Highly efficient and thermal stable La2Si2O7:Ce3+,Tb3+,Eu3+ phosphors: Emission color tuning through terbium bridge

Journal of Alloys and Compounds 785 (2019) 53e61

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Highly efficient and thermal stable La2Si2O7:Ce3þ,Tb3þ,Eu3þ phosphors: Emission color tuning through terbium bridge Wentao Ma a, b, Jilin Zhang a, b, *, Xujian Zhang a, b, Xinguo Zhang c, Yongfu Liu d, **, Shuzhen Liao e, Shixun Lian a, b a

Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), Hunan Normal University, Changsha 410081, China Key Laboratory of Sustainable Resources Processing and Advanced Materials of Hunan Province College, Hunan Normal University, Changsha 410081, China c Guangdong Provincial Key Laboratory of New Drug Screening, School of Pharmaceutical Sciences, Southern Medical University, Guangzhou 510515, China d Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Ningbo, 315201, China e School of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 August 2018 Received in revised form 29 December 2018 Accepted 15 January 2019 Available online 17 January 2019

A series of emission color tunable Ce3þ, Tb3þ, and Eu3þ co-doped La2Si2O7 (LSO) phosphors have been prepared via a solid state reaction. The crystal structure, luminescence properties, and thermal stability of the phosphors were studied in this paper. There exists two kinds of energy transfer (ET) modes under UV excitation, namely, electric dipole-dipole interaction for Ce3þ/Tb3þ and terbium bridge model [Ce3þ/(Tb3þ)n/Eu3þ] for LSO:Ce3þ,Tb3þ,Eu3þ system. The emitting color of the phosphors can be tuned from green to yellow and eventually to orange-red through the Ce3þ/Tb3þ/Eu3þ energy transfer. Although codoping Tb3þ and Eu3þ into La2Si2O7:Ce3þ leads to the reduction of thermal stability, it still remains 80% for LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ at 150  C when compared to that at room temperature. The absolute quantum yield (QY) increases from 71.04% (LSO:0.05Ce3þ) to 95.60% (LSO:0.05Ce3þ, 0.6 Tb3þ), while those of LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ samples remain more than 70%. These results indicate that the phosphors can be candidates for application in phosphor-converted white lightemitting diodes (pc-WLEDs). © 2019 Elsevier B.V. All rights reserved.

Keywords: Phosphor Energy transfer Luminescence pc-LEDs Terbium bridge

1. Introduction The new generation solid-state lighting, white light-emitting diodes (w-LEDs) possess advantages such as energy conservation, environmental friendliness, and long lifetime, etc, and have been used worldwide [1e8]. The commercial phosphor-converted wLEDs (pc-WLEDs) are the combination of a blue LED chip and the yellow phosphor (Y3Al5O12:Ce3þ). However, the color-rendering index (CRI) of this combination is usually lower than 80 owing to the deficiency in red-emitting component [9]. Alternatively, an ultraviolet (UV) or near-UV LED chip combined with blue, green,

* Corresponding author. Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), Hunan Normal University, Changsha 410081, China. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected]. cn (Y. Liu). https://doi.org/10.1016/j.jallcom.2019.01.194 0925-8388/© 2019 Elsevier B.V. All rights reserved.

and red phosphors is used to realize warm white light that has a high CRI value by adjusting the radio of tricolor phosphors. Eu3þ activated red phosphor have been paid much attention due to excellent color purity produced by 5D0/7F2 transition. Nevertheless, it necessary to find sensitizers for Eu3þ luminescence due to the weak and narrow linear absorption originated from the forbidden 4f-4f transitions. As is known to all, Ce3þ is an excellent activator that has strong and broad tunable absorption bands in nUV or blue region due to its allowed 4f-5d transition [10e15]. Furthermore, Ce3þ is an efficient sensitizer that can transfer its excitation energy to coactivators [16e19]. Unfortunately, Eu3þ can hardly be sensitized directly by Ce3þ owing to metal-metal charge transfer effect (MMCT) [20]. Recently, a Ce3þ/(Tb3þ)n/Eu3þ terbium chain has been put forward by A.A. Setlur to alleviate the MMCT effect and to enhance red emission by energy transfer [21], where Tb3þ serves as an intermediate bridge to transfer the excitation energy of Ce3þ to Eu3þ. Some novel red and color-tunable phosphors have been successfully synthesized by using this

