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Eu3+-triactivated Ca3Gd(GaO)3(BO3)4 phosphors via Ce3+ → Tb3+ → Eu3+ energy transfer for near-UV WLEDs applications
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Multicolour tunable luminescence of thermal-stable Ce3+/Tb3+/Eu3+triactivated Ca3Gd(GaO)3(BO3)4 phosphors via Ce3+ → Tb3+ → Eu3+ energy transfer for near-UV WLEDs applications Bin Li, Xiaoyong Huang
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Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Phosphors Rare-earth White LEDs Photoluminescence Energy transfer Emission tuning
A series of single-component blue, green and red phosphors have been fabricated based on the Ca3Gd (GaO)3(BO3)4 host through doping of the Ce3+/Tb3+/Eu3+ ions, and their crystal structure and photoluminescence properties have been discussed in detail. A terbium bridge model via Ce3+ → Tb3+ → Eu3+ energy transfer has been studied. The emission colours of the phosphors can be tuned from blue (0.1661, 0.0686) to green (0.3263, 0.4791) and eventually to red (0.5284, 0.4040) under a single 344 nm UV excitation as the result of the Ce3+ → Tb3+ → Eu3+ energy transfer. The energy transfer mechanisms of Ce3+ → Tb3+ and Tb3+ → Eu3+ were found to be dipole-dipole interactions. Importantly, Ca3Gd(GaO)3(BO3)4:Ce3+,Tb3+,Eu3+ phosphors had high internal quantum efficiency. Moreover, the study on the temperature-dependent emission spectra revealed that the Ca3Gd(GaO)3(BO3)4:Ce3+,Tb3+,Eu3+ phosphors possessed good thermal stability. The above results indicate that the phosphors can be applied into white light-emitting diodes as single-component multicolour phosphors.
1. Introduction During the past decades, rare-earth ions based inorganic luminescent materials with excellent luminescent properties have been widely studied and used in various application fields, including lightings, displays, lasers, solar cells, sensors and bioimaging [1–8]. As the situation of global energy shortage and environmental deterioration, much attention has been paid to the research of green lighting. White lightemitting diodes (WLEDs) as a new type of environment friendly lighting source have many advantages, such as low electricity consumption, long service life, small volume and non-pollution. At present, the commercial WLEDs are generally fabricated by combining the yellow Y3Al5O12:Ce3+ phosphors with the blue chips, which suffer from a low colour rendering index (Ra < 80) and a high correlated colour temperature (CCT > 6000 K) due to the absence of red component [9]. Therefore, various solutions to this defect are proposed, like “blue LED combines green and red phosphors” and “near-ultraviolet (NUV) LED combines blue, green and red phosphors” [10–13]. Obviously, red phosphor is essential to all approaches, but the commercially red phosphor (such as Y2O3:Eu3+) shows lower efficiency under UV light excitation [14]. So, the research on red phosphors is urgent.
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Eu3+ ion is very suitable to fabricate red phosphor, due to its perfect colour purity of narrow line red emission [15–18]. However, Eu3+activated phosphors possess narrow-line-shaped excitation peaks in UV region due to the 4f-4f transition, which are not desirable for NUV LED application. Therefore, broadening the excitation band of Eu3+ is an urgent issue. Recently, much attention has been paid to Ce3+ ion as a sensitizer to broaden the excitation of activators ions. As reported by many scientists [19–21]. Ce3+ ions possess strong and broad absorption in NUV area for it spin- and parity-allowed 4f−5d transition, which makes it possible to broaden the excitation of Eu3+ ion via Ce3+ ion. However, the metal-metal charge transfer (MMCT; Ce3+ + Eu3+ → Ce4+ + Eu2+) always exists to prevent the energy transfer from Ce3+ to Eu3+, so sensitizing Eu3+ ions by using the Ce3+ ions in Ce3+/Eu3+ co-doped phosphors has been proven to be unsuccessful [22]. As is known, Tb3+ ion is not only a good sensitizer to Eu3+ ion [23]. but also a good acceptor to Ce3+ [19]. So (Tb3+)n can be described as a bridge to connect Ce3+ with Eu3+ [24]. Furthermore, by controlling the concentration ratio of Ce3+/Tb3+/Eu3+ ions, the emission colour of Ce3+/Tb3+/Eu3+ tri-doped phosphors can be tuned from blue to green and then to red, meaning that it would be easy to control the colour output satisfying application requirements.
Corresponding author. E-mail address: [email protected] (X. Huang).
https://doi.org/10.1016/j.ceramint.2017.12.082 Received 5 December 2017; Received in revised form 11 December 2017; Accepted 12 December 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Li, B., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.12.082
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Borates have been considered as outstanding host materials for inorganic phosphors due to the merits of themselves, such as, high chemical stability, good thermal stability, and low synthetic temperature. Recently, the Ca3Y(GaO)3(BO3)4 was reported as an excellent host for rare-earth luminescence [25,26]. In this host, due to the existence of [BO3] and [GaO6] groups, rare-earth activators can be heavily doped into the matrix without concentration quenching, which is benefit for achieving high luminescence efficiency. However, to the best of our knowledge, there is no report on the luminescent properties of rareearth ions doped Ca3Gd(GaO)3(BO3)4 (hereafter abbreviated as CGGB). In this study, we reported broad-band excited, multicolour tunable emissions from the CGGB:Ce3+,Tb3+,Eu3+ system. Because of energy transfer from Ce3+ to Tb3+ and then to Eu3+ ions, intense visible light is emitted under a single 344 nm excitation wavelength and the emission colour is continuously tuned from blue to red through adjustment of the concentration ratio of Ce3+/Tb3+/Eu3+ ions. The energy transfer mechanism, emission tuning, internal quantum efficiency (IQE) and thermal stability of the as-prepared phosphors were investigated in detail. These broad-band excited, multicolour tunable CGGB:Ce3+, Tb3+,Eu3+ phosphors are promising materials for applications in NUVbased WLEDs.
Fig. 1. XRD patterns of CGGB host and Ce3+, Tb3+, or Eu3+ doped CGGB samples.
2. Experimental A series of Ca3Gd(1-x-y-z)(GaO)3(BO3)4:xCe3+,yTb3+,zEu3+ (hereafter abbreviated as CGGB:xCe3+,yTb3+,zEu3+) samples were successfully fabricated via a conventional high-temperature solid-state reaction technique. H3BO3 (analytical reagent), CaCO3 (analytical reagent), Gd2O3 (99.99%), Ce(NO3)3·6H2O (99.99%), Tb(NO3)3·6H2O (99.99%), Ga2O3 (99.99%) and Eu2O3 (99.99%) were used as the raw materials. According to the stoichiometric ratio, these raw materials were weighted and ground in an agate mortar to achieve uniformity. In order to compensate the volatilization, the amount of H3BO3 is in excess of 5 wt%. Then, these uniform mixtures were put in the alumina crucibles and sintered at 1000 °C for 4 h in CO atmosphere to reduce the Ce ion into tri-valence ion. After that, the furnace cooled down naturally to room temperature, and the final products were ground and collected for further characterization. The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 X-ray diffractometer using Cu Kα radiation ranging with 5–90° at step rate of 0.02°. The morphology properties of the samples were obtained by a field-emission scanning electron microscope (FESEM; MAIA3 TESCAN) equipped with an energy dispersive X-ray (EDX) spectrum. The diffuse reflection spectra were measured on an UV–vis–NIR spectrophotometer (SHIMADZU UV-2600) attached to an integral sphere and BaSO4 was used as a reference standard. The roomtemperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra and luminescence decay lifetimes of phosphors were measured by Edinburgh FS5 spectrometer equipped with a 150 W continued-wavelength Xenon lamp and a pulsed Xenon lamp, respectively. Temperature-dependent PL spectra were recorded using same spectrophotometer and detectors equipped with a temperature controller. The IQEs of all samples were measured on an Edinburgh FS5 spectrometer equipped with an integrating sphere coated with BaSO4.
Fig. 2. Rietveld XRD refinement of CGGB host.
Table 1 Rietveld refinement results and lattice parameters for Ca3Gd (GaO)3(BO3)4 from the GSAS Rietveld refinement.
