Thermoluminescence studies of ultraviolet and gamma irradiated erbium(III)- and ytterbium(III)-doped gadolinium oxide phosphors

Materials Science in Semiconductor Processing 33 (2015) 169–188

Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Thermoluminescence studies of ultraviolet and gamma irradiated erbium(III)- and ytterbium(III)-doped gadolinium oxide phosphors Raunak Kumar Tamrakar a,n, D.P. Bisen b a Department of Applied Physics, Bhilai Institute of Technology (Seth Balkrishan Memorial), Near Bhilai House, Durg, Chhattisgarh 491001, India b School of Studies in Physics and Astrophysics, Pt. Ravishankar Shukla University, Raipur, Chhattisgarh 492010, India

a r t i c l e i n f o

abstract

Available online 20 February 2015

Present paper deals with the thermoluminescence (TL) properties of Gd2O3:Er3 þ , Yb3 þ phosphor synthesized by a conventional solid state reaction method. The TL measurements were performed under UV and gamma irradiations. The structural and morphological analysis of the resulting phosphor was carried out by X-ray diffraction (XRD) study and field emission gun scanning electron microscopic (FEGSEM) technique. The functional group determination of prepared phosphor was carried out by Fourier transform infrared (FTIR) analysis. Elemental analysis was carried out by energy dispersive X-ray analysis. The particle size was determined by transmission electron microscopic (TEM) technique. For UV irradiation UV source providing 254 nm wavelength was used. Whereas Co60 gamma source was used for gamma irradiation. The TL response of Gd2O3:Er3 þ , Yb3 þ phosphor for two different radiations was compared and studied in detail. The process and possible mechanism for TL were investigated and discussed with the help of energy level models. The kinetic parameters such as order of kinetics, activation energy and frequency factors were evaluated by peak shape method and curve fitting technique. Effects of varying concentrations of Er3 þ and Yb3 þ was also investigated. The TL studies were further investigated by applying computerized glow curve deconvolution (CGCD). & 2015 Elsevier Ltd. All rights reserved.

Keywords: Gd2O3:Er3 þ /Yb3 þ Thermoluminescence UV and gamma irradiated phosphor Solid state reaction synthesis CGCD

1. Introduction One to hundred nanometers materials are defined as nanomaterials. Comparing with its bulk size materials, nanoscale materials have considerably different physical, chemical, electrical and optical properties. These properties of materials are size, length and morphology dependent and they often exhibit important differences with bulk materials. The materials at such scale have attracted many researchers in various fields from material science to biotechnology and genetics

n

Corresponding author. Tel.: þ91 9827850113. E-mail addresses: [email protected] (R.K. Tamrakar), [email protected] (D.P. Bisen). http://dx.doi.org/10.1016/j.mssp.2015.01.044 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

[1–3]. Currently, the application and importance of nanomaterials or nanocomposites in the field, of different branches of luminescence has been increased, especially, as they exhibit enhanced optical, electronic and structural properties. The presence of impurity or dopants also change the physical and optical properties such as strength, hardness, luminescence behavior, and lots of another factors also related to it. Now a days, nanomaterials have potential to be used as an efficient phosphors in so many applications like display applications, for example new flat panel displays with low energy excitation sources, solar energy converters, optical amplifiers, and thermoluminescent dosimeters [4–7]. Thermoluminescence (TL) is the phenomenon of light emission during heating a material (materials is like anything synthetic such as phosphor, nanophosphor or natural such as

170

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

from exposure to radiation through the trapping of carriers and (ii) Release of stored energy as visible light due to recombination of trapped carriers at the luminescent centers provided by impurity atoms or defects in the solid under thermal stimulation. In recent times, the phenomenon has been correctly termed as thermally stimulated luminescence (TSL) [8–12]. Gd2O3 is a well know rare earth oxide material, used in different optical, nuclear and electrical applications, because of its tremendous properties such as high melting point ( E2320 1C), chemical durability, thermal stability and low phonon energy ( E600 cm  1). Furthermore, the high density of Gd2O3 (ρ¼7.6 g/cm3) is also suitable as a host for higher concentration of dopent [13–18].

calcite, quartz, etc.), which has been previously excited by using the different sources. These radiation sources are natural such as UV ray by Sun or gamma rays, X-rays, alpha rays, beta rays and light rays. These rays can ‘excite’ a material but to widely different extents and phenomenons. Out of all the excitation energies imparted a very large portion is almost instantaneously dissipated by different conditions and only the required amount of the energies is absorbed and relevant energy is stored in it. By the thermal heating process, the energy may be released, which is stored previously by absorbing the radiation by different mechanisms and some of it may be in the form of light or glowing the material, which we call thermoluminescence. The underlying mechanism involves the role of (i) crystal defects or doped impurity which allows the storing of energy derived

Relative Intensity (Arb Units)

Gd 2O 3 (94%)+ Er2O 3(1%) +Yb 2O 3(5%) +

15

15

20

20

25

25

30

30

35

2θ

35

40

45

50

55

60

JCPDS 43-1014

40

45

50

2θ Fig. 1. XRD Patterns of Gd2O3:Er3 þ (1%)/Yb3 þ (5%) at 1400 1C.

Fig. 2. FTIR spectra of Gd2O3:Er3 þ /Yb3 þ .

