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Tunable emission of Sr3Sc(PO4)3: Tb3+, Eu3+ phosphors with efficient energy transfer and high thermal stability
Optical Materials 97 (2019) 109397
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Tunable emission of Sr3Sc(PO4)3: Tb3+, Eu3+ phosphors with efficient energy transfer and high thermal stability
T
Jialiang Niua, Nathan Sosb, Ze Zhanga, Wei Zhoua,b,∗ a b
Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing, 100048, China School of Chemistry, Monash University, Melbourne, 3800, Australia
ARTICLE INFO
ABSTRACT
Keywords: Phosphor Luminescence Energy transfer Thermal stability
The color-tunable Sr3Sc(PO4)3:Tb3+, Eu3+ phosphors were synthesized by high-temperature solid-phase method. XRD results indicate that all phosphors doped with and without rare ions are pure orthophosphate in structure. Under 376 nm excitation, the photoluminescence emission spectra show the increase in Eu3+ concentration caused the intensity of Tb3+ emission peak to decrease, and the intensity of Eu3+ emission peak to increase, indicating that the energy transfer occurs from Tb3+ to Eu3+. Based on theoretical calculation, the energy transfer efficiency was found to be about 97.7% and the dipole-quadrupole interaction is assigned to the energy transfer mechanism. By adjusting the Tb3+/Eu3+ ratio, yellow-green, yellow and yellow-orange color shift can be achieved. Furthermore, the phosphor has a quenching temperature of about 553 K and an activation energy of 0.1708 eV.
1. Introduction In recent years, rare earth doped phosphors have attracted great attention in the fields of light-emitting diodes, full-color displays, optical storage, fluorescent lamps, cathode ray tubes and in vivo imaging [1–4]. Particularly, white light-emitting diodes (w-LEDs) have become the fourth-generation lighting source in the global semiconductor and lighting field due to their small size, low energy consumption, high luminous efficiency, long service life and lack of pollution [5–7]. Phosphors are an important part of w-LED and their performance have a great influence on the LED illumination. At present, the most important method to obtain white LEDs commercially is to combine blue InGaN LED chips with yellow phosphors (YAG: Ce3+), but color temperature of the w-LED materials obtained by this method is cold due to lack of red light emission. Cool white light is not conducive to human eye health, and there are other shortages such as low color rendering index (Ra) and high correlated color temperature (Tc) [8,9]. In order to improve Tc, it is adjusted from the cold range (6000–8000 K) to the warm range (3000–5000 K) by adjusted the phosphor concentration, thickness, particle position, morphology, and so on [10]. To improve Ra, one solution is to apply quantum dots (QD) in WLEDs, which can produce Ra as high as about 95 [11]. Another one is to simultaneously red/green/blue (RGB) three-color phosphors by near-ultraviolet (n-UV) chips to emit white light. However, due to the inconsistent
∗
luminescence properties between the various phosphors, the luminous efficiency is low [12,13]. Therefore, co-doped, single-phase, color-adjustable phosphors have attracted interest in improving the color rendering index and correlated color temperature for n-UV w-LEDs, compared to mixed-component phosphors. Studies have shown that by co-doping the sensitizer and activator into the matrix, the energy transfer between the sensitizer and the activator can be used to achieve color-tunable light emission. Eu3+ activator ions can provide strong red emission in the matrix due to their characteristic 5D0→7F2 transition, but their absorption in the ultraviolet (UV) region is weak due to the prohibition of parity transition at 4f-4f [14]. Ce3+ ion is a good sensitizer with strong absorption in the nearultraviolet region, but metal-metal charge transfer (MMCT) in the Ce3+ and Eu3+ co-doped systems hinders the energy transfer from the Ce3+ ions to Eu3+ ions [15]. Tb3+ has previously been reported to be an Eu3+ sensitizer, and imparts a more effective energy transfer from sensitizer to activator species [16]. The Tb3+ emits characteristic blue and green light due to its 5D4→7FJ (J = 6–3) transition under UV/n-UV [17]. When Tb3+ ions and Eu3+ ions are co-doped, due to the wide spectral overlap between their emission bands, efficient energy transfer can be attained in many substrates. This can result in rich sample luminescence properties, as has been demonstrated by Y3Al2Ga3O12: Tb3+, Eu3+ [18], Sr7Zr(PO4)6: Tb3+, Eu3+ [19], CaLaAlO4: Tb3+, Eu3+ [20], GdPO4: Tb3+, Eu3+ [21] and SrLa2(MoO4)4: Tb3+, Eu3+ [22].
Corresponding author. Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing, 100048, China. E-mail address: [email protected] (W. Zhou).
https://doi.org/10.1016/j.optmat.2019.109397 Received 14 June 2019; Received in revised form 23 July 2019; Accepted 18 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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For rare earth luminescent materials, in addition to the doped rare earth ions and the preparation conditions, the matrix also has an important influence on the luminescent properties. An orthophosphate of the formula A3B(PO4)3 (wherein A represents a divalent cation, B represents a trivalent cation) has excellent properties such as a large band gap, covalent bond energy, and a strong disordered structure, which has aroused widespread interest in recent years, such as Sr3Sc(PO4)3:Dy3+ [23], Ba3Lu(PO4):Dy3+ [24]. As far as we know, however, Tb3+, Eu3+ co-doped Sr3Sc(PO4)3 phosphors not been reported yet. In this paper, a series of Sr3Sc(PO4)3: Tb3+, Eu3+ phosphors were synthesized by a conventional solid-phase reaction, which have high energy transfer efficiency, easy preparation, and good chemical stability. The luminescence properties and energy transfer mechanism of Sr3Sc (PO4)3:Tb3+, Eu3+ were systematically analyzed by photoluminescence excitation (PLE) and emission (PL) spectroscopy, concentration quenching, lifetime and CIE chromaticity diagram. The results show that the Sr3Sc(PO4)3: Tb3+, Eu3+ sample is a potential n-UV w-LED phosphor.
formed in the sample preparation and the two rare earth ions entered the crystal lattice without changing the structure of the lattice. Fig. 1(b) is a schematic view showing the crystal structure of Sr3Sc(PO4)3. Sr3Sc (PO4)3 is an orthophosphate, and its crystal structure is cubic, crystallizing in a cubic system of space group I43d (No. 220) [25]. Fig. 2 shows a SEM image of the Sr3Sc(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphor. As can be seen from Fig. 2, the sample consists mainly of solid crystallites. Due to the high temperature solid phase reaction, some agglomeration was induced between the grains. The average particle size is less than 10 μm and the crystalline powder can be used for illumination. 3.2. Luminescence properties of Sr3Sc(PO4)3:Tb3+ Fig. 3 shows the dependence of PL intensity on Tb3+ ion doping concentration. As the dopant content of Tb3+ ions in the matrix increases from 0.03 to 0.25, the overall luminescence intensity of Tb3+ gradually increases. When the concentration of Tb3+ ions exceeds 0.25, the emission intensity of Tb3+ begins to decrease due to self-quenching. Therefore, the optimum doping content of Tb3+ is 0.25. A typical reason for concentration quenching is that as Tb3+ doping increases, an interaction occurs between Tb3+ ions. When the paired distance (Tb3+Tb3+) reaches a certain level, non-radiative energy transfer occurs, resulting in a decrease in luminous intensity [26]. Therefore, the interaction distance equation (1) is used to calculate the critical distance of Tb3+ in this study [27]:
2. Experimental 2.1. Material synthesis The raw materials SrCO3 (AR, Sinopharm Chemical Reagent Co., Ltd., China), Sc2O3 (99.99%, Quzhou Jinding Xincheng Rare Earth New Materials Co., Ltd., China), NH4H2PO4 (AR, Beijing Hongxing Chemical Plant, China), Tb4O7 (99.99%, Shanghai Chemical Reagent Purchasing and Supply Station, China) and Eu2O3 (99.99%, Huizhou High Purity Rare Earth Metal Materials Co., Ltd., China), were weighed according to stoichiometric ratio of Sr3Sc1-x(PO4)3: xTb3+ (x = 0.03, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35 mol) and Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ (y = 0.05, 0.10, 0.20, 0.25, 0.35, 0.45, 0.55 mol). The reagents were transferred to a mortar and ground for 30 min, mixed uniformly. The mixture was transferred to a crucible, then placed in a tube furnace and sintered at 1400 °C for 5 h. The sample was allowed to cool to room temperature, removed and ground again to the appropriate size for characterization.
