- Email: [email protected]
Eu3+
Journal of Luminescence xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Full Length Article
White, yellow and reddish-orange light generation in lithium-aluminum-zinc phosphate glasses co-doped with Dy3þ/Tb3þ and tri-doped with Dy3þ/Tb3þ/Eu3þ E.F. Huerta a, A.N. Meza-Rocha b, R. Lozada-Morales c, A. Speghini d, e, S. Bordignon d, ~ o a, * U. Caldin a
Departamento de Física, Universidad Aut� onoma Metropolitana-Iztapalapa, P.O. Box 55-534, M�exico, D.F 09340, Mexico CONACYT-Benem�erita Universidad Aut� onoma de Puebla, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matem� aticas, Av. San Claudio y Av. 18 Sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue, 72570, Mexico c Facultad de Ciencias Físico-Matem� aticas, Benem�erita Universidad Aut� onoma de Puebla, Av. San Claudio y Av. 18 Sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue, 72570, Mexico d NRG, Department of Biotechnology, University of Verona and INSTM, RU Verona, Strada Le Grazie 15, 37314, Verona, Italy e Institute of Applied Physics “Nello Carrara”, IFAC-CNR, Via Madonna del Piano, 10, 50019, Sesto Fiorentino, FI, Italy b
A B S T R A C T
A spectroscopic investigation of Dy3þ/Tb3þ and Dy3þ/Tb3þ/Eu3þ doped lithium-aluminum-zinc phosphate glasses, focused on generation of tunable visible light, is performed through their photoluminescence spectra and decay time measurements. White, yellow and reddish-orange light emissions can be tuned with the exci tation wavelength. In the Dy3þ/Tb3þ co-doped glass emission of neutral white light of 4204 and 4517 K is achieved upon 393 and 348 nm excitations, respectively. The Dy3þ/Tb3þ/Eu3þ tri-doped glass can generate emission of warm white light of 3275 and 2866 K upon 348 and 446 nm excitations, respectively. The tri-doped glass can also generate yellow light of 2922 K with a high (577 nm) color purity of 98.1% upon 483 nm excitation, and reddish-orange light of 2235 K with a (604 nm) color purity of 96.3% upon 393 nm excitation, whose (0.641, 0.347) CIE1931chromaticity coordinates are close to those (0.67, 0.33) of the red phosphor proposed by the National Television Standard Committee. In the co-doped glass excited at 393 nm non-radiative energy transfer from Dy3þ to Tb3þ could be taking place into Dy3þ-Tb3þ clusters, whereas in the tri-doped glass non-radiative energy transfers Dy3þ→Tb3þ/Eu3þ and Dy3þ→Eu3þ through Tb3þ (upon 446 nm excitation), and Tb3þ→Eu3þ (upon 483 nm excitation), could be occurring into Dy3þ-Tb3þ-Eu3þ clusters.
1. Introduction White light-emitting diodes (WLEDs), with the advantages of long lifetime, saving energy consumption and environmentally friendly, are thought to be the most important solid state light sources for substituting the widely used incandescent and fluorescent lamps [1]. This WLED light has some drawbacks: firstly, the degradation rates of WLEDs and phosphor are not isochronous, thus the white light point in the color coordinate scheme shifts with working time; secondly, the white light from this combination route has an undesirable color balance [2]. Additionally, these WLEDs exhibit drawbacks like high correlated color temperature and poor color-rendering index (Ra < 82) due to the low emission intensity, which restricts their versatility in many applications [3–5]. There is a new non-traditional way to improve the color prop erties of the WLED devices to obtain white light with higher color index. It has been worked on single-component white light emitting phosphors
by co-doping with different sensitizer and activator ions into the single host to fabricate efficient white light sources over the conventional WLEDs [5–8]. Among the several trivalent rare-earth (RE3þ) ions, Dy3þ, Tb3þ and Eu3þ ions have been utilized due to their special luminescence properties for the development of WLEDs. In general, Dy3þ generates blue, yellow and red emissions corresponding to 4F9/2 → 6H15/2, 4 F9/2 → 6H13/2 and 4F9/2 → 6H11/2 transitions, respectively [7]. Dy3þ ions occupying non-inversion symmetry sites in the host lead to yellow hypersensitive (forced electric-dipole) emission considerably more intense than the blue insensitive emission, so that phosphors activated with Dy3þ can be very attractive as sources for yellow solid-state lasers [9]. Due to the emission color rendering index of Dy3þ singly-doped phosphors is low, it is common to co-dope these phosphors with Eu3þ ions to overcome this deficiency because their orange and red emissions corresponding to 5D0→7F0,1,2,3,4 transitions [7,10]. In the same way, Eu3þ ions occupying non-centrosymmetric sites in the host, have been
* Corresponding author. E-mail addresses: [email protected], [email protected] (U. Caldi~ no). https://doi.org/10.1016/j.jlumin.2019.116882 Received 16 August 2019; Received in revised form 25 October 2019; Accepted 6 November 2019 Available online 9 November 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: E.F. Huerta, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.116882
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
widely used to activate reddish-orange phosphors due to their intense (610 nm) 5D0→7F2 hypersensitive electric dipole emission relative to their (590 nm) 5D0→7F1 insensitive magnetic-dipole emission [11]. Further, Dy3þ ions can be activated by near UV-blue light, acting as good sensitizers and transferring a part of their energy to activator ions such as Tb3þ [10–16] and Eu3þ [5,7,10,17–23]. Intense green light emission can be generated through the 5D4→7F5 transition of Tb3þ ions, as well they can act as good sensitizers to Eu3þ activator ions transferring part of the energy from the terbium 5D4 level to the europium 5D0 level [8,10, 16,24]. The lithium-aluminum-zinc phosphate glass host is very attractive for potential WLED applications, since the combination of lithium, aluminum and zinc in phosphate glasses offers high trans parency, and thermal and chemical stability [5]. The luminescent properties based on excitation and emission spectra and decay time profiles of lithium-aluminum-zinc phosphate glasses activated with Dy3þ/Tb3þ and Dy3þ/Tb3þ/Eu3þ were investigated to obtain their application as tunable visible light emitting phosphors with the excita tion wavelength. 2. Experimental details The glasses were synthesized by mixing stoichiometric amounts of reagent grade NH4H2PO4, ZnO, Li2CO3, Al2O3, Dy2O3, Tb2O3 and Eu2O3 in a sintered alumina crucible and melting the mixture for 6 h at 1250 � C. The melts then were quenched onto a copper plate. The glasses were annealed at 350 � C for 12 h to obtain thermal and mechanical stability. Table 1 lists the molar compositions of the lith ium–aluminum–zinc phosphate glasses under investigation and they are labelled as LAZD, LAZT, LAZE, LAZDT and LAZDTE. Excitation and emission spectra and emission decay time profiles were recorded by a FLS 1000 photoluminescence spectrometer (Edin burgh Instruments). All measurements were carried out at room temperature.
Fig. 1. Emission spectra of the LAZDT glass excited at 348 and 393 nm.
chromaticity diagram (Fig. 3). Thus, the correlated color temperatures (CCT), calculated by using the McCamy’s approximate expression [25], are 4204 and 4517 K under excitations at 393 and 348 nm, respectively, which correspond to neutral white light. Neutral white light emission can also be generated from the 97.0 Zn(PO3)2-2.0 Dy2O3-1.0 Tb2O3 glass upon 392 nm excitation, with a CCT value of 4920 K [13]. The LAZDT glass excitation spectrum monitored at 541 nm, within the terbium 5D4 → 7F5 emission and wherein no Dy3þ emission exists, is shown in Fig. 4. The Tb3þ excitation spectrum exhibits dysprosium 4f 9 →
4f 9 transitions in addition to terbium 4f 8 →4f 7 5d and 4f 8 →4f 8 transi tions. The Tb3þ excitation through Dy3þ pumped at 393 nm (Fig. 1), and the presence of Dy3þ excitation bands in the excitation spectrum of Tb3þ (Fig. 4), show evidence of an energy transfer from Dy3þ to Tb3þ, simi larly to that reported for Dy3þ/Tb3þco-doped Zn(PO3)2 glasses [10,12]. This energy transfer is feasible taking into account the overlap between the dysprosium 4F9/2 → 6H15/2 emission and terbium 7F6→5D4 (excita tion) absorption transitions, as it can be appreciated from Fig. 5. Ac cording to the Dexter theory [26], the energy transfer rate is proportional to the spectral overlap integral, denoted as Ω, between the normalized line-shape functions of FDy ðEÞ sensitizer emission and FTb ðEÞ R activator absorption transitions: Ω ¼ ½FDy ðEÞFTb ðEÞ =E4 �dE, E being the average energy of the overlapping transitions. The Ω value between bands of dysprosium 4F9/2 → 6H15/2 emission and terbium 7F6→5D4 (excitation) absorption is 18.1 � 10 2 eV 5, see Fig. 5 inset. Therefore, the Dy3þ→Tb3þ energy transfer from the dysprosium 4F9/2 level to the terbium 5D4 level is favored by the resonance between the transitions (see energy level diagram in Fig. 2): h i h i 4 (i) F9=2 →6 H15=2 Dy3þ and 7 F6 →5 D4 Tb3þ
3. Results and discussion 3.1. LAZDT glass Fig. 1 shows the emission spectra of the LAZDT glass excited at 348 and 393 nm on the 400–750 nm range. The spectra show several bands centered at about 485, 573 and 664 nm, corresponding to Dy3þ ion 4F9/ 6 4 6 4 6 2 → H15/2, F9/2 → H13/2 and F9/2 → H11/2 transitions respectively, and 541 and 620 nm, corresponding to Tb3þ ion 5D4→7F5 and 5D4→7F3 transitions, respectively. The dysprosium (4F9/2 → 6H13/2)/(4F9/ 6 2 → H15/2) emission intensity ratios are 2.1 and 2.6, upon 348 and 393 nm excitations, respectively, suggesting that Dy3þ ions are mostly distributed into non-inversion symmetry sites. Tb3þ ions cannot be excited at 393 nm (25,445 cm 1), as it can be appreciated from the Tb3þ energy level diagram portrayed in Fig. 2. Nevertheless, Tb3þ emissions, in addition to Dy3þ emissions, are observed, which reveals that terbium can be excited through the excitation of dysprosium. The emitting color of the LAZDT glass was analyzed by its CIE1931 chromaticity coordinates using the emission spectra portrayed in Fig. 1. Such coordinates resulted to be (x ¼ 0.398, y ¼ 0.491), upon 393 nm excitation, and (x ¼ 0.384, y ¼ 0.511), upon 348 nm excitation, corre sponding to “a” and “b” points, respectively, on the CIE1931
The decay time of the dysprosium 4F9/2 emitting level was obtained from the decay curve of the 4F9/2 → 4H13/2 (564 nm) emission transition for the LAZD and LAZDT glasses excited at 348 nm. In both glasses such decay does not follow a single-exponential function (Fig. 6), so that an effective decay time, τ, was obtained through the known relation τ ¼ R R tIðtÞdt= IðtÞdt, where I(t) is the emission intensity at time t. In Dy3þ singly-doped glasses the non-exponential decay of the 4F9/2 level emis sion has been attributed to cross-relaxation processes from Dy3þ ions excited up to the 4F9/2 level to nearby Dy3þ ions in the 6H15/2 ground state [12]. The dysprosium 4F9/2 level effective decay time slightly in creases from 0.92 to 0.99 ms for the LAZD and LAZDT glasses,
Table 1 Molar compositions of the synthesized glasses (mol %). Glass
Li2O
Al2O3
ZnO
P2O5
Dy2O3
Tb2O3
Eu2O3
LAZD LAZT LAZE LAZDT LAZDTE
5.0 5.0 5.0 5.0 5.0
5.0 5.0 5.0 5.0 5.0
39.5 39.0 39.0 38.5 37.5
50.0 50.0 50.0 50.0 50.0
0.5 0 0 0.5 0.5
0 1.0 0 1.0 1.0
0 0 1.0 0 1.0
2
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 2. Dy3þ and Tb3þ energy level diagram illustrating the possible energy transfer pathway among the Dy3þ and Tb3þ ions.
