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Dy3+) doped BiLa2O4 phosphors
Optik - International Journal for Light and Electron Optics 170 (2018) 125–131
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Synthesis and multi-colour luminescence spectra of RE3+ (RE3+ =Eu3+, Sm3+, Dy3+, Eu3+/Sm3+/Dy3+) doped BiLa2O4 phosphors
T
⁎
Rajesh Narayana Perumala,b, , Aloysius Xavier Lopeza,b, G. Subalakshmia,b a b
Center for Radiation Environmental Science and Technology, SSN College of Engineering, Kalavakkam, 603 110, India Department of Physics, SSN College of Engineering, Kalavakkam, 603 110, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Phosphors XRD Solid state reaction
Syntheses of BiLa2O4 phosphors is carried out by single doping with Eu3+, Sm3+, Dy3+ and tridoping with Eu3+/Sm3+/Dy3+ ions using high temperature solid reaction method. Characterization studies such as X-ray diffraction, diffuse reflectance spectra, photoluminescence excitation and emission spectra, decay profile measurements are made, in pursuit of a comprehensive analysis of the synthesized materials. As a result of XRD studies, the BiLa2O4 phosphors are found to have monoclinic structure. The luminescence properties of the of BiLa2O4 phosphors varies with different doping elements. The material is subjected to excitation at a wavelength of 464,462 and 422 nm. Strong emission peaks for Eu3+ ions at 626 nm with transition state (5D0 → 7 F3), Sm3+ ions at a wavelength of 651 nm with a transition state (4G5/2 → 6H7/2) and Dy3+ ions at a wavelength of 575 nm with a transition state of (4G5/2 → 6H7/2) are observed. It is evident that the BiLa2O4 phosphors are potential candidates optoelectronic application because of the diverse luminescence properties showed on account of doping with different rare earth elements of Eu3+, Sm3+, Dy3+. The luminescence decay curve and chromaticity coordinates of BiLa2O4 phosphors single and tri-doped with Eu3+, Sm3+, Dy3+ have also been investigated.
1. Introduction Currently, studies have focused on rare earth ion-doped phosphor materials owing to attractive optoelectronic applications that stem from 4f transitions states, with 5 s & 5p outer orbitals that act as a shield [1,2]. Such kinds of rare earth ion-doped luminescent materials are used in light emitting diodes (LED), field emission displays (FEDs) and flat panel displays (FPDs) and other similar applications. Yet, the current focus of researchers pertains to investigation of high efficient luminescent materials for innovations in solid state devices for improving lighting infrastructure [3,4]. Development of white light emitting diodes (WLEDs) have been made by the following ways (i) combining different phosphors of blue, green and red colour luminescence materials (ii) combining a chip of blue emitting InGaN with yellow phosphors (YAG: Ce3+) and (iii) obtaining a mix of blue LED chip in association with red and green phosphors. The disadvantage of these methods is the low colour rendering index and low efficiency property of such WLEDs fabricated with these phosphors [5–7]. In a bid to overcome the disadvantage, suitable phosphors have to be synthesized. The phosphors should also have high chemical and thermal stability. There are many rare earth elements with potential for use in phosphors. However, rare earth ions such as Eu3+, Sm3+ and Dy3+ are commonly used in fabrication of the phosphors. The phosphors could be excited by UV/ blue LEDs chip [8–13]. For LED ⁎
Corresponding author at: Center for Radiation Environmental Science and Technology, SSN College of Engineering, Kalavakkam, 603 110, India. E-mail address: [email protected] (R.N. Perumal).
https://doi.org/10.1016/j.ijleo.2018.05.063 Received 27 March 2018; Accepted 16 May 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 170 (2018) 125–131
R.N. Perumal et al.