R2 C = 2
3V 4 Xc N
1/3
(1)
where XC is the critical concentration, V is the volume of the unit cell and N is the number of cations in the unit cell of the crystal unit. For Sr3Sc1-x(PO4)3:xTb3+ phosphors, V = 994.01 Å3, XC = 0.25, N = 4. To further evaluate their interaction, we use the multipole form of Van Uniter (Equation (2)) [28]: q I = K 1 + (x ) 3 x
1
(2)
where I is the emission intensity of the activator and x is the concentration of the activator. β and k are the constants of a given host lattice under the same excitation conditions. q = 3, 6, 8, or 10, and represents the exchange coupling, dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interaction mechanisms, respectively. To estimate q, Fig. 4 shows the relationship of lg(I/x) versus lg(x), and the slope of the fitted line is -q/3, or −1.0137. Therefore, the q of Sr3Sc (PO4)3:xTb3+ is equal to 3.0411, which is approximately equal to 3, indicating that the concentration quenching mechanism of Sr3Sc (PO4)3:xTb3+ is mainly exchange coupling.
2.2. Characterization X-ray powder diffraction (XRD) data was obtained by using a Bruker Axs D2 (30 kV, 10 mA, CuKα, Karlsruhe, Germany) diffractometer with a scan range of 10°–80° for 2θ and a scan rate of 0.08 (°)/s. Photoluminescence PLE and PL spectra were measured by using a Hitachi F-7000 (400 V, 150 W, Xe lamp) fluorescence spectrophotometer. The morphology of samples was observed by scanning electron microscope (SEM, FEI, Quanta FEG 250). The luminescence decay curve was measured by using a spectrophotometer (FluoroLog-3, HORIBA, Jobin-Yvon) with a 370 nm pulsed laser (Spectral-LED N-370) excitation source, with a pulse width of 12 ns. The temperature-dependent luminescence properties were characterized on the same instrument equipped with a self-made heating accessory and a computer controlled electric furnace (Tianjin Orient KOJI Co. Ltd, TAP-02).
3.3. Luminescence properties of Sr3Sc(PO4)3: Tb3+, Eu3+ Fig. 5 is a diffuse reflectance spectrum of the sample. It can be seen from the figure that Sr3Sc(PO4)3 shows a broad absorption band at about 250–350 nm due to matrix absorption. Rare earth-doped samples have weak absorption in the longer wavelength region due to the 4f -4f transition of Tb3+ and Eu3+ ions. Fig. 6 shows the excitation and emission spectra of Sr3Sc0.75(PO4)3: 0.25 Tb3+, Sr3Sc0.55(PO4)3: 0.45Eu3+, and Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphors. Fig. 6(a) shows the PLE and PL spectra of the Sr3Sc0.75(PO4)3: 0.25 Tb3+ sample. When monitored at an emission wavelength of 544 nm, six emission peaks of Tb3+ can be observed between 300 nm and 500 nm, at 317, 340, 351, 368, 376 and 485 nm. Respectively, the peaks are due to 7F6→5H7, 7F6→5L7, 7F6→5D2, 7 F6→5L3, 7F6→5D3 and 7F6→5D4 transitions, and the strongest excitation peak is at 376 nm. The PL spectra were recorded at 376 nm excitation, consisting of four emission peaks at 488, 544, 583, and
3. Result and discussion 3.1. Phase analysis The phase purity of the prepared phosphors was verified by XRD. Fig. 1(a) shows the standard XRD data for Sr3Sc(PO4)3 and XRD pattern of Sr3Sc0.75(PO4)3: 0.25 Tb3+, Sr3Sc0.55(PO4)3:0.45Eu3+ and Sr3Sc0.3(PO4)3:0.25 Tb3+, 0.45Eu3+. Following the principle of conservation of charge and radius similarity, the Sc3+ ions in the matrix are replaced by Tb3+ and Eu3+. As can be seen from Fig. 1(a), the XRD pattern of the sample is consistent with the standard data without additional diffraction peaks. The results show that no impurities were 2
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Fig. 1. (a) XRD patterns of Sr3Sc0.75(PO4)3:0.25 Tb3+, Sr3Sc0.55(PO4)3:0.45Eu3+, Sr3Sc0.3(PO4)3:0.25 Tb3+, 0.45Eu3+ and the standard data Sr3Sc(PO4)3 (JCPDSNo.33-1351) as a reference. (b) Crystal structure of Sr3Sc(PO4)3.
Fig. 2. SEM image of the Sr3Sc(PO4)3:0.25 Tb3+, 0.45Eu3+ sample.
Fig. 3. PL of the Sr3Sc1-x(PO4)3:xTb3+ phosphors. The inset shows the dependence of emission intensity on the Tb3+-doping concentration.
Fig. 4. The dependence of lg(I/x) on lg(x) in Sr3Sc1-x(PO4)3:xTb3+ (x = 0.20,0.25,0.25,0.30,0.35) phosphors.
621 nm, corresponding to 5D4→7FJ (J = 6–3) transitions [14]; the strongest emission peak was observed at 544 nm. It can be seen in Fig. 6(a) and (c) when monitored by 544 nm emission, the PLE spectra of single-doped Tb3+ and co-doped Tb3+/Eu3+ Sr3Sc(PO4)3 samples are almost identical, However, their PL spectra at 376 nm excitation are significantly different. The emission peak of the PL spectrum of Fig. 6(c) contains the blue and green emission of Tb3+ and the orange-red
emission of Eu3+. Fig. 6(b) shows the PLE and PL spectra of Eu3+ iondoped Sr3Sc(PO4)3 phosphors. When excited at 393 nm, the observed emission peaks at 579, 591, 614, 653 and 702 nm correspond to the 5D0 ground state to 7FJ (J = 0–4) [29]. When the emission is detected by 614 nm, the observed excitation peaks are at 361, 381, 393, 414, 464, 534 nm, corresponding to 7F0 to the excited state 5D4, 5L7, 5L6, 5D3, 5D2 and 5D1 levels [30]. 3
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Fig. 7. Dependence of the PL spectra of Sr3Sc0.75-y(PO4)3:0.25 Tb3+, yEu3+ (y = 0.05,0.10,0.15,0.25,0.35,0.45,0.55) samples on the Eu3+ concentrations.
Fig. 5. Diffuse reflection spectra of Sr3Sc1-x-y(PO4)3:xTb3+, yEu3+.