3. Results and discussion
Formula
Ca3Gd(GaO)3(BO3)4
Crystal system Space group 2θ-interval,° α,° β,° γ,° a (Å) b (Å) c (Å3) V Z Rwp (%) Rp (%)
Hexagonal P63/m 10–80 90 90 120 10.5529(3) 10.5529(3) 5.8239(2) 561.68(3) 6 6.85% 4.19%
space group P63/m. Moreover, crystallographic data and details were summarized in Table 1, and the refined lattice parameters are a = 10.5529(3) Å, b = 10.5529(3) Å, c = 5.8239(2) Å, and cell volume = 561.68(3) Å, which coincides well with those reported in the literature [27]. The crystal structure of CGGB is shown in Fig. 3. There are two different Ca2+ sites, Ca1 site is coordinated with nine oxygen atoms while Ca2 site is surrounded by seven oxygen atoms. Otherwise, two kinds of tunnels exist in this lattice, in which Ca2 ions occupy hexagonal tunnel site while Ca1 ions occupy trigonal tunnel. According to
3.1. Phase and structure Fig. 1 compares the power XRD patterns of CGGB host and Ce3+/ Tb /Eu3+ doped CGGB phosphors. All XRD patterns can be well indexed to the Ca3Y(GaO3)(BO3)4 (ICSD-172155), and no impurity or significant changes were detected in the host structure. Rietveld refinements of the CGGB host were performed to further analyze the crystal structure details of the as-prepared samples, as shown in Fig. 2. It can be found that CGGB crystallized in a hexagonal unit cell with 3+
2
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Subsequently, we synthesized a series of Ce3+ and Eu3+ co-doped phosphors to study the energy transfer between Ce3+ and Eu3+ ions. Fig. 7 compares the PL spectra of CGGB:0.05Ce3+,yEu3+ (y = 0–0.05) and CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ phosphors under 344 nm excitation. As above-mentioned, efficient energy transfer from Ce3+ to Eu3+ ions in CGGB:Ce3+,Eu3+ phosphors was possible due to the good spectral overlap. But disappointedly, under Ce3+-excitation at 344 nm, the emission of Eu3+ ions was subdued throughout. Moreover, the emission of Ce3+ ions was dramatically quenched with increasing Eu3+ doping concentration, which can be ascribed to the existence of MMCT that quenched the luminescence of the sensitizer (Ce3+ + Eu3+ → Ce4+ + Eu2+) [22]. In order to overcome this MMCT effect, we then introduced Tb3+ ions into CGGB:Ce3+,Eu3+ phosphors because the energy can be transferred though Ce3+ → (Tb3+)n → Eu3+ process. As shown in Fig. 7, tri-doping 5 mol% Tb3+ ions into CGGB:0.05Ce3+ ,0.05Eu3+ phosphor can overwhelmingly improve the emission of Eu3+. The peak emission intensity at 621 nm (5D0 → 7F2 transition of Eu3+ ions) in CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ phosphors was about 56 times as high as that of CGGB:0.05Ce3+,0.05Eu3+ phosphors. Moreover, in the PL spectrum of CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ phosphors, the characteristic transitions of both Tb3+ (5D4 → 7F6 at 485 nm, 5D4 → 7F5 at 544 nm, 5D4 → 7F4 at 585 nm, and 5D4 → 7F3 at 614 nm) and Eu3+ ions (5D0 → 7F1 at 595 nm, 5D0 → 7F2 at 621 nm, 5 D0 → 7F3 at 649 nm, and 5D0 → 7F4 at 692 nm) were observed. In order to further confirm the occurrence of Ce3+ → (Tb3+)n → Eu3+ energy transfer process, we compared the PLE spectra of CGGB:0.1Eu3+ with CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ by monitoring at 621 nm emission, as shown in Fig. 8. The PLE spectrum of CGGB:0.1Eu3+ consisted of the O2+-Eu3+ CT band in the 220–280 nm region and the characteristic PLE peaks in the 270–500 nm wavelength range. In sharp contrast, in the PLE spectrum of CGGB:0.05Ce3+, 0.5Tb3+,0.1Eu3+ phosphors, besides of the PLE peaks of Eu3+ ions, the broad PLE band of Ce3+ ions (4f → 5d at 344 nm) and sharp PLE peaks of Tb3+ ions (7F6 → 5L10 at 370 nm and 7F6 → 5D4 at 485 nm) were also presented. These results strongly revealed the existence of “terbium bridge” in the Ce3+ → (Tb3+)n → Eu3+ energy transfer process. Fig. 9 shows the PL spectra of CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5) and CGGB:0.05Ce3+,0.50Tb3+,zEu3+ (z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors under excitation at 344 nm. As can be seen, the PL spectra of Ce3+/Tb3+ co-doped CGGB samples consisted of blue emission band of Ce3+ ions and the characteristic green emission peaks of Tb3+ ions, due to the energy transfer from Ce3+ to Tb3+. Moreover, with increasing Tb3+ concentration from y = 0.05 to y = 0.5, the emission intensity of Ce3+ ions decreased monotonically, while the emission intensity of Tb3+ ions gradually increased without emission quenching (see the inset (a) of Fig. 9). Therefore, the high threshold of quenching concentration of Tb3+ ions in this CGGB host was very desirable for establishing the Ce3+ → (Tb3+)n → Eu3+ energy transfer system via the terbium bridge, which could realize the narrow-line red emission from Eu3+ ions under broad-band NUV excitation. For the CGGB:0.05Ce3+,0.50Tb3+,zEu3+ samples, once Eu3+ ions were introduced, the emission intensities of Ce3+ and Tb3+ ions decreased continuously, whereas the emission intensity of Eu3+ gradually increased until z = 0.10. However, further increase in the Eu3+ concentration in CGGB:0.05Ce3+,0.50Tb3+,zEu3+ samples resulted in reduced emission intensity of Eu3+ ions, owing to concentration quenching effect, as shown in the inset (b) of Fig. 9.
Fig. 3. Crystal structure of CGGB host.
bond valence sum (BVS) [28], BVS of Ca2+ in trigonal tunnel is much higher than it in hexagonal one, for the charge balance, Ln3+ (Ln = Gd，Ce, Tb and Eu) ions prefer to occupy Ca1 site. Moreover, isolated by [BO3] and [GaO6] groups, the distance of two Ca1 ions is large enough to allow Ln3+ ions highly doping. Fig. 4(a) shows the representative FE-SEM image of CGGB: 0.05Ce3+,0.5Tb3+,0.1Eu3+ phosphors. As can be seen, the studied sample is made up of irregular and aggregated microparticles with the size ranging from 5 to 10 µm. Fig. 4(b) shows the EDX spectrum of the CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ sample. As demonstrated, elements of Ca, Ga, Gd, O, B, Ce, Tb and Eu was observed, which clearly indicated the formation of CGGB:Ce3+,Tb3+,Eu3+ phosphors. Besides, the elemental mapping result (Fig. 4(c)) revealed that the components of Ca, Ga, Gd, O, B, Ce, Tb and Eu were uniformly distributed over the whole range of particles. 3.2. Luminescence properties Fig. 5(a) shows the diffuse reflection spectrum, PLE and PL spectra of CGGB:0.05Ce3+ phosphors. By monitoring at 400 nm, there were two principle excitation bands in the PLE spectrum: the first one was located in the 250–300 nm region and the second one was located in the range of 305–375 nm with a peak at ~ 344 nm, due to 4f → 5d transitions of Ce3+ ions. The sharp PLE peak at 275 nm was attributed to the 8S7/2 → 6I7/2 transition of Gd3+ ions, indicating the energy transfer from Gd3+ to Ce3+ ions in the CGGB:Ce3+ phosphors. The PLE spectrum agreed well with the reflection spectrum. The PL spectrum was a broad emission band centered at 400 nm, corresponding to the transition from the 5d level to the ground state of the Ce3+ ion. In order to obtain the strongest blue emission, we changed the doping concentrations of Ce3+ in CGGB and found the optimal concentration was 5 mol %, as presented in Fig. 5(b). Fig. 6 shows the PLE spectra of CGGB:0.1Tb3+ and CGGB:0.1Eu3+ phosphors. For the CGGB:0.1Eu3+ phosphor by monitored at 621 nm, there are several excitation bands centered at 300, 319, 364, 378, 385, 397, 415 and 466 nm, which were assigned to the electronic transition from the 7F0 ground state to the excited states of 5F2, 5H6, 5D4, 5G3, 5G2, 5 L6, 5D3 and 5D2, respectively. While monitored at 544 nm, the CGGB:0.1Tb3+ phosphor presented a series of excitation bands centered at 309, 315, 320, 344, 354, 370, 381 and 485 nm, corresponding to the 7F6 → 5H5, 7F6 → 5H6, 7F6 → 5H7, 7F6 → 5L7, 7F6 → 5D2, 7F6 → 5 L10, 7F6 → 5D3, and 7F6 → 5D4 transition, respectively. Generally speaking, the energy transfer from sensitizer to activator needs manifest spectral overlap between sensitizer's emission and activator's excitation. Herein this present work, the excitation bands of Tb3+ and Eu3+ ions indeed have large spectral overlaps with the emission of Ce3+, suggesting the possibility of the efficient energy transfer from Ce3+ to Tb3+/Eu3+ in CGGB host.
3.3. Decay curves and energy transfer Fig. 10(a) shows the luminescence decay curves of Ce3+ ions in CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5). The decay curve of the CGGB:0.05Ce3+ can be fitted well with a biexponential function by the following equation:
I = A1 exp(−t / τ1) + A2 exp(−t / τ2) 3
(1)
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Fig. 4. (a) FE-SEM image, (b) EDX spectrum and (c) elemental mapping of CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ phosphors.
which provided strong evidence for the energy transfer from Ce3+ to Tb3+ ions. General speaking, energy transfer via exchange interactions needs a large direct or indirect overlapping between donor and acceptor orbitals leading to easy electronic exchange [29]. However, both Ce3+ and Tb3+ ions are reducing ions, so exchange interaction between Ce3+ and Tb3+ would require too high energies and thus the possibility of such an exchange is much low. Consequently, the mechanism of Ce3+ → Tb3+ energy transfer could be attributed to multipolar interaction. According to Dexter's energy-transfer formula of multipolar interaction, the following relation can be obtained [30]:
where I represents the luminescent intensity and A1 and A2 are constants; t is time, and τ1 and τ2 are the decay times for the exponential components. This result indicated that the Ce3+ ions occupied two Gd3+ sites in the CGGB host, which was in accordance with the practical coordination environments of the Gd3+ ion in the crystal lattice. When the Ce3+ and Tb3+ ions were co-doped into the host, the decay curves deviated from the biexponential function. Therefore, the average decay time τ* was determined by the formula: ∞
τ *=
∫0 tI (t ) dt ∞ ∫0 I (t ) dt
(2)
τS0 ∝ C a/3 τS
The average decay times τ* were calculated to be 26.91, 25.35, 23.54, 18.75 and 14.11 ns for CGGB:0.05Ce3+,yTb3+ phosphors with y = 0, 0.05, 0.1, 0.3 and 0.5, respectively. Thus, the decay lifetimes for the Ce3+ ions decreased monotonically as Tb3+ content increased,
(3)
where C(Tb) is the concentration of Tb3+, and a = 6, 8, or 10 is for dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole interactions, respectively. Plots of τS0/τS and C(Tb)a/3 based on the above 4
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Fig. 7. PL spectra of CGGB:0.05Ce3+,yEu3+ (y = 0–0.05) and CGGB:0.05Ce3+, 0.5Tb3+,0.05Eu3+ phosphors under 344 nm excitation.