applications, special interest is shown by the research fraternity on oxide doped rare earth elements [14]. In this work, investigation is carried out on the structure and luminescence of the BiLa2O4 phosphors doped with Dy3+, Sm3+ and Eu3+ ions which are synthesized by high temperature solid state reaction method using a tubular furnace. Under different excitation wavelength, Dy3+ gives near white light emission, Sm3+ displays orange emission spectra and Eu3+ emits red luminescence, which are studied in detail. So, the synthesized materials are found ideal for LED applications. 2. Experimental section A tubular furnace is used for sintering at 1200 °C for synthesis of rare earth doped BiLa2O4 phosphors using high temperature solid state reaction method. Stoichiometric composition of Bi2O3 (Sigma-Aldrich, 99.99%), La2O3 (Sigma-Aldrich, 99.99%), Sm2O3 (Merck, 99%), Dy2O3 (Merck, 99%), and Eu2O3 (Merck, 99%) are used directly as a starting materials without prior purification process. A fine powder of the composite of BiLa2-xO4:xRE3+ (x = 0.03) obtained by grinding using an agate mortar. A muffle furnace is used to heat the powder sample in alumina crucible at 600 °C for 5 h in air atmosphere. On cooling the sample at room temperature, the materials are subjected yet again to a grinding process before reheating at 1200 °C for 6 h, leading to sintering of the material. The sintered material is cooled again, before grinding the samples and transferring them into an alumina crucible. Then, the samples are sintered for the second time at 1200 °C for another 6 h. Then, the composite of BiLa2-xO4 :xRE3+ (x = 0.03) materials are allowed to cool on its own at room temperature before grinding into a fine powder for further characterization studies. Analyses of the XRD pattern of synthesized phosphors is made with PANalytical’s X-ray Diffractometer with Cu Kα radiation and λ = 0.1540 Å, in the range of 2θ from 20 to 60˚ as the X-ray source. Study of the samples for diffuse reflectance spectra made and data collected using Varian Model 5000 UV–Vis spectrophotometer in the wavelength range of 200–800 nm. A 450 W xenon lamp is used as excitation source for Spectroflurometer (JY Fluorolog-FL3-11) for gauging the photoluminescence excitation and emission spectra. CIE-1931 software in MATLAB is used to calculate the chromaticity coordinates. The entire range of samples is maintained at room temperature during the process of study for characterization of the materials. 3. Results and discussion 3.1. X-ray diffraction characterization Fig. 1 shows the X-ray diffraction patterns of the BiLa2O4 phosphors that were subjected to single and tri-doping of Ln3+(Eu, Dy and Sm) with comparison of the samples with JCPDS card no.47-0298. The host materials in the single phase are well indexable with JCPDS card no.47-0298. Owing to the insignificant difference in ionic radius and same electrical charge of La3+ and RE3+ ions, impurity peaks did not present themselves when doped with RE3+, showing that there is only a small impact on the host composition devoid of structure changes. The high temperature solid state reaction is adopted to prepare the single phase host material. As per the JCPDS card no. 47-0298, the host material of BiLa2O4 has a composition with monoclinic crystal structure and a space group C2/m and lattice parameters of a = 6.829, b = 3.988 and c = 4.052 and volume 90.32. In the analysis, peak intensity increases when the host material is single doped with RE3+ ions, showing that the enrichment of the crystalline nature of the composition has taken place. However, peak intensity decreases after tri-doping owing to lattice deterioration in the host material. In order to find the movement in the peak between 25° and 30° in Fig. 2, the XRD pattern is enlarged, which shows that the diff ;raction peaks of synthesized samples deviate slightly to a higher 2θ position because of incorporation of the RE3+ ions into the host lattice. The results show that RE3+ ions have positioned themselves in the host lattice. 3.2. Diffuse reflectance spectra characterization Analyses of diffuse reflectance spectra estimates the optical characteristics of the synthesized samples for several RE3+ ions as
Fig. 1. X-ray diffraction pattern of single, triple doped RE3+ ions in BiLa2O4 host lattice. 126
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Fig. 2. Enlarge spectrum of XRD forms between 25° and 30° angle.
depicted in Fig. 3a. On comparison of the host material and single doping elements, the peak shifts to the higher side in the red region, showing that the samples have high photon efficiency as well as production of more electron-hole pair to strengthen the luminescence. Following equation is used to arrive at the band gap value [15]
αh ϑ = A (h ϑ−Eg )n Where, A symbolize the constant, α stands optical absorption coefficient, h implies Planck’s constant, ϑ standpoints for frequency of light, Eg describes band gap energy value and n is transition parameter. A plot of the curve between (αh ϑ)1/2 and energy is made in Fig. 3b. The band gap energy values are 2.69, 2.62, 2.40, 2.55 and 2.60 eV of the prepared samples of doping with Eu3+, Sm3+ and Dy3+ & Eu3+/Sm3+/Dy3+ ions respectively. 3.3. Photoluminescence characterization Fig. 4 explains the photoluminescence excitation and emission spectra of single and tri-doped BiLa2O4:RE3+(Eu3+, Sm3+ and Dy ) ions. In the samples, observation of high intense excitation peak is made around 400 nm because of the Bi3+ ions in the host lattice. The Bi3+ ions transfer energy efficiently to the activator ions, giving a modified emission peak which depends on doping elements. As a result, Bi3+ ions perform as an effective energy donor to the doped Ln3+ ions. On doping, BiLa2O4:Eu3+ gives red emission; BiLa2O4:Sm3+ gives orange emission and BiLa2O4:Dy3+ gives near white light emission. Explaining about doping with Eu3+, BiLa2O4:Eu3+ sample with doping concentration x = 0.03 of Eu3+ ions, analysis of the excitation spectra is made by using the emission wavelength of Eu3+ (5D0 to 7F3) at 626 nm, shown in Fig. 4a. After finding excitation spectra at 626 nm, intense and broad peaks are found in 464 nm (7F0→5D2). In addition some other peaks present themselves also. Eu3+ ions are not the sole reason for the peaks but Bi3+ ions in the host lattice are also a reason for the peaks obtained in the intense excitation at 464 nm. The peaks presented in the Eu3+ ions are due to f-f transition and the excitation spectra is obtained by the transfer of energy from 3+
Fig. 3. a) Diffuse reflectance spectra BiLa2O4:Ln3+ phosphors b) Band gap value of synthesized phosphors. 127
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Fig. 4. Photoluminescence excitation and emission spectra of single and tri-doping phosphors under different excitation wavelength.
