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Violet blue emission and thermoluminescence glow curve analysis of Gd2SiO5:Ce3+ phosphor
Optik 127 (2016) 6243–6252
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Optik journal homepage: www.elsevier.de/ijleo
Violet blue emission and thermoluminescence glow curve analysis of Gd2 SiO5 :Ce3+ phosphor Yogita Parganiha a,∗ , Jagjeet Kaur a , Vikas Dubey b,∗ , Ravi Shrivastava c , Deepika Chandrakar a a b c
Department of Physics, Govt. Vishwanath Yadav Tamaskar Post Graduate Autonomous College, Durg (C.G.), 491001, India Department of Physics, Bhilai Institute of Technology, Kendri, Raipur, C.G., India ICFAI University, Kumhari, Raipur, C.G., India
a r t i c l e
i n f o
Article history: Received 22 February 2016 Accepted 15 April 2016 Keywords: Violet-blue emission Ce doped GSO phosphor TL glow curve analysis
a b s t r a c t This paper reports synthesis of Ce3+ ions-doped Gd2 SiO5 phosphor powders prepared by modified solid state reaction method, which is most suitable method for large-scale production. Starting materials used for sample preparation were Gd2 O3 , SiO2 and CeO2 with variable concentration of cerium (0.1 mol% to 2.5 mol%) with fixed amount of boric acid (0.05 mol%) as flux. PL excitation spectra was found at 260 nm while the emission spectra was found at 365, 378, 555, 602, 607 and 627 nm for lower Ce3+ doping concentration which are attributed to the Gd3+ ion transitions. For higher concentration of Ce3+ we get broad emission spectra from 350 to 550 nm which is characteristics of Ce3+ ion transition. Commission Internationale de l’Eclairage co-ordinate (CIE) of this phosphor is x = 0.170, y = 0.029, in the violet region for 2.5 mol% of cerium. TL glow curves were recorded for different doses of UV exposure and gamma ray dose at a heating rate of 5◦ C s−1 . The kinetic parameters such as activation energy “E”, the order of kinetics “b”, and the frequency factor “s” of Gd2 SiO5 :Ce3+ have been calculated by using peak shape method. Sample shows general order kinetics for most of the peaks. The effect of Ce3+ concentration on TL intensity was also studied and found that the intensity is maximum for 0.1 mol% and then decreases with increasing concentration of Ce3+ . The present phosphor can acts as a single host for violetblue emission (2.5 mol%) for display devices as well as may be used for thermoluminescence dosimetric material under UV & gamma dose. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction In past few years, research on luminescent nanophosphors provide challenge to development of technologies in various areas such as electronics, photonics, display, lasing, detection, optical amplification, fluorescent sensing in biomedical engineering and environmental control [1–5]. Nanoscale phosphors may have a number of potential advantages over micronsized phosphors. Rare earths (RE) are well known for their extensive use in luminescent materials. The rare earth ions doped inorganic nanophosphor is one of the most promising materials for a variety of applications in solid state lighting (SSL), solid-state lasers, lighting and displays and optical communication fields such as fluorescent lamps, cathode ray tubes, and field emission displays. Light emitting diodes (LEDs) are the ultimate light source in the solid state lighting (SSL) technol-
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Parganiha), [email protected] (V. Dubey). http://dx.doi.org/10.1016/j.ijleo.2016.04.064 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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ogy. The LED technology has flourished for the past few decades. High efficiency, reliability, low power consumption, and durability are among the key factors for the rapid development of the SSL based high-brightness visible LEDs. Silicate family is an attractive class of materials among inorganic phosphors due to their special properties such as water, chemical resistance and visible light transparency. They show superior properties due to thermal stability, low cost, wide band gap, chemical resistance, non toxicity [6]. Among silicate based phosphors, Ce-doped Gd2 SiO5 (GSO:Ce) found its application as a promising and attractive scintillator with a high absorption coefficient, high luminescence output, fast decay time and excellent irradiation hardness (109 rad) [7,8]. As a scintillator it has attracted interest for extensive applications such as nuclear medical imaging, nuclear physics, in ␥-ray detection for oil well dogging and in the field of high energy physics research [9–19]. In this paper, we study the Photoluminescence and thermoluminescence of GSO:Ce3+ phosphor. Phosphor was prepared using modified solid state reaction method. The sample was characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), Field Emission Gun Scanning electron microscopy (FEGSEM), Transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX).
2. Experimental Phosphor of Gd2 SiO5 doped with Ce3+ ions with variable molar concentration of Ce3+ (0.1% to 2.5%) was prepared by solid state reaction method. The precursors Gd2 O3, SiO2 , CeO2 and H3 BO3 (as flux) used for synthesis of Gd2 SiO5 :Ce3+ . The composition of each chemical weighed in proper stoichiometric ratio then mixed thoroughly for 45 min using mortar and pestle. The grinded sample was placed in an alumina crucible and subsequently fired at 1000 ◦ C for 1 h for calcinations and then heated at 1250 ◦ C for 3 h for sintering in a muffle furnace. Every heating was followed by intermediate grinding. Finally the samples were cooled slowly to room temperature in the furnace and ground again into powder for subsequent characterization. Complete reaction is given as:
The FTIR spectrum of a sample was recorded at room temperature in the wave number range of 4000–400 cm−1 on a Bruker spectrophotometer. Observation of particle morphology and elemental analysis was investigated by FEGSEM (field emission gun scanning electron microscope) JEOL JSM7600F. TEM (transmission electron microscope) analysis was carried out by PHILIPS CM200. The photoluminescence (PL) emission and excitation spectra were recorded at room temperature by use of a Shimadzu RF-5301 PC spectrofluorophotometer. The excitation source was a Xenon lamp. The obtained phosphor under the TL examination is given UV radiation using 254 nm UV source Thermoluminescence glow curves were recorded at room temperature by using TLD reader I1009 supplied by Nucleonix Sys. Pvt. Ltd. Hyderabad [20–43].
3. Results and discussion 3.1. XRD analysis of Gd2 SiO5 :Ce3+ In order to obtain the lattice parameters of the sample the powder diffraction pattern was analyzed by Rietveld fitting method [22]. Lattice parameters were refined until to obtain a fairly good agreement between observed and calculated XRD spectra. The pattern is characterized by few prominent peaks found at different glancing angles [43–46]. The size of the crystallite has been computed from the full width half maximum (FWHM) of every peak using the Scherer’s formula [23,24]. The Scherer formula is given by: D=
0.9 Cos
(1)
where D is crystallite size, is the wavelength of X-ray,  is the FWHM, and is the diffraction angle. Calculated crystallite size for different angle of glancing is shown in Table 1b. The average crystallite size is 34.525 nm. The indexing and refinement of lattice parameters are done using Celref v. 3 software. The XRD pattern of the sample is shown in Fig. 1. It shows a monoclinic body centered structure with space group P21 /c and unit cell dimensions a = 9.1045 Å, b = 6.9934 Å, c = 6.8325 Å, ␣ = 90◦ ,  = 107.29◦ and ␥ = 90◦ . It is observed that the peak of XRD of the sample showed resemblance with the XRD pattern of Gd2 SiO5 , reported by Dramicanin et al. [25]. There are few more peaks in observed XRD pattern which could be due to a great number of stacking faults induced by the presence of the doping ions and also due to secondary phases and impurities formed during the elaboration process. The calculated lattice parameters are given in Table 1a.