Fig. 5. (a) Diffuse reflection, excitation and emission spectra of the CGGB:0.05Ce3+ sample. (b) PL spectra of CGGB:xCe3+ (x = 0.01, 0.03, 0.05, 0.07, and 0.1) phosphors under 344 nm excitation. Fig. 8. PLE spectra of the CGGB:0.1Eu3+ (λem = 621 nm) and CGGB:0.05Ce3+,0.5Tb3+, 0.1Eu3+ (λem = 621 nm) phosphors.
Fig. 6. Comparison of PL spectrum of CGGB:0.05Ce3+ sample (λex = 344 nm) and PLE spectra of CGGB:0.1Tb3+ (λem = 544 nm) and CGGB:0.1Eu3+ (λem = 621 nm).
Fig. 9. PL spectra of the CGGB:Ce3+,Tb3+ and CGGB:Ce3+,Tb3+,Eu3+ (λex = 344 nm). The insets are the dependence of (a) Tb3+ (green) and (b) Eu3+ (red) emission intensity on Tb3+ and Eu3+ contents, respectively.
equation were shown in Fig. 11. The best linear behavior was achieved at a = 6, indicating that energy transfer from Ce3+ to Tb3+ took place via the dipole–dipole interaction. The [Tb3+]n bridge not only can facilitate the energy transfer process in Ce3+ → (Tb3+)n → Eu3+ system, but also as can service as a barrier to prevent the undesirable MMCT effect, which can further be proved by the variation in the decay lifetimes of Ce3+ and Tb3+ activators. Fig. 10(b) presents the luminescence decay curves of Ce3+ ions
in CGGB:0.05Ce3+,0.5Tb3+,zEu3+ (z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors. Compared to the CGGB:0.05Ce3+,0.50Tb3+ sample, the decay times of Ce3+ ions dramatically dropped in the Ce3+/Tb3+/ Eu3+ triactivated CGGB phosphors, revealing that the Ce3+ → Eu3+ energy transfer occurred via “terbium bridge”. In order to further confirm such Ce3+ → (Tb3+)n → Eu3+ energy transfer, the decay 5
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As we know, the mechanism of energy transfer from Tb3+ to Eu3+ ions can be attributed to exchange interaction or electric multipolar interaction. To figure out which interaction dominated in the energy transfer process, the average distance (Rc) between the Tb3+ donors and Eu3+ acceptor ions in CGGB:Ce3+,Tb3+,Eu3+ phosphors was evaluated by using the following equation [31]:
3V 1/3 ⎤ Rc = 2 ⎡ ⎣ 4πCN ⎦
(4) 3+
3+
3+
where C is the total concentration of Ce , Tb and Eu ions, N is coordination number, and V the cell volume. For the CGGB host, the V and N are 561.68 Å3 and 2, respectively. The total concentrations C were found to be 0.56, 0.58, 0.60, 0.65, 0.75 and 0.85 for samples with z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3, respectively. Accordingly, Rc was determined to be 9.86, 9.74, 9.63, 9.38, 8.94 and 8.57 Å for samples with z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3, respectively. Generally, exchange interaction requires a smaller Rc value (< 5 Å), and consequently, the Tb3+ → Eu3+ energy-transfer in CGGB:Ce3+,Tb3+,Eu3+ phosphors would take place via electric multipolar interaction. In order to further determine the Tb3+ → Eu3+ energy-transfer mechanism, by using Eq. (3), the electric multipolar interaction parameter a taking the values 6 (dipole–dipole), 8 (dipole–quadrupole), and 10 (quadrupole–quadrupole) were compared by the dependence of τs0/τs of Tb3+ on C(Eu)a/3, as demonstrated in Fig. 12. A line relation was well-fitted at a = 6, so the energy-transfer mechanism from Tb3+ to Eu3+ ions could be the dipole–dipole interaction. Fig. 13(a) depicts the scheme of energy level model for the Ce3+ → 3+ Tb → Eu3+ energy-transfer process. Upon UV irradiation at Ce3+ ions, electrons from the 2F5/2 ground state are excited into the 5d excited state. Some of these electrons return to the ground states (2F7/2 and 2F5/2) of Ce3+ ions, resulting in the violet-blue emissions due to the 5d → 4f transition. At the same time, owing to the nonradiative resonant energy-transfer, other excited electrons transfer into the 5D4 excited state of Tb3+ ions, and subsequently, the electrons relax to the 7 FJ ground state of Tb3+ ions, giving rise to green emission corresponding to 5D4 → 7FJ (J = 3–6) transitions. With heavily doped Tb3+, Ce3+ → Tb3+ → Eu3+ energy transfer can efficiently take place via the [Tb3+]n bridge. Some of the excited electrons at 5D4 excited state of Tb3+ ions can transfer to the 5D1 excited state of Eu3+ ions, and finally, the electrons relax to Eu3+: 5D0 state and then red emissions due to 5D0 → 7FJ (J = 1–4) transitions are realized. Fig. 13(b) shows the CIE diagram of the CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5) and CGGB:0.05Ce3+,0.50Tb3+,zEu3+ (z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors. The CIE chromaticity details of these samples were shown in the Table 2. Clearly, the corresponding colours of these samples can be tuned from blue (0.1661, 0.0686) to green (0.3263, 0.4791), and finally to red (0.5284, 0.4040) through codoping Tb3+ and Eu3+ with Ce3+ ions. Moreover, the IQEs of all the phosphor samples were also listed in Table 2. The CGGB:0.05Ce3+ sample showed an IQE as great as 60%, and moreover, codoping Tb3+ into CGGB:Ce3+ phosphor improved the IQE. The CGGB:0.05Ce3+,0.5Tb3+,0.01Eu3+ possessed a high IQE of 55.5%, but further increasing Eu3+ concentration in CGGB:Ce3+,Tb3+,Eu3+ phosphors resulted in reduced IQEs, owing to the energy loss in the Ce3+ → (Tb3+)n → Eu3+ successive energy-transfer process.
Fig. 10. (a) Decay curves of Ce3+ ions in CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5) phosphors monitoring 400 nm emission. (b) Decay curves of Ce3+ ions in CGGB:0.05Ce3+,0.5Tb3+,zEu3+ (y = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors ion in monitoring 400 nm emission. (c) Decay curves of Tb3+ CGGB:0.05Ce3+,0.5Tb3+,zEu3+ (z = 0, 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors monitoring 544 nm emission.
lifetimes of Tb3+ ions in CGGB:0.05Ce3+,0.50Tb3+,zEu3+ phosphors were also measured, as shown in Fig. 10(c). It can be seen that the decay times of Tb3+ ions decreased fast as the Eu3+ concentration increased. For example, the lifetime of Tb3+ ions in CGGB:0.05Ce3+,0.50Tb3+,0.1Eu3+ sample was reduced to be 0.633 ms, in comparison to 1.575 ms of CGGB:0.05Ce3+,0.50Tb3+ sample. Therefore, the red emission peaks from Eu3+ ions can be attributed to the energy transfer from Tb3+ to Eu3+, due to the shortened decay time of Tb3+ emission with increasing Eu3+ ions.
3.4. Thermal stability of CGGB:Ce3+,Tb3+,Eu3+ phosphors As an important technology parameter of phosphors application, the thermal stability has significant impact on the light output and colour rendering index value of WLEDs. Therefore, the thermal stability of CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ phosphors was investigated. Fig. 14(a) shows the temperature-dependent emission spectra of the CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ sample under 344 nm excitation. With increasing temperature, the profile of the emission bands were 6
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Fig. 11. Dependence of τS0/τS of Ce3+ on (a) CTb6/3, (b) CTb8/3 and (c) CTb10/3.
Fig. 12. Dependence of τS0 /τS of Tb3+ on (a) CEu6/3, (b) CEu8/3 and (c) CEu10/3.
Fig. 13. (a) Illustration of energy levels of Ce3+, Tb3+, Eu3+ ions and Ce3+ → Tb3+ → Eu3+ energy transfer mechanism. (b) CIE chromaticity diagram showing emission colour tuning in CGGB:Ce3+,Tb3+,Eu3+ phosphors under single 344 nm UV excitation. Insets are photographs of the representative phosphors upon excitation under a 365 nm UV lamp.
by following expression [32]:
found to be almost the same, while the emission intensity gradually reduced as the result of the thermal quenching effect. Inset of Fig. 14(a) presents the normalized integrated PL intensity as a function of temperature. The PL intensity at 423 K was calculated to be about 70.96% of that at 298 K, demonstrating that CGGB:Ce3+,Tb3+,Eu3+ phosphors possessed good thermal stability. The activation energy for the thermal quenching can be calculated
I (T ) =
I0 1 + c exp(−Ea/ kT )
(5)
In this expression, I0 and I(T) are the emission intensity at initial temperature and measured temperature T, respectively, c is constant and Ea is the activation energy. Fig. 14(b) shows the plot of ln(I0/I-1) vs. 7
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0.4040) by adjusting the concentration ratio of Ce3+/Tb3+/Eu3+ activators. Furthermore, the CGGB:Ce3+,Tb3+,Eu3+ phosphors possessed high IQEs. Thermal quenching curves showed that the phosphors had good thermal stability. These results indicated that CGGB:Ce3+, Tb3+,Eu3+ can be promising as single-component multi-colour phosphors for application in WLEDs.
