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Dy3+ doped tellurite glass phosphors containing Al3+ ions
Optical Materials 79 (2018) 289e295
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Tailored white light emission in Eu3þ/Dy3þ doped tellurite glass phosphors containing Al3þ ions Michalina Walas a, *, Patryk Piotrowski a, Tomasz Lewandowski a, Anna Synak b, ski a, Wojciech Sadowski a, Barbara Koscielska a Marcin Łapin a sk University of Technology, ul. Gabriela Narutowicza 11/12, 80-233, Faculty of Applied Physics and Mathematics, Department of Solid State Physics, Gdan sk, Poland Gdan b sk, ul. Wita Stwosza 57/246, 80-952, Gdan sk, Poland Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2017 Received in revised form 6 March 2018 Accepted 7 March 2018
Tellurite glass systems modified by addition of aluminum fluoride AlF3 have been successfully synthesized as host matrices for optically active rare earth ions RE3þ (RE3þ ¼ Eu3þ, Dy3þ). Samples with different Eu3þ to Dy3þ molar ratio have been studied in order to determine possibility of white light emission via UV excitation. Structural investigations confirmed amorphous character of materials whereas spectroscopic studies brought more insight into glass network's nature. FTIR results shown presence of two features related to tellurite glass matrix (in 490e935 cm1 spectral region) and another one (940-1250 cm1) due to aluminum addition. Especially, Al-O and Te-O-Al bonds of AlO4 tetrahedrons have been found. AlO4 units are considered as glass formers that improve network's strength and thermal resistivity against devitrification. Based on XPS studies of Al3þ photoelectron band the existence of Al-O and also Al-F bonds have been examined. Moreover, signals originating from Eu3þ and Dy3þ have been found confirming their valence state. Luminescence results revealed possibility of simultaneous UV excitation of Eu3þ and Dy3þ ions. Excitation with lexc ¼ 390 and 393 nm resulted in white light generation starting from warm white to neutral and cool white depending on Eu3þ concentration and used excitation wavelength. Additionally, increase of decay lifetime of Eu3þ induced by Al3þ presence have been revealed based on luminescence decay analysis. Thus, tellurite glass systems modified by AlF3 and doped with Eu3þ/Dy3þ may be considered as promising candidates for white light emitting sources. © 2018 Elsevier B.V. All rights reserved.
Keywords: Eu Dy Luminescence WLED Tellurite glass Al3þ
1. Introduction White light-emitting diodes (WLEDs) have recently attracted considerable attention [1e4] as a next generation light sources and replacement for conventional incandescent and fluorescent lamps. WLEDs are definitely more environmental friendly, energy save, reliable and smaller than traditional light sources but more importantly, they may be characterized by high luminescence efficiency, long lifetimes etc. One of the most popular ways to achieve WLEDs is to combine near UV LED with the glass or crystal phosphor emitting in blue, green and red spectral regions [5e9]. For this purpose, phosphors doped with different RE3þ ions are particularly interesting. Neutral white light can be achieved from Dy3þ singly doped glasses or ceramics [10e12] but in order to adjust the
* Corresponding author. E-mail address: [email protected] (M. Walas). https://doi.org/10.1016/j.optmat.2018.03.015 0925-3467/© 2018 Elsevier B.V. All rights reserved.
emission color temperature and obtain warm white light it is necessary to involve additional red light emitters, e.g. Eu3þ ions. By excitation in the near UV in case of Dy3þ and Eu3þ ions it is possible to obtain simultaneous emission in blue and greenish-yellow (4F9/ 6 3þ 2 / H15/2, 13/2 transitions of Dy ) and in the orange-red spectral regions (5D0/7F1,0 transitions of Eu3þ). For example, A.N. MezaRocha et al. [13] reported white light generation from Eu3þ/Dy3þ Zn(PO3)2 glass system upon 348 and 445 nm excitation. Among variety of host matrices for optically active RE3þ ions tellurite glasses seem to be one of the most promising choices. Low phonon energies (the lowest among oxide glasses), high refractive index, transparency in visible region, good mechanical and chemical resistance combined with high solubility of RE3þ ions make them excellent candidates for RE3þ doped phosphors for WLEDs applications [14,15]. In our previous work [16] we found that Al3þ addition lead to increase of RE3þ luminescence intensity in tellurite-based glasses doped with Eu3þ, Tb3þ and Tm3þ ions. It is
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due to fact that Al3þ ions are known to prevent RE3þ clustering (agglomeration) which leads to decrease of the non-radiative transitions and consequently, to increase in the emission intensity. Moreover, addition of Al3þ ions results in the formation of AlO4 tetrahedral or AlO6 octahedral units. Hereby, Al3þ ions may be considered as glass former or modifier, additionally improving glass tendency against devitrification [17,18]. In this work, we successfully synthesized tellurite-based glasses containing Al3þ ions and co-doped with Eu3þ and Dy3þ ions. By changing the Eu3þ/Dy3þ ratio and adjusting the excitation wavelengths we achieved simultaneous, color-tunable, long-lifetime luminescence originating from both, Eu3þ and Dy3þ ions in blue, green and red spectral regions. Additionally, we studied structural and spectroscopic properties of achieved samples. Results confirmed that tellurite-based glass containing Al3þ ions is suitable host for optically active rare earth ions. 2. Experimental Tellurite-based glass samples with nominal composition (in mol %) 73 TeO2 e 4 BaO e 3 Bi2O3 e 18 AlF3 e x Eu3þ/(2-x) Dy3þ (where x ¼ 2.0; 1.5; 1.0; 0.5; 0.0) were synthesized using melt-quenching technique. At first, appropriate amounts of precursor reagents: TeO2, BaCO3, Bi5OH (OH)9(NO3)4, AlF3, Eu(NO3)3 and Dy(NO3)3 were mixed in an agate mortar to achieve homogenous powders. Samples in the porcelains crucibles were placed in the furnace and decomposed at 700 C for 1 h at first and subsequently heated to 940 C for another 0.5 h. After that time samples were poured onto pre-heated stainless steel plate and subsequently pressed by another steel plate in order to achieve thin, transparent glass species. All steps of synthesis procedure were carried out in air