Energy transfer between Ce3+ and Tb3+ and the enhanced luminescence of a green phosphor SrB2O4:Ce3+, Tb3+, Na+

Shaojun Sun; Li Wu; Huan Yi; Liwei Wu; Jingyuan Ji; Chunling Zhang; Yi Zhang; Yongfa Kong; Jingjun Xu

doi:10.1364/OME.6.001172

1. Introduction

White-light emitting diode (w-LED) has attracted increasing attention because of its superior advantages, such as energy saving, high energy efficiency, environmental friendliness, and long operation time [1–6 ]. The traditional method used to obtain wLEDs is to combine a GaN blue LED chip with yellow phosphor YAG:Ce³⁺. However, the deficiency of red emission results in low color-rendering index (CRI<80) and high correlated color temperature (CCT>7000 K), which are important parameters for its application. An alternative way is to combine red, green and blue tricolor phosphors with a GaN/ InGaN chip, which can produce warm white-light emission with high CRI and good color uniformity. Thus, to study the luminescence properties is very important for exploring the new phosphors, especially the UV or NUV converted phosphors [7–11 ].

Tb³⁺ ion is widely used as an important activator for luminescent materials. It emits blue light with low doping concentration and strong green light with high doping concentration because of the transitions of ⁵D₃–⁷F_J and ⁵D₄–⁷F_J (J = 6, 5, and 3), respectively. However, the Tb³⁺ ion exhibits weak absorption in the range of 300–410 nm because 4f–4f transitions are forbidden by the parity selection rule, suggesting that it does not match well with a UV chip. One of the strategies to enhance the intensity of Tb³⁺ emission is co-doping with ions which show broad absorption and emission bands, such as Ce³⁺ or Eu²⁺. Ce³⁺ has a strong excitation band that originates from parity-allowed 4f–5d transitions, which can efficiently absorb the NUV light and transfer the excitation energy to Tb³⁺, and thus resulting in strong sensitized green emission [12–15 ]. Up-to-now, some Ce³⁺ and Tb³⁺ co-doped phosphors have been reported, such as Lu₂Al₅O₁₂:Ce³⁺, Tb³⁺ [16], NaCaBO₃:Ce³⁺, Tb³⁺ [17], Ca₂Al₃O₆F:Ce³⁺, Tb³⁺ [18], AlN:Ce³⁺, Tb³⁺ [19].

SrB₂O₄ was firstly reported by J. B. Kim [20]. It crystallizes in the orthorhombic space group Pnca with lattice parameters a = 6.589 Å, b = 12.018 Å, and c = 4.3373 Å. Though the energy transfer between Ce³⁺ to Tb³⁺ has been studied in some borates phosphors, they normally consist of (BO₃)^3- or (BO₄)^5- group. In this compound, however, the functional group is (B₂O₄)_n ^2n-, which will afford different crystal field environment. Thus, studying the energy transfer from Ce³⁺ to Tb³⁺ in SrB₂O₄ is very important to enlarge the knowledge on the energy transfer from Ce³⁺ to Tb³⁺. Considering the charge balance of the Ce³⁺, Tb³⁺doped phosphor, alkali metal ions, as described in Ref [21], Li⁺, Na⁺ and K⁺, are codoped into the SrB₂O₄:xCe³⁺, yTb³⁺ to compensate the charge unbalance. In this study, Sr_1-x-y-zB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ (x = 0-0.07, y = 0-0.10, z = 0-0.12) were synthesized by solid stated reaction method. The structure analysis and the photoluminescence properties of all these samples have been studied. The energy transfer from Ce³⁺ to Tb³⁺ in this compound has also been elucidated.

2. Experimental

2.1. Samples preparation

The Sr_1-xB₂O₄:xCe³⁺ (0.00≤x≤0.07), SrB₂O₄:0.02Ce³⁺, 0.02M⁺ (M = Li⁺, Na⁺, K⁺), SrB₂O₄:0.03Tb³⁺, Sr_1-x-y-zB₂O₄:xCe³⁺,yTb³⁺,zNa⁺ powder samples were synthesized by a conventional solid-state reaction. CeO₂ (99.95%), Tb₄O₇ (99.99%), SrCO₃ (A.R.), Li₂CO₃ (A.R.), Na₂CO₃ (A.R.), K₂CO₃ (A.R.) and H₃BO₃ (A.R.) were used as starting materials. The materials were weighed and ground in an agate mortar. The mixtures of BaCO₃, Y₂O₃ and H₃BO₃ (excess 10 mol. %) were first heated at 600 °C for 20 h to decompose H₃BO₃ and BaCO₃, and then reground and heated in a platinum crucible at 900 °C for 48 h. The obtained samples were sintered at 900 °C for 6 h in the reducing atmosphere of H₂ (15%) + Ar (85%).

