Sr9Mg1.5(PO4)7:Eu2+: A Novel Broadband Orange-Yellow-Emitting

Oct 27, 2015 - Phosphor for Blue Light-Excited Warm White LEDs .... orange-yellow-emitting Sr9Mg1.5(PO4)7:Eu2+ phosphor that can be excited efficientl...
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Sr9Mg1.5(PO4)7:Eu2+: A Novel Broadband Orange-Yellow-Emitting Phosphor for Blue Light-Excited Warm White LEDs Wenzhi Sun,†,‡ Yonglei Jia,†,‡ Ran Pang,† Haifeng Li,†,‡ Tengfei Ma,† Da Li,† Jipeng Fu,†,‡ Su Zhang,† Lihong Jiang,*,† and Chengyu Li*,† †

State key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: A new orange-yellow-emitting Sr9Mg1.5(PO4)7:Eu2+ phosphor was prepared via high-temperature solid-state reaction. The structure and optical properties of it were studied systematically. Sr9Mg1.5(PO4)7:Eu2+ can be well-excited by 460 nm blue InGaN chips and exhibit a wide emission band covering from 470 to 850 nm with two main peaks centered at 523 and 620 nm, respectively, which originate from 5d−4f dipole-allowed transitions of Eu2+ in different crystallographic sites. The sites attribution, concentration quenching, fluorescence decay analysis, and temperature-dependent luminescence properties were investigated in detail. Furthermore, a warm white LED device was fabricated by combining a 460 nm blue InGaN chip with the optimized orange-yellow-emitting Sr9Mg1.5(PO4)7:Eu2+. The color coordinate, correlated color temperature and color rendering index of the fabricated LED device were (0.393, 0.352), 3437 K, and 86.07, respectively. Sr9Mg1.5(PO4)7:Eu2+ has great potential to serve as an attractive candidate in the application of blue light-excited warm white LEDs. KEYWORDS: Sr9Mg1.5(PO4)7:Eu2+, orange-yellow-emitting phosphor, photoluminescence, blue InGaN chips, warm white LEDs

1. INTRODUCTION White light-emitting diodes (WLEDs) have received far-ranging attention not only in scientific areas but also in technical industry, because of their superior advantages, such as high brightness, low power consumption, long operating time, great stability, and good environmentally friendly traits.1−4 They are deemed as lighting sources of the next generation and have been used extensively in display lighting and illuminating systems.5 Generally, the pervasive approach to assemble WLEDs is by combining blue InGaN chips with yellowemitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor. However, due to the short of red emission in such WLEDs, this approach usually generates cool white light with the chromaticity coordinate of (0.292, 0.325), relatively high correlated color temperature (CCT) of 7756 K, and poor color rendering index (CRI, Ra) of 75, which restricts their application in many fields, such as, room illuminating and commercial lighting.6,7 To achieve warm white light, new orange-yellow-emitting phosphors containing more red emission than YAG:Ce3+ are badly needed. In recent years, several such phosphors have been investigated, such as, Ba3Lu(PO4)3:Eu2+,Mn2+,8 Sr9Sc(PO4)7:Eu2+,Mn2+,9 NaCaBO3:Ce3+,Mn2+,10 and Ca3Si2O7:Eu2+.11 However, most of these phosphors have to be excited by near-ultraviolet (n-UV) light, which is more or less harmful to human skin and eyes. Therefore, it is still © 2015 American Chemical Society

