Physics Research InternationalVolume 2013 (2013), Article ID 494807, 5 pageshttp://dx.doi.org/10.1155/2013/494807

Research Article

Luminescence Studies of Eu³⁺ Doped Calcium Bromofluoride Phosphor

Jagjeet Kaur, Yogita Parganiha, and Vikas Dubey

Department of Physics, Govt. V.Y.T.P.G. Autonomous College, Durg, Chhattisgarh 491001, India

Received 13 April 2013; Accepted 17 July 2013

Academic Editor: Lorenzo Pavesi

Copyright © 2013 Jagjeet Kaur et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The present paper reports photoluminescence (PL) and thermoluminescence (TL) properties of rare earth-doped calcium bromo-fluoride phosphor. The europium (Eu³⁺) was used as rare earth dopant. The phosphor was prepared by Solid state reaction method (conventional method). The PL emission spectrum of the prepared phosphor shows intense peaks in the red region at 611 nm for ⁵D₀→⁷F₂ transitions, and the PL excitation spectra show a broad band located around 220–400 nm for the emission wavelength fixed at 470 nm. The TL studies were carried out after irradiating the phosphor by UV rays with different exposure time. The glow peak shows second-order kinetics. The present phosphor can act as host for red light emission in display devices.

1. Introduction

Rare earth-doped phosphors have been the center of attraction as luminescent materials from the last many decades. These phosphors have caused great attention to use as the host materials for X-ray screens, neutron detectors, alpha-particle scintillators, and so forth, due to their high luminescence efficiency and are also used as emissive materials in various display devices like LCD, FET, and CRT or as illumination sources [1–3].

Spectroscopic studies of these phosphors play a vital role in characterizing the specific luminescence properties such as photoluminescence and thermoluminescence. The rare earths are usually incorporated in these materials as divalent or trivalent cation for the realization of optically active materials in photonics and optoelectronic applications. The europium is efficiently used as luminescent center in phosphors for various purposes. Phosphors doped with europium ions are of greater importance for observing red colors on the monitors of various display devices [4].

2. Experimental Method

By solid state reaction process, CaF₂, KBr, and Eu₂O₃ were mixed in stoichiometric ratio by dry grinding in mortar and pestle for nearly 45 minutes. The mixture is taken in quartz boat and is fired in air at 730°C for 3 hours in presence of urea.

The photoluminescence studies were carried out using RF5301 spectrophotofluorometer in the wavelength range 400–650 nm at room temperature. The thermoluminescence studies were carried out using TLD reader I1009 supplied by Nucleonix System Pvt. Ltd., Hyderabad [5]. The sample was irradiated by UV radiation 365 nm. The heating rate used for TL measurement is 3°C/s. Curves were analyzed by using computer glow curve deconvolution program.

3. Result and Discussion

3.1. Photoluminescence Studies

3.1.1. PL Excitation Spectra

Figure 1 shows the PL excitation spectra of CaFBr (pure) phosphor monitored at 470 nm. It exhibits a broad excitation band in the range of 220–400 nm. The broad peak maximum at about 265 nm region corresponds to Eu³⁺, while the weak excitation peak seen at 358 nm may be due to crystal field effect.

Figure 1: PL excitation spectra of CaFBr (pure) monitored with 470 nm.

3.1.2. PL Emission Spectra

The PL emission spectra of Eu³⁺ doped CaFBr phosphor were recorded at an excitation wavelength of = 265 nm. Figures 2(a), 2(b), and 2(c) show the room temperature PL emission spectra of CaFBr:Eu³⁺ phosphor with 0.1%, 0.2%, and 0.5% concentrations of Eu, respectively, in the wavelength range 400 to 650 nm. The distinct emission lines lying between 578 and 628 nm are observed due to transitions from excited ⁵D₁ to the ⁷F₁ and ⁵D₀ to the ⁷Fj (). The origin of these transitions (electric dipole or magnetic dipole levels of Eu³⁺ ions) from emitting levels to terminating levels depends upon the location of Eu ion in CaFBr lattice, and the type of transition is determined by selection rules [6]. The intense peak at 611 nm and a small peak at 628 nm correspond to the hypersensitive transition between the ⁵D₀ and ⁷F₂ levels of Eu³⁺ ion in calcium bromofluoride host aroused due to forced electric dipole transition mechanism. The weak emission in the vicinity of 590 nm (590–600 nm) is ascribed to the magnetic dipole transition of ⁵D₀ to ⁷F₁. The weak peak near 580 nm (584 nm) is ascribed due to transition from ⁵D₀ to ⁷F₀ of Eu³⁺ ion in the CaFBr [7]. The wide band peaking at 474 nm may be due to crystal field or host compound. All possible transitions of Eu³⁺ ion in CaFBr host are listed in Table 1. The energy level diagram of Eu³⁺ ion in CaFBr lattice with all possible dipole transitions is shown in Figure 3. Figure 4 shows the comparison of PL intensity with the variation of Eu concentration. It shows that the intensity decreases with increasing the concentration of Eu.

