ARTICLE pubs.acs.org/JPCC
Warm-White-Emitting from Eu2+/Mn2+-Codoped Sr3Lu(PO4)3 Phosphor with Tunable Color Tone and Correlated Color Temperature Ning Guo,†,‡ Yuhua Zheng,†,‡ Yongchao Jia,†,‡ Hui Qiao,†,‡ and Hongpeng You*,† †
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China
bS Supporting Information ABSTRACT:
A series of single-component Sr3Lu(PO4)3:Eu2+,Mn2+ phosphors were successfully synthesized by solid-state reaction, and their photoluminescence properties were investigated. The Sr3Lu(PO4)3:Eu2+,Mn2+ phosphor system was efficiently excited at wavelength ranging from 250 to 420 nm, which is well-matched with ultraviolet (UV) light-emitting diode (LED) chips. As a result of fine-tuning of the emission composition of the Eu2+ and Mn2+ ions, warm white light emission can be realized by combining the emission of Eu2+ and Mn2+ in a single host lattice under UV light excitation. Efficient resonant energy transfer from the Eu2+ to Mn2+ ions was demonstrated to be a dipolequadrupole mechanism in such system, and the energy transfer efficiency increases with an increase in the Mn2+ doping content, which was confirmed by the luminescence spectra and fluorescence decay curves. In addition, the energy transfer efficiency and critical distance were also calculated. The results indicate that the developed phosphor can be used as a potential white-light-emitting phosphor for white LEDs.
1. INTRODUCTION Recently, the use of white light-emitting diodes (LEDs) for solid-state lighting application are attracting extensive research and commercial interest because of their long lifetime, high luminescence efficiency, low power consumption, and environment friendly characteristics.15 Currently, most commercially available white LEDs are fabricated by combining a blue diode chip with Y3Al5O12:Ce3+ yellow phosphor. This approach to generating a relative cool white light that is unsuitable for room lighting is due to the red deficiency of the spectral emission in such LED system.6,7 For general lighting applications it is essential to emit a warmer white light with low correlated color temperature (CCT). The combination of a UV LED chip with red, green, and blue phosphors is an alternative way to generate warm white light. However, the luminescence efficiency is low in this system owing to the reabsorption of emission colors and different aging rates for each phosphor.810 Therefore, the design of single-component phosphor pumped by UV chips is r 2011 American Chemical Society
of significance for white LEDs because single-component phosphor has the advantages of excellent color rendering indexes and the electro-optical design is simple to control the different colors in comparison with mixed phosphors. Up to now, several single-component white-light-emitting phosphors suitable for UV-pumped white LED have been reported, such as Sr3Al2O5Cl2:Ce3+,Eu2+,11 Ca2P2O7:Eu2+,Mn2+,12 and Ca9Gd(PO4)7:Eu2+,Mn2+.13 However, few single-component materials have been reported to emit satisfactory warm whitelight emission. Phosphors based on phosphates show excellent chemical stability and are considered to be efficient luminescent materials because the tetrahedral rigid 3D matrix of phosphate compounds is thought to be ideal for charge stabilization.14 Hence, there is an increasing interest in the synthesis of novel efficient single-component warm white phosphors having structures Received: October 14, 2011 Revised: November 22, 2011 Published: November 29, 2011 1329
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derived from the phosphate family. In this work, we report a facile method to achieve color tone and CCT modulation based on the energy transfer from the Eu2+ to Mn2+ ions in Sr3Lu(PO4)3 host. A series of single-component color-tunable white-light-emitting Sr3Lu(PO4)3:Eu2+,Mn2+ phosphors was synthesized. The corresponding photoluminescence (PL) and energy transfer mechanism of the Eu2+ and Mn2+ codoped Sr3Lu(PO4)3 phosphors are investigated in detail. In particular, the dependence of whitelight-emitting with different CCT on the concentration of Mn2+ activators was discussed.