essential to develop novel orange-yellow-emitting phosphors that can be effectively excited by blue light for warm white LEDs with low CCT and high CRI. Among the high-efficiency phosphors, Eu2+ ions turn out to be one kind of outstanding activators, which have been investigated extensively. The 5d electrons of Eu2+ are easily affected by their coordination surroundings because they are in the outer orbitals. And the emissions of Eu2+ (attributed to 5d− 4f electronic transitions) are therefore sensitive to crystal field environment. Hence, Eu2+-doped phosphors usually have broad excitation bands and strong emissions ranging from n-UV to red region.12−14 In addition, phosphates are widely explored as luminescence materials for their advantages of low synthesis temperature and excellent optical properties.15,16 Recently, a series of whitlockite structure phosphors, isotypic with βCa3(PO4)2, have been reported in the literature, such as, Sr 1.75 Ca 1.25 (PO 4 ) 2 :Eu 2+ ,16 Ca 2 Sr(PO 4 ) 2 :Eu 2+,Mn 2+ ,17 and Sr8MgSc(PO4)7:Eu2+.18 But as far as we know, the photoluminescence properties of activators-doped Sr9Mg1.5(PO4)7 have not been explored yet. Though the Sr9Mg1.5(PO4)7 host is also one variation of the whitlockite-type structure, its Received: July 30, 2015 Accepted: October 27, 2015 Published: October 27, 2015 25219

DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226

Research Article

ACS Applied Materials & Interfaces composition and structure are different from any other variations. In fact, there is no standard data of Sr9Mg1.5(PO4)7 reported in the database of the Joint Committee on Powder Diffraction Standards (JCPDS). But we found that all the diffraction peaks of Sr9Mg1.5(PO4)7 can be well indexed to the standard data of Sr9Fe1.5(PO4)7 (JCPDS card No. 51−0427), indicating that Sr 9 Mg 1.5 (PO 4 ) 7 is isostructural with Sr9Fe1.5(PO4)7, which will be further identified in the text. Meanwhile, the structure of Sr9Fe1.5(PO4)7 is highly disordered,19 and it is well-known that structure determines the properties, so interesting optical properties of Sr9Mg1.5(PO4)7:Eu2+ phosphor are expected. Inspired by the above knowledge, we developed a novel orange-yellow-emitting Sr9Mg1.5(PO4)7:Eu2+ phosphor that can be excited efficiently by 460 nm blue light for warm white LEDs. The structure and optical properties of the phosphor were studied systematically. Meanwhile, we found that the emission spectrum of the obtained orange-yellow-emitting phosphor contained more red emission than that of YAG:Ce3+ under 460 nm excitation. Furthermore, a warm white LED device was fabricated by coupling a 460 nm blue InGaN chip with Sr9Mg1.5(PO4)7:Eu2+ phosphor and its spectral properties were also discussed.

Figure 1. Representative powder XRD profiles of SMPO:xEu2+ (x = 0, 0.02, 0.06, 0.10, and 0.16) together with the standard data of Sr9Fe1.5(PO4)7 (JCPDS card No. 51−0427).

2. EXPERIMENTAL SECTION

phase of SMPO:xEu2+ samples is pure; SMPO:Eu2+ are isostructural with Sr9Fe1.5(PO4)7; and the doping of Eu2+ ions does not cause significant impurities to the crystal structure. To get the detailed crystal structure information on the obtained samples, we carried out Rietveld refinement of SMPO:0.06Eu2+ sample with the single crystal structure data of Sr9Fe1.5(PO4)7 (ICSD No. 93870) as the initial model. Figure 2a shows the observed (crosses) and calculated (red)

