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Energy transfer and multiple photoluminescence of LuNbO4 co-doped with Eu3+ and Tb3+
Materials Research Bulletin 112 (2019) 399–405
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Energy transfer and multiple photoluminescence of LuNbO4 co-doped with Eu3+ and Tb3+ Min Hyuk Im, Young Jin Kim
T
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Department of Advanced Materials Engineering, Kyonggi University, Suwon, 16227, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: A. Optical materials B. Luminescence C. X-ray diffraction D. Phosphors
The energy transfer and multiple photoluminescence of LuNbO4:Eu3+,Tb3+ powders were investigated. The photoluminescence spectra of the powders mainly consisted of green and red emission peaks. The green emission peaks (551 nm) were assigned to the 5D4 → 7F5 transition of Tb3+, and the strong red emission peaks (614 nm) originated from Eu3+ (5D0 → 7F2) via a Tb3+ → Eu3+ energy transfer. Tb3+–Eu3+ co-doping greatly enhanced the red emission under UV excitation that could rarely activate Eu3+ ions. The emission of the powders was tunable from green to red by adjusting the ratio of Eu3+/Tb3+, and thus warm white-light-emitting powders could be obtained. The Tb3+ → Eu3+ energy transfer mechanism was explained using the energy transfer efficiency, the critical distance for the Tb3+ → Eu3+ energy transfer, fluorescence decay curves, and multipolar interactions. The results demonstrate that LuNbO4:Eu3+,Tb3+ powders have a high potential for use in UVpumped white-light-emitting diodes.
1. Introduction Single-phase multicolor-emitting phosphors, which can be achieved by co-doping multiple activators into single-phase hosts, have been extensively investigated because of their various merits. For example, they can exhibit tunable white-light emissions via an energy transfer (ET) between sensitizers and activators in the host, leading to some advantages over multi-phosphor blends for use in phosphor-converted white-light-emitting diodes (pc-WLEDs), such as high luminous efficacy, high color rendering index, and tunable correlated color temperature. Representative co-doping ions for tunable white-light emissions are Eu2+–Mn2+, Ce3+–Mn2+, Ce3+–Eu2+, and Ce3+–Tb3+ [1–7]. In addition, the tunable (multiple) luminescence can be obtained by co-doping Tb3+and Eu3+ [8–22], whereas Sm3+–Eu3+ co-doping into YNbO4 can modify the photoluminescence excitation (PLE) spectra [23]. LuNbO4 has a monoclinic M-fergusonite (distorted T-scheelite) structure at low temperature [24,25] and is an efficient host for down[26–28] and up-conversion phosphors [29]. The photoluminescence (PL) spectra of Eu3+-doped LuNbO4 exhibit sharp emission peaks in the red region, which are assigned to the 5D0→7FJ transition of Eu3+; the 5 D0→7F2 electric-dipole transition, which is associated with an asymmetry of the host lattices, results in the strongest red emission at 614 nm. This demonstrates that the Eu ions occupy non-centro
⁎
symmetric Lu3+ sites. Each Lu atom is coordinated with 8 oxygens. The PLE spectrum of LuNbO4:Eu3+ consists of a broad band peaking at 275 nm and sharp peaks in the near-ultraviolet (NUV) region; the former is a charge transfer band (CTB) and the latter is assigned to the ff transition of Eu3+. For practical use of LuNbO4:Eu3+ in pc-WLEDs pumped by NUV chips, the f-f excitation peaks are too sharp to permit a peak wavelength tolerance of the NUV chips. Accordingly, the PLE spectra need to be modified by co-doping Eu3+ and sensitizers into LuNbO4. Lee et al. [23] suggested that the PL spectra of (Y,Al) NbO4:Sm3+,Eu3+ could exhibit strong red emission (614 nm) under 406 nm excitation via an ET from Sm3+ to Eu3+, which could not be obtained from (Y,Al)NbO4 doped with Eu3+ alone. In this study, LuNbO4 was co-doped with Tb3+ and Eu3+ and the effects of Tb3+ on the luminescence spectra were investigated. The ET from Tb3+ to Eu3+ caused the modification of the PLE spectra, resulting in additional excitation wavelengths, in addition to those of Eu3+, for the red emission of Eu3+. Also, multiple-emission spectra were achieved by adjusting the ratio of Tb3+ to Eu3+. 2. Experiment LuNbO4:xEu3+,yTb3+ powders were synthesized by a conventional solid-state reaction process using Lu2O3 (Molycorp, 99.99%), Nb2O5 (Kojundo Chemical Lab., 99.9%), Eu2O3 (Grand Chemical & Material,
Corresponding author. E-mail address: [email protected] (Y.J. Kim).
https://doi.org/10.1016/j.materresbull.2019.01.009 Received 18 September 2018; Received in revised form 5 December 2018; Accepted 7 January 2019 Available online 07 January 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 112 (2019) 399–405
M.H. Im, Y.J. Kim
Fig. 1. XRD patterns of (a) LuNbO4:xEu3+ and (b) LuNbO4:yTb3+ powders.
99.99%), and Tb4O7 (Alfa Aesar, 99.9%) as starting materials. Stoichiometric starting mixtures with 7 wt% LiCl flux were ball-milled for 24 h and calcined at 1350 °C for 12 h under nitrogen atmosphere. The crystal structure was determined using an X-ray diffractometer (XRD, Rigaku Miniflex II) with CuKα radiation (λ = 1.5406 Å). The PL spectra were measured at room temperature using a PL system equipped with a 500 W xenon lamp (PSI Darsa-5000). Thermal quenching was determined using a fluorescence spectrophotometer (Hitach F-4500). PL decay curves were obtained using a time-resolved PL (TRPL) system (Picoquant MicroTime-200). A single-mode pulsed diode laser (wavelength: 375 nm; pulse width: ˜30 ps; average power: < 1 μW) was used as an excitation source. Fluorescence photons were counted using a time-correlated single-photon counting technique. 3. Results and discussion The XRD patterns of LuNbO4:xEu3+ and LuNbO4:yTb3+ powders are shown in Fig. 1(a) and (b), respectively. The XRD peaks correspond to those of ICDD #00-023-1207, indicating that the synthesized powders consist of a LuNbO4 single phase with a M-fergusonite structure. The other impurity phases are not observed. Lu is coordinated with 8 oxygens. Nb is tetrahedrally coordinated with 4 oxygens, while it can be 6-coordinated with oxygens in a highly distorted octahedron if more nearest-neighbor oxygens are considered. Upon increasing x and y values, the positions of the XRD peaks slightly shifted to the lower diffraction angles because of the larger ionic sizes of the dopants [Eu3+ (r = 1.066 Å for a coordination number of 8) and Tb3+ (r = 1.04 Å)] than that of the Lu3+ ion (r = 0.977 Å). In addition, the peak intensity
Fig. 2. PLE and PL spectra of (a) LuNbO4:xEu3+ and (b) LuNbO4:yTb3+ powders. (c) Emission intensity as a function of x and y.
irregularly changes. Especially, the relative intensity of the (040) peak is significantly stronger compared with that of ICDD #00-023-1207; it is abnormally intense for x = 0.1 and y = 0.05. The XRD peak intensities are correlated with some factors such as the crystal structure, multiplicity, and temperature. It is thought that the multiplicity and temperature factors are negligible in this study, and the structure factor is mainly responsible for the change in the XRD peak intensity. Accordingly, it was inferred from the finding that a difference in the ionic 400
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Fig. 3. PLE spectra of LuNbO4:0.1Eu3+ and LuNbO4:0.1Eu3+,0.1Tb3+ powders.