55

60

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

In this paper, a solid state reaction method was used to synthesize Er3þ /Yb3þ doped Gd2O3 phosphor. This synthesis has the advantages of inexpensive precursors, convenient process control and large mass production. The Er3þ / Yb3þ doped Gd2O3 phosphor was synthesized at a high temperature of 1400 1C. The structure and TL of Er3þ / Yb3þ doped Gd2O3 phosphors were investigated in detail with variable concentration of Er3 þ and Yb3þ . 2. Experimental method The conventional solid state reaction method was used for the synthesis of the Gd2O3:Er3 þ , Yb3 þ phosphor. Gadolinium Oxide (Gd2O3), Erbium Oxide (Er2O3) and Ytterbium Oxide (Yb2O3) of high purity (99.99%) chemicals were purchased by Sigma-Aldrich and used as precursor

171

material. The stoichiometric ratios of these chemicals were weighed and ground in to a fine powder by using agate mortar and pestle. The ground sample was placed in an alumina crucible and heated at 1000 1C for 1 h and 1400 1C for 4 h in a muffle furnace. The sample is allowed to cool at room temperature in the same furnace. Every heating is followed by intermediate grinding using agate mortar and pestle. The mixtures of reagents were grounded together for 45 min to obtain a homogeneous powder. The Gd2O3:Er3 þ /Yb3 þ phosphor containing variable Yb3 þ concentration with fixed Er3 þ concentration and vice versa have been synthesized by the above mentioned method [15,19]. The crystal size of the phosphor was monitored by X-ray diffraction measurement. The X-ray powder diffraction data was collected by using Bruker D8 Advanced X-ray

Fig. 3. (a–c) Scanning electron microscope images of Gd2O3:Er3 þ /Yb3 þ .

172

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

Fig. 4. EDX of Gd2O3:Er3 þ /Yb3 þ at 1400 1C.

Fig. 5. TEM images of Gd2O3:Er3 þ /Yb3 þ phosphor.

diffractometer using Cu Kα radiation. The X-rays were produced using a sealed tube and the wavelength of X-ray was 0.154 nm. The X-rays were detected using a fast counting detector based on silicon strip technology

(Bruker Lynx Eye detector). Molecular structure was determined by FTIR analysis done by Nicolet Instruments Corporation USA MAGNA-550. The surface morphology of the prepared phosphor was determined by field emission

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

173

3.2. Fourier transformation infrared spectroscopy (FTIR) analysis

Fig. 6. Block diagram of the Integral TL Reader of Nucleonix TL1009I.

scanning electron microscopy (FESEM) JSM-7600F. Energy dispersive X-ray analysis (EDX) was used for elemental analysis of the phosphor. Particle diameter and surface morphology of prepared phosphor were determined by Transmission Electron Microscopy (TEM) using Philips CM200. Thermally stimulated luminescence glow curves were recorded at room temperature by using TLD reader I1009 supplied by Nucleonix Sys. Pvt. Ltd., Hyderabad [20,21]. The obtained phosphor under the TL examination is given UV radiation using 254 nm UV source. For gamma irradiation gamma Co60 source was used 0.5 kGy–2 kGy.

The presence of water and organic residues in the Gd2O3: Er3þ /Yb3 þ phosphor was investigated by IR spectral analysis. Fig. 2 shows the FTIR spectrum of the Gd2O3:Er3 þ /Yb3þ phosphor. Common bands do not exist in the spectrum such as the broad O–H band around 3400 cm  1, the1630 cm  1H2O  vibrational band or the bands related to NO3 groups at 1 1384 cm . The spectrum expresses strong peaks at 458 cm  1 and 550 cm  1 which are the characteristics of Gd–O vibrations. 1634–2921 cm  1 may be due to Er–O and Yb–O respectively. All these discussed peaks confirms the formation of Er3þ , Yb3 þ doped Gd2O3 phosphor [13,14,24,25]. This indicates that the nitrates used in the starting materials were completely eliminated and that the product is free of water. There is no evidence for the presence of any organic intermediates in the sample. On the basis of above discussion, we conclude that temperature affect the luminescent efficiency by improving the crystallinity and eliminating the –OH surface groups. At 1400 1C, as can be seen from the XRD, FEGSEM patterns and TEM, best crystal behaviors was achieved, resulting in highest luminescent intensity.

3.3. Scanning electron microscope (SEM) analysis The powder morphology of the solid state reaction synthesized phosphor was characterized by the FE-SEM technique. Fig. 3a–c presents FE-SEM images of the powder under various magnifications. At low magnification, as shown in Fig. 3a and b, the FE-SEM image clearly shows that the crystal have no uniform shapes and sizes. Also, there are some crystals containing pores and cracks. The FE-SEM image also shows the existence of small particles which is the inherent nature of combustion synthesized products.

3. Result and discussion

3.4. Energy-dispersive X-ray spectroscopy (EDX) analysis

3.1. XRD analysis

The energy dispersive X-ray analysis gives both qualitative and quantitative information about the elemental composition of the materials. From the EDX spectra we can conclude that the presence of other materials such as impurities or adducts in the samples. These impurities occur either accidently due to the reagent molecules or added for modification of the basic materials. The EDX spectra represent elemental analysis of the prepared sample (Fig. 4). In the spectrum intense peak of Gd, Yb, Er and O are present which confirms the formation of Gd2O3:Er3 þ /Yb3 þ phosphor.

Fig. 1 shows the X-ray diffraction pattern of Gd2O3: Er3 þ , Yb3 þ phosphor. The XRD pattern shows welldefined peaks, which indicate a high crystalline behavior of the synthesized compound. All the reflections in Fig. 1 could be indexed to those of standard cubic Gd2O3 (JCPDS File no. 43-1014) phase in addition to weak reflex lines [22]. The size of the crystal was computed from the full width half-maximum (FWHM) of the intense peak using Debye Scherer formula [23]. Formula used for calculation is as follows: D¼

0:9λ β cos θ

Here D is particle size; β is FWHM, λ is the wavelength of X-ray source and θ is angle of diffraction. The average particle size was found to be 42 nm.

3.5. Transmission electron microscopy (TEM) analysis Fig. 5 shows the TEM image of the phosphor [14]. Pattern is indexed with the presence of cubic Gd2O3 phase of the material. The image clearly depicts that most of the particle are of nanosize (40–45 nm). Particles do not posses any well defined shape; however, most of the particles are tending towards spherical shape. Particles are attached to one another, i.e. agglomerated.

174

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

1250

15 Min

(131)

Intesnity (Arb. units)

1000

(293)

750

500

250

0 50

100

150

200

250

300

350

400

Temperature in C 1 kGy

40000

Intensity (Arb. Units)

(173)

30000 (306)

20000

10000

0 50

100

150

200

250

300

350

Temperature in C Fig. 7. (a) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (3%) for UV exposure with heating rate 6 1C/s. (b) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (3%) for 1 kGy Gamma with heating rate 6 1C/s.