2.1. Materials and Synthesis. Sr9Mg1.5(PO4)7:xEu2+ (SMPO:xEu2+) (x = 0.02−0.16) phosphors were synthesized via hightemperature solid-state reaction in CO reducing atmosphere. The starting materials were SrCO3 (A.R.), Mg(OH)2•4MgCO3·6H2O (A.R.), NH4H2PO4 (A.R.), and Eu2O3 (99.99%). Stoichiometric amounts of them were ground uniformly and presintered at 450 °C for 3 h in air. Then the reground powders were placed in an alumina crucible to calcine at 1300 °C for 6 h in CO reducing atmosphere. Eventually, the obtained samples were ground for subsequent measurements. 2.2. Materials Characterization. A Bruker D8 focus diffractometer (operating at 40 kV and 40 mA) with graphitemonochromatized Cu Kα radiation (0.15405 nm) was used to record the powder X-ray diffraction (XRD) data. The general structure analysis system (GSAS) software was adopted to accomplish the Rietveld refinements. The diffuse reflectance spectra were measured by a 3600 UV−vis-NIR spectrophotometer with the reference of barium sulfate. A Hitachi F-7000 spectrophotometer with a 150 W xenon lamp as the excitation source was employed to detect the photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The luminescence decay curves were collected from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) with the excitation source of a tunable laser (pulse width = 4 ns; gate = 50 ns). The temperature-dependent PL spectra were measured by a TOSL-3DS thermoluminescence spectrophotometer (Sun Yat-Sen University, Guangzhou). The Photonic Multichannel Analyzer C10027 (Hamamatsu, Japan) was used to measure the quantum efficiency. The spectral properties of the fabricated WLED device were detected by a Rainbow-Light MR-16 v3 Portable Light Meter (Rainbow Light Technology CO. Ltd.).

Figure 2. (a) Observed (crosses) and calculated (red line) powder XRD patterns of SMPO:0.06Eu2+ phosphor. The green sticks stand for the positions of Bragg reflection and the blue line marks the difference between observed and calculated data; (b) crystal structure of Sr9Mg1.5(PO4)7.

3. RESULTS AND DISCUSSION 3.1. Phase Identification and Crystal Structure. Figure 1 depicts powder XRD profiles of SMPO:xEu2+ (x = 0, 0.02, 0.06, 0.10 and 0.16) samples along with the standard data Sr9Fe1.5(PO4)7 (JCPDS card No. 51−0427). As indicated above, there is no standard data of Sr9Mg1.5(PO4)7 reported in the database of JCPDS. But from Figure 1, we can clearly find that all the diffraction peaks of the samples can be well indexed to the standard data of Sr9Fe1.5(PO4)7, which proves that the

XRD patterns together with their difference (blue) for the refinement of SMPO:0.06Eu2+ sample. The refinement results reveal that SMPO:0.06Eu2+ has the trigonal structure with the space group of R3̅m, cell parameters of a = b = 10.582 Å, c = 19.723 Å, Z = 3, and cell volume of V = 1912.727 Å3. And the refinement finally converged to χ2 = 4.643, Rp = 5.25%, and Rwp = 7.29% (Table 1), which again demonstrates that SMPO:0.06Eu2+ is isotypic with Sr9Fe1.5(PO4)7 and Eu2+ ions have been doped into the host lattice successfully. 25220

DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226

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ACS Applied Materials & Interfaces Table 1. Rietveld Refinement and Crystallographic Data of Sr8.94Mg1.5(PO4)7:0.06Eu2+ Sample formula

Sr8.94Mg1.5(PO4)7:0.06Eu2+

space group a = b (Å) c (Å) α = β (deg) γ (deg) Z V (Å3) Rp (%) Rwp (%) χ2

R3̅m (No. 166), trigonal 10.582 19.723 90 120 3 1912.727 5.25 7.29 4.643

Figure 3. Diffuse reflection spectra of Sr9Mg1.5(PO4)7 host and SMPO:xEu2+ (x = 0.04, 0.08, and 0.10) samples.

The crystal structure model of Sr9Mg1.5(PO4)7 is depicted in Figure 2b. In the unit cell of Sr9Mg1.5(PO4)7, there are three Sr crystallographic sites (Sr1, Sr31, and Sr32), two Mg sites (Mg4 and Mg5) and two P sites (P1 and P2).19 The site numeration of Sr9Mg1.5(PO4)7 is the same as other β-Ca3(PO4)2 type compounds. 20,21 Like Sr 9 Fe 1.5 (PO 4 ) 7 , the structure of Sr9Mg1.5(PO4)7 is highly disordered and has a center of symmetry. Sr3 sites are disordered over two 18h sites (Sr31 and Sr32) near the symmetry center; P1O4 tetrahedra are disordered in orientation; the Mg4 site is 75% vacant and the Mg5 octahedral site is completely occupied. The coordination number (CN) of Sr1 site is fixed to 8, while those of Sr31 and Sr32 sites vary with the orientation of P1O4 tetrahedra: 8 or 9 for Sr31 site, and 6 or 7 for Sr32 site. The lattice parameters and atom positions of SMPO:0.06Eu2+ are listed in Table 2.