Fig. 5. (a) PL spectra and (b) variation of the emission intensity of LuNbO4:xEu3+,0.1Tb3+ powders.
Fig. 4. (a) PL spectra and (b) variation of the emission intensity of LuNbO4:0.1Eu3+,yTb3+ powders.
Fig. 6. Schematic energy diagram of Eu3+ and Tb3+.
3+
radius between the Lu and dopant ions led to the change in the atomic positions, resulting in the abnormally strong intensities of the (040) peaks. Further crystal refinement is required for the in-depth explanation. Similar behavior was observed for the LuNbO4:Yb3+,Er3+ phosphors [29]. The PLE and PL spectra of LuNbO4:xEu3+ and LuNO:yTb3+ powders are shown in Fig. 2(a) and (b), respectively. The PLE spectrum of
LuNbO4:xEu3+ consisted of a broad CTB of [NbO4]3− centered at approximately 272 nm and sharp peaks assigned to the f-f transition of Eu3+ in the NUV region, as shown in Fig. 2(a). The corresponding PL spectrum exhibited sharp emission peaks in the red region, which were assigned to the 5D0 → 7FJ transition of Eu3+; the 5D0 → 7F2 electricdipole transition appeared as the strongest peak owing to the lack of 401
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compared with those of earlier studies [20,21], which reported that the excitation peaks assigned to the f-d transitions of Tb3+ were observed at approximately 266 and 259/294 nm for YTaO4:Tb3+ and YNbO4:Tb3+, respectively. This demonstrates that the excitation wavelengths of the f-d transitions of Tb3+ are correlated with the host materials. Under 324 nm excitation, the PL spectra exhibited multipleemission peaks assigned to the 5D4 → 7FJ transition; the 5D4 → 7F5 green peak (551 nm) appeared as the strongest one. The variations of the emission intensities as a function of x and y are shown in Fig. 2(c). The strongest intensity was obtained for x = 0.1 and y = 0.1-0.15; a subsequent decrease in the intensity was attributed to a concentration quenching effect. The PLE spectrum (λem = 614 nm) of LuNbO4 co-doped with 0.1Eu3+ and 0.1Tb3+ was compared with that of LuNbO4:0.1Eu3+, as shown in Fig. 3. The CTB intensities of LuNbO4:0.1Eu3+,0.1Tb3+ were markedly reduced as compared to those of LuNbO4:0.1 Eu3+, because the ET occurred from [NbO4]3− to Tb3+ as well as to Eu3+. However, it was interesting that they were much higher in the 307–316 nm and 328–357 nm ranges than those of LuNbO4:0.1Eu3+. This behavior was ascribed to the ET from Tb3+ to Eu3+. To verify this phenomenon, the PL spectra of LuNbO4:0.1Eu3+,yTb3+ were measured under 312 nm excitation that could rarely activate Eu3+ ions, as shown in Fig. 4(a); the spectra exhibited the weak green bands of Tb3+ and the strong red peaks of Eu3+. The peak intensity as a function of y is shown in Fig. 4(b). For y = 0–0.15, the red emission (614 nm) of Eu3+ sharply increased despite the fixed Eu content (x = 0.1). On the other hand, the green emission (551 nm) of Tb3+ continuously decreased for y = 0.05–0.2. This behavior demonstrated that the ET process occurred between Tb3+ to Eu3+. As a result, increasing y values enhanced the ET from Tb3+ to Eu3+ more and thus increased the red emission of Eu3+. The reason for the drop of the red emission intensity for y = 0.15–0.2 was probably attributed to a back ET from Eu3+ to Tb3+ owing to the high Tb concentration. Mukherjee et al. [12] suggested that the back ET is possible, but it is weak compared with the ET from Tb3+ to Eu3+. The back ET was observed in some phosphors including YNbO4:Sm3+,Eu3+ [23] as well. The PL spectra of LuNbO4:xEu3+,0.1Tb3+ are shown in Fig. 5(a). For x = 0, the predominant green band of Tb3+ was observed. Increasing x values (0–0.1) led to a linear increase in the red emission of Eu3+ and a non-linear decrease in the green emission [Fig. 5(b)]. The opposite behavior of the red and green band indicated that the increasing x value caused the energy absorbed by the Tb3+ ions to be increasingly transferred to the Eu3+ ions. This is a decisive evidence for the ET from Tb3+ to Eu3+. The decrease in the emission intensity for x = 0.1–0.2 was attributed to a concentration quenching effect. Nazarov et al. [19] suggested a three-level model [two excited states of
Fig. 7. Decay curves of LuNbO4:xEu3+,0.1Tb3+ powders.
Fig. 8. ET efficiency (η) of LuNbO4:xEu3+,0.1Tb3+ powders.
