Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3+,Tb3+ phosphors

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CERAMICS INTERNATIONAL

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Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3 þ , Tb3 þ phosphors Jicheng Zhu, Shiqiang Qin, Zhiguo Xia, Quanlin Liun School of Materials Science, University of Science and Technology Beijing, Beijing 100083, China Received 1 June 2015; received in revised form 18 June 2015; accepted 19 June 2015

Abstract The exploration of new color-tunable materials for solid-state lighting based on InGaN semiconductors has attracted great attention. Herein a series of Ce3 þ , Tb3 þ and Ce3 þ /Tb3 þ co-doped Y2Si3O3N4 phosphors have been developed. The intense green emission peaking at 543 nm was observed in the Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors. Energy transfer (ET) process from Ce3 þ to Tb3 þ was verified via the variations of the lifetime values of Ce3 þ , and the ET mechanism is ascribed to the dipole–dipole interaction. The ET critical distance has been calculated by the concentration quenching method. It is believed that the Ce3 þ /Tb3 þ -activated Y2Si3O3N4 phosphors can serve as potential materials for near ultraviolet (n-UV) light emitting diode (LED). & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Optical properties; Oxynitride; Phosphor; Energy transfer

1. Introduction White light emitting diodes (w-LEDs) have made remarkable achievements in solid state lighting due to their long lifetime, small bulk, high luminous efficiency, energy saving, and environmental friendly characteristics [1,2]. Nowadays commercial w-LEDs combines a blue LED chip and yellow lightemitting phosphor, which are very poor in the color rendering index (CRI) because of the color deficiency in the red region [3]. As a consequence, an alternative method is realized by combining n-UV-emitting chips ( 400 nm) with tricolor (blue, green and red) phosphors, which can provide superior color uniformity with a high color rendering index and high quality of white light. Therefore, it is essential to study excellent phosphors with tricolor emission upon n-UV excitation. Among reported tricolor phosphors, rare-earth silicon-oxynitrides of Y–Si–O–N quaternary systems have been found and characterized in many references [4–6]. Typically, the structures of nitride silicates consist of SiN4 tetrahedra. Part of the nitrogen n

Corresponding author. Tel./fax: þ 86 10 6233 4705. E-mail address: [email protected] (Q. Liu).

atoms of SiN4 tetrahedra can be replaced by oxygen to form Si[O/ N]4 tetrahedra. Among them, the SiN4 or Si[O/N]4 units are stacked together by sharing their corners or edges to form a condensed frame work, resulting in excellent chemical stability of the nitridosilicates and oxonitridosilicates [7]. Accordingly, these compounds are interesting host lattices for phosphors. As a highly efficient activator in the host, the Ce3 þ ion has been widely investigated [8], because the emission and absorption spectra of Ce3 þ ions usually consist of broad bands due to the transition between the 4f1 ground state and the crystal field components of the 5d excited state configuration [9,10]. On the other hand, Tb3 þ could be also an ideal activator for phosphors emitting sharp green light. However, the Tb3 þ ion has only weak absorption peaks at about 300–400 nm due to the 4f–4f absorption transitions [11,12]. Therefore, co-doping Ce3 þ and Tb3 þ can achieve broadband absorption to ensure the luminous efficiency and reliability of the w-LEDs device. 2. Experimental Ce3 þ , Tb3 þ and Ce3 þ /Tb3 þ -activated Y2Si3O3N4 phosphors were prepared by a high temperature solid-state reaction.

http://dx.doi.org/10.1016/j.ceramint.2015.06.092 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: J. Zhu, et al., Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.06.092

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2

The starting materials used for the studied phosphors were Y2O3 (99.9%), SiO2 (99.9%), Si3N4 (99.9%), CeO2 (99.995%) and Tb4O7 (99.995%), and they were mixed and ground in an agate mortar according to the given stoichiometric ratio. Then the mixture was sintered at about 2073 K for 2 h under highpurity nitrogen atmosphere ( 4 99.999%) in a boron nitride furnace. The samples were identified by the X-ray powder diffraction (Philips X'PertPW-3040) with Cu Kα radiation (λ=0.15406 nm) operating at 45 kV and 40 mA. The excitation and emission spectra at room temperature were measured on an Edinburgh FLSP920 Fluorescence Spectrophotometer with a Hamamatsu S900-R (red sensitive version) photomultiplier tube (PMT). The lifetimes were recorded on the same instrument (FLSP920), and a nF900 flash lamp was used as the excitation resource. 3. Results and discussions 3.1. Crystal structure

λem = 543 nm

Intensity (a.u.)

