Structural, optical and luminescence properties of

Journal of Luminescence 187 (2017) 360–367

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Structural, optical and luminescence properties of Sm3 þ and Eu3 þ doped calcium borophosphate phosphors for reddish-orange and red emitting light applications V. Reddy Prasad, S. Damodaraiah, S. Babu, Y.C. Ratnakaram n Sri Venkateswara University, Tirupati, India

art ic l e i nf o

a b s t r a c t

Article history: Received 13 November 2016 Received in revised form 28 February 2017 Accepted 22 March 2017 Available online 24 March 2017

Sm3 þ and Eu3 þ doped calcium borophosphate phosphors with the formula 2CaO-B2O3-P2O5: xRE (where, RE¼ Sm3 þ and Eu3 þ , x ¼0.2, 0.4, 0.6, 0.8 and 1.0 mol %) were synthesized by solid state reaction method. 2CaO-B2O3-P2O5: Sm and Eu phosphors were characterized by SEM with EDS, XRD, DRS, excitation, photoluminescence (PL) and decay profiles. SEM results showed that the particles are more irregular morphologies and XRD profiles showed that the prepared phosphors exhibit a hexagonal phase in crystal structure. Using different excitation wavelengths, intense emissions were observed at 403 and 394 nm for Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors respectively. The PL measurements showed that the intensity of luminescence increased with increasing doping concentrations upto 0.6 mol% and then decreased at higher concentrations due to the concentration quenching effect for both the ions. From the decay profiles, it was observed that at lower concentrations of Sm3 þ (0.2 and 0.4 mol%), the decay curves were well fitted to exponential behavior and for higher concentrations of Sm3 þ (0.6, 0.8 and 1.0 mol%), the decay profiles became non-exponential due to the existence of nonradiative channels. In case of Eu3 þ ion, decay curves indicated exponential nature for all concentrations. From CIE chromaticity diagram, reddish-orange and red color emissions were observed at 0.6 mol% of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors. & 2017 Elsevier B.V. All rights reserved.

Key words: Borophosphate Solid sate reaction method Luminescence Decay lifetime CIE color coordinates

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discursion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. X-ray diffraction (XRD) of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. SEM and EDS of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Diffuse reflection spectra (DRS) of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. (a). Photoluminescence of 2CaO-B2O3-P2O5:Sm3 þ phosphor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. (b). Photoluminescence of 2CaO-B2O3-P2O5:Eu3 þ phosphor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Decay lifetime of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. CIE chromaticity coordinates of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n

Corresponding author. E-mail address: [email protected] (Y.C. Ratnakaram).

http://dx.doi.org/10.1016/j.jlumin.2017.03.050 0022-2313/& 2017 Elsevier B.V. All rights reserved.

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V.R. Prasad et al. / Journal of Luminescence 187 (2017) 360–367