inversion symmetry at the Eu3+ (Lu3+) sites. A more detailed explanation for the spectrum of Eu3+ can be found in earlier studies [19,23]. As shown in Fig. 2(b) for LuNbO4:yTb3+, the sharp PLE peaks in 350–390 nm are assigned to the f-f transitions of Tb3+ from the ground state (7F6) to the 5D2 (352 nm), 5L10 (371 nm), and 5G6 (379 nm) states [16,19–21]. On the other hand, the asymmetrically broad PLE band with a peak at approximately 325 nm can be decomposed into two Gaussian bands centered at 272 and 322 nm (dotted lines for y = 0.2), which correspond to the host self-absorption (CTB) and 4f8→4f75d transition of Tb3+, respectively. The CTB intensities were less significant compared with those of LuNbO4:xEu3+. The 4f8→4f75d transitions of LuNbO4:yTb3+ were achieved at the longer wavelengths
Fig. 9. Dependence of Is/Isa on Cn/3. 402
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xc (0.102) is the critical concentration for η = 50%. The calculated Rc value was approximately 11 Å, which was much longer than that (∼5 Å) for the exchange interaction mechanism, indicating that the ET for LuNbO4:Eu3+,Tb3+ is associated with the multipolar interaction mechanism. Previous studies reported Rc values of 15.24 and 15.58 Å for Tb3+-Eu3+ co-doped La3GaGe5O16 [15] and Sr2P2O7 [18], respectively. The multipolar interaction can be determined using Dexter’s Eq. (3) [32]:
Is Isa
C n/3
(3) 3+
where C is the sum of the Tb and Eu concentration, while n = 6, 8, and 10 correspond to the dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. Corresponding plots to Eq. (3) are shown in Fig. 9; the coefficients of determination (R2) of the linear fits are 0.98692, 0.99659, and 0.99626 for n = 6, 8, and 10, respectively. It was difficult to determine whether the d-q or q-q interaction is more suitable for the ET mechanism of LuNbO4: Eu3+,Tb3+, because the R2 values for n = 8 and 10 are nearly equal. Earlier studies have suggested d-d [14,15,17,18] or d-q [13] multipolar interactions for the Tb3+ → Eu3+ ET, depending on the host materials. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates for the PL spectra of LuNbO4:xEu3+,0.1Tb3+ [Fig. 5(a)] are shown in Fig. 10. The coordinate is located in the greenish-yellow region for x = 0.005 and moves toward the color temperature locus for x = 0.01. Thereafter, the coordinate shifts toward the red region for x = 0.05 − 0.2 owing to the predominant red emission of Eu3+. These findings indicate that the emission of LuNbO4:Eu3+,Tb3+ powders is tunable from green to red depending on the ratio of Eu3+ to Tb3+ and that the powders have a high potential for use in pc-WLEDs as whitelight-emitting phosphors. For practical use in UV-pumped WLEDs, LuNbO4:xEu3+,0.1Tb3+ powders were activated by the radiation of different wavelengths in the UV range, as well as the 312 nm radiation (Fig. 11). As described in Fig. 3, LuNbO4:Eu3+,Tb3+ exhibited lower PLE intensity in most excitation wavelengths than LuNbO4:Eu3+. However, for the 307–316 nm and 328–357 nm ranges, the PLE intensity of LuNbO4:Eu3+,Tb3+ was higher than that of LuNbO4:Eu3+, which resulted from the ET from Tb3+ to Eu3+. Accordingly, the emission intensity of the LuNbO4:xEu3+,0.1Tb3+ powders was measured with different excitation wavelengths (312, 335, 350, and 355 nm). Compared with LuNbO4:xEu3+, the red emission intensities of LuNbO4:xEu3+,0.1Tb3+ were significantly higher by 340–440% for x = 0.1. These findings
Fig. 10. CIE chromaticity coordinates of LuNbO4:xEu3+,0.1Tb3+ powders prepared with different x values of (a) 0.005, (b) 0.01, (c) 0.05, (d) 0.1, (e) 0.15, and (f) 0.2. 5
D4 (Tb3+) and 5D0 (Eu3+), and the ground state (Eu3+)] for the Tb3+–Eu3+ ET system for high activator (Eu3+) concentrations, as shown in Fig. 6; non-radiative transitions occur for the 5D4 (Tb3+) → 5 D1 (Eu3+) → 5D0 (Eu3+) transitions, resulting in the 5D0 → 7FJ red emission. Based on this model, they theoretically proposed that upon increasing x values, the red emission intensity linearly increased, whereas the green emission showed a non-linear decrease in intensity. They confirmed that the theoretical formula matched with the experimental findings. Our experimental results [Fig. 5(b)] for x = 0–0.1 also coincide with Nazarov’s model, indicating that the ET mechanism for LuNbO4:Eu3+,Tb3+ is associated with the three-level model. The PL decay curves of LuNbO4:xEu3+,0.1Tb3+ powders were monitored for the green emission band of Tb3+, as shown in Fig. 7. Exponential fitting for the measured PL decay curves was performed using the Symphotime-64 software (Ver. 2.2). The average lifetimes (τavg) were 565.8, 409.7, 333.8, 142.0, and 91.7 μs for x = 0, 0.005, 0.01, 0.05, and 0.1, respectively; τavg values gradually decreased as x increased. This is also a crucial evidence for the Tb3+ → Eu3+ energy transfer. The ET efficiency (η) can be calculated using Eq. (1) [30]:
= 1
sa
3+
(1)
s
where τsa and τs are the τavg values of the green emission (monitoring: 551 nm) of the powders doped with Tb3+ + Eu3+ and Tb3+ alone, respectively. Upon increasing x, η increased and reached approximately 84%, as shown in Fig. 8, indicating that the ET of LuNbO4:Eu3+,Tb3+ is very efficient compared with those (53–91%) of earlier studies [14–18]. The ET from sensitizers to activators typically occurs via a multipolar or exchange interaction mechanism. If the exchange interaction mechanism is responsible for the ET process, the distance between a sensitizer and an activator is shorter than 5 Å. The critical distance (Rc) for the Tb3+ → Eu3+ ET can be calculated using the Blasse formula, as shown in Eq. (2) [31]:
Rc = 2
3V 4 xc N
1/3
Fig. 11. Intensity of the red emission (614 nm) of LuNbO4:xEu3+,0.1Tb3+ powders activated by the radiation of different wavelengths (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(2)
where V (284.48 Å3) is the unit cell volume, N (4) is the number of lattice sites in the unit cell that can be occupied by the dopant ions, and 403
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formula (Eq. (4)) [33]:
IT =
Io 1 + Aexp ( E /kB T )
(4)
