Cyan-Green Phosphor (Lu2M)(Al4Si)O12:Ce3+ for High-Quality LED

Dec 31, 2018 - Cyan-Green Phosphor (Lu2M)(Al4Si)O12:Ce3+ for High-Quality LED Lamp: Tunable Photoluminescence Properties and Enhanced Thermal ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Cyan-Green Phosphor (Lu2M)(Al4Si)O12:Ce3+ for High-Quality LED Lamp: Tunable Photoluminescence Properties and Enhanced Thermal Stability Yunan Zhou, Weidong Zhuang,* Yunsheng Hu,* Ronghui Liu, Huibing Xu, Mingyue Chen, Yuanhong Liu, Yanfeng Li, Yaling Zheng, and Guantong Chen

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National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem Advanced Materials Co., Ltd., Beijing 100088, People’s Republic of China S Supporting Information *

ABSTRACT: High-quality white light-emitting diodes (w-LEDs) are mainly determined by conversion phosphors and the enhancement of cyan component that dominates the high color rendering index. New phosphors (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba), showing a cyan-green emission, have been achieved via the co-substitution of Lu3+-Al3+ by M2+-Si4+ pair in Lu3Al5O12:Ce3+ to compensate for the lack of cyan region and avoid using multiple phosphors. The excitation bands of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba) show a red-shift from 434 to 445 nm which is attributed to the larger centroid shift and crystal field splitting. The enhanced structural rigidity associated with the accommodation of larger M2+ leads to a decreasing Stokes shift and the corresponding blue-shift (533 → 511 nm) in emission spectra, along with an improvement in thermal stability (keeping ∼93% at 150 °C). The cyan-green phosphor Lu2BaAl4SiO12:Ce3+ enables to fabricate a superhigh color rendering w-LED (Ra = 96.6), verifying its superiority and application prospect in high-quality solid-state lightings.

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INTRODUCTION White light-emitting diodes (w-LEDs), based on yellow phosphor Y3Al5O12:Ce3+ (YAG:Ce3+) excited by blue InGaN chip, have received wide attention due to the outstanding advantages, such as long lifetimes, high efficiencies, and environmental friendliness.1−6 However, the lack of red spectral component leads to poor color rendering index (CRI, Ra < 80) and high correlated color temperature (CCT > 4500 K).7−9 Therefore, a mixture of green Lu3Al5O12:Ce3+ (LuAG:Ce3+) and red (Ca,Sr)AlSiN3:Eu2+ phosphors has been introduced instead of a single yellow phosphor to produce higher CRI and lower CCT. But unfortunately, the cavity of cyan component dominating the superhigh CRI (Ra > 95) still exists in the 480−520 nm emission region (Figure 1), making them unsuitable for high-quality w-LEDs. Therefore, several cyan-emitting phosphors peaking at 480−520 nm, such as BaSi 2 O 2 N 2 :Eu 2 + , La 3 Br(SiS 4 ) 2 :Ce 3 + , BaSi 7 N 1 0 :Eu 2 + , RE2Si4N6C:Ce3+, and Ca2LaZr2Ga3O12:Ce3+, are proposed to compensate for the cyan cavity.10−14 Among these, the commercial BaSi2O2N2:Eu2+ shows an effective cyan emission peaking at 494 nm and enables to achieve high-quality w-LEDs (Ra = 95, CCT = 4280 K),15 but it still suffers from poor thermal stability.10 Besides, the other cyan-emitting phosphors are also restricted for high-quality w-LEDs because of their shortcomings, such as structural instability for La3Br(SiS4)2:Ce3+,11 harsh synthesis conditions for BaSi7N10:Eu2+,12 © XXXX American Chemical Society

Figure 1. Electroluminescent spectrum of the w-LED device fabricated using green LuAG:Ce3+ and red (Ca,Sr)AlSiN3:Eu2+ phosphors with a blue InGaN chip.

and poor thermal stability as well as low quantum efficiency for RE2Si4N6C:Ce3+ and Ca2LaZr2Ga3O12:Ce3+.13,14 In a word, cyan phosphors with efficient emission and high thermally stability are still absent. Additionally, the use of multiple phosphors will result in the fabricated complexity and reReceived: October 24, 2018

A

DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. XRD patterns (a) and magnified XRD patterns around 39° (b) of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba). The lattice parameters and the unit cell volume against ion radius (IR) of M cations (c).