3.4. Energy transfer of Sr3Sc(PO4)3: Tb3+, Eu3+ In general, the energy transfer efficiency (ηT) from Tb3+ to Eu3+ can be calculated by equation (3) [31]: T
=1
(Is / IS 0 )
(3)
where IS0 is the PL intensity of Tb3+ when single-doped, Is is the PL intensity of Tb3+ when Tb3+ and Eu3+ are co-doped and ηT is energy transfer efficiency. It can be seen from Fig. 8 that the energy conversion efficiency (ηT) increases with the increase of the Eu3+ ion-doped concentration. When the Eu3+ ion content is 0.55 mol, the ηT reaches 97.7%, which was higher than the other Tb3+ and Eu3+ are co-doped Y3Al2Ga3O12 (74.2. %) [18], Sr7Zr(PO4)6 (56.7%) [19], CaLaAlO4 (63.77%) [20] phosphors. The results show that there is an effective energy transfer pathway between Tb3+ and Eu3+ in the Tb3+ and Eu3+ co-doped phosphor. Fig. 9 shows the main energy levels of Tb3+ and Eu3+ and the energy transfer process from Tb3+ to Eu3+ in Sr3Sc0.753+ , yEu3+ phosphors. When Tb3+ ions are doped, the y(PO4)3:0.25 Tb ground state electrons are excited to the excited state of 5G5 upon excitation by near-ultraviolet light at 376 nm. Then, through non-radiative relaxation, they reach the lower energy 5D4 level, followed by the
Fig. 6. Emission and excitation spectra of (a) Sr3Sc0.75(PO4)3:0.25 Tb3+, (b) Sr3Sc0.55(PO4)3:0.45Eu3+, and (c) Sr3Sc0.55(PO4)3:0.25 Tb3+, 0.20Eu3+.
Comparing Fig. 6(a), (b), (c), the Eu3+ ions have no absorption peak at 376 nm. This result indicates that Eu3+ absorbs energy from Tb3+. Similarly, compared with Fig. 5(b) and (c), the PLE spectrum of the Tb3+, Eu3+ co-doped sample shows the characteristic peak from the 4f4f transition of the Eu3+ ion, monitored by the 614 nm emission. The emission peaks at 376 and 485 nm from the 4f-4f transition of the Tb3+ ion are shown. Therefore, the above results give evidence that the excitation energy is transferred from Tb3+ to Eu3+. Fig. 7 shows the PL emission spectrum of Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ at 376 nm excitation. The results show that with the increase of Eu3+ concentration, the luminescence intensity (544 nm) of Tb3+ decreases gradually. Conversely, the luminescence intensity (614 nm) of Eu3+ increases monotonously in the concentration range of 0.05–0.45, indicating that energy transfer is occurring. The subsequent intensity decreases after 0.45 Eu3+ concentration is a consequence of the self-quenching effect between Eu3+ ions. The trend of Tb3+ and Eu3+ luminescence intensity with Eu3+ concentration is shown in the inset of Fig. 6. The above results indicate an efficient energy transfer process from Tb3+ to Eu3+.
Fig. 8. Energy transfer efficiency (ηT) from Tb3+ to Eu3+ in Sr3Sc0.753+ , yEu3+ (y = 0.05,0.10,0.15,0.25,0.35,0.45,0.55). y(PO4)3:0.25 Tb 4
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6 3 Eu3 +
Fig. 10. Dependence of IS0/IS of Tb3+ on (a) C
8 3 Eu3 +
(b) C
10 3 . Eu3 +
and (c) C
Fig. 9. Schematic energy-level diagram of Tb3+ and Eu3+ in Sr3Sc(PO4)3 and the energy transfer form Tb3+ to Eu3+. 5
D4→7FJ (J = 3–6) emission transition to the 7FJ ground state. The sample mainly shows green light emission. When co-doped, the energy of the excited electrons of the Tb3+ ion at 5D4 can be transferred to the Eu3+ ion through the non-radiative transfer process to reach the 5D1 excited state of the Eu3+ ion. Then, the excited electrons of the Eu3+ ions relax to the 5D0 level non-radiatively and a subsequent transition to the 7FJ ground state by 5D0→7FJ (J = 3–6), emitting characteristic red light [30]. To gain a deeper understanding of the energy transfer mechanism of Sr3Sc0.75-y(PO4)3:0.25 Tb3+,yEu3+, Equation (1) is used to calculate the critical distance (RC). Here XC represents the total concentration of Tb3+ and Eu3+ and the RC is calculated to be about 8.79 Å, indicating that the energy transfer of the co-doped phosphors is unlikely to proceed through the exchange interaction. The Dexter formula and the approximate formula of Reisfeld (Equation (4)) [32] can be used to calculate the multipole interaction of co-doped phosphors:
Is0 IS
C n /3
(4)
Fig. 11. Luminescence decay curves of Tb3+ in Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ phosphors (λex = 376 nm and λem = 544 nm).
3+
at 376 nm when where Is and Is0 are the emission intensities of Tb Eu3+ doped and undoped, C is the total concentration of Tb3+ and Eu3+, n = 6, 8, 10 respectively correspond to the dipole-dipole, dipolequadrupole and quadrupole-quadrupole interactions with each other. As shown in Fig. 10, when n = 8, the linear relationship between IS0/IS and Cn/3 gives the best match (R2 = 0.99663), indicating the energy transfer from Tb3+ to Eu3+ in the Sr3Sc(PO4)3 matrix is mainly due to the dipole-quadrupole interaction mechanism. In order to further demonstrate the energy transfer process from Tb3+ to Eu3+ in the co-doped sample, Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ (y = 0.05, 0.10, 0.25,0.35,0.45,0.55) was tested at 544 nm. The lifetime values are shown in Fig. 11. The fluorescence lifetime decay curve of the sample was conformed to the double exponential equation (Equation (5)) [33]:
lifetime values were determined to be 2.583, 2.072, 1.901, 0.107, 0.102, 0.097 and 0.094 ms, corresponding to y = 0.05, 0.10, 0.20, 0.25, 0.35, 0.45, 0.55 respectively in the sample. It can be seen that as the concentration of Eu3+ increases, the lifetime decreases gradually, indicating that energy transfer from Tb3+ to Eu3+ in the co-doped samples. 3.5. Thermal stability and CIE of Sr3Sc(PO4)3: Tb3+, Eu3+ The thermal stability of the phosphor has a large influence on the output of brightness and color, which considered to be one of the important indexes of the LED value. Therefore, we measured the temperature-dependent emission spectrum of Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ samples at 376 nm excitation, with temperatures ranging from 293 K to 573 K. The trend of change is shown in Fig. 12. Due to the thermal quenching effect, the emission intensity monotonically decreases with increasing temperature. On the one hand, for the operating temperature of the phosphor in the LED, it will vary with phosphor concentration, thickness, package configuration, drive current, and the like. Ma et al. measured the phosphor temperature used a modified bidirectional thermal resistance model. The driving current interval is
(5)
It = A1 exp( t / 1) + A2 exp( t / 2)
where t is time, I is the luminous intensity of time t, A1 and A2 are constants, and τ1 and τ2 are decay times of the exponential component. The average life (τ) can be calculated by the following equation (6) [33]: avg
= (A1
2 1
+ A2 22)/(A1 1 + A2 2)
(6) 3+
For a series of samples of Sr3Sc0.75-y(PO4)3: 0.25 Tb
, yEu
3+
, the 5
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Table 1 CIE chromaticity coordinates of Sr3Sc(PO4)3: xTb3+, yEu3+ phosphors. Sample no.