3.6. Thermoluminescence analysis The PC controlled TLD Reader was used during investigation. Which was procured from M/S Nucleonix System Pvt. Ltd., Hyderabad, India. The TL Analyzer Type TL1009I, is suitable to retrieve our research goal. This system essentially has two parts; (1) the integral TLD reader and, (2) personnel computer system with TL data acquisition and analysis software. The entire electronic hardware of the integral TL reader consists of low voltage (LV) and high voltage (HV) supplies, temperature controller and thermocouple amplifier circuits, microcontroller based data acquisition circuits, Photomultiplier Tube (PMT), heater transformer, PMT current to Frequency (I to F)

converter and kanthal strip for sample loading with drawer assembly, all housed in to a single enclosure. The unit has LED display to show up temperature of kanthal strip and biasing voltage of the PMT. The software features include file handling, configuration, data acquisition, graph processing and reports. Here Fig. 6 presents the block diagram of TL Reader of Nucleonix TL1009I. 4. Determination of kinetic parameters The TL glow curve is related to the trap levels lying at different depths in the band gap between the conduction and the valence band of a solid. These trap levels are characterized by different trapping parameters such as

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

7 HT 6 HT 5 HT 4 HT

3500 3000

Intensity (Arb Units)

175

2500 2000 1500 1000 500 0 0

50

100

150

200

250

300

350

400

Temperature in 0C HT Vs Peak Temperature

For 1st Peak

145 140

Peak Temperature

135 130 125 120 115 110 105 100 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Heating Rate HT Vs Peak Temperature

305

For 2nd Peak

Peak Temperature

300 295 290 285 280 275 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Heating Rate Fig. 8. (a) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 25 min UV exposure time with different heating rates in 1C/s. (b) TL peak temperature of Gd2O3: Er3 þ (1%), Yb3 þ (5%) for 25 min UV exposure time with different heating rates in 1C/s for 1st peak. (c) TL peak temperature of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 25 min UV exposure time with different heating rates in 1C/s for 2nd peak.

176

Table 1 (a) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 25 min UV exposure time with different heating rates in 1C/s for 1st glow peak. Heating rate

T1 (1C)

Tm (1C)

T2 (1C)

τ

δ

ω

μ¼ δ⧸ω

Activation energy E in eV

Frequency factor S in s  1

7 6 5 4

116 104 91 77

144 131 122 110

187 172 161 148

28 27 31 33

43 41 39 38

71 68 70 71

0.608 0.60294 0.55714 0.535

0.83 0.80848 0.66307 0.5878

1.7  1011 1.9  1011 3.9  109 5.6  108

(b) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 25 min UV exposure time with different heating rates in 1C/s for 2nd glow peak. 241 233 228 222

301 293 289 283

332 324 316 311

60 60 61 61

31 31 27 28

91 91 88 89

0.34 0.34 0.30 0.31

0.67 0.65 0.62 0.61

5.5  106 4.5  106 2.7  106 2.5  106

Table 2 (a) Kinetic parameters of Gd2O3:Er3 þ (1%),Yb3 þ (5%) for 2 kGy gamma exposure with different heating rate in 1C/s for 1st glow peak. Heating rate

T1 (1C)

Tm (1C)

T2 (1C)

τ

δ

ω

μ¼δ⧸ω

Activation energy E in eV

Frequency factor S in s  1

7 6 5 4

147 139 128 115

180 173 164 149

208 200 189 176

33 34 36 34

28 27 25 27

61 61 61 61

0.454 0.44262 0.40984 0.44262

0.79 0.74578 0.6696 0.6667

9.1  109 3.2  109 5.9  109 7.4  109

1.11 1.155 0.989 0.945

4.5  1010 1.2  1011 6.2  109 2.9  109

(b) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 2 kGy gamma exposure with different heating rates in 1C/s for 2nd glow peak. 7 6 5 4

271 269 256 246

310 306 297 288

339 333 323 315

39 37 41 42

29 27 26 27

68 64 67 69

0.421 0.421 0.388 0.391

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

7 6 5 4

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

177

7 HT 6 HT 5 HT 4 HT

60000

Intensity (Arb. unit)

50000

40000

30000

20000

10000

0 50

100

150

200

250

300

350

Temperature in C

For 1st Peak

HT Vs Peak Temperature

175

Peak Temperature

170

165

160

155

150

145 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Heating Rate 335

For 2nd Peak

HT Vs Peak Temperature

Peak Temperature

330

325

320

315

310 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

Heating Rate Fig. 9. (a) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 2 kGy gamma exposure with different heating rates in 1C/s. (b) TL peak temperature of Gd2O3: Er3 þ (1%), Yb3 þ (5%) for 2 kGy gamma exposure with different heating rates in 1C/s for 1st peak. (c) TL peak temperature of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for 2 kGy gamma exposure with different heating rates in 1C/s for 2nd peak.

178

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

3500

1% Yb 2% Yb 3% Yb 4% Yb 5% Yb 6% Yb

Intensity (Arb Units)

3000 2500

(131)

HT=6C/s

(293)

2000 1500 1000 500 0 0

50

100

150

200

250

300

350

400

Temperature in C

Intensity (Arb Units)

3500 3000 2500 2000 1500 1000 1

2

3

4

5

6

Yb 3+ Concentration in mol% Fig. 10. (a) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (1–6%) for 25 min UV exposure time with heating rate 6 1C/s. (b) TL glow curve intensities forGd2O3: Er3 þ (1%), Yb3 þ (1–6%) for 25 min UV exposure time with heating rate 6 1C/s.

trap depth, order of kinetics, and frequency factor [26]. The loss of dosimetry information stored in the materials after irradiation is strongly dependent on the position of trapping levels within the forbidden gap which is known as trap depth or activation energy (E). The mechanism of recombination of detrapped charge carriers with their counter-parts is known as the order of kinetics (b). The frequency factor (s) represents the product of the number of times an electron hits the wall and the wall reflection coefficient, treating the trap as a potential well. Thus, liable dosimetry study of thermoluminescent material is based on its trapping parameters [27]. The TL glow curves of the phosphors were recorded with a constant heating rate 6 1C/s in the temperature range from 50 1C to 400 1C. The TL glow curve of crystalline phosphor irradiated with UV 254 nm source was recorded. The method used to

determine the kinetic parameters is as peak shape method. It extract information from a glow peak utilizing the temperature peak Tm and two temperatures (T1 and T2) on either side of Tm, which are half of peak intensity [8]. 4.1. TL glow curve for UV and gamma exposure The thermoluminescence property of the prepared Gd2O3:Er3 þ /Yb3 þ phosphor for UV and gamma irradiation were recorded. The TL glow curve shows dual peak for both the irradiations (Fig. 7a and b). Presence of dual peaks indicates that the dual dopant have more trap formations. The TL glow curve recorded under 254 nm UV irradiation has peaks at 131 1C and 293 1C where as the TL glow curve for gamma irradiated sample the peaks were found at 173 1C and 306 1C.