absorption.3 Therefore, a cyan-green phosphor combining cyan and green color emission and robust thermal stability should be developed to overcome the above drawbacks for highquality w-LEDs. The green phosphor LuAG:Ce3+ has drawn much attention for its high luminescence efficiency and excellent thermal stability. For example, the LuAG:Ce3+ phosphor shows a colortuning from green to yellow and poor thermal stability by increasing Ce3+ content.16 The incorporation of Si4+-N3− into LuAG:Ce3+ results in a red-shift from green to yellow-orange and broadening of photoluminescent spectrum assigned to the increased covalency and crystal field strength.17 A cyan lightemitting phosphor LuCa2Hf2Al3O12:Ce3+ designed by the introduction of Ca2+-Hf4+ into LuAG:Ce3+ shows a cyan to blue-green emission and a lower quantum efficiency.18 Ji et al. discovered the new solid solution phosphor Lu3Al5−2xMgxSixO12:Ce3+ based on the substitution of Mg2+Si4+ for Al3+-Al3+, which exhibits a red-shift from green to yellow accompanied by a degraded thermal stability and luminescence efficiency.19 To the best of our knowledge, there has been no reported study on the new cyan-green phosphors based on the LuAG:Ce3+, which is included in the mechanism of emission tuning, the relationship between structural evolution and thermal stability and its application for high-quality w-LEDs. Therefore, we design the new cyan-green phosphors (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba) via the cosubstitution of Lu3+-Al3+ by M2+-Si4+ pair in LuAG:Ce3+. The crystal structure and luminescent properties of solid solution (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, Ba) are investigated as a function of M ions. The variation in thermal stability depending on M ions is examined in detail, demonstrating that the enhanced structural rigidity associated with the accommodation of larger M2+ in dodecahedral site leads to a decreasing Stokes shift and an improvement in thermal stability. Furthermore, a w-LED lamp adopting the new cyan-green phosphor Lu2BaAl4SiO12:Ce3+ and red phosphor (Ca,Sr)AlSiN3:Eu2+ exhibits a superhigh color rendering (Ra = 96.6), verifying its superiority and application prospect in highquality solid-state lightings.

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Al2O3 (99.99%), SiO2 (A.R.), and CeO2 (99.995%) were used. The raw materials weighed out in the desired stoichiometry were mixed and ground in an agate mortar thoroughly. Then, the mixtures were transferred to an alumina crucible and continually sintered at 1420 °C for 3 h in a reducing atmosphere created by burning activated carbon. Finally, the samples were furnace-cooled to room temperature and ground well for the subsequent measurements. The commercial red (Ca,Sr)AlSiN3:Eu2+ phosphor used for w-LED fabrication was produced by Grirem Advanced Materials Co., Ltd. The blue InGaN chips (λem = 455 nm) were manufactured by Hang Zhou Silan Azure Co., Ltd. Characterization. The phase determination of the as-prepared samples was conducted out on a MiniFlex 600 powder X-ray diffractometer (Rigaku) with Co−Kα radiation (λ = 1.78752 Å), and the dates were recorded over 2θ range of 10°−80° using a scan speed of 6° min−1. The X-ray diffraction (XRD) data for structural refinement were collected on a PW3040/60 powder X-ray diffractometer (PANalytical) with CuKα radiation (λ = 1.54056 Å) over 2θ range of 10°−120° using a scan speed of 0.0170° s−1. The crystallization characteristics were researched by using a S4300 scanning electron microscopy (SEM, Hitachi) facility equipped with a MP32S/M cathodoluminescence (CL) system (Horiba). The elemental composition was analyzed using the energy dispersive Xray spectroscopy (EDS) which is attached to the SEM. The roomtemperature and temperature-dependent photoluminescence spectra were measured by a Fluoromax-4 spectrofluorometer (Horiba) equipped with a 200 W xenon lamp and a photomultiplier tube and a heating attachment. The Raman spectra were collected on a DXRxi Micro Raman imaging spectrometer (Thermo Scientific) with 780 nm excitation wavelength. The fluorescent decay measurements were conducted out on a Deltaflex ultrafast lifetime spectrofluorometer system (Horbia Jobin Yvon). The quantum efficiency (QE) value was determined on an intensified multichannel QY-2000 fluorescence spectrometer (Orient KOJI) equipped with a Xe lamp as excitation source and BaSO4 powder served as a reference. The Commission International de I’Eclairage (CIE) chromaticity coordinates were checked out by a HAAS-2000 high-accuracy spectroradiometer (Everfine) with a CCD detector. A w-LED device was fabricated by coating a smooth mixture of the as-prepared green Lu2BaAl4SiO12:Ce3+ and the commercial red (Ca,Sr)AlSiN3:Eu2+ phosphors and transparent silicon resin to the blue InGaN chips (λem = 455 nm).