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1. Introduction

2. Experimental

Rare earth (RE) ions doped phosphate materials were studied extensively because of their potential applications in green technology, organic LEDs, plasma display panels (PDPs), filed emission displays (FEDs) [1–4], scintillators after glow and dosimetry [5,6]. Particularly, phosphors that contain phosphate have advantages such as low production cost, high rare earth ion solubility and large emission cross-sections attracted much interest in recent years [7,8]. As host materials, phosphates have proven their place in phosphor industry due to their excellent thermal and chemical stability. It is well known that number of phosphate phosphors e.g., Li2BaP2O7: Eu3 þ and Sm3 þ , LiEu(PO3)4: Eu3 þ , Sr3Y(PO4)3: Eu3 þ and Ca9Y(PO4)3: Eu3 þ have shown that they could be effectively excited in the near UV region to generate white light [9–12]. RE ion activated phosphors have attracted considerable research interest because of their excellent luminescent properties. RE ions have intra 4f-4f luminescence spectra characterized by narrow lines with high color purity because of the shielding of 4f electrons by outer 5s2 and 5p6 electrons [13]. For optical devices working for reddish-orange and red emissions, the Sm3 þ and Eu3 þ ions were used since they exhibit prominent reddish-orange and red luminescence through 4G5/2-6H7/2 and 5D2-7F2 transitions, respectively. Moreover, Sm3 þ and Eu3 þ ions are excellent activators with high luminescence quantum efficiencies, good color purity and great radiation stability [14]. Recently, Xiao et al. studied color tenability of Sm3 þ doped antimony–phosphate glass phosphors showing broadband fluorescence [15]. Sr2MoO4: Sm3 þ phosphor prepared by solid state reaction method have potential applications in the solid-state lighting based on the InGN LED chips [16]. Red emitting Mg2GeO4: Sm3 þ phosphors reported by Yang shows promising luminescence properties and suggest an attractive oxide phosphor for LEDs applications [17]. Yal3(BO3)4: Sm3 þ phosphor might be a good candidate for orange-red luminescence when compared to the other reported systems due to their low phonon frequency, high thermal stability and attractive physical, chemical and optical properties [18]. Similarly, Beltaif et al. reported synthesis, optical spectroscopy and Judd–Ofelt analysis of Eu3 þ doped Li2BaP2O7 phosphors [19]. Fang et al. studied photoluminescence properties of blue light excited Ca8La2(PO4)6O2: Eu3 þ red phosphors [20]. Tong et al. studied preliminary observation of self-reduction of Eu ions in αCa3(PO4)2 phosphors prepared in air condition [21]. Guangsheng et al. reported effect of Eu3 þ contents on structure and luminescence properties of Na3Bi2–x(PO4)3:xEu3 þ and Na3Bi1–x(PO4)2: xEu3 þ phosphors [22]. Borophosphate phosphors show significance for white LED (WLED) applications and some related research work was done recently [23,24]. However, there were few investigations on the correlation between synthesis conditions and local structure surrounding Eu3 þ ions in borophosphate phosphors. In this paper, an attempt is made to report the influence of different excitations and concentrations on the photoluminescence properties of Sm3 þ and Eu3 þ doped borophospahte phosphors. Changes in spectroscopic properties as a function of concentration were observed and discussed in terms of local environment of the Sm3 þ and Eu3 þ ions. The present work, aims at analyzing the structural, thermal and optical properties of new reddish-orange and red emitting borophosphate phosphor activated with Sm3 þ and Eu3 þ ions. Together with the concentration dependent fluorescence, the color perception and fluorescence decay of 4G5/2 level of Sm3 þ and 5D0 level of Eu3 þ ions were also studied. The fluorescence properties revealed that these phosphors were an excellent candidates for applications in the field of photonics.

Sm3 þ and Eu3 þ doped calcium borophosphate phosphors with the formula 2CaO-B2O3-P2O5: xRE (where, RE ¼ Sm3 þ and Eu3 þ , x¼ 0.2, 0.4, 0.6, 0.8 and 1.0 mol %) were synthesized by solid state reaction method. The raw materials used in present work are CaCO3, H3BO3, NH4H2PO4, Sm2O3 and Eu2O3. Rare earth oxides, Sm2O3 and Eu2O3 have purity of 99.99% and other materials are of analytical grade. The batch of 10 g was weighed according to the nominal compositions given below, 2Ca0.9O-B2O3-P2O5: 0.2Sm2O3 2Ca0.8O-B2O3-P2O5: 0.4Sm2O3 2Ca0.7O-B2O3-P2O5: 0.6Sm2O3 and 2Ca0.6O-B2O3-P2O5: 0.8Sm2O3 2Ca0.5O-B2O3-P2O5: 1.0Sm2O3

2Ca0.9O-B2O3-P2O5: 0.2 Eu2O3 2Ca0.8O-B2O3-P2O5: 0.4 Eu2O3 2Ca0.7O-B2O3-P2O5: 0.6 Eu2O3 2Ca0.6O-B2O3-P2O5: 0.8 Eu2O3 2Ca0.5O-B2O3-P2O5: 1.0 Eu2O3

The starting materials were taken in an agate motor and after homogenization the batch was first preheated at 650 °C for 2 h in porcelain crucible and then cooled down slowly to room temperature, and fully ground. The preheated batches were again heated 950 °C for 4 h in an electrical furnace. The obtained phosphor powders were pulverized for further characterization and analysis. The crystal structure and phase purity of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors were characterized by X-ray diffraction (XRD) on a diffractometer (Advance-D8, Bruker, Germany) operating at 40 kV and 100 mA with a Cu tube with Kα

radiation (λ ¼1.5406 Ǻ). The SEM morphologies and EDS spectra of Sm3 þ and Eu3 þ doped phosphors were taken by Caral Zeiss EVOMA15 scanning electron microscope. The UV-visible diffuse reflectance spectra (DRS) were measured for all the Sm3 þ and Eu3 þ doped phosphors using JASCO V570 UV–vis-NIR spectrometer. Excitation (0.6 mol% of Sm and Eu), emission and decay lifetimes of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors for different concentrations were recorded by FLS-920 Edinburg- fluorimeter (Horiba FL3- 22iHR320).