ground state 7F0 to higher energy states of 5G4 (376 nm) and 5L6 (392 nm). In the excitation spectra, the 464 nm is considered excitation wavelength, analyzing emission spectra, mentioning the presentation of peaks at 5D0→ 7F1 (590 nm), 5D0 → 7F2 (626 nm), 5 D0 → 7F2 (659) and 5D0 →7F4 (706) electronic states. The electric dipole transition state of 5D0-7F1 (626 nm) is stronger than the magnetic dipole transition of 5D0-7F1 state (590). The magnetic dipole transition is hypersensitive to the environs. On investigating BiLa2O4:Sm3+ ions for the photoluminescence property as per Fig. 4b, shows excitation and emission spectra, monitoring excitation spectra at 651 nm emission wavelength. The excitation spectra tell that intense emission is obtained at 462 nm. Not only that, but many excitation peaks are also presenting themselves, because of the closely spaced energy states of Sm3+ ions. The energy is generated from the ground state 6H5/2 to higher electronic states of 4G7/2 (406 nm), 4G9/2 (448 nm), 6P5/2 (464 nm) and 4I13/2 (472 nm). The excitation spectra shows that the emission peaks can be obtained using blue chip as a source of excitation. The emission wavelength of Sm3+ is similar to Eu3+. Emission spectra of Sm3+ are to be found out and the energy populates from lower energy state to higher energy state of 6P5/2 and non-radiative transition takes place to reach the electronic state of 4G5/2. From the state, it reaches the ground state, offering emission peaks of 574 nm, 608 nm and 651 nm. Reasonable emission is obtained at 574 nm, the weak spectra at 608 nm and strong emission spectra is obtained at 651 nm and their corresponding transition states are given by 4 G5/2 →6H5/2, 4G5/2 →6H7/2 and 4G5/2 →6H9/2. Fig. 4c illustrates the excitation and emission spectra of Dy3+ ions. The excitation spectra is found out at 575 nm emission wavelength. Peaks of excitation spectra are obtained at 351 and 422 nm denoting the 6H15/2→6P7/2 and 6H15/2→4G11/2 transition states. Of the two peaks, 422 nm is stronger, so it is considered as the excitation wavelength to obtain emission spectra. Following excitation of 422 nm wavelength, the emission spectrum throws light on lines 482, 575 and 667 nm wavelength, sourced from the transition states of 4F9/2 level to 6H15/2, 6H13/2 and 6H11/2 states of Dy3+ ions. In the case of tri-doping of RE3+ ions, excitation wavelength of 464 nm, 462 nm and 422 nm are used for obtaining emission spectra shown in Fig. 4d–f. The tri-doping phosphors are less intense when compared with the single doped host element emission spectra, because of the cross-relaxation among the rare earth elements, reducing the peak intensity of the tri-doped synthesized sample. A decrease in radiative transition is because of the shrinking of the distance between activator ions and the creation of crossrelaxation process. From the photoluminescence analysis, it is observed that excitation spectra are around 400 nm because of proper energy transfer from Bi3+ ions in the host lattice. So, blue laser diodes are used for efficient excitation of RE3+ ions doped host. Following the above electronic energy state, a schematic diagram is drawn for the possible modes of energy transfer from host lattice and Ln3+ ions in Fig. 5 and energy is observed in parts to be conveyed to RE3+ ions. The energy level of the Bi3+ ions overlap with the excitation spectra of RE3+ ions in 4f transition states, eliciting intense spectra at 400 nm. From the above aspects, it is clear that the emission wavelength colour could be tuned as per various RE3+ ions.