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RESULTS AND DISCUSSION Crystal and Phase Analysis. Figure 2a depicts the XRD patterns of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba). All the diffraction peaks of the four samples are indexed to LuAG:Ce3+, demonstrating the production of a solid solution. Figure 2b shows the detailed XRD patterns from 38.5° to 40°. Clearly, the diffraction peaks shift to higher angles with doping the small Mg2+-Si4+ instead of Lu3+-Al3+ and shift toward lower

EXPERIMENTAL SECTION

Materials and Synthesis. Powder samples with nominal compositions of (Lu1.96M)(Al4Si)O12: 0.04Ce3+ (M = Mg, Ca, Sr, and Ba) were prepared via the conventional high-temperature solidstate reaction method. The raw materials, that is, Lu2O3 (99.99%), MgO (99.99%), CaCO3 (99.99%), SrCO3 (A.R.), BaCO3 (A.R.), B

DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry angles when M = Ca, Sr, and Ba, which is owing to size mismatch between Lu3+ and M2+. The Rietveld refinements of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba) are performed by GSAS software package using the crystallographic data of LuAG as an initial model.20,21 The Rietveld refinement results and crystal structure data for samples are given in Figure S1 and Table S1 (in Supporting Information), which reveals that the structure of solid solution keeps the same, but the lattice constants and lattice volume change with the varying M2+ ions. Commonly, the XRD peaks should exhibit a continuous shift, and the cell volume and parameters of the solid solutions change linearly with the substitution of M2+ ions (ionic radius Mg2+ < Lu3+ < Ca2+ < Sr2+ < Ba2+).22 However, it is observed that the diffraction peaks of (Lu2M)(Al4Si)O12:Ce3+ (M = Ca, Sr and Ba) stay unchanged with increasing M cation size (Figures 2b and S2), and correspondingly the lattice parameters (a = b = c) and cell volume (V) of M = Sr and M = Ba samples are quite close to that of M = Ca as shown in Figure 2c. To understand this situation, the magnified XRD patterns of (Lu2M)(Al4Si)O12:Ce3+ (M = Sr and Ba) are provided in Figure S3. It can be found that a broad diffraction band in the 25−40° (2θ) range is detected for M = Sr and Ba. Some amorphous glass phase perhaps co-exists in the M = Sr and Ba samples.23 Given the solubility property of glass in HF, a soakage experiment of the M = Sr and Ba samples in HF solution was conducted. After 2 h soakage processing with stirring, it is found that such broad diffraction band diminishes obviously (Figure S3), which confirms the co-existence of glass phase in the M = Sr and M = Ba samples. Therefore, the formation of glass phase leads to the abnormal variation of lattice constants against cation size. The following luminescence analysis are based on the samples without glass phase. To confirm the introduction of large Sr2+ and Ba2+ ions into the crystal lattice, we carefully analyzed the variation of XRD patterns of (Lu2M)(Al4Si)O12:Ce3+ (M = Sr and Ba) and Lu3Al5−xSixO12:Ce3+ (x = 0, 0.04, 0.08, and 0.16) compared to LuAG:Ce3+, as presented in Figures 2b and S4. Clearly, the XRD peaks of Lu3Al5−xSixO12:Ce3+ shift to higher angles with singly doping the smaller Si4+ into Al3+, whereas that of (Lu2M)(Al4Si)O12:Ce3+ (M = Sr and Ba) shift to lower angles compared to LuAG:Ce3+, indicating that the bigger size Sr2+ and Ba2+ substitution for Lu3+ into LuAG dominates the expansion of crystal lattice for M = Sr and Ba samples. Moreover, the EDS elemental mapping images (Figure S5) exhibit that Lu, Sr, Al, Si, and O are homogeneously distributed within the particle selected from HF-soaked M = Sr sample, and atomic ratio (Lu/Sr/Al/Si/O = 12.6/4.3/20.6/ 4.1/58.3) by EDS analysis is close to that of the formula Lu2SrAl4SiO12:Ce3+. It also suggests that most of the Sr2+ or Ba2+ ions successfully enter into the LuAG crystal lattice. For further discussion on the structural evolution by the chemical composition modification, Figure 3a,b−e shows the crystal structure of the initial LuAG viewing along the a axis and the local coordination environment of the targeted (Lu2M)(Al4Si)O12:Ce3+ according to the refinement results, respectively. LuAG crystal structure belongs to the cubic symmetry with space group of Ia3̅d (230). The Altet, Aloct, and Lu atoms are (4-fold) tetrahedrally, (6-fold) octahedrally, and (8-fold) dodecahedrally coordinated to O atoms, respectively. The AlO4 tetrahedrons and AlO6 octahedrons share O2− vertexes or edges with LuO8 dodecahedrons, which form a three-dimensional network. As the chemical substitution of

Figure 3. Structure of LuAG along the a axis (a) and a schematic connection between (Lu/M)O8 and (Al/Si)O4 polyhedrons in (Lu2M)(Al4Si)O12:Ce3+ for M = Mg (b), Ca (c), Sr (d), and Ba (e).