atmosphere. The series of glasses was denoted as TBBA: x Eu/(2-x) Dy. Detailed description of samples along with their chemical composition is presented in Table 1. Structural and spectroscopic properties of samples were investigated using several techniques. In order to confirm an amorphous structure of glasses X-ray diffraction (XRD) measurements were performed on powder samples on Philips X’PERT PLUS diffractometer with Cu-Ka radiation (l ¼ 0.154 nm). Fourier transform infrared spectroscopy (FTIR) measurements were carried out on Perkin-Elmer Frontier MIR/FIR spectrometer with TGS detector in the mid-infrared range to establish the types of chemical bonds in the structural units present in the samples. The measurements were performed on pellet samples mixed with potassium bromide KBr in weight ratio (Sample: KBr) 1:100. X-ray Photoelectron Spectroscopy analysis (XPS) was carried out with X-ray photoelectron spectrometer (Omnicron NanoTechnology) with 128channel collector. Investigated samples were pre-cleaned by Ar ion beam. XPS measurements were performed in ultra-high vacuum conditions, below 1.1 108 mBar. The photoelectrons were excited by an Mg-Ka X-ray source with X-ray anode operated at 15 keV and 300 W. The absorption spectra were measured using a Perkin-Elmer Lambda 35 UVeVis spectrophotometer. Luminescence emission and excitation spectra of glass pieces were collected
in a Horiba Yvon spectrofluorometer (Fluorolog 3) on glass samples. Both spectra were corrected by the spectral response of the equipment and the intensity of the excitation source. Lifetimes were obtained from luminescence decay curves recorded in the same equipment using a pulsed lamp. The decay curves were registered at the characteristic emission wavelength of Eu3þ ions (611 nm) using an excitation wavelength of 393 nm. 3. Results 3.1. XRD X-Ray diffraction studies have been performed in order to confirm the amorphous character of prepared glass samples. Fig. 1 shows the diffraction patterns of TBBA: x Eu/(2-x) Dy glass series. All of the diffraction patterns show only broad amorphous halos. They are connected with presence of short-range order in the glass structure. These features are characteristic to glassy materials [19]. 3.2. FTIR FTIR measurements were undertaken to acquire information about the glass network structure. Infrared spectra of samples are shown in Fig. 2 (left). In case of all samples obtained FTIR spectra consist of two main features that remain unchanged with the various molar ratio of the Eu3þ/Dy3þ dopants. First broad band present in the 490e935 cm1 spectral region is characteristic to tellurite glasses and it might be deconvoluted into three features (Fig. 2 right). First one with maximum at approximately 611 cm1 can be assigned to Te-O-Te vibrations in TeO4 trigonal bipyramidal units [20]. The next component at approximately 725 cm1 is due to symmetric stretching vibrations of Te-O bonds in TeO4 or Te-O-Te linkages between two fourfold coordinated Te atoms. The last feature at around 865 cm1 is most likely caused by stretching vibrations of Te-O in TeO3 trigonal pyramids and/or TeO3þ1 polyhedral groups. Presence of different structural units may be explained as follows. It is well known that glass network modifiers ions e.g. Bi3þ are easily coordinated with Te-Oax bonds resulting in elongation of Te-Oax bonds in TeO4 units. When length of these bonds increase from 0.208 (in TeO4) to 0.280 nm then the TeO3þ1 units are formed. If the Te-Oax length exceeds 0.298 nm structural unit transforms into TeO [21]. The second broad band in the FTIR spectra of TBBA: x Eu/(2-x) Dy samples in spectral range 940e1250 cm1 is connected with the presence of Al atoms in the tellurite glass matrix. The main feature may be also deconvoluted into two signals. The one at 1031 cm1 is
Table 1 Calculated x and y parameters and CCT for lexc ¼ 390 and 393 nm excitation. x Eu/(2-x) Dy
2.0 1.5 1.0 0.5 2.0
Eu Eu/0.5 Dy Eu/1.0 Dy Eu/1.5 Dy Dy
lexc ¼ 390 nm x
y
0.57 0.42 0.38 0.36 0.34
0.35 0.38 0.39 0.40 0.39
CCT (K)
1383 3057 4172 4595 5196
lexc ¼ 393 nm x
y
0.54 0.54 0.51 0.43 0.35
0.34 0.35 0.37 0.38 0.39
CCT (K)
1489 1520 1825 2987 5110
Fig. 1. XRD patterns of TBBA: x Eu/(2-x) Dy.
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Eu4d region is shown in Fig. 3 (middle). Bands observed at 135.9 and 141.7eV with energy separation 5.8 eV may be attributed to Eu3þ spin-orbit 4d3/2 and 4d5/2 doublet, respectively whereas Fig. 3 (right) presents Dy4p spectral region with two peaks due to Dy3þ spin-orbit doublet. First peak at 296.7 eV is related with 4p1/2 in Dy3þ and the second one at 301.7 eV originates from 4p3/2. Therefore, it may be concluded that in tellurite glass systems rare earth dopants exist in þ3 oxidation states. 3.4. Luminescence study
Fig. 2. FTIR spectra of TBBA: x Eu/(2-x) Dy (left) and deconvolution of main features in sample with x ¼ 0.0 (right).
related with Te-O-Al asymmetric stretching vibrations. The latter band at around 1123 cm1 is attributed to Al-O vibrations in AlO4 units. Due to similar size of Te and Al atomic radii it is possible that part of Al atoms replace Te atoms and therefore form AlO4 tetrahedrons. It suggests that Al atoms in studied glass systems act as glass formers [22] additionally improving glass network strength and thermal stability what was found in our previous work [16].
3.3. XPS In order to get further insight into valence states of elements XPS measurements were performed. Due to the fact that compositions of all samples are similar, XPS spectra for one composition: TBBA:1.0Eu/1.0Dy is presented in Fig. 3. Fig. 3 (left) presents Al2p region. Band in spectral region 72e76.5 eV is related with presence of Al3þ ions. However, due to asymmetry of the band deconvolution has been prepared and it revealed two main signals. First one (labeled as 1 in the figure) with maximum at 73.6 eV may be assigned to the presence of Al-O bonds in glass network whereas second one (labeled as 2) at 74.3 eV is due to Al-F bonds in AlF3 [23,24]. Presence of Al-O bonds in samples was previously confirmed in FTIR study. However, due to high concentration of aluminum ions in examined glass systems it is possible that at least part of Al3þ remains connected with fluoride instead of creating bonds with oxygen.
Fig. 3. XPS spectra of TBBA: x Eu/(2-x) Dy (x ¼ 1.0).
