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Cite this: DOI: 10.1039/c4dt03906h
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Optical properties and energy transfer of a novel KSrSc2(PO4)3:Ce3+/Eu2+/Tb3+ phosphor for white light emitting diodes† Mengmeng Jiao,a,b Wenzhen Lv,a,b Wei Lü,a Qi Zhao,a,b Baiqi Shaoa,b and Hongpeng You*a A new langbeinite-type phosphate KSrSc2(PO4)3 has been synthesized by conventional high temperature solid state reaction. Rietveld structure refinement, a field emission scanning electron microscope, photoluminescence spectra, quantum efficiency as well as lifetimes were used to characterize the samples. Structure refinement reveals that KSrSc2(PO4)3 has two kinds of Sr2+ and Sc3+ sites for the doped ions to occupy, forming emission centers. The KSrSc2(PO4)3:Ce3+ and KSrSc2(PO4)3:Eu2+ phosphors both have broad excitation and emission bands due to spin- and orbit-allowed electron transitions. Phosphors with tunable
blue
to
blue-green 3+
Received 18th December 2014, Accepted 14th January 2015 DOI: 10.1039/c4dt03906h www.rsc.org/dalton
colors
were
obtained
by
codoping
the
Tb3+
ions
into
the
2+
KSrSc2(PO4)3:0.03Ce and KSrSc2(PO4)3:0.03Eu phosphors with varying contents. The mechanism of Eu2+→Tb3+ energy transfer is determined to be a dipole–quadrupole interaction in terms of the experimental results and analysis of photoluminescence spectra and decay curves of the phosphors by using the Inokuti–Hirayama theoretical model. Our prepared KSrSc2(PO4)3:Ce3+,Tb3+ and KSrSc2(PO4)3:Eu2+, Tb3+ phosphors are of potential value for UV excited WLEDs.
Introduction In recent years, white light emitting diodes (WLEDs) have been a research focus in both scientific areas and technical industries, due to their much more efficient conversion of electric energy to visible light compared with traditional lighting sources. The WLEDs are thought to be the most superior candidate for replacing incandescent light bulbs and fluorescent lamps and are expected to dominate the market soon.1–6 Now, the most commonly used method to obtain white light is coating the YAG:Ce3+ phosphors on InGaN chips. However, the high correlated colour temperature and low colour rendering index which are caused by the deficiency of the red component in the emission restrict its further application.7,8 Researchers have made much effort to improve the performance of WLEDs fabricated with blue chips coated with yellow phosphors. For example, Zhang et al. have incorporated Cr3+ and Pr3+ ions into the YAG: Ce3+ phosphor and You et al. have codoped Mn2+ and Ce3+ ions into the Y3Al5O12 host;9,10 Li et al. have synthesized novel
a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, P. R. China. E-mail: [email protected] b University of the Chinese Academy of Science, Beijing 100049, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4dt03906h
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Ba0.93Eu0.07Al2O4 yellow phosphor with emission centred at 580 nm under 440 nm excitation.11 Another approach to obtain white light is the use of UV chips coated with blue, green and red phosphors. Since this combination can have smoother spectral distribution over the whole visible range, the obtained WLEDs will have an excellent colour rending index.12,13 Although the UV-A chips with the emission wavelength ranging from 320 to 400 nm are mainly used in this kind of fabrication, the deep UV LED chips with the emission wavelength shorter than 320 nm have attracted much attention and are being developed for usage due to their potential value in biochemical and germicidal applications.14–17 In this case, it is of great importance to explore efficient UV excited phosphors. Researchers have investigated many kinds of inorganic materials including silicate, aluminate, phosphate, nitride, molybdate, tungstate, etc. Among them, rare earth ion doped phosphate materials have attracted much attention since they have low sintering temperature, good thermal stability, and luminescence properties.18–22 It is known that both the Eu2+ and Ce3+ ions have broad emission and excitation bands with high efficiency since the 4f–5d electron transition is spin- and orbit-allowed. Moreover, their excitation and emission wavelengths can be tuned on a large scale depending on different host lattices.23,24 The Tb3+ ions are also important activators since they can have intense green emission due to their 5 D4–7FJ electron transition. Based on the above reasons, in this
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paper we have prepared Eu2+, Ce3+ and Tb3+ doped novel KSrSc2(PO4)3 phosphate phosphors and studied their structure and photoluminescence properties in detail. The obtained KSrSc2(PO4)3:Ce3+,Tb3+ and KSrSc2(PO4)3:Eu2+,Tb3+ phosphors might be of potential value for UV excited WLEDs.
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2. Experimental section 2.1
Materials and synthesis
A series of KSrSc2(PO4)3:Ce3+/Eu2+/Tb3+ (KSSP: Ce3+/Eu2+/Tb3+) phosphors have been synthesized by the traditional solid state reaction. The raw materials KHCO3 (A.R.), SrCO3 (A.R.), Sc2O3 (A.R.), (NH4)2HPO4 (A.R.), CeO2 (99.99%), Eu2O3 (99.99%) and Tb4O7 (99.99%) were weighed according to the stoichiometric ratio and ground in an agate mortar for 15 min to mix thoroughly. After heating in an oven at 180 °C for 4 h, the obtained mixtures were transferred to a tube furnace with the temperature being 1100 °C for 3 h under a reductive atmosphere (20% H2 + 80% N2). The obtained samples were finally cooled to room temperature and ground again for further measurements. 2.2
Measurements and characterization
The powder X-ray diffraction patterns of the prepared samples were obtained using the Bruker AXS D8 operating at 40 kV and 40 mA using monochromated Cu Kα radiation (λ = 1.5405 Å). The size and surface morphology were inspected with a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). Photoluminescence excitation and emission spectra were obtained using a Hitachi F-7000 spectrophotometer and a 150 W xenon lamp was used as the excitation source. The decay curves of the samples were obtained using a LeCroy Wave Runner 6100 digital oscilloscope (1 GHz), and a tunable laser ( pulse width = 4 ns, gate = 50 ns) was used as the excitation source. The photoluminescence quantum yield of the phosphors was determined on a quantum yield measurement system (C9920-02, Hamamatsu Photonics K.K., Japan).
3. Results and discussion 3.1
XRD refinement and the crystal structure
To study the structure of the phosphor, Rietveld structure refinement of KSrSc2(PO4)3:0.01Eu2+ has been performed by using the general structure analysis system (GSAS) program.25 In the refinement, the crystallographic data of KBaFe2(PO4)3 were used as the initial structure model. The langbeinite-type phosphate KBaFe2(PO4)3 was reported to have the cubic system with the P213 (no. 198) space group by Petter D. Battle in 1986.26 Fig. 1 shows the observed and calculated XRD patterns as well as their difference for the Rietveld refinement of the KSrSc2(PO4)3:0.01Eu2+ phosphor. The Rietveld refinement results and cell parameters for this phosphor are illustrated in Table 1. The phosphor was found to crystallize in the cubic system with the space group of P213 (no. 198). The refinement
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Fig. 1 (a) Observed (crosses) and calculated (red solid line) powder XRD pattern of the KSSP:0.01Eu2+ sample. The blue solid lines represent the difference between experimental and calculated data and the green sticks mark the Bragg reflection positions. (b) The coordination environment of Sr2+ and Sc3+ ions.