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method, such as Ce3þ,Tb3þ,Eu3þ co-doping in YBO3 [22], Na2Y2B2O7 [23], GdBO3 [24], and KBaY(BO3)2 [25], etc. Li et al. summarized the utilization of this method towards the enhancement of red emission [26]. La2Si2O7 is well-known for its polymorphism [27]. Monoclinic Ce3þ-doped La2Si2O7 exhibits excellent light yield as a novel scintillator material [28], while tetragonal Ce3þ-doped La2Si2O7 has been synthesized and applied as a novel protective pigment [29]. As far as we know, there are no reports on the energy transfer and other luminescence properties of La2Si2O7:Ce3þ,Tb3þ,Eu3þ phosphors. Therefore, the phase characterization, photoluminescence properties of La2Si2O7:Ce3þ,Tb3þ,Eu3þ are investigated in this work. The results suggest that the terbium bridge has been successfully built up and emission of Eu3þ is greatly enhanced. Besides, the emitting color can be adjusted from ultraviolet to green and finally orange-red by altering the concentrations of the three activators. The good thermal stability and high quantum yield of the color tunable phosphors make them applicable to pc-WLEDs. 2. Experimental 2.1. Materials and synthesis La2-x-y-zSi2O7:xCe3þ,yTb3þ,zEu3þ (abbreviated as LSO:xCe3þ,yTb3þ,zEu3þ) powders were synthesized by a solid-state reaction. In a typical process, stoichiometric amounts of rare earth oxides (La2O3, CeO2, Tb4O7, and Eu2O3, all with a purity of 99.99%) and SiO2 (AR) were mixed thoroughly and ground in an agate mortar for 30 min with a certain volume of ethanol. Then the dried mixed powders were placed in corundum crucibles and maintained at 1400  C in a tube furnace for 3 h under an atmosphere containing 5% H2 and 95% N2. Finally, phosphors were obtained after cooled down to room temperature in the furnace. 2.2. Measurements and characterization An X-ray powder diffractometer (PANalytical X'Pert Pro) with Cu Ka radiation was used to collect the X-ray diffraction (XRD) patterns of the as-prepared phosphors, which is operated at 40 kV and 40 mA. An F4500 spectrophotometer (Hitachi) equipped with a TAP-02 high temperature controller (Orient KOJI) was used to record the photoluminescence excitation (PLE), photoluminescence (PL), and the temperature-dependent PL spectra of the phosphors. An FLS980 Fluorescence Spectrometer (Edinburgh) was also used to record the PL spectra, decay curves and absolute quantum efficiencies (QE).