3. Results and discursion 3.1. X-ray diffraction (XRD) of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors X-ray diffraction patterns of 0.6 mol% Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors were measured in the range, 10 80° and were shown in Fig. 1. By comparing the XRD patterns of Sm3 þ and Eu3 þ doped phosphors with host pattern, it was found that all samples display the same diffraction peaks reported in JCPDS card No: 0-018-0283. It was found that when the doping concentrations varied, the positions of all diffraction peaks are in good agreement and symmetry with those appearing in the JCPDS card No: 0-018-0283, thus implying the formation of pure crystalline phase of 2CaO-B2O3-P2O5: Sm3 þ and Eu3 þ phosphors. Generally, borophosphate crystal contains CaO9, BO4 and PO4 polyhedron, generating three-dimensional composite framework with large tunnels inside. In 2CaO-B2O3-P2O5 crystal, insertion of Sm3 þ and Eu3 þ take place in the nine-fold coordinate calcium site due to similar ionic radii of Sm3 þ , Eu3 þ and Ca2 þ ions. Sm3 þ and Eu3 þ ions occupy a site in nine coordinated large polyhedron occurring in tortuous vertical columns formed by BO4 and PO4 tetrahedra. The chemical environments around Sm3 þ and Eu3 þ ions in the 2CaO-B2O3-P2O5 phosphors have higher local symmetry, leading to a lower refinement factor [25,26]. Fig. 1a and b indicate that

362


Fig. 1. XRD profiles of Sm3 þ (0.6 mol %) and Eu3 þ (0.6 mol %) doped 2CaO-B2O3P2O5 phosphors.

Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphor powders are well crystallized into a hexagonal symmetry with group space P6cc (184). 3.2. SEM and EDS of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors SEM and EDS studies were carried out to study the surface morphology and elemental analysis of prepared phosphors and were shown in Figs. 2 and 3 for Sm3 þ and Eu3 þ doped 2CaO-B2O3P2O5 phosphors, respectively. From Figs. 2(a-e) and 3(a-e), it was found that the particle size is drastically increased as the Sm3 þ and Eu3 þ concentration increases from 0.2 to 1.0 mol%, and the particles are more irregular morphologies due to Sm3 þ and Eu3 þ incorporation. Most commercial phosphor particles currently available in the market are in the range of 2–10 μm. The present particle sizes vary from few microns to several tens of microns. The particle shape and size of the phosphors made them suitable for use in the WLEDs applications. 3.3. Diffuse reflection spectra (DRS) of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors UV-visible diffuse reflection spectra of 2CaO-B2O3-P2O5 phosphors doped with 0.6 mol% Sm3 þ and Eu3 þ ions were measured in the range, 200–800 nm and are shown in Fig. 4. From the figure it was observed that reflectance of 2CaO-B2O3-P2O5:0.6 Sm and 2CaO-B2O3-P2O5:0.6 Eu is almost same indicating that the introduction of large particles has almost no influence on the amount of Sm3 þ and Eu3 þ doped. The band gaps were calculated using the formula given in Ref [27] from DRS of 2CaO-B2O3-P2O5, 2CaO-B2O3-P2O5: 0.6 Sm and 2CaO-B2O3-P2O5: 0.6 Eu and these values are 4.72 eV, 4.25 eV and 4.78 eV, respectively. 3.4. (a). Photoluminescence of 2CaO-B2O3-P2O5:Sm3 þ phosphor The excitation spectrum of Sm3 þ (0.6 mol%) activated 2CaO-B2O3P2O5 phosphor was measured in the wavelength range, 350–550 nm with an emission wavelength of 599 nm and is shown in Fig. 5. From the figure, it was observed that the spectrum consist of five excitation bands peaked at 362, 376, 403, 417 and 476 nm corresponding to the transitions from the ground state 6H5/2 to different excited states 4D3/2, 6 P7/2, 6P3/2, 6P5/2 and 4I9/2 þ 4I11/2 þ 4I13/2, respectively. The excited band, 6 P3/2 centered at 403 nm is the most intense in the visible region. The effect of different excitation wavelengths on emission spectra of Sm3 þ (0.6 mol%) doped 2CaO-B2O3-P2O5 phosphor was studied by recording