to host lattice absorption. When Eu2+ ions are incorporated into Sr9Mg1.5(PO4)7, intense broad absorption bands ranging from 200 to 520 nm are observed, which are attributed to 4f7− 4f65d1 absorption of Eu2+ ions. As Eu2+ concentration increasing, the absorption intensity is enhanced gradually and the absorption edge shows red shift. The wide absorption range of SMPO:Eu2+ samples matches well with the excitation spectra that will be discussed later. Figure 4 exhibits the PLE and PL spectra of SMPO:0.06Eu2+. The excitation spectra monitored at 523 and 620 nm both

Table 2. Lattice Parameters and Atom Positions of Sr8.94Mg1.5(PO4)7:0.06Eu2+ Sample atom

site

occup.

x

y

z

Uiso × 100

Sr1 Sr31 Sr32 Eu1 Eu31 Eu32 Mg4 Mg5 P1 P2 O1 O2 O3 O4

18h 18h 18h 18h 18h 18h 6c 3a 3b 18h 36i 18h 36i 18h

0.96 0.24 0.24 0.04 0.01 0.01 0.25 1.0 1.0 1.0 0.3333 1.0 1.0 1.0

0.18993 0.4907 0.4682 0.18993 0.4907 0.4682 0 0 0 0.49101 0.8989 0.5388 0.2625 0.9109

0.81007 0.5093 0.5318 0.81007 0.5093 0.5318 0 0 0 0.50899 0.038 0.4612 0.0087 0.0891

0.53794 0.0063 0.0148 0.53794 0.0063 0.0148 0.355 0 0.5 0.39797 0.5398 0.6764 0.23304 0.0669

0.77(26) 0.59(53) 0.59(53) 0.77(26) 0.59(53) 0.59(53) 2.78(63) 0.59(53) 3.64(76) 1.01(32) 4.81(28) 1.79(85) 0.43(06) 0.29(13)

Figure 4. PLE and PL spectra of SMPO:0.06Eu2+ phosphor, along with the fitted curve (red dash line) and the decomposed Gaussian components (green dash lines).

consist of broad bands ranging from 220 nm−550 nm with a maximum at ∼460 nm, which are assigned to the 4f7−4f65d1 transition of Eu2+ ions. Clearly, the PLE spectra nearly cover the region from UV to blue, which indicates that SMPO:Eu2+ phosphor can be efficiently excited by n-UV and blue light and can serve as a potential orange-yellow-emitting phosphor in application of blue-InGaN chips based WLEDs. From Figure 4, one can see that the profiles of the PLE spectra monitored at 523 and 620 nm are different, which is due to the emission bands are attributed to different Eu2+ emitting centers.23,24 Moreover, the PL spectra are measured and studied to further analyze the Eu2+ emitting centers in SMPO:Eu2+. As depicted in Figure 4, the 620 nm emission peak dominates the PL spectrum excited at 460 nm, while the intensity of it decreases obviously under the 335 nm excitation. These results both demonstrate that there exist different Eu2+ emitting centers in SMPO:Eu2+. Under 460 nm excitation, SMPO:0.06Eu2+ phosphor shows bright orange-yellow emission. The PL spectrum of SMPO:0.06Eu2+ consists of a wide emission band covering from 470 to 850 nm with two main peaks