3. Results and discussion 3.1. Phase characterization Fig. 1a and Fig. S1 in Supporting Information illustrate the XRD patterns of LSO:xCe3þ, LSO:0.05Ce3þ,yTb3þ, and LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ. The diffraction peaks of the Ce3þdoped samples as shown in Fig. 1a can be indexed to monoclinic La2Si2O7 [space group P21/c (14), JCPDS no. 82-0729], except the additional peak belonging to SiO2 around 22 . The introduction of Ce3þ ions leads to little changes of the host lattice. The XRD profiles of LSO:0.05Ce3þ,yTb3þ and LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ show different variation by changing doping content from Ce3þ singledoped samples. The pure La2Si2O7 phase can be obtained when y is in the range from 0 to 1.0, while some excess diffraction peaks arise that refers to (Y,Th)2Si2O7 monoclinic phase [space group P21/ n (14), JCPDS no. 24-1427] when y is larger than 1.0, as shown in Fig. S1a and Fig. S1b. Fig. 1b presents a unit cell of La2Si2O7 and coordination polyhedra of La and Si atoms. In the lattice of La2Si2O7, there are two La3þ crystallographic sites, namely, La(1) and La(2) both coordinated with eight O atoms. The polyhedra of La(1) and La(2) are similar with each other as shown in Fig. 1b. The effective ionic radii (IR) for the trivalent rare earth ions with the same coordination number (CN) are close to each other. Therefore, Ce3þ, Tb3þ, and Eu3þ may have probability to occupy both La sites. 3.2. Photoluminescence properties The PL spectra of LSO:xCe3þ (x ¼ 0.01  0.07) contain two emission bands peaking at 355 and 378 nm under the excitation by the peak value of PLE band at 332 nm, which are shown in Fig. 2a. These two PL bands have an energy difference of 1714 cm1, which is close to that between the sublevels of 4f, namely, 2F5/2 and 2F7/2 indicating the typical PL bands of Ce3þ. The excitation spectra monitored at 355 and 378 nm present similar profiles, which both contain four bands peaking at 332, 315, 290, and 260 nm corresponding to the transition from 4f to the four different 5d levels of Ce3þ, respectively. The PL spectra under excitation at the above four different wavelengths are shown in Fig. S2, which suggest that there should exist only one Ce3þ luminescence center inferred from the same profile of the PL bands. This can be also demonstrated by the similar decay curves monitored at 355 and 378 nm, which are shown in both Fig. 2bec and Fig. S2c. The decay curves are well fitted with a single-exponential function as shown in Fig. S3, which verifies one Ce3þ luminescence center formed in LSO host. Perhaps the similarity of La(1) and La(2) sites leads to the one-site

Fig. 1. (a) XRD patterns of LSO:xCe3þ (x ¼ 0.01e0.07), (b) Crystal structure of La2Si2O7 host and coordinated environments of La atoms.

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Fig. 2. (a) The PLE and PL spectra of LSO:xCe3þ, (b, c) decay curves of Ce3þ in LSO:xCe3þ monitored at 355 and 378 nm, respectively.

luminescent behavior for Ce3þ. Moreover, the PL intensities obviously increase with Ce3þ concentration and maximize at x ¼ 0.05, then decrease over 0.05 ascribed to the concentration quenching effect (see Fig. 2a and Fig. S2b). Generally, Tb3þ acts as a green emitting center in phosphors with the transitions of 5D4/7FJ (J ¼ 6, 5, 4, 3, and 2) in green region [30,31]. The PLE spectrum of LSO:0.02 Tb3þ contains several excitation bands in the 200e500 nm range when monitored at 542 nm (Fig. 3a). Under excitation of 233 nm, LSO:0.02 Tb3þ emits green light, whose emission spectrum consists of transitions lines of the Tb3þ f/f transitions at 380, 415, 436, and 458 nm in the blue region, and 490, 542, 584, and 620 nm in 475e650 nm region (Fig. 3a). The former relates to 5D3/7FJ transitions of Tb3þ and the

latter belongs to 5D4/7FJ transitions. An overlap between the PLE band of Tb3þ and the PL band of Ce3þ as shown in Fig. 3b and c suggests the possibility of energy transfer (ET) from Ce3þ ion to Tb3þ. It can be proved by the similar PLE spectra in profile monitored at 380 and 542 nm in LSO:Ce3þ,Tb3þ phosphors from Fig. 3d and e. Note that excitation band intensity belonging to Tb3þ f/d transitions decreases in Ce3þ, Tb3þ co-doped samples, which indicates that the emission of Tb3þ occurs mainly through the energy transfer from Ce3þ. Fig. 4a shows the PL spectra of LSO:Ce3þ,Tb3þ. Upon excitation at 332 nm, the PL spectra consist of not only the typical Ce3þ emission band but also the sharp emission bands of Tb3þ at 490, 542, 584, and 620 nm. The emission of Ce3þ become weakens gradually with increasing the content of Tb3þ (Fig. 4b),

Fig. 3. (a) The PLE and PL spectra of LSO:0.02 Tb3þ, Comparison in PLE spectra of (b) Tb3þ single-doped LSO phosphor (lem ¼ 542 nm), (c) PL (lex ¼ 332 nm) and PLE (lem ¼ 380 nm) spectra of Ce3þ single-doped phosphor, and (d, e) PLE spectra of LSO:Ce3þ,Tb3þ co-doped phosphor.