Sr3Sc(PO4)3: 0.25 Tb3+, yEu3+
CIE(x, y)
1 2 3 4 5 6 7
x = 0.25 x = 0.25 x = 0.25 x = 0.25 x = 0.25 x = 0.25 x = 0.25
(0.2767, 0.5173) (0.3079, 0.489) (0.3739, 0.4311) (0.399, 0.3982) (0.4402, 0.3787) (0.4575, 0.3694) (0.4413, 0.346)
y = 0.05 y = 0.10 y = 0.20 y = 0.25 y = 0.35 y = 0.45 y = 0.55
Fig. 12. Emission spectra (λex = 376 nm) of Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ at temperature between 20 and 300 °C.
0.05–0.65 A, and the obtained phosphor temperature is 35–134 °C [34]. So, the prepared sample must have excellent thermal stability under such conditions in order to achieve a good application prospect. Generally, the thermal stability is evaluated using the PL intensity of the phosphor at 423K (relative to room temperature). As shown in the inset of Fig. 12, the emission efficiency of the Sr3Sc(PO4)3 phosphor at 423 K is about 81.2%, which was higher than the other reported phosphors, like SrF2: Tb3+, Eu3+ (66.01%) [29], LaBWO6: Tb3+, Eu3+ (51.8%) [35], and CaGd2(WO4)4: Tb3+, Eu3+ (64.8%) [36]. On the other hand, the quenching temperature (which is defined as the temperature at which the emission intensity is half of the room temperature intensity [37]) is calculated to be about 553 K. The above results indicate that the Sr3Sc0.75-y(PO4)3: Tb3+, Eu3+ phosphor has excellent thermal stability. Activation energy is one of the important factors for evaluating thermal stability. To understand the phenomenon of thermal quenching, the activation energy is estimated by the Arrhenius formula [38] (7):
IT E = 1 + Aexp I0 KT
Fig. 14. The CIE chromaticity diagram for Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ phosphors s and insets shows the luminescent photos of (a) Sr3Sc0.7(PO4)3: 0.25 Tb3+, 0.05Eu3+,(b) Sr3Sc0.55(PO4)3: 0.25 Tb3+, 0.20Eu3+,(c) Sr3Sc0.4(PO4)3: 0.25 Tb3+, 0.35Eu3+ and (d) Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.55Eu3+ samples by 254 nm UV-lamp illumination.
1
(7)
In order to better understand the change of luminescent color, the CIE chromaticity coordinates of Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ phosphors were calculated according to their PL emission intensities. The coordinates (x, y) are listed in Table 1. As can be seen from Fig. 14 and Table 1, by adjusting the doping content of Eu3+, the color of Sr3Sc (PO4)3: 0.25 Tb3+, yEu3+ phosphor can be changed from yellowish green to yellow to yellowish orange. In addition, Ra is a number from 0 to 100, and a higher color rendering index means better color rendering or less color shift. CRIs in the range of 75–100 are considered excellent, while 65–75 are good. The range of 55–65 is fair, and 0–55 is poor [39]. By calculation, the Ra of the Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphor was obtained to be 65, which was in a good range. Its Tc was obtained to be 2879K. The results show that the obtained Sr3Sc (PO4)3:Tb3+, Eu3+ phosphor can achieve color regulation under the illumination of single-wavelength light and may potentially be a candidate for the multi-color phosphor of n-UV w-LEDs.
where I0 is the initial PL intensity at room temperature T, IT is the PL intensity at temperature T, K is the Boltzmann constant and ΔE is the activation energy. As shown in Fig. 13, a linear fit of ln(I0/IT-1) to 1/KT yields a slope of −0.1708, so thermally quenched ΔE is 0.1708 eV.
4. Conclusion In summary, this study reported Sr3Sc(PO4)3:Tb3+, Eu3+ phosphors prepared by conventional solid phase synthesis. The PLE and PL spectra indicate that there is an effective energy transfer in the phosphors from Tb3+ to Eu3+ with high efficiency of 97.7%. Under n-UV excitation, the energy transfer mechanism of Tb3+ to Eu3+ ions is a dipole-quadrupole interaction. Both high operating temperature efficiency and high
Fig. 13. The plot of ln[(I0/IT)-1] against 1/kT for Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphor. 6
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quenching temperature indicated excellent heat resistance. In the codoped sample, the sample exhibited a change from yellow-green to yellow to yellow-orange as the concentration of Eu3+ ions increased. These results demonstrate that phosphors have great potential for wLED applications.
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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by Scientific Research Ability Promotion Plan of Graduate Student in 2019 of Beijing Technology and Business University. References [1] J. Zhou, Q.L. Liu, Z.G. Xia, Structural construction and photoluminescence tuning via energy transfer in apatite-type solid-state phosphors, J. Mater. Chem. C. 6 (2018) 4371–4383 https://doi.org/10.1039/C8TC01043A. [2] Z.B. Wang, W.X. Wang, H. Zhou, J.C. Zhang, S.L. Peng, Z.Y. Zhao, Y.H. Wang, Superlong and color-tunable red persistent luminescence and photostimulated luminescence properties of NaCa2GeO4F:Mn2+,Yb3+ phosphor, Inorg. Chem. 55 (2016) 12822–12831 https://doi.org/10.1021/acs.inorgchem.6b02136. [3] J.F. Ruan, Z.W. Yang, A.J. Huang, Z.Z. Chai, J.B. Qiu, Z.G. Song, Red photoluminescent property and modification of WO3:Eu3+ inverse opal for blue light converted LEDs, Opt. Mater. 75 (2018) 224–229 https://doi.org/10.1016/j.optmat. 2017.10.028. [4] S.K. Sharma, D. Gourier, E. Teston, D. Scherman, C. Richard, B. Viana, Persistent luminescence induced by near infra-red photostimulation in chromium-doped zinc gallate for in vivo optical imaging, Opt. Mater. 63 (2017) 51 https://doi.org/10. 1016/j.optmat.2016.06.053. [5] J. Li, J. Yan, D. Wen, W.U. Khan, J.X. Shi, M.M. Wu, Q. Su, P.A. Tanner, Advanced red phosphors for white light-emitting diodes, J. Mater. Chem. C. 4 (2016) 8611–8623 https://doi.org/10.1039/C6TC02695H. [6] Q.Q. Zhu, L. Wang, N. Hirosaki, L.Y. Hao, X. Xu, R.J. Xie, Extra-broad band orangeemitting Ce3+-doped Y3Si5N9O phosphor for solid-state lighting: electronic, crystal structures and luminescence properties, Chem. Mater. 28 (2016) 4829–4839 https://doi.org/10.1021/acs.chemmater.6b02109. [7] Y.X. Zou, L.X. Yu, Z.Y. Gao, J.L. Zhong, Q.H. Guo, Z.J. Liu, Synthesis, crystal structure, and photoluminescence of Eu2+, Ce3+, Mn2+ doped oxynitride phosphors, Opt. Mater. 92 (2019) 411–417 https://doi.org/10.1016/j.optmat.2019.04. 017. [8] V. Bachmann, C. Ronda, A. Meijerink, Temperature quenching of yellow Ce3+ luminescence in YAG:Ce3+, Chem. Mater. 