the lower temperature side at 414 K and CaS:Ce0.8Sm0.2 shows a peak at 456 K. Similar glow curve structure for all CaS:Ce, Sm systems show that with the varying concentration of the dopant the defect structure of the phosphor is not much affected; only the intensity varies. We have investigated the effect of different heating rates on the TL glow curves. Fig. 5 shows the TL glow curve at different heating rates (5 K/s, 10 K/s, 15 K/s) recorded after a UV exposure of 560 mJ/ cm2. With increasing heating rate the glow peaks shifts toward higher temperature and the TL intensity reduces, which may be due to the well known phenomena of thermal quenching of TL due to increase in heating rates [18e21]. These results are also in agreement with our earlier results [15]. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s is shown in Fig. 6. All the glow
3.2. Thermoluminescence characteristics Higher emission intensity is an essential requirement for a good phosphor. Hence we have investigated the effect of different concentrations of cerium and smarium in CaS host on the TL intensity. Fig. 4 shows the TL glow curves with varying cerium and smarium concentrations in CaS host for a UV exposure of 6.5 J/cm2.
Fig. 6. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s.
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based on the First Order Kinetic model [28]. The first order kinetic equation is given by Equation (1)
! E T Tm T2 IðTÞ ¼ Im exp 1 þ kT Tm T2m # E T Tm ð1 DÞ Dm exp Tm kT "
ð1Þ
where, D ¼ 2 kT/E, Dm ¼ 2kTm/E. Fig. 8 shows the experimental glow curve for CaS:Ce0.4Sm0.6 nanoparticles (exposed to 280 mJ/cm2 of UV dose at a heating rate of 10 K/s), deconvoluted into four peaks (using glow curve deconvolution software). The trapping parameters obtained from the deconvoluted glow peaks are shown in Table 2. The goodness of fit is obtained by figure of merit (FOM) which is 0.019 indicating a good fit between experimental and theoretical peaks. The frequency factor (s) [29] was calculated from Equation (2): Fig. 8. The experimental glow curve for CaS:Ce0.4Sm0.6 nanoparticles (exposed to 280 mJ/cm2 of UV dose) deconvoluted into four peaks at a heating rate of 10 K/s.
curves show a peak around 440 K. TL intensities of the UV irradiated samples have also been plotted against UV dose in Fig. 7. TL intensity increases linearly upto 280 mJ/cm2 of the dose and afterward a small change in the TL intensity is noticed upto 13.5 J/cm2, hence it may be the region of saturation. Beyond it a fall is recorded (upto 80 J/cm2) in the TL intensity with further increase in the dose of the UV radiations. In case of UV irradiated phosphors the TL response mainly generates from the surface traps, since these radiations cannot penetrate deeper and hence will not induce lattice defects. The density of surface defects increases with increase in the UV exposure leading to increase in peak intensity. The fall in the TL intensity at higher doses has been reported earlier by several authors [22,23] and is usually a consequence of competition between radiative and non radiative centers or between different kinds trapping centers [24]. There is a slight variation in the peak position with variation of the dose. Some authors have also reported such shifts in the TL peak position with ion beam bombardment and attributed this effect to disorganization of the initial energy bands [25,26]. 3.3. Analysis of the TL glow curve and calculation of the trapping parameters The TL glow curve of CaS:Ce0.4Sm0.6 is a single broad peak at 452 K, hence it seems to be due to the superimposition of other peaks. Using peak shape method, geometric form factor mg ¼ (T2Tm)/(T2T1), where temperatures T1 and T2 correspond to half of the intensities on either side of the maximum, was calculated. Theoretically the form factor which ranges from 0.37 to 0.56 is close to 0.42 for first order kinetics and 0.52 for second order kinetics [27]. The value of mg in the present case is found to be 0.42 which indicates that this glow curve involves first order kinetics. Hence for analysis of TL glow curve, it was deconvoluted using glow fit deconvolution software which is Table 2 Trapping parameters for CaS:Ce0.4Sm0.6 nanostructures exposed to 280 mJ/cm2 UV dose at a heating rate of 10 K/s. Peak number
Tm(K)
E (eV)
Frequency factor (s)
1 2 3 4
378 406 445.4 490.8
0.8193 0.5884 0.5215 0.5004
7.5 109 1.23 105 2.49 104 1.17 103
bE kT2m
¼ s exp E=kTm ½1 þ ðb 1ÞD
(2)
where, b is the heating rate, b is the order of kinetics, D ¼ 2kTm/E and other symbols have their usual meanings. 4. Conclusions CaS:Ce, Sm nanoparticles were synthesized via solid state diffusion method. Particle sizes was calculated from the FWHM of the most intense XRD (200) peak. TEM confirms the cubic crystalline phase of the nanoparticles. XRD results are in close agreement with the TEM results. Thermoluminescence Characteristics of CaS:Ce,Sm system have been investigated. Effect of different doses of UV on the TL glow curve of CaS:Ce0.4Sm0.6 at a heating rate of 10 K/s has been studied and all the glow curves show single peak around 440 K. For analysis of TL glow curve, it was deconvoluted using glow fit deconvolution software, which is based on the First Order Kinetic model. Acknowledgement One of the authors GS is thankful to the Director, I.U.A.C., New Delhi for financial help in the form of a fellowship under the project UFUP-44305. References [1] A. Pandey, P.D. Sahare, J.S. Bakare, S.P. Lochab, F. Singh, D. Kanjilal, J. Phys. D Appl. Phys. 36 (2003) 2400. [2] A. Pandey, P.D. Sahare, Phys. Stat. Sol. (a) 199 (2003) 533. [3] A. Pandey, P.D. Sahare, Shahnawaz, D. Kanjilal, J. Phys. D Appl. Phys. 37 (2004) 842. [4] S. Xiaolin, H. Guangyan, D. Xinyong, X. Dong, Z. Guilan, T. Guoqing, C. Wenju, J. Phys. Chem. Solid. 62 (2001) 807. [5] Y. Kojima, T. Toyama, J. Alloy. Comp. 475 (2009) 524. [6] M. Suchea, S. Christoulakis, M. Androulidaki, E. Koudoumas, Mater. Sci. Eng. B 150 (2008) 130. [7] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R. Kumar, S.P. Lochab, V. Mishra, N. Singh, Opt. Mat. 32 (2009) 164. [8] G. Cao, Nanostructures & Nanomaterials e Synthesis, Properties & Applications. Imperial College Press, London, 2004. [9] G.L. Marwaha, N. Singh, V.K. Mathur, Radiat. Eff. 53 (1980) 25. [10] G.L. .Marwaha, N. Singh, V.K. Mathur, Mater. Res. Bull. 14 (1979) 1489. [11] G.L. Marwaha, N. Singh, J.S. Nagpal, V.K. Mathur, Radiat. Eff. 55 (1981) 85. [12] L.L. Beecroft, C.K. Ober, Chem. Mater. 9 (1997) 1302. [13] L.N. Lewis, Chem. Rev. 93 (1993) 2693. [14] S.S. Pitale, S.K. Sharma, R.N. Dubey, M.S. Qureshi, M.M. Malik, Opt. Mater. 32 (2010) 461. [15] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Chal. Lett. 6 (2009) 705. [16] G. Sharma, P. Chawla, S.P. Lochab, N. Singh, Radiat. Eff. Defect. Solid. 164 (2009) 763.
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