respectively. This decay time increment in presence of Tb3þ has been attributed to changes in the phonon energies of the host, which leads to a decrease in the non-radiative decay rate of the dysprosium 4F9/2 emit ting level [13,27]. Such non-radiative decay rate decrease of the dysprosium 4F9/2 level could overcome the increase in the Dy3þ to Tb3þ non-radiative energy transfer rate. The energy transfer critical distance RC is the donor-acceptor sepa ration such that the rate of energy transfer between donors and accep tors becomes equal to the rate of donor decay in absence of acceptors. Such distance can be estimated assuming an electric dipole-dipole interaction mechanism from the following relation [28]: RC ¼
� 4 4 �1=6 3ħ c QTb Ω 4πn4
coefficient QTb was estimated through the relationship derived by Blasse, QTb ¼ 4.8 � 10 20 eV m2 � fd [29]. In the 460–500 nm region of dysprosium 4F9/2 → 6H15/2 emission, the fd electric dipole oscillator strength of Tb3þ is low (3 � 10 7 [30]) due to its partially allowed electric dipole transitions, so that QTb � 1:44 � 10 26 eV m2. Using a refractive index value n ¼ 1.56 (measured previously [8]) and the values obtained for QTb and Ω in Eq. (1), a RC value of 7 Å is found. The Dy3þ Tb3þ interaction distance for a random ion distribution [31], consid ering ρDy þ ρTb � 1:3 � 1020 ions/cm3, is around 24 Å, which is more than three times the value of RC . Therefore, it can be concluded that the Dy3þ to Tb3þ energy transfer could be taking place in Dy3þ-Tb3þ clusters.
(1)
3.2. LAZDTE glass
where n is the refractive index of the glass host and QTb is the oscillator strength of the acceptor absorption transition. The integrated absorption
Fig. 7 shows the emission spectra upon excitations at 348, 393, 446 and 483 nm of the LAZDTE glass on the 440–750 nm range. Upon ex citations at 348 and 446 nm, the spectra consist of several bands centered at about 484, 574 and 667 nm, corresponding to Dy3þ ion 4F9/ 6 3þ ion 2 → H15/2,13/2,11/2 transitions; 542 nm, associated with the Tb 5 7 D4→ F5 transition, and 592, 611, 653 and 701 nm, corresponding to Eu3þ ion 5D0→7F1,2,3,4 transitions. Upon 348 nm (28,736 cm 1) excita tion, Dy3þ and Tb3þ are excited through their 6H15/2→(4M15/2,6P7/2) and 7F6→(5L9,5D2,5G5) transitions, respectively, and Eu3þ does not achieve to be excited, as it can be visualized from the Dy3þ, Tb3þ and Eu3þ energy level diagram portrayed in Fig. 8. However, europium 5 D0→7F1,2,3,4 emissions are observed, which denotes that Eu3þ ions are sensitized by Dy3þ and/or Tb3þ ions. Upon 446 nm (22,422 cm 1) excitation, Dy3þ is excited through its 6H15/2 → 4I15/2 transition, but both Tb3þ and Eu3þ do not achieve to be excited, see Fig. 8. Neverthe less, emissions 5D0→7F1,2,3,4 of Eu3þ and 5D4→7F5 of Tb3þ are observed, in addition to emissions 4F9/2 → 6H15/2,13/2,11/2 of Dy3þ, which reveals that Tb3þ:5D4 and Eu3þ:5D0 energy levels can be populated through excitation of Dy3þ. Upon direct Eu3þ (7F0→5L7 transition) excitation at 393 nm, the spectrum shows predominantly the typical 5D0→7F0,1,2,3,4 emissions of Eu3þ. The (5D0→7F2)/(5D0→7F1) emission intensity ratio (asymmetry ratio) is 3.6, suggesting that the Eu3þ ions are predomi nantly distributed into non-inversion symmetry sites. The very intense 5 D0→7F2 transition relative to the 5D0→7F1 one gives rise to bright
Fig. 3. Chromaticity coordinates in CIE1931 diagram of the global emission observed in the LAZDT glass upon excitations at a) 348 nm and b) 393 nm. 3
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 4. Excitation spectrum of the LAZDT glass monitored at 541 nm.
reddish-orange emission observed with the naked eye. Exciting directly the Tb3þ ions at 483 nm (7F6→5D4 transition), the spectrum displays the typical Tb3þ emission band centered at 542 nm. Dy3þ and Eu3þ do not achieve to be excited at this wavelength (483 nm), as it can be noted from the energy level diagram of Dy3þ and Eu3þ portrayed in Fig. 8. Nonetheless, the 5D0→7F0,1,2,3,4 emissions of Eu3þ are observed, which reveals that Eu3þ ions can be sensitized by Tb3þ ions. The tonality of the LAZDTE glass excited at 348 and 446 nm is warm
white of 3275 and 2866 K, respectively, see CIE1931 chromaticity dia gram portrayed in Fig. 9. Such warm white tonality can be shifted to yellow (2922 K) or reddish-orange (2235 K) one upon 483 and 393 nm excitations, respectively (Fig. 9). Both yellow and reddish-orange to nalities have associated high color purities of 98.1 and 96.3%, respec tively, with 577 and 604 nm dominant wavelengths. The (0.641, 0.347) CIE1931chromaticity coordinates of the reddish-orange emission are close to those (0.67, 0.33) of the red phosphor proposed by the National Television Standard Committee. Table 2 lists the CIE1931 chromaticity coordinates, correlated color temperatures and color purities of the different tonalities exhibited by the tri-doped glass excited at 348, 393,
Fig. 5. Overlap region between terbium 7F6→5D4 absorption and dysprosium 4 F9/2 → 6H15/2 emission bands. The inset shows the Tb3þ absorption and Dy3þ emission normalized line-shape functions. The Tb3þ absorption spectrum was taken from the LAZT excitation spectrum monitored at 541 nm.
Fig. 6. Decay time profiles of the dysprosium 4F9/2 → 6H13/2 emission in the LAZD and LAZDT glasses (λem ¼ 564 nm and λex ¼ 348 nm). 4
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 9. Chromaticity coordinates in CIE1931 diagram of the global emission observed in the LAZDTE glass upon excitations at a) 348 nm, b) 393 nm, c) 446 nm and d) 483 nm.
Fig. 7. Emission spectra of the LAZDTE glass excited at 348, 393, 446 and 483 nm.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 CP ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% ðxd xi Þ2 þ ðyd yi Þ2
446 and 483 nm. Warm white tonality can also be obtained in the 97.5 Zn(PO3)2-1.0 Tb2O3-1.0 Eu2O3-0.5 Dy2O3 glass excited at 322 nm, with a CCT value of 3429 K [10]. Color purity values were estimated from the weighted average be tween the phosphor emission color coordinates (x, y) relative to the CIE1931 Standard Source I illuminant coordinates (xi ¼ 0.310, yi ¼ 0.316) and the dominant wavelength coordinates (xd ¼ 0.139, yd ¼ 0.036) relative to the coordinates (xi, yi) according to the rela tionship [32]:
(2)
The dominant wavelength being the monochromatic one such that its coordinates are same color as those of the phosphor, and therefore they are on the same straight line connecting the (x, y) and (xi, yi) points. Excitation at 348 nm is appropriate as it can be attained with AlGaN based LED chips, whereas excitations at 393, 446 and 483 nm can be attained with InGaN based LED chips, so that LAZDTE glasses could be very attractive to fulfill the requirements for solid state lighting tech nology as reddish-orange, yellow and warm white light sources.
