M8MgSc(PO4)7:x

Aug 24, 2017 - The crystal structure, photoluminescence (PL), PL excitation (PLE), and thermal PL ... such as street lights.7,8 However, natural plant...
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M8MgSc(PO4)7:xDy3+ (M = Ca/Sr) Single-Phase Full-Color Phosphor with High Thermal Emission Stability Jiaqi Long, Fujuan Chu, Yuzhen Wang, Chong Zhao, Wenfeng Dong, Xuanyi Yuan, Chaoyang Ma, Zicheng Wen, Ran Ma, Miaomiao Du, and Yongge Cao* Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices, Department of Physics, Renmin University of China, Beijing 100872, China ABSTRACT: Two series of phosphors of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ single-phase white-emitting phosphors with high thermal emission stability are synthesized by the high-temperature solid-state reaction. The crystal structure, photoluminescence (PL), PL excitation (PLE), and thermal PL quenching spectra of Ca8MgSc(PO4)7:xDy3+ and Sr8MgSc(PO4)7:xDy3+ were investigated and compared in detail. Upon excitation at 387 nm, M8MgSc(PO4)7:xDy3+ (M = Ca/Sr) showed white emission centered at 480, 571, 660, and 754 nm. The white-emitting Dy-phosphor Ca8MgSc(PO4)7:Dy3+ (CMSP:Dy) had good terminal stability. The emission intensity of Ca8MgSc(PO4)7:Dy3+ still remained 95.2% of that at room temperature at 160 °C, and remained 77.3% at 300 °C under 387 nm excitation.

1. INTRODUCTION White (W) light-emitting diodes (LEDs) have attracted much attention in recent years because of their applications in liquid crystal displays and solid-state lighting.1,2 High-power LED lamps have begun to be applied to outdoor lighting systems3−6 such as street lights.7,8 However, natural plant habitats, particularly at dark sites such as caves and forests, can be destroyed by outdoor lighting systems because the increase in illumination intensity allows different species to thrive.9 This is a problem to be solved, for the protection of native plants. In addition, extra night lighting also prevents the blooming of the flowers of short-day plants, but, on the contrary, promotes the blooming of the flowers of long-day plants. In a word, LEDs break the biological balance and damage natural plant habitats.10 Recently, Tomohiko Nakajima reported that Dyphosphors can be very promising candidates for plant habitatconscious white LEDs for outdoor lights that can protect both natural plant habitats and crop yield.11 With this aim, we have fabricated a plant habitat-conscious WLED (PHC-WLED) using Dy3+ luminescence. In this paper, we describe the structural and optical properties of a white-emitting Dyphosphor Ca8MgSc(PO4)7:Dy3+ (CMSP:Dy) that had good terminal stability of 95.2% at 160 °C and 77.3% at 300 °C under 387 nm excitation. This terminal stability is among the highest when compared with the previously reported values for Dy-phosphors (Table 1). To the best of our knowledge, the crystal structures and luminescence properties of color tunable Ca8MgSc(PO4)7:Dy3+ white-emitting phosphor and Sr8MgSc(PO4)7:Dy3+ whiteemitting phosphor have not yet been reported in the literature. In this study, we compared the crystal structure of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ phosphors. After that, the luminescence properties of Ca8MgSc(PO4)7:Dy3+ and © 2017 American Chemical Society

Table 1. Thermal Stability of Dy-Doped Phosphors formula

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Ca8MgSc(PO4)7:Dy3+ Sr8MgSc(PO4)7:Dy3+ Ca3Sc2Si3O12:Dy3+ Sr3Gd(PO4)3:Dy3+ MgY4Si3O13:Dy3+ KMgLa(PO4)2:Dy3+ K2YDy(WO4)(PO4):Dy3+ Sr3Lu(PO4)3:Dy3+

95.2% (160 °C) 86.7% (160 °C) ∼86% (150 °C) 84.3% (150 °C) ∼82% (150 °C) ∼73.6% (150 °C) 73% (120 °C) 68% (200 °C)

this work this work 12 13 14 15 16 17

Sr8MgSc(PO4)7:Dy3+ phosphors are investigated and compared. The thermal stabilities of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ are compared with commercial phosphor YAG:Ce3+. In addition, white-light near-UV LEDs were fabricated using single-phase full-color phosphors Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+, and their optical properties were investigated.