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

179

3.4  105 1.6  105 5.1  105 9.2  105 2.5  106 1.5  106

5.2  108 1.4  109 7.9  108 2.8  109 3.0  109 1.6  109

0.28283 0.26471 0.28866 0.28261 0.31111 0.2967 99 102 97 92 90 91 28 27 28 26 28 27 71 75 69 66 62 64 321 320 321 319 321 320

In TL measurements thermal quenching is a major problem that significantly depends on the heating rate used and therefore optimization of heating is needed. The heating rates were optimized for both UV and gamma irradiation for the prepared phosphor. For optimization the TL glow curve were recorded at heating rates of 4 1C s  1, 5 1C s  1, 6 1C s  1 and 7 1C s  1. The effect of heating rate at constant UV exposure time (25 min) and the constant concentration of dopant and codopant Er (1%) and Yb (5%) is presented in Fig. 8(a). It shows that with increasing heating rate the peak position shifted towards the higher temperature side. Due to the effect of heating rate the position of first peak temperature (Tm) and the second glow peak position remain same (Fig. 8b and c ). The kinetic parameter, activation energy and frequency factor for different heating rates for both peaks are mentioned in Table 1(a) and (b). It was found that, with increasing heating rate the peak intensities as well as the total area of the peaks remain constant. As the heating rate was increased from 3 1C/s to 6 1C/s, a slight shift in peak position was observed. The peak temperature shifted to higher temperature side with increase in the peak intensity. After 6 1C/s heating rate peak temperature shifts towards the higher temperature but intensity decreases. This behavior can be explained by the fact that with lower heating rates the charge carries traveling towards the recombination centers for producing the desired luminescence have enough time to get retrapped and do not involve in producing actual luminescence. Whereas, when the heating rate is high, phenomena such as thermal quenching of TL intensity due to larger heating rates arises. The effect of heating rate at constant 2 kGy gamma exposure at the constant concentration of dopant and co-dopant Er (1%) and Yb (5%) are presented in Fig. 9(a)– (c). It shows that with increasing HT rate. The peak position and intensity shows similar behavior as its shows for the UV exposure. The kinetic parameter, activation energy and frequency factor for different heating rates for both peaks are mentioned in Table 2(a) and (b).

222 218 224 227 231 229

293 293 293 293 293 293

4.3. Effect of Er3 þ and Yb3 þ concentration

1% 2% 3% 4% 5% 6%

(b) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (1–6%) for 25 min UV exposure time with heating rate 61C/s for 2nd glow peak.

0.53885 0.50514 0.55671 0.58355 0.62829 0.60522

0.61182 0.64511 0.62586 0.66706 0.67008 0.6495 0.54545 0.52857 0.52778 0.53623 0.5493 0.54795 77 70 72 69 71 73 35 33 34 32 32 33 1% 2% 3% 4% 5% 6%

96 98 97 99 99 98

131 131 131 131 131 131

173 168 169 168 170 171

42 37 38 37 39 40

μ¼δ⧸ω ω δ τ T2 (1C) Tm (1C) T1 (1C) Yb3 þ concentration

Table 3 (a) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (1–6%) for 25 min UV exposure time with heating rate 6 1C/s for 1st glow peak.

Activation energy E in eV

Frequency factor S in s  1

4.2. Effect of heating rates

4.3.1. Effect of Yb3 þ concentration for Gd2O3:Er3 þ (1% fixed), Yb3 þ At constant heating rate, constant UV exposure time and constant concentration of Er3 þ 1 mol% fixed, the variable concentration of co-dopant Yb3 þ from 1–6 mol% was studies. The effect of Yb3 þ concentration shows increase in peak intensity up to 5 mol% of Yb3 þ after that concentration quenching occurs and destroys the trap levels of sample. With increasing concentration of Yb3 þ only the intensity increases with concentration up to 5 mol% of Yb3 þ , no variation was observed in the peak positions (Fig. 10(a)). The TL glow curve recorded for Fixed Er3 þ ¼1% and Yb3 þ ¼(1–6)% Gd2O3 powder shows a strong peak at about 131 1C with a small shoulder at higher temperatures at 293 1C (Fig. 10(a) and (c)). This might suggest a linkage of the defects and TL mechanism.

180

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

4000 3500

.5% Er 1% Er 1.5% 2% Er 2.5% Er

(131)

Intensity (Arb Units)

3000 2500

(292) 2000 1500 1000 500 0 0

50

100

150

200

250

300

350

400

0

Temperature in C 3+

Er Concentration Vs Intensity

3500

For 1st Peak For 2nd Peak

Intensity (Arb. Units)

3000

2500

2000

1500

1000 0.5

1.0

1.5

2.0

2.5

3+

Er Concentration in % Fig. 11. (a) TL glow curve of Gd2O3:Er3 þ (1–2.5%), Yb3 þ (5%) for 25 UV exposure time with heating rate 6 1C/s. (b) TL glow curve intensities for Gd2O3:Er3 þ (1–2.5%), Yb3 þ (5%) for 25 UV exposure time with heating rate 6 1C/s.