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Table 1a Indexing and lattice parameters of Gd2 SiO5 :Ce3+ . Standard Values Zero
Lambda
a
b
c
alpha
beta
gamma
Vol.
0 0
1.5419 0
9.1105 1
6.9783 1
6.8544 1
90 0
107.14 1
90 0
416.4
Final Values: (Standard errors on 2nd line) Zero
Lambda
a
b
c
alpha
beta
gamma
Vol.
0 0
1.5419 0
9.1045 0.1183
6.9934 0.0142
6.8325 0.094
90 0
107.29 1.749
90 0
415.4
h
k
l
2 (Obs)
2 (Cal)
Dif
−1 −1 2 1 3 −3 0 −2 −2 3 −3 −3
1 2 1 2 1 2 3 2 3 3 3 1
2 1 1 1 0 1 1 3 2 0 2 4
29.100 29.442 30.644 32.060 33.742 39.554 41.230 48.473 49.049 50.000 52.814 57.557
29.1721 29.3577 30.6914 32.1497 33.4751 39.5875 41.1211 48.4530 48.9709 50.2319 52.9358 57.5382
−0.0721 0.0843 −0.0474 −0.0897 0.2669 −0.0335 0.1089 0.0200 0.0781 −0.2319 −0.1218 0.0188
160
Relativity Intensity (arb. unit)
140 120 100 80 60 40 20 0 20
30
40
2
50
60
degree
Fig. 1. XRD pattern of Gd2 SiO5 :Ce3+ (2.5 mol%) phosphor.
Fig. 2. FTIR Spectrum of Gd2 SiO5 :Ce3+ (2.5%).
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3.2. FTIR analysis of Gd2 SiO5 :Ce3+ Fig. 2 shows FTIR spectra of prepared sample. In the spectrum the absorption band of silicate groups are clearly evident. The intense broad band centered at 879 cm−1 is assigned to the presence of SiO4 2− group. The band at 409, 420, 427 and 438 cm−1 are assigned to Si-O-Si bending mode of vibration. The band from 448 to 541 cm−1 is due to vibration of Gd-O bonds. The peaks at 420 and 505 cm−1 are assigned to Ce-O vibration. Small peak originated at 2362 cm−1 may be due to presence of CO2 , absorbed from atmosphere and 2923 cm−1 may be due to filter contamination of the FTIR instrument. All these discussed peak found together confirms the formation of Gd2 SiO5 :Ce3+ . 3.3. FEGSEM analysis of Gd2 SiO5 :Ce3+ (2.5 mol%) The particle size and surface morphology of prepared sample was carried out with a Field emission gun scanning electron microscope (FEGSEM) at four different magnifications (10k, 25k, 50k and 100k). Fig. 3[a–d] shows the morphology with different agglomerate shapes, such as sponge like and small spherical. The surface morphology is good and particle size is estimated as of few microns. 3.4. HRTEM analysis of Gd2 SiO5 :Ce3+ (2.5 mol%) To investigate exact particle size and surface morphology of phosphor transmission electron microscopy images was obtained (Fig. 4[a–b]). Here it is clear from HRTEM images that it is formation of spherical shape particles. The high temperature synthesis method maintains the morphology of the prepared phosphor and particle size was found nearly 200 nm. It is clear that the FEGSEM images support the result of HRTEM image (Fig. 4[a–b]).
Table 1b Determination of Crystallite size of Gd2 SiO5 :Ce3+ . 2
 (FWHM)
d spacing
D (Crystallite Size) nm
29.100 29.442 30.644 32.060 33.742 39.554 41.230 48.473 49.049 50.000 52.814 57.557
0.197 0.170 0.210 0.210 0.210 0.170 0.341 0.262 0.183 0.275 0.275 0.328
3.066 3.031 2.915 2.789 2.654 2.276 2.187 1.876 1.855 1.822 1.732 1.600
41.765 48.237 39.301 37.913 39.607 49.569 24.917 33.246 47.607 31.859 32.235 27.658
Fig. 3. FEGSEM images of Gd2 SiO5 :Ce3+ (2.5%) (a) ×10k (b) ×25k (c) ×50k (d) ×100k.
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Fig. 4. [a–b] HRTEM images of Gd2 SiO5 :Ce3+ (2.5%).
The phosphor particles should have a spherical shape and high luminescence efficiency for successful applications. Indeed, phosphor particles with a spherical shape minimize light scattering on their surfaces and therefore, improve the efficiency of light, emission and the brightness of such a phosphor [45,46]. 3.5. Photoluminescence (PL) studies 3.5.1. PL excitation spectra Fig. 5[a] shows the PL excitation spectra of Gd2 SiO5 :Ce3+ phosphor monitored at 615 nm. The peak centered at 260 nm corresponds to higher energy 8 S7/2 -6 DJ transitions of Gd3+ . 3.5.2. PL emission spectra For lower Ce3+ doping concentration (0.1% to 2 mol%) in Gd2 SiO5 host, the peak in UV region (365 & 378 nm) is due to radiative recombination of holes and electrons in Gd2 SiO5 (Fig. 5[b]). The peaks in the visible region in the emission spectra may be due to transition from 6 GJ state. The peak at 555 nm may be due to stark level transition from the 6 GJ state of Gd3+ ion. The red emission peaks at 602, 607, 627 and 640 nm corresponds to 6 GJ -6 PJ transition. It is clear that for lower concentration of Ce3+ ion, dominant peaks are related to Gd3+ ion. For higher Ce3+ doping concentration (2.5 mol%), transition due to Ce3+ ion dominates over transition due to Gd3+ ion and we get emission spectra of Ce3+ ion. The emission spectrum of Gd2 SiO5 :Ce3+ (2.5 mol%) can be decompose into two bands peaked at 413 and 440 nm, which assigned to 5d-4f (2 F5/2 ) and 5d-4f (2 F7/2 ) transitions of Ce3+ , respectively [15,24,25]. So the 2.5 mol% was optimized cerium concentration for GSO:Ce phosphor which may be applicable for blue light emission in display device application for more detail decomposed emission spectra shown in Fig. 5[c] which shows blue emission for cerium doped phosphor.
a
b
300 260 nm
440nm
800
250
413nm
700 Intensity (arb. unit)
Intensity (arb. unit)
900
200 150 100 50
600 500 400 300 200
365nm
378nm
602nm 627nm 607nm
555nm
100
640nm
0
0 200 220 240 260 280 300 320 340 360
400
c
440nm
600
Experimental peak Fitted peak 1 Fitted peak 2
800
413nm
Intensity (arb. unit)
500 Wavelength (nm)
Wavelength (nm)
600
400
200
0 350
400
450
500
550
600
650
Wavelength (nm)
Fig. 5. [a] PL excitation spectra of Gd2 SiO5 :Ce3+ monitored with 615 nm. [b] PL emission spectra of Gd2 SiO5 :Ce3+ with variable Ce3+ concentration. [c] Decomposed emission spectra of Gd2 SiO5 :Ce3+ (2.5 mol%).