Fig. 8. Dy3þ, Tb3þ and Eu3þ energy level diagram illustrating the possible energy transfer pathways among the Dy3þ, Tb3þ and Eu3þ ions. 5
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
The excitation spectrum of the LAZDTE glass monitored at 541 nm (18,484 cm 1), inside the terbium 5D4→7F5 emission and wherein Dy3þ and Eu3þ do not emit, is shown in Fig. 10(a). The excitation spectrum of the LAZT glass monitored at 541 nm is also displayed for the sake of comparison. The excitation spectrum of the LAZDTE glass shows, in addition to terbium 4f 8 →4f 7 5d and 4f 8 →4f 8 transitions, dysprosium 6 H15/2 → 4K13/2, 6H15/2 → 4K15/2, 6H15/2 → 4K17/2þ4M19/2,21/2þ4I13/ 4 6 4 6 4 6 4 2þ F7/2, H15/2 → G11/2, H15/2 → I15/2 and H15/2 → F9/2 transitions. 3þ The presence of Dy excitation bands in the excitation spectrum of Tb3þ shows a clear evidence of an energy transfer from Dy3þ to Tb3þ. As in the LAZDT glass the Dy3þ to Tb3þ energy transfer from the dyspro sium 4F9/2 level to the terbium 5D4 level could take place by the reso nance between the transitions (i), see Fig. 8. Fig. 10(b) shows the excitation spectrum of the LAZDTE glass monitored at 701 nm (14,265 cm 1), inside the europium 5D0→7F4 emission, and wherein Tb3þ and Dy3þ do not emit. The excitation spectrum of the LAZE glass monitored at 701 nm is also portrayed for the sake of comparison. The excitation spectrum of the LAZDTE glass shows,
Table 2 CIE1931 chromaticity coordinates, correlated color temperature (CCT), color purity (CP) and emission color of the LAZDT glass excited at 348 and 393 nm, and LAZDTE glass excited at 348, 393, 446 and 483 nm.
in addition to Eu3þ 4f 6 →4f 6 transitions, also Dy3þ (6H15/2 → 4M15/ 6 6 4 3þ 7 ( F6→5D4) transitions, which 2þ P7/2 and H15/2 → I15/2) and Tb shows clear evidence of Dy3þ to Eu3þ and Tb3þ to Eu3þ energy transfers. Taking into account that the dysprosium 4F9/2 → 6H13/2 emission band overlaps with europium 7F1→5D0 and 7F0→5D0 (excitation) absorption bands (Fig. 11), it can be inferred that the Dy3þ to Eu3þ energy transfer from the dysprosium 4F9/2 level to the europium 5D0 level takes place by the resonance between the transitions (Fig. 8) [5]: h i h i 4 F9=2 →6 H13=2 Dy3þ and 7 F0;1 →5 D0 Eu3þ (ii)
h h h i D4 →7 F4 Tb3þ and7 F0;1 →5 D0 Eu3þ
(iv)
η¼1
CIE (x, y)
CCT (K)
CP (%)
λd (nm)
Emission color
LAZDT
348
4517
72.4
-
LAZDT
393
4204
70.7
-
LAZDTE
348
3275
76.4
-
neutral white neutral white warm white
LAZDTE
393
2235
96.3
604
LAZDTE
446
2866
78.1
-
reddishorange warm white
LAZDTE
483
(0.384, 0.511) (0.398, 0.491) (0.447, 0.465) (0.641, 0.347) (0.469, 0.450) (0.490, 0.500)
2922
98.1
577
yellow
τTb
The Ω value between terbium 5D4→7F5 emission and europium 7 F1→5D1 (excitation) absorption bands is 9.0 � 10 2 eV 5, and that between terbium 5D4→7F4 emission and europium 7F0,1 → 5D0 (excita tion) absorption bands is 56.0 � 10 2 eV 5, see Fig. 12 inset. Considering the possibility that Dy3þ transfers part of its energy to Tb3þ and subsequently Tb3þ transfers part of the energy received to Eu3þ due to the resonant transitions (i) and (iii)/(iv), respectively, then it is feasible that the Dy3þ to Eu3þ energy transfer could also take place via Tb3þ. The decay time profile of the dysprosium 4F9/2 → 4H13/2 emission at 564 nm upon 348 nm excitation was analyzed to obtain additional evi dence on the Dy3þ to Tb3þ and Dy3þ to Eu3þ energy transfer processes. The decay profile does not follow a single-exponential function (Fig. 13), so that an effective decay time of 0.83 ms was obtained by using the above mentioned formula. This decay time is faster than that (0.92 ms) obtained in the LAZD glass. Such faster decay time of Dy3þ in presence of Tb3þ and Eu3þ could be attributed to non-radiative energy transfers Dy3þ→Tb3þ and/or Dy3þ→Eu3þ. The efficiency of Dy3þ to Tb3þ/Eu3þ energy transfer was estimated using the dysprosium 4F9/2 level decay time data in presence (τd ¼ 0:83 ms) and absence (τod ¼ 0:92 ms) of acceptor ions (Tb3þ and Eu3þ) through the expression [10]:
τd τ0d
λex (nm)
so that the energy transfer efficiency is 0.10. In the LAZT and LAZDTE glasses decay time measurements of the terbium 5D4 level were recorded upon 377 nm excitation and monitoring the terbium 5D4→7F5 emission at 541 nm. In the LAZT glass the 5D4 level decay time profile is single exponential, with a lifetime value of 2.99 ms, whereas in the LAZDTE glass the 5D4 level decay time profile is not single exponential as a consequence of energy transfer from Tb3þ to Eu3þ, see Fig. 14. A faster average decay time of 2.45 ms is obtained for the Tb3þ emission in the LAZDTE glass. Such faster decay time reveals a non-radiative energy transfer from Tb3þ to Eu3þ. In consequence, the efficiency of energy transfer from Tb3þ to Eu3þ results to be 0.18, as estimated from Eq. (3). The Tb3þ to Dy3þ energy transfer can be considered inefficient, since the terbium 5D4 level is at lower energy (~550 cm 1) than the dyspro sium 4F9/2 level. Hence, it can be assumed that Tb3þ ions excited to the 5 D4 level can transfer some energy only to Eu3þ ions [10]. Considering that such energy transfer could be much faster than the energy diffusion among Tb3þ ions, the dominant electric multipolar interaction mecha nism taking place in the Tb3þ to Eu3þ energy transfer was determined following the Inokuti-Hirayama model [33]. Thus, the time decay pro file of the terbium 5D4 level emission was fit through the relation [33]: � � �3=m � t t γ IðtÞ ¼ I0 exp (4) m o o
The Ω value between the dysprosium 4F9/2 → 6H13/2 emission and europium 7F0,1 → 5D0 (excitation) absorption bands is 22.0 � 10 2 eV 5, see Fig. 11 inset. From the overlap between terbium 5D4→7F5 and 5D4→7F4 emission bands and europium 7F1→5D1 and 7F0,1 → 5D0 (excitation) absorption bands, respectively (see Fig. 12), it can be inferred that the energy transfer from the terbium 5D4 level to the europium 5D1 and 5D0 levels takes place by the resonant transitions (Fig. 8) [8,9]: � � � 5 D4 →7 F5 Tb3þ and 7 F1 →5 D1 Eu3þ �; (iii) 5
Glass
τTb
where I0 is the initial intensity at t ¼ 0, τoTb (2.99 ms) is the decay time of the terbium 5D4 level in absence of Eu3þ, γ m is a measure of the direct Tb3þ to Eu3þ energy transfer, and m ¼ 6, 8 and 10 for electric dipoledipole, dipole-quadrupole and quadrupole-quadrupole couplings, respectively. The R-squared values of the fits through Eq. (4) resulted to be 0.9986 (m ¼ 6), 0.9980 (m ¼ 8) and 0.9968 (m ¼ 10). Therefore, best fits are attained with m ¼ 6 and m ¼ 8, for which γ6 ¼ 0.41 and γ 8 ¼ 0.44, as it can be seen from Fig. 14. Thus, an electric dipole-dipole or dipole-quadrupole interaction mechanism might be the dominant mechanism in the Tb3þ to Eu3þ energy transfer like in the Tb3þ and Eu3þ co-doped glass (5.0 Li2O-5.0 Al2O3-39.0 ZnO-50.0 P2O5-1.0 Tb2O3-1.0 Eu2O3), in which R-squared values of the fits resulted to be 0.9992 and 0.9989 for electric dipole-dipole and dipole-quadrupole interactions, respectively [8]. The RC energy transfer critical interaction distance between terbium and europium for an electric dipole-dipole or dipole-quadrupole inter action can be obtained from its relation with the γm parameter with m ¼ 6 and m ¼ 8, respectively: 11=3 0 3γ B � m� C RC ¼ @ A 4πΓ 1 m3 ρEu
(3) 6
(5)
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 10. Excitation spectra of (a) LAZT and LAZDTE glasses monitoring at 541 nm and (b) LAZE and LAZDTE glasses monitoring at 701 nm.
7
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 11. Overlap region between europium 7F0,1 → 5D0 (excitation) absorption and dysprosium 4F9/2 → 6H13/2 emission bands. The inset shows the Eu3þ (excitation) absorption and Dy3þ emission normalized line-shape functions. The Eu3þ absorption spectrum was taken from the LAZE glass excitation spectrum monitored at 701 nm.
Fig. 13. Decay time profiles of the dysprosium 4F9/2 → 6H13/2 emission (λem ¼ 564 nm and λex ¼ 348 nm) in the LAZD and LAZDTE glasses.
4. Conclusions Spectroscopic analysis of Dy3þ/Tb3þ and Dy3þ/Tb3þ/Eu3þ doped lithium-aluminum-zinc phosphate glasses, focused on generation of visible light tunable with the excitation, is carried out from emission and excitation spectra and decay time measurements. The Dy3þ/Tb3þ codoped glass emits neutral white light of 4204 and 4517 K under exci tations at 393 and 348 nm, respectively. Such neutral white emission from the co-doped glass upon Dy3þ excitation at 393 nm was achieved with the Tb3þ sensitization by Dy3þ through a non-radiative energy transfer taking place into Dy3þ-Tb3þ clusters. The Dy3þ/Tb3þ/Eu3þ tridoped glass emits warm white light of 3275 and 2866 K upon 348 and 446 excitations, respectively, yellow light of 2922 K upon 483 nm excitation, and reddish-orange light of 2235 K upon 393 nm excitation. The yellow and reddish-orange emission color purities are 98.1 and 96.3%, respectively, with 577 and 604 nm dominant wavelengths. The CIE1931chromaticity coordinates of the reddish-orange emission
where ρEu is the Eu3þ ions/cm3 concentration (8.6�1019 ions/cm3). RC resulted to be 8.6 and 9.5 Å assuming electric dipole-dipole and dipolequadrupole interactions, respectively. The Tb3þ-Eu3þ interaction dis tance for a random ion distribution [31], considering ρTb þ ρEu � 1:7� 1020 ions/cm3, is around 22 Å, which is twice more than the value of RC . Therefore, it can be concluded that in the LAZDTE glass the energy transfer from Tb3þ to Eu3þ might be occurring into Tb3þ-Eu3þ clusters. This, along with the fact that in the LAZDT glass the Dy3þ to Tb3þ energy transfer could take place into Dy3þ-Tb3þ clusters, it is reasonable to conclude that in the tri-doped glass the Dy3þ to Eu3þ energy transfer through Tb3þ could be occurring into Dy3þ-Tb3þ-Eu3þ clusters.