Indeed, oxygen related defects sites as singly occupied oxygen vacancies (isolated, or associated with the other impurities or defect sites) or vacancy-interstitial pairs have been observed. The kinetic parameter, activation energy and frequency factor for different Yb3 þ concentrations for both peaks are mentioned in Table 3(a) and (b).

The TL response as a function of varying Er3 þ concentration (0.5–2.5%) with fixed concentration at 5 mol% shows no change in peak position. The intensity of lower peak at 131 1C remains same for different Er3 þ concentrations. Whereas the higher temperature at 293 1C increasing with increasing Er3 þ concentration up to 1 mol% and then get quenched (Fig. 11(a) and (b)).

4.3.2. Effect of Er3 þ concentration for Gd2O3: Er3 þ (0.5–2.5)% and Yb3 þ (5% fix) The effect of variable concentration of Er3 þ (0.5– 2.5 mol%) and the fixed concentration 5 mol% of Yb3 þ on TL glow curve was studied. This indicated that the recorded TL glow curve is suitable for UV dose detection by TLD reader. It might useful for the exposure coming from sunlight with UV radiation (Fig. 11(a)–(b)).

4.3.3. Effect of UV exposure time for Gd2O3:Er3 þ (1%), Yb3 þ (5%) Fig. 12(a) and (b) shows the TL glow curve of Gd2O3: Er3 þ (1%), Yb3 þ (5%) for the variation of different UV exposure times. It shows dual peak at 131 and 293 1C which indicates that the dual dopant have more traps formation and the intensity increases with increasing UV exposure time up to 25 min than intensity decreases due

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

(131)

2000

5 Min 10 Min 15 Min 20 Min 25 Min 30Min

1750

Intensity (Arb. units)

1500

181

(293)

1250 1000 750 500 250 0 0

50

100

150

200

250

300

350

400

450

0

Temperature in C For 1st Peak For 2nd Peak

Dose V s Intensity

3500

Intensity (Arb. Units)

3000

2500

2000

1500

1000

500

0 5

10

15

20

25

30

Time in Minute Fig. 12. (a) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for different UV exposure times with heating rate 6 1C/s. (b) TL glow curve intensities for Gd2O3: Er3 þ (1%), Yb3 þ (5%) for different UV exposure times with heating rate 6 1C/s.

to thermal quenching occurs on the sample. The kinetic parameters for the TL glow curve for various UV exposure times were given in Table 4(a) and (b).

gamma exposure has glow peaks at 173 1C and 306 1C. TL intensity increased with increasing gamma exposure (Fig. 13(a) and (b)).

4.4. TL glow curve for gamma exposure

4.5. UV vs gamma

The effect of gamma exposure on Gd2O3:Er3 þ (1%), Yb (5%) was recorded from 0.5 to 2 kGy. TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for different Gamma exposures were recorded under identical condition of optimized concentration of Er3 þ and Yb3 þ for UV exposure for the comparative studies. The TL glow curve by

The TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for both UV and γ exposure by a C60 source for comparative studies of two different ionizing radiations were presented (Fig. 14). The glow curves of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for UV exposure are entirely different to that of the gamma irradiated sample. The TL glow curve structures of the

3þ

2.7  104 1.9  106 1.3  105 1.7  104 5.6  104 0.42614 0.61586 0.4973 0.40654 0.44981 0.21101 0.31522 0.27619 0.23077 0.2381 109 92 105 117 105 23 29 29 27 25 86 63 76 90 80 315 321 321 319 308 292 292 292 292 283 206 229 216 202 203 0.5% 1% 1.5% 2% 2.5%

0.64511 0.60908 0.60496 0.62303 0.62303 0.52857 0.53333 0.51389 0.51429 0.51429 glow peak. 70 75 72 70 70 6 1C/s for 2nd 0.5% 98 131 168 33 37 1% 96 131 171 35 40 1.5% 96 131 168 35 37 2% 97 131 167 34 36 2.5% 97 131 167 34 36 3þ 3þ (b) Kinetic parameters of Gd2O3:Er (1–2.5%),Yb (15–6%) for 25 UV exposure time with heating rate

Activation energy E in eV μ¼δ⧸ω ω δ τ T2 (1C) Tm (1C) T1 (1C) Er3 þ concentration

Table 4 (a) Kinetic parameters of Gd2O3:Er3 þ (1–2.5%),Yb3 þ (15–6%) for 25 UV exposure time with heating rate 6 1C/sfor 1st glow peak.

1.4  109 4.8  108 4.2  108 7.2  108 7.3  108

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

Frequency factor S in s  1

182

samples irradiated with gamma rays consists of a dual peak around 173 1C and 306 1C while that exposed to UV has prominent peak at 131 1C and 293 1C. There is a significant variation in the glow curve structure, the intensity and the position of the peaks for the two irradiations. It is well known that the interaction of two different types of radiations with matter is based on different principles and this can lead to the alteration in the luminescence and trapping centers [12]. Comparison between the TL glow curve of gamma-rays irradiated and UV ray irradiated TL glow curve illustrates the change in the peak positions along with the small change in glow curve structure which can also be observed by the modifications in the trapping parameters (Tables 5 and 6(a) and (b)). Though, some change was observed in the relative intensity of the two peaks of the glow curves. The position of the TL peak in case of gamma irradiation can be reasonably related to the high concentration of the surface trapping centers whereas, the observed change in the TL glow peak positions of the nanomaterial could be attributed to the disorganization of the initial Trapping Canters/Luminescence Centers, due to the use of highly energetic ions for bombardments [24]. Also, higher rate of energy deposition results in a high degree of ionization and also may lead to alterations in the traps sites. The appearance of low temperature peak in case of UV irradiated phosphor and high temperature peaks for gamma irradiated phosphor is due to different energies of two radiations. The response of Gd2O3:Er3 þ (1%), Yb3 þ (5%) phosphor towards electron and photons is different this can be attributed to different linear energy transfer (LET) or disorganization of the initial trap centers (TC)/ luminescence center (LC) [10]. Comparison between the TL glow curve of gamma-rays irradiated and that induced by UV rays illustrates the change in the peak positions in conjunction with the glow curve structure which can also be observed by the modifications in the trapping parameters (Tables 4 and 5(a) and (b)). Gamma radiation is indirectly ionizing radiation. It was noted that gamma radiation gives higher luminescence efficiency when compared with UV radiation. However, gamma radiation, heavier charged particles may induce additional defects in the host material as compared to UV radiations. Further, with increase in the radiation exposure the density of defects increases leading to increase in peak intensity. 4.6. Analysis of glow curves by GCCD curve fitting and trapping parameters The escaping electron from the trap has equal probability of either being retrapped or of recombining with hole in a recombination center. For this, computerized glow curve fitting methods have been used by physicists studying TL mechanisms and have helped better understanding and important advances in TLD. Considering this fact, the glow curve convolution deconvolution (GCCD) curve fitting in Gd2O3:Er3 þ (1%), Yb3 þ (5%), nananocrystalline material was done using glow curve deconvolution (GCD) functions suggested by Kitis et al. [28,29] for first, second and general order glow curves, respectively. They