Fig. 4. (a) PL spectra of LSO:0.05Ce3þ,yTb3þ (y ¼ 0e1.0) and (b) emission intensities of Ce3þ and Tb3þ versus Tb3þ content, the inset in (a) represents the schematic diagram of Ce3þto-Tb3þ ET.

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Fig. 5. (a) Decay curves of Ce3þ in LSO:0.05Ce3þ,yTb3þ phosphors monitored at 355 nm. (b) Ce3þ-to-Tb3þ ET efficiency versus Tb3þ content.

while that of Tb3þ increases firstly, maximizes at y ¼ 0.6, and then decreases. These results further demonstrate the existence of Ce3þto-Tb3þ energy transfer. Finally, LSO:Ce3þ,Tb3þ emits green light because the green emission peaking at 542 nm dominates. Furthermore, 5D3/7FJ transitions emissions of Tb3þ do not show similar intensity trend with the increasing content of Tb3þ and maximize at y ¼ 0.2. It can be explained by concentration quenching of 5D3/7FJ emission at low doping concentration due to cross relaxation between two Tb3þ ions [30,32]. Fig. 5a depicts the decay curves of Ce3þ in LSO:0.05Ce3þ,yTb3þ. In the time range of 0e200 ns, faster degradation of Ce3þ emission is observed along with increasing of Tb3þ content, and the decay curves can be fitted with a bi-exponential function. The decrease of decay times with increasing Tb3þ content indicate the existence of Ce3þ-to-Tb3þ ET. To further understand the ET process, the ET efficiency (hT) were calculated by the equation [33,34],

I hT ¼ 1  S IS0

(1)

herein, IS0 and IS correspond to the PL intensities of Ce3þ in singledoped and Ce3þ,Tb3þ co-doped phosphor, respectively. The PL intensity at 378 nm is adopted in the calculation. The calculated ET efficiency is plotted as a function of Tb3þ content (Fig. 5b), which increases gradually with Tb3þ content and reaches approximately 92% at y ¼ 1.0. The results indicate the existence of efficient Ce3þto-Tb3þ ET. In general, there are two kinds of non-radiation ET processes, namely, exchange interaction and multi-polar interaction [31,35]. Overlap between the electron clouds is needed for the exchange interaction, which means very short distance between the Ce3þ and Tb3þ. The average distance (Rc) between Ce3þ and Tb3þ can be obtained by the formula [36].


Rc z2

3V 4pXc N

1=3 (2)

The unit cell volume (V) and the number (N) of La3þ ions in the unit cell of La2Si2O7 are 627.2 Å3 and 8, respectively. The critical concentration (Xc) is 0.125 for LSO:0.05Ce3þ,0.2 Tb3þ, at which the intensity of Ce3þ emission is nearly half of the Ce3þ single-doped one. Then the calculated critical distance of ET is 10.62 Å accordingly, which is larger than 4 Å that exchange interaction requires. In order to figure out which multi-polar ET process dominates, Reisfeld's approximation and Dexter's energy transfer formula is given and expressed as follows [37,38],

Fig. 6. Relationship between IS0/IS of Ce3þ and (a) C6/3, (b) C8/3 and (c) C10/3.

hS0 n=3 fC hS

(3)

wherehS0 and hS refer to the quantum efficiencies of Ce3þ in singledoped and Ce3þ,Tb3þ co-doped phosphor, respectively, C is the sum of Ce3þ and Tb3þ content, n refers to different interaction modes that can be 6, 8, or 10. The value of hS0/hS is approximately equal to the ratio of corresponding PL intensities, namely, IS0/IS [39,40]. Consequently, the formula can be presented as

