Synthesis and characterization of monodisperse yttrium aluminum garnet (YAG) micro-crystals with rhombic dodecahedron

Journal of Alloys and Compounds 762 (2018) 537e547

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Synthesis and characterization of monodisperse yttrium aluminum garnet (YAG) micro-crystals with rhombic dodecahedron Tao Xu a, Rui Yuan a, Peng-Cheng Xu a, Dong-Jie Pan a, Woochul Yang c, Hai-Bo Guo a, Yu-Fang Shen b, Jian-Feng Hu a, Zhi-Jun Zhang a, *, Jing-Tai Zhao a, ** a

School of Materials Science and Engineering, Shanghai University, China Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin, 541004, China c Department of Physics, Dongguk University, Pildong-ro, Choong-gu, Seoul, 04620, South Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 January 2018 Received in revised form 22 April 2018 Accepted 22 May 2018 Available online 23 May 2018

Uniform, monodisperse yttrium aluminum garnet (YAG) rhombic dodecahedron micro-crystals were synthesized using coprecipitation and hydrothermal methods. The Na2SO4 as a surfactant was helpful in improving the dispersity of the crystallites and forming well-faceted, micro-sized dodecahedral YAG crystallites. The effect of the reaction time and the solvent has also been studied. The crystal growth mechanism called dissolution/crystallization was determined by analyzing the experimental and simulated results. And the specific well-developed crystal facets in the {110} family have been demonstrated by the theoretical calculation. A higher integrated emission intensity can be achieved for the YAG:Ce crystallites by improving the phase purity, morphology with better developed dodecahedrons, and dispersion. Furthermore, a rather high luminous efficacy (LE ¼ 104.14 lm/W) for the white LED fabricated using the YAG:Ce3þ phosphor prepared in this work was realized. The correlation between the crystal growth, morphology and luminescence properties of YAG crystallites has been established, which exhibits a great importance for the synthesis and application of mono-dispersed YAG crystallites. © 2018 Elsevier B.V. All rights reserved.

Keywords: Monodisperse Optical materials Coprecipitation & hydrothermal methods Crystal growth & structure Morphology Luminescence

1. Introduction YAG is widely used as a host material for phosphors [1e3], and preparing monodisperse nanometer-sized YAG phosphors is the focus of this research [4e6]. Also, in recent years, the rapid development of video encoding, planar displays, and lighting, have allowed higher resolutions and better luminous efficacy, so phosphors with suitable luminescence, particle size and morphology need to be developed. Monodisperse nano-phosphors can reduce the scattering loss and improve the external quantum efficiency, and this is helpful to improve the display resolution. In addition, hosts with good crystallinity, which can be characterized according to their morphology, can produce phosphors with suitable luminescence properties. Therefore, studying the particle size and morphology or crystallinity of the phosphors has attracted a

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.-J. Zhang), [email protected] (J.-T. Zhao). https://doi.org/10.1016/j.jallcom.2018.05.266 0925-8388/© 2018 Elsevier B.V. All rights reserved.

significant amount attention. In addition to rare-earth-activated YAG phosphors, transparent YAG ceramics have also been widely used in solid state lighting, displays, solid state lasers, scintillation detectors, window materials and other applications [7,8]. In 1995, Ikesue prepared transparent YAG:Nd ceramics and was able to obtain a laser output [9,10]. Research into transparent YAG ceramics has become popular [11e18], and these can now be used to achieve large-sized glasslike materials with high thermal conductivity and good mechanical strength that allow doping at high concentrations. Notably, in order to achieve these attractive applications, the most important thing is to obtain excellent quality nano-powders with a fine grain and superior performance at a lower sintering temperature, which is the first step in the synthesis of ceramics. However, for the reason that exact process conditions influence the growth drastically, obtaining high quality material is still more of an art than science despite using plenty of methods. These related issues need to be further explored by researchers. Overall, to control the properties of the YAG phosphors and transparent ceramics, it is important to prepare monodispersed

538

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

YAG crystallites. Synthesis is well known to directly determine the particle size distribution, dispersion and nano/micro-crystalline morphology of the powders. Therefore, the preparation technology for YAG crystallites has been the focus of researchers. At present, researchers have developed various preparation methods, such as: the solid-state [19,20], coprecipitation [21e23], sol-gel combustion [24,25], spray pyrolysis [26,27], microemulsion [28,29], hydrothermal [30,31], solvent thermal [32e36] and other wet chemical methods. The solid-state method is simple, making it easy to realize bulk production of powders. However, this method requires multiple milling, and it is easy to introduce impurities and cause lattice defects. The formation of a YAG phase requires a higher temperature. The particle size of the resulting YAG crystallites is up to the micron level and the agglomeration is serious. Compared to the solid-phase method, the wet chemical method can be used to mix the elements at an atomic level. Pure phase YAG can be obtained at a lower temperature, and the particle size of powders is small with better dispersity. Therefore, the wet chemical method (typically the hydrothermal or solvothermal method) is considered to be ideal to prepare the YAG crystallites. Furthermore, the hydrothermal method or solvothermal reaction produces a crystallization process without further heat treatment, making it is possible to avoid the hard agglomeration introduced by subsequent heat treatment and beneficial to obtaining YAG micro/nano crystallites with good dispersity. The YAG micro/nano crystallites with mono-dispersity and complete crystal development in this work were prepared by a hydrothermal reaction of the precursors obtained via coprecipitation. The morphology of the YAG crystallites can be adjusted by adding the surfactant Na2SO4, which is an important anionic surfactant that is widely used in many industrial processes, including for colloid stabilization, metal treatment, mineral flotation, dailyused detergents, and pesticides [37]. The optimal conditions to synthesize the YAG powders are summarized. The integrity of the developed facet is calibrated with further verifications through a simulation. The theoretical basis for the reaction growth process of the YAG crystallites was further elaborated, and the relationship between the morphology and the luminescence was also investigated. The connection of the growth process, morphology and luminescence properties of YAG is not only helpful to understand its mechanism, but can also guide its application. 2. Experiment 2.1. Precursor preparation Aluminum nitrate hydrate (Al(NO3)3$9H2O, A.R.), yttrium nitrate hexahydrate (Y(NO3)3$6H2O, A.R.), and cerium nitrate hexahydrate (Ce(NO3)3$6H2O, A.R.) were dissolved in deionized water. The solutions are mixed according to the molar ratio of (Y1xCex)3Al5O12 (x ¼ 0.02) while keeping the concentration of aluminum nitrate at 0.15 mol/L and the total volume of the solution is approximately 40 ml. The mixed nitrate solution was added dropwise to a 3 M ammonium hydrogen carbonate solution under mild agitation of 400 r/min at room temperature. After aging for 12 h, the precipitate was repeatedly washed with distilled water until the pH is 7 and dried at 80  C for 4 h. 2.2. Synthesis of YAG micro/nano-crystallites The dried precipitate as a precursor was dispersed in different solvents and placed in an autoclave. The autoclave was heated to 300  C and was kept at this temperature for 12e60 h. Then, the autoclave was cooled to room temperature in air. The resulting suspension was centrifugated, repeatedly washed with distilled