2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. The powder samples Sr3(1‑m‑n)Lu(PO4)3:mEu2+,nMn2+ (SLuP:mEu2+,nMn2+) were synthesized by the solid-state reaction method. The reactants SrCO3 (A.R. (Analytical Reagent)), Lu2O3 (99.99%), NH4H2PO4 (A.R.), Eu2O3 (99.99%), and MnCO3 (A.R.) were weighed according to stoichiometric ratio. After mixing and grinding, the mixtures were first heated to 1300 °C for 3 h in a CO atmosphere and slowly cooled to room temperature. Then, the powders were ground and sintered at 1200 °C for 3 h under a 10% H290%N2 gas mixture. Finally, the as-synthesized samples were slowly cooled to room temperature inside the tube furnace under H2N2 flow. 2.2. Measurements and Characterization. The phase purity of SLuP:mEu2+,nMn2+ phosphors were carefully checked by using powder X-ray diffraction (XRD) analysis (Bruker AXS D8) in the 2θ range from 10 to 80°, with graphite monochromatized Cu Kα radiation (λ = 0.15405 nm) operating at 40 kV and 40 mA. The PL emission and photoluminescence excitation (PLE) spectra of the obtained powders were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The quantum efficiency (QE) was analyzed with a PL quantum-efficiency measurement system (C9920-02, Hamamatsu Photonics, Shizuoka) by a 150 W xenon lamp. All measurements were performed at room temperature.
3. RESULTS AND DISCUSSION 3.1. Phase Identification and Crystal Structure. As an eulytite structure mineral, Sr3Lu(PO4)3 has a cubic unit cell with lattice parameters a = 10.095 Å, V = 1028.77 Å3, and space group I43d (no. 220). In this structure, the Sr2+/Lu3+ pairs of cations are disordered on a single crystallographic site, whereas the oxygen atoms of the phosphate groups are distributed over three partially occupied sites.15,16 The phase purity of all samples was analyzed by XRD measurements. Figure 1 illustrates the XRD patterns of SLuP:0.005Eu2+,nMn2+ phosphors with varied doping Mn2+ contents (n). All observed peaks can be indexed to the standard data of Sr3Lu(PO4)3 with JCPDS card no 33-1344. No obvious impurity phase was detected when Eu2+ and Mn2+ were doped into the host lattice, indicating that the samples are complete solid solution. Besides, one can notice that the peaks of the samples shift toward higher angles with the increment of Mn2+ content, which is ascribed to the substitution of the larger Sr2+ by the smaller Mn2+. Additionally, the unit cell parameters a and V of SLuP:0.005Eu2+,nMn2+ samples were computed by indexing XRD data. From Figure 2, we can see that the unit cell parameters a and V for SLuP:0.005Eu2+,nMn2+ samples decrease
Figure 1. XRD patterns of SLuP:0.005Eu2+,nMn2+ samples. The standard data for Sr3Lu(PO4)3 (JCPDS card no. 33-1344) is shown as a reference.
Figure 2. Variation in lattice parameters a and V as a function of Mn2+ content (n) for SLuP:0.005Eu2+,nMn2+ samples.
Figure 3. PLE and PL spectra of SLuP:0.005Eu2+ phosphor.
linearly when n is gradually increased from 0.005 to 0.08, which is consistent with Vegard’s law.17 These observations confirm that the Eu2+ and Mn2+ ions have been effectively built into the host lattice, and the SLuP:0.005Eu2+,nMn2+ (from n = 0.005 to 0.08) samples form complete solid solutions. 1330
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Figure 4. PLE and PL spectra of SLuP:0.005Eu2+, 0.03Mn2+ phosphor.