2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. Polycrystalline phosphors with compositions of Ca8MgSc(PO4)7:Dy3+ (CMSP:Dy) and Sr8MgSc(PO4)7:Dy3+ (SMSP:Dy) described in this work were prepared with a high-temperature solid-state reaction. Briefly, the constituent raw materials SrCO3 (A.R., 99.9%), CaCO3 (A.R., 99.9%), MgO (A.R., 99%), Sc2O3 (A.R., 99.99%), NH4H2PO4 (A.R., 99%), and Dy2O3 (A.R., 99.99%) were weighed according to the stoichiometric ratio. Individual batches of 10 g were weighed according to the designed stoichiometry and mixed homogeneously with the same mass of absolute ethyl alcohol as the dispersant. After a planetary ball-milling Received: May 21, 2017 Published: August 24, 2017 10381

DOI: 10.1021/acs.inorgchem.7b01268 Inorg. Chem. 2017, 56, 10381−10386

Article

Inorganic Chemistry

Figure 1. (a) Observed (crosses) and calculated (red dots) powder XRD patterns of the Ca8MgSc(PO4)7:Dy3+ phosphor. The pink sticks stand for the positions of Bragg reflection, and the blue line marks the difference between observed and calculated data. (b) Crystal structure of Ca8MgSc(PO4)7. (c) Observed (×) and calculated (red ●) powder XRD patterns of Sr8MgSc(PO4)7:Dy3+ phosphor. (d) Crystal structure of Sr8MgSc(PO4)7. process, the obtained homogeneous slurry was placed in a Petri dish and dried in an oven. Then, the dried mixtures were put into a crucible with a lid and heated in a tubular furnace at 1250 °C for 6 h under a reducing atmosphere of 5% H2/95% N2. When cooled down to room temperature, the prepared phosphors were crushed and ground for subsequent measurements. 2.2. Characterization. All crystal structure compositions were checked for phase formation by using powder X-ray diffraction (XRD) analysis with a Rigaku X-ray diffractometer (Tokyo, Japan) with a graphite monochromator using Cu Kα radiation (λ = 1.54056 Å), over the angular range 10° < 2θ < 80°, operating at 40 kV and 40 mA. XRD Rietveld profile refinements of the structural models and textural analysis were performed with the use of TOPAS 4.2 software. The schematic crystal structure of M8MgSc(PO4)7:Dy3+ (M = Ca/Sr) was drawn in VESTA.18 The photoluminescence (PL), photoluminescence excitation (PLE), and thermal photoluminescence (TPL) spectra of the samples were analyzed by using a Hitachi F-7000 spectrophotometer (Tokyo, Japan) with a 150 W Xe lamp and a temperature controller. Optical properties such as luminescence spectra, correlated color temperature (CCT), color-rendering index, and the Commission International de I’Eclairage (CIE) color coordinates of the initial mixed phosphors and the fabricated white LEDs were characterized using a DARSA PRO 5100 PL system (PSI Trading Co. Ltd., Korea) and evaluated under a forward-bias current of 60 mA at room temperature.

together with their difference (blue) for the refinement of the M8MgSc(PO4)7:Dy3+ (M = Ca/Sr) sample. The refinement results reveal that Ca8MgSc(PO4)7:Dy3+ has a trigonal structure with the space group R3cH; cell parameters a = b = 10.33 Å, c = 36.97 Å; and cell volume V = 3416.6 Å3. Also, the refinement finally converged to GOF = 2.72, Rp = 12.41%, and Rwp = 17.36% (Table 2), which again demonstrates Table 2. Rietveld Refinement and Crystallographic Data of M8MgSc(PO4)7:Dy3+ (M = Ca/Sr) Sample formula

Ca8MgSc(PO4)7:Dy3+

Sr8MgSc(PO4)7:Dy3+

space group a = b (Å) c (Å) α = β (deg) γ (deg) V (Å3) Rexp (%) Rwp (%) GOF

R3cH (No.161), trigonal 10.33022 36.97014 90 120 3416.649 6.36 16.95 2.66

R3cH (No.161), trigonal 10.60145 39.47915 90 120 3842.638 6.21 24.61 3.96

that Ca8MgSc(PO4)7 is isotypic with Ca9MgH(PO4)7 and Dy3+ ions have been doped into the host lattice successfully. Similarly, the refinement results reveal that Sr 8MgSc(PO4)7:Dy3+ also has a trigonal structure with space group R3cH; cell parameters a = b = 10.6 Å, c = 39.48 Å; and cell volume V = 3842.69 Å3. Since the radius of Ca2+ (1 Å) is smaller than the radius of Sr2+ (1.18 Å), the lattice constant of Ca8MgSc(PO4)7 is smaller than that of Sr8MgSc(PO4)7, as shown in Table 2.