Considering the refinement results as well as the similarity of ionic radii between Sr2+ (1.32, 1.35, 1.4, and 1.45 Å for CN= 6, 7, 8, and 9, respectively) and Eu2+ (1.31, 1.34, 1.39, and 1.44 Å for CN = 6, 7, 8, and 9, respectively) ions,22 we think that Eu2+ will preferably substitute for the three Sr2+ sites homogeneously, resulting in three Eu2+-emitting centers in SMPO:Eu2+ phosphor. 3.2. Photoluminescence Properties of SMPO:Eu2+ Phosphor. The diffuse reflection spectra of Sr9Mg1.5(PO4)7 matrix and Eu2+-doped Sr9Mg1.5(PO4)7 phosphors are illustrated in Figure 3. The Sr9Mg1.5(PO4)7 matrix shows high reflection in the range of 245−700 nm, and then exhibits obvious energy absorption from 200 to 245 nm, corresponding 25221

DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226

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ACS Applied Materials & Interfaces centered at 523 and 620 nm, respectively, which originate from 4f65d1−4f7 dipole-allowed transition of Eu2+ in different crystallographic sites. The emission spectrum of SMPO:0.06Eu 2+ can be resolved into three Gaussian components with peaks centered at 520 nm, 605 and 695 nm, respectively, which is shown in Figure 4. These peaks are attributed to the three distinct emission sites of Eu2+. On the basis of the refinement results, Eu2+ ions occupy the three Sr2+ sites with different coordination numbers and bond distances in the crystal lattice of SMPO:0.06Eu2+. The bond distances of Sr/ Eu−O in SMPO:0.06Eu2+ sample are listed in Table S1 in detail, and the average bond distances of Sr1/Eu1−O, Sr31/ Eu31−O and Sr32/Eu32−O are 2.6590, 2.7402, and 2.6043 Å, respectively. As is well-known, the 5d electrons of Eu2+ are in the outer orbitals and thus sensitive to the crystal field environment. Doping Eu2+ ions into the Sr2+ sites with stronger crystal field strength (shorter Sr−O bond distance) will result in a red shift of the excitation and emission bands.12,13 Furthermore, in accordance with Van Uiterts̀ report, the position of the d-band edge (E) of Eu2+ ions relies on their local environment strongly. Thus, the relationship between emission peaks and Eu2+-emitting sites can be inferred from the following equation25 E = Q [1 − (V /4)1/ V 10−nEar /80]

Figure 5. (a) PL spectra of SMPO:xEu2+ (x = 0.02−0.16) under the excitation of 460 nm; (b) the corresponding PL intensities as a function of Eu2+ concentration (x).

transfer among Eu2+ ions in SMPO:xEu2+ phosphor should not be controlled by exchange interaction, but by electric multipolar-multipolar interaction.26 According to Dexter̀s theory, the PL intensity (I) per activator keeps to the equation below27,28

(1)

where E refers to the energy position for the emission peak of Eu2+ ion (cm−1), Q is the energy position for the lower d-band edge of free ion (for Eu2+, Q = 34000 cm−1), V represents the valence of Eu2+ and thus equals to 2, r is the radius of the host cation substituted by Eu2+, n stands for the number of anions in the immediate shell around Eu2+, and Ea (a constant in the same host) is the electron affinity of the atoms that form anions (eV). On the basis of the above analysis, the value of E is in direct proportion to the product of n and r. Sr1/Eu1 atoms are 8-coordinated with the average Sr1/Eu1−O bond distance of 2.6590 Å, whereas the coordination numbers of Sr31 and Sr32 sites vary with the orientation of P1O4 tetrahedra: 8 or 9 for Sr31 site (average Sr31/Eu31−O bond distance is 2.7402 Å), and 6 or 7 for Sr32 site (average Sr32/Eu32−O bond distance is 2.6043 Å). Therefore, it is rational to deduce that the emission peaks centered at 520, 605, and 695 nm are assigning to Eu2+ ions occupying the Sr31, Sr1, and Sr32 sites, respectively. A variety of SMPO:xEu2+ (x = 0−0.16) samples have been prepared to get the best doping concentration of Eu2+ ions. Figure 5 shows the PL spectra of SMPO:xEu2+ as a function of various Eu2+ concentrations (x) under the excitation of 460 nm. PL intensity ascends with x increasing, reaches a maximum at x = 0.06 and then decreases when x > 0.06, because of the concentration quenching effect. The critical distance (Rc) between Eu2+ ions can be estimated by the following formula proposed by Blasse26 ⎡ 3V ⎤1/3 R c ≈ 2⎢ ⎥ ⎣ 4πxcZ ⎦