M2+-Si4+ pair for Lu3+-Al3+ pair, the dodecahedral and tetrahedral sites of (Lu2M)(Al4Si)O12:Ce3+ are randomly distributed by Lu3+/M2+ and Al3+/Si4+ with the same ratio of 2/1, as depicted in Figure 3b−e, respectively. The substitution will result in the local structural distortions due to the cation size mismatch between M2+ (Si4+) and Lu3+ (Al3+). Compared to LuAG, the tetrahedral polyhedron of (Lu2M)(Al4Si)O12:Ce3+ gets compressed arising from the substitution of Al3+ by smaller Si4+. However, the distortions in dodecahedral change differently depending on the size mismatch between M2+ and Lu3+, which can theoretically tune Ce3+ luminescent properties. The structure−property correlation between local coordination structure and luminescence properties will be detailed in the next section. Photoluminescence Properties. Figure 4a,b provides the normalized excitation and emission spectra of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba). The normalized excitation spectra (monitored at 530 nm) of all samples present two broad excitation bands located at around 340 and 440 nm, which are ascribed to the transitions from 4f ground state to the two lowest 5d excited levels (labeled as 5d1 and 5d2) of Ce3+.24 With the variation of M from Mg to Ba, the two excitation bands show a red-shift, and the low-energy side shifts from 434 to 445 nm, as shown in Figure 4a. The spectroscopic parameter details of (Lu2M)(Al4Si)O12:Ce3+ are listed in Table S2. The red-shift is determined by two possible factors: the nephelauxetic effect and the crystal field splitting.25 According to the model proposed by Morrison and developed by Dorenbos, the centroid shift (εc) of Ce3+ 5d levels is positively correlated with the anion polarizability (αsp).26 The anion polarizability in Ce3+-doped oxide compounds can be estimated by27 4.8 αsp = 0.33 + 2 χav (1) Accordingly, the anion polarizability is influenced by the average electronegativity χav of the cations which is defined as following: χav = C

1 Na

Nc

∑ 1

ziχi γi

(2) DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Normalized excitation spectra (a) and emission spectra (b) of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba). Stokes shift (c) and thermal quenching properties of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba) (d). Schematic graph of correlation between thermal stability and Stokes shift (e) and luminescence decay profiles of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba) (f).

where Nc is the summation over all cations in the compound, Na is the number of anions in the formula, χi represents the electronegativity of cation i with formal charge zi, and γ is the formal negative charge of the anion. By utilizing Pauling-type electronegativity values,28 χav values are calculated to be 1.548, 1.523, 1.518, and 1.513 in (Lu2M)(Al4Si)O12:Ce3+ for M = Mg, Ca, Sr, and Ba, respectively. Based on eq 1, the decreasing χav will lead to the increase of the anion polarizability αsp. As a result, the centroid of Ce3+ 5d levels shifts to a lower-energy position, resulting in the red-shift of the excitation band from M = Mg to M= Ba. Another factor for the red-shift depends on the crystal field splitting of 5d levels for Ce3+. Normally, the lattice expansion generates a decrease of the crystal field splitting and causes a blue-shift of the lowest-energy excitation band.29 However, this is not the case in this work. Given the local coordination environment of Ce3+ ion in the compounds, we can find that

each CeO8 polyhedron is simultaneously connected with six (Al/Si)O4 tetrahedrons, four AlO6 octahedrons, and four (Lu/ M)O8 square antiprism by sharing nodes or edges. As the Al/Si content keeps invariable in the solid solution, they should have no significant effect on the geometry of CeO8 polyhedron. When the M cation takes the place of Lu3+, the (Lu/M)O8 polyhedron will expand with increasing M cation size, which influences the Ce−O bond distances in CeO8. As depicted in Figure 5, the model clearly diaplays the shrinkage of CeO8 polyhedron from M = Mg to Ba. Therefore, the decrease in Ce−O bond length strengthens the crystal field splitting of Ce3+ 5d levels, which also contributes to the red-shift of the 4f1−5d1 excitation band. The excitation spectra of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba) show a red-shift from M = Mg to M = Ba. However, the maxima of emission band exhibit a blue-shift from 533 to 511 nm as shown in Figure 4b and Table S2. It is D

DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Schematic shrinkage mechanism of Ce−O with the introduction of bigger size M cations into the dodecahedral site.