3. RESULTS AND DISCUSSION 3.1. Microstructure. To get the detailed crystal structure information on the obtained samples, we carried out Rietveld refinement of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ samples with the single crystal structure data of Ca9MgH(PO4)7 (ICSD no. 6190) and Sr9MgH(PO4)7 (ICSD no. 5113), respectively, as the initial model. Figure 1a,c shows the observed (crosses) and calculated (red dots) XRD patterns 10382

DOI: 10.1021/acs.inorgchem.7b01268 Inorg. Chem. 2017, 56, 10381−10386

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ions, and the Dy2/Dy3 sites both have seven coordinating Ca/ Sr ions, which are marked by numbers in Figure 2. 3.2. Luminescence Property. Figure 3a shows PLE and PL spectra of M7.9MgSc(PO4)7:0.1Dy3+(M = Ca/Sr) samples at room temperature. The excitation spectra monitored at 572 nm consist of many peaks ranging from 250 to 500 nm with a maximum at ∼350 nm, which are assigned to the 6H15/2 → 6 P7/2 transition of Dy3+ ions. The PLE spectra nearly cover the region from UV to blue. Moreover, the PL spectra, as shown in Figure 3a, are measured and studied to further analyze the Dy3+ emitting centers in M7.9MgSc(PO4)7:0.1Dy3+ (M = Ca/Sr). In the PL spectrum, four sharp peaks around 480, 571, 660, and 754 nm were obtained due to transitions of 4F9/2 → 6H15/2, 4 F9/2 → 6H13/2, 4F9/2 → 6H11/2, and 4F9/2 → 6H9/2, respectively. The red shift (about 2 nm) of the emission spectrum as a whole occurs with Sr replacing Ca in Ca7.9MgSc(PO4)7:0.1Dy3+. The lattice constants of Sr8MgSc(PO4)7 (a = b = 10.60145 Å, c = 39.47915 Å) are larger than those of Ca8MgSc(PO4)7 (a = b = 10.33022 Å, c = 36.97014 Å), which results in a lower crystal field strength of Dy3+ in Sr8MgSc(PO4)7 than in Ca8MgSc(PO4)7. Lower-energy splitting causes the transition energy from excited states to ground states to be lower, which makes the emission spectrum red-shifted. Two main emission peaks are located in the blue (480 nm) and yellow (571 nm) regions. As Figure 3b shows, the energy level scheme of Dy3+ shows the mechanism for different observed emissions in detail. Figure 4 shows the PL emission intensity of Dy3+ as a function of the Dy3+ doping concentration (x = 0, 0.01, 0.02, 0.04, 0.08, 0.1, 0.12, 0.14, 0.2, 0.3, 0.4) in the sample under excitation at 387 nm. It can be seen clearly in the inset of Figure 4 that the emission intensity increases first and reaches the maximum value when the Dy3+ doping concentration rises to 0.14; it then decreases gradually with the concentration increasing from 0.14 to 0.4. This phenomenon contributed to concentration quenching which usually is due to the occurrence of the energy transfer within the nearest Dy3+ ions with the terminal step ending at a killer site. Similarly, the dependence of PL emission intensity is clearly seen in Figure 5. The emission intensity increases first and reaches the maximum value when the Dy3+ doping concentration rises to 0.1; it then decreases gradually with the

The crystal structure model of M8MgSc(PO4)7 (M = Ca/Sr) is depicted in Figure 1b,d. The primitive part of the M8MgSc(PO4)7 (M = Ca/Sr) unit cell has three Ca/Sr crystallographic sites (Ca1/Sr1, Ca2/Sr2, and Ca3/Sr3). Concerning the local environment of Dy3+ ions in the M8MgSc(PO4)7 (M = Ca/Sr) crystals, the three possible Dy3+-containing sites have different coordination environments. The coordination situation in the first three nearest coordination spheres around the Dy3+ sites is shown in Figure 2. As can be seen, the first, second, and third coordination