3.2. Photoluminescence Properties. The PL and PLE spectra for the Eu2+-doped SLuP are presented in Figure 3. The PLE spectrum shows a broad absorption band from 250 to 420 nm, indicating that this phosphor can well match with the emission light of UV-LED chips. The Eu2+ singly doped sample exhibits a broad asymmetric blue-green emission band centering at 496 nm attributed to the typical 4f65d1-4f7 transition of the Eu2+ ions. As shown in Figure S1 of the Supporting Information, this broad emission band can be decomposed into three well-separated Gaussian components with maxima at 2.824, 2.504, and 2.221 eV (corresponding to 439, 495, and 558 nm, respectively) on an energy scale. These results indicate that there are three lattice sites occupied by Eu2+ ions, which are also supported by crystal structure of the host lattice.15,16,18 As shown in Figure S2 (Supporting Information), it is observed that there is a significant spectral overlap between the PL spectrum of Eu2+ with the PLE spectrum of Mn2+, indicating that a resonance-type energy transfer from the Eu2+ to Mn2+ ions is expected in the codoped samples.12,19 Figure 4 illustrates the PLE and PL spectra of SLuP:0.005Eu2+,0.03Mn2+ samples. It can be seen that the PLE spectrum monitoring the red emission of the Mn2+ is similar to that monitoring the blue-green emission of the Eu2+ ions, which means that the Mn2+ ions are essentially excited through the Eu2+ ions, thereby demonstrating the existence of an efficient energy transfer from the Eu2+ to Mn2+ ions in SLuP lattice. As a result, the excitation into the PLE band of the Eu2+ ions at 355 nm yields both the emission of the Eu2+ and Mn2+ ions, which consists of a blue-green band corresponding to the f-d transition of the Eu2+ ions and a red band attributed to the 4 T1-6A1 transition of the Mn2+ ions, respectively. Therefore, warm white light emission can be realized by combining the emission of the Eu2+ and Mn2+ ions in a single host lattice under UV light excitation by properly tuning the emission composition of the Eu2+ and Mn2+ ions through the principle of energy transfer. To investigate the dependence of tunable emission on Mn2+ content, a series of samples has been prepared. The Mn2+ content is varied, whereas the content of Eu2+ is fixed at 0.005. The PL spectra of SLuP:0.005Eu2+,nMn2+ (from n = 0 to 0.08) samples under an excitation of 355 nm are depicted in Figure 5. The intensity of the Eu2+ emission decreases with an increasing doped Mn2+ content. In contrast, the PL intensity of the Mn2+ emission increases until the Mn2+ content quenching is above 0.04. The observed variations in the emission intensity of the Eu2+ and Mn2+ ions strongly indicate the energy transfer from Eu2+ to Mn2+. In addition, it can be seen that the emission peak of the Mn2+ ions shifts toward long-wavelength from 605 to 633 nm with the increase in Mn2+ content n from 0.005 to 0.08, which could be ascribed to the change of crystal field strength. The emission of the Mn2+ ion varies from green to red, depending on
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Figure 5. PL spectra for SLuP:0.005Eu2+,nMn2+ phosphors on Mn2+ doping content (n).
Figure 6. Photoluminescence decay curves of Eu2+ in SLuP:0.005Eu2+, nMn2+ displayed on a logarithmic intensity scale (excited at 355 nm, monitored at 490 nm).
the influence of the crystal field. In this regard, the crystal field around Mn2+ has been suggested as obeying20 Dq ¼
ze2 r 4 6R 5
ð1Þ
where Dq is measurement of the crystal field strength, R is the distance between the central ion and its ligands, z is the charge or valence of the anion, e is the charge of an electron, and r is the radius of the d wave function. For the Sr3Lu(PO4)3 lattice, a substitution of smaller Mn2+ ions for larger Sr2+ ions can cause a shortening of bond length, as confirmed by the fact that the lattice constants V and a for samples decrease linearly with increasing Mn2+ content (Figure 2). According to eq 1, we know that this leads to the enhancement of crystal field strength surrounding Mn2+ ions and further results in a larger crystal field splitting of Mn2+ 3d energy levels, which makes the lowest 3d state of Mn2+ closer to its ground state and finally gives a red shift of the PL emission peak of the Mn2+ ion.21 The decay curves of SLuP:0.005Eu2+,nMn2+ samples excited at 355 nm and monitored at 490 nm are shown in Figure 6. The decay curve of SLuP:0.005Eu2+ has been analyzed by curve fitting, and it can be well fitted through a triple-exponential function (Figure S3 in Supporting Information). Therefore, we 1331
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Figure 7. Dependence of the energy transfer efficiency ηT and the fluorescence lifetime of Eu2+ on Mn2+ content (n).
have obtained three decay components τ1 = 0.049 μs, τ2 = 0.334 μs, and τ3= 1.099 μs, which are attributed to the Eu2+ substitution for three different Sr2+ sites, respectively.2 These results are also supported by the crystal structure and spectral deconvolution. When Mn2+ ions are introduced, the decays deviate from exponential. This deviation is more evident with the increase in the Mn2+ doping content, and the decay of the Eu2+ ions becomes faster and faster, attributed to the energy transfer from the Eu2+ to Mn2+ ions. Because of the deviations from exponential decay, the average fluorescence lifetime was defined as the following formula22,23 τavg ¼
Z ∞ 0
Figure 8. CIE chromaticity diagram for SLuP:0.005Eu2+,nMn2+ phosphors (point 1 to 8) excited at 355 nm.