probably due to the decreasing of Stokes shift. These emission bands can be decomposed into two Gaussian sub-bands (the inset of Figure 4b) which normally correspond to the 2D3/2 → 2 F5/2 and 2F7/2 spin−orbit coupled transition of Ce3+.30 By evaluating the energy difference between the 2F5/2 →2D3/2 excitation maxima and the 2D3/2 → 2F5/2 emission maxima,31 the Stokes shift are estimated to be 3395 cm−1, 3068 cm−1, 2670 cm−1, and 2670 cm−1 for M = Mg, Ca, Sr, and Ba, respectively, as shown in Figure 4c. With increasing M cation size, the internal QE exhibits an increasing trend from 47% for M = Mg to 81% for M = Ba at room temperature (Table S2). The internal QE of (Lu2M)(Al4Si)O12:Ce3+ for M = Ba (81%) is obviously higher than that isostructural cyan-emitting Ca2GdZr2Al3O12:Ce3+ (40.26%)32 and Ca2LaZr2Ga3O12:Ce3+ (35.2%)14 and comparable to that of the commercial cyan BaSi2O2N2:Eu2+ (71%).10 Figure 4d presents the variation of the integrated emission intensity as a function of temperature from 25 to 250 °C. The relative intensity at 150 °C remains about 76.0% for M = Mg, 85.2% for M = Ca, 91.6% for M = Sr, and 92.9% for M = Ba, respectively. Moreover, it is desirable that the thermal stability of (Lu2M)(Al4Si)O12:Ce3+ for M = Ba is even slightly better than that of the commercial LuAG:Ce3+ and YAG:Ce3+, as shown in Figure S6. We can also observe that the dramatically improved thermal stability is accompanied by the decreasing Stokes shift from M = Mg to Ba. To understand the underlying mechanism for this behavior, thermal quenching process caused by phonon-assisted relaxation is illustrated by the configurational coordinate model,33 as shown in Figure 4e. In Ce3+-activated phosphors, the excitation is followed by internal conversion from the equilibrium position of the 4f7 ground state to the 4f65d1 excited state, as indicated by the process ① in Figure 4e. Ideally, most of the excited-state electrons return to the ground state along ways ② following a photon emission with high quantum yield. However, in systems where a large departure of the excited-state potential energy surface (ΔR) occurs along R axis upon excitation, the potential energy surfaces corresponding to the excited state and ground state can intersect each other. From this crossover point, a phononassisted nonradiative transition can occur with the activation energy (ΔE). At this point, it follows that limiting ΔR corresponding to a smaller Stokes shift is one way to ensure a relatively bigger ΔE and inhibit this thermal quenching. Normally, ΔR is associated with the rigidity of the crystal structure; materials containing more rigid chemical bonds tend to have a smaller ΔR.34 On that basis, the local structural evolution model of Ce3+-doped (Lu2M)(Al4Si)O12 (M = Mg, Ca, Sr, and Ba) is proposed to elucidate the relationship between the structural rigidity and photoluminescence properties, as shown in Figure 6. In LuAG:Ce3+, the host possesses a relatively stable and stiff crystal structure, and one LuO8 dodecahedron is coordinated with six AlO4 tetrahedrons by

Figure 6. Schematic polyhedrons distortion caused by the substitution of M2+-Si4+ for Lu3+-Al3+ in (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba).

sharing vertex or edge. The (Lu2M)(Al4Si)O12:Ce3+ are achieved via the co-substitution of one-third of LuO8 and AlO4 by MO8 and SiO4 polyhedrons starting from LuAG:Ce3+. The proposed model (Figure 6) clearly shows the shrinkage of (Lu/M)O8 polyhedron with the introduction of Mg2+ in these compounds, causing a great shrinkage distortion and lower structural rigidity of crystal lattice, which lead to a larger Stokes shift and poor thermal stability. On the contrary, the bigger M cations (M = Ca, Sr, and Ba) occupying Lu site lead to the expansion of (Lu/M)O8 polyhedron. This effect neutralizes the tetrahedron shrinkage arising from the substitution of Al3+ by smaller Si4+. The crystal structure becomes more stable, thus the Stokes shift is gradually decreased and the thermal stability is improved from M = Mg to M = Ba. To further explore the evolution of structural rigidity in the compounds, the Raman spectra (depicted in Figure 7a) of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba) were collected to provide information on the variation of external lattice and chemical bonds. As indicated by the recent work of Karlsson et al. with YAG:Ce3+, the Raman spectra can be divided to three separate regions: low-frequency range (0−350 cm−1), medium-frequency range (350−600 cm−1), and highfrequency range (600−800 cm−1), and the peaks at 600−800 cm−1 are considered as the bending and stretching modes of (Al/Si)O4 and AlO6.35 In the low- and medium-frequency regions, the positions of all peaks are largely unaffected by M cation. However, in the high-frequency region, we can observe a simultaneous upward shift of modes of the main peak around 790 cm−1 and other minor peaks (such as 735 cm−1) with the successive substitution of M from Mg to Ba, as illustrated in Figure 7b. It reflects the changes in either the average Al/Si−O distances or the local structural symmetry of the (Al/Si)O4 and AlO6 caused by the accommodation of M cation with different size. Karlsson et al. pointed out that the upward shift of highfrequency modes is correlated with the higher structural rigidity.35 That is to say, the Raman spectra results further explain that M = Ba sample exhibits the highest structural rigidity and the best performance in thermal stability and quantum efficiency. Meanwhile, the luminescence lifetime measurement is adopted to verify the variation of Ce3+ coordination environE

DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Raman spectra (a) and peak positions around 735 cm−1 (triangle) and 790 cm−1 (sphere) (b) of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba).