Table 1: Various transitions of Eu³⁺ ion in CaFBr host.

Transitions
Emission wavelength

5D₀→⁷F₂
611 nm, 628 nm

5D₀→⁷F₁
590–600 nm

5D₀→⁷F₀
584 nm

(a)

(b)

(c)

Figure 2: (a) PL emission spectra of CaFBr:Eu (0.1%) monitored with 265 nm. (b) PL emission spectra of CaFBr:Eu (0.2%) monitored with 265 nm. (c) PL emission spectra of CaFBr:Eu (0.5%) monitored with 265 nm.

Figure 3: Energy level diagram of all possible transitions of Eu³⁺ ions.

Figure 4: Comparative variation of Eu concentration monitored with 265 nm.

3.1.3. CIE

As illustrated in Figure 5, the emission color of CaFBr:Eu³⁺ phosphor can be expressed by the Commission International de l’Eclairage (CIE) chromaticity coordinates. The prepared CaFBr:Eu³⁺ phosphor exhibits red light, and its chromaticity coordinate is ,  .

Figure 5: CIE chromaticity coordinate diagram of CaFBr:Eu³⁺ phosphor.

3.2. Thermoluminescence Studies

Thermoluminescence (TL) glow curves of CaFBr:Eu³⁺ phosphor were recorded after UV ray irradiation. The TL glow curves for different UV doses and for different Eu concentration at a heating rate of 3°C s⁻¹ are shown in Figure 6. Figure 6(a) shows TL glow curve for pure CaFBr for 5, 10, 15, and 30 minute UV irradiation. Prominent glow peaks are found at 109, 115, 89, and 95°C. Sample shows the second-order kinetics. Figure 6(b) shows TL glow curve for CaFBr doped with 0.1% Eu for 5 and 10 minute UV irradiation. Prominent glow peaks areThe origin of these transition found at 157°C. This sample shows first-order kinetics. Figure 6(c) shows TL glow curve of CaFBr doped with 0.2% Eu for 5, 10, 15, 20, 25, and 30 minute UV irradiation. Prominent glow peaks found at 111°C, 111°C, 111°C, 182°C, 126°C, and 111°C respectively. Sample shows second order kinetics. Figure 6(d) shows TL glow curve of CaFBr doped with 0.5% Eu for 10, 15, 20, 25, and 30 minute UV irradiation. For all the doses, there is no significant change in position of glow peaks, and prominent glow peaks are found at 94°C. Sample shows-second order kinetics.

(a)

(b)

(c)

(d)

Figure 6: (a) TL glow curve of pure CaFBr phosphor. (b) TL glow curve of CaFBr doped with 0.1% Eu. (c) TL glow curve of CaFBr doped with 0.2% Eu. (d) TL glow curve for 0.5% Eu CaFBr.

3.3. 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 bands of a solid. These trap levels are characterized by different trapping parameters such as trap depth, order of kinetics, and frequency factor [3]. 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 (). The mechanism of recombination of detrapped charge carriers with their counter parts is known as the order of kinetics (). The frequency factor () 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.

The values of kinetic parameters for pure CaFBr, CaFBr:Eu (0.1%), CaFBr:Eu (0.2%), and CaFBr:Eu (0.5%) are given in Tables 2, 3, 4, and 5, respectively. The peak shape factor for the TL glow curve of the prepared phosphor was found to be ~0.5 (for maximum peaks).