water, and dried in air at 80  C for 12 h. 2.3. Characterization For phase identification, X-ray powder diffraction (XRD) data of the samples were collected on a Rigaku D\max-2200 (Cu Ka radiation, l ¼ 1.54178 Å, 40 kV and 200 mA) at ambient temperature. The 2q ranges of all data are from 10 to 90 . The Fourier transform infrared spectra (FT-IR) of the samples were measured using Thermo Nicolet 50 ATR in the wavenumber range of 400e4000 cm1. The morphology, particle sizes and crystallinity of the powders were studied using a scanning electronic microscope (SEM, FEI Magellan 400). The particle size distributions were characterized using a particulate size description analyser (PSDA, Mastersizer, 2000). An Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES, PERKINE 7300DV) was used to analyze the elemental composition. The photoluminescence of the YAG:Ce samples was measured using a Hitachi F-4600 fluorescence spectrometer. The scan speed was fixed at 240 nm min1, the voltage was 400 V, and spectral bandwidth was set as 2.5 nm. The luminescence decay profiles were collected on an Edinburgh Instruments (FLS 980) spectrometer. The light source consists of a continuous xenon lamp with an nF920 lamp as the excitation source. All luminescence spectra were recorded at room temperature. 3. Results and discussion The work to prepare the YAG samples under different conditions was conducted as shown in Table 1. 3.1. Effect of the reaction conditions on formation and morphology 3.1.1. Effect of variations in Y/Al and the amount of Na2SO4 on the product Fig. 1 presents XRD patterns for samples synthesized in water at 300  C for 24 h with different amounts of Na2SO4 surfactant with changes in the ratio of Y/Al. The phase formation of YAG crystallites is quite similar, as shown in the XRD patterns, even when using different Y/Al ratios (3:4 or 3:5). Without the addition of Na2SO4, the diffraction peaks can be well indexed to Y3Al5O12 (PDF No. 330040), which has a cubic structure with the space group Ia3d (230). However, there is a trace of Y(OH)3 as an impurity. With the addition of Na2SO4, the amount of Y(OH)3 increases, and the final results changes as the ratio of Y/Al changes in terms of the intensity of diffraction peaks for impurities. For samples obtained with Y/ Al ¼ 3:4, as shown in Fig. 1a, an increase in the amount of Na2SO4 to 0.2 g results in an increase in the intensity of the diffraction peaks of

Table 1 The YAG samples prepared under different conditions in 100 ml solution. Y/Al

Na2SO4 (g)

pH

T ( C), time (h)

Solvent

Phase Purity

Doping

No.

3:4 3:4 3:4 3:4 3:5 3:5 3:5 3:5 3:5 3:5 3:5 3:5 3:5

0 0.04 0.08 0.2 0 0.04 0.08 0.2 0.2 0.2 0.2 0.2 0.2

7 7 7 7 7 7 7 7 12 e 7 7 7

300, 300, 300, 300, 300, 300, 300, 300, 300, 300, 300, 300, 300,

Water Water Water Water Water Water Water Water Water Ethanol Water Water Water

YAG YAG, YAG, YAG, YAG YAG, YAG, YAG, YAG, YAG, YAG, YAG, YAG,

None None None None None None None None None None Ce3% Ce3% Ce3%

1 2 3 4 5 6 7 8 9 10 11 12 13

24 24 24 24 24 24 24 24 24 24 12 48 60

Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3 Y(OH)3

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

539

Fig. 1. XRD patterns of YAG synthesized in water at 300  C for 24 h with different amounts of the surfactant Na2SO4, (a) Y/Al ¼ 3:4; (b) Y/Al ¼ 3:5; the diamond labeled peaks are the Y(OH)3 impurity phase.

Y(OH)3. However, for samples with Y/Al ¼ 3:5, the intensity of the diffraction peaks of Y(OH)3 increases at first and then decreases. This indicates that not only the Y/Al ratio but also the amount of Na2SO4 exhibits an influence on the phase formation of the product. Fig. 2 shows the morphologies of the products synthesized at 300  C for 24 h with different amounts of the Na2SO4 surfactant when the ratio of Y/Al changes. The two particle types, prism or sheet particles and polyhedral particles, can be attributed to Y(OH)3 and YAG, respectively [38]. As shown in Fig. 2a and e, with the absence of Na2SO4, the prepared particles are more seriously agglomerate than those obtained with the addition of Na2SO4, as shown in Fig. 2bed and 2f-h. In this condition, the crystal face is not fully developed, and only few grains develop completely into dodecahedrons, with an average size of 200e300 nm. The morphologies of the samples are fairly different after Na2SO4 was added into the solution at 0.04 g, 0.08 g, and 0.2 g, as shown in Fig. 2bed and 2f-h. The particle dispersion significantly changed and particles become more dispersed. The mean particle size of these monodispersed YAG crystallites was approximately 500e700 nm, larger than that prepared without Na2SO4. More importantly, these crystal grains are fully developed, and many that developed rhombic dodecahedrons appear. Na2SO4, as an anionic surfactant, adsorbs onto the surface of the particles through a charge repulsion to adjust the distance between the particles [39], leading to different morphologies, better dispersion and crystallinity [40e42]. However, some rod-like or sheet-like impurities can be observed after the addition of Na2SO4 (Fig. 2bed, f-h), which is in good agreement with the results of the XRD showing that Y(OH)3 is the impurity phase. Also, as seen from Fig. 2, the amount of impurities increased when Y/Al ¼ 3:4, while that of the samples obtained when Y/Al ¼ 3:5 increase first and then decrease with an increasing amount of Na2SO4. An analysis of the particle size was carried out to further study the dispersity of the two series of samples. Fig. 3a and b depict the particle size distribution of the samples with different amounts of Na2SO4 when the ratio of Y/Al was equal to 3:4 or 3:5, respectively. The particles are small and are easy to agglomerate without Na2SO4. There are two peaks that correspond to the particle size distribution of the particles, with large particles observed after agglomeration. Actually, the average particle size obtained using this method is larger than that observed from the SEM images due to the agglomeration between the primary particles. As mentioned earlier, some rod-like or sheet-like impurities are