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Fig. 5. Energy level illustration.
3.4. Decay time A record of the luminescence decay curve of the BiLa2O4 doped Eu3+,Dy3+ and Sm3+ ions is made under their spotted excitation and emission spectra as part of photoluminescence spectra analysis as shown in Fig. 6. For getting the decay curve of Eu3+ ions, the excitation wavelength is 464 nm and emission wavelength is 626 nm. In the case of decay curve for Sm3+ ions, the excitation wavelength is 462 nm and emission wavelength is 651 nm. For Dy3+ ions, the excitation wavelength is 422 nm and emission wavelength is 572 nm. As per Fig. 6, this decay curve is well suited for double-exponential fitting [16]: t
t
I (t ) = A1 e(− τ1 ) + A2 e(− τ2 )
I (t ) luminescence intensity, A1 and A2 amplitudes and τ1 and τ2 decay times. The average decay times (τ*) are evaluated from given equation: τ* =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
Fig. 6. Decay curve of BiLa2O4:RE3+ composition. 129
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Fig. 7. CIE diagram of BiLa2O4:RE3+ phosphors.
The decay times are estimated at 0.0013, 0.0109 and 0.014 ms for Eu3+ Sm3+ and Dy3+ doped host materials respectively. 3.5. CIE, colour purity and CCT Fig. 7 showing the chromaticity co-ordinates which are found using CIE 1931 colour matching function. The chromaticity coordinates of synthesized materials BiLa2O4:Eu3+, BiLa2O4:Sm3+ and BiLa2O4: Dy3+ are (0.656, 0.342), (0.583, 0.415) and (0.328, 0.373) respectively. In a bid to get the colour purity of the synthesized materials, measurement is made based on the given equation [17]:
Colour Purity =
(x −x i )2 + (y−yi )2 (x d−x i )2 + (yd −yi )2
. 100%
where (x, y) points to chromaticity coordinate of the light source, (xi, yi) denotes the CIE of equal-energy illuminant value (0.3333, 0.3333), and (xd, yd) pertains to the chromaticity coordinate corresponding to the intense wavelength. In Eu3+, Sm3+ and Dy3+ ions, the wavelength with high intensity is present at of 626, 651 and 575 nm, considering them as the dominant wavelength. Colour coordinates are found out for the dominant wavelength. They are (0.702, 0.292), (0.726, 0.273) and (0.455, 0.541) for Eu3+, Sm3+ and Dy3+ ions. By substituting the colour coordinates in the colour purity equation, efficiency of colour purity is found to be 16%, 86% and 66% in the synthesized sample. This result shows that BiLa2O4:Eu3+ gives deep red colour and a higher colour attention as against the purity of other materials, making it a fit candidate for LEDs applications. The estimation of correlated colour temperature (CCT) is made as per the following formula [18]: CCT=−449n3 +3525n2−6823.3n+5520.33 n=(x−xe/y−ye) mentions the reciprocal slope and (xe = 0.332, ye = 0.186) fixes the epicenter. The correlated colour temperature is estimated at 2531 K, 1685 K and 5667 K for Eu3+, Sm3+ and Dy3+ ions doped host material. The correlated colour temperature of BiLa2O4:Eu3+ is 2531 K and the colour coordinates are found to be (0.656, 0.342), showing that the material giving red luminiscence is the best candidate for WLEDs applications. 4. Conclusion Single and tri-doped with BiLa2O4 host lattice is synthesized by high temperature solid state reaction method by doping with RE3+ (Eu3+ Sm3+ and Dy3+) ions. Excited in the blue region, the phosphors gives intense emission at 626, 651 and 575 nm because of their transition states 5D0 → 7F2, 4G5/2 →6H9/2 and 4F9/2 → 6H13/2 of Eu3+,Sm3+ and Dy3+ ions that are doped in the host lattice. The cross relaxation process explains the low emission intensity of tri-doping phosphors. The chromaticity co-ordinates of Eu3+,Sm3+ and Dy3+ ions are (0.656, 0.342), (0.583, 0.415) and (0.328, 0.373) and its correlated colour temperatures are 2531 K, 1685 K and 5667 K. The high purity is found in BiLa2O4 doped Eu3+ among the RE3+ ions, with colour purity of 86%, giving an emission of deep red. The chromaticity co-ordinates are x = 0.656, y = 0.342 and correlated colour coordinates are 2531 K. In white LED applications, the BiLa2O4 doped Eu3+ red phosphors could be used. Acknowledgments The authors gratefully acknowledge the Department of Science and Technology, India for granting financial support for the 130