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Fig. 6. CIE coordinate diagram of Gd2 SiO5 :Ce3+ (2.5 mol%). Table 2 Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 5 min UV dose at heating rate 5 ◦ C s−1 . Peak
T1 (K)
Tm (K)
T2 (K)
␦
= ␦/
Activation energy (Eav )
Frequency factor (s)
Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7
337.70 345.00 358.60 389.10 438.20 480.90 478.00
348.50 360.90 386.80 413.70 456.30 500.40 515.00
357.20 372.40 410.80 431.70 469.30 514.20 543.20
10.80 15.90 28.20 24.60 18.10 19.50 37.00
8.70 11.50 24.00 18.00 13.00 13.80 28.20
19.50 27.40 52.20 42.60 31.10 33.30 65.20
0.45 0.42 0.46 0.42 0.42 0.41 0.43
1.44 0.96 0.65 0.79 1.36 1.46 0.82
4 × 1020 1 × 1013 7 × 108 1 × 109 3 × 1015 1 × 1014 1 × 107
3.6. CIE co-ordinates The luminous colour was depicted by studying colour co-ordinates and colour ratios of Gd2 SiO5 :Ce3+ phosphor. The values of chromaticity coordinates of the GSO powder has been estimated from 1931CIE system and is shown in Fig. 6. It was observed that the Commission Internationale de l’Eclairage (CIE) co-ordinates of GSO phosphor were measured as (x, y) and the value is found in violet-blue region [26,27]. Their corresponding location has been marked in Fig. 6 with cross in violet-blue region.
ColourPurity=
(xs − xi )2 + (ys − yi )2
(xd − xi )2 + (yd − yi )2
× 100%
(2)
where (xs , ys ) are the coordinates of a sample point, (xd , yd ) are the coordinates of the dominant wavelength and (xi ,yi ) are the coordinates of the illuminant point. The calculation was carried out.It indicate that colour purity increased with rare earth concentration and was a maximum at 2.5 mol% of Ce3+ concentration reaches to 89% [22–42]. The results indicate that GSO:Ce3+ (2.5%) phosphors gave the maximum violet-blue emission and good violet-blue chromaticity with high colour purity and therefore have promising applications in solid-state lighting and display devices (Fig. 6). 3.7. Thermoluminescence (TL) studies For variable Ce3+ ion (0.1–1 mol%) at fixed UV exposure time i.e. 5 min, TL glow curve was recorded (Fig. 7[a]). For Ce3+ (0.1 mol%) the TL glow curve intensity was maximum after that TL intensity decreases with increasing concentration of Ce3+ . Fig. 7[b] shows the CGCD curve of Gd2 SiO5 :Ce3+ (0.1%) for 5 min UV dose at heating rate 5 ◦ C s−1 and Table 2 presents its calculated kinetic parameters for seven deconvoluted peaks (Fig. 7[b]). It shows first order kinetics. The recorded glow curves of the prepared phosphors exposed to 5 min UV exposure with different concentration of cerium ion seem to be composite in nature as they exhibits three broad peaks (Fig. 7[a]). So, CGCD method was used to deconvolute and calculate the kinetic parameters. Here in present case, the glow curves of maximum intense peak i.e. 0.1 mol% of cerium ion irradiated with 5 min UV exposure was deconvoluted by CGCD method using Glowfit programme [28,32].
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Fig. 7. [a] TL glow curve of Gd2 SiO5 :Ce3+ with variable Ce3+ concentration for 5 min UV dose at heating rate 5 ◦ C s−1 . [b] CGCD curve of Gd2 SiO5 :Ce3+ (0.1%) for 5 min. UV dose at heating rate 5 ◦ C s−1 . [c] TL glow curve of Gd2 SiO5 :Ce3+ (0.1%) with variable UV dose at heating rate 5 ◦ C s−1 . [d] UV dose vs TL intensity plot of Gd2 SiO5 :Ce3+ (0.1%). [e] CGCD curve of Gd2 SiO5 :Ce3+ (0.1%) for 10 min UV dose at heating rate 5 ◦ C s−1 . [f] TL Glow curve of Gd2 SiO5 :Ce3+ for variable concentration for 0.1 kGy gamma dose at heating rate 5 ◦ C s−1 . [g] TL Glow curve of Gd2 SiO5 :Ce3+ (0.1%) for variable gamma dose at heating rate 5 ◦ C s−1 . [h] Gamma dose vs intensity plot for Gd2 SiO5 :Ce3+ (0.1%). [i] CGCD Glow curve of Gd2 SiO5 :Ce3+ (0.1%) for 2 kGy gamma dose at heating rate 5 ◦ C s−1 .