I /x = k[1 + β(x)θ /3 ]−1

(3)

where x is the concentration of Eu ions; k and β are constants for a given host lattice and excitation condition; θ = 10, 8, and 6 represents quadrupole−quadrupole, dipole−quadrupole, and dipole−dipole interactions, respectively, and θ = 3 stands for energy migration between nearest or next nearest Eu2+ ions. The dependence of lg[I/x(Eu2+)] on lg[x(Eu2+)] can be fitted as a line and the slope is −θ/3, by processing eq 3. It is shown in Figure 6 that the slope is −2.072. Hence, θ is found to be 2+

Figure 6. Dependence of lg(I/xEu2+) on lg(xEu2+) for SMPO:xEu2+ phosphor.

6.216 and approximately equal to 6, which demonstrates that the concentration quenching mechanism predominant in SMPO:xEu2+ phosphor is dipole−dipole interaction. Figure 7 shows the normalized PL spectra of orange-yellowemitting SMPO:0.06Eu2+ and commercial yellow-emitting YAG:Ce3+ phosphors under the excitation of 460 nm. It is clear that the orange-yellow-emitting SMPO:0.06Eu2+ phosphor we prepared contains more red emission than commercial yellow-emitting YAG:Ce3+ phosphor. Therefore, high-quality warm white light could be obtained by coupling 460 nm blue

(2) 2+

where xc stands for the critical concentration of Eu , V represents the volume of unit cell, and Z refers to the number of formula units per unit cell. Z = 3, xc = 0.06, and V = 1912.727 Å3 for SMPO:xEu2+ samples. Thus, Rc is determined to be 27.28 Å. As the exchange interaction commonly happens in a forbidden transition (Rc is about 5 Å), the nonradiative energy 25222

DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226

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ACS Applied Materials & Interfaces

3.3. Fluorescence Lifetime Properties of SMPO:Eu2+ Phosphor. In general, an important approach to analyze energy transfer process in phosphors is by investigating their fluorescence decay curves. Therefore, decay curves of SMPO:xEu2+ varied with different Eu2+ concentrations (x) were measured and displayed in Figure 9. The corresponding

Figure 7. Normalized PL spectra of orange-yellow-emitting SMPO:0.06Eu2+ and commercial yellow-emitting YAG:Ce3+ phosphors (λex = 460 nm).

InGaN chips with the orange-yellow-emitting phosphor SMPO:0.06Eu2+. Under 460 nm excitation, the Commission Internationalede L’Eclairage (CIE) chromaticity coordinate of SMPO:0.06Eu2+ is (0.492, 0.478), which is marked in Figure 8

Figure 9. Fluorescence decay curves of Eu2+ ions in SMPO:xEu2+ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12 and 0.14) samples excited at 460 nm and monitored at (a) 523 nm, (b) 620 nm.

luminescence lifetimes were also investigated. SMPO:xEu2+ phosphors were excited at 460 nm and monitored at 523 nm (Figure 9a) or 620 nm (Figure 9b), respectively. Commonly, with the concentration of activators increasing, the distance between them decreases and thus energy transfer among them occurs. Hence, the decay times are different with various concentrations of activators. 3,29,30 As there are three luminescence centers in SMPO:Eu2+, the decay curves cannot simply be fitted by single-exponential function. Therefore, the average decay time can be obtained by the following equation31,32

Figure 8. CIE coordinates of SMPO:0.06Eu2+ (point 2, λex = 460 nm), commercial YAG:Ce3+ (point 1, λex = 460 nm), and the fabricated warm white LED device (point 3).