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project ref-SR/S2/CMP/117/2012 and SSN management for giving junior research fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Synthesis and multi-colour luminescence spectra of RE3+ (RE3+ =Eu3+, Sm3+, Dy3+, Eu3+/Sm3+/Dy3+) doped BiLa2O4 phosphors
T
⁎
Rajesh Narayana Perumala,b, , Aloysius Xavier Lopeza,b, G. Subalakshmia,b a b
Center for Radiation Environmental Science and Technology, SSN College of Engineering, Kalavakkam, 603 110, India Department of Physics, SSN College of Engineering, Kalavakkam, 603 110, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Phosphors XRD Solid state reaction
Syntheses of BiLa2O4 phosphors is carried out by single doping with Eu3+, Sm3+, Dy3+ and tridoping with Eu3+/Sm3+/Dy3+ ions using high temperature solid reaction method. Characterization studies such as X-ray diffraction, diffuse reflectance spectra, photoluminescence excitation and emission spectra, decay profile measurements are made, in pursuit of a comprehensive analysis of the synthesized materials. As a result of XRD studies, the BiLa2O4 phosphors are found to have monoclinic structure. The luminescence properties of the of BiLa2O4 phosphors varies with different doping elements. The material is subjected to excitation at a wavelength of 464,462 and 422 nm. Strong emission peaks for Eu3+ ions at 626 nm with transition state (5D0 → 7 F3), Sm3+ ions at a wavelength of 651 nm with a transition state (4G5/2 → 6H7/2) and Dy3+ ions at a wavelength of 575 nm with a transition state of (4G5/2 → 6H7/2) are observed. It is evident that the BiLa2O4 phosphors are potential candidates optoelectronic application because of the diverse luminescence properties showed on account of doping with different rare earth elements of Eu3+, Sm3+, Dy3+. The luminescence decay curve and chromaticity coordinates of BiLa2O4 phosphors single and tri-doped with Eu3+, Sm3+, Dy3+ have also been investigated.
1. Introduction Currently, studies have focused on rare earth ion-doped phosphor materials owing to attractive optoelectronic applications that stem from 4f transitions states, with 5 s & 5p outer orbitals that act as a shield [1,2]. Such kinds of rare earth ion-doped luminescent materials are used in light emitting diodes (LED), field emission displays (FEDs) and flat panel displays (FPDs) and other similar applications. Yet, the current focus of researchers pertains to investigation of high efficient luminescent materials for innovations in solid state devices for improving lighting infrastructure [3,4]. Development of white light emitting diodes (WLEDs) have been made by the following ways (i) combining different phosphors of blue, green and red colour luminescence materials (ii) combining a chip of blue emitting InGaN with yellow phosphors (YAG: Ce3+) and (iii) obtaining a mix of blue LED chip in association with red and green phosphors. The disadvantage of these methods is the low colour rendering index and low efficiency property of such WLEDs fabricated with these phosphors [5–7]. In a bid to overcome the disadvantage, suitable phosphors have to be synthesized. The phosphors should also have high chemical and thermal stability. There are many rare earth elements with potential for use in phosphors. However, rare earth ions such as Eu3+, Sm3+ and Dy3+ are commonly used in fabrication of the phosphors. The phosphors could be excited by UV/ blue LEDs chip [8–13]. For LED ⁎
Corresponding author at: Center for Radiation Environmental Science and Technology, SSN College of Engineering, Kalavakkam, 603 110, India. E-mail address: [email protected] (R.N. Perumal).
https://doi.org/10.1016/j.ijleo.2018.05.063 Received 27 March 2018; Accepted 16 May 2018 0030-4026/ © 2018 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 170 (2018) 125–131
R.N. Perumal et al.