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Table 3 Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 10 min UV dose at heating rate 5 ◦ C s−1 . Peak
T1 (K)
Tm (K)
T2 (K)
␦
= ␦/
Activation energy (Eav )
Frequency factor (s)
Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6
353.40 386.90 421.20 468.30 483.60 466.50
376.90 408.50 436.60 487.20 508.10 525.30
393.20 424.80 446.60 499.90 527.00 571.40
23.50 21.60 15.40 18.90 24.50 58.80
16.30 16.30 10.00 12.70 18.90 46.10
39.80 37.90 25.40 31.60 43.40 104.90
0.41 0.4 3 0.39 0.40 0.44 0.44
0.67 0.92 1.33 1.39 1.30 0.52
2 × 108 7 × 1010 9 × 1014 8 × 1013 2 × 1012 1 × 104
This computer programme is based on Halperin and Barner equations which describe the flow of the charges between the various energy levels during a trap emptying by thermal heating. The kinetic parameters of trap levels were determined for each deconvoluted peak by this programme [29]. The theoretical generated glow curves were fitted with the experimnatal glow curves and the quality of fitting was checked by calculating the figure of merit (FOM) for each fitting defined by [29–31]
FOM =
− TLExp − TLThe
(3)
here TLexp and TLThe represent the TL intensity of experimental and theoretical glow curves respectively. The summation extends over all the available experimental data points. Quality of fitting and choice of the appropriate number of peaks was refined by repeating the process of fitting in order to get the minimum FOM with minimum number of possible peaks. The fits were considered adequate when the FOM values were observed below 5% with most actual being below 2%. In the present studies, FOM was found 1.89% which confirms a very good agreement between theoretical generated and experimentally recorded glow curves. The fitted TL glow curves are shown in Fig. 7[b] and the values of trap depths and frequency factors of trapped charges calculated by CGCD method are summarized in Table 2. Fig. 7[c] shows the TL glow curve of Gd2 SiO5 :Ce3+ (0.1%) with different UV exposure time at constant heating rate i.e. 5 ◦ C s−1 . The sample shows three peaks at 84.14 ◦ C, 138.51 ◦ C and 229.92 ◦ C with different UV exposure time. Three distinct peaks show composite nature and it show formation of number of traps. Some may be shallow and some deep traps, low temperature peak shows formation of shallower trap and high temperature peak shows deep trapping. The TL intensity increases up to 10 min of UV exposure than it decreases (Fig. 7[d]). The estimated kinetic parameters for the phosphor is calculated by curve fitting techniques CGCD curve of experimental data (Fig. 7[e]) using peak shape method proposed by Chen [32]. The activation energy is found in between 0.52–1.39 eV and the frequency factor is range of 1 × 104 to 9 × 1014 for 10 min UV irradiated phosphor. Table 3 shows the Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 10 min UV dose at heating rate 5 ◦ C s−1 . Fig. 7[f] shows the TL glow curve for Gd2 SiO5 :Ce3+ (0.1–2.0%) irradiated by gamma Co60 source with dose 0.1 kGy. It shows very good TL glow curve at 222.08 ◦ C. Here the maximum intense peak found for 0.1 mol% of Ce3+ concentration for gamma irradiated sample after that the trap level destroy because of higher concentration Ce3+ . To calculate the trap levels, Ce3+ doped Gd2 SiO5 phosphor were irradiated with different gamma doses and also the TL glow curve of irradiated phosphors were recorded at different concentration of cerium ions. Significant change in the temperature of the glow curve peak as well as intensity was observed after gamma irradiation. This may be due to rearrangement of existing traps and formation of new traps for charge trapping. The TL glow curve for gamma doses ranging from 0.1 kGy–2 kGy for cerium ion doped phosphor at constant heating rate 5 ◦ C s−1 are shown in Fig. 7[g]. The shape of the glow peak did not alter significantly an increase of gamma dose. Fig. 7[h] shows the linear response with gamma dose up to 1 kGy gamma dose and high temperature peak centered at 222 ◦ C shows formation of deep trapping. Kinetic parameters are calculated by CGCD technique here in the present case, the glow curve of 2 kGy gamma dose was deconvoluted by CGCD method using Glowfit programme. Figure of merit can be calculated by CGCD technique (Fig. 7[i]). Four different peaks are found when fitted the glow curve by CGCD technique and kinetic parameters are presented in Table 4. Calculated activation energy is high and it varies from 0.7–2.46 eV which confirms the formation of deep trapping in the prepared phosphor for gamma irradiation. Order of kinetic is found near first order kinetics for prepared sample.
Table 4 Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 2 kGy gamma dose at heating rate 5 ◦ C s−1 . Peak
T1 (K)
Tm (K)
T2 (K)
␦
= ␦/
Activation energy (Eav )
Frequency factor (s)
Peak 1 Peak 2 Peak 3 Peak 4
389.80 451.40 468.30 476.40
420.20 462.10 491.60 507.60
445.20 470.20 508.50 531.70
30.40 10.70 23.30 31.20
25.00 08.10 16.90 24.10
55.40 18.80 40.20 55.30
0.45 0.43 0.42 0.44
0.70 2.46 1.21 1.00
5 × 107 4 × 1026 7 × 1011 1 × 109
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4. Conclusions 1. Gd2 SiO5 :Ce3+ doped phosphor successfully synthesized by modified solid state reaction method. XRD pattern confirms that synthesized sample shows monoclinic structure. The crystallites size was found to be 34.525 nm. XRD studies confirm the phosphors are mostly in single phase and nano crystallites. 2. Peaks found in FTIR spectrum confirm the formation of Gd2 SiO5 :Ce3+ . 3. FEGSEM images show the morphology with different agglomerate shapes, such as sponge like and small spherical shape. The surface morphology is good and particle size is of few microns. 4. HRTEM images show formation of spherical shape particles and particle size was found nearly 200 nm. 5. In PL emission spectra, for lower Ce3+ doping concentration (0.1–2 mol%), transitions occur due to Gd3+ ion. The peak at 555 nm may be due to stark level transition from the 6 GJ state of Gd3+ ion. The red emission peaks at 602, 607, 627 and 640 nm corresponds to 6 GJ -6 PJ transition. For higher Ce3+ doping concentration (2.5 mol%), transition due to Ce3+ ion dominates over transition due to Gd3+ ion and we get emission spectra of Ce3+ ion. The emission spectrum of Gd2 SiO5 :Ce3+ (2.5 mol%) can be decompose into two bands peaked at 413 and 440 nm, which assigned to 5d-4f (2 F5/2 ) and 5d-4f (2 F7/2 ) transitions of Ce3+ , respectively. The results indicate that Gd2 SiO5 :Ce3+ (2.5%) phosphors can be selected as a potential candidate for blue light emission in solid-state lighting and display devices. 6. The composited TL glow curves of UV irradiated doped with cerium ion phosphor GSO shows three peaks which indicates that three different sets of traps were being activates having their own kinetic parameters. 7. Gamma irradiated sample shows high temperature peak (222 ◦ C) which shows formation of deep trapping in the prepared sample for gamma irradiation and verified by high activation energy varies from 0.70–2.46 eV. 8. The phosphor show linear response with gamma ray exposure with high intensity and show near first order kinetics which is characteristics of TL dosimetry phosphor. So, the phosphor may be useful in TL dosimetry. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
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[39] Y. Parganiha, J. Kaur Saluja, V. Dubey, R. Shrivastava, Synthesis and luminescence properties of Gd2 SiO5 phosphor, J. Disp. Technol. 12 (1) (2016) 66–70. [40] Yogita Parganiha, Jagjeet Kaur, Vikas Dubey, Ravi Shrivastava, YAlO3 :Ce3+ powders: synthesis, characterization, thermoluminescence and optical studies, Superlattices Microstruct. 85 (2015) 410–417. [41] Ravi Shrivastava, Jagjeet Kaur, Vikas Dubey, Beena Jaykumar, Luminescence studies on europium- and dysprosium-doped di-strontium magnesium silicate phosphor, Res. Chem. Intermed. 41 (6) (2015) 3699–3708. [42] Jagjeet Kaur, Vikas Dubey, Yogita Parganiha, Deepti Singh, N.S. Suryanarayana, Review of the preparation, characterization, and luminescence properties of Pr3+ -doped CaTiO3 phosphor, Res. Chem. Intermed. 41 (6) (2015) 3597–3621. [43] J. Kaur, D. Singh, V. Dubey, N.S. Suryanarayana, Y. Parganiha, P. Jha, Review of the synthesis, characterization, and properties of LaAlO3 phosphors, Res. Chem. Intermed. 40 (8) (2014) 2737–2771. [44] R. Shrivastava, J. Kaur, V. Dubey, B. Jaykumar, Photoluminescence, trap states and thermoluminescence decay process study of Ca2 MgSi2 O7 : Eu2+ , Dy3+ , Bull. Mater. Sci. 37 (4) (2014) 925–929. [45] V. Dubey, Thermoluminescence Study of Semaria Limestone of Cg Basin: Thermoluminescence and Photoluminescence Studies of Beta Irradiated Limestone Collected from Semaria Mines of Durg Distt, LAP LAMBERT Academic Publishing, 2012. [46] J. Kaur, Y. Singh, V. Dubey, D. Suryanarayana, A review report on medical imaging phosphors, Res. Chem. Intermed. 40 (8) (2014) 2837–2858.