Figure 2. First three nearest coordination spheres of Dy ions which are determined to be located in the three Ca/Sr sites in the unit cell of M8MgSc(PO 4)7 (M = Ca/Sr). The first, second, and third coordination spheres of Dy3+ consist of O atoms, PO4 tetrahedrons, MgO6 octahedrons, and Ca/Sr ions, respectively. Dy1 site has eight coordinating Ca/Sr ions, and Dy2/Dy3 sites both have seven coordinating Ca/Sr ions, which are marked by numbers.

spheres of Dy3+ ions consist of O atoms, PO4 tetrahedrons, and Ca/Sr ions, respectively. At these three sites, different forms of DyOn polyhedrons, different numbers of coordinating PO4 tetrahedrons, and different numbers of neighboring Ca/Sr ions are observed. The Dy1 site has eight coordinating Ca/Sr

Figure 3. (a) PL and PLE spectra of M8MgSc(PO4)7:0.1Dy3+ (M = Ca/Sr). Inset: λex = 387 nm. (b) The energy level scheme of Dy3+ shows the mechanism for different observed emissions. 10383

DOI: 10.1021/acs.inorgchem.7b01268 Inorg. Chem. 2017, 56, 10381−10386

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Inorganic Chemistry

Figure 6. Emission spectra of Dy3+ as a function of temperature (25− 300 °C) in CMSP:0.14Dy3+ (λex = 387 nm). Inset: Temperaturedependent integrated PL intensity of CMSP:0.14Dy3+ during heating processes upon 387 nm excitation.

Figure 4. Emission spectra of Ca8MgSc(PO4)7:xDy3+ (x = 0, 0.01, 0.02, 0.04, 0.08, 0.1, 0.12, 0.14, 0.2, 0.3, 0.4) (λex = 387 nm). Inset: Intensities of 571 nm emission as a function of Dy3+ concentration (λex = 387 nm).

Figure 7. Emission spectra of Dy3+ as a function of temperature (25− 300 °C) in SMSP:0.1Dy3+ (λex = 387 nm). Inset: Temperaturedependent integrated PL intensity of SMSP:0.1Dy3+ during heating processes upon 387 nm excitation.

Figure 5. Emission spectra of Sr8MgSc(PO4)7:xDy3+ (x = 0, 0.02, 0.04, 0.1, 0.2, 0.3) (λex = 387 nm). Inset: Intensities of 573 nm emission as a function of Dy3+ concentration (λex = 387 nm).

quenching. The emission intensity remains over 86.7% of that at room temperature at 433 K (160 °C), and remains over 59.2% of that at room temperature at 573 K (300 °C) as shown in the inset of Figure 7. The decrease is attributed to thermal quenching, and the mechanism can be explained by evaluating the activation energy, ΔE, according to the Arrhenius equation:19 I0 I= 1 + A e(−Ea / KBT )

concentration increasing from 0.1 to 0.3. This similar phenomenon is also ascribed to concentration quenching. 3.3. Temperature-Dependent Emission Spectra of M8MgSc(PO4)7:Dy3+ (M = Ca/Sr). The analysis of thermal stability beyond 298 K is necessary before applications with practical use because the operating temperature of LEDs is much higher than room temperature. The temperature dependence of PL spectra is shown in Figure 6. With an increase in temperature from 298 K (25 °C) to 573 K (300 °C), the PL emission intensity decreases continuously due to the thermal quenching. Note that the emission intensity still remains over 95.2% of that at room temperature at 433 K (160 °C), as shown in the inset of Figure 6. Surprisingly, even when the temperature reached 573 K (300 °C), the emission intensity still remains over 77.3% of that at room temperature. As far as we know, the thermal stability of CMSP:Dy3+ is one of the best from the previously reported results among Dy-doped phosphors. Contrast details are listed in Table 1. Similar to those of CMSP:0.1Dy3+, emission spectra of Dy3+ as a function of temperature (25−300 °C) in SMSP:0.1Dy3+ (λex = 387 nm) are shown in Figure 7. With an increase in temperature from 298 K (25 °C) to 573 K (300 °C), the PL emission intensity decreases continuously due to the thermal