Figure 8. Normalized excitation spectra (a) and emission spectra (b) of (Lu3−xMgx)(Al5−xSix)O12:Ce3+ (x = 0, 0.25, 0.5, 0.75, and 1). Stokes shift values (c) and thermal quenching properties of (Lu3−xMgx)(Al5−xSix)O12:Ce3+ (d).

ment in (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr, and Ba). The decay curves of Ce3+ with different M ions excited by 450 nm and monitored at the respective maximum emission wavelength are also presented in Figure 4f. All curves obey a single exponential. The lifetime values are evaluated to be 58.58 ns, 59.31 ns, 61.58 ns, and 61.96 ns for M = Mg, Ca, Sr, and Ba. The increasing lifetime of Ce3+ can be ascribed to the suppression of nonradiative relaxation according to Xie’s work;36 and this trend coincides with the thermal quenching behavior. These results demonstrate the nonradiative processes (including thermal quenching and luminescence decay behaviors) of activators are sensitive to the local coordination environment. According to the above discussion, we proposed that the bad thermal stability of Lu2MgAl4SiO12:Ce3+ results from poor

structural rigidity due to the introduction of smaller Mg2+ ions in LuAG:Ce3+. To validate the speculation, we investigated the evolution of photoluminescence properties and thermal stability in the (Lu3−xMgx)(Al5−xSix)O12:Ce3+ solid solution as a function of x. The normalized excitation and emission spectra of (Lu3−xMgx)(Al5−xSix)O12:Ce3+ (x = 0, 0.25, 0.5, 0.75, and 1) are depicted in Figure 8a,b. The maxima of the 4f5d1 excitation band have a obvious blue-shift from 447 to 434 nm, but the emission band almost stays the same, which indicates the Stokes shift increases with the increasing of Mg concentration, as illustrated in Figure 8c. The temperaturedependent emission intensity of (Lu3−xMgx)(Al5−xSix)O12:Ce3+ for x = 0.25 and 1 is monitored and given in Figure 8d. It can be observed that the thermal stability of x = 0.25 is superior to that of x = 1. The results prove that the more F

DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX

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LED chip. Figure 10a presents the final electroluminescence spectrum of the as-fabricated w-LED device under a driven current 50 mA. The CCT, the CIE chromaticity coordinates, and the luminous efficiency were checked out to be 4577 K, (0.358, 0.363), and 67.46 lm/W, respectively, as shown in Figure 10b. The CRI (Ra) of this w-LED device reaches up to 96.6, obviously higher than that of the w-LED fabricated by coupling a blue InGaN chip with green LuAG:Ce3+ and red (Ca,Sr)AlSiN3:Eu2+ phosphors (Ra = 91.7) for the enhancement of cyan emission. Additionally, the full set of 15 CRIs and the Ra is given in Table 1. Except for R12, all components of the

substitution of Mg2+-Si4+ for Lu3+-Al3+ leads to a great shrinkage distortion and a lower structural rigidity which is related to thermal stability. The above discussion further verifies that the structural rigidity of (Lu2M)(Al4Si)O12:Ce3+ can be enhanced with the accommodation of larger M2+. CL Emission. To further investigate the morphology as well as luminescence uniformity of the solid solution, the combined technique of SEM and CL (SEM-CL) was employed to characterize the cross-section of the particles taken from the Lu2BaAl4SiO12:Ce3+ sample. The SEM image (Figure 9a)

Table 1. Full Set of 15 CRIs and the General Ra of the Fabricated w-LED Ra

R1

R2

R3

R4

R5

R6

R7

96.6 R8

98 R9

98 R10

98 R11

95 R12

97 R13

97 R14

95 R15

95

96

96

99

81

98

99

96

CRIs are beyond 95, further indicating that excellent white light was obtained. The results demonstrate that the Lu2BaAl4SiO12:Ce3+ phosphor plays an effective role in improving Ra value and hence may be a great potential cyangreen phosphor for use in w-LEDs.

Figure 9. Cross-sectional SEM image of the Lu2BaAl4SiO12:Ce3+ particles (a) (scale: 5 μm). Point CL study on the particles (b) and the corresponding emission spectra (c).