where IT is the emission intensity at a given temperature T, Io is the initial emission intensity, A is a constant, and kB is Boltzmann’s constant (8.617 × 10−5 eV/K). ΔE was estimated by plotting ln[(Io/IT)‒1] vs. 1/ kBT, as shown in Fig. 12(c). ΔE values were obtained to be approximately 0.29 and 0.44 eV for the red and green bands, respectively, indicating that the thermal quenching for the red emission of Eu3+ was lower than that for the green emission of Tb3+. The obtained ΔE for the red emission of Eu3+ was similar to those for silicate-based red phosphors doped with Eu3+ including CaAl2Si2O8:Eu3+ (0.27 eV) [34]. 4. Conclusions The PL spectra of LuNbO4:xEu3+,yTb3+ consisted of weak green bands of Tb3+ and strong red peaks of Eu3+. Upon increasing x or y, the red emission (614 nm) of Eu3+ significantly increased, whereas the green emission (551 nm) of Tb3+ decreased. In addition, τav value for the green band gradually decreased as x increased. These behaviors demonstrated that the red emission occurred via the ET process from Tb3+ to Eu3+. The PLE intensity of LuNbO4:Eu3+,Tb3+ was higher in the 307–316 nm and 328–357 nm ranges as compared to LuNbO4:Eu3+, resulting in the greatly enhanced intensity of the red emission when the powders were activated by UV light within these wavelength ranges. Upon increasing x up to 0.1, η increased and reached approximately 84%. The calculated Rc value was approximately 10.80 Å, indicating that the ET for LuNbO4:Eu3+,Tb3+ is associated with the multipolar interaction mechanism (d-q or q-q). The emission of LuNbO4:Eu3+,Tb3+ was tunable from green to red by adjusting the ratio of Eu3+ to Tb3+, and white-light emission, in turn, could be obtained. The findings indicated that LuNbO4:Eu3+,Tb3+ powders have a significant potential for use in UV-pumped WLEDs. Acknowledgements This work was supported by Kyonggi University Research Grant 2017. We are grateful to Dr. Chae at KBSI Daegu Center in Korea for a TRPL measurement. References [1] K. Li, M.M. Shang, H.Z. Liana, J. Lin, Recent development in phosphors with different emitting colors via energy transfer, J. Mater. Chem. C 4 (2016) 5507–5530. [2] W. Ahn, J. Park, Y.J. Kim, Multiple-photoluminescence of Ba3MgSi2O8:Eu2+,xMn2+ nanopowders prepared by a sol-gel-combustion hybrid process, Sci. Adv. Mater. 8 (2016) 2022–2027. [3] J. Park, W. Ahn, Y.J. Kim, Phase transitions and luminescence of Ba2-xCaxSiO4 codoped with Eu2+/Mn2+, Sci. Adv. Mater. 8 (2016) 2008–2013. [4] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host? Chem. Soc. Rev. 43 (2014) 1372–1386. [5] C.-H. Hsu, S. Das, C.-H. Lu, Color-tunable, single phased MgY4Si3O13:Ce3+,Mn2+ phosphors with efficient energy transfer for white-light-emitting diodes, J. Electrochem. Soc. 159 (2012) J193–J199. [6] C.-K. Chang, T.-M. Chen, Sr3B2O6:Ce3+,Eu2+: a potential single-phased whiteemitting borate phosphor for ultraviolet light-emitting diodes, Appl. Phys. Lett. 91 (2007) 081902-1–081902-3. [7] J.M. Yang, Y. Zhang, Z.Y. Cheng, J. Lin, Color tuning via energy transfer in Sr3In (PO4)3:Ce3+/Tb3+/Mn2+ phosphor, J. Mater. Chem. 22 (2012) 14262–14271. [8] K. Thomas, D. Alexander, S. Sisira, S. Gopi, P.R. Biju, N.V. Unnikrishnan, C. Joseph, Energy transfer driven tunable emission of Tb/Eu co-doped lanthanum molybdate nanophosphors, Opt. Mater. 80 (2018) 37–46. [9] N. Jain, B.P. Singh, R.K. Singh, J. Singh, R.A. Singh, Enhanced photoluminescence behaviour of Eu3+activated ZnMoO4 nanophosphors via Tb3+ co-doping for light emitting diode, J. Lumin. 188 (2017) 504–513. [10] J. Yang, J. Dong, R. Wu, H. Wu, H. Song, S. Gan, L. Zou, A novel color-tunable phosphor, Na5Gd9F32:Ln3+ (Ln = Eu, Tb, Dy, Sm, Ho) sub-microcrystals: structure, luminescence and energy transfer properties, Dalton Trans. 47 (2018) 9795–9803. [11] B. Wang, Q. Ren, O. Hai, X. Wu, Luminescence properties and energy transfer in Tb3+ and Eu3+ co-doped Ba2P2O7 phosphors, RSC Adv. 7 (2017) 15222–15227. [12] S. Mukherjee, V. Sudarsan, R.K. Vatsa, S.V. Godbole, R.M. Kadam, U.M. Bhatta, A.K. Tyagi, Effect of structure, particle size and relative concentration of Eu3+ and
Fig. 12. (a) Temperature dependence of the PL spectra of LuNbO4:0.01Eu3+,0.1Tb3+. (b) Normalized emission intensities as a function of temperatures and (c) Arrhenius plots for (b): ln[(Io/IT)-1] vs. 1/kBT.
demonstrate that the LuNbO4:Eu3+,Tb3+ powders have a significant potential for use in UV-pumped WLEDs. Temperature dependence (thermal quenching) of the PL spectra of LuNbO4:0.01Eu3+,0.1Tb3+ was measured, as shown in Fig. 12(a). The intensities of the green and red emissions gradually decreased over temperatures of 25-200 °C; both emission intensities remained at approximately 30% of the initial intensity [Fig. 12(b)]. On the other hand, the peak wavelengths for both emission bands did not change, because LuNbO4 has a rigid structure. The thermal quenching is attributed to the nonradiative transition from the excited states to the ground states of the Eu3+ and Tb3+ ions. The activation energy (ΔE) for the thermal quenching of the emission intensity can be calculated using Arrhenius 404
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[23] E.Y. Lee, Y.J. Kim, The effects of M ions and Sm3+ co-doping on the luminescent properties of (Y,M)NbO4:Eu3+, Electrochem. Solid-State Lett. 13 (2010) J110–J113. [24] J.M. Jehng, I.E. Wachs, Structural chemistry and Raman spectra of niobium oxides, Chem. Mater. 3 (1991) 100–107. [25] O. Yamaguchi, K. Matsui, T. Kawabe, Crystallization and transformation of distorted tetragonal YNbO4, J. Am. Ceram. Soc. 68 (1985) C275–C276. [26] X. Xiao, B. Yan, Synthesis and luminescent properties of novel RENbO4:Ln3+ (RE = Y, Gd, Lu; Ln = Eu, Tb) micro-crystalline phosphors, J. Non-Cryst. Solids 351 (2005) 3634–3639. [27] S.K. Mahesh, P.P. Rao, T.L. Francis, V.R. Reshmi, P. Koshy, Intense red line emitting phosphor LuNbO4:Eu3+ for white light emitting diode applications, Mater. Lett. 120 (2014) 115–117. [28] A.H. Krumpel, P. Boutinaud, E. van der Kolk, P. Dorenbos, Charge transfer transitions in the transition metal oxides ABO4:Ln3+ and APO4:ln3+ (A = La, Gd, Y, Lu, Sc; B = V, Nb, Ta; Ln = lanthanide), J. Lumin. 130 (2010) 1357–1365. [29] J. Park, Y.J. Kim, Upconversion luminescence of (Lu,M)NbO4:Yb3+,Er3+ (M: Al3+, Ga3+), Mater. Res. Bull. 96 (2017) 270–274. [30] P.I. Paulose, G. Jose, V. Thomas, N.V. Unnikrishnan, M.K.R. Warrier, Sensitized fluorescence of Ce3+/Mn2+ system in phosphate glass, J. Phys. Chem. Solids 64 (2013) 841–846. [31] G. Blasse, Energy transfer in oxidic phosphors, Phys. Lett. A 28 (1968) 444–445. [32] D.L. Dexter, J.A. Schulman, Theory of concentration quenching in inorganic phosphors, J. Chem. Phys. 22 (1954) 1063. [33] S. Bhushan, M.V. Chukichev, Temperature dependent studies of cathodoluminescence of green band of ZnO crystals, J. Mater. Sci. Lett. 7 (1988) 319–321. [34] W.B. Dai, M. Zhou, Z.Y. Xian, L.K. Zeng, Structure and photoluminescence characteristics of europium(III) doped in CaAl2Si2O8 phosphors, RSC Adv. 4 (2014) 25470–25478.