Fig. 12. Overlap region between europium 7F1→5D1 and 7F0,1 → 5D0 (excita tion) absorption bands and terbium 5D4→7F5 and 5D4→7F4 emission bands, respectively. The inset shows the Eu3þ (excitation) absorption and Dy3þ emis sion normalized line-shape functions. The Eu3þ absorption spectrum was taken from the LAZE excitation spectrum monitored at 701 nm.
Fig. 14. Decay time profiles of the terbium 5D4 → 7F5 emission at 541 nm in the LAZT and LAZDTE glasses excited at 377 nm, which are fit to a singleexponential function (LAZT glass) and Eq. (4) for electric dipole-dipole (dd), dipole-quadrupole (dq) and quadrupole-quadrupole (qq) interactions (LAZ DTE glass). 8
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
(0.641, 0.347) are close to those (0.67, 0.33) of the red phosphor pro posed by the National Television Standard Committee. The emissions of warm white light and yellow light are attained through non-radiative energy transfers from Dy3þ to Tb3þ and Eu3þ (upon 446 nm excita tion) and from Tb3þ to Eu3þ (upon 483 nm excitation), respectively. Tb3þ to Eu3þ energy transfer might be dominated by an electric dipoledipole or dipole-quadrupole interaction as suggested from the InokutiHirayama model. The Dy3þ to Eu3þ energy transfer could also be occurring through Tb3þ, in addition to the direct Dy3þ to Eu3þ energy transfer, into Dy3þ-Tb3þ-Eu3þ clusters. Excitation at 348 nm is appro priate as it can be attained with AlGaN based LED chips, whereas exci tations at 393, 446 and 483 nm can be achieved with InGaN based LED chips. Thus, LAZDT and LAZDTE glasses excited by these LEDs could be appropriate materials for the development of phosphors with neutral white light emission (LAZDT glass), and warm white, yellow and reddish-orange light emissions (LAZDTE glass).
[10] A.N. Meza-Rocha, R. Lozada-Morales, A. Speghini, M. Bettinelli, U. Caldi~ no, White light generation in Tb3þ/Eu3þ/Dy3þ triply-doped Zn(PO3)2 glass, Opt. Mater. 51 (2016) 128. [11] G. Blasse, On the Eu3þ fluorescence of mixed metal oxides. IV. The photoluminescent efficiency of Eu3þ-activated oxides, J. Chem. Phys. 45 (1966) 2356. [12] U. Caldi~ no, G. Mu~ noz, H.I. Camarillo, A. Speghini, M. Bettinelli, Down-shifting by energy transfer in Tb3þ/Dy3þ co-doped zinc phosphate glasses, J. Lumin. 161 (2015) 142. [13] J. Ju� arez-Batalla, A.N. Meza-Rocha, G. Mu~ noz H, Ulises Caldi~ no, Green to white tunable light emitting phosphors: Dy3þ/Tb3þ in zinc phosphate glasses, Opt. Mater. 64 (2017) 33. [14] G. Lakshminarayana, Kawa M. Kaky, S.O. Baki, A. Lira, U. Caldi~ no, I.V. Kityk, M. A. Mahdi, Optical absorption, luminescence, and energy transfer processes studies for Dy3þ/Tb3þ-codoped borate glasses for solid-state lighting applications, Opt. Mater. 72 (2017) 380. [15] J. Pisarska, A. Kos, E. Pietrasik, W.A. Pisarski, Energy transfer from Dy3þ to Tb3þ in lead borate glass, Mater. Lett. 129 (2014) 146. [16] J. Pisarska, A. Kos, W.A. Pisarski, Spectroscopy and energy transfer in lead borate glasses doubly doped with Dy3þ-Tb3þ and Tb3þ-Eu3þ ions, Spectrochim. Acta A – Mol. Biomol. Spectrosc. 129 (2014) 649. [17] D. Rajesh, K. Brahmachary, Y.C. Ratnakaram, N. Kiran, A.P. Baker, G.G. Wang, Energy transfer based emission analysis of Dy3þ/Eu3þ co-doped ZANP glasses for white LED applications, J. Alloy. Comp. 646 (2015) 1096. [18] M. Vijayakumar, K. Marimuthu, Tailoring the luminescence of Eu3þ co-doped Dy3þ incorporated aluminofluoro-borophosphate glasses for white light applications, J. Lumin. 178 (2016) 414. [19] A.N. Meza-Rocha, A. Speghini, M. Bettinelli, U. Caldi~ no, White light generation through Zn(PO3)2 glass activated with Eu3þ and Dy3þ, J. Lumin. 176 (2016) 235. [20] David A. Rodríguez-Carvajal, A.N. Meza-Rocha, U. Caldi~ no, R. Lozada-Morales, � E. Alvarez, MaE. Zayas, Reddish-orange, neutral and warm white emissions in Eu3 þ 3þ 3þ 3þ , Dy and Dy /Eu doped CdO-GeO2-TeO2 glasses, Solid State Sci. 61 (2016) 70. [21] V. Kumar, A. Pandey, O.M. Ntwaeaborwa, V. Dutta, H.C. Swart, Structural and luminescence properties of Eu3þ/Dy3þ embedded sodium silicate glass for multicolour emission, J. Alloy. Comp. 708 (2017) 922. [22] A.N. Meza-Rocha, I. Camarillo, R. Lozada-Morales, U. Caldi~ no, Reddish-orange and neutral/wam white light emitting phosphors: Eu3þ, Dy3þ and Dy3þ/Eu3þ in potassium-zinc phosphate glasses, J. Lumin. 183 (2017) 341. [23] U. Caldi~ no, A. Lira, A.N. Meza-Rocha, I. Camarillo, R. Lozada-Morales, Development of sodium-zinc phosphate glasses doped with Dy3þ, Eu3þ and Dy3þ/ 3þ Eu for yellow laser medium, reddish-orange and white phosphor applications, J. Lumin. 194 (2018) 231. _ [24] J. Pisarska, A. Kos, M. Sołtys, L. Zur, W.A. Pisarski, Energy transfer from Tb3 þ to Eu3 þ in lead borate glass, J. Non-Cryst. Solids 388 (2014) 1. [25] C.S. McCamy, Correlated color temperature as an explicit function of chromaticity coordinates, Color Res. Appl. 17 (1992) 142. [26] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836. [27] S. Surendra Babu, P. Babu, C.K. Jayasankar, Th Tr€ oster, W. Sievers, G. Wortmann, Optical properties of Dy3þ-doped phosphate and fluorophosphate glasses, Opt. Mater. 31 (2009) 624. [28] U. Caldi~ no, A. Speghini, M. Bettinelli, Optical spectroscopy of zinc metaphosphate glasses activated by Ce3þ and Tb3þ ions, J. Phys. Condens. Matter 18 (2006) 3499. [29] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24 (1969) 131. [30] J.M.P.J. Verstegen, J.L. Sommerdijk, J.G. Verriet, Cerium and terbium luminescence in LaMgAl11O19, J. Lumin. 6 (1973) 425. [31] U. Caldi~ no G, Energy transfer in CaF2 doped with Ce3þ, Eu2þ and Mn2þ ions, J. Phys. Condens. Matter 15 (2003) 7127. [32] Z. Lou, J. Hao, Cathodoluminescence of rare-earth-doped zinc aluminate films, Thin Solid Films 450 (2004) 334. [33] M. Inokuti, F. Hirayama, Influence of energy transfer by the exchange mechanism on donor luminescence, J. Chem. Phys. 43 (1965) 1978.
Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgments ~ o thank CONACyT, M�exico, for the A.N. Meza-Rocha and U. Caldin CB-2016-01 project (grant 286218). A. Speghini and S. Bordignon thank University of Verona, Verona, Italy, for financial support in the frame work of “Ricerca di Base 2015” project. References [1] R. Vijayakumar, H. Guo, X.Y. Huang, Energy transfer and color-tunable luminescence properties of Dy3þ and Eu3þ, co-doped Na3Sc2(PO4)3 phosphors for near-UV LED-based warm white LEDs, Dyes Pigments 156 (2018) 8. [2] W.J. Yang, L.Y. Luo, T.M. Chen, N.S. Wang, Luminescence and energy transfer of Eu- and Mn-coactivated CaAl2Si2O8 as a potential phosphor for white-light UVLED, Chem. Mater. 17 (2005) 3883. [3] T. Ogi, A.B.D. Nandiyanto, W.N. Wang, F. Iskandar, K. Okuyama, Direct synthesis of spherical YAG:Ce phosphor from precursor solution containing polymer and urea, Chem. Eng. J. 210 (2012) 461. [4] Y. Liu, G. Liu, J. Wang, X. Dong, W. Yu, Single-component and warm-whiteemitting phosphor NaGd(WO4)2:Tm3þ, Dy3þ, Eu3þ: synthesis, luminescence, energy transfer, and tunable color, Inorg. Chem. 53 (2014) 11457. [5] A.N. Meza-Rocha, A. Speghini, J. Franchini, R. Lozada-Morales, U. Caldi~ no, Multicolor emission in lithium-aluminum-zinc phosphate glasses activated with Dy3þ, Eu3þ and Dy3þ/Eu3þ, J. Mater. Sci. Mater. Electron. 28 (2017) 10564. [6] J. Li, Y. Wang, Tunable emission with efficient energy transfer in Na2SrSi2O6:Ce3þ, Tb3þ phosphor for near-UV LED, Opt. Mater. 88 (2019) 648. [7] Y. Zheng-Wei, S. Xiao-Yu, W. Zhen-Qing, Luminescent properties of Dy3þ and/or Eu3þ doped Mg2Al4Si5O18 phosphors and energy transfer between Dy3þ/Eu3þ ion pairs, J. Lumin. 197 (2018) 164. [8] H.I. Francisco-Rodriguez, A. Lira, O. Soriano-Romero, A.N. Meza-Rocha, S. Bordignon, A. Speghini, R. Lozada-Morales, U. Caldi~ no, Lithium-aluminum-zinc phosphate glasses activated with Tb3þ and Tb3þ/Eu3þ for green laser medium, reddish-orange and white phosphor applications, Opt. Mater. 79 (2018) 358. [9] A. Lira, A. Speghini, E. Camarillo, M. Bettinelli, U. Caldi~ no, Spectroscopic evaluation of Zn(PO3)2:Dy3þ glass as an active medium for solid state yellow laser, Opt. Mater. 38 (2014) 188.