21 (2009) 2077–2084 http://pubs.acs.org/ doi/abs/10.1021/cm8030768. [9] M. Li, J. Zhang, J. Han, Z.X. Qiu, W.L. Zhou, L.P. Yu, Z.Q. Li, S.X. Lian, Changing Ce3+ content and co-doping Mn2+ induced tunable emission and energy transfer in Ca2.5Sr0.5Al2O6:Ce3+, Mn2+, Inorg. Chem. 56 (2016) 241–251 https://doi.org/10. 1021/acs.inorgchem.6b02082. [10] X.B. Luo, R. Hu, S. Liu, K. Wang, Heat and fluid flow in high-power LED packaging and applications, Prog. Energy Combust. Sci. 56 (2016) 1–32 https://doi.org/10. 1016/j.pecs.2016.05.003. [11] B. Xie, R. Hu, X. Luo, Quantum dots-converted light-emitting diodes packaging for lighting and display: status and perspectives, J. Electron. Packag. 138 (2) (2016) 020803https://doi.org/10.1115/1.4033143. [12] Q. Zhou, Y. Zhou, Y. Liu, L.J. Luo, Z.L. Wang, J.H. Peng, J. Yan, M.M. Wu, A new red phosphor BaGeF6:Mn4+: hydrothermal synthesis, photo-luminescence properties, and its application in warm white LED devices, J. Mater. Chem. C. 3 (2015) 3055–3059 https://doi.org/10.1039/C4TC02956A. [13] C. Yoon, T. Kim, M.H. Shin, Y.G. Song, K. Shin, Y.J. Kim, K. Lee, Highly luminescent and stable white light-emitting diodes created by direct incorporation of Cd-free quantum dots in silicone resins using the thiol group, J. Mater. Chem. C. 3 (2015) 6908–6915 https://doi.org/10.1039/C5TC00660K. [14] [a] W.G. Ran, H.M. Noh, B.K. Moon, S.H. Park, J.H. Jeong, J.H. Kim, G.Z. Liu, J.S. Shi, Crystal structure, electronic structure and photoluminescence properties of KLaMgWO6:Eu3+ phosphors, J. Lumin. 197 (2018) 270–276 https://doi.org/10. 1016/j.jlumin.2018.01.053; [b] X.B. Qiao, J. Xin, X.M. Nie, J. Xu, S. Qi, Z.S. Jiang, The structure-dependent luminescence of Eu3+-activated MBaY(BO3)2 (M=Na, K) nanoparticles, Dyes Pigments 143 (2017) 103–111 https://doi.org/10.1016/j.dyepig.2017.04.008. [15] G. Blasse, Energy transfer from Ce3+ to Eu3+ in (Y, Gd)F3, Phys. Status Solidi 75 (1983) K41–K43 https://doi.org/10.1002/pssa.2210750150. [16] B. Li, X. Huang, 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, Ceram. Int. 44 (2018) 4915–4923 https:// doi.org/10.1016/j.ceramint.2017.12.082.
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Tunable emission of Sr3Sc(PO4)3: Tb3+, Eu3+ phosphors with efficient energy transfer and high thermal stability
T
Jialiang Niua, Nathan Sosb, Ze Zhanga, Wei Zhoua,b,∗ a b
Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing, 100048, China School of Chemistry, Monash University, Melbourne, 3800, Australia
ARTICLE INFO
ABSTRACT
Keywords: Phosphor Luminescence Energy transfer Thermal stability
The color-tunable Sr3Sc(PO4)3:Tb3+, Eu3+ phosphors were synthesized by high-temperature solid-phase method. XRD results indicate that all phosphors doped with and without rare ions are pure orthophosphate in structure. Under 376 nm excitation, the photoluminescence emission spectra show the increase in Eu3+ concentration caused the intensity of Tb3+ emission peak to decrease, and the intensity of Eu3+ emission peak to increase, indicating that the energy transfer occurs from Tb3+ to Eu3+. Based on theoretical calculation, the energy transfer efficiency was found to be about 97.7% and the dipole-quadrupole interaction is assigned to the energy transfer mechanism. By adjusting the Tb3+/Eu3+ ratio, yellow-green, yellow and yellow-orange color shift can be achieved. Furthermore, the phosphor has a quenching temperature of about 553 K and an activation energy of 0.1708 eV.
1. Introduction In recent years, rare earth doped phosphors have attracted great attention in the fields of light-emitting diodes, full-color displays, optical storage, fluorescent lamps, cathode ray tubes and in vivo imaging [1–4]. Particularly, white light-emitting diodes (w-LEDs) have become the fourth-generation lighting source in the global semiconductor and lighting field due to their small size, low energy consumption, high luminous efficiency, long service life and lack of pollution [5–7]. Phosphors are an important part of w-LED and their performance have a great influence on the LED illumination. At present, the most important method to obtain white LEDs commercially is to combine blue InGaN LED chips with yellow phosphors (YAG: Ce3+), but color temperature of the w-LED materials obtained by this method is cold due to lack of red light emission. Cool white light is not conducive to human eye health, and there are other shortages such as low color rendering index (Ra) and high correlated color temperature (Tc) [8,9]. In order to improve Tc, it is adjusted from the cold range (6000–8000 K) to the warm range (3000–5000 K) by adjusted the phosphor concentration, thickness, particle position, morphology, and so on [10]. To improve Ra, one solution is to apply quantum dots (QD) in WLEDs, which can produce Ra as high as about 95 [11]. Another one is to simultaneously red/green/blue (RGB) three-color phosphors by near-ultraviolet (n-UV) chips to emit white light. However, due to the inconsistent
∗
luminescence properties between the various phosphors, the luminous efficiency is low [12,13]. Therefore, co-doped, single-phase, color-adjustable phosphors have attracted interest in improving the color rendering index and correlated color temperature for n-UV w-LEDs, compared to mixed-component phosphors. Studies have shown that by co-doping the sensitizer and activator into the matrix, the energy transfer between the sensitizer and the activator can be used to achieve color-tunable light emission. Eu3+ activator ions can provide strong red emission in the matrix due to their characteristic 5D0→7F2 transition, but their absorption in the ultraviolet (UV) region is weak due to the prohibition of parity transition at 4f-4f [14]. Ce3+ ion is a good sensitizer with strong absorption in the nearultraviolet region, but metal-metal charge transfer (MMCT) in the Ce3+ and Eu3+ co-doped systems hinders the energy transfer from the Ce3+ ions to Eu3+ ions [15]. Tb3+ has previously been reported to be an Eu3+ sensitizer, and imparts a more effective energy transfer from sensitizer to activator species [16]. The Tb3+ emits characteristic blue and green light due to its 5D4→7FJ (J = 6–3) transition under UV/n-UV [17]. When Tb3+ ions and Eu3+ ions are co-doped, due to the wide spectral overlap between their emission bands, efficient energy transfer can be attained in many substrates. This can result in rich sample luminescence properties, as has been demonstrated by Y3Al2Ga3O12: Tb3+, Eu3+ [18], Sr7Zr(PO4)6: Tb3+, Eu3+ [19], CaLaAlO4: Tb3+, Eu3+ [20], GdPO4: Tb3+, Eu3+ [21] and SrLa2(MoO4)4: Tb3+, Eu3+ [22].