IS0 fC n=3 IS

(4)

The dependence of IS0/IS for Ce3þ on Cn/3 in LSO:0.05Ce3þ,yTb3þ is displayed in Fig. 6. The best linear relationship for IS0/IS  Cn/3 appears when n ¼ 6, which suggests that the Ce3þ-to-Tb3þ energy transfer may be dominated by the dipolar-dipolar interaction. The PL intensity of Ce3þ for LSO:0.05Ce3þ,zEu3þ decreases rapidly with increasing the Eu3þ content as shown in Fig. S4, however, the emission peaks of Eu3þ are hardly observed. As a result, the ET from Ce3þ to Eu3þ does not occur and the introduction of Eu3þ even results in Ce3þ emission quenching due to the presence of metalmetal charge transfer (MMCT) [20,41]. Tb3þ is introduced to alleviate the MMCT effect and enhance the red emission of Eu3þ based on researches of A. A. Setlur [21] and Wen [23,42]. The optimal content of Eu3þ was chosen by the study of the luminescent properties of LSO:0.05Ce3þ,0.8 Tb3þ,zEu3þ. Fig. 7a displays the PL spectra of LSO:0.05Ce3þ,0.8 Tb3þ,zEu3þ under 332 nm excitation. The range from 340 nm to 450 nm is not given here because emission band of Ce3þ are not obvious. Upon increasing Eu3þ content, emission intensities of Eu3þ firstly

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Fig. 7. PL spectra of (a) LSO:0.05Ce3þ,0.8 Tb3þ,zEu3þ (z ¼ 0, 0.01, 0.02, 0.03, 0.04) and (b) LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ under 332 nm excitation.

increase and then decrease with a maximum PL intensity at z ¼ 0.01, while those of Tb3þ gradually decrease. Fig. 7b depicts the typical PL spectrum of LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ consisting the emission bands of Tb3þ and Eu3þ. Emission of Eu3þ consists of several narrow lines at 592, 611, 652 and 704 nm relating to 5 D0/7FJ transitions and the peak at 611 nm (5D0/7F2) is strongest. In order to study the role of Tb3þ content for energy transfer, the luminescent properties of LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ (y ¼ 0.2e1.0) are studied. The corresponding PL spectra under excitation at 332 nm are shown in Fig. 8 and the relative PL intensities of Tb3þ and Eu3þ at 542 and 611 nm, respectively, are illustrated as inset in Fig. 8. The PL intensity of Tb3þ increases firstly with its content until maximizing at y ¼ 0.4. When Tb3þ concentration is low (y ¼ 0.2), emission of Eu3þ is fairly weak, which demonstrates that energy transfer is unable to take place with low Tb3þ concentration and terbium bridge could not form to enhance

emission of Eu3þ greatly in the LSO host. Tb3þ emission decreases with further increasing Tb3þ content over y ¼ 0.4, while emission of Eu3þ gets stronger with y increasing from 0.2 to 1.0. The greatly enhanced Eu3þ emission indicates ET from Ce3þ to Eu3þ through Tb3þ. The existence ET of Tb3þ to Eu3þ can also be indicated by the decrease of Tb3þ lifetime (the inset in Fig. 8). The decay curves of Tb3þ change from a well single-exponential form to nonexponential form, and the lifetime of Tb3þ reduces with increasing Tb3þ content, which demonstrate that efficient ET from Ce3þ to Eu3þ can be realized in La2Si2O7 host by appropriate increasing the content of Tb3þ. Fig. 9 illustrates a diagram to explain the ET processes in LSO:Ce3þ,Tb3þ,Eu3þ. Firstly, the 4f electrons of Ce3þ ions are excited effectively to the 5d level under UV irradiation. After returning to the lowest vibrational level of 5d level, the excited electrons of Ce3þ ions may return to the ground state giving out UV light emission or

Fig. 8. PL spectra of LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ under excitation of 332 nm. Left inset: corresponding PL intensity of Tb3þ (542 nm) and Eu3þ (611 nm) versus Tb3þ content. Right inset: decay curves of Tb3þ in LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ monitored at 542 nm.