Table 1 Rietveld refinement results KSrSc2(PO4)3:0.01Eu2+ phosphor
Formula Radiation type 2θ range Symmetry Space group Cell parameters Reliability factors
and
crystal
data
for
the
KSr0.99Eu0.01Sc(PO4)3 Cu Kα radiation with λ = 1.5405 Å 10–90° Cubic system P213 (no. 198) a = 10.01 Å, b = 10.01 Å, c = 10.01 Å, α = β = γ = 90°, V = 1003.055 Å3; Z = 4 χ2 = 5.217, Rwp = 7.87%, Rp = 5.37%
finally converged to Rp = 5.37%, Rwp = 7.87%, and χ2 = 5.217, indicating that all the observed peaks satisfy the reflection conditions and our prepared phosphor is of single phase. In the KSrSc2(PO4)3 host, there are two kinds of Sr and Sc sites and their coordination environment is shown in Fig. 1(b). The Sr2+ ions are coordinated by nine oxygen atoms, while the Sc3+ ions are coordinated by six oxygen atoms forming distorted [ScO6] octahedra further connected by the [PO4] tetrahedra. Considering the ionic radii and valence state, the doped Ce3+ and Tb3+ ions are thought to substitute the Sc3+ ions, while the Eu2+ ions are expected to substitute Sr2+ ions forming emission centres. The SEM image of the prepared KSSP sample is shown in Fig. 2. The sample presents an irregular shape with the particle size ranging from 1 to 5 μm. The composition of the sample has been analysed by energy-dispersive X-ray spectroscopy (EDX). From the result demonstrated in Fig. 2, we can see that the sample is composed of K, Sr, Sc, P, and O atoms. The atomic ratio in the sample is K : Sr : Sc : P = 1 : 1.15 : 2.21 : 3.09, which is similar to their ratio in the formula.
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Fig. 2 EDX spectrum and surface morphology of the prepared KSSP sample.
3.2 Luminescence properties, colour hue tuning, and the quantum efficiency The Ce3+ ion as an activator has been doped into the host and the luminescence properties have been investigated. Fig. 3(a) shows the excitation and emission spectra of the KSSP:0.03Ce3+ phosphor. The excitation spectrum covers the region from 200 to 350 nm and the emission spectrum ranges from 325 to 450 nm with a maximum at 380 nm. The broad excitation and emission spectra are caused by the spin and orbital allowed electron transition between the 4f 15d0 ground state and the 4f 05d1 excitation state. The emission wavelength shows a small red shift with the increasing Ce3+ content as shown in Fig. 3(b). The red shift can be caused by the reabsorption process or energy transfer between the different sites of the Ce3+ ions. Since the overlap between the excitation and emission spectra is small, the reabsorption process can be excluded and the energy transfer between the different sites of
Fig. 3 (a) PL and PLE spectra of the KSSP:0.03Ce3+ phosphor; (b) emission spectra of Ce3+ doped KSSP phosphors with different Ce3+ contents (the inset shows the dependence of the emission intensity on the Ce3+ content).
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Fig. 4 (a) PL and PLE spectra of the KSSP:0.02Eu2+ phosphor; (b) emission spectra of Eu2+ doped KSSP phosphors with different Eu2+ contents. (c) Relationship between the emission intensity of KSSP:yEu2+ phosphors and the Eu2+ content.
the Ce3+ ions is responsible for this red shift. With the increasing Ce3+ content, the emission intensity of the phosphors first increased gradually to a maximum and then decreased due to the concentration quenching. The dependence of the emission intensity on the doping content is also shown in the inset of Fig. 3(b). The luminescence properties of the Eu2+ doped phosphor have also been investigated. As shown in Fig. 4(a), monitored at 445 nm, the photoluminescence excitation (PLE) spectrum of the KSSP:0.02Eu2+ phosphor shows a broad excitation band ranging from 225 to 375 nm, which can be ascribed to the electron transition from the 4f 7 ground state to the 4f 65d1 excited state of the Eu2+ ion. Under 300 nm irradiation, the sample can emit intense blue light with an emission band in the range from 380 to 525 nm with a maximum at 445 nm. The full-width at half-maximum for the asymmetric band is about 75 nm. Moreover, the PL spectrum of the KSSP:0.02Eu2+ phosphor is decomposed into two separate Gaussian components and is shown in Fig. S1 (ESI†). The two splitting bands centered at 2.47 and 2.78 eV can be ascribed to different emission sites in the KSSP host. Fig. 4(b) shows the emission spectra of KSSP:yEu2+ samples with y varying from 0.005 to 0.05 under irradiation at 300 nm. One can see that the optimal doping content occurs at y = 0.03, beyond which the emission intensity will decrease due to concentration quenching. The relationship between the emission intensity and the doping content of Eu2+ ions is also shown in Fig. 4(c) for an intuitional view. The Tb3+ ion as an important activator can be well incorporated into many host lattices giving intense emission. It is known to us that the Tb3+ has a simple 4f-configurational energy level structure with low-energy state 7FJ ( J = 6, 5…, 0) and excited states 5D3 and 5D4. Generally, the blue emissions resulting from transitions of 5D3→7FJ will dominate at a very low doping concentration of the Tb3+ ion. As the Tb3+ concentration increases, the cross relaxation between the 5D3–5D4 and
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Fig. 5 (a) PL and PLE spectra of the KSSP:0.05Tb3+ phosphor. (b) PL and PLE spectra of the KSSP:0.03Eu2+,0.10Tb3+ phosphor.
F0–7F6 occurs, which can increase the population of the 5D4 energy level, correspondingly enhancing the transitions of 5D4 to 7FJ and producing green light. Fig. 5(a) represents the photoluminescence excitation and emission spectra of the KSSP:0.05Tb3+ phosphor. Monitored at 550 nm, the excitation spectrum shows a broad band ranging from 200 to 280 nm and several weak peaks in the range of 300 to 500 nm (enlarged and shown in the inset of Fig. 5(a)). The broad band results from the 4f 8–4f 75d1 transition of the Tb3+ ions, while the excitation peaks are caused by the spin and orbital forbidden intra-4f transitions. Under 230 nm irradiation, the as-prepared sample has two emission sets, one is the blue emission caused by the 5D3→7FJ transition, and the other is the green emission caused by the 5D4→7FJ transition. As we know that the 4f–5d transition of Eu2+ is much stronger than the f–f transition of Tb3+ ions according to Laporte parity selection rules,27 it is possible to enhance the Tb3+ emission by energy transfer from the Eu2+ to Tb3+ ions. Fig. 5(b) demonstrates the excitation and emission spectra of the Eu2+ and Tb3+ codoped phosphors. When monitored at 445 nm, the phosphor shows the excitation spectra of the Eu2+ ions, while when monitored at 550 nm which is the typical emission of the Tb3+ ions, the excitation spectrum contains not only the excitation band of the Tb3+ ions but also that of the Eu2+ ions, which can validate the occurrence of the energy transfer from the Eu2+ to Tb3+ ions. To further study the energy transfer between the Eu2+ and 3+ Tb ions, a series of KSSP:Eu2+,Tb3+ phosphors with various Eu2+ and Tb3+ contents have been prepared. Fig. 6(a) shows the variation of emission spectra of KSSP:0.03Eu2+,zTb3+ phosphors with z varying from 0 to 0.18. With the increasing Tb3+ content, the green emission peaks caused by the 5D4–7FJ transition reached a maximum value at z = 0.15, and then decreased due to concentration quenching. Meanwhile, the emission intensity of the Eu2+ ions dropped greatly as z changed from 0 to 0.18. Fig. 6(b) shows the emission spectra of the KSSP:yEu2+,0.10Tb3+ phosphor. From this figure, we can 7
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Fig. 6 Emission spectra of (a) KSSP:0.03Eu2+,zTb3+ phosphors with different Tb3+ contents. (b) KSSP:yEu2+, 0.10Tb3+ phosphors with different Eu2+ contents.