Fig. 1 presents the powder XRD patterns of as-prepared Y2Si3O3N4 compounds accompanied with different doping rare earth ions. The XRD patterns of the Y2Si3O3N4 compounds are matched well with the reported Y2Si3O3N4 phase (JCPDS card 01-088-0123), which crystallize in a tetragonal cell with space group P4̄21m. The lattice parameters of nondoped Y2Si3O3N4 powder are a ¼ b ¼ 7.6137 Å, c¼ 4.9147 Å, V ¼ 284.90 Å3 and Z ¼ 2. The structure of Y2Si3O3N4 has two formula units in a unit cell, i.e. the unit cell contains 4Y þ 6Si þ 6O þ 8N atoms. Y, Si, O and N atoms occupy 4e, 2a þ 4e, 2c þ 4e and 8f Wyckoff positions, respectively. There is only one crystallographic yttrium site in the Y2Si3O3N4 lattice, and Y atom is eight-coordinated by four oxygen and four nitrogen atoms [8]. Fig. 2a depicts the PLE and PL spectra of the Y2Si3O3N4: Tb3 þ phosphors. The PLE spectrum monitored at 542 nm exhibits a broad band from 250 to 450 nm, which has characteristic transitions of forbidden 4f–4f transitions within

the Tb3 þ configuration in the wavelength range of 300– 450 nm. In addition, there is broad excitation peak at around 274 nm which can be assigned to the spin-allowed 4f8–4f75d (ΔS ¼ 0) transition [13]. Under 274 nm excitation, the characteristic emissions of Tb3 þ at 413, 436, 461, 493, 543, 584 and 626 nm are observed which can be attributed to the transitions 5D3–7F5, 5D3–7F4, 5D3–7F3, 5D4–7F6, 5D4–7F5, 5 D4–7F4, and 5D4–7F3, respectively. Fig. 2b illustrates the PLE and PL spectra of the Y2Si3O3N4: Ce3 þ phosphors. Upon the excitation of 395 nm, the PL spectrum shows a broad band between 400 and 650 nm. The PLE spectrum of Y2Si3O3N4:Ce3 þ comprises of two bands centered at 307 and 395 nm which corresponds to the electronic transitions from the ground state to the different crystal field splitting bands of the excited 5d states of Ce3 þ ions [14]. This excitation band matches well with the emission of n-UV chips, which make it a potential n-UV LED converting phosphor. Additionally, the PL spectrum shows no difference except for the relative intensity under 307 and 395 nm excitation, implying that Ce3 þ ions only occupy one site in the host lattice, which is consistent with the crystal structure. In order to enhance the absorption intensity in the n-UV region for the Tb3 þ emission, Ce3 þ ions can be co-doped as sensitizers to transfer the excitation energy to the Tb3 þ ions. We can observe that there is an overlap between the emission band of Ce3 þ and the absorptions of Tb3 þ indicating the possible resonance type energy transfer from Ce3 þ to Tb3 þ in the Y2Si3O3N4 host (Fig. 2c). After co-doping Ce3 þ and Tb3 þ ions in Y2Si3O3N4, the PL spectrum exhibits a broad emission centered at 480 nm corresponding to the allowed f–d transition of the Ce3 þ ions, and an intense green emission assigned to

λex = 274 nm

λex = 395 nm λem = 480 nm λex = 307 nm

λem = 543 nm

250

300

350

λem = 480 nm

400

450

λex= 395 nm

500

550

600

650

Wavelength (nm) 3þ

Fig. 1. The XRD patterns of Y2Si3O3N4:Ce

3þ

,Tb

samples.

Fig. 2. PLE and PL spectra of Y2Si3O3N4:4%Tb3 þ (a), Y2Si3O3N4:2%Ce3 þ (b) and Y2Si3O3N4:2%Ce3 þ ,4%Tb3 þ (c).