introduced after adding Na2SO4. They are more easily agglomerated than regular dodecahedrons, resulting in a larger particle size. As a consequence, the peak on the right of the corresponding particle size distribution curve should become stronger, as expected. As shown in Fig. 3, when 0.04 g Na2SO4 was added, the particle size distribution is wider, and both peaks are less noticeable due to the introduction of impurities. When the Na2SO4 increased up to 0.08 g, the right peak was significantly enhanced, indicating that the product contains more impurities. When the amount continues to increase up to 0.2 g, the left peak is enhanced while the right peak is weakened, indicating that there is enough Na2SO4 to adjust the formation of YAG particles and dispersity of these particles. The size distributions for these products revealed that in the optimal condition of Y/Al ¼ 3:5 with 0.2 g Na2SO4, shown in the XRD and SEM results, the product has the narrowest particle size distribution, indicating that the particle size distribution is uniform. 3.1.2. Effect of different solvents on the phase and morphology To further optimize the product, we tried to change the reaction solvent environment to explore its impact on the phases and morphology. Fig. 4 presents the XRD patterns and SEM images of samples (No. 8, 9, 10) synthesized in different solvents with 0.2 g Na2SO4 added. We combine the information of these two characterization methods to obtain integrated information on the phase and morphology of these samples. When water is used as solvent, the samples prepared under a condition of pH ¼ 7 are purer and have a more regular morphology with well-developed crystal faces than those with pH ¼ 12 due to the fact that when the pH becomes large, it will promote the occurrence of side reactions, as will be described below. Therefore, this will lead to more impurities and an irregular morphology. The sample prepared in alcohol consists of a nearly single phase of YAG with a relative high crystallinity, as shown in the XRD patterns (Fig. 4a), but the crystal faces have not been well developed, as shown in the SEM images in Fig. 4d. 3.1.3. The effect of the reaction time on the phase and morphology As shown in Fig. 1S in the supporting information, with the extension of the reaction time, the diffraction peaks of the impurities are fewer and the intensity is lower, indicating that a longer reaction time leads to a purer phase. In addition, Fig. 5a clearly shows that the product obtained when the reaction time is 12 h has many small impurities and a particle size of about 500 nm. With an increase in the reaction time, the impurities are fewer, and the

540

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

Fig. 2. SEM images of the products synthesized at 300  C for 24 h with different amounts of the surfactant Na2SO4, (a)(b)(c)(d) for Y/Al ¼ 3:4, 0, 0.04, 0.08, 0.2 g Na2SO4; (e)(f)(g)(h) for Y/Al ¼ 3:5, 0, 0.04, 0.08, 0.2 g Na2SO4, respectively.

Fig. 3. Particle size distribution of the samples synthesized at 300  C for 24 h with different amounts of the surfactant Na2SO4: (a)Y/Al ¼ 3:4, (b) Y/Al ¼ 3:5.

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

541

Fig. 4. XRD patterns (a) and SEM images of samples synthesized at 300  C for 24 h with the addition of 0.2 g Na2SO4 when Y/Al ¼ 3:5, (b) water (pH ¼ 12), (c) water (pH ¼ 7), and (d) ethanol as solvent.

Fig. 5. SEM images of the samples synthesized at 300  C for different reaction time in water with 3% Ce doped (Y/Al ¼ 3:5, 0.2 g Na2SO4): (a) 12 h, (b) 24 h, (c) 48 h, (d) 60 h.

particle size increases to the micron level. When the reaction time is of up to 48 h or even 60 h, the YAG micro-nanocrystals with a dodecahedral shape are arranged in a very regular way. In short, the results obtained from XRD, SEM and particle size distribution show that the optimal conditions to synthesize uniform, mono-dispersed YAG crystallites with well-developed faces and high crystallinity were Y/Al ¼ 3:5, 0.2 g Na2SO4, 48 h, water

(pH ¼ 7).

3.2. The determination of the crystal face The formation energy of a free-standing crystal particle due to surface free energies can be simplified as [43,44].

542

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

M X DG ¼ q fi gi

(1)

r

where M is the molar mass and r is the density of the bulk phase, q is the surface-to-volume ratio of the particle, fi is the fractional area P of facet i such that fi ¼ 1, and gi is the surface free energy of facet i

i, where the index i runs through all surrounding facets of the particle. The surface energies gi are evaluated through density functional theory calculations implemented in the VASP code [45,46]. In our calculations, the interactions of the electrons are depicted using the PBE form of an exchange-correlation functional [47]; the effects of the nuclei and core electrons are depicted using the projectoraugmented wave (PAW) method [48,49]. The PAW datasets for the elements (Al, O, Y, H) are obtained from the PAW dataset library that ships with VASP, and the core radii are 1.90 Bohr for Al, 1.52 Bohr for O, 1.10 Bohr for H, and 2.60 Bohr for Y. The cut-off energy of the plane wave functions is 500.0 eV. The occupation of the electronic states is modeled using the Gaussian smearing method with a smearing width of 0.05 eV. The first Brillouin zone is sampled on special k-points generated using the Monkhorst-Pack scheme [50]. The grids for used to generate special k-points are 4  4  4 for the bulk phases (cubic YAG, rhombohedral a-Al2O3, and cubic Y2O3), and 3  3  1 for the surfaces. We consider two surface orientations, the {100} family and {110} family, which respectively encloses a cubic or dodecahedral