Corresponding author. Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing, 100048, China. E-mail address: [email protected] (W. Zhou).
https://doi.org/10.1016/j.optmat.2019.109397 Received 14 June 2019; Received in revised form 23 July 2019; Accepted 18 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
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For rare earth luminescent materials, in addition to the doped rare earth ions and the preparation conditions, the matrix also has an important influence on the luminescent properties. An orthophosphate of the formula A3B(PO4)3 (wherein A represents a divalent cation, B represents a trivalent cation) has excellent properties such as a large band gap, covalent bond energy, and a strong disordered structure, which has aroused widespread interest in recent years, such as Sr3Sc(PO4)3:Dy3+ [23], Ba3Lu(PO4):Dy3+ [24]. As far as we know, however, Tb3+, Eu3+ co-doped Sr3Sc(PO4)3 phosphors not been reported yet. In this paper, a series of Sr3Sc(PO4)3: Tb3+, Eu3+ phosphors were synthesized by a conventional solid-phase reaction, which have high energy transfer efficiency, easy preparation, and good chemical stability. The luminescence properties and energy transfer mechanism of Sr3Sc (PO4)3:Tb3+, Eu3+ were systematically analyzed by photoluminescence excitation (PLE) and emission (PL) spectroscopy, concentration quenching, lifetime and CIE chromaticity diagram. The results show that the Sr3Sc(PO4)3: Tb3+, Eu3+ sample is a potential n-UV w-LED phosphor.
formed in the sample preparation and the two rare earth ions entered the crystal lattice without changing the structure of the lattice. Fig. 1(b) is a schematic view showing the crystal structure of Sr3Sc(PO4)3. Sr3Sc (PO4)3 is an orthophosphate, and its crystal structure is cubic, crystallizing in a cubic system of space group I43d (No. 220) [25]. Fig. 2 shows a SEM image of the Sr3Sc(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphor. As can be seen from Fig. 2, the sample consists mainly of solid crystallites. Due to the high temperature solid phase reaction, some agglomeration was induced between the grains. The average particle size is less than 10 μm and the crystalline powder can be used for illumination. 3.2. Luminescence properties of Sr3Sc(PO4)3:Tb3+ Fig. 3 shows the dependence of PL intensity on Tb3+ ion doping concentration. As the dopant content of Tb3+ ions in the matrix increases from 0.03 to 0.25, the overall luminescence intensity of Tb3+ gradually increases. When the concentration of Tb3+ ions exceeds 0.25, the emission intensity of Tb3+ begins to decrease due to self-quenching. Therefore, the optimum doping content of Tb3+ is 0.25. A typical reason for concentration quenching is that as Tb3+ doping increases, an interaction occurs between Tb3+ ions. When the paired distance (Tb3+Tb3+) reaches a certain level, non-radiative energy transfer occurs, resulting in a decrease in luminous intensity [26]. Therefore, the interaction distance equation (1) is used to calculate the critical distance of Tb3+ in this study [27]:
2. Experimental 2.1. Material synthesis The raw materials SrCO3 (AR, Sinopharm Chemical Reagent Co., Ltd., China), Sc2O3 (99.99%, Quzhou Jinding Xincheng Rare Earth New Materials Co., Ltd., China), NH4H2PO4 (AR, Beijing Hongxing Chemical Plant, China), Tb4O7 (99.99%, Shanghai Chemical Reagent Purchasing and Supply Station, China) and Eu2O3 (99.99%, Huizhou High Purity Rare Earth Metal Materials Co., Ltd., China), were weighed according to stoichiometric ratio of Sr3Sc1-x(PO4)3: xTb3+ (x = 0.03, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35 mol) and Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ (y = 0.05, 0.10, 0.20, 0.25, 0.35, 0.45, 0.55 mol). The reagents were transferred to a mortar and ground for 30 min, mixed uniformly. The mixture was transferred to a crucible, then placed in a tube furnace and sintered at 1400 °C for 5 h. The sample was allowed to cool to room temperature, removed and ground again to the appropriate size for characterization.
R2 C = 2
3V 4 Xc N
1/3
(1)
where XC is the critical concentration, V is the volume of the unit cell and N is the number of cations in the unit cell of the crystal unit. For Sr3Sc1-x(PO4)3:xTb3+ phosphors, V = 994.01 Å3, XC = 0.25, N = 4. To further evaluate their interaction, we use the multipole form of Van Uniter (Equation (2)) [28]: q I = K 1 + (x ) 3 x
1
(2)
where I is the emission intensity of the activator and x is the concentration of the activator. β and k are the constants of a given host lattice under the same excitation conditions. q = 3, 6, 8, or 10, and represents the exchange coupling, dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interaction mechanisms, respectively. To estimate q, Fig. 4 shows the relationship of lg(I/x) versus lg(x), and the slope of the fitted line is -q/3, or −1.0137. Therefore, the q of Sr3Sc (PO4)3:xTb3+ is equal to 3.0411, which is approximately equal to 3, indicating that the concentration quenching mechanism of Sr3Sc (PO4)3:xTb3+ is mainly exchange coupling.
2.2. Characterization X-ray powder diffraction (XRD) data was obtained by using a Bruker Axs D2 (30 kV, 10 mA, CuKα, Karlsruhe, Germany) diffractometer with a scan range of 10°–80° for 2θ and a scan rate of 0.08 (°)/s. Photoluminescence PLE and PL spectra were measured by using a Hitachi F-7000 (400 V, 150 W, Xe lamp) fluorescence spectrophotometer. The morphology of samples was observed by scanning electron microscope (SEM, FEI, Quanta FEG 250). The luminescence decay curve was measured by using a spectrophotometer (FluoroLog-3, HORIBA, Jobin-Yvon) with a 370 nm pulsed laser (Spectral-LED N-370) excitation source, with a pulse width of 12 ns. The temperature-dependent luminescence properties were characterized on the same instrument equipped with a self-made heating accessory and a computer controlled electric furnace (Tianjin Orient KOJI Co. Ltd, TAP-02).
3.3. Luminescence properties of Sr3Sc(PO4)3: Tb3+, Eu3+ Fig. 5 is a diffuse reflectance spectrum of the sample. It can be seen from the figure that Sr3Sc(PO4)3 shows a broad absorption band at about 250–350 nm due to matrix absorption. Rare earth-doped samples have weak absorption in the longer wavelength region due to the 4f -4f transition of Tb3+ and Eu3+ ions. Fig. 6 shows the excitation and emission spectra of Sr3Sc0.75(PO4)3: 0.25 Tb3+, Sr3Sc0.55(PO4)3: 0.45Eu3+, and Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphors. Fig. 6(a) shows the PLE and PL spectra of the Sr3Sc0.75(PO4)3: 0.25 Tb3+ sample. When monitored at an emission wavelength of 544 nm, six emission peaks of Tb3+ can be observed between 300 nm and 500 nm, at 317, 340, 351, 368, 376 and 485 nm. Respectively, the peaks are due to 7F6→5H7, 7F6→5L7, 7F6→5D2, 7 F6→5L3, 7F6→5D3 and 7F6→5D4 transitions, and the strongest excitation peak is at 376 nm. The PL spectra were recorded at 376 nm excitation, consisting of four emission peaks at 488, 544, 583, and
3. Result and discussion 3.1. Phase analysis The phase purity of the prepared phosphors was verified by XRD. Fig. 1(a) shows the standard XRD data for Sr3Sc(PO4)3 and XRD pattern of Sr3Sc0.75(PO4)3: 0.25 Tb3+, Sr3Sc0.55(PO4)3:0.45Eu3+ and Sr3Sc0.3(PO4)3:0.25 Tb3+, 0.45Eu3+. Following the principle of conservation of charge and radius similarity, the Sc3+ ions in the matrix are replaced by Tb3+ and Eu3+. As can be seen from Fig. 1(a), the XRD pattern of the sample is consistent with the standard data without additional diffraction peaks. The results show that no impurities were 2
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Fig. 1. (a) XRD patterns of Sr3Sc0.75(PO4)3:0.25 Tb3+, Sr3Sc0.55(PO4)3:0.45Eu3+, Sr3Sc0.3(PO4)3:0.25 Tb3+, 0.45Eu3+ and the standard data Sr3Sc(PO4)3 (JCPDSNo.33-1351) as a reference. (b) Crystal structure of Sr3Sc(PO4)3.