Table 2 Variations of CIE chromaticity coordinates (x, y) for CGGB:Ce3+,Tb3+,Eu3+ phosphors excited at 344 nm. Sample
CIE coordinates (x, y)
IQE
CGGB:0.05Ce3+ CGGB:0.05Ce3+,0.05Tb3+ CGGB:0.05Ce3+,0.1Tb3+ CGGB:0.05Ce3+,0.3Tb3+ CGGB:0.05Ce3+,0.5Tb3+ CGGB:0.05Ce3+,0.5Tb3+,0.01Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.03Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.2Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+
(0.1661, (0.2068, (0.2370, (0.2980, (0.3263, (0.3459, (0.3763, (0.4001, (0.4681, (0.5138, (0.5284,
60.0% 65.2% 69.3% 75.5% 61.2% 55.5% 50.4% 49.6% 40.1% 25.4% 10.2%
0.0686) 0.1852) 0.2618) 0.4105) 0.4791) 0.4753) 0.4605) 0.4533) 0.4444) 0.4210) 0.4040)
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, the Startup Research Grant of Taiyuan University of Technology (No. Tyutrc201489a), the Excellent Young Scholars Research Grant of Taiyuan University of Technology (Nos. 2014YQ009, 2015YQ006, and 2016YQ03), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, No. 2017-skllmd-01). References [1] L. Huang, Y. Zhu, X. Zhang, R. Zou, F. Pan, J. Wang, M. Wu, HF-free hydrothermal route for synthesis of highly efficient narrow-band red emitting phosphor K2Si1–xF6:xMn4+ for warm white light-emitting diodes, Chem. Mater. 28 (2016) 1495–1502. [2] X. Huang, S. Han, W. Huang, X. Liu, Enhancing solar cell efficiency: the search for luminescent materials as spectral converters design, Chem. Soc. Rev. 44 (2013) 173–201. [3] S. Gai, C. Li, P. Yang, J. Lin, Recent progress in rare earth micro/nanocrystals: soft chemical synthesis, luminescent properties, and biomedical applications, Chem. Rev. 114 (2014) 2343–2389. [4] S.F. Lai, Z.W. Yang, J. Li, B. Shao, J.Z. Yang, Y.D. Wang, J.B. Qiu, Z.G. Song, Photoluminescence enhancement of Eu3+ ions by Ag species in SiO2 three-dimensionally ordered macroporous materials, J. Mater. Chem. C 3 (2015) 7699–7708. [5] X. Huang, Enhancement of near-infrared to near-infrared upconversion luminescence in sub-10 nm ultra-small LaF3:Yb3+/Tm3+ nanoparticles through lanthanide doping, Opt. Lett. 40 (2015) 5231–5234. [6] Z. Mao, J. Chen, J. Li, D. Wang, Dual-responsive Sr2SiO4:Eu2+Ba3MgSi2O8:Eu2+,Mn2+ composite phosphor to human eyes and plant chlorophylls applications for general lighting and plant lighting, Chem. Eng. J. 284 (2016) 1003–1007. [7] P. Du, X. Huang, J.S. Yu, Yb3+-concentration dependent upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped Gd2MoO6 nanocrystals prepared by a facile citric-assisted sol–gel method, Inorg. Chem. Front. 4 (2017) 1987–1995. [8] X.Y. Huang, Broadband dye-sensitized upconversion: a promising new platform for future solar upconverter design, J. Alloy. Compd. 690 (2017) 356–359. [9] X.Y. Huang, B. Li, H. Guo, Highly efficient Eu3+-activated K2Gd(WO4)(PO4) redemitting phosphors with superior thermal stability for solid-state lighting, Ceram. Int. 43 (2017) 10566–10571. [10] H. Guan, Y. Sheng, C. Xu, Y. Dai, X. Xie, H. Zou, Energy transfer and tunable multicolor emission and paramagnetic properties of GdF3:Dy3+,Tb3+,Eu3+ phosphors, Phys. Chem. Chem. Phys. 18 (2016) 19807–19819. [11] A. Huang, Z. Yang, C. Yu, Z. Chai, J. Qiu, Z. Song, Tunable and white light emission of a single-phased Ba2Y(BO3)2Cl:Bi3+,Eu3+ phosphor by energy transfer for ultraviolet converted white LEDs, J. Phys. Chem. C 121 (2017) 5267–5276. [12] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host? Chem. Soc. Rev. 43 (2014) 1372–1386. [13] X.Y. Huang, B. Li, H. Guo, Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors, J. Alloy. Compd. 695 (2017) 2773–2780. [14] J.D. Axe, P.F. Weller, Fluorescence and energy transfer in Y2O3:Eu3+, J. Chem. Phys. 40 (1964) 3066–3069. [15] X. Huang, Solid-state lighting: red phosphor converts white LEDs, Nat. Photonics 8 (2014) 748–749. [16] X. Huang, H. Guo, B. Li, Eu3+ -activated Na2Gd(PO4)(MoO4): a novel highbrightness red-emitting phosphor with high color purity and quantum efficiency for white light-emitting diodes, J. Alloy. Compd. 720 (2017) 29–38. [17] P. Du, J.S. Yu, Self-activated multicolor emissions in Ca2NaZn2(VO4)3:Eu3+ phosphors for simultaneous warm white light-emitting diodes and safety sign, Dyes Pigment. 147 (2017) 16–23. [18] R. Cao, T. Fu, Y. Cao, H. Ao, S. Guo, G. Zheng, Photoluminescence properties and energy transfer of novel red phosphor Sr3P4O13:Eu3+,Bi3+, Mater. Lett 155 (2015) 68–70. [19] M. Shang, S. Liang, H. Lian, J. Lin, Luminescence properties of Ca19Ce(PO4)14:A (A = Eu3+/Tb3+/Mn2+) phosphors with abundant colors: abnormal coexistence of Ce4+/3+-Eu3+ and energy transfer of Ce3+→Tb3+/Mn2+ and Tb3+-Mn2+, Inorg. Chem. 56 (2017) 6131–6140.
Fig. 14. (a) Temperature dependent PL spectra of the CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ phosphors (inset shows normalized PL emission intensity of CGGB:0.05Ce3+, 0.5Tb3+,0.3Eu3+ phosphors as a function of temperature). (b) The Ln[(I0/I) − 1] versus 1/(kT) plot and the calculated activation energy (Ea) for CGGB:0.05Ce3+,0.5Tb3+, 0.3Eu3+ phosphors.