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

Intensity (Arb. Units)

2 kGy 1.5 kGy 1 kGy .5 kGy

(173)

60000

183

50000

(306) 40000

30000

20000

10000

0 50

100

150

200

250

300

350

Temperature in 0C 6000

Dose Vs Intensity

For 1st Peak For 2nd Peak

Intensity (Arb. Units)

5000

4000

3000

2000

1000 0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Gamma Dose in kGy Fig. 13. (a) TL glow curve of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for gamma exposure with heating rate in 6 1C/s. (b) TL glow curve intensities for Gd2O3:Er3 þ (1%), Yb3 þ (5%) for gamma exposure with heating rate 6 1C/s.

are applied to the experimentally obtained glow curves to isolate each peak. μg ¼

T2 Tm T 2 T 1

The order of kinetics, activation energy and other trapping parameters were found by applying Chen's formulae. To determine the order of kinetics, the frequency factor which includes T1 and T2 (temperature corresponding to the half of the intensities on either side of the maximum) was calculated and Tm corresponds to the temperature at the maximum TL intensity. Theoretically the frequency factor, which ranges between 0.42 and 0.52, is close to 0.42 for first order kinetics and 0.52 for second order kinetics [29,30].

4.6.1. For UV-ray with optimize concentration of Gd2O3:Er3 þ (1% fixed), Yb3 þ (5%) The TL glow of Gd2O3:Er3 þ , Yb3 þ phosphor could be deconvoluted in to three glow peaks, with peaks at 131,229 and 298 K. The estimated kinetic parameters for Er3 þ , Yb3 þ doped Gd2O3 phosphor were calculated by curve fitting techniques for CGCD curve of experimental data. The overlapping glow peaks (main peak) can lead to the broadening of the glow peak and consequently appear to lower the value of trap depth and higher the value of frequency factor [28]. Trap parameters were calculated using Chen's method [27]. The glow peaks were deconvoluted using the software “peak fit” results on TL parameters is tabulated in Table 7. The activation energy is found in between 0.59 and 1.26 eV and the frequency

184

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

UV Ray Gamma Ray

(173)

60000

Intensity (Arb. unit)

50000

(306) 40000

30000

20000

10000

(131)

(293)

0 50

100

150

200

250

300

350

400

0

Temperature in C 3þ

Fig. 14. TL glow curve of Gd2O3:Er

(1%), Yb3 þ (5%) for both UV exposure and gamma dose.

factor is in the range of 5.59  1008–5.57  1014 for UV irradiated phosphor (Figs. 15 and 16). 4.6.2. For gamma with optimize concentration of Gd2O3: Er3 þ (1% fixed), Yb3 þ (5%) The estimated kinetic parameters for Er3 þ , Yb3 þ doped Gd2O3 phosphor is calculated by curve fitting techniques CGCD curve of experimental data and the peak shape method proposed by Chen and others [8– 10,25,29,30,32]. The activation energy is found in between 0.82 and 2.24 eV and the frequency factor is in the range of 3.15  1010–4.81  1023 for UV irradiated phosphor (Table 8). 4.7. Models for thermoluminescence The presence of defects in material can upset the periodicity of the lattice and disturb the normal band structure, results in the formation of localized energy levels within the forbidden gap. Moving electrons (holes) through the conduction (valence) band can become trapped at these defect sites. While irradiating the material with ionizing radiations, some electrons gain sufficient energy to be raised from the valence band to the conduction band from which they may subsequent be trapped at available defect centers. When their radiated material is heated at sufficiently higher temperatures, these trapped electrons can gain enough thermal energy to escape from the traps back to the conduction band. From here they may make direct transitions back to the valence band or alternatively, they may become re-trapped or may combine with trapped holes. If the electron trap energy levels are close to the conduction band, then this thermal de-trapping may occur at lower temperatures and recombination with holes leads to emission of light. Deeper laying traps will require heating at elevated

temperature in order to release electrons from traps [27,28]. In this study we tried some models to explain the TL response of Gd2 O3:Er, Yb phosphor for two different irradiations. The occurrence of two peaks in case of UV irradiated Gd2 O3:Er, Yb as well as two peaks in case of gamma irradiated phosphor is explained here using the model shown schematically in Fig. 17. According to the given models, irradiation of sample by gamma rays creates two types of traps (one type are of shallower traps and other of deeper traps) in the forbidden band gap of material, as shown in Fig. 17. In electron irradiated samples shallow as well as deep taps are produced due to irradiation as evident from the deconvolution analysis. Similar behavior shown by UV irradiated phosphor. The higher intensity of low temperature peak in gamma and UV irradiated phosphor is due to attribution from shallow traps, which are getting emptied earlier and the deep traps (acting as a reservoir) may replace the m successively (step 2) or also they can go directly to the conduction band (step 1) and recombine with the trapped holes/luminescence centers during their back journey giving rise to a high temperature peak (second peak) [27,28,31,32], as shown in Fig. 17. For low doses of radiation, either the shallow traps get emptied at relatively low temperatures during TL readout and the deep traps may replace some of them or the deep traps go directly (step 1) to the conduction band and recombine subsequently giving rise to the high temperature peak. Irradiating the materials with radiations create free electron hole pairs which moves freely through the lattice and become trapped at defect center. The lifetime of these traps vary from few seconds to years. Usually the shallow/low temperature traps may get emptied during storage of material at room temperature.