the emission spectra at different excitation wavelengths. The emission spectra consist of four emission peaks corresponding to transitions, 4 G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4G5/2-6H11/2. It was observed that among various excitation wavelengths, Sm3 þ doped 2CaO-B2O3-P2O5 phosphor excited by a light of wavelength 403 nm results in an intense reddish orange light emission corresponding to a wavelength of 599 nm. Intense emission at 403 nm also indicated that Sm3 þ doped 2CaO-B2O3-P2O5 phosphor can be effectively excited in near UV LEDs. The excitation wavelength of 403 nm corresponded to 6 H5/2-6P3/2 transition of Sm3 þ ion. Using the above optimized excitation wavelength, i.e. 403 nm, emission spectra of Sm3 þ doped 2CaO-B2O3-P2O5 phosphors were measured at different concentrations and are shown in Fig. 6. The spectra possesses four emission peaks in visible region at 562, 599, 646 and 707 nm, which are associated with the transitions from 4 G5/2 to different levels 6H5/2, 6H7/2, 6H9/2 and 6H11/2, respectively. Among the three observed emission transitions, 4G5/2-6H7/2 transition is the most intense one. Such high intensity obviously suggests that the energy loss due to cross relaxation is insignificant. In other words during the process of energy transfer from the excited state of Sm3 þ ion (by electric multiple interaction) to the nearest Sm3 þ ion lying in the ground state, the energy loss is minimum [28]. The first one at 562 nm (4G5/2-6H5/2) is a magnetic-dipole transition, the second at 599 nm (4G5/2-6H7/2) is a partly magnetic and partly a forced electric-dipole transition, and the other at 646 nm (4G5/2-6H9/2) is purely electric-dipole transition which is sensitive to crystal filed [29–31]. The intensity of luminescence in phosphors is usually affected by the variation in concentration of rare earth ions. In the present work, from Fig. 6, it was observed that with the increased dopant concentration, luminescence intensity also increased due to increase in optically active centers upto 0.6 mol%, but at concentrations of Sm3 þ 4 0.6 mol%, the photoluminescence intensity declined due to well known concentration quenching phenomenon. The theory behind this phenomenon is that as the concentration of dopant reaches to a certain value called critical value (0.6 mol%), the average interaction distance between dopant ions decreased and they come closer, which in turn enhances non-radiative transitions thereby decreasing overall photoluminescence intensity. 3.5. (b). Photoluminescence of 2CaO-B2O3-P2O5:Eu3 þ phosphor The excitation spectra of Eu3 þ (0.6 mol%) doped 2CaO-B2O3P2O5 phosphor was measured in the wavelength range, 350– 500 nm at an emission wavelength of 613 nm, and is shown in Fig. 7. Figure show four excitation transitions (populated from 7F0 and 7F1 ground states) such as 7F0-5D4, 7F1-5L7, 7F0-5L6 and 7 F0-5D2 corresponding to wavelengths 362, 379, 394 and 465 nm respectively. Among all the excitation bands, the band observed at 394 nm is sharper than the remaining bands. In the present work, an intense emission is observed at 394 nm for Eu3 þ doped phosphor. The spectrum showed narrow line shape due to excitations from both 7F0 and 7F1 states to various excited states. In the present work, the sharp peak at 465 nm was due to the pure electronic transition (PET) 7F0-5D2. The intensity of luminescence peaks vary with the variation of excitation wavelengths. The emission spectra of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor using different excitation wavelengths were also obtained. It was observed that the present phosphor produced an intense red photoluminescence at 613 nm for all excitation wavelengths. Among all excitation wavelengths, the most intense red emission appeared at 394 nm. Using this excitation wavelength (394 nm), the emission spectra were measured in the wavelength range, 550–725 nm for all Eu3 þ doped 2CaO-B2O3-P2O5 phosphors at different concentrations and are shown in Fig. 8. This emission spectra consisted of