2.2. Characterizations

The phase identification of as-prepared powders was checked at room temperature by a PANalytical X’Pert Pro powder X-ray diffractometer with Cu-Kα radiation (40 kV, 40 mA). Diffuse reflectance spectra were measured by a UV-visible spectrophotometer (Hitachi U-4100) using white BaSO₄ powder as a reference standard. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured by a FLS920 Fluorescence Spectrometer (Edinburgh Instruments) equipped with a Xe light source and double excitation monochromators. The lifetimes were recorded using a hydrogen lamp as a light source and a photomultiplier (R928P) was used as detector. The temperature-dependent luminescence properties were measured on a ﬂuorescence spectrophotometer (F-4600, HITACHI, Japan) with a photomultiplier tube operated at 400 V and a 150 W Xe lamp used as the excitation lamp. The internal quantum efficiency of optimized-composition phosphor SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ was determined on an FLS920 spectrometer under excitation of 319 nm.

3. Results and discussion

3.1. Phase of the as-grown phosphor

The XRD patterns of the SrB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ are shown in Fig. 1(a) . All the patterns are well consistent with that of SrB₂O₄ (ICSD #203226) [20] besides two very small peaks of TbBO₃ (JCPDS No. 24-1272) indexed in the Fig. 1(a) as the doped content is high. The impurity does not affect the result of the experiment because it cannot be activated under 319 nm [22]. As introduced above, SrB₂O₄ crystallizes in the orthorhombic space group Pnca with lattice parameters a = 6.589 Å, b = 12.018 Å, and c = 4.3373 Å. The crystal structure of SrB₂O₄ is shown in Fig. 1(b). It consists of slightly puckered layers with the composition of (B₂O₄)_n ^2n- and each Sr²⁺ atom is coordinated by eight O atoms from neighbouring (B₂O₄)_n ^2n- layers. There are only two cations in the host, Sr²⁺ and B³⁺. The radius of B³⁺ is much smaller than that of Ce³⁺ and Tb³⁺, thus the doped ions will not occupy B³⁺ site. As reported by Shannon [23], the effective ionic radius (r) of Sr²⁺ is 1.26 Å as the coordination number (CN) equals to 8, while r_Ce ³⁺ = 1.143 Å, r_Tb ³⁺ = 1.04 Å, r_Na ⁺ = 1.18 Å, r_K ⁺ = 1.51 Å, r_Li ⁺ = 0.92 Å as CN = 8. Considering the charge balance as well as the crystal environment of the cations, the doped cations are expected to occupy the Sr²⁺ sites, that is the doped Ce³⁺, Tb³⁺ and Na⁺ is coordinated by eight O^2-. The crystal environment of the doped ions is shown in Fig. 2(b) .

Fig. 1 (a) XRD patterns of samples SrB₂O₄:xCe³⁺, yTb³⁺, zNa⁺. (b) Crystal structure of SrB₂O₄ (along Z axis) and the coordination environment of Sr with O atoms (M⁺ = Li⁺, Na⁺, K⁺).

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Fig. 2 Diffuse reflection spectra of sample SrB₂O₄:xCe³⁺, yTb³⁺, zNa⁺.

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3.2. Diffuse reflection spectra of Sr_1-x-y-zB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ powder samples

Figure 2 shows the diffuse reflection spectra of Sr_1-x-y-zB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ powder samples. The absorption around 237 nm and 340 nm are attributed to the typical 4f→5d transition of Tb³⁺ and Ce³⁺, respectively. As doped by Ce³⁺, the absorption around 340 nm is very strong, while the absorption of Tb³⁺ is low as the host doped by these ions. However, the absorption of Tb³⁺ increases greatly with the Tb³⁺ co-doping concentrations changing from 0 mol. % to 10 mol. % when the Ce³⁺ doping concentration is fixed on 2 mol. %.