Table 5 (a) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for different UV exposure times with heating rate 6 1C/s for 1st glow peak. T1 (1C)

Tm (1C)

T2 (1C)

τ

δ

ω

μ ¼δ⧸ω

Activation energy E in eV

Frequency factor S in s  1

5 10 15 20 25 30

105 100 101 102 101 100

131 131 131 131 131 131

166 171 172 172 172 173

26 31 30 29 30 31

35 40 41 41 41 42

61 71 71 70 71 73

0.57377 0.56338 0.57746 0.58571 0.57746 0.57534

0.83045 0.69513 0.72184 0.74873 0.72184 0.69826

3.7  1011 6.4  109 1.4  1010 3.2  1010 1.4  1010 7.1  109

0.54921 0.60828 0.62935 0.59572 0.6551 0.60726

4.3  105 1.6  106 2.5  106 1.2  106 4.5  106 1.5  106

(b) Kinetic parameters of Gd2O3: Er3 þ (1%),Yb3 þ (5%) for different UV exposure times with heating rate 6 1C/s for 2nd glow peak. 5 10 15 20 25 30

224 229 231 228 233 229

293 293 293 293 293 293

313 323 322 321 324 322

69 64 62 65 60 64

20 30 29 28 31 29

89 94 91 93 91 93

0.22472 0.31915 0.31868 0.30108 0.34066 0.31183

Table 6 (a) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for gamma exposure with heating rate 6 1C/s for 1st glow peak. Dose in kGy

T1

Tm

T2

τ

δ

ω

μ¼ δ⧸ω

Activation energy E in eV

Frequency factor S in s  1

0.5 1 1.5 2

142 141 143 139

173 173 173 173

201 199 201 200

31 32 30 34

28 26 28 27

59 58 58 61

0.47458 0.44828 0.48276 0.44262

0.82565 0.79456 0.85523 0.74578

2.8  1010 1.2  1010 6.4  1010 3.2  109

1.18933 1.2195 1.18541 1.15315

2.5  1011 4.8  1011 2.3  1011 1.2  1011

(b) Kinetic parameters of Gd2O3:Er3 þ (1%), Yb3 þ (5%) for gamma exposure with heating rate 6 1C/s for 2nd glow peak. 0.5 1 1.5 2

270 271 270 269

306 306 306 306

333 330 331 332

36 35 36 37

27 24 25 26

63 59 61 63

0.42857 0.40678 0.40984 0.4127

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

UV exposure time

185

186

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

Table 7 Typical trapping parameters of the deconvolution peaks. The sample was exposed for 25 min UV exposure time. Peaks T1 (1C) Tm (1C) T2 (1C) 131 229 298

β

E (eV)

S (s  1)

T1 (1C) Tm (1C) T2 (1C) lg

Peaks

0.53 1.73 0.66706 2.78  109 0.50 1.59 1.268055 5.57  1014 0.50 1.59 0.855908 5.59  108

168 259 345

Peak 1 139 Peak 2 220 Peak 3 270

170 235 301

E (eV)

For UV Ray Experimental Glow Curve Theoretical Fitted Glow Curve Deconvoluted Peak 1 Deconvoluted Peak 2 Deconvoluted Peak 3

3000

TL Intensity (Arb Units)

b

2500

2000

1500

1000

500

0 50

100

150

200

250

300

350

400

0

Temperature in C Fig. 15. CGCD curve of experimental TL glow peak of Er3 þ , Yb3 þ doped Gd2O3 phosphor with heating rate 6 1C/s.

For Gamma Ray 60000

Experimental Glow Curve Theoretical Fitted Glow Curve Deconvoluted Peak 1 Deconvoluted Peak 2 Deconvoluted Peak 3

50000

40000

30000

20000

10000

0 50

100

150

200

250

300

S (s  1)

0.51 1.80 0.823143 3.15  1010 0.51 1.82 2.240887 4.81  1023 0.50 1.59 1.381159 1.82  1013

203 251 333

3500

TL Intensity (Arb Units)

Peak 1 199 Peak 2 199 Peak 3 250

lg

Table 8 Typical trapping parameters of the deconvolution peaks. The sample was exposed for 2 kGy.

350

Temperature in 0C Fig. 16. CGCD curve of experimental TL glow peak of Er3 þ , Yb3 þ doped Gd2O3 phosphor with heating rate 6 1C/s.

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

187

Fig. 17. Schematic representation of TL glow process for UV and gamma irradiated phosphor.

5. Conclusion Er3 þ , Yb3 þ co-doped Gd2O3 phosphor has been prepared by the solid state reaction method. The structural characterization revealed cubic structure of the prepared phosphor. The average particle size of the prepared phosphor was found to be 42 nm. FTIR studies shows that the phosphor surface was free from killer centers like H2O, CO2 etc. The absence of these molecules may be explained due the preparation of phosphor at very high temperature. The TL response of Gd2O3:Er3 þ , Yb3 þ phosphor for two different radiations was compared and studied in detail. Detail process and possible mechanism for TL are studied and discussed with the help of energy level models. The TL glow curve recorded under UV and gamma radiations. It has two glow peaks for both radiations. This dual peak may be due to presence of more number of trap centers. The increase in TL intensity may be the result of high surface to volume ratio in nanoparticles; as a result high surface energy barrier was formed. For lower dose the defects/traps generated were low due to less surface barrier energy. As c-dose increases the energy density crosses the barrier as a result large number of defects was produced in the nanoparticles which ultimately keep on increasing with the dose till saturation was achieved. The computerized glow curve deconvolation (CGCD) was applied on the TL glow curve of the optimized conditions for both UV and gamma radiations. The deconvolated curve have three peaks at 131 1C, 229 1C, and 298 1C for UV radiation. Similarly deconvolated curve for gamma radiation also have three peaks at 170 1C, 235 1C, 301 1C. The kinetic parameters such as order of kinetics, activation energy and frequency factors for Er3 þ , Yb3 þ doped Gd2O3 phosphor were evaluated by peak shape method after applying curve fitting technique. The frequency factor for the TL glow curve of UV radiation was 2.78  109 s  1, 5.57  1014 s  1, 5.59  108 s  1 for peak 1, peak 2 and peak 3 respectively. The activation energy for the peaks 1, 2 and 3 were 0.667 eV, 1.268 eV, 0.855 eV respectively. For gamma radiated TL glow curve frequency factor for all the three

deconvolated peaks 1, 2 and 3 was 3.15  1010 s  1, 4.81  1023 s  1, 1.82  1013 s  1 respectively and activation energy for these peaks was 0.823 eV, 2.240 eV, and 1.381 eV, respectively.