applications, special interest is shown by the research fraternity on oxide doped rare earth elements [14]. In this work, investigation is carried out on the structure and luminescence of the BiLa2O4 phosphors doped with Dy3+, Sm3+ and Eu3+ ions which are synthesized by high temperature solid state reaction method using a tubular furnace. Under different excitation wavelength, Dy3+ gives near white light emission, Sm3+ displays orange emission spectra and Eu3+ emits red luminescence, which are studied in detail. So, the synthesized materials are found ideal for LED applications. 2. Experimental section A tubular furnace is used for sintering at 1200 °C for synthesis of rare earth doped BiLa2O4 phosphors using high temperature solid state reaction method. Stoichiometric composition of Bi2O3 (Sigma-Aldrich, 99.99%), La2O3 (Sigma-Aldrich, 99.99%), Sm2O3 (Merck, 99%), Dy2O3 (Merck, 99%), and Eu2O3 (Merck, 99%) are used directly as a starting materials without prior purification process. A fine powder of the composite of BiLa2-xO4:xRE3+ (x = 0.03) obtained by grinding using an agate mortar. A muffle furnace is used to heat the powder sample in alumina crucible at 600 °C for 5 h in air atmosphere. On cooling the sample at room temperature, the materials are subjected yet again to a grinding process before reheating at 1200 °C for 6 h, leading to sintering of the material. The sintered material is cooled again, before grinding the samples and transferring them into an alumina crucible. Then, the samples are sintered for the second time at 1200 °C for another 6 h. Then, the composite of BiLa2-xO4 :xRE3+ (x = 0.03) materials are allowed to cool on its own at room temperature before grinding into a fine powder for further characterization studies. Analyses of the XRD pattern of synthesized phosphors is made with PANalytical’s X-ray Diffractometer with Cu Kα radiation and λ = 0.1540 Å, in the range of 2θ from 20 to 60˚ as the X-ray source. Study of the samples for diffuse reflectance spectra made and data collected using Varian Model 5000 UV–Vis spectrophotometer in the wavelength range of 200–800 nm. A 450 W xenon lamp is used as excitation source for Spectroflurometer (JY Fluorolog-FL3-11) for gauging the photoluminescence excitation and emission spectra. CIE-1931 software in MATLAB is used to calculate the chromaticity coordinates. The entire range of samples is maintained at room temperature during the process of study for characterization of the materials. 3. Results and discussion 3.1. X-ray diffraction characterization Fig. 1 shows the X-ray diffraction patterns of the BiLa2O4 phosphors that were subjected to single and tri-doping of Ln3+(Eu, Dy and Sm) with comparison of the samples with JCPDS card no.47-0298. The host materials in the single phase are well indexable with JCPDS card no.47-0298. Owing to the insignificant difference in ionic radius and same electrical charge of La3+ and RE3+ ions, impurity peaks did not present themselves when doped with RE3+, showing that there is only a small impact on the host composition devoid of structure changes. The high temperature solid state reaction is adopted to prepare the single phase host material. As per the JCPDS card no. 47-0298, the host material of BiLa2O4 has a composition with monoclinic crystal structure and a space group C2/m and lattice parameters of a = 6.829, b = 3.988 and c = 4.052 and volume 90.32. In the analysis, peak intensity increases when the host material is single doped with RE3+ ions, showing that the enrichment of the crystalline nature of the composition has taken place. However, peak intensity decreases after tri-doping owing to lattice deterioration in the host material. In order to find the movement in the peak between 25° and 30° in Fig. 2, the XRD pattern is enlarged, which shows that the diff ;raction peaks of synthesized samples deviate slightly to a higher 2θ position because of incorporation of the RE3+ ions into the host lattice. The results show that RE3+ ions have positioned themselves in the host lattice. 3.2. Diffuse reflectance spectra characterization Analyses of diffuse reflectance spectra estimates the optical characteristics of the synthesized samples for several RE3+ ions as
Fig. 1. X-ray diffraction pattern of single, triple doped RE3+ ions in BiLa2O4 host lattice. 126
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Fig. 2. Enlarge spectrum of XRD forms between 25° and 30° angle.
depicted in Fig. 3a. On comparison of the host material and single doping elements, the peak shifts to the higher side in the red region, showing that the samples have high photon efficiency as well as production of more electron-hole pair to strengthen the luminescence. Following equation is used to arrive at the band gap value [15]
αh ϑ = A (h ϑ−Eg )n Where, A symbolize the constant, α stands optical absorption coefficient, h implies Planck’s constant, ϑ standpoints for frequency of light, Eg describes band gap energy value and n is transition parameter. A plot of the curve between (αh ϑ)1/2 and energy is made in Fig. 3b. The band gap energy values are 2.69, 2.62, 2.40, 2.55 and 2.60 eV of the prepared samples of doping with Eu3+, Sm3+ and Dy3+ & Eu3+/Sm3+/Dy3+ ions respectively. 3.3. Photoluminescence characterization Fig. 4 explains the photoluminescence excitation and emission spectra of single and tri-doped BiLa2O4:RE3+(Eu3+, Sm3+ and Dy ) ions. In the samples, observation of high intense excitation peak is made around 400 nm because of the Bi3+ ions in the host lattice. The Bi3+ ions transfer energy efficiently to the activator ions, giving a modified emission peak which depends on doping elements. As a result, Bi3+ ions perform as an effective energy donor to the doped Ln3+ ions. On doping, BiLa2O4:Eu3+ gives red emission; BiLa2O4:Sm3+ gives orange emission and BiLa2O4:Dy3+ gives near white light emission. Explaining about doping with Eu3+, BiLa2O4:Eu3+ sample with doping concentration x = 0.03 of Eu3+ ions, analysis of the excitation spectra is made by using the emission wavelength of Eu3+ (5D0 to 7F3) at 626 nm, shown in Fig. 4a. After finding excitation spectra at 626 nm, intense and broad peaks are found in 464 nm (7F0→5D2). In addition some other peaks present themselves also. Eu3+ ions are not the sole reason for the peaks but Bi3+ ions in the host lattice are also a reason for the peaks obtained in the intense excitation at 464 nm. The peaks presented in the Eu3+ ions are due to f-f transition and the excitation spectra is obtained by the transfer of energy from 3+
Fig. 3. a) Diffuse reflectance spectra BiLa2O4:Ln3+ phosphors b) Band gap value of synthesized phosphors. 127
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Fig. 4. Photoluminescence excitation and emission spectra of single and tri-doping phosphors under different excitation wavelength.