Please cite this article as: J. Zhu, et al., Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.06.092

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Fig. 4 shows the representative Ce3 þ decay curves (excited at 395 nm and monitored at 480 nm) of Y2Si3O3N4:2%Ce3 þ , xTb3 þ phosphors. The luminescent decay curve of all the samples can be fitted well with a biexponential function by the following equation [19]:

ð2Þ I ¼ A1 exp  t=τ1 þ A2 exp  t=τ2

3+

2% Ce 3+ 3+ 2% Ce , 1% Tb 3+ 3+ 2% Ce , 4% Tb 3+ 3+ 2% Ce , 8% Tb 3+ 3+ 2% Ce , 12% Tb 3+ 3+ 2% Ce , 20% Tb 3+ 3+ 2% Ce , 30% Tb

Intensity (a.u.)

where I represents the luminescent intensity and A1 and A2 are constants; t is time, and τ1 and τ2 are the decay times for the exponential components. Using these parameters, the average decay time τ can be determined by the formula given in the following equation:
τ ¼ A1 τ21 þ A2 τ22 =ðA1 τ1 þ A2 τ2 Þ ð3Þ 450

500

550

600

650

Wavelength (nm) Fig. 3. PL spectra of Y2Si3O3N4:2%Ce3 þ ,xTb3 þ phosphors (x¼ 0–30%).

the 5D4–7FJ forbidden transition of the Tb3 þ ions. It is obvious that the excitation spectrum of Y2Si3O3N4:2%Ce3 þ ,4%Tb3 þ monitored at 480 nm (Ce3 þ emission) is similar to that monitored at 543 nm (Tb3 þ emission) except for the difference of the relative intensity, which also provides another evidence for the energy transfer from Ce3 þ to Tb3 þ . It is very interesting to find from Fig. 3 that the Ce3 þ emission decreases when the concentration of Tb3 þ increases in Y2Si3O3N4:2%Ce3 þ ,xTb3 þ . Nevertheless, the sample of x ¼ 1% has stronger intensity than the sample of x ¼ 0 due to the emission overlaps at 480 nm (Ce3 þ emission) and 493 nm (Tb3 þ emission). The green emission of Tb3 þ reaches maximum when x ¼ 20%, then decreases because of the concentration quenching of Tb3 þ . In general, the resonant-type energy transfer from a sensitizer to an activator in a phosphor may take place by the exchange interaction and electric multipolar interaction [15]. It is known that if energy transfer takes the exchange interaction, the critical distance between the sensitizer and activator should be shorter than 5 Å [16]. The critical distance Rc for energy transfer from the Ce3 þ to Tb3 þ ions was calculated using the concentration quenching method. The critical distance Rc between Tb3 þ and Tb3 þ can be estimated by the following formula suggested by Blasse [17,18]: 

3V Rc  2 4πxc N

The values of average decay times of the Ce3 þ excited-state for Y2Si3O3N4:2%Ce3 þ ,xTb3 þ phosphors (x ¼ 0, 4%, 8%, 12%, 20% and 30%) were calculated to be 25.2, 24.6, 20.2, 17.3, 14.4, 9.8 and 6.4 ns, respectively. It can be seen that the lifetime values for the Ce3 þ ions decreases monotonically as Tb3 þ content increased, which act as the strong evidence for the energy transfer from Ce3 þ to Tb3 þ ions. The energy transfer efficiency from Ce3 þ to Tb3 þ can be calculated based on the following expression [20]: τx ηT ¼ 1  ð4Þ τ0 where τ0 and τx are the corresponding lifetimes of donor Ce3 þ in the absence and the presence of the acceptor Tb3 þ , respectively, and ηT is the calculation of the energy transfer efficiency. It can be seen from Fig. 5 that the energy transfer efficiency (ηT) increases with an increasing Tb3 þ concentration. The results of the energy transfer efficiency from Ce3 þ to Tb3 þ under UV excitation were calculated by Eq. (4) to be 2.08%, 19.87%, 31.53%, 43.05%, 60.93% and 74.58%, respectively. According to Dexter's energy-transfer formula of multipolar interaction, the following relation can be obtained [21]: τS0 p Ca=3 ð5Þ τS where C is the concentration of Tb3 þ , and a ¼ 6, 8, or 10 for dipole–dipole, dipole–quadrupole, or quadrupole–quadrupole

1=3

3+

Tb concentration 0 1% 4% 8% 12% 20% 30%

1000

ð1Þ

where N is the number of molecules in the unit cell, V is the unit cell volume and xc is the critical concentration. For Y2Si3O3N4 host, N=2, V=284.9 Å3 and the critical concentration is about 0.35 from the total concentration of Ce3 þ and Tb3 þ at which the energy transfer efficiency is 0.5. According to the above equation, the critical distance of energy transfer is estimated to be about 9.25 Å. This value is much longer than 3–4 Å, indicating little possibility of energy transfer via the exchange interaction mechanism. Thus, energy transfer mechanism in this system is governed by the electric multipolar interaction.