Table 2: Kinetic parameters for pure CaFBr.

UV min

Activation energy (E)
Frequency factor (s)

10 min UV
72
95
125
23
30
53
0.566
0.777

5 min UV
66
89
110
23
21
44
0.477
0.735

15 min UV
67
105
163
38
58
96
0.604
0.508

30 min UV
72
116
183
44
67
111
0.604
0.465

Table 3: Kinetic parameters for 0.1% Eu.

UV min

Activation energy (E)
Frequency factor (s)

5 min UV
128
157
174
29
17
46
0.37
0.804

10 min UV
103
157
189
54
32
86
0.37
0.419

Table 4: Kinetic parameters for CaFBr doped with 0.2% Eu.

UV min

Activation energy (E)
Frequency factor (s)

20 min UV
135
182
214
47
32
79
0.405
0.55

30 min UV
80
111
143
31
32
63
0.508
0.61

15 min UV
67
111
130
44
19
63
0.302
0.40

20 min UV
71
111
118
40
7
47
0.149
0.43

25 min UV
96
126
152
30
26
56
0.464
0.68

Table 5: Kinetic parameters for CaFBr doped with 0.5% Eu.

UV min

Activation energy (E)
Frequency factor (s)

10 min
70
94
118
24
24
48
0.50
0.72

15 min
71
94
121
23
27
50
0.54
0.76

20 min
71
94
123
23
29
52
0.55
0.77

25 min
72
94
118
22
24
46
0.52
0.79

30 min
75
99
120
24
21
45
0.46
0.74

4. Conclusions

CaFBr:Eu³⁺ with different (0.1%, 0.2%, 0.5%) Eu concentration, was successfully synthesized by solid state reaction method. Photoluminescence measurement ( nm) shows an intense red emission composed of three bands centered at 578 nm, 611 nm, and 628 nm, associated with Eu³⁺ allowed transition ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, and ⁵D₀→⁷F₂, respectively. The CIE of CaFBr:Eu³⁺ exhibits red light, and its chromaticity coordinate is and . The chromaticity point is in the deep red region, indicating its high color purity. Moreover, the photoluminescence emission peak at 611 nm of this phosphor indicates it as a strong red emitting phosphor. The value of activation energy is highest for Eu (0.1%) CaFBr. The value of activation energy for all Eu concentrations is between 0.40 and 0.804 eV. Corresponding frequency factor value is 6 × 10⁵ to 2 × 10¹² s⁻¹.

References

M. Nazarov, J. H. Kang, D. Y. Jeon, S. Bukesov, and T. Akmaeva, “Synthesis and luminescent performances of some europium activated yttrium oxide based systems,” Optical Materials, vol. 27, no. 10, pp. 1587–1592, 2005.

J. S. Bae, K. S. Shim, B. K. Moon et al., “Enhanced luminescent characteristics of Y2-xGdxO₃:Eu³⁺ ceramic phosphors by Li-doping,” Journal of the Korean Physical Society, vol. 46, no. 5, pp. 1193–1197, 2005.

G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer, Berlin, Germany, 1994.

W. Köstler, A. Winnacker, W. Rossner, and B. C. Grabmaier, “Effect of Pr-codoping on the X-ray induced afterglow of (Y,Gd)₂O₃:Eu,” Journal of Physics and Chemistry of Solids, vol. 56, no. 7, pp. 907–913, 1995.

J. Kaur, N. S. Suryanarayana, and V. Dubey, “Effect of temperature on the ML of Au doped (Zn,Cd)S mixed phosphors,” Chinese Chemical Letters, vol. 22, no. 6, pp. 709–712, 2011.

B. K. Gupta, D. Haranath, S. Saini, V. N. Singh, and V. Shanker, “Synthesis and characterization of ultra-fine Y₂O₃:Eu³⁺ nanophosphors for luminescent security ink applications,” Nanotechnology, vol. 21, no. 5, Article ID 055607, 2010.

C. Gorller-Walrand, E. Huygen, K. Binnemans, and L. Fluyt, “Optical absorption spectra, crystal-field energy levels and intensities of Eu³⁺ in GdAl₃(BO₃)₄,” Journal of Physics Condensed Matter, vol. 6, no. 38, pp. 7797–7812, 1994.