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Energy transfer and multiple photoluminescence of LuNbO4 co-doped with Eu3+ and Tb3+ Min Hyuk Im, Young Jin Kim
T
⁎
Department of Advanced Materials Engineering, Kyonggi University, Suwon, 16227, Republic of Korea
ARTICLE INFO
ABSTRACT
Keywords: A. Optical materials B. Luminescence C. X-ray diffraction D. Phosphors
The energy transfer and multiple photoluminescence of LuNbO4:Eu3+,Tb3+ powders were investigated. The photoluminescence spectra of the powders mainly consisted of green and red emission peaks. The green emission peaks (551 nm) were assigned to the 5D4 → 7F5 transition of Tb3+, and the strong red emission peaks (614 nm) originated from Eu3+ (5D0 → 7F2) via a Tb3+ → Eu3+ energy transfer. Tb3+–Eu3+ co-doping greatly enhanced the red emission under UV excitation that could rarely activate Eu3+ ions. The emission of the powders was tunable from green to red by adjusting the ratio of Eu3+/Tb3+, and thus warm white-light-emitting powders could be obtained. The Tb3+ → Eu3+ energy transfer mechanism was explained using the energy transfer efficiency, the critical distance for the Tb3+ → Eu3+ energy transfer, fluorescence decay curves, and multipolar interactions. The results demonstrate that LuNbO4:Eu3+,Tb3+ powders have a high potential for use in UVpumped white-light-emitting diodes.
1. Introduction Single-phase multicolor-emitting phosphors, which can be achieved by co-doping multiple activators into single-phase hosts, have been extensively investigated because of their various merits. For example, they can exhibit tunable white-light emissions via an energy transfer (ET) between sensitizers and activators in the host, leading to some advantages over multi-phosphor blends for use in phosphor-converted white-light-emitting diodes (pc-WLEDs), such as high luminous efficacy, high color rendering index, and tunable correlated color temperature. Representative co-doping ions for tunable white-light emissions are Eu2+–Mn2+, Ce3+–Mn2+, Ce3+–Eu2+, and Ce3+–Tb3+ [1–7]. In addition, the tunable (multiple) luminescence can be obtained by co-doping Tb3+and Eu3+ [8–22], whereas Sm3+–Eu3+ co-doping into YNbO4 can modify the photoluminescence excitation (PLE) spectra [23]. LuNbO4 has a monoclinic M-fergusonite (distorted T-scheelite) structure at low temperature [24,25] and is an efficient host for down[26–28] and up-conversion phosphors [29]. The photoluminescence (PL) spectra of Eu3+-doped LuNbO4 exhibit sharp emission peaks in the red region, which are assigned to the 5D0→7FJ transition of Eu3+; the 5 D0→7F2 electric-dipole transition, which is associated with an asymmetry of the host lattices, results in the strongest red emission at 614 nm. This demonstrates that the Eu ions occupy non-centro
⁎
symmetric Lu3+ sites. Each Lu atom is coordinated with 8 oxygens. The PLE spectrum of LuNbO4:Eu3+ consists of a broad band peaking at 275 nm and sharp peaks in the near-ultraviolet (NUV) region; the former is a charge transfer band (CTB) and the latter is assigned to the ff transition of Eu3+. For practical use of LuNbO4:Eu3+ in pc-WLEDs pumped by NUV chips, the f-f excitation peaks are too sharp to permit a peak wavelength tolerance of the NUV chips. Accordingly, the PLE spectra need to be modified by co-doping Eu3+ and sensitizers into LuNbO4. Lee et al. [23] suggested that the PL spectra of (Y,Al) NbO4:Sm3+,Eu3+ could exhibit strong red emission (614 nm) under 406 nm excitation via an ET from Sm3+ to Eu3+, which could not be obtained from (Y,Al)NbO4 doped with Eu3+ alone. In this study, LuNbO4 was co-doped with Tb3+ and Eu3+ and the effects of Tb3+ on the luminescence spectra were investigated. The ET from Tb3+ to Eu3+ caused the modification of the PLE spectra, resulting in additional excitation wavelengths, in addition to those of Eu3+, for the red emission of Eu3+. Also, multiple-emission spectra were achieved by adjusting the ratio of Tb3+ to Eu3+. 2. Experiment LuNbO4:xEu3+,yTb3+ powders were synthesized by a conventional solid-state reaction process using Lu2O3 (Molycorp, 99.99%), Nb2O5 (Kojundo Chemical Lab., 99.9%), Eu2O3 (Grand Chemical & Material,
Corresponding author. E-mail address: [email protected] (Y.J. Kim).
https://doi.org/10.1016/j.materresbull.2019.01.009 Received 18 September 2018; Received in revised form 5 December 2018; Accepted 7 January 2019 Available online 07 January 2019 0025-5408/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. XRD patterns of (a) LuNbO4:xEu3+ and (b) LuNbO4:yTb3+ powders.