Excitation spectra of TBBA: x Eu/(2-x) Dy under 573 and 611 nm observation are shown in Fig. 4 left and right, respectively. When applying lem ¼ 573 nm all samples containing Dy3þ ions exhibit excitation in 350e440 nm spectral region. Several bands at 351, 366, 387 and 426 nm are attributed to the electron transitions in Dy3þ from 6H15/2 state to higher excitation states: 6P7/2, 4P7/2, 4F7/2 and 4G15/2, respectively [4,25] with the most intense feature at 387 nm. Additionally, for samples with x ¼ 2.0, 1.5 and 1.0 band at 393 nm due to Eu3þ 7F0/5L6 transition is confirmed with intensity decreasing with smaller value of x. For sample with x ¼ 2.0 also a few weak bands due to Eu3þ excitation are visible at 362, 382 and 414 nm. After employment of lem ¼ 611 nm bands at 362, 382, 393 and 415 nm may be distinguished which correspond to 7F0/5D7, 5 L7, 5L6 and 5D3 transitions of Eu3þ with the most intense excitation at 393 nm. For emission study in visible range two excitation wavelengths 390 and 393 nm have been chosen. Spectra for lexc ¼ 390 and 393 nm are shown in Fig. 5 (left) and (right), respectively. After excitation with 390 nm wavelength emission spectrum of x ¼ 2.0 sample shown emission bands due to Eu3þ transitions only, which can be distinguished at: 511 nm (5D2/7F3), 533 (5D1/7F1), 553 nm (5D1/7F2), 577 nm (5D0/7F0), 590 nm(5D0/7F1), 611 nm (5D0/7F2), 653 nm (5D0/7F3) and 699 nm (5D0/7F4) [26,27] whereas in spectra of x ¼ 0 sample three features at 479, 573 and 663 nm due to Dy3þ: 4F9/2/6HJ (J ¼ 15/2, 13/2, 11/2) can be observed [28,29]. However, in emission spectra of x ¼ 1.5; 1.0 and 0.5 samples recorded for lexc ¼ 390 nm bands originating from both, Eu3þ and Dy3þ transitions are observed. Intensity ratio between main features at 573 (Dy3þ) and 611 nm (Eu3þ) increases with decreasing of x value in samples due to smaller concentration of Eu3þ ions. Emission spectra of TBBA:xEu/(2-x) Dy glass series achieved using lexc ¼ 393 nm revealed similar bands originating from Eu3þ and Dy3þ transitions from excited to ground states in these ions. Moreover, for sample with x ¼ 2.0 additional low intensity features due to electronic transitions from higher excitation states (5D2, 5D1) are present in the spectrum. In general, they may be attributed to low phonon energy environment of studied glass system. With changing the x value from 2.0 to 0 intensity ratio between Dy3þ: 4F9/2 / 6H13/2 and Eu3þ: 5D0/7F2 increases but in this case features due to Eu3þ remain significantly high even for low concentration of Eu3þ. To evaluate the TBBA: x Eu/(2-x) Dy glasses' performance on color luminescent emission, CIE chromaticity coordinates (x, y) were determined from the emission spectra and plotted in Fig. 6. Calculated parameters x and y as well as color temperature are shown in Table 1. Usually, warm white emission is considered to have color temperature around 2700e3500 K. With CCT around 4000 K emission is turning to neutral white, whereas above 5000 K it changes to cool white [13,30,12]. In studied TBBA: x Eu/(2-x) Dy glass systems after lexc ¼ 390 nm irradiation cool white light emission color is acquired with Dy3þ doping only. Neutral white is observed in case of x ¼ 0.5 and 1.0 content of Eu3þ reaching warm white for x ¼ 1.5. Further increase of x value leads to achieving red light emission. Similar behavior is
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Fig. 4. Excitation spectra of TBBA: x Eu/(2-x) Dy for lem ¼ 573 nm (left) and for lem ¼ 611 nm (right).
Fig. 5. Emission spectra of TBBA: x Eu/(2-x) Dy for lexc ¼ 390 nm (left) and for lexc ¼ 393 nm (right).
observed when applying lexc ¼ 393 nm excitation. Cool white is achieved for x ¼ 0.0 however, when part of Dy3þ is replaced by Eu3þ (x ¼ 0.5) emission color turns to warm white. For x ¼ 1.0, 1.5 and 2.0
emission color changes to orange-red and red. Hereby, addition of Eu3þ and simultaneous emission from Eu3þ and Dy3þ gives possibility to achieve desirable emission color trough adjusting Eu3þ/
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Fig. 6. CIE chromaticity diagrams of TBBA: x Eu/(2-x) Dy for lexc ¼ 390 nm (left) and lexc ¼ 393 nm (right).
Dy3þ concentration and excitation wavelength in proposed glass systems.
3.5. Luminescence lifetimes In our previous work we showed that presence of Al3þ in the glass matrix leads to increase of RE3þ ions emission bands intensities. In the present study, additional measurements of luminescence lifetime decays were executed to get further insight into that matter. For this purpose Eu3þ decays in TBBA glass matrix and in the matrix without addition of AlF3 (labeled as TBB:2Eu) have been examined for lexc ¼ 393 nm with 611 nm observation. Results are shown in Fig. 7 for TBBA: 2 Eu (left) and TBB: 2Eu (right). Because of double-exponential character of luminescence lifetime curves, the average luminescence lifetimes of Eu3þ have been calculated using equation (1):
t〈avg〉 ¼ A1 t21 þ A2 t22
. ðA1 t1 þ A2 t2 Þ
(1)
where A1 and A2 are amplitudes of two exponentials (slow and fast component, respectively) and t1, t2 correspond to decay lifetimes [31]. Values of parameters A1,2; t1,2 and t for both samples as well as fitting parameters R are present in Table 2. Achieved values of t are with good agreement with other results presented in several works where measured lifetimes of 5D0 levels of Eu3þ in different tellurite glass matrices, e.g. 0.68e0.80 m s in PbF2-WO3TeO2 glasses with various Eu2O3 concentration [32]; ~1.1. ms in TeO2-ZnF2-PbO-Nb2O5-Eu2O3 glasses [33]. In general, average lifetimes of 5D0 levels depend on Eu3þ concentration and host matrices. For example, in number of reports concerning oxyfluoride glasses and glass ceramics doped with Eu3þ the average lifetime values varied from around 1.1 to 1.3 m s in TeO2-B2O3-AO-AF2 (where A ¼ Pb, Ba, Zn, Cd, Sr) glass matrix with constant amount of Eu2O3 equal to 1mol% [34]. In [19] values of Eu3þ lifetimes increase from approximately 1.0 to 1.2 m s with increasing Eu3þ amount
from 0.05 to 1.6 mol% in TeO2-K2O-P2O5-B2O3-ZnF2 glass matrix. Similar behavior have been reported in [32] and in [35]. Additionally, increase of quantum efficiency with greater amount of Eu3þ have been reported [35]. On the other hand, in [33] Eu3þ lifetime of 5 D0 remained almost unchanged for different Eu3þ concentration (0.5, 2.0 and 5.0 mol% of Eu3þ). Double-exponential character of measured Eu3þ luminescence lifetimes may be described as follows. First, lifetime component t1 may be related to slow decay mechanism as a result of direct return from 5D0 excitation to 7F2 ground level in Eu3þ whereas second short lifetime component t2 can be ascribed to fast mechanism of Eu3þ decay as a result of most likely non-radiative interactions. Similar double exponential decay characteristics have been already observed in multicomponent tellurite glasses doped with Eu3þ [19]. According to Sajna et al. [19] the non-radiative interactions are most likely caused by Eu3þ clustering or due to structural defects in glass host which absorb energy from excited ions. In other words, part of absorbed by Eu3þ energy may be transferred through nonradiative process to other Eu3þ ions agglomerated in surroundings (via cross-relaxation process) or to host matrix because energy gap between oxygen vacancies and 5D0 levels of Eu3þ is relatively low. Al3þ addition leads to RE-O-Al bonds formation instead of REO-RE thus, the spacing between RE3þ becomes significant and therefore promotes radiative transitions over non-radiative processes [36,37]. Moreover, addition of fluorides such as AlF3 into glass system results in decrease of host matrix phonon energies which promotes radiative transitions of RE3þ [38]. On the other hand, in fluoroborate glass systems [39] authors proposed that two possible sites of Eu3þ with different symmetry also might result in double exponential luminescence decay. In examined glass systems, values of A1 and t1 increase with presence of Al3þ ions. Additionally, percentage of short lifetime due to non-radiative transitions is smaller for sample TBBA: 2 Eu and value of t2 is slightly higher in this case. Hereby, addition of Al3þ leads to significant increase of effectiveness and lifetime of 5D0/ 7F2 radiative transition most likely by reducing RE3þ clustering and minimizing
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Fig. 7. Decay curves of TBBA: 2 Eu (left) and glass without Al3þ addition TBB: 2Eu (right).