9
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: http://www.elsevier.com/locate/jlumin
Full Length Article
White, yellow and reddish-orange light generation in lithium-aluminum-zinc phosphate glasses co-doped with Dy3þ/Tb3þ and tri-doped with Dy3þ/Tb3þ/Eu3þ E.F. Huerta a, A.N. Meza-Rocha b, R. Lozada-Morales c, A. Speghini d, e, S. Bordignon d, ~ o a, * U. Caldin a
Departamento de Física, Universidad Aut� onoma Metropolitana-Iztapalapa, P.O. Box 55-534, M�exico, D.F 09340, Mexico CONACYT-Benem�erita Universidad Aut� onoma de Puebla, Postgrado en Física Aplicada, Facultad de Ciencias Físico-Matem� aticas, Av. San Claudio y Av. 18 Sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue, 72570, Mexico c Facultad de Ciencias Físico-Matem� aticas, Benem�erita Universidad Aut� onoma de Puebla, Av. San Claudio y Av. 18 Sur, Col. San Manuel Ciudad Universitaria, Puebla, Pue, 72570, Mexico d NRG, Department of Biotechnology, University of Verona and INSTM, RU Verona, Strada Le Grazie 15, 37314, Verona, Italy e Institute of Applied Physics “Nello Carrara”, IFAC-CNR, Via Madonna del Piano, 10, 50019, Sesto Fiorentino, FI, Italy b
A B S T R A C T
A spectroscopic investigation of Dy3þ/Tb3þ and Dy3þ/Tb3þ/Eu3þ doped lithium-aluminum-zinc phosphate glasses, focused on generation of tunable visible light, is performed through their photoluminescence spectra and decay time measurements. White, yellow and reddish-orange light emissions can be tuned with the exci tation wavelength. In the Dy3þ/Tb3þ co-doped glass emission of neutral white light of 4204 and 4517 K is achieved upon 393 and 348 nm excitations, respectively. The Dy3þ/Tb3þ/Eu3þ tri-doped glass can generate emission of warm white light of 3275 and 2866 K upon 348 and 446 nm excitations, respectively. The tri-doped glass can also generate yellow light of 2922 K with a high (577 nm) color purity of 98.1% upon 483 nm excitation, and reddish-orange light of 2235 K with a (604 nm) color purity of 96.3% upon 393 nm excitation, whose (0.641, 0.347) CIE1931chromaticity coordinates are close to those (0.67, 0.33) of the red phosphor proposed by the National Television Standard Committee. In the co-doped glass excited at 393 nm non-radiative energy transfer from Dy3þ to Tb3þ could be taking place into Dy3þ-Tb3þ clusters, whereas in the tri-doped glass non-radiative energy transfers Dy3þ→Tb3þ/Eu3þ and Dy3þ→Eu3þ through Tb3þ (upon 446 nm excitation), and Tb3þ→Eu3þ (upon 483 nm excitation), could be occurring into Dy3þ-Tb3þ-Eu3þ clusters.
1. Introduction White light-emitting diodes (WLEDs), with the advantages of long lifetime, saving energy consumption and environmentally friendly, are thought to be the most important solid state light sources for substituting the widely used incandescent and fluorescent lamps [1]. This WLED light has some drawbacks: firstly, the degradation rates of WLEDs and phosphor are not isochronous, thus the white light point in the color coordinate scheme shifts with working time; secondly, the white light from this combination route has an undesirable color balance [2]. Additionally, these WLEDs exhibit drawbacks like high correlated color temperature and poor color-rendering index (Ra < 82) due to the low emission intensity, which restricts their versatility in many applications [3–5]. There is a new non-traditional way to improve the color prop erties of the WLED devices to obtain white light with higher color index. It has been worked on single-component white light emitting phosphors
by co-doping with different sensitizer and activator ions into the single host to fabricate efficient white light sources over the conventional WLEDs [5–8]. Among the several trivalent rare-earth (RE3þ) ions, Dy3þ, Tb3þ and Eu3þ ions have been utilized due to their special luminescence properties for the development of WLEDs. In general, Dy3þ generates blue, yellow and red emissions corresponding to 4F9/2 → 6H15/2, 4 F9/2 → 6H13/2 and 4F9/2 → 6H11/2 transitions, respectively [7]. Dy3þ ions occupying non-inversion symmetry sites in the host lead to yellow hypersensitive (forced electric-dipole) emission considerably more intense than the blue insensitive emission, so that phosphors activated with Dy3þ can be very attractive as sources for yellow solid-state lasers [9]. Due to the emission color rendering index of Dy3þ singly-doped phosphors is low, it is common to co-dope these phosphors with Eu3þ ions to overcome this deficiency because their orange and red emissions corresponding to 5D0→7F0,1,2,3,4 transitions [7,10]. In the same way, Eu3þ ions occupying non-centrosymmetric sites in the host, have been
* Corresponding author. E-mail addresses: [email protected], [email protected] (U. Caldi~ no). https://doi.org/10.1016/j.jlumin.2019.116882 Received 16 August 2019; Received in revised form 25 October 2019; Accepted 6 November 2019 Available online 9 November 2019 0022-2313/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: E.F. Huerta, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2019.116882
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
widely used to activate reddish-orange phosphors due to their intense (610 nm) 5D0→7F2 hypersensitive electric dipole emission relative to their (590 nm) 5D0→7F1 insensitive magnetic-dipole emission [11]. Further, Dy3þ ions can be activated by near UV-blue light, acting as good sensitizers and transferring a part of their energy to activator ions such as Tb3þ [10–16] and Eu3þ [5,7,10,17–23]. Intense green light emission can be generated through the 5D4→7F5 transition of Tb3þ ions, as well they can act as good sensitizers to Eu3þ activator ions transferring part of the energy from the terbium 5D4 level to the europium 5D0 level [8,10, 16,24]. The lithium-aluminum-zinc phosphate glass host is very attractive for potential WLED applications, since the combination of lithium, aluminum and zinc in phosphate glasses offers high trans parency, and thermal and chemical stability [5]. The luminescent properties based on excitation and emission spectra and decay time profiles of lithium-aluminum-zinc phosphate glasses activated with Dy3þ/Tb3þ and Dy3þ/Tb3þ/Eu3þ were investigated to obtain their application as tunable visible light emitting phosphors with the excita tion wavelength. 2. Experimental details The glasses were synthesized by mixing stoichiometric amounts of reagent grade NH4H2PO4, ZnO, Li2CO3, Al2O3, Dy2O3, Tb2O3 and Eu2O3 in a sintered alumina crucible and melting the mixture for 6 h at 1250 � C. The melts then were quenched onto a copper plate. The glasses were annealed at 350 � C for 12 h to obtain thermal and mechanical stability. Table 1 lists the molar compositions of the lith ium–aluminum–zinc phosphate glasses under investigation and they are labelled as LAZD, LAZT, LAZE, LAZDT and LAZDTE. Excitation and emission spectra and emission decay time profiles were recorded by a FLS 1000 photoluminescence spectrometer (Edin burgh Instruments). All measurements were carried out at room temperature.
Fig. 1. Emission spectra of the LAZDT glass excited at 348 and 393 nm.
chromaticity diagram (Fig. 3). Thus, the correlated color temperatures (CCT), calculated by using the McCamy’s approximate expression [25], are 4204 and 4517 K under excitations at 393 and 348 nm, respectively, which correspond to neutral white light. Neutral white light emission can also be generated from the 97.0 Zn(PO3)2-2.0 Dy2O3-1.0 Tb2O3 glass upon 392 nm excitation, with a CCT value of 4920 K [13]. The LAZDT glass excitation spectrum monitored at 541 nm, within the terbium 5D4 → 7F5 emission and wherein no Dy3þ emission exists, is shown in Fig. 4. The Tb3þ excitation spectrum exhibits dysprosium 4f 9 →
4f 9 transitions in addition to terbium 4f 8 →4f 7 5d and 4f 8 →4f 8 transi tions. The Tb3þ excitation through Dy3þ pumped at 393 nm (Fig. 1), and the presence of Dy3þ excitation bands in the excitation spectrum of Tb3þ (Fig. 4), show evidence of an energy transfer from Dy3þ to Tb3þ, simi larly to that reported for Dy3þ/Tb3þco-doped Zn(PO3)2 glasses [10,12]. This energy transfer is feasible taking into account the overlap between the dysprosium 4F9/2 → 6H15/2 emission and terbium 7F6→5D4 (excita tion) absorption transitions, as it can be appreciated from Fig. 5. Ac cording to the Dexter theory [26], the energy transfer rate is proportional to the spectral overlap integral, denoted as Ω, between the normalized line-shape functions of FDy ðEÞ sensitizer emission and FTb ðEÞ R activator absorption transitions: Ω ¼ ½FDy ðEÞFTb ðEÞ =E4 �dE, E being the average energy of the overlapping transitions. The Ω value between bands of dysprosium 4F9/2 → 6H15/2 emission and terbium 7F6→5D4 (excitation) absorption is 18.1 � 10 2 eV 5, see Fig. 5 inset. Therefore, the Dy3þ→Tb3þ energy transfer from the dysprosium 4F9/2 level to the terbium 5D4 level is favored by the resonance between the transitions (see energy level diagram in Fig. 2): h i h i 4 (i) F9=2 →6 H15=2 Dy3þ and 7 F6 →5 D4 Tb3þ
3. Results and discussion 3.1. LAZDT glass Fig. 1 shows the emission spectra of the LAZDT glass excited at 348 and 393 nm on the 400–750 nm range. The spectra show several bands centered at about 485, 573 and 664 nm, corresponding to Dy3þ ion 4F9/ 6 4 6 4 6 2 → H15/2, F9/2 → H13/2 and F9/2 → H11/2 transitions respectively, and 541 and 620 nm, corresponding to Tb3þ ion 5D4→7F5 and 5D4→7F3 transitions, respectively. The dysprosium (4F9/2 → 6H13/2)/(4F9/ 6 2 → H15/2) emission intensity ratios are 2.1 and 2.6, upon 348 and 393 nm excitations, respectively, suggesting that Dy3þ ions are mostly distributed into non-inversion symmetry sites. Tb3þ ions cannot be excited at 393 nm (25,445 cm 1), as it can be appreciated from the Tb3þ energy level diagram portrayed in Fig. 2. Nevertheless, Tb3þ emissions, in addition to Dy3þ emissions, are observed, which reveals that terbium can be excited through the excitation of dysprosium. The emitting color of the LAZDT glass was analyzed by its CIE1931 chromaticity coordinates using the emission spectra portrayed in Fig. 1. Such coordinates resulted to be (x ¼ 0.398, y ¼ 0.491), upon 393 nm excitation, and (x ¼ 0.384, y ¼ 0.511), upon 348 nm excitation, corre sponding to “a” and “b” points, respectively, on the CIE1931
The decay time of the dysprosium 4F9/2 emitting level was obtained from the decay curve of the 4F9/2 → 4H13/2 (564 nm) emission transition for the LAZD and LAZDT glasses excited at 348 nm. In both glasses such decay does not follow a single-exponential function (Fig. 6), so that an effective decay time, τ, was obtained through the known relation τ ¼ R R tIðtÞdt= IðtÞdt, where I(t) is the emission intensity at time t. In Dy3þ singly-doped glasses the non-exponential decay of the 4F9/2 level emis sion has been attributed to cross-relaxation processes from Dy3þ ions excited up to the 4F9/2 level to nearby Dy3þ ions in the 6H15/2 ground state [12]. The dysprosium 4F9/2 level effective decay time slightly in creases from 0.92 to 0.99 ms for the LAZD and LAZDT glasses,
Table 1 Molar compositions of the synthesized glasses (mol %). Glass
Li2O
Al2O3
ZnO
P2O5
Dy2O3
Tb2O3
Eu2O3
LAZD LAZT LAZE LAZDT LAZDTE
5.0 5.0 5.0 5.0 5.0
5.0 5.0 5.0 5.0 5.0
39.5 39.0 39.0 38.5 37.5
50.0 50.0 50.0 50.0 50.0
0.5 0 0 0.5 0.5
0 1.0 0 1.0 1.0
0 0 1.0 0 1.0
2
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 2. Dy3þ and Tb3þ energy level diagram illustrating the possible energy transfer pathway among the Dy3þ and Tb3þ ions.