shape. For each orientation a large number of terminations can be constructed with different atomic structure and chemical compositions [51]. The terminations obtained by cleaving the bulk phase contain no hydrogen, corresponding to anhydrous conditions. We hydrogenate (i.e., attach hydrogen atoms to) unsaturated oxygen atoms at the surfaces to simulate hydrous conditions. Among the numerous terminations we select stoichiometric ones with compositions that can be expressed as xY3Al5O12þ yAl2O3þ zY2O3þ rH2O. Thus, excess Al atoms in a surface system are from the aAl2O3 phase, excess Y atoms are from cubic Y2O3 phase, and H atoms are from the H2O gas phase. Only one of y and z, not both, can be non-zero. Other nonstoichiometric terminations may require an additional O2 phase in the chemical reservoir, and are excluded from the present study for simplicity. The surface energies are calculated by

g¼

EðslabÞ  xEðY3 Al5 O12 Þ  yEðAl2 O3 Þ  zEðY2 O3 Þ  rEðH2 OÞ 2A (2)

where E(slab)is the total energy of the slab model that represent a termination, A is the surface area, and 2A means that each slab contains two free surfaces. The terminations are presented in Table 2 in a scheme similar to that reported by U. Aschauer [51]. The chemical compositions and surface energies are also shown in Table 2. We are most interested in the termination with the lowest surface energy for each surface orientation, because this is the

Table 2 Schematic representation of the possible stoichiometric non-polar cuts in a repeat unit of each surface. The first column lists the distance of cleaving planes relative to a reference plane, in unit of Å. The symbol Osc stands for singly-coordinated O, Odc for doubly coordinated O, and Otc for triply coordinated O atom.

(100) surface 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 (110) surface 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8

Top layer

Name and modification

#Al2O3

#Y2O3

#H2O

g (J$m2)

-Y2-Al2-O4e e e -O4-O4e -Al8-Y4-O4e -O4e e -O4-

(a) Remove one Al and one Y. (b) Hydrogenate 4 Odc and 2 Otc. e e e e e e e e e e e e (c) Hydrogenate 2 Odc.

0 2/3 e e e e e e e e e e e e 0

0 0 e e e e e e e e e e e e 0.4

0 6 e e e e e e e e e e e e 2

1.366 0.800 e e e e e e e e e e e e 1.659

-O2-Al2-O2e -O2-O2-O2-Al2-Y2e -O2-O2e e -Al2-Y2e -O2-O2e -O2-Al2-Y2 -O2-O2-

e (a) Remove one Al. e e (b) Hydrogenate 2 Odc. e e e (c) Hydrogenate 2 Osc. e e e (d) Remove 1 Al and 1 Y. e e (e) Hydrogenate 2 Odc. e e (f) Hydrogenate 6 Odc. (g) Hydrogenate 2 Odc.

e 0 e e 0 e e e 0 e e e 0 e e 2/3 e e 2 2

e 0 e e 1.2 e e e 0.4 e e e 0 e e 0 e e 0 0

e 0 e e 2 e e e 2 e e e 0 e e 2 e e 6 2

e 1.847 e e 1.640 e e e 1.320 e e e 2.196 e e 1.323 e e 0.638 1.035

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

thermodynamically stable termination. Under anhydrous conditions, the surface energy of stoichiometric termination (a) of (100) is 1.366 J m2, lower than the terminations of (110), 1.847 J m2 for termination (a) and 2.196 J m2 for termination (d). In contrast, under hydrous conditions, the (110) surface's lowest-energy termination has a lower surface energy than that of the (100) surface (0.638 J m2 v.s. 0.800 J m2). Hydrogenation decreases surface energies for both orientations, but the extent of decrease is much larger for the (110) surface. With the surface energies, we can now use Eq. (1) to calculate and compare the formation of crystal particles in two shapes: a cube surrounded solely by {100} surfaces, and a rhombic dodecahedron that is surrounded by {110} surfaces. The results in Fig. 6a show that under anhydrous conditions cubic particles have a lower formation energy than rhombic dodecahedral particles, while under hydrous conditions rhombic dodecahedral particles have lower formation than cubic particles. For the hydrothermal synthesis, nanoparticle nucleation and growth proceed under hydrous conditions, thus the YAG crystal particles of the rhombic dodecahedron shape, which is enclosed by surfaces of the {110} family, is thermodynamically more stable than that of cube shape surrounded by {100} surfaces. Fig. 6c shows the reconstructed shape of the rhombic dodecahedron, and Fig. 6d presents an SEM image of the resulting product. Obviously, these are very consistent. As we all know, the angle between the two adjacent {100} facets of the cubic crystal system is 90 , and the {100} facets surround a cube. In contrast, the polyhedron formed by the {110} facets of a cubic crystal is a rhombic dodecahedron. In addition to the crystallographic considerations, the thermodynamics from the above-mentioned calculations also prove that, in the aqueous environment of our hydrothermal synthesis, the resulting crystals' cut-off surfaces are in the {110} family. TEM analyses were carried out to investigate the morphology and crystallinity of the YAG micro-nanocrystals. As shown in Fig. 6b, the mean size of the YAG crystals is about 1 mm, and the crystallinity is very good. From the high-resolution TEM image in the inset of Fig. 6b, the distance measured between adjacent fringes is about 0.41 nm. According to the indice spacing formula, the