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CONCLUSIONS The new cyan-green phosphors (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba) have been achieved via the co-substitution of Lu3+-Al3+ by M2+-Si4+ pair starting from Lu3Al5O12:Ce3+. The excitation bands of (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba) show a red-shift from 434 to 445 nm, which is attributed to the larger centroid shift and crystal field splitting depending on the size mismatch. The relationship between thermal stability and variation of structural rigidity in (Lu2M)(Al4Si)O12:Ce3+ (M = Mg, Ca, Sr and Ba) has been demonstrated. The more substitution of small Mg2+ for Lu3+ leads to a great shrinkage distortion and a lower structural rigidity. The enhanced structural rigidity associated with the accommodation of larger M2+ (M = Ca, Sr, and Ba) leads to a decreasing Stokes shift and the corresponding blue-shift (533 → 511 nm) in emission spectra, along with an improvement in thermal stability (keeping ∼93% at 150 °C). The w-LED d e v i c e a d o p t i n g t h e n e w c y a n - g r e e n ph o s p h o r Lu2BaAl4SiO12:Ce3+ and red phosphor (Ca,Sr)AlSiN3:Eu2+

shows that the particles have lamellar shape with a size of about 6 μm. The point CL spectrum that is more sensitive than emission spectrum to the localized variations could even distinguish luminescence information on different areas in several tens of nanometers. As shown in Figure 9b,c, the point CL spectra of the measured phosphor particle present quite similar emission profiles, comprising a main band peaking at ∼510 nm and a side shoulder peaking at ∼320 nm. The main band (∼510 nm) is deemed to the transitions of Ce3+ in Lu2BaAl4SiO12 coinciding to the photoluminescence emission results. The side shoulder (∼320 nm) can be ascribed to the recombination luminescence of the antisite defect (AD) Lu3+Al3+, which has been observed in LuAG:Ce single crystal before.37,38 The similar point spectra demonstrate relative luminescence uniformity within the grain.39 w-LED Device. The optimal Lu2BaAl4SiO12:Ce3+ cyangreen phosphor is utilized to fabricate a white LED device with red-emitting CaAlSiN3:Eu2+ phosphor and a 455 nm InGaN

Figure 10. Electroluminescent spectrum of the w-LED device fabricated using the as-prepared cyan-green Lu2BaAl4SiO12:Ce3+ and the commercial (Ca,Sr)AlSiN3:Eu2+ red phosphor with a blue InGaN chip (a). CIE chromaticity coordinates of the fabricated w-LED and the used phosphors (the insets show the photographs of the used phosphors under blue light excitation) (b). G