Table 2 Fitting parameters of luminescence decays of TBB: 2Eu and TBBA: 2Eu. Sample
R2
A1 (%)
t1 (ms)
A2 (%)
t2 (ms)
t (ms)
TBB:2Eu TBBA:2Eu
0.9998 0.9996
44 70
0.45 0.82
56 30
0.14 0.26
0.36 0.76
phonon energy in Eu3þ environment. 4. Conclusions Tellurite glass systems containing aluminum fluoride were found to be an attractive host for optically active rare earth ions. Due to Al3þ ions presence of AlO4 tetrahedrons in glass structure have been confirmed. AlO4 units were found to act as a glass formers and they have positive influence on some glass parameters. Additionally, because of Al band asymmetry in XPS spectrum two different surroundings of aluminum ions have been proposed. Except of creating Al-O bonds, they may partially bond with fluoride ions. Luminescence excitation spectra revealed possibility of simultaneous excitation of Eu3þ and Dy3þ by UV radiation taking into account excitations spectra overlapping in this region. After irradiation by 390 and 393 nm wavelengths emission spectra containing bands due to Eu3þ and Dy3þ radiative transitions have been obtained. In spectra achieved with 390 nm dominant emission is due to Dy3þ: 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 transitions. However, with increasing of x value emission originating from Eu3þ becomes more significant. Application of 393 nm excitation leads to emission predominantly due to Eu3þ: 5D0/7F1 and 5D0/7F2 but emission due to Dy3þ is also observable for low x values. Additionally, presence of Eu3þ transitions from higher excited states such as 5D2 and 5D1 suggests low phonon energy environmental of studied tellurite glass systems. Luminescence emission color investigations confirm that after applying UV excitation with different value of x in samples composition it is possible to achieve white light generation with different color temperature from warm white to neutral (with increasing x value) and reaching cool white when only Dy3þ doping is used. Moreover, Al3þ ions have found to have significant influence on Eu3þ luminescence decay time
elongation from t ¼ 0.36 for (in glass matrix TBB:2 Eu without Al3þ) to t ¼ 0.76 m s in TBBA:2Eu glass. It may be caused by presence of aluminum ions which are known to prevent RE3þ clustering and agglomeration and/or decrease phonon energy by aluminum fluoride which has lower phonon energy than tellurite glasses. This leads to conclusion that tellurite glasses containing aluminum ions are promising matrix to optically active RE3þ ions and may be considered as a potential phosphor for white light emitting sources. Acknowledgements The authors would like to thank Dr. Ana Isabel Becerro for performed luminescence measurements and her helpful remarks concerning this topic. This research has been supported by the grant 2015/17/B/ST5/03143 financed by National Science Centre (A.S.). References [1] M. Gong, X. Liang, Y. Wang, H. Xu, L. Zhang, W. Xiang, Novel synthesis and optical characterization of phosphor-converted WLED employing Ce:YAGdoped glass, J. Alloy. Comp. 664 (2016) 125e132, https://doi.org/10.1016/ j.jallcom.2015.12.239. [2] Y. Woo Seo, S. Heum Park, S. Hyoung Chang, J. Hyun Jeong, K. Ho Kim, J.-S. Bae, Tunable single-phased white-emitting Sr 3 Y(PO 4 ) 3:Dy 3þ phosphors for near-ultraviolet white light-emitting diodes, Ceram. Int. 43 (2017) 8497e8501, https://doi.org/10.1016/j.ceramint.2017.03.205. [3] S.H. Lee, S.R. Bae, Y.G. Choi, W.J. Chung, Eu2þ/Eu3þ-doped oxyfluoride glass ceramics with LaF3 for white LED color conversion, Opt. Mater. 41 (2015) 71e74, https://doi.org/10.1016/j.optmat.2014.10.018. [4] Y. Il Jeon, L. Krishna Bharat, J.S. Yu, Synthesis and luminescence properties of Eu3þ/Dy3þ ions co-doped Ca2La8(GeO4)6O2 phosphors for white-light applications, J. Alloy. Comp. 620 (2015) 263e268, https://doi.org/10.1016/ j.jallcom.2014.09.135. [5] C.J. Zhao, J.L. Cai, R.Y. Li, S.L. Tie, X. Wan, J.Y. Shen, White light emission from Eu 3 þ/Tb 3 þ/Tm 3 þ triply-doped aluminoborate glass excited by UV light, J. Non-Cryst. Solids 358 (2012) 604e608, https://doi.org/10.1016/ j.jnoncrysol.2011.11.011. [6] M. Mungra, F. Steudel, B. Ahrens, S. Schweizer, Tm/Tb/Eu triple-doped lithium aluminoborate glass for white light generation, J. Lumin. 192 (2017) 71e76, https://doi.org/10.1016/j.jlumin.2017.06.028. [7] V. Naresh, K. Gupta, C. Parthasaradhi Reddy, B.S. Ham, Energy transfer and colour tunability in UV light induced Tm(3þ)/Tb(3þ)/Eu(3þ): ZnB glasses generating white light emission, Spectrochim. Acta. A. Mol. Biomol. Spectrosc 175 (2016) 43e50, https://doi.org/10.1016/j.saa.2016.12.023. ski, M. De˛ bowski, A. Synak, [8] B. Koscielska, M. Walas, T. Lewandowski, M. Łapin
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Tailored white light emission in Eu3þ/Dy3þ doped tellurite glass phosphors containing Al3þ ions Michalina Walas a, *, Patryk Piotrowski a, Tomasz Lewandowski a, Anna Synak b, ski a, Wojciech Sadowski a, Barbara Koscielska a Marcin Łapin a sk University of Technology, ul. Gabriela Narutowicza 11/12, 80-233, Faculty of Applied Physics and Mathematics, Department of Solid State Physics, Gdan sk, Poland Gdan b sk, ul. Wita Stwosza 57/246, 80-952, Gdan sk, Poland Institute of Experimental Physics, Faculty of Mathematics, Physics and Informatics, University of Gdan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 November 2017 Received in revised form 6 March 2018 Accepted 7 March 2018