Intensity (a.u.)

400

3

100

τ(ns) 25.2 24.7 20.2 17.3 14.3 9.8 6.4

λ ex = 395 nm λem = 480 nm

10

0

100

200

300

400

500

Time Decay (ns) Fig. 4. Ce3 þ decay curves of Y2Si3O3N4:2%Ce3 þ ,xTb3 þ (x¼ 0–30%) phosphors monitoring at 480 nm.

Please cite this article as: J. Zhu, et al., Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.06.092

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4

ET efficiency (%)

100

3+

Relative intensity (a.u.)

3+

ET efficiency of Ce -Tb 3+ 5 7 Tb D4- F5 transition

80 60 40 20 0 0

5

10

Tb

3+

15

20

25

30

concentration (mol%) 3þ

interactions, respectively. Plots of τs0/τs and Ca/3 based on the above equation are shown in Fig. 6. The best linear behavior was observed when a is equal to 6, indicating that energy transfer from Ce3 þ to Tb3 þ took place via the dipole–dipole mechanism. The CIE chromaticity diagram and coordinates for the Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors are calculated according to the relevant PL spectra and are exhibited in Fig. 7. The color tone of the phosphors can be tuned from blue to green with the increase of the Tb3 þ content because of efficient Ce3 þ –Tb3 þ energy transfer. The corresponding CIE coordinates are calculated, and the values are changed from (0.18, 0.30) to (0.34, 0.54). 4. Conclusions

3þ

Fig. 5. The dependence of the Ce emission, the Tb emission and the energy transfer efficiency of Ce3 þ –Tb3 þ doping concentration for Y2Si3O3N4:2%Ce3 þ ,xTb3 þ phosphors (x¼ 0–30%).

In summary, we have synthesized a series of Y2Si3O3N4: Ce3 þ ,Tb3 þ phosphors by solid state reaction, and the phase

4.0

4.0 2

3.0

2.5

3.0

2.5

2.0

2.0

1.5

1.5 0.5

1.0

1.5

2.0

2.5

CTb3+ /10

R = 0.99696

3.5

τs0/τs of Ce 3+

τs0/τs of Ce 3+

2

R = 0.98149

3.5

3.0

0

2

4

6 6/3

1

CTb3+ /10

8

10

2

4.0

4.0

2

R = 0.94177

2

R = 0.97491

3.5

τs0/τs of Ce 3+

τs0/τs of Ce 3+

3.5

3.0

2.5

3.0

2.5

2.0

2.0

1.5

1.5 0

10

20 8/3 3+ Tb

C

30

/10

3

40

0

40

80

120

CTb

10/3 3+

/10

160

200

4

Fig. 6. The dependence of τs0/τs of Ce3 þ on (a) C, (b) C6/3, (c) C8/3 and (d) C10/3.

Please cite this article as: J. Zhu, et al., Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.06.092

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Fig. 7. A representation of the CIE chromaticity coordinates for the Y2Si3O3N4:2%Ce3 þ ,xTb3 þ phosphors (x¼ 0–30%).

structure and luminescence properties were investigated in detail. The energy transfer from the Ce3 þ to Tb3 þ ions has been demonstrated to be a resonant type via the dipole–dipole mechanism in the Ce3 þ /Tb3 þ co-doped Y2Si3O3N4 phosphors. The energy transfer critical distance has also been evaluated by the concentration quenching method. Owing to the energy transfer from Ce3 þ to Tb3 þ , the emission could be tuned from the blue (0.18, 0.30) to the green (0.34, 0.53) region in the Ce3 þ /Tb3 þ co-doped Y2Si3O3N4 phosphors, showing the phosphors would have potential use in the technology of white light-emitting diodes. Acknowledgments This present work was supported by the National Natural Science Foundation of China (Grant no. 51272027, 51472028).

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Please cite this article as: J. Zhu, et al., Synthesis and color-tunable emission studies of Y2Si3O3N4:Ce3 þ ,Tb3 þ phosphors, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.06.092