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Violet blue emission and thermoluminescence glow curve analysis of Gd2 SiO5 :Ce3+ phosphor Yogita Parganiha a,∗ , Jagjeet Kaur a , Vikas Dubey b,∗ , Ravi Shrivastava c , Deepika Chandrakar a a b c
Department of Physics, Govt. Vishwanath Yadav Tamaskar Post Graduate Autonomous College, Durg (C.G.), 491001, India Department of Physics, Bhilai Institute of Technology, Kendri, Raipur, C.G., India ICFAI University, Kumhari, Raipur, C.G., India
a r t i c l e
i n f o
Article history: Received 22 February 2016 Accepted 15 April 2016 Keywords: Violet-blue emission Ce doped GSO phosphor TL glow curve analysis
a b s t r a c t This paper reports synthesis of Ce3+ ions-doped Gd2 SiO5 phosphor powders prepared by modified solid state reaction method, which is most suitable method for large-scale production. Starting materials used for sample preparation were Gd2 O3 , SiO2 and CeO2 with variable concentration of cerium (0.1 mol% to 2.5 mol%) with fixed amount of boric acid (0.05 mol%) as flux. PL excitation spectra was found at 260 nm while the emission spectra was found at 365, 378, 555, 602, 607 and 627 nm for lower Ce3+ doping concentration which are attributed to the Gd3+ ion transitions. For higher concentration of Ce3+ we get broad emission spectra from 350 to 550 nm which is characteristics of Ce3+ ion transition. Commission Internationale de l’Eclairage co-ordinate (CIE) of this phosphor is x = 0.170, y = 0.029, in the violet region for 2.5 mol% of cerium. TL glow curves were recorded for different doses of UV exposure and gamma ray dose at a heating rate of 5◦ C s−1 . The kinetic parameters such as activation energy “E”, the order of kinetics “b”, and the frequency factor “s” of Gd2 SiO5 :Ce3+ have been calculated by using peak shape method. Sample shows general order kinetics for most of the peaks. The effect of Ce3+ concentration on TL intensity was also studied and found that the intensity is maximum for 0.1 mol% and then decreases with increasing concentration of Ce3+ . The present phosphor can acts as a single host for violetblue emission (2.5 mol%) for display devices as well as may be used for thermoluminescence dosimetric material under UV & gamma dose. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction In past few years, research on luminescent nanophosphors provide challenge to development of technologies in various areas such as electronics, photonics, display, lasing, detection, optical amplification, fluorescent sensing in biomedical engineering and environmental control [1–5]. Nanoscale phosphors may have a number of potential advantages over micronsized phosphors. Rare earths (RE) are well known for their extensive use in luminescent materials. The rare earth ions doped inorganic nanophosphor is one of the most promising materials for a variety of applications in solid state lighting (SSL), solid-state lasers, lighting and displays and optical communication fields such as fluorescent lamps, cathode ray tubes, and field emission displays. Light emitting diodes (LEDs) are the ultimate light source in the solid state lighting (SSL) technol-
∗ Corresponding authors. E-mail addresses: [email protected] (Y. Parganiha), [email protected] (V. Dubey). http://dx.doi.org/10.1016/j.ijleo.2016.04.064 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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ogy. The LED technology has flourished for the past few decades. High efficiency, reliability, low power consumption, and durability are among the key factors for the rapid development of the SSL based high-brightness visible LEDs. Silicate family is an attractive class of materials among inorganic phosphors due to their special properties such as water, chemical resistance and visible light transparency. They show superior properties due to thermal stability, low cost, wide band gap, chemical resistance, non toxicity [6]. Among silicate based phosphors, Ce-doped Gd2 SiO5 (GSO:Ce) found its application as a promising and attractive scintillator with a high absorption coefficient, high luminescence output, fast decay time and excellent irradiation hardness (109 rad) [7,8]. As a scintillator it has attracted interest for extensive applications such as nuclear medical imaging, nuclear physics, in ␥-ray detection for oil well dogging and in the field of high energy physics research [9–19]. In this paper, we study the Photoluminescence and thermoluminescence of GSO:Ce3+ phosphor. Phosphor was prepared using modified solid state reaction method. The sample was characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), Field Emission Gun Scanning electron microscopy (FEGSEM), Transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDX).
2. Experimental Phosphor of Gd2 SiO5 doped with Ce3+ ions with variable molar concentration of Ce3+ (0.1% to 2.5%) was prepared by solid state reaction method. The precursors Gd2 O3, SiO2 , CeO2 and H3 BO3 (as flux) used for synthesis of Gd2 SiO5 :Ce3+ . The composition of each chemical weighed in proper stoichiometric ratio then mixed thoroughly for 45 min using mortar and pestle. The grinded sample was placed in an alumina crucible and subsequently fired at 1000 ◦ C for 1 h for calcinations and then heated at 1250 ◦ C for 3 h for sintering in a muffle furnace. Every heating was followed by intermediate grinding. Finally the samples were cooled slowly to room temperature in the furnace and ground again into powder for subsequent characterization. Complete reaction is given as:
The FTIR spectrum of a sample was recorded at room temperature in the wave number range of 4000–400 cm−1 on a Bruker spectrophotometer. Observation of particle morphology and elemental analysis was investigated by FEGSEM (field emission gun scanning electron microscope) JEOL JSM7600F. TEM (transmission electron microscope) analysis was carried out by PHILIPS CM200. The photoluminescence (PL) emission and excitation spectra were recorded at room temperature by use of a Shimadzu RF-5301 PC spectrofluorophotometer. The excitation source was a Xenon lamp. The obtained phosphor under the TL examination is given UV radiation using 254 nm UV source Thermoluminescence glow curves were recorded at room temperature by using TLD reader I1009 supplied by Nucleonix Sys. Pvt. Ltd. Hyderabad [20–43].