Tellurite glass systems modified by addition of aluminum fluoride AlF3 have been successfully synthesized as host matrices for optically active rare earth ions RE3þ (RE3þ ¼ Eu3þ, Dy3þ). Samples with different Eu3þ to Dy3þ molar ratio have been studied in order to determine possibility of white light emission via UV excitation. Structural investigations confirmed amorphous character of materials whereas spectroscopic studies brought more insight into glass network's nature. FTIR results shown presence of two features related to tellurite glass matrix (in 490e935 cm1 spectral region) and another one (940-1250 cm1) due to aluminum addition. Especially, Al-O and Te-O-Al bonds of AlO4 tetrahedrons have been found. AlO4 units are considered as glass formers that improve network's strength and thermal resistivity against devitrification. Based on XPS studies of Al3þ photoelectron band the existence of Al-O and also Al-F bonds have been examined. Moreover, signals originating from Eu3þ and Dy3þ have been found confirming their valence state. Luminescence results revealed possibility of simultaneous UV excitation of Eu3þ and Dy3þ ions. Excitation with lexc ¼ 390 and 393 nm resulted in white light generation starting from warm white to neutral and cool white depending on Eu3þ concentration and used excitation wavelength. Additionally, increase of decay lifetime of Eu3þ induced by Al3þ presence have been revealed based on luminescence decay analysis. Thus, tellurite glass systems modified by AlF3 and doped with Eu3þ/Dy3þ may be considered as promising candidates for white light emitting sources. © 2018 Elsevier B.V. All rights reserved.
Keywords: Eu Dy Luminescence WLED Tellurite glass Al3þ
1. Introduction White light-emitting diodes (WLEDs) have recently attracted considerable attention [1e4] as a next generation light sources and replacement for conventional incandescent and fluorescent lamps. WLEDs are definitely more environmental friendly, energy save, reliable and smaller than traditional light sources but more importantly, they may be characterized by high luminescence efficiency, long lifetimes etc. One of the most popular ways to achieve WLEDs is to combine near UV LED with the glass or crystal phosphor emitting in blue, green and red spectral regions [5e9]. For this purpose, phosphors doped with different RE3þ ions are particularly interesting. Neutral white light can be achieved from Dy3þ singly doped glasses or ceramics [10e12] but in order to adjust the
* Corresponding author. E-mail address: [email protected] (M. Walas). https://doi.org/10.1016/j.optmat.2018.03.015 0925-3467/© 2018 Elsevier B.V. All rights reserved.
emission color temperature and obtain warm white light it is necessary to involve additional red light emitters, e.g. Eu3þ ions. By excitation in the near UV in case of Dy3þ and Eu3þ ions it is possible to obtain simultaneous emission in blue and greenish-yellow (4F9/ 6 3þ 2 / H15/2, 13/2 transitions of Dy ) and in the orange-red spectral regions (5D0/7F1,0 transitions of Eu3þ). For example, A.N. MezaRocha et al. [13] reported white light generation from Eu3þ/Dy3þ Zn(PO3)2 glass system upon 348 and 445 nm excitation. Among variety of host matrices for optically active RE3þ ions tellurite glasses seem to be one of the most promising choices. Low phonon energies (the lowest among oxide glasses), high refractive index, transparency in visible region, good mechanical and chemical resistance combined with high solubility of RE3þ ions make them excellent candidates for RE3þ doped phosphors for WLEDs applications [14,15]. In our previous work [16] we found that Al3þ addition lead to increase of RE3þ luminescence intensity in tellurite-based glasses doped with Eu3þ, Tb3þ and Tm3þ ions. It is
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due to fact that Al3þ ions are known to prevent RE3þ clustering (agglomeration) which leads to decrease of the non-radiative transitions and consequently, to increase in the emission intensity. Moreover, addition of Al3þ ions results in the formation of AlO4 tetrahedral or AlO6 octahedral units. Hereby, Al3þ ions may be considered as glass former or modifier, additionally improving glass tendency against devitrification [17,18]. In this work, we successfully synthesized tellurite-based glasses containing Al3þ ions and co-doped with Eu3þ and Dy3þ ions. By changing the Eu3þ/Dy3þ ratio and adjusting the excitation wavelengths we achieved simultaneous, color-tunable, long-lifetime luminescence originating from both, Eu3þ and Dy3þ ions in blue, green and red spectral regions. Additionally, we studied structural and spectroscopic properties of achieved samples. Results confirmed that tellurite-based glass containing Al3þ ions is suitable host for optically active rare earth ions. 2. Experimental Tellurite-based glass samples with nominal composition (in mol %) 73 TeO2 e 4 BaO e 3 Bi2O3 e 18 AlF3 e x Eu3þ/(2-x) Dy3þ (where x ¼ 2.0; 1.5; 1.0; 0.5; 0.0) were synthesized using melt-quenching technique. At first, appropriate amounts of precursor reagents: TeO2, BaCO3, Bi5OH (OH)9(NO3)4, AlF3, Eu(NO3)3 and Dy(NO3)3 were mixed in an agate mortar to achieve homogenous powders. Samples in the porcelains crucibles were placed in the furnace and decomposed at 700 C for 1 h at first and subsequently heated to 940 C for another 0.5 h. After that time samples were poured onto pre-heated stainless steel plate and subsequently pressed by another steel plate in order to achieve thin, transparent glass species. All steps of synthesis procedure were carried out in air atmosphere. The series of glasses was denoted as TBBA: x Eu/(2-x) Dy. Detailed description of samples along with their chemical composition is presented in Table 1. Structural and spectroscopic properties of samples were investigated using several techniques. In order to confirm an amorphous structure of glasses X-ray diffraction (XRD) measurements were performed on powder samples on Philips X’PERT PLUS diffractometer with Cu-Ka radiation (l ¼ 0.154 nm). Fourier transform infrared spectroscopy (FTIR) measurements were carried out on Perkin-Elmer Frontier MIR/FIR spectrometer with TGS detector in the mid-infrared range to establish the types of chemical bonds in the structural units present in the samples. The measurements were performed on pellet samples mixed with potassium bromide KBr in weight ratio (Sample: KBr) 1:100. X-ray Photoelectron Spectroscopy analysis (XPS) was carried out with X-ray photoelectron spectrometer (Omnicron NanoTechnology) with 128channel collector. Investigated samples were pre-cleaned by Ar ion beam. XPS measurements were performed in ultra-high vacuum conditions, below 1.1 108 mBar. The photoelectrons were excited by an Mg-Ka X-ray source with X-ray anode operated at 15 keV and 300 W. The absorption spectra were measured using a Perkin-Elmer Lambda 35 UVeVis spectrophotometer. Luminescence emission and excitation spectra of glass pieces were collected