ground state 7F0 to higher energy states of 5G4 (376 nm) and 5L6 (392 nm). In the excitation spectra, the 464 nm is considered excitation wavelength, analyzing emission spectra, mentioning the presentation of peaks at 5D0→ 7F1 (590 nm), 5D0 → 7F2 (626 nm), 5 D0 → 7F2 (659) and 5D0 →7F4 (706) electronic states. The electric dipole transition state of 5D0-7F1 (626 nm) is stronger than the magnetic dipole transition of 5D0-7F1 state (590). The magnetic dipole transition is hypersensitive to the environs. On investigating BiLa2O4:Sm3+ ions for the photoluminescence property as per Fig. 4b, shows excitation and emission spectra, monitoring excitation spectra at 651 nm emission wavelength. The excitation spectra tell that intense emission is obtained at 462 nm. Not only that, but many excitation peaks are also presenting themselves, because of the closely spaced energy states of Sm3+ ions. The energy is generated from the ground state 6H5/2 to higher electronic states of 4G7/2 (406 nm), 4G9/2 (448 nm), 6P5/2 (464 nm) and 4I13/2 (472 nm). The excitation spectra shows that the emission peaks can be obtained using blue chip as a source of excitation. The emission wavelength of Sm3+ is similar to Eu3+. Emission spectra of Sm3+ are to be found out and the energy populates from lower energy state to higher energy state of 6P5/2 and non-radiative transition takes place to reach the electronic state of 4G5/2. From the state, it reaches the ground state, offering emission peaks of 574 nm, 608 nm and 651 nm. Reasonable emission is obtained at 574 nm, the weak spectra at 608 nm and strong emission spectra is obtained at 651 nm and their corresponding transition states are given by 4 G5/2 →6H5/2, 4G5/2 →6H7/2 and 4G5/2 →6H9/2. Fig. 4c illustrates the excitation and emission spectra of Dy3+ ions. The excitation spectra is found out at 575 nm emission wavelength. Peaks of excitation spectra are obtained at 351 and 422 nm denoting the 6H15/2→6P7/2 and 6H15/2→4G11/2 transition states. Of the two peaks, 422 nm is stronger, so it is considered as the excitation wavelength to obtain emission spectra. Following excitation of 422 nm wavelength, the emission spectrum throws light on lines 482, 575 and 667 nm wavelength, sourced from the transition states of 4F9/2 level to 6H15/2, 6H13/2 and 6H11/2 states of Dy3+ ions. In the case of tri-doping of RE3+ ions, excitation wavelength of 464 nm, 462 nm and 422 nm are used for obtaining emission spectra shown in Fig. 4d–f. The tri-doping phosphors are less intense when compared with the single doped host element emission spectra, because of the cross-relaxation among the rare earth elements, reducing the peak intensity of the tri-doped synthesized sample. A decrease in radiative transition is because of the shrinking of the distance between activator ions and the creation of crossrelaxation process. From the photoluminescence analysis, it is observed that excitation spectra are around 400 nm because of proper energy transfer from Bi3+ ions in the host lattice. So, blue laser diodes are used for efficient excitation of RE3+ ions doped host. Following the above electronic energy state, a schematic diagram is drawn for the possible modes of energy transfer from host lattice and Ln3+ ions in Fig. 5 and energy is observed in parts to be conveyed to RE3+ ions. The energy level of the Bi3+ ions overlap with the excitation spectra of RE3+ ions in 4f transition states, eliciting intense spectra at 400 nm. From the above aspects, it is clear that the emission wavelength colour could be tuned as per various RE3+ ions.
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Fig. 5. Energy level illustration.