1/kT. The activation energy of CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ phosphors was 0.146 eV. The relatively high activation energy obtained in present work further reveals that the CGGB:Ce3+,Tb3+,Eu3+ phosphors possess good thermal stability and are promising red-emitting phosphors for WLEDs. 4. Conclusions In summary, a series of emission colour tunable CGGB:Ce3+,Tb3+, Eu3+ phosphors have been successfully fabricated by a high temperature solid-state reaction. The Ce3+ → Tb3+ → Eu3+ energy transfer with terbium bridge model was studied in detail. The energy transfer mechanisms of Ce3+ → Tb3+ and Tb3+ → Eu3+ have been demonstrated to be dipole-dipole interactions. Because of the energy transfer, the emission colours of the phosphors can be tuned from blue (0.1661, 0.0686) to green (0.3263, 0.4791) and eventually to red (0.5284, 8
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B. Li, X. Huang [20] S. Lee, S.Y. Seo, Optimization of yttrium aluminum garnet:Ce3+ phosphors for white light-emitting diodes by combinatorial chemistry method, J. Electrochem. Soc. 149 (2002) J85–J88. [21] W.B. Im, Y. Fourre, S. Brinkley, J. Sonoda, S. Nakamura, S.P. DenBaars, R. Seshadri, Substitution of oxygen by fluorine in the GdSr2AlO5:Ce3+ phosphors: Gd1−xSr2+ x AlO5−xFx solid solutions for solid state white lighting, Opt. Express 17 (2009) 22673–22679. [22] G. Blasse, Energy transfer from Ce3+ to Eu3+ in (Y, Gd)F3, Phys. Status Solidi 75 (1983) K41–K43. [23] B. Li, X. Huang, H. Guo, Y. Zeng, Energy transfer and tunable photoluminescence of LaBWO6:Tb3+,Eu3+ phosphors for near-UV white LEDs, Dyes Pigment. 150 (2018) 67–72. [24] D. Wen, J. Shi, M. Wu, Q. Su, Studies of terbium bridge: saturation phenomenon, significance of sensitizer and mechanisms of energy transfer, and luminescence quenching, ACS Appl. Mater. Interfaces 6 (2014) 10792–10801. [25] W.U. Khan, J.H. Li, X.H. Li, Q.L. Wu, J. Yan, Y.Q. Xu, F.Y. Xie, J.X. Shi, M.M. Wu, Efficient energy transfer and luminescence properties of Ca3Y (GaO)3(BO3)4:Tb3+,Eu3+ as a green-to-red colour tunable phosphor under near-UV excitation, Dalton Trans. 46 (2017) 1885–1891. [26] C.H. Huang, T.M. Chen, A novel single-composition trichromatic white-light Ca3Y
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(GaO)3(BO3)4:Ce3+,Mn2+,Tb3+ phosphor for UV-light emitting diodes, J. Phys. Chem. C 115 (2011) 2349–2355. Y. Yu, Q.S. Wu, R.K. Li, Structure of two new borates YCa3(AlO)3(BO3)4 and YCa3(GaO)3(BO3)4, J. Solid State Chem. 179 (2006) 429–432. J.P. Naskar, S. Hati, D. Datta, New bond-valence sum model, Acta Crystallogr. 53 (2010) 885–894. A.M. Srivastava, M.T. Sobieraj, A. Valossis, S.K. Ruan, E. Banks, Luminescence and energy transfer phenomena in Ce3+,Tb3+ doped K 3La (PO4)2, J. Electrochem. Soc. 137 (1990) 2959–2962. K. Li, M.M. Shang, Y. Zhang, J. Fan, H.Z. Lian, J. Lin, Photoluminescence properties of single-component white-emitting Ca9Bi(PO4)7:Ce3+,Tb3+,Mn2+ phosphors for UV LEDs, J. Mater. Chem. C 3 (2015) 7096–7104. X. Zhang, L. Zhou, Q. Pang, J. Shi, M. Gong, Tunable luminescence and Ce3+→Tb3+→ Eu3+ energy transfer of broadband-excited and narrow line red emitting Y2SiO5:Ce3+,Tb3+,Eu3+ phosphor, J. Phys. Chem. C 118 (2014) 7591–7598. X. Huang, B. Li, H. Guo, D. Chen, Molybdenum-doping-induced photoluminescence enhancement in Eu3+-activated CaWO4 red-emitting phosphors for white lightemitting diodes, Dyes Pigment. 143 (2017) 86–94.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Multicolour tunable luminescence of thermal-stable Ce3+/Tb3+/Eu3+triactivated Ca3Gd(GaO)3(BO3)4 phosphors via Ce3+ → Tb3+ → Eu3+ energy transfer for near-UV WLEDs applications Bin Li, Xiaoyong Huang
⁎
Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Phosphors Rare-earth White LEDs Photoluminescence Energy transfer Emission tuning
A series of single-component blue, green and red phosphors have been fabricated based on the Ca3Gd (GaO)3(BO3)4 host through doping of the Ce3+/Tb3+/Eu3+ ions, and their crystal structure and photoluminescence properties have been discussed in detail. A terbium bridge model via Ce3+ → Tb3+ → Eu3+ energy transfer has been studied. The emission colours of the phosphors can be tuned from blue (0.1661, 0.0686) to green (0.3263, 0.4791) and eventually to red (0.5284, 0.4040) under a single 344 nm UV excitation as the result of the Ce3+ → Tb3+ → Eu3+ energy transfer. The energy transfer mechanisms of Ce3+ → Tb3+ and Tb3+ → Eu3+ were found to be dipole-dipole interactions. Importantly, Ca3Gd(GaO)3(BO3)4:Ce3+,Tb3+,Eu3+ phosphors had high internal quantum efficiency. Moreover, the study on the temperature-dependent emission spectra revealed that the Ca3Gd(GaO)3(BO3)4:Ce3+,Tb3+,Eu3+ phosphors possessed good thermal stability. The above results indicate that the phosphors can be applied into white light-emitting diodes as single-component multicolour phosphors.
1. Introduction During the past decades, rare-earth ions based inorganic luminescent materials with excellent luminescent properties have been widely studied and used in various application fields, including lightings, displays, lasers, solar cells, sensors and bioimaging [1–8]. As the situation of global energy shortage and environmental deterioration, much attention has been paid to the research of green lighting. White lightemitting diodes (WLEDs) as a new type of environment friendly lighting source have many advantages, such as low electricity consumption, long service life, small volume and non-pollution. At present, the commercial WLEDs are generally fabricated by combining the yellow Y3Al5O12:Ce3+ phosphors with the blue chips, which suffer from a low colour rendering index (Ra < 80) and a high correlated colour temperature (CCT > 6000 K) due to the absence of red component [9]. Therefore, various solutions to this defect are proposed, like “blue LED combines green and red phosphors” and “near-ultraviolet (NUV) LED combines blue, green and red phosphors” [10–13]. Obviously, red phosphor is essential to all approaches, but the commercially red phosphor (such as Y2O3:Eu3+) shows lower efficiency under UV light excitation [14]. So, the research on red phosphors is urgent.
⁎
Eu3+ ion is very suitable to fabricate red phosphor, due to its perfect colour purity of narrow line red emission [15–18]. However, Eu3+activated phosphors possess narrow-line-shaped excitation peaks in UV region due to the 4f-4f transition, which are not desirable for NUV LED application. Therefore, broadening the excitation band of Eu3+ is an urgent issue. Recently, much attention has been paid to Ce3+ ion as a sensitizer to broaden the excitation of activators ions. As reported by many scientists [19–21]. Ce3+ ions possess strong and broad absorption in NUV area for it spin- and parity-allowed 4f−5d transition, which makes it possible to broaden the excitation of Eu3+ ion via Ce3+ ion. However, the metal-metal charge transfer (MMCT; Ce3+ + Eu3+ → Ce4+ + Eu2+) always exists to prevent the energy transfer from Ce3+ to Eu3+, so sensitizing Eu3+ ions by using the Ce3+ ions in Ce3+/Eu3+ co-doped phosphors has been proven to be unsuccessful [22]. As is known, Tb3+ ion is not only a good sensitizer to Eu3+ ion [23]. but also a good acceptor to Ce3+ [19]. So (Tb3+)n can be described as a bridge to connect Ce3+ with Eu3+ [24]. Furthermore, by controlling the concentration ratio of Ce3+/Tb3+/Eu3+ ions, the emission colour of Ce3+/Tb3+/Eu3+ tri-doped phosphors can be tuned from blue to green and then to red, meaning that it would be easy to control the colour output satisfying application requirements.
Corresponding author. E-mail address: [email protected] (X. Huang).
https://doi.org/10.1016/j.ceramint.2017.12.082 Received 5 December 2017; Received in revised form 11 December 2017; Accepted 12 December 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Li, B., Ceramics International (2017), https://doi.org/10.1016/j.ceramint.2017.12.082
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Borates have been considered as outstanding host materials for inorganic phosphors due to the merits of themselves, such as, high chemical stability, good thermal stability, and low synthetic temperature. Recently, the Ca3Y(GaO)3(BO3)4 was reported as an excellent host for rare-earth luminescence [25,26]. In this host, due to the existence of [BO3] and [GaO6] groups, rare-earth activators can be heavily doped into the matrix without concentration quenching, which is benefit for achieving high luminescence efficiency. However, to the best of our knowledge, there is no report on the luminescent properties of rareearth ions doped Ca3Gd(GaO)3(BO3)4 (hereafter abbreviated as CGGB). In this study, we reported broad-band excited, multicolour tunable emissions from the CGGB:Ce3+,Tb3+,Eu3+ system. Because of energy transfer from Ce3+ to Tb3+ and then to Eu3+ ions, intense visible light is emitted under a single 344 nm excitation wavelength and the emission colour is continuously tuned from blue to red through adjustment of the concentration ratio of Ce3+/Tb3+/Eu3+ ions. The energy transfer mechanism, emission tuning, internal quantum efficiency (IQE) and thermal stability of the as-prepared phosphors were investigated in detail. These broad-band excited, multicolour tunable CGGB:Ce3+, Tb3+,Eu3+ phosphors are promising materials for applications in NUVbased WLEDs.
Fig. 1. XRD patterns of CGGB host and Ce3+, Tb3+, or Eu3+ doped CGGB samples.
2. Experimental A series of Ca3Gd(1-x-y-z)(GaO)3(BO3)4:xCe3+,yTb3+,zEu3+ (hereafter abbreviated as CGGB:xCe3+,yTb3+,zEu3+) samples were successfully fabricated via a conventional high-temperature solid-state reaction technique. H3BO3 (analytical reagent), CaCO3 (analytical reagent), Gd2O3 (99.99%), Ce(NO3)3·6H2O (99.99%), Tb(NO3)3·6H2O (99.99%), Ga2O3 (99.99%) and Eu2O3 (99.99%) were used as the raw materials. According to the stoichiometric ratio, these raw materials were weighted and ground in an agate mortar to achieve uniformity. In order to compensate the volatilization, the amount of H3BO3 is in excess of 5 wt%. Then, these uniform mixtures were put in the alumina crucibles and sintered at 1000 °C for 4 h in CO atmosphere to reduce the Ce ion into tri-valence ion. After that, the furnace cooled down naturally to room temperature, and the final products were ground and collected for further characterization. The X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 X-ray diffractometer using Cu Kα radiation ranging with 5–90° at step rate of 0.02°. The morphology properties of the samples were obtained by a field-emission scanning electron microscope (FESEM; MAIA3 TESCAN) equipped with an energy dispersive X-ray (EDX) spectrum. The diffuse reflection spectra were measured on an UV–vis–NIR spectrophotometer (SHIMADZU UV-2600) attached to an integral sphere and BaSO4 was used as a reference standard. The roomtemperature photoluminescence (PL) and photoluminescence excitation (PLE) spectra and luminescence decay lifetimes of phosphors were measured by Edinburgh FS5 spectrometer equipped with a 150 W continued-wavelength Xenon lamp and a pulsed Xenon lamp, respectively. Temperature-dependent PL spectra were recorded using same spectrophotometer and detectors equipped with a temperature controller. The IQEs of all samples were measured on an Edinburgh FS5 spectrometer equipped with an integrating sphere coated with BaSO4.
Fig. 2. Rietveld XRD refinement of CGGB host.