in a Horiba Yvon spectrofluorometer (Fluorolog 3) on glass samples. Both spectra were corrected by the spectral response of the equipment and the intensity of the excitation source. Lifetimes were obtained from luminescence decay curves recorded in the same equipment using a pulsed lamp. The decay curves were registered at the characteristic emission wavelength of Eu3þ ions (611 nm) using an excitation wavelength of 393 nm. 3. Results 3.1. XRD X-Ray diffraction studies have been performed in order to confirm the amorphous character of prepared glass samples. Fig. 1 shows the diffraction patterns of TBBA: x Eu/(2-x) Dy glass series. All of the diffraction patterns show only broad amorphous halos. They are connected with presence of short-range order in the glass structure. These features are characteristic to glassy materials [19]. 3.2. FTIR FTIR measurements were undertaken to acquire information about the glass network structure. Infrared spectra of samples are shown in Fig. 2 (left). In case of all samples obtained FTIR spectra consist of two main features that remain unchanged with the various molar ratio of the Eu3þ/Dy3þ dopants. First broad band present in the 490e935 cm1 spectral region is characteristic to tellurite glasses and it might be deconvoluted into three features (Fig. 2 right). First one with maximum at approximately 611 cm1 can be assigned to Te-O-Te vibrations in TeO4 trigonal bipyramidal units [20]. The next component at approximately 725 cm1 is due to symmetric stretching vibrations of Te-O bonds in TeO4 or Te-O-Te linkages between two fourfold coordinated Te atoms. The last feature at around 865 cm1 is most likely caused by stretching vibrations of Te-O in TeO3 trigonal pyramids and/or TeO3þ1 polyhedral groups. Presence of different structural units may be explained as follows. It is well known that glass network modifiers ions e.g. Bi3þ are easily coordinated with Te-Oax bonds resulting in elongation of Te-Oax bonds in TeO4 units. When length of these bonds increase from 0.208 (in TeO4) to 0.280 nm then the TeO3þ1 units are formed. If the Te-Oax length exceeds 0.298 nm structural unit transforms into TeO [21]. The second broad band in the FTIR spectra of TBBA: x Eu/(2-x) Dy samples in spectral range 940e1250 cm1 is connected with the presence of Al atoms in the tellurite glass matrix. The main feature may be also deconvoluted into two signals. The one at 1031 cm1 is
Table 1 Calculated x and y parameters and CCT for lexc ¼ 390 and 393 nm excitation. x Eu/(2-x) Dy
2.0 1.5 1.0 0.5 2.0
Eu Eu/0.5 Dy Eu/1.0 Dy Eu/1.5 Dy Dy
lexc ¼ 390 nm x
y
0.57 0.42 0.38 0.36 0.34
0.35 0.38 0.39 0.40 0.39
CCT (K)
1383 3057 4172 4595 5196
lexc ¼ 393 nm x
y
0.54 0.54 0.51 0.43 0.35
0.34 0.35 0.37 0.38 0.39
CCT (K)
1489 1520 1825 2987 5110
Fig. 1. XRD patterns of TBBA: x Eu/(2-x) Dy.
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Eu4d region is shown in Fig. 3 (middle). Bands observed at 135.9 and 141.7eV with energy separation 5.8 eV may be attributed to Eu3þ spin-orbit 4d3/2 and 4d5/2 doublet, respectively whereas Fig. 3 (right) presents Dy4p spectral region with two peaks due to Dy3þ spin-orbit doublet. First peak at 296.7 eV is related with 4p1/2 in Dy3þ and the second one at 301.7 eV originates from 4p3/2. Therefore, it may be concluded that in tellurite glass systems rare earth dopants exist in þ3 oxidation states. 3.4. Luminescence study
Fig. 2. FTIR spectra of TBBA: x Eu/(2-x) Dy (left) and deconvolution of main features in sample with x ¼ 0.0 (right).
related with Te-O-Al asymmetric stretching vibrations. The latter band at around 1123 cm1 is attributed to Al-O vibrations in AlO4 units. Due to similar size of Te and Al atomic radii it is possible that part of Al atoms replace Te atoms and therefore form AlO4 tetrahedrons. It suggests that Al atoms in studied glass systems act as glass formers [22] additionally improving glass network strength and thermal stability what was found in our previous work [16].
3.3. XPS In order to get further insight into valence states of elements XPS measurements were performed. Due to the fact that compositions of all samples are similar, XPS spectra for one composition: TBBA:1.0Eu/1.0Dy is presented in Fig. 3. Fig. 3 (left) presents Al2p region. Band in spectral region 72e76.5 eV is related with presence of Al3þ ions. However, due to asymmetry of the band deconvolution has been prepared and it revealed two main signals. First one (labeled as 1 in the figure) with maximum at 73.6 eV may be assigned to the presence of Al-O bonds in glass network whereas second one (labeled as 2) at 74.3 eV is due to Al-F bonds in AlF3 [23,24]. Presence of Al-O bonds in samples was previously confirmed in FTIR study. However, due to high concentration of aluminum ions in examined glass systems it is possible that at least part of Al3þ remains connected with fluoride instead of creating bonds with oxygen.
Fig. 3. XPS spectra of TBBA: x Eu/(2-x) Dy (x ¼ 1.0).