3.3. Photoluminescence properties of Sr_1-xB₂O₄:xCe³⁺

Figure 3 shows the PLE and PL spectra of SrB₂O₄:0.02Ce³⁺ phosphor. The excitation spectrum consists of broad absorption bands located in the range of 3.8-5 eV (250-320 nm), which is attributed to the 4f-5d transition of Ce³⁺ ions. The transition at about 319 nm shows the maximum intensity. The PL spectrum exhibits a strong asymmetric broad emission band from 2.7 eV (450 nm) to 3.8 eV (325 nm) centered at 360 nm, which corresponds to the transitions from the lowest 5d level to the spin-orbit split ²F_5/2 and ²F_7/2 states of the 4f¹ configuration [12], The emission spectrum can be fitted into two peaks centered at A (3.28 eV, 378 nm) and B (3.52 eV, 352 nm) by Gaussian fitting, labeled as bands A and B. The energy gap between the bands A and B is 0.24 eV (1936 cm⁻¹), which is in good agreement with the theoretical value of Ce³⁺ (2000 cm⁻¹) [12,18 ]. As seen in the inset of Fig. 3, the emission intensity increases with the increasing doping concentration until reaching a maximum of 2 mol. %. After that, the intensity decreases because of the concentration quenching.

Fig. 3 The PLE and PL spectra of phosphor SrB₂O₄:0.02Ce³⁺. The dotted lines A and B are double-peak fitting of Ce³⁺ by Gauss fitting. The inset is photoluminescence intensities of SrB₂O₄:xCe³⁺ as a function of Ce³⁺ contents.

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The normalized emission bands of the SrB₂O₄:xCe³⁺ (x = 0.005–0.07) phosphors indicate that the emission wavelength of Ce³⁺ shows a red shift with increasing doping concentration, as shown in Fig. 4(a) . The inset in Fig. 4(a) shows clearly a red shift of about 12 nm in the emission spectra according to the barycentric wavelength when the x value increases from 0.005 to 0.07, which is mainly ascribed to the expansion of the crystal coordination. As the Sr²⁺ ion with large radius is substituted by the Ce³⁺ ion with small radius, the small Sr²⁺ ion will results in a lattice contraction and results in an increase of the crystal field. Meanwhile, reabsorption and clustering can also lead to a red shift in Ce³⁺ doped phosphor. As a consequence, the emission energies of Ce³⁺ ions decrease [24, 25 ]. Considering of the charge balance of the Ce³⁺ doped phosphor, alkali metal ions, Li⁺, Na⁺ and K⁺, are codoped into the SrB₂O₄:0.02Ce³⁺ to compensate the charge unbalancing. The PL emission spectra of SrB₂O₄:0.02Ce³⁺, 0.02M⁺ (M⁺ = L{Xia, 2013 #417}i⁺, Na⁺, K⁺) are shown in Fig. 4(b). Obviously the luminescent intensity of SrB₂O₄:Ce³⁺ is significantly enhanced as codoped Li⁺, Na⁺, or K⁺ ion. Besides, an obvious red shift can be observed in the emissions of the phosphors with charge compensators and increase in the order of SrB₂O₄:0.02Ce³⁺, 0.02Na⁺ < SrB₂O₄:0.02Ce³⁺, 0.02K⁺ < SrB₂O₄:0.02Ce³⁺, 0.02Li⁺. It is well known that the ionic radius of Li⁺, Na⁺, and K⁺ are 0.92 Å, 1.18 Å, and 1.51 Å, respectively as CN = 8. Thus the ionic radius differences of △₁ = Na⁺- Sr²⁺, △₂ = K⁺- Sr²⁺ and △₃ = Li⁺- Sr²⁺ are 0.08 Å, 0.25 Å and 0.34 Å, respectively, which results in the crystal field with different strength in the order of SrB₂O₄:0.02Ce³⁺, 0.02Na⁺ < SrB₂O₄:0.02Ce³⁺, 0.02K⁺ < SrB₂O₄:0.02Ce³⁺, 0.02Li⁺.So the red shift induced by emission energy decreasing can be seen in the same order. Besides, the intensity of the luminescent emission of the phosphor codoped by Li⁺, Na⁺, and K⁺ has the same variation because of the difference of the crystal field strength from Li⁺, Na⁺, and K⁺.

Fig. 4 (a) The normalized emission bands of the SrB₂O₄:xCe³⁺ phosphors (λ_ex = 319 nm). The inset is the maximum wavelength for each of the SrB₂O₄:xCe³⁺ phosphors (λ_ex = 319 nm). (b) The emission bands of the SrB₂O₄:0.02Ce³⁺, 0.02M⁺ (M⁺ = Li⁺, Na⁺, K⁺) (λ_ex = 319 nm).