see that the codoping of the Eu2+ ions has greatly improved the Tb3+ emission under irradiation by comparing the emission spectra of the KSSP:yEu2+,0.10Tb3+ and KSSP:0.10Tb3+ phosphors. Although the Tb3+ content was fixed, the emission intensity of the Tb3+ ion varied with the changing Eu2+ concentration. The emission intensity of both the Eu2+ and Tb3+ ions reached a maximum value at y = 0.02, and then decreased with a further increment of the Eu2+ ion. The changing Tb3+ emission intensity in KSSP:yEu2+,0.10Tb3+ phosphors can further validate the efficient energy transfer from the Eu2+ to Tb3+ ions. The variation of the Eu2+ and Tb3+ emission intensity in KSSP:0.03Eu2+,zTb3+ and KSSP:yEu2+,0.10Tb3+ phosphors is also shown in Fig. S2 (ESI†). Meanwhile, we have synthesized the Ce3+ and Tb3+ codoped phosphors and investigated their photoluminescence spectra and energy transfer properties. Fig. 7 demonstrates the excitation and emission spectra of the KSSP:0.03Ce3+,0.12Tb3+ phosphor. When monitored at 380 nm, the phosphor shows an excitation band of Ce3+ ions, while when monitored at 550 nm the excitation spectrum consists of both the excitation band of the Tb3+ ions ascribed to the f–d transition and the excitation band of the Ce3+ ions, indicating that there is energy transfer from the Ce3+ to Tb3+ ions. The variation of emission spectra on changing the doping content of Ce3+ and Tb3+ ions has also been studied and is shown in Fig. 8. In Fig. 8(a), one can see that the emission intensity of Ce3+ ions decreased monotonously with the increasing Tb3+ content, while the
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Fig. 7 Photoluminescence excitation and emission spectra of the KSSP:0.03Ce3+,0.12Tb3+ phosphor.
Fig. 8 Emission spectra of Ce3+ and Tb3+ codoped samples with various (a) Tb3+ contents and (b) Ce3+ contents.
emission intensity of Tb3+ ions first increased to a maximum and then decreased due to concentration quenching. For the KSSP:xCe3+,0.10Tb3+ phosphors, with the concentration of Ce3+ increasing from 0.005 to 0.04, both the emission intensity of Ce3+ and Tb3+ ions increased to a maximum at x = 0.03 and then decreased with further increment of the Ce3+ content. The variation of the Tb3+ emission intensity in the KSSP:
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Fig. 9 Variation of emission of doped ions in (a) KSSP:0.03Ce3+,0.09Tb3+ and (b) KSSP:0.03Eu2+,0.10Tb3+ samples.
xCe3+,0.10Tb3+ phosphors with a fixed Tb3+ content further validates the energy transfer from the Ce3+ to Tb3+ ions. As is known that the WLEDs usually work at a temperature as high as 150 °C, thus, it is essential to investigate the thermal stability of the prepared phosphors. Fig. 9(a) and 9(b) show the variation of the emission intensity of the doped rare earth ions in KSSP:0.03Ce3+,0.09Tb3+ and KSSP:0.03Eu2+,0.10Tb3+ phosphors, respectively. In the figure, we can see that both phosphors have good thermal stability and the emission intensity of the phosphors decreased gradually with the increasing temperature. At 423 K, the emission intensity of Ce3+ ions in KSSP:0.03Ce3+,0.09Tb3+ decreased to 81% of its initial value at 298 K, while the Tb3+ ions decreased to 88% of the emission intensity at 298 K. For the KSSP:0.03Eu2+,0.10Tb3+ phosphor, the emission intensity of Eu2+ and Tb3+ ions at T = 423 K decreased to 55% and 79% of the value at T = 298 K, respectively. From the investigation results, we can see that the KSSP:0.03Ce3+,0.09Tb3+ phosphor has a better thermal stability than the KSSP:0.03Eu2+,0.10Tb3+ phosphor. The temperature quenching for the phosphors can be ascribed to the more intense electron–phonon interaction in both ground and excited states at higher temperature. The CIE coordinates of the typical prepared samples have been calculated and are listed in Table 2. The coordinates of Ce3+ and Eu2+ singly doped samples were determined to be (0.163, 0.027) and (0.153, 0.104), respectively. With the increasing Tb3+ content, the CIE coordinates of KSSP:0.03Eu2+,zTb3+ samples changed from (0.153, 0.104) to (0.197, 0.271) due to different emission components of the Eu2+ and Tb3+ ions resulting from the energy transfer from the Eu2+ to Tb3+ ions. For the KSSP:0.03Ce3+,zTb3+ phosphors, the CIE coordinates varied from (0.163, 0.027) to (0.272, 0.502) when the Tb3+ content was increased from 0 to 0.15. Moreover, the quantum efficiency as an important factor for phosphors has also been measured and is listed in Table 2. For the KSSP:0.03Ce3+ phosphor the quantum efficiency is as high as 56.1% and for the Eu2+ singly doped sample the quantum efficiency is 36.9%.
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Table 2 CIE coordinates and quantum efficiency of the prepared samples
Sample
CIE coordinates (x, y)
Quantum efficiency (%)
KSSP:0.03Ce3+ KSSP:0.03Eu2+ KSSP:0.03Eu2+,0.02Tb3+ KSSP:0.03Eu2+,0.05Tb3+ KSSP:0.03Eu2+,0.07Tb3+ KSSP:0.03Eu2+,0.10Tb3+ KSSP:0.03Eu2+,0.12Tb3+ KSSP:0.03Eu2+,0.15Tb3+ KSSP:0.03Eu2+,0.18Tb3+ KSSP:0.03Ce3+,0.03Tb3+ KSSP:0.03Ce3+,0.06Tb3+ KSSP:0.03Ce3+,0.09Tb3+ KSSP:0.03Ce3+,0.12Tb3+ KSSP:0.03Ce3+,0.15Tb3+
(0.163, 0.027) (0.153, 0.104) (0.162, 0.139) (0.172, 0.183) (0.178, 0.204) (0.185, 0.229) (0.189, 0.244) (0.195, 0.265) (0.197, 0.271) (0.229, 0.308) (0.255, 0.429) (0.264, 0.471) (0.268, 0.486) (0.272, 0.502)
56.1 36.9 44.4 41.3 44.8 39.2 43.2 39.4 37.6 63.3 68.9 66.8 77.2 73.1
The codoping of Tb3+ ions into the host with Eu2+ and Ce3+ ions has increased the quantum efficiency of the phosphors. The quantum efficiency of the KSSP:0.03Ce3+,0.12Tb3+ phosphor can reach 77.2%. From Table 2, we can see that KSSP:0.03Ce3+ and KSSP:0.03Ce3+,zTb3+ phosphors have higher quantum efficiencies than the KSSP:0.03Eu2+ and KSSP:0.03Eu2+,zTb3+ phosphors, respectively. Moreover, the quantum efficiency of the phosphors can be further evaluated by optimizing the synthesis conditions. 3.3 Analysis of the energy transfer mechanism for Eu2+→Tb3+ The nonradiative Eu2+→Tb3+ energy transfer has been justified by the above study on luminescence properties of the obtained Eu2+ and Tb3+ codoped samples. To further obtain evidence of the Eu2+→Tb3+ energy transfer and study the luminescence dynamics, the decay curves of the Eu2+ ions in KSSP:0.03Eu2+, zTb3+ samples with different Tb3+ contents have been obtained and are shown on the logarithmic intensity scale in Fig. 10. We can see that all the decay curves deviated from the single exponential function and the doping of Tb3+ ions has modified