3. Results and discussion 3.1. XRD analysis of Gd2 SiO5 :Ce3+ In order to obtain the lattice parameters of the sample the powder diffraction pattern was analyzed by Rietveld fitting method [22]. Lattice parameters were refined until to obtain a fairly good agreement between observed and calculated XRD spectra. The pattern is characterized by few prominent peaks found at different glancing angles [43–46]. The size of the crystallite has been computed from the full width half maximum (FWHM) of every peak using the Scherer’s formula [23,24]. The Scherer formula is given by: D=
0.9 Cos
(1)
where D is crystallite size, is the wavelength of X-ray,  is the FWHM, and is the diffraction angle. Calculated crystallite size for different angle of glancing is shown in Table 1b. The average crystallite size is 34.525 nm. The indexing and refinement of lattice parameters are done using Celref v. 3 software. The XRD pattern of the sample is shown in Fig. 1. It shows a monoclinic body centered structure with space group P21 /c and unit cell dimensions a = 9.1045 Å, b = 6.9934 Å, c = 6.8325 Å, ␣ = 90◦ ,  = 107.29◦ and ␥ = 90◦ . It is observed that the peak of XRD of the sample showed resemblance with the XRD pattern of Gd2 SiO5 , reported by Dramicanin et al. [25]. There are few more peaks in observed XRD pattern which could be due to a great number of stacking faults induced by the presence of the doping ions and also due to secondary phases and impurities formed during the elaboration process. The calculated lattice parameters are given in Table 1a.
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Table 1a Indexing and lattice parameters of Gd2 SiO5 :Ce3+ . Standard Values Zero
Lambda
a
b
c
alpha
beta
gamma
Vol.
0 0
1.5419 0
9.1105 1
6.9783 1
6.8544 1
90 0
107.14 1
90 0
416.4
Final Values: (Standard errors on 2nd line) Zero
Lambda
a
b
c
alpha
beta
gamma
Vol.
0 0
1.5419 0
9.1045 0.1183
6.9934 0.0142
6.8325 0.094
90 0
107.29 1.749
90 0
415.4
h
k
l
2 (Obs)
2 (Cal)
Dif
−1 −1 2 1 3 −3 0 −2 −2 3 −3 −3
1 2 1 2 1 2 3 2 3 3 3 1
2 1 1 1 0 1 1 3 2 0 2 4
29.100 29.442 30.644 32.060 33.742 39.554 41.230 48.473 49.049 50.000 52.814 57.557
29.1721 29.3577 30.6914 32.1497 33.4751 39.5875 41.1211 48.4530 48.9709 50.2319 52.9358 57.5382
−0.0721 0.0843 −0.0474 −0.0897 0.2669 −0.0335 0.1089 0.0200 0.0781 −0.2319 −0.1218 0.0188
160
Relativity Intensity (arb. unit)
140 120 100 80 60 40 20 0 20
30
40
2
50
60
degree
Fig. 1. XRD pattern of Gd2 SiO5 :Ce3+ (2.5 mol%) phosphor.
Fig. 2. FTIR Spectrum of Gd2 SiO5 :Ce3+ (2.5%).
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3.2. FTIR analysis of Gd2 SiO5 :Ce3+ Fig. 2 shows FTIR spectra of prepared sample. In the spectrum the absorption band of silicate groups are clearly evident. The intense broad band centered at 879 cm−1 is assigned to the presence of SiO4 2− group. The band at 409, 420, 427 and 438 cm−1 are assigned to Si-O-Si bending mode of vibration. The band from 448 to 541 cm−1 is due to vibration of Gd-O bonds. The peaks at 420 and 505 cm−1 are assigned to Ce-O vibration. Small peak originated at 2362 cm−1 may be due to presence of CO2 , absorbed from atmosphere and 2923 cm−1 may be due to filter contamination of the FTIR instrument. All these discussed peak found together confirms the formation of Gd2 SiO5 :Ce3+ . 3.3. FEGSEM analysis of Gd2 SiO5 :Ce3+ (2.5 mol%) The particle size and surface morphology of prepared sample was carried out with a Field emission gun scanning electron microscope (FEGSEM) at four different magnifications (10k, 25k, 50k and 100k). Fig. 3[a–d] shows the morphology with different agglomerate shapes, such as sponge like and small spherical. The surface morphology is good and particle size is estimated as of few microns. 3.4. HRTEM analysis of Gd2 SiO5 :Ce3+ (2.5 mol%) To investigate exact particle size and surface morphology of phosphor transmission electron microscopy images was obtained (Fig. 4[a–b]). Here it is clear from HRTEM images that it is formation of spherical shape particles. The high temperature synthesis method maintains the morphology of the prepared phosphor and particle size was found nearly 200 nm. It is clear that the FEGSEM images support the result of HRTEM image (Fig. 4[a–b]).
Table 1b Determination of Crystallite size of Gd2 SiO5 :Ce3+ . 2
 (FWHM)
d spacing
D (Crystallite Size) nm
29.100 29.442 30.644 32.060 33.742 39.554 41.230 48.473 49.049 50.000 52.814 57.557
0.197 0.170 0.210 0.210 0.210 0.170 0.341 0.262 0.183 0.275 0.275 0.328
3.066 3.031 2.915 2.789 2.654 2.276 2.187 1.876 1.855 1.822 1.732 1.600
41.765 48.237 39.301 37.913 39.607 49.569 24.917 33.246 47.607 31.859 32.235 27.658
Fig. 3. FEGSEM images of Gd2 SiO5 :Ce3+ (2.5%) (a) ×10k (b) ×25k (c) ×50k (d) ×100k.
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Fig. 4. [a–b] HRTEM images of Gd2 SiO5 :Ce3+ (2.5%).
The phosphor particles should have a spherical shape and high luminescence efficiency for successful applications. Indeed, phosphor particles with a spherical shape minimize light scattering on their surfaces and therefore, improve the efficiency of light, emission and the brightness of such a phosphor [45,46]. 3.5. Photoluminescence (PL) studies 3.5.1. PL excitation spectra Fig. 5[a] shows the PL excitation spectra of Gd2 SiO5 :Ce3+ phosphor monitored at 615 nm. The peak centered at 260 nm corresponds to higher energy 8 S7/2 -6 DJ transitions of Gd3+ . 3.5.2. PL emission spectra For lower Ce3+ doping concentration (0.1% to 2 mol%) in Gd2 SiO5 host, the peak in UV region (365 & 378 nm) is due to radiative recombination of holes and electrons in Gd2 SiO5 (Fig. 5[b]). The peaks in the visible region in the emission spectra may be due to transition from 6 GJ state. The peak at 555 nm may be due to stark level transition from the 6 GJ state of Gd3+ ion. The red emission peaks at 602, 607, 627 and 640 nm corresponds to 6 GJ -6 PJ transition. It is clear that for lower concentration of Ce3+ ion, dominant peaks are related to Gd3+ ion. For higher Ce3+ doping concentration (2.5 mol%), transition due to Ce3+ ion dominates over transition due to Gd3+ ion and we get emission spectra of Ce3+ ion. The emission spectrum of Gd2 SiO5 :Ce3+ (2.5 mol%) can be decompose into two bands peaked at 413 and 440 nm, which assigned to 5d-4f (2 F5/2 ) and 5d-4f (2 F7/2 ) transitions of Ce3+ , respectively [15,24,25]. So the 2.5 mol% was optimized cerium concentration for GSO:Ce phosphor which may be applicable for blue light emission in display device application for more detail decomposed emission spectra shown in Fig. 5[c] which shows blue emission for cerium doped phosphor.