Excitation spectra of TBBA: x Eu/(2-x) Dy under 573 and 611 nm observation are shown in Fig. 4 left and right, respectively. When applying lem ¼ 573 nm all samples containing Dy3þ ions exhibit excitation in 350e440 nm spectral region. Several bands at 351, 366, 387 and 426 nm are attributed to the electron transitions in Dy3þ from 6H15/2 state to higher excitation states: 6P7/2, 4P7/2, 4F7/2 and 4G15/2, respectively [4,25] with the most intense feature at 387 nm. Additionally, for samples with x ¼ 2.0, 1.5 and 1.0 band at 393 nm due to Eu3þ 7F0/5L6 transition is confirmed with intensity decreasing with smaller value of x. For sample with x ¼ 2.0 also a few weak bands due to Eu3þ excitation are visible at 362, 382 and 414 nm. After employment of lem ¼ 611 nm bands at 362, 382, 393 and 415 nm may be distinguished which correspond to 7F0/5D7, 5 L7, 5L6 and 5D3 transitions of Eu3þ with the most intense excitation at 393 nm. For emission study in visible range two excitation wavelengths 390 and 393 nm have been chosen. Spectra for lexc ¼ 390 and 393 nm are shown in Fig. 5 (left) and (right), respectively. After excitation with 390 nm wavelength emission spectrum of x ¼ 2.0 sample shown emission bands due to Eu3þ transitions only, which can be distinguished at: 511 nm (5D2/7F3), 533 (5D1/7F1), 553 nm (5D1/7F2), 577 nm (5D0/7F0), 590 nm(5D0/7F1), 611 nm (5D0/7F2), 653 nm (5D0/7F3) and 699 nm (5D0/7F4) [26,27] whereas in spectra of x ¼ 0 sample three features at 479, 573 and 663 nm due to Dy3þ: 4F9/2/6HJ (J ¼ 15/2, 13/2, 11/2) can be observed [28,29]. However, in emission spectra of x ¼ 1.5; 1.0 and 0.5 samples recorded for lexc ¼ 390 nm bands originating from both, Eu3þ and Dy3þ transitions are observed. Intensity ratio between main features at 573 (Dy3þ) and 611 nm (Eu3þ) increases with decreasing of x value in samples due to smaller concentration of Eu3þ ions. Emission spectra of TBBA:xEu/(2-x) Dy glass series achieved using lexc ¼ 393 nm revealed similar bands originating from Eu3þ and Dy3þ transitions from excited to ground states in these ions. Moreover, for sample with x ¼ 2.0 additional low intensity features due to electronic transitions from higher excitation states (5D2, 5D1) are present in the spectrum. In general, they may be attributed to low phonon energy environment of studied glass system. With changing the x value from 2.0 to 0 intensity ratio between Dy3þ: 4F9/2 / 6H13/2 and Eu3þ: 5D0/7F2 increases but in this case features due to Eu3þ remain significantly high even for low concentration of Eu3þ. To evaluate the TBBA: x Eu/(2-x) Dy glasses' performance on color luminescent emission, CIE chromaticity coordinates (x, y) were determined from the emission spectra and plotted in Fig. 6. Calculated parameters x and y as well as color temperature are shown in Table 1. Usually, warm white emission is considered to have color temperature around 2700e3500 K. With CCT around 4000 K emission is turning to neutral white, whereas above 5000 K it changes to cool white [13,30,12]. In studied TBBA: x Eu/(2-x) Dy glass systems after lexc ¼ 390 nm irradiation cool white light emission color is acquired with Dy3þ doping only. Neutral white is observed in case of x ¼ 0.5 and 1.0 content of Eu3þ reaching warm white for x ¼ 1.5. Further increase of x value leads to achieving red light emission. Similar behavior is
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Fig. 4. Excitation spectra of TBBA: x Eu/(2-x) Dy for lem ¼ 573 nm (left) and for lem ¼ 611 nm (right).
Fig. 5. Emission spectra of TBBA: x Eu/(2-x) Dy for lexc ¼ 390 nm (left) and for lexc ¼ 393 nm (right).
observed when applying lexc ¼ 393 nm excitation. Cool white is achieved for x ¼ 0.0 however, when part of Dy3þ is replaced by Eu3þ (x ¼ 0.5) emission color turns to warm white. For x ¼ 1.0, 1.5 and 2.0
emission color changes to orange-red and red. Hereby, addition of Eu3þ and simultaneous emission from Eu3þ and Dy3þ gives possibility to achieve desirable emission color trough adjusting Eu3þ/
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Fig. 6. CIE chromaticity diagrams of TBBA: x Eu/(2-x) Dy for lexc ¼ 390 nm (left) and lexc ¼ 393 nm (right).
Dy3þ concentration and excitation wavelength in proposed glass systems.
3.5. Luminescence lifetimes In our previous work we showed that presence of Al3þ in the glass matrix leads to increase of RE3þ ions emission bands intensities. In the present study, additional measurements of luminescence lifetime decays were executed to get further insight into that matter. For this purpose Eu3þ decays in TBBA glass matrix and in the matrix without addition of AlF3 (labeled as TBB:2Eu) have been examined for lexc ¼ 393 nm with 611 nm observation. Results are shown in Fig. 7 for TBBA: 2 Eu (left) and TBB: 2Eu (right). Because of double-exponential character of luminescence lifetime curves, the average luminescence lifetimes of Eu3þ have been calculated using equation (1):
t〈avg〉 ¼ A1 t21 þ A2 t22
. ðA1 t1 þ A2 t2 Þ
(1)
where A1 and A2 are amplitudes of two exponentials (slow and fast component, respectively) and t1, t2 correspond to decay lifetimes [31]. Values of parameters A1,2; t1,2 and t
from 0.05 to 1.6 mol% in TeO2-K2O-P2O5-B2O3-ZnF2 glass matrix. Similar behavior have been reported in [32] and in [35]. Additionally, increase of quantum efficiency with greater amount of Eu3þ have been reported [35]. On the other hand, in [33] Eu3þ lifetime of 5 D0 remained almost unchanged for different Eu3þ concentration (0.5, 2.0 and 5.0 mol% of Eu3þ). Double-exponential character of measured Eu3þ luminescence lifetimes may be described as follows. First, lifetime component t1 may be related to slow decay mechanism as a result of direct return from 5D0 excitation to 7F2 ground level in Eu3þ whereas second short lifetime component t2 can be ascribed to fast mechanism of Eu3þ decay as a result of most likely non-radiative interactions. Similar double exponential decay characteristics have been already observed in multicomponent tellurite glasses doped with Eu3þ [19]. According to Sajna et al. [19] the non-radiative interactions are most likely caused by Eu3þ clustering or due to structural defects in glass host which absorb energy from excited ions. In other words, part of absorbed by Eu3þ energy may be transferred through nonradiative process to other Eu3þ ions agglomerated in surroundings (via cross-relaxation process) or to host matrix because energy gap between oxygen vacancies and 5D0 levels of Eu3þ is relatively low. Al3þ addition leads to RE-O-Al bonds formation instead of REO-RE thus, the spacing between RE3þ becomes significant and therefore promotes radiative transitions over non-radiative processes [36,37]. Moreover, addition of fluorides such as AlF3 into glass system results in decrease of host matrix phonon energies which promotes radiative transitions of RE3þ [38]. On the other hand, in fluoroborate glass systems [39] authors proposed that two possible sites of Eu3þ with different symmetry also might result in double exponential luminescence decay. In examined glass systems, values of A1 and t1 increase with presence of Al3þ ions. Additionally, percentage of short lifetime due to non-radiative transitions is smaller for sample TBBA: 2 Eu and value of t2 is slightly higher in this case. Hereby, addition of Al3þ leads to significant increase of effectiveness and lifetime of 5D0/ 7F2 radiative transition most likely by reducing RE3þ clustering and minimizing
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M. Walas et al. / Optical Materials 79 (2018) 289e295
Fig. 7. Decay curves of TBBA: 2 Eu (left) and glass without Al3þ addition TBB: 2Eu (right).