the Eu2+ fluorescence dynamics. The effective lifetime of the Eu2+ ions in different phosphors has been calculated by using the following equation: Ð1 tIðtÞdt τ ¼ Ð01 ð1Þ 0 IðtÞdt where I(t ) stands for the emission intensity of the Eu2+ ions at time t. For the KSSP:0.03Eu2+,zTb3+ samples with the Tb3+ content being 0, 0.02, 0.05, 0.07, 0.10, 0.12, 0.15 and 0.18, the effective lifetimes were determined to be 600.9, 583.2, 513.8, 500.6, 482.3, 452.7, 424.2 and 395.0 ns, respectively. With the increasing Tb3+ content, the lifetime decreased monotonously which is shown in Fig. S3 (ESI†). This phenomenon can be ascribed to the increasing energy transfer rate from the Eu2+ to Tb3+ ions since the measured lifetime τ can be demonstrated as28–30 1 1 ¼ þ Anr þ P t τ τ0
ð2Þ
in which τ0 is the radiative lifetime; Anr stands for the nonradiative rate caused by the multiphonon relaxation, and Pt stands for the energy transfer rate between the doped ions. When the Tb3+ ions as activators were codoped into the phosphor and distributed randomly, the excited Eu2+ ions will transfer their energy to the closer Tb3+ ions, accelerating the decay process. Thus, the lifetime of the Eu2+ ions will decrease gradually with the increasing Tb3+content. The energy transfer efficiency for the Eu2+→Tb3+ can be calculated by the equation ηT ¼ 1 τs =τs0
ð3Þ
in which τs is the lifetime of Eu2+ in the presence of Tb3+ ions; τs0 is the lifetime of Eu2+ in the phosphor without Tb3+ions. The calculated energy transfer efficiency is shown in Fig. S3 (ESI†). To investigate the mechanism of Eu2+→Tb3+ energy transfer, we employed the Inokuti–Hirayama (I–H) model.31 As is known, the energy migration process can be negligible compared with the energy transfer between donors and acceptors if the donors and acceptors in the host are distributed uniformly in the host. The normalized intensity of the Eu2+ fluorescence in KSSP:0.03Eu2+,zTb3+ phosphors can be written as I D ðtÞ ¼ I D0 ðtÞf ðtÞ
ð4Þ
where ID0(t ) is the decay function of donors after excitation without the acceptors, the function f (t ) represents the loss of excited donors due to one way energy transfer to the acceptors. If the energy transfer rate between the donor and the acceptor is inversely proportional to the distance, according to the I–H formula, we can obtain 4 3 f ðtÞ ¼ exp πΓ 1 ð5Þ nA α3=m t3=m 3 m Fig. 10 Decay curves of KSSP:0.03Eu2+,zTb3+ phosphors with different Tb3+ doping contents (excited at 300 nm, monitored at 445 nm).
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where nA is the number of acceptor ions per unit volume, α is the rate constant for energy transfer, m = 6, 8, and 10 corre-
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Acknowledgements
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Our work was financially supported by the National Natural Science Foundation of China (grant nos. 21271167 and 11304309) and the Fund for Creative Research Groups (grant no. 21221061), and the National Basic Research Program of China (973 Program, grant no. 2014CB643803).
Notes and references
Fig. 11 Dependence of log{ln[ID0(t )/ID(t )]} on log(t ) for KSSP:0.03Eu2+, zTb3+ (z = 0.07, 0.12, 0.15) samples. The red lines indicate the fitting behaviours.
sponds to electric dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. After calculation, we can obtain logfln½I D0 ðtÞ=I D ðtÞg /
3 logðtÞ m
ð6Þ
which represents that log{ln[ID0(t )/ID(t )]} acts as a linear function of log(t ) with a slope of 3/m. The log{ln[ID0(t )/ID(t )]} for KSSP:0.03Eu2+,zTb3+ phosphors was plotted versus log(t ) and is shown in Fig. 11. The slopes of fitting lines were calculated to be 0.401, 0.369 and 0.395 for KSSP:0.03Eu2+,zTb3+ samples with z = 0.07, 0.12 and 0.15, respectively, and thus the values of m were determined to be 7.48, 7.59, and 8.13, respectively, indicating that the electric dipole–quadrupole interaction is responsible for the energy transfer from the Eu2+ to Tb3+ions.
Conclusions In summary, we have synthesized a series of novel Ce3+, Eu2+ and Tb3+ doped KSrSc2(PO4)3 phosphors by traditional high temperature solid state reaction. The KSrSc2(PO4)3 crystallized in the cubic system with the space group of P213 (no. 198). The KSrSc2(PO4)3:Ce3+ and KSrSc2(PO4)3:Eu2+ phosphors show intense broad emission bands due to their spin and orbital allowed 5d→4f transitions, while the KSrSc2(PO4)3:Tb3+ phosphor shows the typical Tb3+ emissions due to the 5D3–7FJ and 5 D4–7FJ transitions. Phosphors with tunable colour hues were obtained by codoping the Tb3+ ions into the Eu2+ and Ce3+ singly doped samples, respectively. The occurrence of energy transfer from Eu2+ to Tb3+ ions had been confirmed by systematical optical property and decay lifetime studies. By using the Inokuti–Hirayama theoretical model, the dipole–quadrupole interaction mechanism was validated to be responsible for the Eu2+→Tb3+ energy transfer. Moreover, the quantum efficiency of the prepared phosphors had been measured.
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1 P. Pust, V. Weiler, C. Hecht, A. Tücks, A. S. Wochnik, A.-K. Hen, D. Wiechert, C. Scheu, P. J. Schmidt and W. Schnick, Nat. Mater., 2014, 13, 891–896. 2 S. Schmiechen, H. Schneider, P. Wagatha, C. Hecht, P. J. Schmidt and W. Schnick, Chem. Mater., 2014, 26, 2712–2719. 3 S.-P. Lee, C.-H. Huang and T.-M. Chen, J. Mater. Chem. C, 2014, 2, 8925–8931. 4 W. B. Park, S. P. Singh, C. Yoonb and K.-S. Sohn, J. Mater. Chem. C, 2013, 1, 1832–1839. 5 A. J. F. -Carrión, M. Ocaña, J. G. -Sevillano, E. Cantelar and A. I. Becerro, J. Phys. Chem. C, 2014, 118, 18035–18043. 6 T. Suehiro, R.-J. Xie and N. Hirosaki, Ind. Eng. Chem. Res., 2014, 53, 2713–2717. 7 Q. Wu, Z. Yang, Z. Zhao, M. Que, X. Wang and Y. Wang, J. Mater. Chem. C, 2014, 2, 4967–4973. 8 S. Miao, Z. Xia, J. Zhang and Q. Liu, Inorg. Chem., 2014, 53, 10386–10393. 9 L. Wang, X. Zhang, Z. Hao, Y. Luo, X.-j. Wang and J. Zhang, Opt. Express, 2010, 18, 25177–25182. 10 Y. Jia, Y. Huang, Y. Zheng, N. Guo, H. Qiao, Q. Zhao, W. Lv and H. You, J. Mater. Chem., 2012, 22, 15146–15152. 11 X. Li, J. D. Budai, F. Liu, J. Y. Howe, J. Zhang, X.-J. Wang, Z. Gu, C. Sun, R. S. Meltzer and Z. Pan, Light: Sci. Appl., 2013, 2, e50. 12 K. Y. Jung, H. W. Lee and H.-K. Jung, Chem. Mater., 2006, 18, 2249–2255. 13 M. Shang, G. Li, D. Geng, D. Yang, X. Kang, Y. Zhang, H. Lian and J. Lin, J. Phys. Chem. C, 2012, 116, 10222–10231. 14 B. Vinila, P. D. Dimple, M. Mohapatra, S. V. Godbole, R. Ghildiyal and A. K. Tyagi, Nanotechnology, 2009, 20, 125707. 15 E. Pavitra, G. S. R. Raju, Y. H. Ko and J. S. Yu, Phys. Chem. Chem. Phys., 2012, 14, 11296–11307. 16 A. J. Fischer, A. A. Allerman, M. H. Crawford, K. H. A. Bogart, S. R. Lee, R. J. Kaplar, W. W. Chow, S. R. Kurtz, K. W. Fullmer and J. J. Figiel, Appl. Phys. Lett., 2004, 84, 3394–3396. 17 V. Adivarahan, S. Wu, J. P. Zhang, A. Chitnis, M. Shatalov, V. Mandavilli, R. Gaska and M. A. Khan, Appl. Phys. Lett., 2004, 84, 4762–4764. 18 T.-S. Chan, R.-S. Liu and I. Baginskiy, Chem. Mater., 2008, 20, 1215–1217. 19 N. Guo, W. Lü, Y. Jia, W. Lv, Q. Zhao and H. You, ChemPhysChem, 2013, 14, 192–197.