363

Fig. 2. SEM images (a ¼ 0.2, b¼0.4, c ¼0.6, d ¼0.8 and e¼ 1.0 mol %) of Sm3 þ doped 2CaO-B2O3-P2O5 phosphor.

five emission bands at 578, 592, 613, 654 and 702 nm, related to the 5D0-7FJ (J ¼0, 1, 2, 3 and 4) transitions respectively. Luminescence transitions between 4 f levels of rare earth ions are predominantly due to electric dipole or magnetic dipole interactions. The intensity of the electric dipole transitions depends strongly on the site symmetry of the host. Magnetic dipole transitions were not affected much by the site symmetry because they were parity allowed [32,33]. In the present work, the emission spectra showed the most intense and dominant red luminescence at 613 nm for 5D0-7F2 electric-dipole transition over the 5D0-7F1 magnetic-dipole transition, which occurred at 591 nm of Eu3 þ ion. The intensity of strong emission peak for 5D0-7F2 transition increased with increasing Eu3 þ concentration upto 0.6 mol% and

decreased on further doping of Eu3 þ ions due to concentration quenching. Generally, the quenching mechanism was associated with the exchange interaction between a numbers of excited ions and depends mainly on the crystal structure and homogeneity of dopant concentration in the host environment [34]. This quenching effect leads to suppress the emission intensity by the non-radiative relaxation process between two adjacent Eu3 þ ions. The intensity ratio of 5D0-7F2 and 5D0-7F1 peaks was used to estimate the site asymmetry of Eu3 þ ions for all concentrations. The intensity ratios of these two transitions (R/O ratio) were calculated for all concentrations. Among all concentrations, 0.6 mol% of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor has high R/O ratio value as 6.19, which provided clear evidence that Eu3 þ ions mainly

364


Fig. 3. SEM images (a ¼ 0.2, b¼0.4, c ¼0.6, d ¼0.8 and e¼ 1.0 mol%) of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor.

occupy the lattice sites of Ca2 þ without inversion symmetry. The fluorescence intensity ratio (R) of 5D0- 7F2 and 5D0-7F1 peaks is used to establish the degree of asymmetry in the vicinity of Eu3 þ ions and Eu-O covalence for various Eu3 þ doped systems [35]. Moreover, R value also depends on the Judd-Ofelt parameter Ω2, which is used to explain the short range effects. Therefore, the variation of R and in turn Ω2 gives the information about the short range effect on local structure around Eu3 þ ions and Eu-O covalency. The higher the value of R, lower the symmetry around the Eu3 þ ions and higher the Eu-O covalency, and vice-versa. The R value for 2CaO-B2O3-P2O5:Eu3 þ phosphor is found to be higher for 0.6 mol% of Eu3 þ (6.19), suggesting that the Eu3 þ ions are located in an asymmetric environment. The appearance of the non-

degenerate 5D0-7F0 transition indicates that the Eu3 þ ion is in an environment of low symmetry in the 2CaO-B2O3-P2O5 phosphor [35]. Further, the magnetic dipole transition 5D0-7F1 splits into three components indicating that the crystallographic sites of the Eu3 þ ions in the present system are as low as orthorhombic, monoclinic or triclinic in a crystalline lattice. It is also evident from the appearance of the electric-dipole transition at 613 nm (5D0-5F2) that the Eu3 þ ions occupied the site of non-inversion symmetry [36]. In addition, the occupation of Ca2 þ sites by smaller Eu3 þ ions resulting in creation of oxygen vacancy to compensate the negative charge of Eu-Ca in the lattice, such as 3Ca2 þ -2Eu3 þ þ vacancy (Ca2 þ ) leading to lowering the symmetry of the surroundings of Eu3 þ ions, and thus enhance the


365

Fig. 7. Excitation spectrum of Eu3 þ (0.6 mol %) doped 2CaO-B2O3-P2O5 phosphor. Fig. 4. DRS spectra of Sm P2O5 phosphors.