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Fig. 9. Simple energy level diagram of ET processes of Ce3þ/Tb3þ/Eu3þ in La2Si2O7.

transferring their excited energy to the 5D3 level of Tb3þ due to similar energy. The Ce3þ-to-Tb3þ ET probability increases with Tb3þ content. Whereafter, the excited 5D3 level of Tb3þ transfers the energy to 5D4 level via cross relaxation between two Tb3þ ions [43,44], followed by the characteristic green emission (5D4/7FJ) of Tb3þ. Besides, the excited Tb3þ ions in 5D3 level can generate a weak emission of 5D3/7FJ in blue region as shown in Fig. 4. On the other hand, increasing Tb3þ content will shorten the distances between Tb3þ and Tb3þ/Eu3þ, which leads to enhancement of Eu3þ emission due to the occurrence of ET from Tb3þ to Eu3þ and simultaneously quenches the emission of Tb3þ. Finally, the excited Eu3þ in 5D2 and 5 D1 relaxes to 5D0 level nonradiatively and followed by the characteristic emission of Eu3þ originating from 5D0/7FJ.

Fig. 10. CIE chromaticity diagram with coordinates for LSO:0.05Ce3þ, LSO:0.05Ce3þ,0.8 Tb3þ, and LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ based on the PL spectra excited at 332 nm, insets are photographs of LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ under a 302 nm UV lamp.

Table 1 Quantum Yields (QYs) for LSO:0.05Ce3þ,yTb3þ and LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ Phosphors Excited at 332 nm UV Light. Samples

3.3. CIE coordinates, quantum yield, and thermal stability Fig. 10 displays the chromaticity coordinates of LSO:0.05Ce3þ, LSO:0.05Ce3þ,0.8 Tb3þ, and LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ phosphors. The insets illustrate the photographs of LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ phosphors under a 302 nm UV lamp, which exhibits a tunable color emission intuitively. It is quite apparent that the calculated coordinated point shifts from the n-UV region for LSO:0.05Ce3þ to the green region after introduction of Tb3þ. The emission color of LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ varies from green to orange-red by changing the Tb3þ content, which corresponds to the variation of ratio for green and red emission intensities. Besides, the absolute quantum yields (QYs) for the phosphors excited at 332 nm are measured and listed in Table 1. The QY values for LSO:0.05Ce3þ,yTb3þ phosphors increase from 71.04% to 95.60% with Tb3þ content and then decrease. The maximum QY is obtained when y ¼ 0.6, which is higher than LSO:0.05Ce3þ. This result can be explained by the efficient Ce3þ-toTb3þ energy transfer, which reduces the nonradiative transition of Ce3þ. As for LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ phosphors, QY values remain 70.08%e76.83% with the maximum at y ¼ 0.6, lower than Ce3þ and Tb3þ co-doped phosphors apparently, which should be related to the energy loss within ET process between Tb3þ and Eu3þ [45], and the increase of nonradiative probability for Eu3þ. It is well-known that the thermal quenching property is an important parameter for luminescent materials. Therefore, the PL spectra of LSO:0.05Ce3þ, LSO:0.05Ce3þ,0.6 Tb3þ, and 3þ 3þ 3þ LSO:0.05Ce ,0.8 Tb ,0.02Eu phosphors versus temperature are measured and depicted in Fig. 11. At room temperature, the emission spectrum of Ce3þ shows its well-known double emission band

LSO:0.05Ce

3þ

3þ

,yTb

LSO:0.05Ce3þ,yTb3þ,0.02Eu3þ

Tb3þ content (y)

QYs (%)