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PAPER
Cite this: DOI: 10.1039/c4dt03906h
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Optical properties and energy transfer of a novel KSrSc2(PO4)3:Ce3+/Eu2+/Tb3+ phosphor for white light emitting diodes† Mengmeng Jiao,a,b Wenzhen Lv,a,b Wei Lü,a Qi Zhao,a,b Baiqi Shaoa,b and Hongpeng You*a A new langbeinite-type phosphate KSrSc2(PO4)3 has been synthesized by conventional high temperature solid state reaction. Rietveld structure refinement, a field emission scanning electron microscope, photoluminescence spectra, quantum efficiency as well as lifetimes were used to characterize the samples. Structure refinement reveals that KSrSc2(PO4)3 has two kinds of Sr2+ and Sc3+ sites for the doped ions to occupy, forming emission centers. The KSrSc2(PO4)3:Ce3+ and KSrSc2(PO4)3:Eu2+ phosphors both have broad excitation and emission bands due to spin- and orbit-allowed electron transitions. Phosphors with tunable
blue
to
blue-green 3+
Received 18th December 2014, Accepted 14th January 2015 DOI: 10.1039/c4dt03906h www.rsc.org/dalton
colors
were
obtained
by
codoping
the
Tb3+
ions
into
the
2+
KSrSc2(PO4)3:0.03Ce and KSrSc2(PO4)3:0.03Eu phosphors with varying contents. The mechanism of Eu2+→Tb3+ energy transfer is determined to be a dipole–quadrupole interaction in terms of the experimental results and analysis of photoluminescence spectra and decay curves of the phosphors by using the Inokuti–Hirayama theoretical model. Our prepared KSrSc2(PO4)3:Ce3+,Tb3+ and KSrSc2(PO4)3:Eu2+, Tb3+ phosphors are of potential value for UV excited WLEDs.
Introduction In recent years, white light emitting diodes (WLEDs) have been a research focus in both scientific areas and technical industries, due to their much more efficient conversion of electric energy to visible light compared with traditional lighting sources. The WLEDs are thought to be the most superior candidate for replacing incandescent light bulbs and fluorescent lamps and are expected to dominate the market soon.1–6 Now, the most commonly used method to obtain white light is coating the YAG:Ce3+ phosphors on InGaN chips. However, the high correlated colour temperature and low colour rendering index which are caused by the deficiency of the red component in the emission restrict its further application.7,8 Researchers have made much effort to improve the performance of WLEDs fabricated with blue chips coated with yellow phosphors. For example, Zhang et al. have incorporated Cr3+ and Pr3+ ions into the YAG: Ce3+ phosphor and You et al. have codoped Mn2+ and Ce3+ ions into the Y3Al5O12 host;9,10 Li et al. have synthesized novel
a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Science, Changchun 130022, P. R. China. E-mail: [email protected] b University of the Chinese Academy of Science, Beijing 100049, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4dt03906h
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Ba0.93Eu0.07Al2O4 yellow phosphor with emission centred at 580 nm under 440 nm excitation.11 Another approach to obtain white light is the use of UV chips coated with blue, green and red phosphors. Since this combination can have smoother spectral distribution over the whole visible range, the obtained WLEDs will have an excellent colour rending index.12,13 Although the UV-A chips with the emission wavelength ranging from 320 to 400 nm are mainly used in this kind of fabrication, the deep UV LED chips with the emission wavelength shorter than 320 nm have attracted much attention and are being developed for usage due to their potential value in biochemical and germicidal applications.14–17 In this case, it is of great importance to explore efficient UV excited phosphors. Researchers have investigated many kinds of inorganic materials including silicate, aluminate, phosphate, nitride, molybdate, tungstate, etc. Among them, rare earth ion doped phosphate materials have attracted much attention since they have low sintering temperature, good thermal stability, and luminescence properties.18–22 It is known that both the Eu2+ and Ce3+ ions have broad emission and excitation bands with high efficiency since the 4f–5d electron transition is spin- and orbit-allowed. Moreover, their excitation and emission wavelengths can be tuned on a large scale depending on different host lattices.23,24 The Tb3+ ions are also important activators since they can have intense green emission due to their 5 D4–7FJ electron transition. Based on the above reasons, in this
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paper we have prepared Eu2+, Ce3+ and Tb3+ doped novel KSrSc2(PO4)3 phosphate phosphors and studied their structure and photoluminescence properties in detail. The obtained KSrSc2(PO4)3:Ce3+,Tb3+ and KSrSc2(PO4)3:Eu2+,Tb3+ phosphors might be of potential value for UV excited WLEDs.
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2. Experimental section 2.1
Materials and synthesis
A series of KSrSc2(PO4)3:Ce3+/Eu2+/Tb3+ (KSSP: Ce3+/Eu2+/Tb3+) phosphors have been synthesized by the traditional solid state reaction. The raw materials KHCO3 (A.R.), SrCO3 (A.R.), Sc2O3 (A.R.), (NH4)2HPO4 (A.R.), CeO2 (99.99%), Eu2O3 (99.99%) and Tb4O7 (99.99%) were weighed according to the stoichiometric ratio and ground in an agate mortar for 15 min to mix thoroughly. After heating in an oven at 180 °C for 4 h, the obtained mixtures were transferred to a tube furnace with the temperature being 1100 °C for 3 h under a reductive atmosphere (20% H2 + 80% N2). The obtained samples were finally cooled to room temperature and ground again for further measurements. 2.2
Measurements and characterization
The powder X-ray diffraction patterns of the prepared samples were obtained using the Bruker AXS D8 operating at 40 kV and 40 mA using monochromated Cu Kα radiation (λ = 1.5405 Å). The size and surface morphology were inspected with a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). Photoluminescence excitation and emission spectra were obtained using a Hitachi F-7000 spectrophotometer and a 150 W xenon lamp was used as the excitation source. The decay curves of the samples were obtained using a LeCroy Wave Runner 6100 digital oscilloscope (1 GHz), and a tunable laser ( pulse width = 4 ns, gate = 50 ns) was used as the excitation source. The photoluminescence quantum yield of the phosphors was determined on a quantum yield measurement system (C9920-02, Hamamatsu Photonics K.K., Japan).