a
b
300 260 nm
440nm
800
250
413nm
700 Intensity (arb. unit)
Intensity (arb. unit)
900
200 150 100 50
600 500 400 300 200
365nm
378nm
602nm 627nm 607nm
555nm
100
640nm
0
0 200 220 240 260 280 300 320 340 360
400
c
440nm
600
Experimental peak Fitted peak 1 Fitted peak 2
800
413nm
Intensity (arb. unit)
500 Wavelength (nm)
Wavelength (nm)
600
400
200
0 350
400
450
500
550
600
650
Wavelength (nm)
Fig. 5. [a] PL excitation spectra of Gd2 SiO5 :Ce3+ monitored with 615 nm. [b] PL emission spectra of Gd2 SiO5 :Ce3+ with variable Ce3+ concentration. [c] Decomposed emission spectra of Gd2 SiO5 :Ce3+ (2.5 mol%).
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Fig. 6. CIE coordinate diagram of Gd2 SiO5 :Ce3+ (2.5 mol%). Table 2 Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 5 min UV dose at heating rate 5 ◦ C s−1 . Peak
T1 (K)
Tm (K)
T2 (K)
␦
= ␦/
Activation energy (Eav )
Frequency factor (s)
Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7
337.70 345.00 358.60 389.10 438.20 480.90 478.00
348.50 360.90 386.80 413.70 456.30 500.40 515.00
357.20 372.40 410.80 431.70 469.30 514.20 543.20
10.80 15.90 28.20 24.60 18.10 19.50 37.00
8.70 11.50 24.00 18.00 13.00 13.80 28.20
19.50 27.40 52.20 42.60 31.10 33.30 65.20
0.45 0.42 0.46 0.42 0.42 0.41 0.43
1.44 0.96 0.65 0.79 1.36 1.46 0.82
4 × 1020 1 × 1013 7 × 108 1 × 109 3 × 1015 1 × 1014 1 × 107
3.6. CIE co-ordinates The luminous colour was depicted by studying colour co-ordinates and colour ratios of Gd2 SiO5 :Ce3+ phosphor. The values of chromaticity coordinates of the GSO powder has been estimated from 1931CIE system and is shown in Fig. 6. It was observed that the Commission Internationale de l’Eclairage (CIE) co-ordinates of GSO phosphor were measured as (x, y) and the value is found in violet-blue region [26,27]. Their corresponding location has been marked in Fig. 6 with cross in violet-blue region.
ColourPurity=
(xs − xi )2 + (ys − yi )2
(xd − xi )2 + (yd − yi )2
× 100%
(2)
where (xs , ys ) are the coordinates of a sample point, (xd , yd ) are the coordinates of the dominant wavelength and (xi ,yi ) are the coordinates of the illuminant point. The calculation was carried out.It indicate that colour purity increased with rare earth concentration and was a maximum at 2.5 mol% of Ce3+ concentration reaches to 89% [22–42]. The results indicate that GSO:Ce3+ (2.5%) phosphors gave the maximum violet-blue emission and good violet-blue chromaticity with high colour purity and therefore have promising applications in solid-state lighting and display devices (Fig. 6). 3.7. Thermoluminescence (TL) studies For variable Ce3+ ion (0.1–1 mol%) at fixed UV exposure time i.e. 5 min, TL glow curve was recorded (Fig. 7[a]). For Ce3+ (0.1 mol%) the TL glow curve intensity was maximum after that TL intensity decreases with increasing concentration of Ce3+ . Fig. 7[b] shows the CGCD curve of Gd2 SiO5 :Ce3+ (0.1%) for 5 min UV dose at heating rate 5 ◦ C s−1 and Table 2 presents its calculated kinetic parameters for seven deconvoluted peaks (Fig. 7[b]). It shows first order kinetics. The recorded glow curves of the prepared phosphors exposed to 5 min UV exposure with different concentration of cerium ion seem to be composite in nature as they exhibits three broad peaks (Fig. 7[a]). So, CGCD method was used to deconvolute and calculate the kinetic parameters. Here in present case, the glow curves of maximum intense peak i.e. 0.1 mol% of cerium ion irradiated with 5 min UV exposure was deconvoluted by CGCD method using Glowfit programme [28,32].
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Fig. 7. [a] TL glow curve of Gd2 SiO5 :Ce3+ with variable Ce3+ concentration for 5 min UV dose at heating rate 5 ◦ C s−1 . [b] CGCD curve of Gd2 SiO5 :Ce3+ (0.1%) for 5 min. UV dose at heating rate 5 ◦ C s−1 . [c] TL glow curve of Gd2 SiO5 :Ce3+ (0.1%) with variable UV dose at heating rate 5 ◦ C s−1 . [d] UV dose vs TL intensity plot of Gd2 SiO5 :Ce3+ (0.1%). [e] CGCD curve of Gd2 SiO5 :Ce3+ (0.1%) for 10 min UV dose at heating rate 5 ◦ C s−1 . [f] TL Glow curve of Gd2 SiO5 :Ce3+ for variable concentration for 0.1 kGy gamma dose at heating rate 5 ◦ C s−1 . [g] TL Glow curve of Gd2 SiO5 :Ce3+ (0.1%) for variable gamma dose at heating rate 5 ◦ C s−1 . [h] Gamma dose vs intensity plot for Gd2 SiO5 :Ce3+ (0.1%). [i] CGCD Glow curve of Gd2 SiO5 :Ce3+ (0.1%) for 2 kGy gamma dose at heating rate 5 ◦ C s−1 .