Acknowledgment We are very grateful to IUC Indore for XRD characterization and also thankful to Dr. Mukul Gupta for his cooperation. I am very thankful to SAIF IIT, Bombay for other characterization such as TEM and EDX. I am very thankful to vikas dubey for TL studies. References [1] N. Salah, S. Habib Sami, H. Khan Zishan, D Jouide Fathi, Radiat. Phys. Chem. 80 (2011) 923–928. [2] NS Xu, Huq S Ejaz, Mater. Sci. Eng. R R 48 (2–5) (2005) 47–189. [3] R.N. Bhargawa, D Gallagher, Phys. Rev. Lett. 62 (3) (1994) 416–419. [4] Yi Guangshun, Baoquan Sun, Fengzhen Yang, Depu Chen, J. Mater. Chem. 11 (12) (2001) 2928–2929. [5] N. Salah, S. Habib Sami, Z.H. Khan, S. Al-Hamedi, S.P Lochab, J. Lumin 129 (3) (2009) 192–196. [6] E. Hemmera, T. Yamano, H. Kishimoto, N. Venkatachalam, H. Hyodo, K. Soga, Cytotoxic aspects of gadolinium oxide nanostructures for up-conversion and NIR bioimaging, Acta Biomat. 9 (2013) 4734–4743. [7] T. Justel, J.C. Krupa, D.U. Wiechert, J. Lumin 93 (3) (2001) 179–189. [8] S W S Mc Keever, Thermoluminescence of Solids (Cambridge Solid State Science Series), , 1985. [9] Nambi K. S. V. Thermo1uminescence its understanding and applications, Health Physics Division, Bhabha Atomic Research Centre, Brazil, 1977. [10] Klaus Becker, Solid State Dosimetry, CRC PressOhio, 1973. [11] McKeever, Chen, Radiat. Meas. 27 (1997) 625–661. [12] S.W.S. McKeever, Radiat. Protect. Dosim. 17 (1981) 431–435. [13] R.K. Tamrakar, D.P. Bisen, AIP Conf. Proc 1536 (2013) 273–276, http: //dx.doi.org/10.1063/1.4810206. [14] R.K. Tamrakar, D.P. Bisen, Brahme Nameeta, Res. Chem. Intermed. 40 (2014) 1771–1779. [15] R.K. Tamrakar, D.P. Bisen, C.S. Robinson, I.P. Sahu, Brahme Nameeta, Indian J. Mater. Sci. (2014)7. [16] I.L. Medintz, H.T. Uyeda, E.R. Goldman, H. Mattoussi, Nat. Mater. 4 (2005) 435–446.

188

R.K. Tamrakar, D.P. Bisen / Materials Science in Semiconductor Processing 33 (2015) 169–188

[17] Tiwari Ratnesh, Awate Vikram, Tolani Smita, Verma Namrata, Dubey Vikas, Raunak Kumar Tamrakar, Results Phys. 4 (2014) 63–68. [18] Tamrakar R. K., Bisen D. P. and N. Bramhe, Influence of Er3 þ concentration on the photoluminescence characteristics and excitation mechanism of Gd2O3:Er3 þ phosphor synthesized via a solidstate reaction method, http://dx.doi.org/10.1002/bio.2803. [19] Tamrakar Raunak, Dubey Vikas, Swamy N. Kumar, Tiwari Ratnesh, S. V.N. Pammi, P.V. Ramakrishna, Thermoluminescence studies of UV-irradiated Y2O3: Eu3+ doped phosphor, Research on Chemical Intermediates 39 (8) (2013) 3919–3923. [20] Tamrakar Raunak Kumar, D.P. Bisen, Optical and kinetic studies of CdS: Cu nanoparticles, Res. Chem. Intermed. 39 (2013) 3043–3048. [21] R. Tiwari, Taunk P. Bala, Tamrakar R. Kumar, Swamy N. Kumar, V. Dubey, Chalcogenide Lett. 11 (3) (2014) 141–158. [22] Grier D., Mccarthy G., North Dakota State University, Frago, North Dakota, USA, ICCD Grant-in-Aid (1991), 1991.

[23] A. Guinier, X-ray diffraction, Freeman, San Francisco, 1963. [24] C. Joshi, A. Rai, Y. Dwivedi, S.B. Rai, J. Lumin. 132 (2012) 806–810. [25] R.K. Tamrakar, D.P. Bisen, Nameeta Brahme, Recent Res. Sci. Technol. 4 (8) (2012) 70–72. [26] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, Germany, 1994 (ISNN: 978-3-540–58019-5). [27] R. Chen, Y krish, Analysis of Thermally Stimulated Process, Pergamon Press, Oxford, 1981, 159–161. [28] G. Kitis, G.J.M. Gomez-Ros, J.W.N. Tuyn, Phys. J. D: Appl. Phys. 31 (1998) 2636. [29] Shaila BahlAnant PandeyS.P. Lochab, V.E. Aleynikov, A.G. Molokanov, Pratik Kumar, J. Lumin. 134 (2013) 691–698. [30] P Bhushan, N.S Kore, S.J Dhoble, Dhoble, J. Lumin. 150 (2014) 59–67. [31] P.D. Sahare, J.S. Bakare, S.D. Dhole, N.B. Ingale, A.A. Rupasov, J. Lumin. 130 (2010) 258. [32] Manveer Singh, P.D. Sahare, Kumar Pratik, Radiat. Meas. 59 (2013) 8–14.