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3.4. Energy transfer from Ce³⁺ to Tb³⁺

Figure 5(a) shows the PLE and PL spectra of SrB₂O₄:0.02Ce³⁺ and Fig. 5(b) is the PLE and PL spectra of SrB₂O₄:0.03Tb³⁺. The absorption peaks in the range of 250-300 nm in Fig. 5(b) is attributed to the allowed 4f–5d transition of Tb³⁺, while the absorptions from 300 to 400 nm is assigned to the f–f transitions of Tb³⁺. The PL spectrum shows a series of sharp line emissions at 486, 541, 582, and 620 nm due to the ⁵D₄→⁷F_J (J = 6, 5, 4 and 3) characteristic transitions of Tb³⁺ ions [17, 26 ]. The comparison of the PL spectrum of SrB₂O₄:Ce³⁺ and PLE spectrum of SrB₂O₄:Tb³⁺ reveals an obvious spectral overlap between the emission band of Ce³⁺ centered at 360 nm and the excitation transitions of Tb³⁺ from 330 to 400 nm. Therefore, an effective resonance-type energy transfer from Ce³⁺ to Tb³⁺ is expected. Figure 6 shows a comparison of PLE and PL spectra of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺(a) and the PLE spectrum of SrB₂O₄:0.03Tb³⁺ (b). When excited at 319 nm, the PL spectrum of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ exhibits both a strong asymmetric broad emission band from 320 to 450 nm which belongs to the 5d-4f transitions of Ce³⁺ ions and a series of sharp line emissions at 486, 541, 582 and 620 nm due to the ⁵D₄→⁷F_J (J = 6, 5, 4 and 3) characteristic transitions of Tb³⁺ ions, implying an efficient energy transfer (ET) from Ce³⁺ to Tb³⁺. Moreover, the PLE spectra of the SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ exhibit two broad peaks with similar shape centered at about 319 nm when monitored at 360 and 541 nm, respectively, which is obviously different from SrB₂O₄:0.03Tb³⁺ (the red dotted line), it further demonstrates the ET process from Ce³⁺ to Tb³⁺.

Fig. 5 The excitation and emission spectra of SrB₂O₄:0.02Ce³⁺ (a) and SrB₂O₄:0.03Tb³⁺ (b).

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Fig. 6 The PLE and PL spectra of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ (a) and the PLE spectrum of SrB₂O₄:0.03Tb³⁺ (b) (for comparison, the red dotted line is timed by 30).

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In order to improve the luminescent intensity, Na⁺ codoped SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ phosphor is prepared because Na⁺ ions exhibit the strongest charge compensation ability, as shown in Fig. 4. A comparison of the PL spectra of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ and SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ phosphors on the excitation of 319 nm are shown in Fig. 7 . Obviously the emissions are enhanced significantly with Na⁺ compensator. The inset of Fig. 7 shows the measured and simulated broad emission (2.8-3.8 eV) spectra and decomposed Gaussian components. The decomposed peaks are centered at 3.33 eV (372 nm) and 3.58 eV (346 nm), respectively and labeled as bands C and D with an energy gap of 0.25 eV (2020 cm⁻¹), which is in good agreement with the theoretical value of 2000 cm⁻¹.

Fig. 7 The PL spectra of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ and SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ phosphors (λ_ex = 319 nm). The inset is the double-peak fitting of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ (2.8-3.5 eV) by Gauss fitting.

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A series of phosphors are prepared to investigate the energy transfer between Ce³⁺ and Tb³⁺. The emission spectra of SrB₂O₄:0.02Ce³⁺, yTb³⁺, zNa⁺ phosphors under 319 nm excitation are shown in Fig. 8(a) . The Ce³⁺ emission intensity decreases monotonously with the Tb³⁺ doping concentrations increasing from 0 mol. % to 10 mol. % while the Ce³⁺ doping concentration is fixed at 2 mol. %. In contrast, the sharp emission of Tb³⁺ corresponding to the ⁵D₄–⁷F_J transitions increases with the increasing doping concentration until reaching a maximum when y = 3 mol. %. The intensity of the emission peak of Tb³⁺ decreases as the content of Tb³⁺ (y) is beyond 3 mol. % because of the increasing mutual interaction between the two nearby Tb³⁺ ions with increasing content of the activator ions. The energy transfer efficiency η_T from Ce³⁺ to Tb³⁺ can be expressed by [12]

η_{T} = 1 - I_{s} / I_{s o}

where I_so and I_s are the luminescent intensity of a sensitizer (Ce³⁺) in the absence and presence of an activator (Tb³⁺), respectively. The obtained results of η_T as a function of Tb³⁺ concentrations are plotted in Fig. 8(b), in which η_T increases gradually with increasing Tb³⁺ concentration and reaches to 80.2% for a Tb³⁺ concentration of 10 mol. %.