(point 2) together with that of commercial YAG:Ce3+ (point 1) for comparison. In order to evaluate the luminescence efficiency of the obtained phosphor, the quantum efficiencies (QE) of SMPO:0.06Eu2+ and commercial YAG:Ce3+ excited by 460 nm were measured, as listed in Table 3. The internal and Table 3. Quantum Efficiencies of SMPO:0.06Eu2+ and Commercial YAG:Ce3+ internal QE absorption external QE

SMPO:0.06Eu2+ (%)

YAG:Ce3+ (%)

42.6 63.2 26.9

95.4 85.6 81.7

∞

τ=

∫0 tI(t )dt ∞

∫0 I(t )dt

(4)

According to the above equation, the average lifetimes of Eu2+ ions in SMPO:xEu2+ (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12, and 0.14) monitored at 523 nm were calculated to be 1.245, 1.162, 1.098, 1.011, 0.944, 0.847, and 0.755 μs, respectively; those monitored at 620 nm were 2.402, 2.269, 2.135, 1.974, 1.806, 1.658, and 1.517 μs, respectively. With the concentration (x) increasing, the average lifetimes monitored at 523 and 620 nm were both found to decline gradually, which demonstrates that energy transfer occurs among Eu2+ ions. The average lifetimes of a given Eu2+ concentration monitored at 523 and 620 nm are

external quantum efficiencies for SMPO:0.06Eu2+ are 44.7 and 32.9% of that for commercial YAG:Ce3+, respectively. However, the quantum efficiency can be advanced by in-depth optimization of the compositions and prepared process, because it is tightly related to the preparation conditions, compositions, and crystallinity, etc.18,32 25223

DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226

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ACS Applied Materials & Interfaces different, and this further testifies that different Eu2+-emitting centers exist in SMPO:Eu2+ phosphor. 3.4. Thermal Stability of SMPO:Eu2+ Phosphor. Generally, thermal stability is an important character for phosphors applied in WLEDs. To explore the thermal stability of SMPO:0.06Eu2+ phosphor, we detected and illustrated the relationship of PL intensity with temperature (excited at 460 nm) in Figure 10a. The emission intensity of SMPO:0.06Eu2+

where IT and I0 stand for emission intensities at experimental and room temperature, respectively; k is the Boltzmann constant (k = 8.617 × 10−5 eV/K); c (constant) represents the rate of thermally activated escape. Figure 10b plots the dependence of ln[(I0/IT) − 1] on 1/T for SMPO:0.06Eu2+ phosphor. Based on eq 5, Ea was determined to be 0.30 eV, which is higher than that of some phosphors investigated recently.10,28 The result reveals that SMPO:Eu2+ phosphor can serve as a potential orange-yellow-emitting phosphor in blue InGaN chips based warm white LED applications. 3.5. Electroluminescence Spectrum of the Fabricated LED Device. In order to further investigate the potential application of SMPO:Eu2+ in warm white LEDs, a phosphorconverted LED device was fabricated by combining a 460 nm blue InGaN chip with SMPO:0.06Eu2+ phosphor. Figure 11

Figure 11. EL spectrum of the warm white-LED device fabricated with a 460 nm blue InGaN chip and orange-yellow-emitting SMPO:0.06Eu2+ phosphor. The inset shows a photograph of the LED device driven by a 350 mA current.