Table 2 Fitting parameters of luminescence decays of TBB: 2Eu and TBBA: 2Eu. Sample
R2
A1 (%)
t1 (ms)
A2 (%)
t2 (ms)
t
TBB:2Eu TBBA:2Eu
0.9998 0.9996
44 70
0.45 0.82
56 30
0.14 0.26
0.36 0.76
phonon energy in Eu3þ environment. 4. Conclusions Tellurite glass systems containing aluminum fluoride were found to be an attractive host for optically active rare earth ions. Due to Al3þ ions presence of AlO4 tetrahedrons in glass structure have been confirmed. AlO4 units were found to act as a glass formers and they have positive influence on some glass parameters. Additionally, because of Al band asymmetry in XPS spectrum two different surroundings of aluminum ions have been proposed. Except of creating Al-O bonds, they may partially bond with fluoride ions. Luminescence excitation spectra revealed possibility of simultaneous excitation of Eu3þ and Dy3þ by UV radiation taking into account excitations spectra overlapping in this region. After irradiation by 390 and 393 nm wavelengths emission spectra containing bands due to Eu3þ and Dy3þ radiative transitions have been obtained. In spectra achieved with 390 nm dominant emission is due to Dy3þ: 4F9/2 / 6H15/2 and 4F9/2 / 6H13/2 transitions. However, with increasing of x value emission originating from Eu3þ becomes more significant. Application of 393 nm excitation leads to emission predominantly due to Eu3þ: 5D0/7F1 and 5D0/7F2 but emission due to Dy3þ is also observable for low x values. Additionally, presence of Eu3þ transitions from higher excited states such as 5D2 and 5D1 suggests low phonon energy environmental of studied tellurite glass systems. Luminescence emission color investigations confirm that after applying UV excitation with different value of x in samples composition it is possible to achieve white light generation with different color temperature from warm white to neutral (with increasing x value) and reaching cool white when only Dy3þ doping is used. Moreover, Al3þ ions have found to have significant influence on Eu3þ luminescence decay time
elongation from t ¼ 0.36 for (in glass matrix TBB:2 Eu without Al3þ) to t ¼ 0.76 m s in TBBA:2Eu glass. It may be caused by presence of aluminum ions which are known to prevent RE3þ clustering and agglomeration and/or decrease phonon energy by aluminum fluoride which has lower phonon energy than tellurite glasses. This leads to conclusion that tellurite glasses containing aluminum ions are promising matrix to optically active RE3þ ions and may be considered as a potential phosphor for white light emitting sources. Acknowledgements The authors would like to thank Dr. Ana Isabel Becerro for performed luminescence measurements and her helpful remarks concerning this topic. This research has been supported by the grant 2015/17/B/ST5/03143 financed by National Science Centre (A.S.). References [1] M. Gong, X. Liang, Y. Wang, H. Xu, L. Zhang, W. Xiang, Novel synthesis and optical characterization of phosphor-converted WLED employing Ce:YAGdoped glass, J. Alloy. Comp. 664 (2016) 125e132, https://doi.org/10.1016/ j.jallcom.2015.12.239. [2] Y. Woo Seo, S. Heum Park, S. Hyoung Chang, J. Hyun Jeong, K. Ho Kim, J.-S. Bae, Tunable single-phased white-emitting Sr 3 Y(PO 4 ) 3:Dy 3þ phosphors for near-ultraviolet white light-emitting diodes, Ceram. Int. 43 (2017) 8497e8501, https://doi.org/10.1016/j.ceramint.2017.03.205. [3] S.H. Lee, S.R. Bae, Y.G. Choi, W.J. Chung, Eu2þ/Eu3þ-doped oxyfluoride glass ceramics with LaF3 for white LED color conversion, Opt. Mater. 41 (2015) 71e74, https://doi.org/10.1016/j.optmat.2014.10.018. [4] Y. Il Jeon, L. Krishna Bharat, J.S. Yu, Synthesis and luminescence properties of Eu3þ/Dy3þ ions co-doped Ca2La8(GeO4)6O2 phosphors for white-light applications, J. Alloy. Comp. 620 (2015) 263e268, https://doi.org/10.1016/ j.jallcom.2014.09.135. [5] C.J. Zhao, J.L. Cai, R.Y. Li, S.L. Tie, X. Wan, J.Y. Shen, White light emission from Eu 3 þ/Tb 3 þ/Tm 3 þ triply-doped aluminoborate glass excited by UV light, J. Non-Cryst. Solids 358 (2012) 604e608, https://doi.org/10.1016/ j.jnoncrysol.2011.11.011. [6] M. Mungra, F. Steudel, B. Ahrens, S. Schweizer, Tm/Tb/Eu triple-doped lithium aluminoborate glass for white light generation, J. Lumin. 192 (2017) 71e76, https://doi.org/10.1016/j.jlumin.2017.06.028. [7] V. Naresh, K. Gupta, C. Parthasaradhi Reddy, B.S. Ham, Energy transfer and colour tunability in UV light induced Tm(3þ)/Tb(3þ)/Eu(3þ): ZnB glasses generating white light emission, Spectrochim. Acta. A. Mol. Biomol. Spectrosc 175 (2016) 43e50, https://doi.org/10.1016/j.saa.2016.12.023. ski, M. De˛ bowski, A. Synak, [8] B. Koscielska, M. Walas, T. Lewandowski, M. Łapin
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