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Table 3 Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 10 min UV dose at heating rate 5 ◦ C s−1 . Peak
T1 (K)
Tm (K)
T2 (K)
␦
= ␦/
Activation energy (Eav )
Frequency factor (s)
Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6
353.40 386.90 421.20 468.30 483.60 466.50
376.90 408.50 436.60 487.20 508.10 525.30
393.20 424.80 446.60 499.90 527.00 571.40
23.50 21.60 15.40 18.90 24.50 58.80
16.30 16.30 10.00 12.70 18.90 46.10
39.80 37.90 25.40 31.60 43.40 104.90
0.41 0.4 3 0.39 0.40 0.44 0.44
0.67 0.92 1.33 1.39 1.30 0.52
2 × 108 7 × 1010 9 × 1014 8 × 1013 2 × 1012 1 × 104
This computer programme is based on Halperin and Barner equations which describe the flow of the charges between the various energy levels during a trap emptying by thermal heating. The kinetic parameters of trap levels were determined for each deconvoluted peak by this programme [29]. The theoretical generated glow curves were fitted with the experimnatal glow curves and the quality of fitting was checked by calculating the figure of merit (FOM) for each fitting defined by [29–31]
FOM =
− TLExp − TLThe
(3)
here TLexp and TLThe represent the TL intensity of experimental and theoretical glow curves respectively. The summation extends over all the available experimental data points. Quality of fitting and choice of the appropriate number of peaks was refined by repeating the process of fitting in order to get the minimum FOM with minimum number of possible peaks. The fits were considered adequate when the FOM values were observed below 5% with most actual being below 2%. In the present studies, FOM was found 1.89% which confirms a very good agreement between theoretical generated and experimentally recorded glow curves. The fitted TL glow curves are shown in Fig. 7[b] and the values of trap depths and frequency factors of trapped charges calculated by CGCD method are summarized in Table 2. Fig. 7[c] shows the TL glow curve of Gd2 SiO5 :Ce3+ (0.1%) with different UV exposure time at constant heating rate i.e. 5 ◦ C s−1 . The sample shows three peaks at 84.14 ◦ C, 138.51 ◦ C and 229.92 ◦ C with different UV exposure time. Three distinct peaks show composite nature and it show formation of number of traps. Some may be shallow and some deep traps, low temperature peak shows formation of shallower trap and high temperature peak shows deep trapping. The TL intensity increases up to 10 min of UV exposure than it decreases (Fig. 7[d]). The estimated kinetic parameters for the phosphor is calculated by curve fitting techniques CGCD curve of experimental data (Fig. 7[e]) using peak shape method proposed by Chen [32]. The activation energy is found in between 0.52–1.39 eV and the frequency factor is range of 1 × 104 to 9 × 1014 for 10 min UV irradiated phosphor. Table 3 shows the Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 10 min UV dose at heating rate 5 ◦ C s−1 . Fig. 7[f] shows the TL glow curve for Gd2 SiO5 :Ce3+ (0.1–2.0%) irradiated by gamma Co60 source with dose 0.1 kGy. It shows very good TL glow curve at 222.08 ◦ C. Here the maximum intense peak found for 0.1 mol% of Ce3+ concentration for gamma irradiated sample after that the trap level destroy because of higher concentration Ce3+ . To calculate the trap levels, Ce3+ doped Gd2 SiO5 phosphor were irradiated with different gamma doses and also the TL glow curve of irradiated phosphors were recorded at different concentration of cerium ions. Significant change in the temperature of the glow curve peak as well as intensity was observed after gamma irradiation. This may be due to rearrangement of existing traps and formation of new traps for charge trapping. The TL glow curve for gamma doses ranging from 0.1 kGy–2 kGy for cerium ion doped phosphor at constant heating rate 5 ◦ C s−1 are shown in Fig. 7[g]. The shape of the glow peak did not alter significantly an increase of gamma dose. Fig. 7[h] shows the linear response with gamma dose up to 1 kGy gamma dose and high temperature peak centered at 222 ◦ C shows formation of deep trapping. Kinetic parameters are calculated by CGCD technique here in the present case, the glow curve of 2 kGy gamma dose was deconvoluted by CGCD method using Glowfit programme. Figure of merit can be calculated by CGCD technique (Fig. 7[i]). Four different peaks are found when fitted the glow curve by CGCD technique and kinetic parameters are presented in Table 4. Calculated activation energy is high and it varies from 0.7–2.46 eV which confirms the formation of deep trapping in the prepared phosphor for gamma irradiation. Order of kinetic is found near first order kinetics for prepared sample.
Table 4 Kinetic parameters for Gd2 SiO5 :Ce3+ (0.1%) for 2 kGy gamma dose at heating rate 5 ◦ C s−1 . Peak
T1 (K)
Tm (K)
T2 (K)
␦
= ␦/
Activation energy (Eav )
Frequency factor (s)
Peak 1 Peak 2 Peak 3 Peak 4
389.80 451.40 468.30 476.40
420.20 462.10 491.60 507.60
445.20 470.20 508.50 531.70
30.40 10.70 23.30 31.20
25.00 08.10 16.90 24.10
55.40 18.80 40.20 55.30
0.45 0.43 0.42 0.44
0.70 2.46 1.21 1.00
5 × 107 4 × 1026 7 × 1011 1 × 109
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4. Conclusions 1. Gd2 SiO5 :Ce3+ doped phosphor successfully synthesized by modified solid state reaction method. XRD pattern confirms that synthesized sample shows monoclinic structure. The crystallites size was found to be 34.525 nm. XRD studies confirm the phosphors are mostly in single phase and nano crystallites. 2. Peaks found in FTIR spectrum confirm the formation of Gd2 SiO5 :Ce3+ . 3. FEGSEM images show the morphology with different agglomerate shapes, such as sponge like and small spherical shape. The surface morphology is good and particle size is of few microns. 4. HRTEM images show formation of spherical shape particles and particle size was found nearly 200 nm. 5. In PL emission spectra, for lower Ce3+ doping concentration (0.1–2 mol%), transitions occur due to Gd3+ ion. The peak at 555 nm may be due to stark level transition from the 6 GJ state of Gd3+ ion. The red emission peaks at 602, 607, 627 and 640 nm corresponds to 6 GJ -6 PJ transition. For higher Ce3+ doping concentration (2.5 mol%), transition due to Ce3+ ion dominates over transition due to Gd3+ ion and we get emission spectra of Ce3+ ion. The emission spectrum of Gd2 SiO5 :Ce3+ (2.5 mol%) can be decompose into two bands peaked at 413 and 440 nm, which assigned to 5d-4f (2 F5/2 ) and 5d-4f (2 F7/2 ) transitions of Ce3+ , respectively. The results indicate that Gd2 SiO5 :Ce3+ (2.5%) phosphors can be selected as a potential candidate for blue light emission in solid-state lighting and display devices. 6. The composited TL glow curves of UV irradiated doped with cerium ion phosphor GSO shows three peaks which indicates that three different sets of traps were being activates having their own kinetic parameters. 7. Gamma irradiated sample shows high temperature peak (222 ◦ C) which shows formation of deep trapping in the prepared sample for gamma irradiation and verified by high activation energy varies from 0.70–2.46 eV. 8. The phosphor show linear response with gamma ray exposure with high intensity and show near first order kinetics which is characteristics of TL dosimetry phosphor. So, the phosphor may be useful in TL dosimetry. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]
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