Fig. 2. SEM image of the Sr3Sc(PO4)3:0.25 Tb3+, 0.45Eu3+ sample.
Fig. 3. PL of the Sr3Sc1-x(PO4)3:xTb3+ phosphors. The inset shows the dependence of emission intensity on the Tb3+-doping concentration.
Fig. 4. The dependence of lg(I/x) on lg(x) in Sr3Sc1-x(PO4)3:xTb3+ (x = 0.20,0.25,0.25,0.30,0.35) phosphors.
621 nm, corresponding to 5D4→7FJ (J = 6–3) transitions [14]; the strongest emission peak was observed at 544 nm. It can be seen in Fig. 6(a) and (c) when monitored by 544 nm emission, the PLE spectra of single-doped Tb3+ and co-doped Tb3+/Eu3+ Sr3Sc(PO4)3 samples are almost identical, However, their PL spectra at 376 nm excitation are significantly different. The emission peak of the PL spectrum of Fig. 6(c) contains the blue and green emission of Tb3+ and the orange-red
emission of Eu3+. Fig. 6(b) shows the PLE and PL spectra of Eu3+ iondoped Sr3Sc(PO4)3 phosphors. When excited at 393 nm, the observed emission peaks at 579, 591, 614, 653 and 702 nm correspond to the 5D0 ground state to 7FJ (J = 0–4) [29]. When the emission is detected by 614 nm, the observed excitation peaks are at 361, 381, 393, 414, 464, 534 nm, corresponding to 7F0 to the excited state 5D4, 5L7, 5L6, 5D3, 5D2 and 5D1 levels [30]. 3
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Fig. 7. Dependence of the PL spectra of Sr3Sc0.75-y(PO4)3:0.25 Tb3+, yEu3+ (y = 0.05,0.10,0.15,0.25,0.35,0.45,0.55) samples on the Eu3+ concentrations.
Fig. 5. Diffuse reflection spectra of Sr3Sc1-x-y(PO4)3:xTb3+, yEu3+.
3.4. Energy transfer of Sr3Sc(PO4)3: Tb3+, Eu3+ In general, the energy transfer efficiency (ηT) from Tb3+ to Eu3+ can be calculated by equation (3) [31]: T
=1
(Is / IS 0 )
(3)
where IS0 is the PL intensity of Tb3+ when single-doped, Is is the PL intensity of Tb3+ when Tb3+ and Eu3+ are co-doped and ηT is energy transfer efficiency. It can be seen from Fig. 8 that the energy conversion efficiency (ηT) increases with the increase of the Eu3+ ion-doped concentration. When the Eu3+ ion content is 0.55 mol, the ηT reaches 97.7%, which was higher than the other Tb3+ and Eu3+ are co-doped Y3Al2Ga3O12 (74.2. %) [18], Sr7Zr(PO4)6 (56.7%) [19], CaLaAlO4 (63.77%) [20] phosphors. The results show that there is an effective energy transfer pathway between Tb3+ and Eu3+ in the Tb3+ and Eu3+ co-doped phosphor. Fig. 9 shows the main energy levels of Tb3+ and Eu3+ and the energy transfer process from Tb3+ to Eu3+ in Sr3Sc0.753+ , yEu3+ phosphors. When Tb3+ ions are doped, the y(PO4)3:0.25 Tb ground state electrons are excited to the excited state of 5G5 upon excitation by near-ultraviolet light at 376 nm. Then, through non-radiative relaxation, they reach the lower energy 5D4 level, followed by the
Fig. 6. Emission and excitation spectra of (a) Sr3Sc0.75(PO4)3:0.25 Tb3+, (b) Sr3Sc0.55(PO4)3:0.45Eu3+, and (c) Sr3Sc0.55(PO4)3:0.25 Tb3+, 0.20Eu3+.
Comparing Fig. 6(a), (b), (c), the Eu3+ ions have no absorption peak at 376 nm. This result indicates that Eu3+ absorbs energy from Tb3+. Similarly, compared with Fig. 5(b) and (c), the PLE spectrum of the Tb3+, Eu3+ co-doped sample shows the characteristic peak from the 4f4f transition of the Eu3+ ion, monitored by the 614 nm emission. The emission peaks at 376 and 485 nm from the 4f-4f transition of the Tb3+ ion are shown. Therefore, the above results give evidence that the excitation energy is transferred from Tb3+ to Eu3+. Fig. 7 shows the PL emission spectrum of Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ at 376 nm excitation. The results show that with the increase of Eu3+ concentration, the luminescence intensity (544 nm) of Tb3+ decreases gradually. Conversely, the luminescence intensity (614 nm) of Eu3+ increases monotonously in the concentration range of 0.05–0.45, indicating that energy transfer is occurring. The subsequent intensity decreases after 0.45 Eu3+ concentration is a consequence of the self-quenching effect between Eu3+ ions. The trend of Tb3+ and Eu3+ luminescence intensity with Eu3+ concentration is shown in the inset of Fig. 6. The above results indicate an efficient energy transfer process from Tb3+ to Eu3+.
Fig. 8. Energy transfer efficiency (ηT) from Tb3+ to Eu3+ in Sr3Sc0.753+ , yEu3+ (y = 0.05,0.10,0.15,0.25,0.35,0.45,0.55). y(PO4)3:0.25 Tb 4
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6 3 Eu3 +
Fig. 10. Dependence of IS0/IS of Tb3+ on (a) C
8 3 Eu3 +
(b) C
10 3 . Eu3 +
and (c) C
Fig. 9. Schematic energy-level diagram of Tb3+ and Eu3+ in Sr3Sc(PO4)3 and the energy transfer form Tb3+ to Eu3+. 5
D4→7FJ (J = 3–6) emission transition to the 7FJ ground state. The sample mainly shows green light emission. When co-doped, the energy of the excited electrons of the Tb3+ ion at 5D4 can be transferred to the Eu3+ ion through the non-radiative transfer process to reach the 5D1 excited state of the Eu3+ ion. Then, the excited electrons of the Eu3+ ions relax to the 5D0 level non-radiatively and a subsequent transition to the 7FJ ground state by 5D0→7FJ (J = 3–6), emitting characteristic red light [30]. To gain a deeper understanding of the energy transfer mechanism of Sr3Sc0.75-y(PO4)3:0.25 Tb3+,yEu3+, Equation (1) is used to calculate the critical distance (RC). Here XC represents the total concentration of Tb3+ and Eu3+ and the RC is calculated to be about 8.79 Å, indicating that the energy transfer of the co-doped phosphors is unlikely to proceed through the exchange interaction. The Dexter formula and the approximate formula of Reisfeld (Equation (4)) [32] can be used to calculate the multipole interaction of co-doped phosphors:
Is0 IS
C n /3
(4)
Fig. 11. Luminescence decay curves of Tb3+ in Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ phosphors (λex = 376 nm and λem = 544 nm).