Table 1 Rietveld refinement results and lattice parameters for Ca3Gd (GaO)3(BO3)4 from the GSAS Rietveld refinement.
3. Results and discussion
Formula
Ca3Gd(GaO)3(BO3)4
Crystal system Space group 2θ-interval,° α,° β,° γ,° a (Å) b (Å) c (Å3) V Z Rwp (%) Rp (%)
Hexagonal P63/m 10–80 90 90 120 10.5529(3) 10.5529(3) 5.8239(2) 561.68(3) 6 6.85% 4.19%
space group P63/m. Moreover, crystallographic data and details were summarized in Table 1, and the refined lattice parameters are a = 10.5529(3) Å, b = 10.5529(3) Å, c = 5.8239(2) Å, and cell volume = 561.68(3) Å, which coincides well with those reported in the literature [27]. The crystal structure of CGGB is shown in Fig. 3. There are two different Ca2+ sites, Ca1 site is coordinated with nine oxygen atoms while Ca2 site is surrounded by seven oxygen atoms. Otherwise, two kinds of tunnels exist in this lattice, in which Ca2 ions occupy hexagonal tunnel site while Ca1 ions occupy trigonal tunnel. According to
3.1. Phase and structure Fig. 1 compares the power XRD patterns of CGGB host and Ce3+/ Tb /Eu3+ doped CGGB phosphors. All XRD patterns can be well indexed to the Ca3Y(GaO3)(BO3)4 (ICSD-172155), and no impurity or significant changes were detected in the host structure. Rietveld refinements of the CGGB host were performed to further analyze the crystal structure details of the as-prepared samples, as shown in Fig. 2. It can be found that CGGB crystallized in a hexagonal unit cell with 3+
2
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Subsequently, we synthesized a series of Ce3+ and Eu3+ co-doped phosphors to study the energy transfer between Ce3+ and Eu3+ ions. Fig. 7 compares the PL spectra of CGGB:0.05Ce3+,yEu3+ (y = 0–0.05) and CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ phosphors under 344 nm excitation. As above-mentioned, efficient energy transfer from Ce3+ to Eu3+ ions in CGGB:Ce3+,Eu3+ phosphors was possible due to the good spectral overlap. But disappointedly, under Ce3+-excitation at 344 nm, the emission of Eu3+ ions was subdued throughout. Moreover, the emission of Ce3+ ions was dramatically quenched with increasing Eu3+ doping concentration, which can be ascribed to the existence of MMCT that quenched the luminescence of the sensitizer (Ce3+ + Eu3+ → Ce4+ + Eu2+) [22]. In order to overcome this MMCT effect, we then introduced Tb3+ ions into CGGB:Ce3+,Eu3+ phosphors because the energy can be transferred though Ce3+ → (Tb3+)n → Eu3+ process. As shown in Fig. 7, tri-doping 5 mol% Tb3+ ions into CGGB:0.05Ce3+ ,0.05Eu3+ phosphor can overwhelmingly improve the emission of Eu3+. The peak emission intensity at 621 nm (5D0 → 7F2 transition of Eu3+ ions) in CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ phosphors was about 56 times as high as that of CGGB:0.05Ce3+,0.05Eu3+ phosphors. Moreover, in the PL spectrum of CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ phosphors, the characteristic transitions of both Tb3+ (5D4 → 7F6 at 485 nm, 5D4 → 7F5 at 544 nm, 5D4 → 7F4 at 585 nm, and 5D4 → 7F3 at 614 nm) and Eu3+ ions (5D0 → 7F1 at 595 nm, 5D0 → 7F2 at 621 nm, 5 D0 → 7F3 at 649 nm, and 5D0 → 7F4 at 692 nm) were observed. In order to further confirm the occurrence of Ce3+ → (Tb3+)n → Eu3+ energy transfer process, we compared the PLE spectra of CGGB:0.1Eu3+ with CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ by monitoring at 621 nm emission, as shown in Fig. 8. The PLE spectrum of CGGB:0.1Eu3+ consisted of the O2+-Eu3+ CT band in the 220–280 nm region and the characteristic PLE peaks in the 270–500 nm wavelength range. In sharp contrast, in the PLE spectrum of CGGB:0.05Ce3+, 0.5Tb3+,0.1Eu3+ phosphors, besides of the PLE peaks of Eu3+ ions, the broad PLE band of Ce3+ ions (4f → 5d at 344 nm) and sharp PLE peaks of Tb3+ ions (7F6 → 5L10 at 370 nm and 7F6 → 5D4 at 485 nm) were also presented. These results strongly revealed the existence of “terbium bridge” in the Ce3+ → (Tb3+)n → Eu3+ energy transfer process. Fig. 9 shows the PL spectra of CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5) and CGGB:0.05Ce3+,0.50Tb3+,zEu3+ (z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors under excitation at 344 nm. As can be seen, the PL spectra of Ce3+/Tb3+ co-doped CGGB samples consisted of blue emission band of Ce3+ ions and the characteristic green emission peaks of Tb3+ ions, due to the energy transfer from Ce3+ to Tb3+. Moreover, with increasing Tb3+ concentration from y = 0.05 to y = 0.5, the emission intensity of Ce3+ ions decreased monotonically, while the emission intensity of Tb3+ ions gradually increased without emission quenching (see the inset (a) of Fig. 9). Therefore, the high threshold of quenching concentration of Tb3+ ions in this CGGB host was very desirable for establishing the Ce3+ → (Tb3+)n → Eu3+ energy transfer system via the terbium bridge, which could realize the narrow-line red emission from Eu3+ ions under broad-band NUV excitation. For the CGGB:0.05Ce3+,0.50Tb3+,zEu3+ samples, once Eu3+ ions were introduced, the emission intensities of Ce3+ and Tb3+ ions decreased continuously, whereas the emission intensity of Eu3+ gradually increased until z = 0.10. However, further increase in the Eu3+ concentration in CGGB:0.05Ce3+,0.50Tb3+,zEu3+ samples resulted in reduced emission intensity of Eu3+ ions, owing to concentration quenching effect, as shown in the inset (b) of Fig. 9.
Fig. 3. Crystal structure of CGGB host.
bond valence sum (BVS) [28], BVS of Ca2+ in trigonal tunnel is much higher than it in hexagonal one, for the charge balance, Ln3+ (Ln = Gd，Ce, Tb and Eu) ions prefer to occupy Ca1 site. Moreover, isolated by [BO3] and [GaO6] groups, the distance of two Ca1 ions is large enough to allow Ln3+ ions highly doping. Fig. 4(a) shows the representative FE-SEM image of CGGB: 0.05Ce3+,0.5Tb3+,0.1Eu3+ phosphors. As can be seen, the studied sample is made up of irregular and aggregated microparticles with the size ranging from 5 to 10 µm. Fig. 4(b) shows the EDX spectrum of the CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ sample. As demonstrated, elements of Ca, Ga, Gd, O, B, Ce, Tb and Eu was observed, which clearly indicated the formation of CGGB:Ce3+,Tb3+,Eu3+ phosphors. Besides, the elemental mapping result (Fig. 4(c)) revealed that the components of Ca, Ga, Gd, O, B, Ce, Tb and Eu were uniformly distributed over the whole range of particles. 3.2. Luminescence properties Fig. 5(a) shows the diffuse reflection spectrum, PLE and PL spectra of CGGB:0.05Ce3+ phosphors. By monitoring at 400 nm, there were two principle excitation bands in the PLE spectrum: the first one was located in the 250–300 nm region and the second one was located in the range of 305–375 nm with a peak at ~ 344 nm, due to 4f → 5d transitions of Ce3+ ions. The sharp PLE peak at 275 nm was attributed to the 8S7/2 → 6I7/2 transition of Gd3+ ions, indicating the energy transfer from Gd3+ to Ce3+ ions in the CGGB:Ce3+ phosphors. The PLE spectrum agreed well with the reflection spectrum. The PL spectrum was a broad emission band centered at 400 nm, corresponding to the transition from the 5d level to the ground state of the Ce3+ ion. In order to obtain the strongest blue emission, we changed the doping concentrations of Ce3+ in CGGB and found the optimal concentration was 5 mol %, as presented in Fig. 5(b). Fig. 6 shows the PLE spectra of CGGB:0.1Tb3+ and CGGB:0.1Eu3+ phosphors. For the CGGB:0.1Eu3+ phosphor by monitored at 621 nm, there are several excitation bands centered at 300, 319, 364, 378, 385, 397, 415 and 466 nm, which were assigned to the electronic transition from the 7F0 ground state to the excited states of 5F2, 5H6, 5D4, 5G3, 5G2, 5 L6, 5D3 and 5D2, respectively. While monitored at 544 nm, the CGGB:0.1Tb3+ phosphor presented a series of excitation bands centered at 309, 315, 320, 344, 354, 370, 381 and 485 nm, corresponding to the 7F6 → 5H5, 7F6 → 5H6, 7F6 → 5H7, 7F6 → 5L7, 7F6 → 5D2, 7F6 → 5 L10, 7F6 → 5D3, and 7F6 → 5D4 transition, respectively. Generally speaking, the energy transfer from sensitizer to activator needs manifest spectral overlap between sensitizer's emission and activator's excitation. Herein this present work, the excitation bands of Tb3+ and Eu3+ ions indeed have large spectral overlaps with the emission of Ce3+, suggesting the possibility of the efficient energy transfer from Ce3+ to Tb3+/Eu3+ in CGGB host.