Fig. 8 (a) The emission spectra of SrB₂O₄:0.02Ce³⁺, yTb³⁺, zNa⁺ phosphors (λ_ex = 319 nm). (b) The normalized emission intensity of Ce³⁺ (5d-4f) and Tb³⁺ (⁵D₄–⁷F_J) as a function of Tb³⁺ contents and the intensities of η_ET as a function of Tb³⁺ contents.

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Generally, two main aspects are considered to be responsible for the resonant energy-transfer mechanism: one is exchange interaction and another one is multi-polar interaction. The exchange interaction energy transfer occurs when the critical distance between sensitizer and activator is shorter than 5 Å [11, 12 ]. Blasse suggests the critical distance R_Ce–Tb can be obtained using the following equation [27]:

R_{C e - T b} \approx 2 {[\frac{3 V}{4 π x_{c} Z}]}^{\frac{1}{3}}

where V is the volume of the unit cell, x_c is the critical concentration, at which the luminescence intensity of Ce³⁺ reduces to half of that for the sample without Tb³⁺, and Z is the number of formula units per unit cell. For SrB₂O₄ host, Z = 4 and V = 343.33 Å³ [20]. The critical concentration in this study is x_c≈0.02 + 0.025 = 0.045. Consequently, the critical distance (R_Ce-Tb) of energy transfer is about 15.4 Å, which is much larger than 5 Å, indicating that the energy transfer should be the multi-polar interaction mechanism.

To further verify the mechanism of ET from Ce³⁺ to Tb³⁺, the relationship between the emission intensity of sensitizer and the doping concentration can be approximately demonstrated by using Dexter's formula of multipolar interaction and Reisfeld's approximation as follows [28]:

\frac{I_{s o}}{I_{s}} \propto C^{n / 3}

where I_so is the intrinsic luminescent intensity of Ce³⁺, I_s is the luminescent intensity of Ce³⁺ in the presence of Tb³⁺ and the factor C is the sum doping concentration of Ce³⁺ and Tb³⁺ ions. In the above expression, n = 6, 8, and 10, corresponding to dipole-dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. The I_so/I_s–C^n/3 curves (n = 6, 8 and 10) are plotted in Fig. 9 . The R² value of the nonlinear curve fit in Fig. 9(a) is estimated to be 0.9709. This value indicates that the curve is most similar to the linear relation when n = 6, suggesting that the energy transfer from the Ce³⁺ to Tb³⁺ ions is the dipole-dipole mechanism. Therefore, the dipole-dipole interaction is the main energy transfer mechanism in Sr_1-x-y-zB₂O₄:xCe³⁺, yTb³⁺, zNa⁺.

Fig. 9 Dependence of I_so/I_s of C^n/3 on the exponent (a) n = 6, (b) n = 8 and (c) n = 10.

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For luminescence application, the quantum efficiency (QE) of a phosphor is often regarded as a measure of its merit. The value of QE can be determined by the integrated method and calculated by the equation [29]:

Q E = \frac{P_{c} - (1 - A_{d i r}) P_{b}}{L_{a} A_{d i r}}

where P_c is the number of photons in the emission wavelength region with sample present in the sphere and in the path of the incident beam, and P_b is the number of photons in the emission wavelength region with the sample present in the sphere, out of the path of the incident beam. The term L_a the integrated excitation profile obtained from the empty integrated sphere (without the sample present). The direct absorbance A_dir can also be calculated by using the equation:

A_{d i r} = 1 - \frac{L_{c}}{L_{b}}

where L_b is the number of photons in the incident wavelength region with the sample present in the sphere but out of the path of the incident beam, and L_c is the number of photons in the incident wavelength region with the sample present in the sphere, directly illuminated by the incident beam. The QE of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphors are determined as 41.3% and 54.7% under 319 nm excitation, respectively. Obviously the QE of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ is significantly large than that of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ due to charge compensation of Na⁺ ion.