543

distance between adjacent (110) planes is calculated to be 0.42 nm, which is close to the results measured from our XRD experiments. Because these (110) atomic planes are parallel to the exposed facets, this further validates the presence of (110) cut-off facets. 3.3. Heterogeneous analysis and reaction mechanism The impurity phase in the SEM images is further analyzed by conducting an element mapping of the sample (Y/Al ¼ 3:5, 0.2 g Na2SO4, 24 h, Sample No. 8 listed in Table 1). Fig. 7a shows SEM images of the selected element analysis area, and Fig. 7b and c presents the element mapping of Y and Al, respectively. It is obvious that the rods and sheets in the SEM images are rich in Y elements and lack of Al element, indicating that the impurity phase is the compound of Y elements. In order to further determine the impurity phases, an infrared analysis of the product has been done. FT-IR spectra of the products synthesized at 300  C for 24 h with different amounts of Na2SO4 are shown in Fig. 7d and e. The absorption peaks at 1395, 1535 and þ  1640 cm1 are attributed to CO2 3 , NH4 , and OH , respectively [52], which is due to the presence of the precipitant NH4HCO3 in the coprecipitation reaction. The absorption peaks at 788, 727 and 696 cm1 are derived from the metal-oxygen vibration characteristics of Al-O, Y-O, and Y-O-Al stretches, respectively [53]. In contrast to the FT-IR spectra of the precursor, the new absorption peaks below 1000 cm1 of the product obtained by the hydrothermal reaction are correlated to the vibrations of metal-oxygen bonds [54], which also demonstrates the generation of the YAG phase. According to the literature [55], the characteristic peak of OH is the broad absorption band centered at 3450 cm1. For the spectra of precursors, this broad absorption band centered at 3450 cm1 derives from water absorbed on the surface of the sample. Meanwhile, for the products synthesized at 300  C for 24 h with different amounts of Na2SO4, this broad absorption band also exists, which is attributed to the OH stretching. With no addition of the surfactant, this broad band centered at 3450 cm1 representing the bond of O-H hardly splits. However, when the surfactant is added, this broad band splits into many small peaks,

Fig. 6. Formation energies of YAG nanocrystal particles in dodecahedron and cube shapes under hydrous and anhydrous conditions (a); (b) TEM image of the product (Y/Al ¼ 3:5, 0.2 g Na2SO4, 48 h, water), the inset is the high resolution TEM image; (c) reconstructed shape of rhombic dodecahedron; (d) shape of rhombic dodecahedron observed by SEM. The facet (110) is labeled.

544

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

Fig. 7. SEM images (a), the mapping images of Y (b) and Al (c) of the samples synthesized at 300  C for 24 h in water; FT-IR spectra of the products synthesized at 300  C for 24 h with different amounts of Na2SO4: (d) Y/Al ¼ 3:4, (e) Y/Al ¼ 3:5, and (f) ICP curves for varying concentration of Y/Al.

indicating the formation of some impurities. The more seriously the broad band splits, the more impurities the product contains. It is noteworthy that this broadband split in the infrared pattern of the synthesized sample (No. 8) was not obvious, indicating that these samples contain the fewest impurities and have the best quality that we need. This conclusion is consistent with that drawn from the X-ray diffraction patterns and the SEM images that were previously mentioned. When the ratio of Y/Al is 3:5, the closest to the YAG molecular formula, it is the most likely to form a pure YAG phase. As we know from previous studies, the reaction mechanism of the solvothermal synthesis of ceramic powders is dissolution/precipitation or dissolution/crystallization [56]. The main reaction process in the water can be summed up as follows: the precursor  decomposes and dissolves as Al(OH) 4 and Y(OH)6 , and there are four reactions in the process:

gradually with time and then almost stabilizes after 48 h, while the concentration of Al is always decreasing. The reaction process is that of precursor's dissolving and recrystallizing. Therefore, when the reaction time comes to 5 h, the concentration of the elements (Y and Al) in the solution is relatively higher because all of the precursor decomposes and dissolves into Al(OH)-4 and Y(OH)-6. As the reaction time is prolonged, the Y/Al compounds participate in the above reactions (3)/(4)/(5)/(6), and their concentrations decrease. The reaction time for the concentration of Y in the solution to be balanced is less than that of Al, indicating that Y is the first solution to the solid phase relative to Al. Therefore, the solid phase is rich in yttrium, which is consistent with the element distribution mapping result. The results of this test strongly demonstrate the process theory (dissolution and recrystallization mechanism) of the hydrothermal synthesis of YAG.

AlðOHÞ 4 /AlOðOHÞ

(3)

3.4. Luminescence properties

YðOHÞ 6 /YðOHÞ3

(4)

 AlðOHÞ 4 þ YðOHÞ6 /Y3 Al5 Ox ðOHÞy

(5)

Y3Al5Ox (OH)y /YAG

(6)

Since reactions (3) and (4) are in a competitive relationship with reaction (5) and (6), the product AlO(OH) or Y(OH)3 may exist. Combined with the aforementioned elemental distribution map and FT-IR analysis, the impurity phase is yttrium-rich and contains an O-H bond. Then, when the above-mentioned reaction equations are taken into account, it can be inferred that most of the impurity phases in the product should be Y(OH)3. In order to further analyze the reaction process, the element concentration of the solution in the reactor was measured. Fig. 7f shows the change in the Y/Al element concentration in the final liquid product in the reactor under different reaction time (5, 24, 48, 96 h). The results show that the concentration of Y decreases

To explore the correlation between the morphology and luminescence, the luminescence intensity and decay curves of samples obtained at different reaction time were measured for comparison. Fig. 8a and b presents the excitation and emission spectra of samples synthesized at 300  C in water for different reaction time, respectively. The excitation and emission spectra have approximately the same position and shape as the reaction time changes, which suggests the crystal field strength around Ce3þ and the covalent bond effect between the Ce3þ-O2- bond are very similar. Three main excitation peaks centered at 334, 409 and 450 nm are observed in the wavelength range of 300e500 nm, related to the 4f (2F5/2) / 5d transitions of Ce3þ. One broad emission peak centered at 540 nm, attributed to the transition from the lowest 5d energy level to the 4f ground state 4f (2F5/2), can been seen in a region of 480e750 nm upon the excitation of 450 nm. Also, a shoulder at a longer wavelength, resulting from another ground state of 4f (2F7/2), was detected. Noticeably, there is a big difference in excitation and emission intensity of different samples prepared under different reaction time. Both the excitation and emission spectra show the same tendency for different samples. In contrast, it is easy to find

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

545

Fig. 8. Excitation (a), emission (b) spectra and the photoluminescence decay curves (c)(d) of YAG: Ce synthesized at 300  C in water for different reaction time.