exhibits the EL spectrum of the fabricated LED device under 350 mA forward-bias current. Three main emission bands can be clearly found in the EL spectrum: the emission band centered at ∼460 nm is attributed to the blue InGaN chip, while the other two bands peaked at ∼523 nm and ∼620 nm are the two emission peaks of SMPO:0.06Eu2+ phosphor. The fabricated SMPO:0.06Eu2+ phosphor-converted LED device can emit bright warm white light with the color coordinate of (0.393, 0.352) and relatively low CCT of 3437 K. The color coordinate is shown in Figure 8 (point 3). The CRI value (Ra) is determined to be 86.07, which is higher than that of the white LED lamp fabricated by coupling a blue InGaN chip with YAG:Ce3+.5,7 The above results further prove that the orangeyellow-emitting SMPO:Eu2+ phosphor has great potential to serve as an excellent candidate in blue-InGaN chips based warm white LEDs.

Figure 10. (a) PL spectra of SMPO:0.06Eu2+ with various temperature ranging from 300 to 448 K; (b) the relationship of ln[(I0/IT) − 1] with 1/T in SMPO:0.06Eu2+ phosphor; the inset plots the PL intensities of SMPO:0.06Eu2+ as a function of temperature (T). (λex = 460 nm).

decreases as the temperature increasing from 300 to 448 K. Compared with the initial intensity, the emission intensity of SMPO:0.06Eu2+ at 373 K declines by 44.32%. Meanwhile, the shape of PL spectra changes with temperature increasing and the change becomes obvious when T > 373 K, which is probably due to the three emission centers experiencing different coordination environments and thus being affected by temperature at different levels.33 It can be observed from Figure 10a that the decreasing rate of Eu31, Eu1, and Eu32 emission peaks are different: Eu31 and Eu32 emission peaks decrease faster than that of the Eu1 one and the difference becomes obvious when T > 373 K, which further demonstrates that there are three Eu2+ emission centers in SMPO:Eu2+ phosphor. However, it should be noted that the CIE coordinates show little shift with the temperature increasing, and it can be discerned from Table S2 and Figure S1. The activation energy (Ea) is a parameter to estimate thermal stability of phosphors and it can be obtained from the following formula34,35 I0 IT = E 1 + c exp − kTa (5)

4. CONCLUSION A new orange-yellow-emitting SMPO:Eu2+ phosphor was synthesized by high-temperature solid-state reaction. With a broad excitation band ranging from 220 to 550 nm, the orangeyellow-emitting SMPO:Eu2+ phosphor can be well excited by 460 nm blue InGaN chips and exhibit a wide emission band covering from 470 to 850 nm with two main peaks centered at 523 and 620 nm, respectively. We found that the emission spectrum of the obtained orange-yellow-emitting phosphor contains more red emission than that of YAG:Ce3+ under 460 nm excitation. Furthermore, high-quality warm white light was obtained by fabricating a WLED device with a 460 nm blue InGaN chip and the orange-yellow-emitting SMPO:0.06Eu2+

( )

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DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226

Research Article

ACS Applied Materials & Interfaces

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phosphor. The color coordinate, CCT and Ra of the fabricated WLED device were (0.393, 0.352), 3437 K, and 86.07, respectively. The excellent properties demonstrate that SMPO:Eu2+ has great potential to serve as an attractive candidate in the application of blue light-excited warm white LEDs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b06961. Bond distances of Sr/Eu−O in SMPO:0.06Eu2+; CIE coordinates of SMPO:0.06Eu2+ at different temperature (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.:+86-0431-85262208. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research is financially supported by the Hong Kong, Macao and Taiwan Science and Technology Cooperation Special Project of Ministry of Science and Technology of China (Grant No. 2014DFT10310), the Program of Science and Technology Development Plan of Jilin Province of China (Grant No. 20140201007GX), the National Basic Research Program of China (973 Program, Grant No. 2014CB643801), and the National Natural Science Foundation of China (Grant Nos. 51102229, 51402288, 21401184).

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REFERENCES

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DOI: 10.1021/acsami.5b06961 ACS Appl. Mater. Interfaces 2015, 7, 25219−25226