To verify an energy transfer existing from Ce³⁺ to Tb³⁺, the decay curves of samples SrB₂O₄:0.02Ce³⁺, yTb³⁺, zNa⁺ (y = 0.00-0.05, z = 0.02-0.07) excited at 319 nm and monitored at 360 nm were measured and shown in Fig. 10 . The luminescent decay curves can be well fitted with a first-order exponential equation described by Blasse and Grabmaier [30]:

I = I_{0} \exp (- \frac{t}{τ})

where I₀ and I are the luminescent intensities at time t = 0 and t = t, respectively, and τ is the fluorescence lifetime. The fact that all the decay curves can be fitted well by single exponential equation confirms that the Ce³⁺ and Tb³⁺ ions occupy only one Sr²⁺ site, which is in accordance with the previous structural discussion. The decay time of Ce³⁺ is found to decrease from 26.50 ns (y = 0.00) to 24.89 ns (y = 0.05) with an increasing concentration of Tb³⁺ ions, which indicates that the co-doping Tb³⁺ ions modify the fluorescent dynamics of the Ce³⁺ ions and give an evidence for the energy transfer from Ce³⁺ to Tb³⁺ in the SrB₂O₄ matrix. However, the decrease of decay time is not particularly large, which is possibly ascribed to the superposition of Ce³⁺ and Tb³⁺ emission (⁵D₃-⁷F₆ transition with longer lifetime) [31] or the doping concentration of Tb³⁺ is too small in comparison to NaCaBO₃:Ce³⁺, Tb³⁺ [17].

Fig. 10 Photoluminescence decay curves of SrB₂O₄:0.02Ce³⁺, yTb³⁺, zNa⁺ phosphors.

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3.5. CIE coordinates of SrB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ phosphors

The CIE chromaticity coordinates of SrB₂O₄:0.02Ce³⁺, yTb³⁺, zNa⁺ phosphors under 319 nm excitation and SrB₂O₄:0.03Tb³⁺ excited at 258 nm were calculated from the emission spectra and presented in Table 1 . Figure 11 shows the CIE chromaticity coordinates of SrB₂O₄:0.02Ce³⁺, 0.02Na⁺ (A), SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ (C), SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ (D) excited at 319 nm and the coordinates of SrB₂O₄:0.03Tb³⁺ (B) excited at 258 nm. The emitting color of phosphor can be tuned from blue to green by changing the codoping Tb³⁺ ions. The CIE chromaticity coordinates of the optimized SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphor is calculated to be (0.3035, 0.4812), located in the green region. Obviously the SrB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ phosphor can be efficiently excited via NUV light to emit strong green light.

Table 1. The CIE Chromaticity Coordinates of SrB₂O₄:xCe³⁺, yTb³⁺, zNa⁺ under the excitation of 319 nm UV and SrB₂O₄:0.03Tb³⁺ excited at 258 nm.

View Table

Fig. 11 CIE chromaticity coordinates of SrB₂O₄:0.02Ce³⁺, 0.02Na⁺ (A), SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺ (C), SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ (D) (λ_ex = 319 nm) and the CIE chromaticity coordinates of SrB₂O₄:0.03Tb³⁺ (B) under (λ_ex = 258 nm).

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3.6. Temperature-dependent PL properties of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺

Figure 12(a) shows the temperature-dependent PL spectra (λ_ex = 319 nm) of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ at the temperature range of 25–300 °C. The blue emission peaks at different temperatures show a similar shape with a slight red shift, as shown in Fig. 12(b). The red shift of the blue emission band is because of the enhancement of the thermal vibration of the host lattice with increasing temperature, which enhances the crystal field splitting and decreases splitting energy of Ce³⁺ (5d-4f). However, the green emission peaks at different temperatures show a similar shape and barely move. These phenomena are attributed to the 5d–4f emissions of the Ce³⁺ ion which depends strongly on the crystal field, while the 4f–4f emissions of the Tb³⁺ ion is almost unaffected by the crystal field. As it can be seen from Fig. 12(c), the intensities of the emission of Ce³⁺ and Tb³⁺ ion are decrease with the temperature increasing and the speed of attenuation is similar, which implies the SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphor has a good color purity. Compared with the intensity of the emission peak at room temperature, the intensities of the green emission peak at 75 °C and 150 °C remained about 81.6% and 59.3%, respectively, which indicates that this phosphor has a good thermal stability.