that the sample synthesized for 48 h exhibits the strongest integrated emission intensity. In general, the luminescence intensity of the sample is associated with its crystallinity and surface morphology. Ce3þ replaces the position of Y in the crystal of YAG, and the complete dodecahedron is conducive to the luminescence of the substance [57,58]. The XRD and SEM results given above indicate that, when the reaction time was 12 h, the obtained samples contained many impurity phases, and the development was incomplete with only few crystal grains developed, which contributes to the weakest luminescence intensity. However, when the reaction time is extended to 24 h or more, the grain development of the product is more complete and the amount of impurities gradually becomes smaller, resulting in a significant increase in the emission intensity of the sample. As described above, the YAG:Ce sample (No. 12) prepared under the optimum conditions (Y/ Al ¼ 3:5, 0.2 g Na2SO4, 48 h, water) with the best morphology shows the highest emission intensity. This result also reveals that the samples with better morphologies may exhibit better optical properties because fewer impurities and better crystallization will reduce the crystal defects and consequently decrease non-radiation recombination. Therefore, the preparation of YAG:Ce with good morphology and crystallinity is conducive to a higher luminous efficacy of the phosphor. The photoluminescence decay curves of YAG:Ce synthesized at 300  C for different reaction time in water are shown in Fig. 8c and d. Using a double-exponential function the curves can be fitted and the time constants (t1 and t2) are shown in Fig. 8c and d. The shorter time constant t1 is due to the quenching of Ce3þ by the defects at the surface, while the longer time constant t2 may be attributed to Ce3þ inside the particle. When the reaction time changes, the change of corresponding decay time is not great, indicating that the effect of the reaction time on luminescence kinetics is not significant.

IðtÞ ¼ A1 expðt=t1 Þ þ A2 expðt=t2 Þ þ I0

(7)

3þ

It is well known that YAG: Ce can achieve strong absorption in the blue light and convert blue light into yellow-red light. When these phosphors are embedded into a single blue InGaN-based LED chip (460 nm) and measured at room temperature, white LEDs can be realized. As shown in Fig. 9, white light with a wide range can be obtained through using the YAG:Ce3þ phosphor (sample No. 12). A rather high luminous efficacy (LE ¼ 104.14 lm/W) is realized at a

Fig. 9. Emission spectra of a white LED using YAG:Ce3þ as the conversion phosphor at room temperature (the inset shows the CIE chromaticity points of white LEDs fabricated with the YAG:Ce3þ phosphor (sample No. 12) and the white LED device we prepared).

546

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547

corrected color temperature (CCT) up to 4224 K and the color rendering index (CRI Ra) is around 68. The inset in Fig. 9 describes that the CIE chromaticity point of white LED lamps fabricated with YAG:Ce3þ is (0.3955, 0.4686). Compared with a white LED using yellow emitting CaAlSiN3:Ce3þ (LE ¼ 50 lm/W) and some YAG phosphors (for example 86.7 lm/W [59]), the obtained YAG:Ce3þLED by this work is a white LED with rather high brightness. 4. Conclusions Monodispersed YAG micro-nanocrystals with few impurities, good dispersity, enhanced crystallinity and improved luminous performance were prepared via coprecipitation and hydrothermal method. The addition of Na2SO4 as a surfactant not only improves the dispersity, but also regulates the surface development of the crystals, leading to the completed dodecahedral YAG micro-crystals with specific well-developed crystal facets. It has been demonstrated by the theoretical calculation that these crystal facets are in the {110} family, which is thermodynamically more stable compared to the {100} family by which the YAG crystallites are enclosed under anhydrous conditions. An extension of the reaction time resulted in larger crystals with a more complete faces development and 48 h is the optimal and relatively efficient time point. Differences in the solvent and pH levels play a role in the morphology and size of the crystallites, and the crystallites were found to develop better in water (pH ¼ 7). The samples prepared under the condition of Y/Al ¼ 3:5 and 0.2 g Na2SO4 in water for 48 h showed the strongest emission intensity among all samples, which means that better luminescence properties are strongly related to the improved phase purity, morphology and dispersion. Therefore, the morphology-controlled synthesis of YAG-based powders has great significance for systematic studies on the fundamental of crystal growth and application of mono-dispersed YAG crystallites. A rather high luminous efficacy (LE ¼ 104.14 lm/W) for the white LED using the phosphors by this work has been realized with a corrected color temperature (CCT) up to 4224 K and the color rendering index (CRI Ra) around 68. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 51772185, 51171239, 11275249, 111 program (D16002), Science and Technology Commission of Shanghai Municipality under Grant No. 15DZ2260300, Shanghai University Innovation Program, and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2015M2B2A4033073, 2016R1D1A1B03933488). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.05.266. References [1] Y. Morihiro, S. Kato, H. Imoto, S. Nomura, K. Harada, K. Kajiwara, M. Fujii, H. Fujisawa, K. Saito, M. Suzuki, Production of phosphor (YAG:Tb) fine particles by hydrothermal synthesis in supercritical water, J. Mater. Chem. 9 (1999) 2671e2674. [2] A. Potdevin, G. Chadeyron, D. Boyer, B. Caillier, R. Mahiou, Sol gel based YAG: Tb3þ or Eu3þ phosphors for application in lighting sources, J. Phys. D Appl. Phys. 38 (2005) 3251e3260. [3] S. Zhou, Z. Fu, J. Zhang, S. Zhang, Spectral properties of rare-earth ions in nanocrystalline YAG:Re (Re¼Ce3þ, Pr3þ, Tb3þ), J. Lumin. 118 (2006) 179e185. [4] M. Nyman, L.E. Shearohwer, J.E. Martin, P. Provencio, Nano-YAG:Ce mechanisms of growth and epoxy-encapsulation, Chem. Mater. 21 (2009)