0 0.2 0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0

71.04 90.62 92.75 95.60 86.64 80.15 71.03 76.83 74.92 70.08

structure. Upon raising temperature, the two emission bands broaden. Although Ce3þ has a thermal quenching, this effect is weaker than the broaden effect in the temperature range from 25 to 100  C, resulting in a slight increase trend of the integrated intensities as shown in Fig. 11d. When the temperature is higher than 100  C, the thermal quenching of Ce3þ prevail the broadening, leading a gradually decrease in PL intensity. The PL intensities of Ce3þ and Tb3þ in LSO:0.05Ce3þ,0.6 Tb3þ and Tb3þ and Eu3þ in LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ decrease gradually with increasing operated temperature. It is obvious that LSO:0.05Ce3þ,0.6 Tb3þ has a lower thermal quenching remaining 89.2% of initial value than LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ retaining 82.2% of original integrated intensity at 150  C, and LSO:0.05Ce3þ shows the best thermal stability among them as exhibited in Fig. 11d. The temperaturedependant emission intensities of Ce3þ and Tb3þ in LSO:0.05Ce3þ,0.6 Tb3þ, and those for Tb3þ and Eu3þ in LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ are shown in Fig. S5. The thermal stability of Ce3þ emission in Ce3þ,Tb3þ co-doped sample is worse than that of LSO:Ce3þ. And the thermal stability of Eu3þ emission is

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Fig. 11. The dependence of emission spectra (lex ¼ 332 nm) on temperature of (a) LSO:0.05Ce3þ, (b) LSO:0.05Ce3þ,0.6 Tb3þ, and (c) LSO:0.05Ce3þ,0.8 Tb3þ,0.02Eu3þ, the inset in (a) is the normalized PL spectra of LSO:0.05Ce3þ. (d) The temperature dependence of integrated emission intensity.

also worse than that of Tb3þ in Ce3þ,Tb3þ,Eu3þ co-doped sample. The introduction of Tb3þ results in the decrease of thermal stability of Ce3þ. This phenomenon can be expressed as follows. The quantum yield (QY) of Ce3þ can by expressed by Ref. [46].



3þ

QY Ce



¼

g0

g0 þ WET þ AeDE=kT



where g0 is the intrinsic decay rate of Ce3þ, WET is the energy transfer rate, and A is a constant, DE is the activation energy of thermal de-excitation for 5d state of Ce3þ, k is the Boltzmann constant. One should note that the QY(Ce3þ) is not the values in Table 1, which involve both Ce3þ and Tb3þ. For LSO:Ce3þ without WET, the thermal de-excitation is responsible for the thermal stability or QY versus temperature. According to the PLE and PL spectra, LSO:Ce3þ has a small Stokes shift, which relates to a small offset between the parabolas of excited and ground states and then a large DE, suggesting reasonable high thermal stability for LSO:Ce3þ. While WET exists for the Ce3þ,Tb3þ co-doped sample. If WET keeps unchanged upon increase temperature, it will reduce the influence of thermal de-excitation on QY or PL intensity of Ce3þ. The bigger the WET value, the higher the reduction of influence, namely, QY or PL intensity of Ce3þ in co-doped one should decrease slower than Ce3þ-single-doped one upon increasing T. However, the thermal stability of Ce3þ emission in Ce3þ,Tb3þ co-doped sample is worse than that of LSO:Ce3þ (comparing Fig. S5 with Fig. 11d). This result suggests that only if WET increases with temperature can the QY or PL intensity of Ce3þ in co-doped one decrease faster than LSO:Ce3þ. The comparison of decay time of Ce3þ in Ce3þ and Ce3þ,Tb3þ codoped samples upon increasing temperature can tell whether if WET increases with T. The decay time can be expressed as follows [47].