3.4. Decay time A record of the luminescence decay curve of the BiLa2O4 doped Eu3+,Dy3+ and Sm3+ ions is made under their spotted excitation and emission spectra as part of photoluminescence spectra analysis as shown in Fig. 6. For getting the decay curve of Eu3+ ions, the excitation wavelength is 464 nm and emission wavelength is 626 nm. In the case of decay curve for Sm3+ ions, the excitation wavelength is 462 nm and emission wavelength is 651 nm. For Dy3+ ions, the excitation wavelength is 422 nm and emission wavelength is 572 nm. As per Fig. 6, this decay curve is well suited for double-exponential fitting [16]: t
t
I (t ) = A1 e(− τ1 ) + A2 e(− τ2 )
I (t ) luminescence intensity, A1 and A2 amplitudes and τ1 and τ2 decay times. The average decay times (τ*) are evaluated from given equation: τ* =
A1 τ12 + A2 τ22 A1 τ1 + A2 τ2
Fig. 6. Decay curve of BiLa2O4:RE3+ composition. 129
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Fig. 7. CIE diagram of BiLa2O4:RE3+ phosphors.
The decay times are estimated at 0.0013, 0.0109 and 0.014 ms for Eu3+ Sm3+ and Dy3+ doped host materials respectively. 3.5. CIE, colour purity and CCT Fig. 7 showing the chromaticity co-ordinates which are found using CIE 1931 colour matching function. The chromaticity coordinates of synthesized materials BiLa2O4:Eu3+, BiLa2O4:Sm3+ and BiLa2O4: Dy3+ are (0.656, 0.342), (0.583, 0.415) and (0.328, 0.373) respectively. In a bid to get the colour purity of the synthesized materials, measurement is made based on the given equation [17]:
Colour Purity =
(x −x i )2 + (y−yi )2 (x d−x i )2 + (yd −yi )2
. 100%
where (x, y) points to chromaticity coordinate of the light source, (xi, yi) denotes the CIE of equal-energy illuminant value (0.3333, 0.3333), and (xd, yd) pertains to the chromaticity coordinate corresponding to the intense wavelength. In Eu3+, Sm3+ and Dy3+ ions, the wavelength with high intensity is present at of 626, 651 and 575 nm, considering them as the dominant wavelength. Colour coordinates are found out for the dominant wavelength. They are (0.702, 0.292), (0.726, 0.273) and (0.455, 0.541) for Eu3+, Sm3+ and Dy3+ ions. By substituting the colour coordinates in the colour purity equation, efficiency of colour purity is found to be 16%, 86% and 66% in the synthesized sample. This result shows that BiLa2O4:Eu3+ gives deep red colour and a higher colour attention as against the purity of other materials, making it a fit candidate for LEDs applications. The estimation of correlated colour temperature (CCT) is made as per the following formula [18]: CCT=−449n3 +3525n2−6823.3n+5520.33 n=(x−xe/y−ye) mentions the reciprocal slope and (xe = 0.332, ye = 0.186) fixes the epicenter. The correlated colour temperature is estimated at 2531 K, 1685 K and 5667 K for Eu3+, Sm3+ and Dy3+ ions doped host material. The correlated colour temperature of BiLa2O4:Eu3+ is 2531 K and the colour coordinates are found to be (0.656, 0.342), showing that the material giving red luminiscence is the best candidate for WLEDs applications. 4. Conclusion Single and tri-doped with BiLa2O4 host lattice is synthesized by high temperature solid state reaction method by doping with RE3+ (Eu3+ Sm3+ and Dy3+) ions. Excited in the blue region, the phosphors gives intense emission at 626, 651 and 575 nm because of their transition states 5D0 → 7F2, 4G5/2 →6H9/2 and 4F9/2 → 6H13/2 of Eu3+,Sm3+ and Dy3+ ions that are doped in the host lattice. The cross relaxation process explains the low emission intensity of tri-doping phosphors. The chromaticity co-ordinates of Eu3+,Sm3+ and Dy3+ ions are (0.656, 0.342), (0.583, 0.415) and (0.328, 0.373) and its correlated colour temperatures are 2531 K, 1685 K and 5667 K. The high purity is found in BiLa2O4 doped Eu3+ among the RE3+ ions, with colour purity of 86%, giving an emission of deep red. The chromaticity co-ordinates are x = 0.656, y = 0.342 and correlated colour coordinates are 2531 K. In white LED applications, the BiLa2O4 doped Eu3+ red phosphors could be used. Acknowledgments The authors gratefully acknowledge the Department of Science and Technology, India for granting financial support for the 130
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