1536e1542. [5] A. Revaux, G. Dantelle, N. George, R. Seshadri, T. Gacoin, J.P. Boilot, A protected annealing strategy to enhanced light emission and photostability of YAG:Ce nanoparticle-based films, Nanoscale 3 (2011) 2015. [6] H.-S. Roh, D.H. Kim, I.-J. Park, H.J. Song, S. Hur, D.-W. Kim, K.S. Hong, Templatefree synthesis of monodispersed Y3Al5O12:Ce3þ nanosphere phosphor, J. Mater. Chem. 22 (2012) 12275. [7] T.I. Mah, T.A. Parthasarathy, H.D. Lee, Polycrystalline YAG; structural or functional? Process. Res. 5 (2004) 369e379. [8] S.F. Wang, J. Zhang, D.W. Luo, F. Gu, D.Y. Tang, Z.L. Dong, G.E.B. Tan, W.X. Que, T.S. Zhang, S. Li, Transparent ceramics:Processing, materials and applications, Prog. Solid State Chem. 41 (2013) 20e54. [9] A. Ikesue, I. Furusato, K. Kamata, Fabrication of polycrystalline, transparent YAG ceramics by a solid-stae reaction method, J. Am. Ceram. Soc. 78 (1995) 225e228. [10] A. Ikesue, I. Furusato, K. Kamata, Fabrication of polycrystalline, transparent YAG ceramics by a solid-stae reaction method, J. Am. Ceram. Soc. 78 (1995) 1033e1040. [11] A. Ikesue, L.A. Yan, T. Yoda, S. Nakayama, T. Kamimura, Fabrication and laser performance of polycrystal and single crystal Nd:YAG by advanced ceramic processing, Opt. Mater. 29 (2007) 1289e1294.
kov

, A. Vedda, K. Nejezchleb, [12] E. Miho a, M. Nikl, J.A. Mares, A. Beitlerova K. Bla zek, C. D'Ambrosio, Luminescence and scintillation properties of YAG:Ce single crystal and optical ceramics, J. Lumin. 126 (2007) 77e80. [13] J. Dong, K. Ueda, H. Yagi, A.A. Kaminskii, Z. Cai, Comparative study the effect of Yb concentrations on laser characteristics of Yb:YAG ceramics and crystals, Laser Phys. Lett. 6 (2009) 282e289. [14] X.-B. Yang, Y. Shi, H.-J. Li, Q.-Y. Bi, L.-B. Su, Q. Liu, Y.-B. Pan, J. Xu, Optical and scintillation properties of Ce:YAG crystal and transparent ceramics, Acta Phys. Sin. 58 (2009) 54e62. [15] J. Dong, G. Xu, J. Ma, M. Cao, Y. Cheng, K.I. Ueda, H. Yagi, A.A. Kaminskii, Investigation of continuous-wave and Q-switched microchip laser characteristics of Yb:YAG ceramics and crystals, Opt. Mater. 34 (2012) 959e9646. [16] R.M. Springer, M.E. Thomas, R.I. Joseph, Analysis and comparison of singlecrystal and polycrystalline Nd:YAG, IEEE J. Quant. Electron. 51 (2015) 1e8. [17] J. Li, Y. Pan, Y. Zeng, W. Liu, B. Jiang, J. Guo, The history, development, and future prospects for laser ceramics: a review, Int. J. Refract. Met. H. 39 (2013) 44e52. [18] S.F. Wang, J. Zhang, D.W. Luo, F. Gu, D.Y. Tang, Z.L. Dong, G.E.B. Tan, W.X. Que, T.S. Zhang, S. Li, L.B. Kong, Transparent ceramics: processing, materials and applications, Prog. Solid State Chem. 41 (2013) 20e54. [19] C.S. Chou, C.Y. Wu, C.H. Yeh, R.Y. Yang, J.H. Chen, The optimum conditions for solid-state-prepared (Y3 xCex)Al5O12 phosphor using the Taguchi method, Adv. Powder Technol. 23 (2012) 97e103. [20] Y. Jia, Y. Huang, Y. Zheng, N. Guo, H. Qiao, Q. Zhao, W. Lv, H. You, Color point tuning of Y3Al5O12:Ce3þ phosphor via Mn2þeSi4þ incorporation for white light generation, J. Mater. Chem. 22 (2012) 15146e15152. [21] J. Su, Q.L. Zhang, S.F. Shao, W.P. Liu, S.M. Wan, S.T. Yin, Phase transition, structure and luminescence of Eu:YAG nanophosphors by co-precipitation method, J. Alloys Compd. 470 (2009) 306e310. [22] T.G. Deineka, A.G. Doroshenko, P.V. Mateychenko, A.V. Tolmachev, E.A. Vovk, O.M. Vovk, R.P. Yavetskiy, V.N. Baumer, D.S. Sofronov, Influence of sulfate ions on properties of co-precipitated Y3Al5O12:Nd3þ nanopowders, J. Alloys Compd. 508 (2010) 200e205. [23] C. Marlot, E. Barraud, S.L. Gallet, M. Eichhorn, F. Bernard, Synthesis of YAG nanopowder by the co-precipitation method: influence of pH and study of the reaction mechanisms, J. Solid State Chem. 191 (2012) 114e120. [24] K. Guo, H.H. Chen, X. Guo, X.X. Yang, F.F. Xu, J.T. Zhao, Morphology investigation of yttrium aluminum garnet nano-powders prepared by a solegel combustion method, J. Alloys Compd. 500 (2010) 34e38. [25] S.A. Hassanzadeh-Tabrizi, Synthesis and luminescence properties of YAG:Ce nanopowder prepared by the Pechini method, Adv. Powder Technol. 23 (2012) 324e327. [26] H.L. Sang, D.S. Jung, J.M. Han, H.Y. Koo, Y.C. Kang, Fine-sized Y3Al5O12:Ce phosphor powders prepared by spray pyrolysis from the spray solution with barium fluoride flux, J. Alloys Compd. 477 (2009) 776e779. [27] L. Mancic, K. Marinkovic, B.A. Marinkovic, M. Dramicanin, O. Milosevic, YAG: Ce3þ nanostructured particles obtained via spray pyrolysis of polymeric precursor solution, J. Eur. Ceram. Soc. 30 (2010) 577e582. [28] C.H. Lu, C.H. Huang, B.M. Cheng, Synthesis and luminescence properties of microemulsion-derived Y3Al5O12:Eu3þ phosphors, J. Alloys Compd. 473 (2009) 376e381. [29] C.H. Hsu, C.H. Huang, B.M. Cheng, C.H. Lu, Influence of microemulsion conditions on the VUV-excited luminescence and microstructures of Y3Al5O12: Eu3þ phosphors, Mater. Chem. Phys. 124 (2010) 632e638. [30] M.N. Danchevskaya, Y.D. Ivakin, A.V. Maryashkin, G.P. Muravieva, Stepwise synthesis of fine crystalline Ce-doped yttrium aluminum garnet in water medium in subcritical and supercritical conditions, Russ. J. Phys. Chem. B. 5 (2011) 1056e1068. [31] A. Sahraneshin, S. Takami, K. Minami, D. Hojo, T. Arita, T. Adschiri, Synthesis and morphology control of surface functionalized nanoscale yttrium aluminum garnet particles via supercritical hydrothermal method, Prog. Cryst. Growth Char. Mater. 58 (2012) 43e50. [32] J.-W. Lee, J.-H. Lee, E.-J. Woo, H. Ahn, J.-S. Kim, C.-H. Lee, Synthesis of nanosized Ce3þ, Eu3þ-Codoped YAG phosphor in a continuous supercritical water