3.3. Decay curves and energy transfer Fig. 10(a) shows the luminescence decay curves of Ce3+ ions in CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5). The decay curve of the CGGB:0.05Ce3+ can be fitted well with a biexponential function by the following equation:
I = A1 exp(−t / τ1) + A2 exp(−t / τ2) 3
(1)
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Fig. 4. (a) FE-SEM image, (b) EDX spectrum and (c) elemental mapping of CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ phosphors.
which provided strong evidence for the energy transfer from Ce3+ to Tb3+ ions. General speaking, energy transfer via exchange interactions needs a large direct or indirect overlapping between donor and acceptor orbitals leading to easy electronic exchange [29]. However, both Ce3+ and Tb3+ ions are reducing ions, so exchange interaction between Ce3+ and Tb3+ would require too high energies and thus the possibility of such an exchange is much low. Consequently, the mechanism of Ce3+ → Tb3+ energy transfer could be attributed to multipolar interaction. According to Dexter's energy-transfer formula of multipolar interaction, the following relation can be obtained [30]:
where I represents the luminescent intensity and A1 and A2 are constants; t is time, and τ1 and τ2 are the decay times for the exponential components. This result indicated that the Ce3+ ions occupied two Gd3+ sites in the CGGB host, which was in accordance with the practical coordination environments of the Gd3+ ion in the crystal lattice. When the Ce3+ and Tb3+ ions were co-doped into the host, the decay curves deviated from the biexponential function. Therefore, the average decay time τ* was determined by the formula: ∞
τ *=
∫0 tI (t ) dt ∞ ∫0 I (t ) dt
(2)
τS0 ∝ C a/3 τS
The average decay times τ* were calculated to be 26.91, 25.35, 23.54, 18.75 and 14.11 ns for CGGB:0.05Ce3+,yTb3+ phosphors with y = 0, 0.05, 0.1, 0.3 and 0.5, respectively. Thus, the decay lifetimes for the Ce3+ ions decreased monotonically as Tb3+ content increased,
(3)
where C(Tb) is the concentration of Tb3+, and a = 6, 8, or 10 is for dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole interactions, respectively. Plots of τS0/τS and C(Tb)a/3 based on the above 4
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Fig. 7. PL spectra of CGGB:0.05Ce3+,yEu3+ (y = 0–0.05) and CGGB:0.05Ce3+, 0.5Tb3+,0.05Eu3+ phosphors under 344 nm excitation.
Fig. 5. (a) Diffuse reflection, excitation and emission spectra of the CGGB:0.05Ce3+ sample. (b) PL spectra of CGGB:xCe3+ (x = 0.01, 0.03, 0.05, 0.07, and 0.1) phosphors under 344 nm excitation. Fig. 8. PLE spectra of the CGGB:0.1Eu3+ (λem = 621 nm) and CGGB:0.05Ce3+,0.5Tb3+, 0.1Eu3+ (λem = 621 nm) phosphors.
Fig. 6. Comparison of PL spectrum of CGGB:0.05Ce3+ sample (λex = 344 nm) and PLE spectra of CGGB:0.1Tb3+ (λem = 544 nm) and CGGB:0.1Eu3+ (λem = 621 nm).
Fig. 9. PL spectra of the CGGB:Ce3+,Tb3+ and CGGB:Ce3+,Tb3+,Eu3+ (λex = 344 nm). The insets are the dependence of (a) Tb3+ (green) and (b) Eu3+ (red) emission intensity on Tb3+ and Eu3+ contents, respectively.
equation were shown in Fig. 11. The best linear behavior was achieved at a = 6, indicating that energy transfer from Ce3+ to Tb3+ took place via the dipole–dipole interaction. The [Tb3+]n bridge not only can facilitate the energy transfer process in Ce3+ → (Tb3+)n → Eu3+ system, but also as can service as a barrier to prevent the undesirable MMCT effect, which can further be proved by the variation in the decay lifetimes of Ce3+ and Tb3+ activators. Fig. 10(b) presents the luminescence decay curves of Ce3+ ions
in CGGB:0.05Ce3+,0.5Tb3+,zEu3+ (z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors. Compared to the CGGB:0.05Ce3+,0.50Tb3+ sample, the decay times of Ce3+ ions dramatically dropped in the Ce3+/Tb3+/ Eu3+ triactivated CGGB phosphors, revealing that the Ce3+ → Eu3+ energy transfer occurred via “terbium bridge”. In order to further confirm such Ce3+ → (Tb3+)n → Eu3+ energy transfer, the decay 5
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As we know, the mechanism of energy transfer from Tb3+ to Eu3+ ions can be attributed to exchange interaction or electric multipolar interaction. To figure out which interaction dominated in the energy transfer process, the average distance (Rc) between the Tb3+ donors and Eu3+ acceptor ions in CGGB:Ce3+,Tb3+,Eu3+ phosphors was evaluated by using the following equation [31]:
3V 1/3 ⎤ Rc = 2 ⎡ ⎣ 4πCN ⎦
(4) 3+
3+
3+
where C is the total concentration of Ce , Tb and Eu ions, N is coordination number, and V the cell volume. For the CGGB host, the V and N are 561.68 Å3 and 2, respectively. The total concentrations C were found to be 0.56, 0.58, 0.60, 0.65, 0.75 and 0.85 for samples with z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3, respectively. Accordingly, Rc was determined to be 9.86, 9.74, 9.63, 9.38, 8.94 and 8.57 Å for samples with z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3, respectively. Generally, exchange interaction requires a smaller Rc value (< 5 Å), and consequently, the Tb3+ → Eu3+ energy-transfer in CGGB:Ce3+,Tb3+,Eu3+ phosphors would take place via electric multipolar interaction. In order to further determine the Tb3+ → Eu3+ energy-transfer mechanism, by using Eq. (3), the electric multipolar interaction parameter a taking the values 6 (dipole–dipole), 8 (dipole–quadrupole), and 10 (quadrupole–quadrupole) were compared by the dependence of τs0/τs of Tb3+ on C(Eu)a/3, as demonstrated in Fig. 12. A line relation was well-fitted at a = 6, so the energy-transfer mechanism from Tb3+ to Eu3+ ions could be the dipole–dipole interaction. Fig. 13(a) depicts the scheme of energy level model for the Ce3+ → 3+ Tb → Eu3+ energy-transfer process. Upon UV irradiation at Ce3+ ions, electrons from the 2F5/2 ground state are excited into the 5d excited state. Some of these electrons return to the ground states (2F7/2 and 2F5/2) of Ce3+ ions, resulting in the violet-blue emissions due to the 5d → 4f transition. At the same time, owing to the nonradiative resonant energy-transfer, other excited electrons transfer into the 5D4 excited state of Tb3+ ions, and subsequently, the electrons relax to the 7 FJ ground state of Tb3+ ions, giving rise to green emission corresponding to 5D4 → 7FJ (J = 3–6) transitions. With heavily doped Tb3+, Ce3+ → Tb3+ → Eu3+ energy transfer can efficiently take place via the [Tb3+]n bridge. Some of the excited electrons at 5D4 excited state of Tb3+ ions can transfer to the 5D1 excited state of Eu3+ ions, and finally, the electrons relax to Eu3+: 5D0 state and then red emissions due to 5D0 → 7FJ (J = 1–4) transitions are realized. Fig. 13(b) shows the CIE diagram of the CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5) and CGGB:0.05Ce3+,0.50Tb3+,zEu3+ (z = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors. The CIE chromaticity details of these samples were shown in the Table 2. Clearly, the corresponding colours of these samples can be tuned from blue (0.1661, 0.0686) to green (0.3263, 0.4791), and finally to red (0.5284, 0.4040) through codoping Tb3+ and Eu3+ with Ce3+ ions. Moreover, the IQEs of all the phosphor samples were also listed in Table 2. The CGGB:0.05Ce3+ sample showed an IQE as great as 60%, and moreover, codoping Tb3+ into CGGB:Ce3+ phosphor improved the IQE. The CGGB:0.05Ce3+,0.5Tb3+,0.01Eu3+ possessed a high IQE of 55.5%, but further increasing Eu3+ concentration in CGGB:Ce3+,Tb3+,Eu3+ phosphors resulted in reduced IQEs, owing to the energy loss in the Ce3+ → (Tb3+)n → Eu3+ successive energy-transfer process.
Fig. 10. (a) Decay curves of Ce3+ ions in CGGB:0.05Ce3+,yTb3+ (y = 0, 0.05, 0.1, 0.3, and 0.5) phosphors monitoring 400 nm emission. (b) Decay curves of Ce3+ ions in CGGB:0.05Ce3+,0.5Tb3+,zEu3+ (y = 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors ion in monitoring 400 nm emission. (c) Decay curves of Tb3+ CGGB:0.05Ce3+,0.5Tb3+,zEu3+ (z = 0, 0.01, 0.03, 0.05, 0.1, 0.2, and 0.3) phosphors monitoring 544 nm emission.
lifetimes of Tb3+ ions in CGGB:0.05Ce3+,0.50Tb3+,zEu3+ phosphors were also measured, as shown in Fig. 10(c). It can be seen that the decay times of Tb3+ ions decreased fast as the Eu3+ concentration increased. For example, the lifetime of Tb3+ ions in CGGB:0.05Ce3+,0.50Tb3+,0.1Eu3+ sample was reduced to be 0.633 ms, in comparison to 1.575 ms of CGGB:0.05Ce3+,0.50Tb3+ sample. Therefore, the red emission peaks from Eu3+ ions can be attributed to the energy transfer from Tb3+ to Eu3+, due to the shortened decay time of Tb3+ emission with increasing Eu3+ ions.