Fig. 12 (a) The temperature-dependent PL spectra of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ (λ_ex = 319 nm). (b) The normalized emission bands of Ce³⁺ (5d-4f) as a function of temperature contents. (c) The intensity of the emission of Ce³⁺ and Tb³⁺ ion as a function of temperature contents. (d) The ln(I₀/I_T −1) vs. 1/kT activation energy graph for thermal quenching of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺.

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The decrease of the emission intensity of the phosphors at different temperature can be described by the Arrhenius Equation [32, 33 ]:

I_{T} = I_{0} / [1 + \exp (- E_{a} / k T)]

where I₀ and I_T are the green luminescent intensities of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ at room temperature and the measuring temperature, respectively. E_a is activation energy and k is the Boltzmann constant (8.617 × 10⁻⁵ eV K⁻¹). Therefore, E_a can be calculated from the slope of the plot of ln(I₀/I_T −1) vs. 1/kT. As is displayed in Fig. 12(d), E_a of Ce³⁺ and Tb³⁺ ion are obtained to be 0.28 eV and 0.21 eV respectively, which also indicate that the SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphor has good thermal stability.

4. Conclusion

In summary, a novel phosphor SrB₂O₄:Ce³⁺, Tb³⁺, Na⁺ was synthesized through solid state reaction method. The doped Ce³⁺, Tb³⁺, Na⁺ ions occupy one emission center of Sr²⁺ site in SrB₂O₄ host. The phosphors can be efficiently excited by near-ultraviolet light with the wavelength of 319 nm and emit bright green light with good color purity. The co-doped alkali metal ions can obviously enhance the intensity of the luminescence peak. An effective energy transfer from Ce³⁺ to Tb³⁺ exists in the phosphor. The energy transfer efficiency reaches as high as 80.2% for a Tb³⁺ concentration of 10 mol. %. The critical distance was calculated to be 15.4 Å and the ET from the Ce³⁺ to Tb³⁺ ions had been demonstrated to be a resonant type via the dipole–dipole interaction mechanism in the SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphor. The 5d–4f emissions of the Ce³⁺ ion depends strongly on the crystal field while the 4f–4f emissions of the Tb³⁺ ion is the opposite by analyzing the temperature-dependent PL spectra. The QE of SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphors are 41.3% and 54.7% under 319 nm excitation, respectively. Compared to the intensity of the emission peak at room temperature, the green emission intensities at 75 °C and 150 °C remained about 81.6% and 59.3%. The CIE chromaticity coordinate of the optimized SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺ phosphor was calculated to be (0.3035, 0.4812), which indicates that the SrB₂O₄:Ce³⁺, Tb³⁺, Na⁺ phosphor had a good thermal stability and stable CIE coordinates.

Acknowledgments

This work was financially supported by National Natural Science Foundation of China (51372121, 61274053, 51572132, U146020005, 91222111 and 51572132), and Tianjin Research Program of Application Foundation and Advanced Technology (14JCYBJC17800, 12JCYBJC10900). It was also supported by International Science and Technology Cooperation Program of China (2013DFG52660), and National Basic Research Program of China (2013CB328706).

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Point	Samples	CIE (X, Y)
1	SrB₂O₄:0.02Ce³⁺, 0.02Na⁺	(0.232, 0.120)
2	SrB₂O₄:0.03Tb³⁺	(0.297, 0.431)
3	SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺	(0.301, 0.516)
4	SrB₂O₄:0.02Ce³⁺, 0.01Tb³⁺, 0.02Na⁺	(0.306, 0.453)
5	SrB₂O₄:0.02Ce³⁺, 0.03Tb³⁺, 0.05Na⁺	(0.304, 0.481)
6	SrB₂O₄:0.02Ce³⁺, 0.05Tb³⁺, 0.07Na⁺	(0.299, 0.466)
7	SrB₂O₄:0.02Ce³⁺, 0.07Tb³⁺, 0.09Na⁺	(0.297, 0.437)
8	SrB₂O₄:0.02Ce³⁺, 0.10Tb³⁺, 0.12Na⁺	(0.299, 0.466)

Energy transfer between Ce³⁺ and Tb³⁺ and the enhanced luminescence of a green phosphor SrB₂O₄:Ce³⁺, Tb³⁺, Na⁺

Abstract

1. Introduction