Rearranging this equation results in ⎛I ⎞ E ln⎜ 0 − 1⎟ = ln A − a ⎝I ⎠ KBT

where I and I0 are the emission intensities at temperature T and initial temperature, respectively. KB is the Boltzmann constant (8.629 × 10−5 eV K−1), and A is a constant. As shown in Figure 8, the linear relationship between ln[(I0/I) − 1] and 1/KBT is obtained, from which the slope, Ea, of Ca8MgSc(PO4)7:Dy3+ is calculated to be ∼0.26 eV, and Ea of Sr8MgSc(PO4)7:Dy3+ is calculated to be ∼0.259 eV. Ea of YAG:Ce3+ is calculated to be ∼0.2 eV. This indicates that the phosphors Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ have a high thermal stability. 10384

DOI: 10.1021/acs.inorgchem.7b01268 Inorg. Chem. 2017, 56, 10381−10386

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Figure 8. (a) Arrhenius fitting of emission intensity of Ca8MgSc(PO4)7:Dy3+. (b) Arrhenius fitting of emission intensity of Sr8MgSc(PO4)7:Dy3+. (c) Arrhenius fitting of emission intensity of YAG:Ce3+.

Figure 9. (a) Temperature-dependent integrated PL intensity of Ca8MgSc(PO4)7:Dy3+, Sr8MgSc(PO4)7:Dy3+, and commercial phosphor YAG:Ce3+. (b) Configurational coordinate diagram of the ground and excited states of Dy3+.

Figure 9a shows temperature-dependent integrated PL intensity of Ca8MgSc(PO4)7:Dy3+, Sr8MgSc(PO4)7:Dy3+, and commercial phosphor YAG:Ce3+. The thermal stabilities of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ are better than that of commercial phosphor YAG:Ce3+ when the temperature is increased. In Figure 9b, the curves of 6 H9/2−6H11/2−6H13/2−6H15/2 and 4G11/2−4I15/2−4F9/2 represent the ground and excited states of Dy3+, respectively. Point A is the lowest position of 4F9/2. Point M is the crossing point of the excited states and ground states. At room temperature, the electrons of 6H9/2−6H11/2−6H13/2−6H15/2 are first excited to their excited states upon 387 nm UV light excitation. For Dy3+, most of the electrons of 4G11/2−4I15/2−4F9/2 return to the ground states from point A by radiative transition. With the increase of temperature, the electrons of excited states might simultaneously overcome the activation energy ΔE2 under stronger phonon−electron coupling and directly tunnel to g. Therefore, activation energy ΔE2 is directly related to thermal quenching. 3.4. Optical Properties, LED Lamp Fabrication, and EL Spectrum. To demonstrate the potential application of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ phosphors, as shown in Figure 10, a phosphor-converted LED was fabricated by combining a 365 nm UV-chip and was driven by a forwardbias current of 60 mA (points A and B, respectively). A standard white light-emitting single-phase full-color phosphor of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ is realized. The insets show photographs of the LED lamp packages driven by 60 mA currents. Color correlated temperature (CCT) values of A and B are 9220 and 12 871 K, respectively.

Figure 10. CIE chromaticity diagram of mixing Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ white phosphors (points A and B, respectively). The insets show photographs of LED package with 365 nm UV-chips driven by 60 mA current and phosphors excited by 365 nm excitation.

white-emitting phosphors with high thermal emission stability by a high-temperature solid-state reaction. The detailed crystal structures of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ are compared and discussed. The red shift (about 2 nm) of the emission spectrum as a whole occurs with Sr replacing Ca in Ca7.9MgSc(PO4)7:0.1Dy3+ which is shown in the PLE and PL spectra of M7.9MgSc(PO4)7:0.1Dy3+ (M = Ca/Sr) samples at room temperature. The analysis of thermal stability of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ is considered. The emission intensity of Ca8MgSc(PO4)7:Dy3+ still remains over 95.2% of that at room temperature at 433 K (160 °C),

4. CONCLUSIONS In summary, we have synthesized two series of phosphors of Ca8MgSc(PO4)7:Dy3+ and Sr8MgSc(PO4)7:Dy3+ single-phase 10385

DOI: 10.1021/acs.inorgchem.7b01268 Inorg. Chem. 2017, 56, 10381−10386

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which shows its high thermal stability, even compared with the commercial phosphors YAG:Ce3+.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yongge Cao: 0000-0002-1250-1641 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the programs of the National Key Research and Development Program of China (2017YFB0403200)

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01268 Inorg. Chem. 2017, 56, 10381−10386