(6)

In LSO:Ce3þ,Tb3þ,



(5)



t Ce3þ ¼ 1=ðg0 þ gnr Þ ¼ 1=½g0 þ A expðDE=kTÞ; 

t Ce3þ ¼ 1=ðg0 þ gnr þ WET Þ;

(7)

where g0, gnr, and WET are the intrinsic decay rate, nonradiative decay rate (thermal de-excitation), and energy transfer rate, respectively. If WET increases with T, t(Ce3þ) in LSO:Ce3þ,Tb3þ should decreases faster than that in LSO:Ce3þ. Temperaturedependent decay curves of LSO:Ce3þ and LSO:Ce3þ,Tb3þ are measured as shown in Fig. 12a and b in Supporting Information, which can be well fitted by a single-exponential and a biexponential function, respectively. The fitted decay times are also listed in Fig. 12. The values of g0 and A can then be obtained from the fitting of t e T curve by equation (6) with the data of LSO:Ce3þ, which are shown in Fig. 12c. Finally, temperature-dependant WET can be obtained from equation (7) and shown in Fig. 12d, indicating an increase of WET with temperature. These results support the phenomenon that the PL intensity of Ce3þ in co-doped sample decreases faster than the single-doped one. For co-doped samples, the PL band of Ce3þ is very weak due to strong energy transfer from Ce3þ to Tb3þ. Therefore, the thermal quenching mainly relates to Tb3þ and Eu3þ. Tb3þ and Eu3þ (4f-4f transitions) with weak coupling both have small energy difference (DE) between the lowest excited and ground states. For the 4f-4f transitions in Tb3þ or Eu3þ, the nonradiative decay rate (W) at low temperature is given by Ref. [48].

W ¼ b exp½  ðDE  2hnmax Þa

(8)

where a and b are constants, nmax is the highest available

60

W. Ma et al. / Journal of Alloys and Compounds 785 (2019) 53e61

Fig. 12. Decay curves of Ce3þ versus temperature. (a) LSO:0.05Ce3þ, (b) LSO:0.05Ce3þ,0.2 Tb3þ, (c) t e T curve for LSO:0.05Ce3þ, (d) WET vs T for LSO:0.05Ce3þ,0.2 Tb3þ.

vibrational frequency of the surroundings of the rare earth ion. The smaller DE corresponds to a higher nonradiative rate (W) in the same host. Therefore, Eu3þ should have a higher W than Tb3þ. The temperature-dependent nonradiative decay rate W(T) for the 4f-4f transitions is given by Refs. [49,50].

h iDE=hn WðTÞ ¼ W 1  ehn=kT Þ

(grant no. 2016YFB0302403). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.01.194.

(9)

where W is the nonradiative decay rate at low temperature, n znmax, h is Planck constant and k is Boltzmann constant. W(T) tends to increase with the temperature. Furthermore, Eu3þ has a larger W than that of Tb3þ. Therefore, the thermal stability of Eu3þ-related phosphor is worse than that of Tb3þ-related one. 4. Conclusions La2Si2O7:Ce3þ,Tb3þ,Eu3þ phosphors with color-tunable emissions were synthesized. An electric dipole-dipole interaction has been verified to play a key role in the Ce3þ-to-Tb3þ energy transfer and the terbium bridge between Ce3þ and Eu3þ was realized to overcome MMCT effect and enhance the Eu3þ emission. Upon UV irradiation, the narrow-band emission of Eu3þ can be greatly enhanced by energy transfer with the assistance of Tb3þ bridge and the emitting color can be adjusted from green to orange-red. Moreover, the quantum yield and thermal stability of selected phosphors were investigated, among which the QY value of the optimum green phosphor La2Si2O7:0.05Ce3þ,0.6 Tb3þ is up to 95.60%. The quantum yields of La2Si2O7:Ce3þ,Tb3þ,Eu3þ phosphors remain higher than 70%. The temperature dependence of luminescence shows that the obtained phosphors have excellent thermal stability. The results indicate that La2Si2O7:Ce3þ,Tb3þ,Eu3þ can be a good candidate for pc-WLEDs. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant nos. 21505038, 51402105, 21601081), National Key Research and Development Program

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