3þ

(0.6 mol %) and Eu

3þ

(0.6 mol %) doped 2CaO-B2O3-

Fig. 8. Emission spectra of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations. Fig. 5. Excitation spectrum of Sm3 þ (0.6 mol %) doped 2CaO-B2O3-P2O5 phosphor.

Fig. 6. Emission spectra of Sm3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations. 5

D0- 7F2 red emission transition. However, excessive calcium vacancies will be created when the Eu3 þ concentration exceeds certain limit, affecting the crystallinity that led to the luminescence quenching [37].

3.6. Decay lifetime of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors The fluorescence decay curves of 4G5/2 level of Sm3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations were recorded with an excitation wavelength 403 nm and emission wavelength 599 nm and are shown in Fig. 9(a). It is observed that at lower concentrations of Sm3 þ (0.2 and 0.4 mol%), the decay curves were well fitted to exponential which indicated the absence of energy transfer between the Sm3 þ ions, whereas at higher concentrations (0.6, 0.8 and 1.0 mol%), the decay profiles became nonexponential due to the existence of non-radiative channels [38]. Measured lifetimes (τmeas) of 4G5/2 level were presented in Table 1. It was observed that, the measured lifetimes of the excited state, 4 G5/2 decreased with an increase of Sm3 þ ion concentration due to energy transfer between excited Sm3 þ ions through cross-relaxation channels. It was found predominant at higher concentrations of Sm3 þ ions i.e. Z 0.6 mol% in 2CaO-B2O3-P2O5 phosphor. The decrease in lifetime and non-exponential fit were the characteristics of the existence of concentration quenching mechanism at higher concentrations of Sm3 þ ion. The decay lifetime curves for the 5D0 state of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations were measured under excitation and emission wavelengths, 394 and 613 nm, respectively and are shown in Fig. 9(b). From the figure, it was observed that the decay curves have exhibited single exponential nature for all concentrations of Eu3 þ doped 2CaO-B2O3P2O5 phosphor. The measured lifetimes (τmeas) were obtained from the decay curves for the excited state; 5D0 for different

366


Fig. 9. Decay profiles of (a) Sm3 þ and (b) Eu3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations.

Table 1 Measured lifetimes (τcal, ms), absolute quantum efficiencies (QE, %) and color coordinates of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations. Formula

2Ca0.9O-B2O3-P2O5:0.2Sm2O3 2Ca0.8O-B2O3-P2O5:0.4Sm2O3 2Ca0.7O-B2O3-P2O5:0.6Sm2O3 2Ca0.6O-B2O3-P2O5:0.8Sm2O3 2Ca0.5O-B2O3-P2O5:1.0Sm2O3 2Ca0.9O-B2O3-P2O5:0.2Eu2O3 2Ca0.8O-B2O3-P2O5:0.4Eu2O3 2Ca0.7O-B2O3-P2O5:0.6Eu2O3 2Ca0.6O-B2O3-P2O5:0.8Eu2O3 2Ca0.5O-B2O3-P2O5:1.0Eu2O3

τmeas

0.352 0.201 0.176 0.175 0.161 2.320 2.314 2.304 2.276 2.266

QE

97.02 75.63 66.58 63.52 46.75 96.52 72.28 58.38 43.60 30.02

Color coordinates x

y

0.50 0.52 0.56 0.55 0.54 0.58 0.61 0.62 0.61 0.60

0.29 0.33 0.31 0.32 0.32 0.30 0.32 0.33 0.31 0.32

concentrations and are presented in Table 1. From the table, it was observed that the measured lifetimes (τmeas) decreased with the increase of Eu3 þ concentration. This was due to the non-radiative relaxation (WNR) of the 5D0 state of Eu3 þ ions. The radiative transition was attributed to the ion-ion interaction of the Eu3 þ ions and it includes all the emission transitions. The total decay rate of 5D0 level was the combination of both radiative and nonradiative processes [39]. The absolute quantum efficiencies (AQE) of Sm3 þ and Eu3 þ doped phosphors were measured under excitation at 403 and 394 nm respectively with the fluorescence spectrometer by using an integrating sphere. The AQE values of Sm3 þ and Eu3 þ doped phosphors are presented in Table 1. From the table, it is observed that AQE values decreased with increasing of Sm3 þ and Eu3 þ ions concentrations. These values are compared with other reported red emitting phosphors. The AQE values are 70% for Eu doped SiAlON phosphor and 86% for Ba1.95Eu0.05SiO4 phosphor [40,41]. 3.7. CIE chromaticity coordinates of 2CaO-B2O3-P2O5:Sm3 þ and Eu3 þ phosphors The Commission International de I’Eclairage (CIE) chromaticity coordinates such as X, Y and Z were calculated using the formula