3. Results and discussion 3.1
XRD refinement and the crystal structure
To study the structure of the phosphor, Rietveld structure refinement of KSrSc2(PO4)3:0.01Eu2+ has been performed by using the general structure analysis system (GSAS) program.25 In the refinement, the crystallographic data of KBaFe2(PO4)3 were used as the initial structure model. The langbeinite-type phosphate KBaFe2(PO4)3 was reported to have the cubic system with the P213 (no. 198) space group by Petter D. Battle in 1986.26 Fig. 1 shows the observed and calculated XRD patterns as well as their difference for the Rietveld refinement of the KSrSc2(PO4)3:0.01Eu2+ phosphor. The Rietveld refinement results and cell parameters for this phosphor are illustrated in Table 1. The phosphor was found to crystallize in the cubic system with the space group of P213 (no. 198). The refinement
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Fig. 1 (a) Observed (crosses) and calculated (red solid line) powder XRD pattern of the KSSP:0.01Eu2+ sample. The blue solid lines represent the difference between experimental and calculated data and the green sticks mark the Bragg reflection positions. (b) The coordination environment of Sr2+ and Sc3+ ions.
Table 1 Rietveld refinement results KSrSc2(PO4)3:0.01Eu2+ phosphor
Formula Radiation type 2θ range Symmetry Space group Cell parameters Reliability factors
and
crystal
data
for
the
KSr0.99Eu0.01Sc(PO4)3 Cu Kα radiation with λ = 1.5405 Å 10–90° Cubic system P213 (no. 198) a = 10.01 Å, b = 10.01 Å, c = 10.01 Å, α = β = γ = 90°, V = 1003.055 Å3; Z = 4 χ2 = 5.217, Rwp = 7.87%, Rp = 5.37%
finally converged to Rp = 5.37%, Rwp = 7.87%, and χ2 = 5.217, indicating that all the observed peaks satisfy the reflection conditions and our prepared phosphor is of single phase. In the KSrSc2(PO4)3 host, there are two kinds of Sr and Sc sites and their coordination environment is shown in Fig. 1(b). The Sr2+ ions are coordinated by nine oxygen atoms, while the Sc3+ ions are coordinated by six oxygen atoms forming distorted [ScO6] octahedra further connected by the [PO4] tetrahedra. Considering the ionic radii and valence state, the doped Ce3+ and Tb3+ ions are thought to substitute the Sc3+ ions, while the Eu2+ ions are expected to substitute Sr2+ ions forming emission centres. The SEM image of the prepared KSSP sample is shown in Fig. 2. The sample presents an irregular shape with the particle size ranging from 1 to 5 μm. The composition of the sample has been analysed by energy-dispersive X-ray spectroscopy (EDX). From the result demonstrated in Fig. 2, we can see that the sample is composed of K, Sr, Sc, P, and O atoms. The atomic ratio in the sample is K : Sr : Sc : P = 1 : 1.15 : 2.21 : 3.09, which is similar to their ratio in the formula.
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Fig. 2 EDX spectrum and surface morphology of the prepared KSSP sample.
3.2 Luminescence properties, colour hue tuning, and the quantum efficiency The Ce3+ ion as an activator has been doped into the host and the luminescence properties have been investigated. Fig. 3(a) shows the excitation and emission spectra of the KSSP:0.03Ce3+ phosphor. The excitation spectrum covers the region from 200 to 350 nm and the emission spectrum ranges from 325 to 450 nm with a maximum at 380 nm. The broad excitation and emission spectra are caused by the spin and orbital allowed electron transition between the 4f 15d0 ground state and the 4f 05d1 excitation state. The emission wavelength shows a small red shift with the increasing Ce3+ content as shown in Fig. 3(b). The red shift can be caused by the reabsorption process or energy transfer between the different sites of the Ce3+ ions. Since the overlap between the excitation and emission spectra is small, the reabsorption process can be excluded and the energy transfer between the different sites of
Fig. 3 (a) PL and PLE spectra of the KSSP:0.03Ce3+ phosphor; (b) emission spectra of Ce3+ doped KSSP phosphors with different Ce3+ contents (the inset shows the dependence of the emission intensity on the Ce3+ content).
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Fig. 4 (a) PL and PLE spectra of the KSSP:0.02Eu2+ phosphor; (b) emission spectra of Eu2+ doped KSSP phosphors with different Eu2+ contents. (c) Relationship between the emission intensity of KSSP:yEu2+ phosphors and the Eu2+ content.
the Ce3+ ions is responsible for this red shift. With the increasing Ce3+ content, the emission intensity of the phosphors first increased gradually to a maximum and then decreased due to the concentration quenching. The dependence of the emission intensity on the doping content is also shown in the inset of Fig. 3(b). The luminescence properties of the Eu2+ doped phosphor have also been investigated. As shown in Fig. 4(a), monitored at 445 nm, the photoluminescence excitation (PLE) spectrum of the KSSP:0.02Eu2+ phosphor shows a broad excitation band ranging from 225 to 375 nm, which can be ascribed to the electron transition from the 4f 7 ground state to the 4f 65d1 excited state of the Eu2+ ion. Under 300 nm irradiation, the sample can emit intense blue light with an emission band in the range from 380 to 525 nm with a maximum at 445 nm. The full-width at half-maximum for the asymmetric band is about 75 nm. Moreover, the PL spectrum of the KSSP:0.02Eu2+ phosphor is decomposed into two separate Gaussian components and is shown in Fig. S1 (ESI†). The two splitting bands centered at 2.47 and 2.78 eV can be ascribed to different emission sites in the KSSP host. Fig. 4(b) shows the emission spectra of KSSP:yEu2+ samples with y varying from 0.005 to 0.05 under irradiation at 300 nm. One can see that the optimal doping content occurs at y = 0.03, beyond which the emission intensity will decrease due to concentration quenching. The relationship between the emission intensity and the doping content of Eu2+ ions is also shown in Fig. 4(c) for an intuitional view. The Tb3+ ion as an important activator can be well incorporated into many host lattices giving intense emission. It is known to us that the Tb3+ has a simple 4f-configurational energy level structure with low-energy state 7FJ ( J = 6, 5…, 0) and excited states 5D3 and 5D4. Generally, the blue emissions resulting from transitions of 5D3→7FJ will dominate at a very low doping concentration of the Tb3+ ion. As the Tb3+ concentration increases, the cross relaxation between the 5D3–5D4 and
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Fig. 5 (a) PL and PLE spectra of the KSSP:0.05Tb3+ phosphor. (b) PL and PLE spectra of the KSSP:0.03Eu2+,0.10Tb3+ phosphor.