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Inorganic Chemistry

(8) Yang, C.-C.; Lin, C.-M.; Chen, Y.-J.; Wu, Y.-T.; Chuang, S.-R.; Liu, R.-S.; Hu, S.-F. Highly Stable Three-Band White Light from An InGaN-Based Blue Light Emitting Diode Chip Precoated with (Oxy)nitride Green/Red Phosphors. Appl. Phys. Lett. 2007, 90, 123503. (9) Zhang, L. L.; Zhang, J. H.; Zhang, X.; Hao, Z. D.; Zhao, H. F.; Luo, Y. S. New Yellow-Emitting Nitride Phosphor SrAlSi4N7:Ce3+ and Important Role of Excessive AlN in Material Synthesis. ACS Appl. Mater. Interfaces 2013, 5, 12839−12846. (10) Bachmann, V.; Ronda, C.; Oeckler, O.; Schnick, W.; Meijerink, A. Color Point Tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+ for White Light LEDs. Chem. Mater. 2009, 21, 316−325. (11) Lee, S.-P.; Liu, S.-D.; Chan, T.-S.; Chen, T.-M. Synthesis and Luminescence Properties of Novel Ce3+- and Eu2+-Doped Lanthanum Bromothiosilicate La3Br(SiS4)2 Phosphors for White LEDs. ACS Appl. Mater. Interfaces 2016, 8, 9218−9223. (12) Qin, J. L.; Zhang, H. R.; Lei, B. F.; Dong, H. W.; Liu, Y. L.; Meng, J. X.; Zheng, M. T.; Xiao, Y. Preparation and Afterglow Properties of Highly Condensed Nitridosilicate BaSi7N10:Eu2+ Phosphor. J. Lumin. 2014, 152, 230−233. (13) Yan, C. P.; Liu, Z. N.; Zhuang, W. D.; Liu, R. H.; Xing, X. R.; Liu, Y. H.; Chen, G. T.; Li, Y. F.; Ma, X. L. YScSi4N6C:Ce3+A Broad Cyan-Emitting Phosphor to Weaken the Cyan Cavity in FullSpectrum White Light-Emitting Diodes. Inorg. Chem. 2017, 56, 11087−11095. (14) Zhong, J. Y.; Zhuang, W. D.; Xing, X. R.; Liu, R. H.; Li, Y. F.; Liu, Y. H.; Hu, Y. S. Synthesis, Crystal Structures, and Photoluminescence Properties of Ce3+-Doped Ca2LaZr2Ga3O12: New Garnet Green-Emitting Phosphors for White LEDs. J. Phys. Chem. C 2015, 119, 5562−5569. (15) Xie, R.-J.; Hirosaki, N.; Sakuma, K.; Kimura, N. White LightEmitting Diodes (LEDs) Using (Oxy)nitride Phosphors. J. Phys. D: Appl. Phys. 2008, 41, 144013. (16) Park, K.; Kim, T.; Yu, Y.; Seo, K.; Kim, J. Y/Gd-Free Yellow Lu3Al5O12:Ce3+ Phosphor for White LEDs. J. Lumin. 2016, 173, 159− 164. (17) Liu, J. Q.; Wang, X. J.; Xuan, T. T.; Wang, C. B.; Li, H. L.; Sun, Z. Lu3(Al,Si)5(O,N)12:Ce3+ Phosphors with Broad Emission Band and High Thermal Stability for White LEDs. J. Lumin. 2015, 158, 322−327. (18) Wang, X. C.; Zhao, Z. Y.; Wu, Q. S.; Li, Y. Y.; Wang, Y. H. Synthesis, Structure and Photoluminescence Properties of Ca2LuHf2(AlO4)3:Ce3+, A Novel Garnet-Based Cyan Light-Emitting Phosphor. J. Mater. Chem. C 2016, 4, 11396−11403. (19) Ji, H. P.; Wang, L.; Molokeev, M. S.; Hirosaki, N.; Xie, R. J.; Huang, Z. H.; Xia, Z. G.; ten Kate, O. M.; Liu, L. H.; Atuchin, V. V. Structure Evolution and Photoluminescence of Lu3(Al,Mg)2(Al,Si)3O12:Ce3+ Phosphors: New Yellow-Color Converter for Blue LED-Driven Solid State Lighting. J. Mater. Chem. C 2016, 4, 6855−6863. (20) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (21) Toby, B. H. EXPGUI, A Graphical User Interface for GSAS. J. Appl. Crystallogr. 2001, 34, 210−213. (22) Shannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 925−946. (23) Ji, H. P.; Wang, L.; Cho, Y.; Hirosaki, N.; Molokeev, M. S.; Xia, Z. G.; Huang, Z. H.; Xie, R.-J. New Y2BaAl4SiO12:Ce3+ Yellow Microcrystal-Glass Powder Phosphor with High Thermal Emission Stability. J. Mater. Chem. C 2016, 4, 9872−9878. (24) Pan, Z. F.; Li, W. Q.; Xu, Y.; Hu, Q. S.; Zheng, Y. F. Structure and Redshift of Ce3+ Emission in Anisotropic Expansion Garnet Phosphor MgY2Al4SiO12:Ce3+. RSC Adv. 2016, 6, 20458−20466. (25) Zhang, J. C.; Zhang, J. L.; Zhou, W. L.; Ji, X. Y.; Ma, W. T.; Qiu, Z. X.; Yu, L. P.; Li, C. Z.; Xia, Z. G.; Wang, Z. L.; Lian, S. X. Composition Screening in Blue-Emitting Li4Sr1+xCa0.97−x(SiO4)2:Ce3+ Phosphors for High Quantum Efficiency and Thermal Stable

exhibits a superhigh color rendering (Ra = 96.6) and lower color temperature 4577 K, verifying its superiority and application prospect in high-quality solid-state lightings.

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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/acs.inorgchem.8b03017. Final refined results of (Lu2M)(Al4Si)O12:Ce3+, spectroscopic parameters of (Lu2M)(Al4Si)O12:Ce3+, Rietveld refinement of XRD patterns of (Lu2M)(Al4Si)O12:Ce3+, normalized magnified XRD patterns of (Lu2M)(Al4Si)O12:Ce3+, XRD patterns of (Lu2M)(Al4Si)O12:Ce3+ (M = Sr and Ba) before and after soakage by HF solution, XRD patterns and magnified XRD patterns of Lu3Al5−xSixO12:Ce3+, SEM image and EDS elemental mapping images of (Lu2M)(Al4Si)O12:Ce3+ (M = Sr), and thermal quenching properties of (Lu2M)(Al4Si)O12:Ce3+ (M = Ba), LuAG:Ce and YAG:Ce (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Weidong Zhuang: 0000-0001-5088-0892 Huibing Xu: 0000-0002-8350-6044 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology of China (grant number: 2016YFB0400605). REFERENCES

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DOI: 10.1021/acs.inorgchem.8b03017 Inorg. Chem. XXXX, XXX, XXX−XXX