respectively. This decay time increment in presence of Tb3þ has been attributed to changes in the phonon energies of the host, which leads to a decrease in the non-radiative decay rate of the dysprosium 4F9/2 emit ting level [13,27]. Such non-radiative decay rate decrease of the dysprosium 4F9/2 level could overcome the increase in the Dy3þ to Tb3þ non-radiative energy transfer rate. The energy transfer critical distance RC is the donor-acceptor sepa ration such that the rate of energy transfer between donors and accep tors becomes equal to the rate of donor decay in absence of acceptors. Such distance can be estimated assuming an electric dipole-dipole interaction mechanism from the following relation [28]: RC ¼
� 4 4 �1=6 3ħ c QTb Ω 4πn4
coefficient QTb was estimated through the relationship derived by Blasse, QTb ¼ 4.8 � 10 20 eV m2 � fd [29]. In the 460–500 nm region of dysprosium 4F9/2 → 6H15/2 emission, the fd electric dipole oscillator strength of Tb3þ is low (3 � 10 7 [30]) due to its partially allowed electric dipole transitions, so that QTb � 1:44 � 10 26 eV m2. Using a refractive index value n ¼ 1.56 (measured previously [8]) and the values obtained for QTb and Ω in Eq. (1), a RC value of 7 Å is found. The Dy3þ Tb3þ interaction distance for a random ion distribution [31], consid ering ρDy þ ρTb � 1:3 � 1020 ions/cm3, is around 24 Å, which is more than three times the value of RC . Therefore, it can be concluded that the Dy3þ to Tb3þ energy transfer could be taking place in Dy3þ-Tb3þ clusters.
(1)
3.2. LAZDTE glass
where n is the refractive index of the glass host and QTb is the oscillator strength of the acceptor absorption transition. The integrated absorption
Fig. 7 shows the emission spectra upon excitations at 348, 393, 446 and 483 nm of the LAZDTE glass on the 440–750 nm range. Upon ex citations at 348 and 446 nm, the spectra consist of several bands centered at about 484, 574 and 667 nm, corresponding to Dy3þ ion 4F9/ 6 3þ ion 2 → H15/2,13/2,11/2 transitions; 542 nm, associated with the Tb 5 7 D4→ F5 transition, and 592, 611, 653 and 701 nm, corresponding to Eu3þ ion 5D0→7F1,2,3,4 transitions. Upon 348 nm (28,736 cm 1) excita tion, Dy3þ and Tb3þ are excited through their 6H15/2→(4M15/2,6P7/2) and 7F6→(5L9,5D2,5G5) transitions, respectively, and Eu3þ does not achieve to be excited, as it can be visualized from the Dy3þ, Tb3þ and Eu3þ energy level diagram portrayed in Fig. 8. However, europium 5 D0→7F1,2,3,4 emissions are observed, which denotes that Eu3þ ions are sensitized by Dy3þ and/or Tb3þ ions. Upon 446 nm (22,422 cm 1) excitation, Dy3þ is excited through its 6H15/2 → 4I15/2 transition, but both Tb3þ and Eu3þ do not achieve to be excited, see Fig. 8. Neverthe less, emissions 5D0→7F1,2,3,4 of Eu3þ and 5D4→7F5 of Tb3þ are observed, in addition to emissions 4F9/2 → 6H15/2,13/2,11/2 of Dy3þ, which reveals that Tb3þ:5D4 and Eu3þ:5D0 energy levels can be populated through excitation of Dy3þ. Upon direct Eu3þ (7F0→5L7 transition) excitation at 393 nm, the spectrum shows predominantly the typical 5D0→7F0,1,2,3,4 emissions of Eu3þ. The (5D0→7F2)/(5D0→7F1) emission intensity ratio (asymmetry ratio) is 3.6, suggesting that the Eu3þ ions are predomi nantly distributed into non-inversion symmetry sites. The very intense 5 D0→7F2 transition relative to the 5D0→7F1 one gives rise to bright
Fig. 3. Chromaticity coordinates in CIE1931 diagram of the global emission observed in the LAZDT glass upon excitations at a) 348 nm and b) 393 nm. 3
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 4. Excitation spectrum of the LAZDT glass monitored at 541 nm.
reddish-orange emission observed with the naked eye. Exciting directly the Tb3þ ions at 483 nm (7F6→5D4 transition), the spectrum displays the typical Tb3þ emission band centered at 542 nm. Dy3þ and Eu3þ do not achieve to be excited at this wavelength (483 nm), as it can be noted from the energy level diagram of Dy3þ and Eu3þ portrayed in Fig. 8. Nonetheless, the 5D0→7F0,1,2,3,4 emissions of Eu3þ are observed, which reveals that Eu3þ ions can be sensitized by Tb3þ ions. The tonality of the LAZDTE glass excited at 348 and 446 nm is warm
white of 3275 and 2866 K, respectively, see CIE1931 chromaticity dia gram portrayed in Fig. 9. Such warm white tonality can be shifted to yellow (2922 K) or reddish-orange (2235 K) one upon 483 and 393 nm excitations, respectively (Fig. 9). Both yellow and reddish-orange to nalities have associated high color purities of 98.1 and 96.3%, respec tively, with 577 and 604 nm dominant wavelengths. The (0.641, 0.347) CIE1931chromaticity coordinates of the reddish-orange emission are close to those (0.67, 0.33) of the red phosphor proposed by the National Television Standard Committee. Table 2 lists the CIE1931 chromaticity coordinates, correlated color temperatures and color purities of the different tonalities exhibited by the tri-doped glass excited at 348, 393,
Fig. 5. Overlap region between terbium 7F6→5D4 absorption and dysprosium 4 F9/2 → 6H15/2 emission bands. The inset shows the Tb3þ absorption and Dy3þ emission normalized line-shape functions. The Tb3þ absorption spectrum was taken from the LAZT excitation spectrum monitored at 541 nm.
Fig. 6. Decay time profiles of the dysprosium 4F9/2 → 6H13/2 emission in the LAZD and LAZDT glasses (λem ¼ 564 nm and λex ¼ 348 nm). 4
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 9. Chromaticity coordinates in CIE1931 diagram of the global emission observed in the LAZDTE glass upon excitations at a) 348 nm, b) 393 nm, c) 446 nm and d) 483 nm.
Fig. 7. Emission spectra of the LAZDTE glass excited at 348, 393, 446 and 483 nm.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 CP ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% ðxd xi Þ2 þ ðyd yi Þ2
446 and 483 nm. Warm white tonality can also be obtained in the 97.5 Zn(PO3)2-1.0 Tb2O3-1.0 Eu2O3-0.5 Dy2O3 glass excited at 322 nm, with a CCT value of 3429 K [10]. Color purity values were estimated from the weighted average be tween the phosphor emission color coordinates (x, y) relative to the CIE1931 Standard Source I illuminant coordinates (xi ¼ 0.310, yi ¼ 0.316) and the dominant wavelength coordinates (xd ¼ 0.139, yd ¼ 0.036) relative to the coordinates (xi, yi) according to the rela tionship [32]:
(2)
The dominant wavelength being the monochromatic one such that its coordinates are same color as those of the phosphor, and therefore they are on the same straight line connecting the (x, y) and (xi, yi) points. Excitation at 348 nm is appropriate as it can be attained with AlGaN based LED chips, whereas excitations at 393, 446 and 483 nm can be attained with InGaN based LED chips, so that LAZDTE glasses could be very attractive to fulfill the requirements for solid state lighting tech nology as reddish-orange, yellow and warm white light sources.