Table 1. Comparison of the CIE Chromaticity Coordinates (x, y), CCT (K), and Quantum Efficiency (QE) for SLuP:0.005Eu2+,nMn2+ Phosphors Excited at 355 nm
ð2Þ
IðtÞ dt
sample no.
where I(t) is the fluorescence intensity at time t with normalized initial intensity. The calculated average lifetimes of Eu2+ are shown in Figure 7. It can be seen that the lifetime of the Eu2+ emission shortens with increasing doping content of Mn2+. According to the data of average lifetime, the energy transfer efficiency from the Eu2+ to Mn2+ ions can be calculated by the following equation24 ηT ¼ 1 τ=τ0
ð3Þ
where τ0 is the lifetime of the sensitizer Eu with the absence of Mn2+ and τ is the lifetime of Eu2+ with the presence of Mn2+. The energy transfer efficiency was calculated as a function of Mn2+ content and was illustrated in Figure 7. With increasing Mn2+ doping content, the energy transfer efficiency was found to increase gradually and reach 40% for Mn2+ content of 0.08. These results support the efficient energy transfer from the Eu2+ to Mn2+ ions. As shown in Figure 8 and Table 1, the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates and CCT of SLuP:0.005Eu2+,nMn2+ samples were determined based on their corresponding PL spectrum. The Eu2+ doping content is fixed at 0.005 as the content of Mn2+ increases from 0 to 0.08; the corresponding color tone of the phosphor shifts from bluegreen to green-yellow, warm-white, and eventually to orange. In particular, the CCT of white light also can be tuned from 5052 (Sample 5) to 3592 K (Sample 7) by appropriately changing the Mn2+ content. Therefore, a warm-white-light emission with CIE coordinates of (0.391, 0.366) and CCTs of 3592 K was realized in SLuP:0.005Eu2+,0.06Mn2+ phosphor. It is clear that white light with different CCT can be produced for different practical
sample composition (n)
CIE coordinates (x, y)
CCT (K)
quantum efficiency (QE)
1
n=0
(0.251, 0.369)
9373
47.5%
2
n = 0.005
(0.269, 0.368)
8394
58.0%
3
n = 0.01
(0.292, 0.368)
7241
53.3%
4
n = 0.02
(0.324, 0.368)
5849
56.3%
5
n = 0.03
(0.345, 0.367)
5052
59.8%
6 7
n = 0.04 n = 0.06
(0.369, 0.363) (0.391, 0.366)
4217 3592
55.3% 43.4%
8
n = 0.08
(0.421, 0.360)
2440
37.7%
2+
applications by varying the Mn2+ content in SLuP:Eu2+,Mn2+ phosphor system. For white LED application, the QE is an important parameter for LED phosphors. Therefore, the QE of the phosphors was measured and is shown in Table 1. In general, a tunable white light emission with suitable QE was obtained in SLuP:0.005Eu2+,nMn2+ samples by the efficient energy transfer from the Eu2+ to Mn2+ ions. In addition, the maximum QE of as-prepared white-light-emitting SLuP:0.005Eu2+,0.03Mn2+ sample with the CIE coordinate (0.345, 0.367) can reach 59.8%. These data indicate that SLuP:0.005Eu2+,nMn2+ samples have relatively appropriate QE and warm-white-light emission, so they could be used as phosphors for white LEDs. Moreover, the QE can be improved by controlling the particle size, size distribution, morphology, and crystalline defects through optimization of the processing conditions and composition. In general, the energy transfer between the Eu2+ and Mn2+ ions mainly takes place via exchange interaction and electric multipolar interaction. According to Dexter’s energy transfer formula for exchange interaction, it is known that if energy transfer takes the exchange interaction, the linear relationship 1332
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are nearly coincident with the conventional value of S = 8, meaning that the dominant interaction mechanism for EuMn energy transfer in SLuP host is based on the dipolequadrupole interaction. For the dipolequadrupole interaction mechanism, the critical distance (RC) for energy transfer from Eu2+ to Mn2+ can be obtained by spectral overlap method28 Z FS ðEÞFA ðEÞ dE 8 12 2 RC ¼ 3:024 10 λS fq ð7Þ E4
Figure 9. Plots of experimental data ln[ln(I(t)/I0) (t/τ0)] versus ln(t/τ0)3 of Eu2+ with the solid lines representing theoretical fits. The calculated S values are given to each line.