F0–7F6 occurs, which can increase the population of the 5D4 energy level, correspondingly enhancing the transitions of 5D4 to 7FJ and producing green light. Fig. 5(a) represents the photoluminescence excitation and emission spectra of the KSSP:0.05Tb3+ phosphor. Monitored at 550 nm, the excitation spectrum shows a broad band ranging from 200 to 280 nm and several weak peaks in the range of 300 to 500 nm (enlarged and shown in the inset of Fig. 5(a)). The broad band results from the 4f 8–4f 75d1 transition of the Tb3+ ions, while the excitation peaks are caused by the spin and orbital forbidden intra-4f transitions. Under 230 nm irradiation, the as-prepared sample has two emission sets, one is the blue emission caused by the 5D3→7FJ transition, and the other is the green emission caused by the 5D4→7FJ transition. As we know that the 4f–5d transition of Eu2+ is much stronger than the f–f transition of Tb3+ ions according to Laporte parity selection rules,27 it is possible to enhance the Tb3+ emission by energy transfer from the Eu2+ to Tb3+ ions. Fig. 5(b) demonstrates the excitation and emission spectra of the Eu2+ and Tb3+ codoped phosphors. When monitored at 445 nm, the phosphor shows the excitation spectra of the Eu2+ ions, while when monitored at 550 nm which is the typical emission of the Tb3+ ions, the excitation spectrum contains not only the excitation band of the Tb3+ ions but also that of the Eu2+ ions, which can validate the occurrence of the energy transfer from the Eu2+ to Tb3+ ions. To further study the energy transfer between the Eu2+ and 3+ Tb ions, a series of KSSP:Eu2+,Tb3+ phosphors with various Eu2+ and Tb3+ contents have been prepared. Fig. 6(a) shows the variation of emission spectra of KSSP:0.03Eu2+,zTb3+ phosphors with z varying from 0 to 0.18. With the increasing Tb3+ content, the green emission peaks caused by the 5D4–7FJ transition reached a maximum value at z = 0.15, and then decreased due to concentration quenching. Meanwhile, the emission intensity of the Eu2+ ions dropped greatly as z changed from 0 to 0.18. Fig. 6(b) shows the emission spectra of the KSSP:yEu2+,0.10Tb3+ phosphor. From this figure, we can 7
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Fig. 6 Emission spectra of (a) KSSP:0.03Eu2+,zTb3+ phosphors with different Tb3+ contents. (b) KSSP:yEu2+, 0.10Tb3+ phosphors with different Eu2+ contents.
see that the codoping of the Eu2+ ions has greatly improved the Tb3+ emission under irradiation by comparing the emission spectra of the KSSP:yEu2+,0.10Tb3+ and KSSP:0.10Tb3+ phosphors. Although the Tb3+ content was fixed, the emission intensity of the Tb3+ ion varied with the changing Eu2+ concentration. The emission intensity of both the Eu2+ and Tb3+ ions reached a maximum value at y = 0.02, and then decreased with a further increment of the Eu2+ ion. The changing Tb3+ emission intensity in KSSP:yEu2+,0.10Tb3+ phosphors can further validate the efficient energy transfer from the Eu2+ to Tb3+ ions. The variation of the Eu2+ and Tb3+ emission intensity in KSSP:0.03Eu2+,zTb3+ and KSSP:yEu2+,0.10Tb3+ phosphors is also shown in Fig. S2 (ESI†). Meanwhile, we have synthesized the Ce3+ and Tb3+ codoped phosphors and investigated their photoluminescence spectra and energy transfer properties. Fig. 7 demonstrates the excitation and emission spectra of the KSSP:0.03Ce3+,0.12Tb3+ phosphor. When monitored at 380 nm, the phosphor shows an excitation band of Ce3+ ions, while when monitored at 550 nm the excitation spectrum consists of both the excitation band of the Tb3+ ions ascribed to the f–d transition and the excitation band of the Ce3+ ions, indicating that there is energy transfer from the Ce3+ to Tb3+ ions. The variation of emission spectra on changing the doping content of Ce3+ and Tb3+ ions has also been studied and is shown in Fig. 8. In Fig. 8(a), one can see that the emission intensity of Ce3+ ions decreased monotonously with the increasing Tb3+ content, while the
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Fig. 7 Photoluminescence excitation and emission spectra of the KSSP:0.03Ce3+,0.12Tb3+ phosphor.
Fig. 8 Emission spectra of Ce3+ and Tb3+ codoped samples with various (a) Tb3+ contents and (b) Ce3+ contents.
emission intensity of Tb3+ ions first increased to a maximum and then decreased due to concentration quenching. For the KSSP:xCe3+,0.10Tb3+ phosphors, with the concentration of Ce3+ increasing from 0.005 to 0.04, both the emission intensity of Ce3+ and Tb3+ ions increased to a maximum at x = 0.03 and then decreased with further increment of the Ce3+ content. The variation of the Tb3+ emission intensity in the KSSP:
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Fig. 9 Variation of emission of doped ions in (a) KSSP:0.03Ce3+,0.09Tb3+ and (b) KSSP:0.03Eu2+,0.10Tb3+ samples.
xCe3+,0.10Tb3+ phosphors with a fixed Tb3+ content further validates the energy transfer from the Ce3+ to Tb3+ ions. As is known that the WLEDs usually work at a temperature as high as 150 °C, thus, it is essential to investigate the thermal stability of the prepared phosphors. Fig. 9(a) and 9(b) show the variation of the emission intensity of the doped rare earth ions in KSSP:0.03Ce3+,0.09Tb3+ and KSSP:0.03Eu2+,0.10Tb3+ phosphors, respectively. In the figure, we can see that both phosphors have good thermal stability and the emission intensity of the phosphors decreased gradually with the increasing temperature. At 423 K, the emission intensity of Ce3+ ions in KSSP:0.03Ce3+,0.09Tb3+ decreased to 81% of its initial value at 298 K, while the Tb3+ ions decreased to 88% of the emission intensity at 298 K. For the KSSP:0.03Eu2+,0.10Tb3+ phosphor, the emission intensity of Eu2+ and Tb3+ ions at T = 423 K decreased to 55% and 79% of the value at T = 298 K, respectively. From the investigation results, we can see that the KSSP:0.03Ce3+,0.09Tb3+ phosphor has a better thermal stability than the KSSP:0.03Eu2+,0.10Tb3+ phosphor. The temperature quenching for the phosphors can be ascribed to the more intense electron–phonon interaction in both ground and excited states at higher temperature. The CIE coordinates of the typical prepared samples have been calculated and are listed in Table 2. The coordinates of Ce3+ and Eu2+ singly doped samples were determined to be (0.163, 0.027) and (0.153, 0.104), respectively. With the increasing Tb3+ content, the CIE coordinates of KSSP:0.03Eu2+,zTb3+ samples changed from (0.153, 0.104) to (0.197, 0.271) due to different emission components of the Eu2+ and Tb3+ ions resulting from the energy transfer from the Eu2+ to Tb3+ ions. For the KSSP:0.03Ce3+,zTb3+ phosphors, the CIE coordinates varied from (0.163, 0.027) to (0.272, 0.502) when the Tb3+ content was increased from 0 to 0.15. Moreover, the quantum efficiency as an important factor for phosphors has also been measured and is listed in Table 2. For the KSSP:0.03Ce3+ phosphor the quantum efficiency is as high as 56.1% and for the Eu2+ singly doped sample the quantum efficiency is 36.9%.