3.4. Thermal stability of CGGB:Ce3+,Tb3+,Eu3+ phosphors As an important technology parameter of phosphors application, the thermal stability has significant impact on the light output and colour rendering index value of WLEDs. Therefore, the thermal stability of CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ phosphors was investigated. Fig. 14(a) shows the temperature-dependent emission spectra of the CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ sample under 344 nm excitation. With increasing temperature, the profile of the emission bands were 6
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Fig. 11. Dependence of τS0/τS of Ce3+ on (a) CTb6/3, (b) CTb8/3 and (c) CTb10/3.
Fig. 12. Dependence of τS0 /τS of Tb3+ on (a) CEu6/3, (b) CEu8/3 and (c) CEu10/3.
Fig. 13. (a) Illustration of energy levels of Ce3+, Tb3+, Eu3+ ions and Ce3+ → Tb3+ → Eu3+ energy transfer mechanism. (b) CIE chromaticity diagram showing emission colour tuning in CGGB:Ce3+,Tb3+,Eu3+ phosphors under single 344 nm UV excitation. Insets are photographs of the representative phosphors upon excitation under a 365 nm UV lamp.
by following expression [32]:
found to be almost the same, while the emission intensity gradually reduced as the result of the thermal quenching effect. Inset of Fig. 14(a) presents the normalized integrated PL intensity as a function of temperature. The PL intensity at 423 K was calculated to be about 70.96% of that at 298 K, demonstrating that CGGB:Ce3+,Tb3+,Eu3+ phosphors possessed good thermal stability. The activation energy for the thermal quenching can be calculated
I (T ) =
I0 1 + c exp(−Ea/ kT )
(5)
In this expression, I0 and I(T) are the emission intensity at initial temperature and measured temperature T, respectively, c is constant and Ea is the activation energy. Fig. 14(b) shows the plot of ln(I0/I-1) vs. 7
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0.4040) by adjusting the concentration ratio of Ce3+/Tb3+/Eu3+ activators. Furthermore, the CGGB:Ce3+,Tb3+,Eu3+ phosphors possessed high IQEs. Thermal quenching curves showed that the phosphors had good thermal stability. These results indicated that CGGB:Ce3+, Tb3+,Eu3+ can be promising as single-component multi-colour phosphors for application in WLEDs.
Table 2 Variations of CIE chromaticity coordinates (x, y) for CGGB:Ce3+,Tb3+,Eu3+ phosphors excited at 344 nm. Sample
CIE coordinates (x, y)
IQE
CGGB:0.05Ce3+ CGGB:0.05Ce3+,0.05Tb3+ CGGB:0.05Ce3+,0.1Tb3+ CGGB:0.05Ce3+,0.3Tb3+ CGGB:0.05Ce3+,0.5Tb3+ CGGB:0.05Ce3+,0.5Tb3+,0.01Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.03Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.05Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.1Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.2Eu3+ CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+
(0.1661, (0.2068, (0.2370, (0.2980, (0.3263, (0.3459, (0.3763, (0.4001, (0.4681, (0.5138, (0.5284,
60.0% 65.2% 69.3% 75.5% 61.2% 55.5% 50.4% 49.6% 40.1% 25.4% 10.2%
0.0686) 0.1852) 0.2618) 0.4105) 0.4791) 0.4753) 0.4605) 0.4533) 0.4444) 0.4210) 0.4040)
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi, the Startup Research Grant of Taiyuan University of Technology (No. Tyutrc201489a), the Excellent Young Scholars Research Grant of Taiyuan University of Technology (Nos. 2014YQ009, 2015YQ006, and 2016YQ03), and the Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology, No. 2017-skllmd-01). References [1] L. Huang, Y. Zhu, X. Zhang, R. Zou, F. Pan, J. Wang, M. Wu, HF-free hydrothermal route for synthesis of highly efficient narrow-band red emitting phosphor K2Si1–xF6:xMn4+ for warm white light-emitting diodes, Chem. Mater. 28 (2016) 1495–1502. [2] X. Huang, S. Han, W. Huang, X. Liu, Enhancing solar cell efficiency: the search for luminescent materials as spectral converters design, Chem. Soc. Rev. 44 (2013) 173–201. [3] S. Gai, C. Li, P. Yang, J. Lin, Recent progress in rare earth micro/nanocrystals: soft chemical synthesis, luminescent properties, and biomedical applications, Chem. Rev. 114 (2014) 2343–2389. [4] S.F. Lai, Z.W. Yang, J. Li, B. Shao, J.Z. Yang, Y.D. Wang, J.B. Qiu, Z.G. Song, Photoluminescence enhancement of Eu3+ ions by Ag species in SiO2 three-dimensionally ordered macroporous materials, J. Mater. Chem. C 3 (2015) 7699–7708. [5] X. Huang, Enhancement of near-infrared to near-infrared upconversion luminescence in sub-10 nm ultra-small LaF3:Yb3+/Tm3+ nanoparticles through lanthanide doping, Opt. Lett. 40 (2015) 5231–5234. [6] Z. Mao, J. Chen, J. Li, D. Wang, Dual-responsive Sr2SiO4:Eu2+Ba3MgSi2O8:Eu2+,Mn2+ composite phosphor to human eyes and plant chlorophylls applications for general lighting and plant lighting, Chem. Eng. J. 284 (2016) 1003–1007. [7] P. Du, X. Huang, J.S. Yu, Yb3+-concentration dependent upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped Gd2MoO6 nanocrystals prepared by a facile citric-assisted sol–gel method, Inorg. Chem. Front. 4 (2017) 1987–1995. [8] X.Y. Huang, Broadband dye-sensitized upconversion: a promising new platform for future solar upconverter design, J. Alloy. Compd. 690 (2017) 356–359. [9] X.Y. Huang, B. Li, H. Guo, Highly efficient Eu3+-activated K2Gd(WO4)(PO4) redemitting phosphors with superior thermal stability for solid-state lighting, Ceram. Int. 43 (2017) 10566–10571. [10] H. Guan, Y. Sheng, C. Xu, Y. Dai, X. Xie, H. Zou, Energy transfer and tunable multicolor emission and paramagnetic properties of GdF3:Dy3+,Tb3+,Eu3+ phosphors, Phys. Chem. Chem. Phys. 18 (2016) 19807–19819. [11] A. Huang, Z. Yang, C. Yu, Z. Chai, J. Qiu, Z. Song, Tunable and white light emission of a single-phased Ba2Y(BO3)2Cl:Bi3+,Eu3+ phosphor by energy transfer for ultraviolet converted white LEDs, J. Phys. Chem. C 121 (2017) 5267–5276. [12] M. Shang, C. Li, J. Lin, How to produce white light in a single-phase host? Chem. Soc. Rev. 43 (2014) 1372–1386. [13] X.Y. Huang, B. Li, H. Guo, Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors, J. Alloy. Compd. 695 (2017) 2773–2780. [14] J.D. Axe, P.F. Weller, Fluorescence and energy transfer in Y2O3:Eu3+, J. Chem. Phys. 40 (1964) 3066–3069. [15] X. Huang, Solid-state lighting: red phosphor converts white LEDs, Nat. Photonics 8 (2014) 748–749. [16] X. Huang, H. Guo, B. Li, Eu3+ -activated Na2Gd(PO4)(MoO4): a novel highbrightness red-emitting phosphor with high color purity and quantum efficiency for white light-emitting diodes, J. Alloy. Compd. 720 (2017) 29–38. [17] P. Du, J.S. Yu, Self-activated multicolor emissions in Ca2NaZn2(VO4)3:Eu3+ phosphors for simultaneous warm white light-emitting diodes and safety sign, Dyes Pigment. 147 (2017) 16–23. [18] R. Cao, T. Fu, Y. Cao, H. Ao, S. Guo, G. Zheng, Photoluminescence properties and energy transfer of novel red phosphor Sr3P4O13:Eu3+,Bi3+, Mater. Lett 155 (2015) 68–70. [19] M. Shang, S. Liang, H. Lian, J. Lin, Luminescence properties of Ca19Ce(PO4)14:A (A = Eu3+/Tb3+/Mn2+) phosphors with abundant colors: abnormal coexistence of Ce4+/3+-Eu3+ and energy transfer of Ce3+→Tb3+/Mn2+ and Tb3+-Mn2+, Inorg. Chem. 56 (2017) 6131–6140.
Fig. 14. (a) Temperature dependent PL spectra of the CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ phosphors (inset shows normalized PL emission intensity of CGGB:0.05Ce3+, 0.5Tb3+,0.3Eu3+ phosphors as a function of temperature). (b) The Ln[(I0/I) − 1] versus 1/(kT) plot and the calculated activation energy (Ea) for CGGB:0.05Ce3+,0.5Tb3+, 0.3Eu3+ phosphors.
1/kT. The activation energy of CGGB:0.05Ce3+,0.5Tb3+,0.3Eu3+ phosphors was 0.146 eV. The relatively high activation energy obtained in present work further reveals that the CGGB:Ce3+,Tb3+,Eu3+ phosphors possess good thermal stability and are promising red-emitting phosphors for WLEDs. 4. Conclusions In summary, a series of emission colour tunable CGGB:Ce3+,Tb3+, Eu3+ phosphors have been successfully fabricated by a high temperature solid-state reaction. The Ce3+ → Tb3+ → Eu3+ energy transfer with terbium bridge model was studied in detail. The energy transfer mechanisms of Ce3+ → Tb3+ and Tb3+ → Eu3+ have been demonstrated to be dipole-dipole interactions. Because of the energy transfer, the emission colours of the phosphors can be tuned from blue (0.1661, 0.0686) to green (0.3263, 0.4791) and eventually to red (0.5284, 8
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