99.99%), and Tb4O7 (Alfa Aesar, 99.9%) as starting materials. Stoichiometric starting mixtures with 7 wt% LiCl flux were ball-milled for 24 h and calcined at 1350 °C for 12 h under nitrogen atmosphere. The crystal structure was determined using an X-ray diffractometer (XRD, Rigaku Miniflex II) with CuKα radiation (λ = 1.5406 Å). The PL spectra were measured at room temperature using a PL system equipped with a 500 W xenon lamp (PSI Darsa-5000). Thermal quenching was determined using a fluorescence spectrophotometer (Hitach F-4500). PL decay curves were obtained using a time-resolved PL (TRPL) system (Picoquant MicroTime-200). A single-mode pulsed diode laser (wavelength: 375 nm; pulse width: ˜30 ps; average power: < 1 μW) was used as an excitation source. Fluorescence photons were counted using a time-correlated single-photon counting technique. 3. Results and discussion The XRD patterns of LuNbO4:xEu3+ and LuNbO4:yTb3+ powders are shown in Fig. 1(a) and (b), respectively. The XRD peaks correspond to those of ICDD #00-023-1207, indicating that the synthesized powders consist of a LuNbO4 single phase with a M-fergusonite structure. The other impurity phases are not observed. Lu is coordinated with 8 oxygens. Nb is tetrahedrally coordinated with 4 oxygens, while it can be 6-coordinated with oxygens in a highly distorted octahedron if more nearest-neighbor oxygens are considered. Upon increasing x and y values, the positions of the XRD peaks slightly shifted to the lower diffraction angles because of the larger ionic sizes of the dopants [Eu3+ (r = 1.066 Å for a coordination number of 8) and Tb3+ (r = 1.04 Å)] than that of the Lu3+ ion (r = 0.977 Å). In addition, the peak intensity
Fig. 2. PLE and PL spectra of (a) LuNbO4:xEu3+ and (b) LuNbO4:yTb3+ powders. (c) Emission intensity as a function of x and y.
irregularly changes. Especially, the relative intensity of the (040) peak is significantly stronger compared with that of ICDD #00-023-1207; it is abnormally intense for x = 0.1 and y = 0.05. The XRD peak intensities are correlated with some factors such as the crystal structure, multiplicity, and temperature. It is thought that the multiplicity and temperature factors are negligible in this study, and the structure factor is mainly responsible for the change in the XRD peak intensity. Accordingly, it was inferred from the finding that a difference in the ionic 400
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Fig. 3. PLE spectra of LuNbO4:0.1Eu3+ and LuNbO4:0.1Eu3+,0.1Tb3+ powders.
Fig. 5. (a) PL spectra and (b) variation of the emission intensity of LuNbO4:xEu3+,0.1Tb3+ powders.
Fig. 4. (a) PL spectra and (b) variation of the emission intensity of LuNbO4:0.1Eu3+,yTb3+ powders.
Fig. 6. Schematic energy diagram of Eu3+ and Tb3+.
3+
radius between the Lu and dopant ions led to the change in the atomic positions, resulting in the abnormally strong intensities of the (040) peaks. Further crystal refinement is required for the in-depth explanation. Similar behavior was observed for the LuNbO4:Yb3+,Er3+ phosphors [29]. The PLE and PL spectra of LuNbO4:xEu3+ and LuNO:yTb3+ powders are shown in Fig. 2(a) and (b), respectively. The PLE spectrum of
LuNbO4:xEu3+ consisted of a broad CTB of [NbO4]3− centered at approximately 272 nm and sharp peaks assigned to the f-f transition of Eu3+ in the NUV region, as shown in Fig. 2(a). The corresponding PL spectrum exhibited sharp emission peaks in the red region, which were assigned to the 5D0 → 7FJ transition of Eu3+; the 5D0 → 7F2 electricdipole transition appeared as the strongest peak owing to the lack of 401
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compared with those of earlier studies [20,21], which reported that the excitation peaks assigned to the f-d transitions of Tb3+ were observed at approximately 266 and 259/294 nm for YTaO4:Tb3+ and YNbO4:Tb3+, respectively. This demonstrates that the excitation wavelengths of the f-d transitions of Tb3+ are correlated with the host materials. Under 324 nm excitation, the PL spectra exhibited multipleemission peaks assigned to the 5D4 → 7FJ transition; the 5D4 → 7F5 green peak (551 nm) appeared as the strongest one. The variations of the emission intensities as a function of x and y are shown in Fig. 2(c). The strongest intensity was obtained for x = 0.1 and y = 0.1-0.15; a subsequent decrease in the intensity was attributed to a concentration quenching effect. The PLE spectrum (λem = 614 nm) of LuNbO4 co-doped with 0.1Eu3+ and 0.1Tb3+ was compared with that of LuNbO4:0.1Eu3+, as shown in Fig. 3. The CTB intensities of LuNbO4:0.1Eu3+,0.1Tb3+ were markedly reduced as compared to those of LuNbO4:0.1 Eu3+, because the ET occurred from [NbO4]3− to Tb3+ as well as to Eu3+. However, it was interesting that they were much higher in the 307–316 nm and 328–357 nm ranges than those of LuNbO4:0.1Eu3+. This behavior was ascribed to the ET from Tb3+ to Eu3+. To verify this phenomenon, the PL spectra of LuNbO4:0.1Eu3+,yTb3+ were measured under 312 nm excitation that could rarely activate Eu3+ ions, as shown in Fig. 4(a); the spectra exhibited the weak green bands of Tb3+ and the strong red peaks of Eu3+. The peak intensity as a function of y is shown in Fig. 4(b). For y = 0–0.15, the red emission (614 nm) of Eu3+ sharply increased despite the fixed Eu content (x = 0.1). On the other hand, the green emission (551 nm) of Tb3+ continuously decreased for y = 0.05–0.2. This behavior demonstrated that the ET process occurred between Tb3+ to Eu3+. As a result, increasing y values enhanced the ET from Tb3+ to Eu3+ more and thus increased the red emission of Eu3+. The reason for the drop of the red emission intensity for y = 0.15–0.2 was probably attributed to a back ET from Eu3+ to Tb3+ owing to the high Tb concentration. Mukherjee et al. [12] suggested that the back ET is possible, but it is weak compared with the ET from Tb3+ to Eu3+. The back ET was observed in some phosphors including YNbO4:Sm3+,Eu3+ [23] as well. The PL spectra of LuNbO4:xEu3+,0.1Tb3+ are shown in Fig. 5(a). For x = 0, the predominant green band of Tb3+ was observed. Increasing x values (0–0.1) led to a linear increase in the red emission of Eu3+ and a non-linear decrease in the green emission [Fig. 5(b)]. The opposite behavior of the red and green band indicated that the increasing x value caused the energy absorbed by the Tb3+ ions to be increasingly transferred to the Eu3+ ions. This is a decisive evidence for the ET from Tb3+ to Eu3+. The decrease in the emission intensity for x = 0.1–0.2 was attributed to a concentration quenching effect. Nazarov et al. [19] suggested a three-level model [two excited states of
Fig. 7. Decay curves of LuNbO4:xEu3+,0.1Tb3+ powders.