3+
at 376 nm when where Is and Is0 are the emission intensities of Tb Eu3+ doped and undoped, C is the total concentration of Tb3+ and Eu3+, n = 6, 8, 10 respectively correspond to the dipole-dipole, dipolequadrupole and quadrupole-quadrupole interactions with each other. As shown in Fig. 10, when n = 8, the linear relationship between IS0/IS and Cn/3 gives the best match (R2 = 0.99663), indicating the energy transfer from Tb3+ to Eu3+ in the Sr3Sc(PO4)3 matrix is mainly due to the dipole-quadrupole interaction mechanism. In order to further demonstrate the energy transfer process from Tb3+ to Eu3+ in the co-doped sample, Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ (y = 0.05, 0.10, 0.25,0.35,0.45,0.55) was tested at 544 nm. The lifetime values are shown in Fig. 11. The fluorescence lifetime decay curve of the sample was conformed to the double exponential equation (Equation (5)) [33]:
lifetime values were determined to be 2.583, 2.072, 1.901, 0.107, 0.102, 0.097 and 0.094 ms, corresponding to y = 0.05, 0.10, 0.20, 0.25, 0.35, 0.45, 0.55 respectively in the sample. It can be seen that as the concentration of Eu3+ increases, the lifetime decreases gradually, indicating that energy transfer from Tb3+ to Eu3+ in the co-doped samples. 3.5. Thermal stability and CIE of Sr3Sc(PO4)3: Tb3+, Eu3+ The thermal stability of the phosphor has a large influence on the output of brightness and color, which considered to be one of the important indexes of the LED value. Therefore, we measured the temperature-dependent emission spectrum of Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ samples at 376 nm excitation, with temperatures ranging from 293 K to 573 K. The trend of change is shown in Fig. 12. Due to the thermal quenching effect, the emission intensity monotonically decreases with increasing temperature. On the one hand, for the operating temperature of the phosphor in the LED, it will vary with phosphor concentration, thickness, package configuration, drive current, and the like. Ma et al. measured the phosphor temperature used a modified bidirectional thermal resistance model. The driving current interval is
(5)
It = A1 exp( t / 1) + A2 exp( t / 2)
where t is time, I is the luminous intensity of time t, A1 and A2 are constants, and τ1 and τ2 are decay times of the exponential component. The average life (τ) can be calculated by the following equation (6) [33]: avg
= (A1
2 1
+ A2 22)/(A1 1 + A2 2)
(6) 3+
For a series of samples of Sr3Sc0.75-y(PO4)3: 0.25 Tb
, yEu
3+
, the 5
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Table 1 CIE chromaticity coordinates of Sr3Sc(PO4)3: xTb3+, yEu3+ phosphors. Sample no.
Sr3Sc(PO4)3: 0.25 Tb3+, yEu3+
CIE(x, y)
1 2 3 4 5 6 7
x = 0.25 x = 0.25 x = 0.25 x = 0.25 x = 0.25 x = 0.25 x = 0.25
(0.2767, 0.5173) (0.3079, 0.489) (0.3739, 0.4311) (0.399, 0.3982) (0.4402, 0.3787) (0.4575, 0.3694) (0.4413, 0.346)
y = 0.05 y = 0.10 y = 0.20 y = 0.25 y = 0.35 y = 0.45 y = 0.55
Fig. 12. Emission spectra (λex = 376 nm) of Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ at temperature between 20 and 300 °C.
0.05–0.65 A, and the obtained phosphor temperature is 35–134 °C [34]. So, the prepared sample must have excellent thermal stability under such conditions in order to achieve a good application prospect. Generally, the thermal stability is evaluated using the PL intensity of the phosphor at 423K (relative to room temperature). As shown in the inset of Fig. 12, the emission efficiency of the Sr3Sc(PO4)3 phosphor at 423 K is about 81.2%, which was higher than the other reported phosphors, like SrF2: Tb3+, Eu3+ (66.01%) [29], LaBWO6: Tb3+, Eu3+ (51.8%) [35], and CaGd2(WO4)4: Tb3+, Eu3+ (64.8%) [36]. On the other hand, the quenching temperature (which is defined as the temperature at which the emission intensity is half of the room temperature intensity [37]) is calculated to be about 553 K. The above results indicate that the Sr3Sc0.75-y(PO4)3: Tb3+, Eu3+ phosphor has excellent thermal stability. Activation energy is one of the important factors for evaluating thermal stability. To understand the phenomenon of thermal quenching, the activation energy is estimated by the Arrhenius formula [38] (7):
IT E = 1 + Aexp I0 KT
Fig. 14. The CIE chromaticity diagram for Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ phosphors s and insets shows the luminescent photos of (a) Sr3Sc0.7(PO4)3: 0.25 Tb3+, 0.05Eu3+,(b) Sr3Sc0.55(PO4)3: 0.25 Tb3+, 0.20Eu3+,(c) Sr3Sc0.4(PO4)3: 0.25 Tb3+, 0.35Eu3+ and (d) Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.55Eu3+ samples by 254 nm UV-lamp illumination.
1
(7)
In order to better understand the change of luminescent color, the CIE chromaticity coordinates of Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, yEu3+ phosphors were calculated according to their PL emission intensities. The coordinates (x, y) are listed in Table 1. As can be seen from Fig. 14 and Table 1, by adjusting the doping content of Eu3+, the color of Sr3Sc (PO4)3: 0.25 Tb3+, yEu3+ phosphor can be changed from yellowish green to yellow to yellowish orange. In addition, Ra is a number from 0 to 100, and a higher color rendering index means better color rendering or less color shift. CRIs in the range of 75–100 are considered excellent, while 65–75 are good. The range of 55–65 is fair, and 0–55 is poor [39]. By calculation, the Ra of the Sr3Sc0.75-y(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphor was obtained to be 65, which was in a good range. Its Tc was obtained to be 2879K. The results show that the obtained Sr3Sc (PO4)3:Tb3+, Eu3+ phosphor can achieve color regulation under the illumination of single-wavelength light and may potentially be a candidate for the multi-color phosphor of n-UV w-LEDs.
where I0 is the initial PL intensity at room temperature T, IT is the PL intensity at temperature T, K is the Boltzmann constant and ΔE is the activation energy. As shown in Fig. 13, a linear fit of ln(I0/IT-1) to 1/KT yields a slope of −0.1708, so thermally quenched ΔE is 0.1708 eV.
4. Conclusion In summary, this study reported Sr3Sc(PO4)3:Tb3+, Eu3+ phosphors prepared by conventional solid phase synthesis. The PLE and PL spectra indicate that there is an effective energy transfer in the phosphors from Tb3+ to Eu3+ with high efficiency of 97.7%. Under n-UV excitation, the energy transfer mechanism of Tb3+ to Eu3+ ions is a dipole-quadrupole interaction. Both high operating temperature efficiency and high
Fig. 13. The plot of ln[(I0/IT)-1] against 1/kT for Sr3Sc0.3(PO4)3: 0.25 Tb3+, 0.45Eu3+ phosphor. 6
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quenching temperature indicated excellent heat resistance. In the codoped sample, the sample exhibited a change from yellow-green to yellow to yellow-orange as the concentration of Eu3+ ions increased. These results demonstrate that phosphors have great potential for wLED applications.
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