given in Ref. [42] and used to get stimulation for each of the three primary colors such as red, green and blue required to match the color. In the present work, the luminescence color of the samples excited at 403 nm was characterized by the CIE chromaticity diagram of Sm3 þ doped 2CaO-B2O3-P2O5 phosphor as shown in Fig. 10(a). The chromaticity coordinates of Sm3 þ doped 2CaOB2O3-P2O5 phosphor for different concentrations were evaluated and are presented in Table 1. From the Fig. 10(a), it was observed that in the samples prepared with lower concentrations of Sm3 þ ion, the reddish-orange color emission was significantly less compared with the 0.6 mol% (0.56 and 0.31) of Sm3 þ doped 2CaOB2O3-P2O5 phosphor. Thus, it was confirmed that the 0.6 mol% of Sm3 þ doped 2CaO-B2O3-P2O5 phosphor might be used as a potential reddish-orange emitting material. The CIE chromaticity diagram of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor under the excitation of 394 nm was shown in Fig. 10(b). The chromaticity coordinates of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations were calculated and are also presented in Table 1. The emission color of Eu3 þ (0.2 mol%) doped 2CaO-B2O3-P2O5 phosphor was pure orange, and the associated color coordinates were 0.58 and 0.30. By introducing Eu3 þ (0.4 mol%) into the host the emitting color changed to orange-red and the color coordinates changed to 0.61 and 0.32. Upon further increasing the concentration of Eu3 þ (upto 0.6 mol%), the emitting color shifted towards pure red region and then to orange-red region for 0.8 and 1.0 mol% of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor. As a consequence, 0.6 mol% of Eu3 þ doped 2CaO-B2O3-P2O5 phosphor may be used as a potential red luminescence material.

4. Conclusions Calcium borophosphate phosphor activated with Sm3 þ and Eu ions for different concentrations were synthesized by solid state reaction method. These prepared phosphors were characterized by various spectroscopic techniques. From SEM images, it was observed that the particles were more irregular morphologies due to Sm3 þ and Eu3 þ incorporation. From XRD profiles, all Sm3 þ and Eu3 þ doped phosphor powders were well crystallized into hexagonal symmetry with group space P6cc (184). Using 3þ


367

Fig. 10. CIE chromaticity diagram of (a) Sm3 þ and (b) Eu3 þ doped 2CaO-B2O3-P2O5 phosphor for different concentrations. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).

different excitation wavelengths, emission spectra of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors were obtained using excitation wavelengths, 403 and 394 nm, respectively. From emission spectra, concentration quenching was observed at 0.6 mol% of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors. From the decay profiles, it was observed that at lower concentrations of Sm3 þ (0.2 and 0.4 mol%), the decay curves were well fitted to exponential behavior and at higher concentrations of Sm3 þ (0.6, 0.8 and 1.0 mol%), the decay profile became non-exponential due to the existence of non-radiative channels. In case of Eu3 þ ion, decay curves exhibited exponential nature for all concentrations. From CIE chromaticity diagram, reddish-orange and red color emissions were observed at 0.6 mol% of Sm3 þ and Eu3 þ doped 2CaO-B2O3P2O5 phosphors. As a consequence, 0.6 mol% of Sm3 þ and Eu3 þ doped 2CaO-B2O3-P2O5 phosphors could be used as potential reddish-orange and red luminescence material.

Acknowledgement One of the authors V. Reddy Prasad expresses his thanks to University Grants Commission (UGC), New Delhi for the sanction of JRF under Rajiv Gandhi National Fellowship (RGNF). The authors acknowledge MoU-DAE-BRNS Project (No. 2009/34/36/BRNS/ 3174), Department of Physics, S.V. University, Tirupati, India foe extending experimental facility.

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