Fig. 8. ET efficiency (η) of LuNbO4:xEu3+,0.1Tb3+ powders.
inversion symmetry at the Eu3+ (Lu3+) sites. A more detailed explanation for the spectrum of Eu3+ can be found in earlier studies [19,23]. As shown in Fig. 2(b) for LuNbO4:yTb3+, the sharp PLE peaks in 350–390 nm are assigned to the f-f transitions of Tb3+ from the ground state (7F6) to the 5D2 (352 nm), 5L10 (371 nm), and 5G6 (379 nm) states [16,19–21]. On the other hand, the asymmetrically broad PLE band with a peak at approximately 325 nm can be decomposed into two Gaussian bands centered at 272 and 322 nm (dotted lines for y = 0.2), which correspond to the host self-absorption (CTB) and 4f8→4f75d transition of Tb3+, respectively. The CTB intensities were less significant compared with those of LuNbO4:xEu3+. The 4f8→4f75d transitions of LuNbO4:yTb3+ were achieved at the longer wavelengths
Fig. 9. Dependence of Is/Isa on Cn/3. 402
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xc (0.102) is the critical concentration for η = 50%. The calculated Rc value was approximately 11 Å, which was much longer than that (∼5 Å) for the exchange interaction mechanism, indicating that the ET for LuNbO4:Eu3+,Tb3+ is associated with the multipolar interaction mechanism. Previous studies reported Rc values of 15.24 and 15.58 Å for Tb3+-Eu3+ co-doped La3GaGe5O16 [15] and Sr2P2O7 [18], respectively. The multipolar interaction can be determined using Dexter’s Eq. (3) [32]:
Is Isa
C n/3
(3) 3+
where C is the sum of the Tb and Eu concentration, while n = 6, 8, and 10 correspond to the dipole-dipole (d-d), dipole-quadrupole (d-q), and quadrupole-quadrupole (q-q) interactions, respectively. Corresponding plots to Eq. (3) are shown in Fig. 9; the coefficients of determination (R2) of the linear fits are 0.98692, 0.99659, and 0.99626 for n = 6, 8, and 10, respectively. It was difficult to determine whether the d-q or q-q interaction is more suitable for the ET mechanism of LuNbO4: Eu3+,Tb3+, because the R2 values for n = 8 and 10 are nearly equal. Earlier studies have suggested d-d [14,15,17,18] or d-q [13] multipolar interactions for the Tb3+ → Eu3+ ET, depending on the host materials. The Commission Internationale de l’Eclairage (CIE) chromaticity coordinates for the PL spectra of LuNbO4:xEu3+,0.1Tb3+ [Fig. 5(a)] are shown in Fig. 10. The coordinate is located in the greenish-yellow region for x = 0.005 and moves toward the color temperature locus for x = 0.01. Thereafter, the coordinate shifts toward the red region for x = 0.05 − 0.2 owing to the predominant red emission of Eu3+. These findings indicate that the emission of LuNbO4:Eu3+,Tb3+ powders is tunable from green to red depending on the ratio of Eu3+ to Tb3+ and that the powders have a high potential for use in pc-WLEDs as whitelight-emitting phosphors. For practical use in UV-pumped WLEDs, LuNbO4:xEu3+,0.1Tb3+ powders were activated by the radiation of different wavelengths in the UV range, as well as the 312 nm radiation (Fig. 11). As described in Fig. 3, LuNbO4:Eu3+,Tb3+ exhibited lower PLE intensity in most excitation wavelengths than LuNbO4:Eu3+. However, for the 307–316 nm and 328–357 nm ranges, the PLE intensity of LuNbO4:Eu3+,Tb3+ was higher than that of LuNbO4:Eu3+, which resulted from the ET from Tb3+ to Eu3+. Accordingly, the emission intensity of the LuNbO4:xEu3+,0.1Tb3+ powders was measured with different excitation wavelengths (312, 335, 350, and 355 nm). Compared with LuNbO4:xEu3+, the red emission intensities of LuNbO4:xEu3+,0.1Tb3+ were significantly higher by 340–440% for x = 0.1. These findings
Fig. 10. CIE chromaticity coordinates of LuNbO4:xEu3+,0.1Tb3+ powders prepared with different x values of (a) 0.005, (b) 0.01, (c) 0.05, (d) 0.1, (e) 0.15, and (f) 0.2. 5
D4 (Tb3+) and 5D0 (Eu3+), and the ground state (Eu3+)] for the Tb3+–Eu3+ ET system for high activator (Eu3+) concentrations, as shown in Fig. 6; non-radiative transitions occur for the 5D4 (Tb3+) → 5 D1 (Eu3+) → 5D0 (Eu3+) transitions, resulting in the 5D0 → 7FJ red emission. Based on this model, they theoretically proposed that upon increasing x values, the red emission intensity linearly increased, whereas the green emission showed a non-linear decrease in intensity. They confirmed that the theoretical formula matched with the experimental findings. Our experimental results [Fig. 5(b)] for x = 0–0.1 also coincide with Nazarov’s model, indicating that the ET mechanism for LuNbO4:Eu3+,Tb3+ is associated with the three-level model. The PL decay curves of LuNbO4:xEu3+,0.1Tb3+ powders were monitored for the green emission band of Tb3+, as shown in Fig. 7. Exponential fitting for the measured PL decay curves was performed using the Symphotime-64 software (Ver. 2.2). The average lifetimes (τavg) were 565.8, 409.7, 333.8, 142.0, and 91.7 μs for x = 0, 0.005, 0.01, 0.05, and 0.1, respectively; τavg values gradually decreased as x increased. This is also a crucial evidence for the Tb3+ → Eu3+ energy transfer. The ET efficiency (η) can be calculated using Eq. (1) [30]:
= 1
sa
3+
(1)
s
where τsa and τs are the τavg values of the green emission (monitoring: 551 nm) of the powders doped with Tb3+ + Eu3+ and Tb3+ alone, respectively. Upon increasing x, η increased and reached approximately 84%, as shown in Fig. 8, indicating that the ET of LuNbO4:Eu3+,Tb3+ is very efficient compared with those (53–91%) of earlier studies [14–18]. The ET from sensitizers to activators typically occurs via a multipolar or exchange interaction mechanism. If the exchange interaction mechanism is responsible for the ET process, the distance between a sensitizer and an activator is shorter than 5 Å. The critical distance (Rc) for the Tb3+ → Eu3+ ET can be calculated using the Blasse formula, as shown in Eq. (2) [31]:
Rc = 2
3V 4 xc N
1/3
Fig. 11. Intensity of the red emission (614 nm) of LuNbO4:xEu3+,0.1Tb3+ powders activated by the radiation of different wavelengths (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
(2)
where V (284.48 Å3) is the unit cell volume, N (4) is the number of lattice sites in the unit cell that can be occupied by the dopant ions, and 403
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formula (Eq. (4)) [33]:
IT =
Io 1 + Aexp ( E /kB T )
(4)
where IT is the emission intensity at a given temperature T, Io is the initial emission intensity, A is a constant, and kB is Boltzmann’s constant (8.617 × 10−5 eV/K). ΔE was estimated by plotting ln[(Io/IT)‒1] vs. 1/ kBT, as shown in Fig. 12(c). ΔE values were obtained to be approximately 0.29 and 0.44 eV for the red and green bands, respectively, indicating that the thermal quenching for the red emission of Eu3+ was lower than that for the green emission of Tb3+. The obtained ΔE for the red emission of Eu3+ was similar to those for silicate-based red phosphors doped with Eu3+ including CaAl2Si2O8:Eu3+ (0.27 eV) [34]. 4. Conclusions The PL spectra of LuNbO4:xEu3+,yTb3+ consisted of weak green bands of Tb3+ and strong red peaks of Eu3+. Upon increasing x or y, the red emission (614 nm) of Eu3+ significantly increased, whereas the green emission (551 nm) of Tb3+ decreased. In addition, τav value for the green band gradually decreased as x increased. These behaviors demonstrated that the red emission occurred via the ET process from Tb3+ to Eu3+. The PLE intensity of LuNbO4:Eu3+,Tb3+ was higher in the 307–316 nm and 328–357 nm ranges as compared to LuNbO4:Eu3+, resulting in the greatly enhanced intensity of the red emission when the powders were activated by UV light within these wavelength ranges. Upon increasing x up to 0.1, η increased and reached approximately 84%. The calculated Rc value was approximately 10.80 Å, indicating that the ET for LuNbO4:Eu3+,Tb3+ is associated with the multipolar interaction mechanism (d-q or q-q). The emission of LuNbO4:Eu3+,Tb3+ was tunable from green to red by adjusting the ratio of Eu3+ to Tb3+, and white-light emission, in turn, could be obtained. The findings indicated that LuNbO4:Eu3+,Tb3+ powders have a significant potential for use in UV-pumped WLEDs. Acknowledgements This work was supported by Kyonggi University Research Grant 2017. We are grateful to Dr. Chae at KBSI Daegu Center in Korea for a TRPL measurement. References [1] K. Li, M.M. Shang, H.Z. Liana, J. Lin, Recent development in phosphors with different emitting colors via energy transfer, J. Mater. Chem. C 4 (2016) 5507–5530. [2] W. Ahn, J. Park, Y.J. Kim, Multiple-photoluminescence of Ba3MgSi2O8:Eu2+,xMn2+ nanopowders prepared by a sol-gel-combustion hybrid process, Sci. Adv. Mater. 8 (2016) 2022–2027. [3] J. Park, W. Ahn, Y.J. Kim, Phase transitions and luminescence of Ba2-xCaxSiO4 codoped with Eu2+/Mn2+, Sci. Adv. Mater. 8 (2016) 2008–2013. [4] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host? Chem. Soc. Rev. 43 (2014) 1372–1386. [5] C.-H. Hsu, S. Das, C.-H. Lu, Color-tunable, single phased MgY4Si3O13:Ce3+,Mn2+ phosphors with efficient energy transfer for white-light-emitting diodes, J. Electrochem. Soc. 159 (2012) J193–J199. [6] C.-K. Chang, T.-M. Chen, Sr3B2O6:Ce3+,Eu2+: a potential single-phased whiteemitting borate phosphor for ultraviolet light-emitting diodes, Appl. Phys. Lett. 91 (2007) 081902-1–081902-3. [7] J.M. Yang, Y. Zhang, Z.Y. Cheng, J. Lin, Color tuning via energy transfer in Sr3In (PO4)3:Ce3+/Tb3+/Mn2+ phosphor, J. Mater. Chem. 22 (2012) 14262–14271. [8] K. Thomas, D. Alexander, S. Sisira, S. Gopi, P.R. Biju, N.V. Unnikrishnan, C. Joseph, Energy transfer driven tunable emission of Tb/Eu co-doped lanthanum molybdate nanophosphors, Opt. Mater. 80 (2018) 37–46. [9] N. Jain, B.P. Singh, R.K. Singh, J. Singh, R.A. Singh, Enhanced photoluminescence behaviour of Eu3+activated ZnMoO4 nanophosphors via Tb3+ co-doping for light emitting diode, J. Lumin. 188 (2017) 504–513. [10] J. Yang, J. Dong, R. Wu, H. Wu, H. Song, S. Gan, L. Zou, A novel color-tunable phosphor, Na5Gd9F32:Ln3+ (Ln = Eu, Tb, Dy, Sm, Ho) sub-microcrystals: structure, luminescence and energy transfer properties, Dalton Trans. 47 (2018) 9795–9803. [11] B. Wang, Q. Ren, O. Hai, X. Wu, Luminescence properties and energy transfer in Tb3+ and Eu3+ co-doped Ba2P2O7 phosphors, RSC Adv. 7 (2017) 15222–15227. [12] S. Mukherjee, V. Sudarsan, R.K. Vatsa, S.V. Godbole, R.M. Kadam, U.M. Bhatta, A.K. Tyagi, Effect of structure, particle size and relative concentration of Eu3+ and
Fig. 12. (a) Temperature dependence of the PL spectra of LuNbO4:0.01Eu3+,0.1Tb3+. (b) Normalized emission intensities as a function of temperatures and (c) Arrhenius plots for (b): ln[(Io/IT)-1] vs. 1/kBT.
demonstrate that the LuNbO4:Eu3+,Tb3+ powders have a significant potential for use in UV-pumped WLEDs. Temperature dependence (thermal quenching) of the PL spectra of LuNbO4:0.01Eu3+,0.1Tb3+ was measured, as shown in Fig. 12(a). The intensities of the green and red emissions gradually decreased over temperatures of 25-200 °C; both emission intensities remained at approximately 30% of the initial intensity [Fig. 12(b)]. On the other hand, the peak wavelengths for both emission bands did not change, because LuNbO4 has a rigid structure. The thermal quenching is attributed to the nonradiative transition from the excited states to the ground states of the Eu3+ and Tb3+ ions. The activation energy (ΔE) for the thermal quenching of the emission intensity can be calculated using Arrhenius 404
Materials Research Bulletin 112 (2019) 399–405
M.H. Im, Y.J. Kim
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