Fig. 8. Dy3þ, Tb3þ and Eu3þ energy level diagram illustrating the possible energy transfer pathways among the Dy3þ, Tb3þ and Eu3þ ions. 5
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
The excitation spectrum of the LAZDTE glass monitored at 541 nm (18,484 cm 1), inside the terbium 5D4→7F5 emission and wherein Dy3þ and Eu3þ do not emit, is shown in Fig. 10(a). The excitation spectrum of the LAZT glass monitored at 541 nm is also displayed for the sake of comparison. The excitation spectrum of the LAZDTE glass shows, in addition to terbium 4f 8 →4f 7 5d and 4f 8 →4f 8 transitions, dysprosium 6 H15/2 → 4K13/2, 6H15/2 → 4K15/2, 6H15/2 → 4K17/2þ4M19/2,21/2þ4I13/ 4 6 4 6 4 6 4 2þ F7/2, H15/2 → G11/2, H15/2 → I15/2 and H15/2 → F9/2 transitions. 3þ The presence of Dy excitation bands in the excitation spectrum of Tb3þ shows a clear evidence of an energy transfer from Dy3þ to Tb3þ. As in the LAZDT glass the Dy3þ to Tb3þ energy transfer from the dyspro sium 4F9/2 level to the terbium 5D4 level could take place by the reso nance between the transitions (i), see Fig. 8. Fig. 10(b) shows the excitation spectrum of the LAZDTE glass monitored at 701 nm (14,265 cm 1), inside the europium 5D0→7F4 emission, and wherein Tb3þ and Dy3þ do not emit. The excitation spectrum of the LAZE glass monitored at 701 nm is also portrayed for the sake of comparison. The excitation spectrum of the LAZDTE glass shows,
Table 2 CIE1931 chromaticity coordinates, correlated color temperature (CCT), color purity (CP) and emission color of the LAZDT glass excited at 348 and 393 nm, and LAZDTE glass excited at 348, 393, 446 and 483 nm.
in addition to Eu3þ 4f 6 →4f 6 transitions, also Dy3þ (6H15/2 → 4M15/ 6 6 4 3þ 7 ( F6→5D4) transitions, which 2þ P7/2 and H15/2 → I15/2) and Tb shows clear evidence of Dy3þ to Eu3þ and Tb3þ to Eu3þ energy transfers. Taking into account that the dysprosium 4F9/2 → 6H13/2 emission band overlaps with europium 7F1→5D0 and 7F0→5D0 (excitation) absorption bands (Fig. 11), it can be inferred that the Dy3þ to Eu3þ energy transfer from the dysprosium 4F9/2 level to the europium 5D0 level takes place by the resonance between the transitions (Fig. 8) [5]: h i h i 4 F9=2 →6 H13=2 Dy3þ and 7 F0;1 →5 D0 Eu3þ (ii)
h h h i D4 →7 F4 Tb3þ and7 F0;1 →5 D0 Eu3þ
(iv)
η¼1
CIE (x, y)
CCT (K)
CP (%)
λd (nm)
Emission color
LAZDT
348
4517
72.4
-
LAZDT
393
4204
70.7
-
LAZDTE
348
3275
76.4
-
neutral white neutral white warm white
LAZDTE
393
2235
96.3
604
LAZDTE
446
2866
78.1
-
reddishorange warm white
LAZDTE
483
(0.384, 0.511) (0.398, 0.491) (0.447, 0.465) (0.641, 0.347) (0.469, 0.450) (0.490, 0.500)
2922
98.1
577
yellow
τTb
The Ω value between terbium 5D4→7F5 emission and europium 7 F1→5D1 (excitation) absorption bands is 9.0 � 10 2 eV 5, and that between terbium 5D4→7F4 emission and europium 7F0,1 → 5D0 (excita tion) absorption bands is 56.0 � 10 2 eV 5, see Fig. 12 inset. Considering the possibility that Dy3þ transfers part of its energy to Tb3þ and subsequently Tb3þ transfers part of the energy received to Eu3þ due to the resonant transitions (i) and (iii)/(iv), respectively, then it is feasible that the Dy3þ to Eu3þ energy transfer could also take place via Tb3þ. The decay time profile of the dysprosium 4F9/2 → 4H13/2 emission at 564 nm upon 348 nm excitation was analyzed to obtain additional evi dence on the Dy3þ to Tb3þ and Dy3þ to Eu3þ energy transfer processes. The decay profile does not follow a single-exponential function (Fig. 13), so that an effective decay time of 0.83 ms was obtained by using the above mentioned formula. This decay time is faster than that (0.92 ms) obtained in the LAZD glass. Such faster decay time of Dy3þ in presence of Tb3þ and Eu3þ could be attributed to non-radiative energy transfers Dy3þ→Tb3þ and/or Dy3þ→Eu3þ. The efficiency of Dy3þ to Tb3þ/Eu3þ energy transfer was estimated using the dysprosium 4F9/2 level decay time data in presence (τd ¼ 0:83 ms) and absence (τod ¼ 0:92 ms) of acceptor ions (Tb3þ and Eu3þ) through the expression [10]:
τd τ0d
λex (nm)
so that the energy transfer efficiency is 0.10. In the LAZT and LAZDTE glasses decay time measurements of the terbium 5D4 level were recorded upon 377 nm excitation and monitoring the terbium 5D4→7F5 emission at 541 nm. In the LAZT glass the 5D4 level decay time profile is single exponential, with a lifetime value of 2.99 ms, whereas in the LAZDTE glass the 5D4 level decay time profile is not single exponential as a consequence of energy transfer from Tb3þ to Eu3þ, see Fig. 14. A faster average decay time of 2.45 ms is obtained for the Tb3þ emission in the LAZDTE glass. Such faster decay time reveals a non-radiative energy transfer from Tb3þ to Eu3þ. In consequence, the efficiency of energy transfer from Tb3þ to Eu3þ results to be 0.18, as estimated from Eq. (3). The Tb3þ to Dy3þ energy transfer can be considered inefficient, since the terbium 5D4 level is at lower energy (~550 cm 1) than the dyspro sium 4F9/2 level. Hence, it can be assumed that Tb3þ ions excited to the 5 D4 level can transfer some energy only to Eu3þ ions [10]. Considering that such energy transfer could be much faster than the energy diffusion among Tb3þ ions, the dominant electric multipolar interaction mecha nism taking place in the Tb3þ to Eu3þ energy transfer was determined following the Inokuti-Hirayama model [33]. Thus, the time decay pro file of the terbium 5D4 level emission was fit through the relation [33]: � � �3=m � t t γ IðtÞ ¼ I0 exp (4) m o o
The Ω value between the dysprosium 4F9/2 → 6H13/2 emission and europium 7F0,1 → 5D0 (excitation) absorption bands is 22.0 � 10 2 eV 5, see Fig. 11 inset. From the overlap between terbium 5D4→7F5 and 5D4→7F4 emission bands and europium 7F1→5D1 and 7F0,1 → 5D0 (excitation) absorption bands, respectively (see Fig. 12), it can be inferred that the energy transfer from the terbium 5D4 level to the europium 5D1 and 5D0 levels takes place by the resonant transitions (Fig. 8) [8,9]: � � � 5 D4 →7 F5 Tb3þ and 7 F1 →5 D1 Eu3þ �; (iii) 5
Glass
τTb
where I0 is the initial intensity at t ¼ 0, τoTb (2.99 ms) is the decay time of the terbium 5D4 level in absence of Eu3þ, γ m is a measure of the direct Tb3þ to Eu3þ energy transfer, and m ¼ 6, 8 and 10 for electric dipoledipole, dipole-quadrupole and quadrupole-quadrupole couplings, respectively. The R-squared values of the fits through Eq. (4) resulted to be 0.9986 (m ¼ 6), 0.9980 (m ¼ 8) and 0.9968 (m ¼ 10). Therefore, best fits are attained with m ¼ 6 and m ¼ 8, for which γ6 ¼ 0.41 and γ 8 ¼ 0.44, as it can be seen from Fig. 14. Thus, an electric dipole-dipole or dipole-quadrupole interaction mechanism might be the dominant mechanism in the Tb3þ to Eu3þ energy transfer like in the Tb3þ and Eu3þ co-doped glass (5.0 Li2O-5.0 Al2O3-39.0 ZnO-50.0 P2O5-1.0 Tb2O3-1.0 Eu2O3), in which R-squared values of the fits resulted to be 0.9992 and 0.9989 for electric dipole-dipole and dipole-quadrupole interactions, respectively [8]. The RC energy transfer critical interaction distance between terbium and europium for an electric dipole-dipole or dipole-quadrupole inter action can be obtained from its relation with the γm parameter with m ¼ 6 and m ¼ 8, respectively: 11=3 0 3γ B � m� C RC ¼ @ A 4πΓ 1 m3 ρEu
(3) 6
(5)
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 10. Excitation spectra of (a) LAZT and LAZDTE glasses monitoring at 541 nm and (b) LAZE and LAZDTE glasses monitoring at 701 nm.
7
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
Fig. 11. Overlap region between europium 7F0,1 → 5D0 (excitation) absorption and dysprosium 4F9/2 → 6H13/2 emission bands. The inset shows the Eu3þ (excitation) absorption and Dy3þ emission normalized line-shape functions. The Eu3þ absorption spectrum was taken from the LAZE glass excitation spectrum monitored at 701 nm.
Fig. 13. Decay time profiles of the dysprosium 4F9/2 → 6H13/2 emission (λem ¼ 564 nm and λex ¼ 348 nm) in the LAZD and LAZDTE glasses.
4. Conclusions Spectroscopic analysis of Dy3þ/Tb3þ and Dy3þ/Tb3þ/Eu3þ doped lithium-aluminum-zinc phosphate glasses, focused on generation of visible light tunable with the excitation, is carried out from emission and excitation spectra and decay time measurements. The Dy3þ/Tb3þ codoped glass emits neutral white light of 4204 and 4517 K under exci tations at 393 and 348 nm, respectively. Such neutral white emission from the co-doped glass upon Dy3þ excitation at 393 nm was achieved with the Tb3þ sensitization by Dy3þ through a non-radiative energy transfer taking place into Dy3þ-Tb3þ clusters. The Dy3þ/Tb3þ/Eu3þ tridoped glass emits warm white light of 3275 and 2866 K upon 348 and 446 excitations, respectively, yellow light of 2922 K upon 483 nm excitation, and reddish-orange light of 2235 K upon 393 nm excitation. The yellow and reddish-orange emission color purities are 98.1 and 96.3%, respectively, with 577 and 604 nm dominant wavelengths. The CIE1931chromaticity coordinates of the reddish-orange emission
where ρEu is the Eu3þ ions/cm3 concentration (8.6�1019 ions/cm3). RC resulted to be 8.6 and 9.5 Å assuming electric dipole-dipole and dipolequadrupole interactions, respectively. The Tb3þ-Eu3þ interaction dis tance for a random ion distribution [31], considering ρTb þ ρEu � 1:7� 1020 ions/cm3, is around 22 Å, which is twice more than the value of RC . Therefore, it can be concluded that in the LAZDTE glass the energy transfer from Tb3þ to Eu3þ might be occurring into Tb3þ-Eu3þ clusters. This, along with the fact that in the LAZDT glass the Dy3þ to Tb3þ energy transfer could take place into Dy3þ-Tb3þ clusters, it is reasonable to conclude that in the tri-doped glass the Dy3þ to Eu3þ energy transfer through Tb3þ could be occurring into Dy3þ-Tb3þ-Eu3þ clusters.
Fig. 12. Overlap region between europium 7F1→5D1 and 7F0,1 → 5D0 (excita tion) absorption bands and terbium 5D4→7F5 and 5D4→7F4 emission bands, respectively. The inset shows the Eu3þ (excitation) absorption and Dy3þ emis sion normalized line-shape functions. The Eu3þ absorption spectrum was taken from the LAZE excitation spectrum monitored at 701 nm.
