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.
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.
(a)
(b)
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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 , .
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)
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).
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.