T. Xu et al. / Journal of Alloys and Compounds 762 (2018) 537e547 system, Ind. Eng. Chem. Res. 47 (2008) 5994e6000. [33] Z. Wu, X. Zhang, W. He, Y. Du, N. Jia, P. Liu, F. Bu, Solvothermal synthesis of spherical YAG powders via different precipitants, J. Alloys Compd. 472 (2009) 576e580. [34] J.Y. Park, H.C. Jung, G.S.R. Raju, J.H. Jeong, B.K. Moon, J.H. Kim, Y.K. Lee, Solvothermal synthesis and luminescence properties of the novel aluminum garnet phosphors for WLED applications, Curr. Appl. Phys. 13 (2013) 441e447. [35] Z. Wu, X. Zhang, W. He, Y. Du, N. Jia, G. Xu, Preparation of YAG:Ce spheroidal phase-pure particles by solvo-thermal method and their photoluminescence, J. Alloys Compd. 468 (2009) 571e574. [36] A. Cabanas, M. Poliakoff, The continuous hydrothermal synthesis of nanoparticulate ferrites in near critical and supercritical water, J. Mater. Chem. 11 (2001) 1408e1416. [37] P.C. Pavan, E.L. Crepaldi, J.B. Valim, Sorption of anionic surfactants on layered double hydroxides, J. Colloid Interface Sci. 229 (2000) 346e352. [38] M. Xu, Z. Zhang, J. Zhao, J. Zhang, Z. Liu, Low temperature synthesis of monodispersed YAG:Eu crystallites by hydrothermal method, J. Alloys Compd. 647 (2015) 1075e1080. [39] L. Zhang, T. Xu, X. Zhao, Y. Zhu, Controllable synthesis of Bi2MoO6 and effect of morphology and variation in local structure on photocatalytic activities, Appl. Catal. B Environ. 98 (2010) 138e146. [40] C.J. Murphy, Nanocubes nanoboxes, Science 298 (2002) 2139e2141. [41] V.F. Puntes, D. Zanchet, C.K. Erdonmez, A.P. Alivisatos, Synthesis of hcp-Co nanodisks, J. Am. Chem. Soc. 124 (2002) 12874e12880. [42] A.B. Panda, A. Garry Glaspell, M.S. Elshall, Microwave synthesis and optical properties of uniform nanorods and nanoplates of rare earth oxides, J. Phys. Chem. C 111 (2007) 1861e1864. [43] A.S. Barnard, Modelling of nanoparticles: approaches to morphology and evolution, Rep. Prog. Phys. 73 (2010), 086502. [44] A.S. Barnard, P. Zapol, A model for the phase stability of arbitrary nanoparticles as a function of size and shape, J. Chem. Phys. 121 (2004) 4276. [45] G. Kresse, J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set, Phys. Rev. B Condens. Matter 54 (1996) 11169.

547

[46] G. Kresse, J. Hafner, Ab initio molecular dynamics for liquid metals, Phys. Rev. B Condens. Matter 47 (1993) 558. [47] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865e3868. €chl, Projector augmented-wave method, Phys. Rev. B Condens. Matter [48] P.E. Blo 50 (1994) 17953. [49] G. Kresse, D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B Condens. Matter 59 (1999) 1758e1775. [50] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys. Rev. B 13 (1976) 5188e5192. [51] U. Aschauer, P. Bowen, S.C. Parker, Atomistic modeling study of surface segregation in Nd:YAG, J. Am. Ceram. Soc. 89 (2006) 3812e3816. [52] J.-G. Li, T. Ikegami, J.-H. Lee, T. Mori, Low-temperature fabrication of transparent yttrium aluminum garnet (YAG) ceramics without additives, J. Am. Ceram. Soc. 83 (2000) 961e963. [53] M. Panneerselvam, G.N. Subanna, K.J. Rao, Translucent yttrium aluminum garnet: microwave-assisted route to synthesis and processing, J. Mater. Res. 16 (2001) 2773e2776. [54] K. Wakino, M. Murata, H. Tamura, Far infrared reflection spectra of Ba(Zn,Ta) O3-BaZrO3 dielectric resonator material, J. Am. Ceram. Soc. 69 (1986) 34e37. [55] S. Deyan, The Application of Infra-red Spectrum in the Polymer Research, Science Press, Beijing, 1990. [56] A. Lukowiak, R.J. Wiglusz, M. Maczka, P. Gluchowski, W. Strek, IR and Raman spectroscopy study of YAG nanoceramics, Chem. Phys. Lett. 494 (2010) 279e283. [57] P. Dorenbos, The 4fn44fn15d transitions of the trivalent lanthanides in halogenides and chalcogenides, J. Lumin. 91 (2000) 91e106. [58] S. Ye, F. Xiao, Y.X. Pan, Y.Y. Ma, Q.Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: recent advances in materials, techniques and properties, Mater. Sci. Eng. R. 71 (2010) 1e34. [59] M. Cantore, N. Pfaff, R.M. Farrell, J.S. Speck, S. Nakamura, S.P. DenBaars, High luminous flux from single crystal phosphor-converted laser-based white lighting system, Optic Express 24 (2016) A215.