Fig. 14. Decay time profiles of the terbium 5D4 → 7F5 emission at 541 nm in the LAZT and LAZDTE glasses excited at 377 nm, which are fit to a singleexponential function (LAZT glass) and Eq. (4) for electric dipole-dipole (dd), dipole-quadrupole (dq) and quadrupole-quadrupole (qq) interactions (LAZ DTE glass). 8
E.F. Huerta et al.
Journal of Luminescence xxx (xxxx) xxx
(0.641, 0.347) are close to those (0.67, 0.33) of the red phosphor pro posed by the National Television Standard Committee. The emissions of warm white light and yellow light are attained through non-radiative energy transfers from Dy3þ to Tb3þ and Eu3þ (upon 446 nm excita tion) and from Tb3þ to Eu3þ (upon 483 nm excitation), respectively. Tb3þ to Eu3þ energy transfer might be dominated by an electric dipoledipole or dipole-quadrupole interaction as suggested from the InokutiHirayama model. The Dy3þ to Eu3þ energy transfer could also be occurring through Tb3þ, in addition to the direct Dy3þ to Eu3þ energy transfer, into Dy3þ-Tb3þ-Eu3þ clusters. Excitation at 348 nm is appro priate as it can be attained with AlGaN based LED chips, whereas exci tations at 393, 446 and 483 nm can be achieved with InGaN based LED chips. Thus, LAZDT and LAZDTE glasses excited by these LEDs could be appropriate materials for the development of phosphors with neutral white light emission (LAZDT glass), and warm white, yellow and reddish-orange light emissions (LAZDTE glass).
[10] A.N. Meza-Rocha, R. Lozada-Morales, A. Speghini, M. Bettinelli, U. Caldi~ no, White light generation in Tb3þ/Eu3þ/Dy3þ triply-doped Zn(PO3)2 glass, Opt. Mater. 51 (2016) 128. [11] G. Blasse, On the Eu3þ fluorescence of mixed metal oxides. IV. The photoluminescent efficiency of Eu3þ-activated oxides, J. Chem. Phys. 45 (1966) 2356. [12] U. Caldi~ no, G. Mu~ noz, H.I. Camarillo, A. Speghini, M. Bettinelli, Down-shifting by energy transfer in Tb3þ/Dy3þ co-doped zinc phosphate glasses, J. Lumin. 161 (2015) 142. [13] J. Ju� arez-Batalla, A.N. Meza-Rocha, G. Mu~ noz H, Ulises Caldi~ no, Green to white tunable light emitting phosphors: Dy3þ/Tb3þ in zinc phosphate glasses, Opt. Mater. 64 (2017) 33. [14] G. Lakshminarayana, Kawa M. Kaky, S.O. Baki, A. Lira, U. Caldi~ no, I.V. Kityk, M. A. Mahdi, Optical absorption, luminescence, and energy transfer processes studies for Dy3þ/Tb3þ-codoped borate glasses for solid-state lighting applications, Opt. Mater. 72 (2017) 380. [15] J. Pisarska, A. Kos, E. Pietrasik, W.A. Pisarski, Energy transfer from Dy3þ to Tb3þ in lead borate glass, Mater. Lett. 129 (2014) 146. [16] J. Pisarska, A. Kos, W.A. Pisarski, Spectroscopy and energy transfer in lead borate glasses doubly doped with Dy3þ-Tb3þ and Tb3þ-Eu3þ ions, Spectrochim. Acta A – Mol. Biomol. Spectrosc. 129 (2014) 649. [17] D. Rajesh, K. Brahmachary, Y.C. Ratnakaram, N. Kiran, A.P. Baker, G.G. Wang, Energy transfer based emission analysis of Dy3þ/Eu3þ co-doped ZANP glasses for white LED applications, J. Alloy. Comp. 646 (2015) 1096. [18] M. Vijayakumar, K. Marimuthu, Tailoring the luminescence of Eu3þ co-doped Dy3þ incorporated aluminofluoro-borophosphate glasses for white light applications, J. Lumin. 178 (2016) 414. [19] A.N. Meza-Rocha, A. Speghini, M. Bettinelli, U. Caldi~ no, White light generation through Zn(PO3)2 glass activated with Eu3þ and Dy3þ, J. Lumin. 176 (2016) 235. [20] David A. Rodríguez-Carvajal, A.N. Meza-Rocha, U. Caldi~ no, R. Lozada-Morales, � E. Alvarez, MaE. Zayas, Reddish-orange, neutral and warm white emissions in Eu3 þ 3þ 3þ 3þ , Dy and Dy /Eu doped CdO-GeO2-TeO2 glasses, Solid State Sci. 61 (2016) 70. [21] V. Kumar, A. Pandey, O.M. Ntwaeaborwa, V. Dutta, H.C. Swart, Structural and luminescence properties of Eu3þ/Dy3þ embedded sodium silicate glass for multicolour emission, J. Alloy. Comp. 708 (2017) 922. [22] A.N. Meza-Rocha, I. Camarillo, R. Lozada-Morales, U. Caldi~ no, Reddish-orange and neutral/wam white light emitting phosphors: Eu3þ, Dy3þ and Dy3þ/Eu3þ in potassium-zinc phosphate glasses, J. Lumin. 183 (2017) 341. [23] U. Caldi~ no, A. Lira, A.N. Meza-Rocha, I. Camarillo, R. Lozada-Morales, Development of sodium-zinc phosphate glasses doped with Dy3þ, Eu3þ and Dy3þ/ 3þ Eu for yellow laser medium, reddish-orange and white phosphor applications, J. Lumin. 194 (2018) 231. _ [24] J. Pisarska, A. Kos, M. Sołtys, L. Zur, W.A. Pisarski, Energy transfer from Tb3 þ to Eu3 þ in lead borate glass, J. Non-Cryst. Solids 388 (2014) 1. [25] C.S. McCamy, Correlated color temperature as an explicit function of chromaticity coordinates, Color Res. Appl. 17 (1992) 142. [26] D.L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phys. 21 (1953) 836. [27] S. Surendra Babu, P. Babu, C.K. Jayasankar, Th Tr€ oster, W. Sievers, G. Wortmann, Optical properties of Dy3þ-doped phosphate and fluorophosphate glasses, Opt. Mater. 31 (2009) 624. [28] U. Caldi~ no, A. Speghini, M. Bettinelli, Optical spectroscopy of zinc metaphosphate glasses activated by Ce3þ and Tb3þ ions, J. Phys. Condens. Matter 18 (2006) 3499. [29] G. Blasse, Energy transfer in oxidic phosphors, Philips Res. Rep. 24 (1969) 131. [30] J.M.P.J. Verstegen, J.L. Sommerdijk, J.G. Verriet, Cerium and terbium luminescence in LaMgAl11O19, J. Lumin. 6 (1973) 425. [31] U. Caldi~ no G, Energy transfer in CaF2 doped with Ce3þ, Eu2þ and Mn2þ ions, J. Phys. Condens. Matter 15 (2003) 7127. [32] Z. Lou, J. Hao, Cathodoluminescence of rare-earth-doped zinc aluminate films, Thin Solid Films 450 (2004) 334. [33] M. Inokuti, F. Hirayama, Influence of energy transfer by the exchange mechanism on donor luminescence, J. Chem. Phys. 43 (1965) 1978.
Declaration of competing interest The authors declare that they have no conflict of interest. Acknowledgments ~ o thank CONACyT, M�exico, for the A.N. Meza-Rocha and U. Caldin CB-2016-01 project (grant 286218). A. Speghini and S. Bordignon thank University of Verona, Verona, Italy, for financial support in the frame work of “Ricerca di Base 2015” project. References [1] R. Vijayakumar, H. Guo, X.Y. Huang, Energy transfer and color-tunable luminescence properties of Dy3þ and Eu3þ, co-doped Na3Sc2(PO4)3 phosphors for near-UV LED-based warm white LEDs, Dyes Pigments 156 (2018) 8. [2] W.J. Yang, L.Y. Luo, T.M. Chen, N.S. Wang, Luminescence and energy transfer of Eu- and Mn-coactivated CaAl2Si2O8 as a potential phosphor for white-light UVLED, Chem. Mater. 17 (2005) 3883. [3] T. Ogi, A.B.D. Nandiyanto, W.N. Wang, F. Iskandar, K. Okuyama, Direct synthesis of spherical YAG:Ce phosphor from precursor solution containing polymer and urea, Chem. Eng. J. 210 (2012) 461. [4] Y. Liu, G. Liu, J. Wang, X. Dong, W. Yu, Single-component and warm-whiteemitting phosphor NaGd(WO4)2:Tm3þ, Dy3þ, Eu3þ: synthesis, luminescence, energy transfer, and tunable color, Inorg. Chem. 53 (2014) 11457. [5] A.N. Meza-Rocha, A. Speghini, J. Franchini, R. Lozada-Morales, U. Caldi~ no, Multicolor emission in lithium-aluminum-zinc phosphate glasses activated with Dy3þ, Eu3þ and Dy3þ/Eu3þ, J. Mater. Sci. Mater. Electron. 28 (2017) 10564. [6] J. Li, Y. Wang, Tunable emission with efficient energy transfer in Na2SrSi2O6:Ce3þ, Tb3þ phosphor for near-UV LED, Opt. Mater. 88 (2019) 648. [7] Y. Zheng-Wei, S. Xiao-Yu, W. Zhen-Qing, Luminescent properties of Dy3þ and/or Eu3þ doped Mg2Al4Si5O18 phosphors and energy transfer between Dy3þ/Eu3þ ion pairs, J. Lumin. 197 (2018) 164. [8] H.I. Francisco-Rodriguez, A. Lira, O. Soriano-Romero, A.N. Meza-Rocha, S. Bordignon, A. Speghini, R. Lozada-Morales, U. Caldi~ no, Lithium-aluminum-zinc phosphate glasses activated with Tb3þ and Tb3þ/Eu3þ for green laser medium, reddish-orange and white phosphor applications, Opt. Mater. 79 (2018) 358. [9] A. Lira, A. Speghini, E. Camarillo, M. Bettinelli, U. Caldi~ no, Spectroscopic evaluation of Zn(PO3)2:Dy3þ glass as an active medium for solid state yellow laser, Opt. Mater. 38 (2014) 188.
9