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Table 2 CIE coordinates and quantum efficiency of the prepared samples
Sample
CIE coordinates (x, y)
Quantum efficiency (%)
KSSP:0.03Ce3+ KSSP:0.03Eu2+ KSSP:0.03Eu2+,0.02Tb3+ KSSP:0.03Eu2+,0.05Tb3+ KSSP:0.03Eu2+,0.07Tb3+ KSSP:0.03Eu2+,0.10Tb3+ KSSP:0.03Eu2+,0.12Tb3+ KSSP:0.03Eu2+,0.15Tb3+ KSSP:0.03Eu2+,0.18Tb3+ KSSP:0.03Ce3+,0.03Tb3+ KSSP:0.03Ce3+,0.06Tb3+ KSSP:0.03Ce3+,0.09Tb3+ KSSP:0.03Ce3+,0.12Tb3+ KSSP:0.03Ce3+,0.15Tb3+
(0.163, 0.027) (0.153, 0.104) (0.162, 0.139) (0.172, 0.183) (0.178, 0.204) (0.185, 0.229) (0.189, 0.244) (0.195, 0.265) (0.197, 0.271) (0.229, 0.308) (0.255, 0.429) (0.264, 0.471) (0.268, 0.486) (0.272, 0.502)
56.1 36.9 44.4 41.3 44.8 39.2 43.2 39.4 37.6 63.3 68.9 66.8 77.2 73.1
The codoping of Tb3+ ions into the host with Eu2+ and Ce3+ ions has increased the quantum efficiency of the phosphors. The quantum efficiency of the KSSP:0.03Ce3+,0.12Tb3+ phosphor can reach 77.2%. From Table 2, we can see that KSSP:0.03Ce3+ and KSSP:0.03Ce3+,zTb3+ phosphors have higher quantum efficiencies than the KSSP:0.03Eu2+ and KSSP:0.03Eu2+,zTb3+ phosphors, respectively. Moreover, the quantum efficiency of the phosphors can be further evaluated by optimizing the synthesis conditions. 3.3 Analysis of the energy transfer mechanism for Eu2+→Tb3+ The nonradiative Eu2+→Tb3+ energy transfer has been justified by the above study on luminescence properties of the obtained Eu2+ and Tb3+ codoped samples. To further obtain evidence of the Eu2+→Tb3+ energy transfer and study the luminescence dynamics, the decay curves of the Eu2+ ions in KSSP:0.03Eu2+, zTb3+ samples with different Tb3+ contents have been obtained and are shown on the logarithmic intensity scale in Fig. 10. We can see that all the decay curves deviated from the single exponential function and the doping of Tb3+ ions has modified
the Eu2+ fluorescence dynamics. The effective lifetime of the Eu2+ ions in different phosphors has been calculated by using the following equation: Ð1 tIðtÞdt τ ¼ Ð01 ð1Þ 0 IðtÞdt where I(t ) stands for the emission intensity of the Eu2+ ions at time t. For the KSSP:0.03Eu2+,zTb3+ samples with the Tb3+ content being 0, 0.02, 0.05, 0.07, 0.10, 0.12, 0.15 and 0.18, the effective lifetimes were determined to be 600.9, 583.2, 513.8, 500.6, 482.3, 452.7, 424.2 and 395.0 ns, respectively. With the increasing Tb3+ content, the lifetime decreased monotonously which is shown in Fig. S3 (ESI†). This phenomenon can be ascribed to the increasing energy transfer rate from the Eu2+ to Tb3+ ions since the measured lifetime τ can be demonstrated as28–30 1 1 ¼ þ Anr þ P t τ τ0
ð2Þ
in which τ0 is the radiative lifetime; Anr stands for the nonradiative rate caused by the multiphonon relaxation, and Pt stands for the energy transfer rate between the doped ions. When the Tb3+ ions as activators were codoped into the phosphor and distributed randomly, the excited Eu2+ ions will transfer their energy to the closer Tb3+ ions, accelerating the decay process. Thus, the lifetime of the Eu2+ ions will decrease gradually with the increasing Tb3+content. The energy transfer efficiency for the Eu2+→Tb3+ can be calculated by the equation ηT ¼ 1 τs =τs0
ð3Þ
in which τs is the lifetime of Eu2+ in the presence of Tb3+ ions; τs0 is the lifetime of Eu2+ in the phosphor without Tb3+ions. The calculated energy transfer efficiency is shown in Fig. S3 (ESI†). To investigate the mechanism of Eu2+→Tb3+ energy transfer, we employed the Inokuti–Hirayama (I–H) model.31 As is known, the energy migration process can be negligible compared with the energy transfer between donors and acceptors if the donors and acceptors in the host are distributed uniformly in the host. The normalized intensity of the Eu2+ fluorescence in KSSP:0.03Eu2+,zTb3+ phosphors can be written as I D ðtÞ ¼ I D0 ðtÞf ðtÞ
ð4Þ
where ID0(t ) is the decay function of donors after excitation without the acceptors, the function f (t ) represents the loss of excited donors due to one way energy transfer to the acceptors. If the energy transfer rate between the donor and the acceptor is inversely proportional to the distance, according to the I–H formula, we can obtain 4 3 f ðtÞ ¼ exp πΓ 1 ð5Þ nA α3=m t3=m 3 m Fig. 10 Decay curves of KSSP:0.03Eu2+,zTb3+ phosphors with different Tb3+ doping contents (excited at 300 nm, monitored at 445 nm).
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where nA is the number of acceptor ions per unit volume, α is the rate constant for energy transfer, m = 6, 8, and 10 corre-
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Acknowledgements
Published on 14 January 2015. Downloaded by Northern Illinois University on 28/01/2015 04:28:04.
Our work was financially supported by the National Natural Science Foundation of China (grant nos. 21271167 and 11304309) and the Fund for Creative Research Groups (grant no. 21221061), and the National Basic Research Program of China (973 Program, grant no. 2014CB643803).
Notes and references
Fig. 11 Dependence of log{ln[ID0(t )/ID(t )]} on log(t ) for KSSP:0.03Eu2+, zTb3+ (z = 0.07, 0.12, 0.15) samples. The red lines indicate the fitting behaviours.
sponds to electric dipole–dipole, dipole–quadrupole, and quadrupole–quadrupole interactions, respectively. After calculation, we can obtain logfln½I D0 ðtÞ=I D ðtÞg /
3 logðtÞ m
ð6Þ
which represents that log{ln[ID0(t )/ID(t )]} acts as a linear function of log(t ) with a slope of 3/m. The log{ln[ID0(t )/ID(t )]} for KSSP:0.03Eu2+,zTb3+ phosphors was plotted versus log(t ) and is shown in Fig. 11. The slopes of fitting lines were calculated to be 0.401, 0.369 and 0.395 for KSSP:0.03Eu2+,zTb3+ samples with z = 0.07, 0.12 and 0.15, respectively, and thus the values of m were determined to be 7.48, 7.59, and 8.13, respectively, indicating that the electric dipole–quadrupole interaction is responsible for the energy transfer from the Eu2+ to Tb3+ions.
Conclusions In summary, we have synthesized a series of novel Ce3+, Eu2+ and Tb3+ doped KSrSc2(PO4)3 phosphors by traditional high temperature solid state reaction. The KSrSc2(PO4)3 crystallized in the cubic system with the space group of P213 (no. 198). The KSrSc2(PO4)3:Ce3+ and KSrSc2(PO4)3:Eu2+ phosphors show intense broad emission bands due to their spin and orbital allowed 5d→4f transitions, while the KSrSc2(PO4)3:Tb3+ phosphor shows the typical Tb3+ emissions due to the 5D3–7FJ and 5 D4–7FJ transitions. Phosphors with tunable colour hues were obtained by codoping the Tb3+ ions into the Eu2+ and Ce3+ singly doped samples, respectively. The occurrence of energy transfer from Eu2+ to Tb3+ ions had been confirmed by systematical optical property and decay lifetime studies. By using the Inokuti–Hirayama theoretical model, the dipole–quadrupole interaction mechanism was validated to be responsible for the Eu2+→Tb3+ energy transfer. Moreover, the quantum efficiency of the prepared phosphors had been measured.
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