should be observed by the following formula25 lnðIS0 =IS Þ µ C
ð4Þ
where IS0 and IS and are the luminescence intensities of the sensitizer Eu2+ without and with activator Mn2+ present and C is the concentration of Mn2+. The ln(IS0/IS)µC plots are illustrated in Figure S4 (Supporting Information) and the linear behavior is not observed, indicating little possibility of energy transfer via the exchange interaction mechanism. Therefore, the electric multipolar interaction should be responsible for energy transfer from the Eu2+ to Mn2+ ions. The excited donor ions can relax by direct energy transfer to acceptor ions or after migration of excitation energy among donor ions until an acceptor is reached. If the donor and acceptor ions are uniformly distributed in the host and the migration process are negligible compared with energy transfer between donors and acceptors, then the donor’s decay curves following the InokutiHirayama (IH) model equation for multipolar interactions26,27 3=S ! t t IðtÞ ¼ I0 exp α ð5Þ τ0 τ0 where I(t) is the emission intensity after pulsed excitation, I0 is the intensity of the emission at t = 0, τ0 is intrinsic decay time of the donor ion, α is a parameter containing the energy probability, and S is an indication of electric multipole character; S = 6, 8, and 10 for dipoledipole, dipolequadrupole, and quadrupole quadrupole interaction, respectively. By modifying eq 5, the value of S can be determined from the following relationship 3 IðtÞ t t ð6Þ µ ln ln ln I0 τ0 τ0 To get a correct S value, we plotted the decay curves using eq 6. This plot should yield a straight line with a slope equal to 1/S. The fitting results of SLuP:0.005Eu2+,nMn2+ samples were illustrated in Figure 9. The value of S estimated from the slope was found to be 7.92, 8.20, and 8.25 for SLuP:0.005Eu2+,nMn2+ samples with n = 0.01, 0.02, and 0.03, respectively. There values
where fq is the oscillator strength of the involved absorption transition of the acceptor (Mn2+), λS (in angstroms) is the wavelength position of the sensitizer’s emission, ERis the energy involved in the transfer (in electronvolts), and FS(E)FA(E) dE/E4 represents the spectral overlap between the normalized shapes of the Eu2+ emission FS(E) and the Mn2+ excitation FA(E), and in our case it is calculated to be ∼0.03634 eV5. Using the above equation with fq = 1010, the critical distance RC was estimated to be 11.3 Å.
4. CONCLUSIONS In summary, we have developed novel single-component warm-white-light Sr3Lu(PO4)3:Eu2+,Mn2+ phosphors by energy transfer and properly tuning activator content. The obtained phosphors show a broad excitation spectral range from 250 to 420 nm and have relatively appropriate QE, which can meet the application for UV LED chips. The energy transfer from the Eu2+ to Mn2+ ions has been demonstrated to be a resonant type via a dipolequadrupole mechanism based on the IH theoretical model, and the critical distance was calculated to be 11.3 Å. Furthermore, we have demonstrated that the emission color of the obtained phosphor can be tuned easily from blue-green to green-yellow, warm-white, and eventually to orange by manipulating the content of Mn2+. More importantly, a warm white light emission from SLuP:0.005Eu2+,0.04Mn2+ phosphor with a CCT of 4217 K, CIE coordinates of (0.369, 0.363), and QE of 55.3% was obtained under 355 nm excitation. Our results show that the developed phosphor has significant potential to be a single-component warm-white-light phosphor for fabricating white LED devices. ’ ASSOCIATED CONTENT
bS
PL spectrum of SLuP:Eu2+ and its Gaussian components; details of the spectral overlap between Eu2+ and Mn2+; PL decay curve of Eu2+ emission in SLuP:0.005Eu2+ and curve-fitting under excitation at 355 nm, monitored at 490 nm; and dependence of ln(IS0/IS) of Eu2+ on Mn2+ content. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (grant no. 20771098) and the Fund for Creative Research Groups (grant no. 20921002), and 1333
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the National Basic Research Program of China (973 Program, grant no. 2007CB935502).
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