Synthesis and Luminescence Properties of Novel Ce3+- and Eu2+-

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Synthesis and Luminescence Properties of Novel Ce3+- and Eu2+-doped Lanthanum Bromothiosilicate La3Br(SiS4)2 Phosphors for White LEDs Szu-Ping Lee, Shuang-De Liu, Ting-Shan Chan, and Teng-Ming Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12125 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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ACS Applied Materials & Interfaces

Synthesis and Luminescence Properties of Novel Ce3+- and Eu2+doped Lanthanum Bromothiosilicate La3Br(SiS4)2 Phosphors for White LEDs Szu-Ping Lee†, Shuang-De Liu†, Ting-Shan Chan‡, and Teng-Ming Chen*,† †

Phosphors Research Laboratory, Department of Applied Chemistry and Institute of Molecular Science, National ‡ Chiao Tung University, Hsinchu 30010, Taiwan. National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan ABSTRACT: Novel Ce3+- and Eu2+-doped lanthanum bromothiosilicate La3Br(SiS4)2:Ce3+and La3Br(SiS4)2:Eu2+ phosphors were 3+

prepared by solid-state reaction in an evacuated and sealed quartz glass ampoule. The La3Br(SiS4)2:Ce phosphor generates a 2+ cyan emission upon excitation at 375 nm, whereas the La3Br(SiS4)2:Eu phosphor could be excited with extremely broad range from UV to blue region (300 to 600 nm) and generates a reddish-orange broadband emission centered at 640 nm. In addition, 3+ 2+ thermal luminescence properties of La3Br(SiS4)2:Ce and La3Br(SiS4)2:Eu phosphors from 20 to 200°C were investigated. The 2+ combination of a 450 nm blue InGaN-based LED chip with the red-emitting La3Br(SiS4)2:Eu phosphor, and green-emitting 2+ BOSE:Eu commercial phosphor produced a warm-white light with the CRI value of ~95 and the CCT of 5,120 K. Overall, these results show that the prepared phosphors may have potential applications in pc-WLED.

KEYWORDS: thiosilicate phosphor, rare-earth activator, luminescence, light emitting diodes, solid-state lighting

INTRODUCTION To serve as white light sources, white light emitting diodes (WLEDs) show a promising future in general illumination with advantages over conventional lighting sources because of their life-lasting, environment-friendly and energy-efficient properties.1−4 The most general approach for generating white light can be achieved by the combination of the blue LED chip with the cerium-doped yttrium aluminum garnet (YAG:Ce3+) phosphor.4−7 However, the deficiency of red spectral region in this way limits its applications, because of the poor color rendering index (CRI) and the high correlated color temperature (CCT).8,9 This limitation can be solved by enhancing the red region of spectrum, for example, the combination of a blue LED chip with the yellow-emitting YAG:Ce3+ and narrow-band red-emitting K2SiF6:Mn4+ phosphors exhibiting high luminous efficacy of radiation (LER) value;10 a blue LED chip with green-emitting β-sialon:Eu2+ and redemitting CaAlSiN3:Eu2+ phosphors presenting high CRI value.11 In addition, pumping the RGB phosphors with a near-ultraviolet (n-UV) LED chip or an ultraviolet (UV) LED chip can also improve the CRI value.12−14 Accordingly, phosphors play a critical role in generating high-quality white light, for this reason, it is significant to develop novel phosphors for phosphor-converted WLEDs (pcWLEDs).7−14 Ce3+ and Eu2+ are two major activators with broadband excitation and emission for luminescent materials, which have been extensively used for solid-state lighting.15–17 This is mainly attributed to their comparatively small Stokes-shift that makes the phosphor excitable in widely broad range from UV to blue region, which matches well with the emission of commercial LEDs. In addition, the

distinct and broadband emission properties that can achieve higher CRI value for WLEDs application, and short decay lifetimes that can prevent saturation effects.18 The Ce3+ and Eu2+ ions exhibit the parity-allowed 4fn−1 → 5d14fn emission from the UV to the visible spectral region according to the host compounds and affected by covalency effects and crystal-field splitting. Until now, phosphors comprising inorganic silicon materials have been studied in applications of solid-sate lighting, such as silicate, silicon oxynitride, silicon nitride, and thiosilicate phosphors.19–23 Especially, thiosilicate phosphors have lower synthetic temperature, and the emission of thiosilicate varies from blue (Ba2SiS4:Ce3+) to red (Ca2SiS4:Eu2+) region.23–25 In 1999, Riccardi et al. first reported the structural investigation and luminescence properties of the cerium halothiosilicate Ce3X(SiS4)2 (X = Cl, Br, I) family.26 In this study, we discover and report two new Ce3+-doped and Eu2+-doped lanthanum bromothiosilicate La3Br(SiS4)2:Ce3+ and La3Br(SiS4)2:Eu2+ phosphors, which were observed isotypically to the Atype lanthanum chlorosilicate La3Cl(SiO4)2.27 For the first time, we described their synthesis strategy, crystal structures, luminescence and spectroscopic properties of the new bromothiosilicates and demonstrated their potential to serve as a LED conversion phosphor for solid-state lighting.

EXPERIMENTAL SECTION Materials. The powder samples of La3Br(SiS4)2:R (R = Ce3+, Eu2+) were prepared from La2S3 (Alfa Aesar, 99%), LaBr3 (Alfa Aesar, 99.9%), Si powder (Alfa Aesar, 99.999%), and S powder (Acros, 99.999%). The dopant sources were from Ce2S3 (Alfa Aesar, 99.99%) and EuBr2

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(Alfa Aesar, 99.99%). In addition, NaBr (J. T. Baker, 99.0%) was used as the charge compensator in the La3Br(SiS4)2:Eu2+ part. The raw materials were intimately ground and loaded into a quartz glass ampoule in the glove box under nitrogen atmosphere. These ampoules were sealed off under dynamic vacuum after evacuated to 10−4 Torr, then placed in a furnace and heated at 5 °C/min to 1100 °C for 8 h and then quenched gently to ambient temperature. In each case, the reactions were summarized in the following equations. 2 LaBr3 + (8 − 9 x) La2 S3 + 9 x Ce2 S3 + 12 Si + 24 S

(La0.9Ce0.1)3Br(SiS4)2. The crystallographic data of the isotypic single-crystal structure of La3Br(SiS4)230 (ICSD No.

(1)

→ 6 ( La1− xCex ) 3 BrSi2 S8 (1 − 9 y ) LaBr3 + 4 La 2 S 3 + 9 y EuBr2 + 9 y NaBr + 6 Si + 12 S → 3 ( La1− y Na y Eu y ) 3 BrSi 2 S 8

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(2)

Characterization. The phase purity of the reaction product was analyzed by Synchrotron X-ray Diffraction (SXRD) using the BL01C2 beamline with an X-ray wavelength of 0.774908 Å at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The General Structure Analysis System (GSAS) software28,29 was used to investigate the X-ray Rietveld refinements. The scanning electron microscopy (SEM) morphological analysis and energy dispersive X-ray spectroscopy (EDS) analysis were performed with a JEOL JSM-7401F operated at voltage of 5 kV. The diffuse reflection (DR) spectra were recorded using a Hitachi 3010 double-beam ultraviolet-visible (UV–vis) spectrometer (Hitachi Co., Tokyo, Japan) in combination with aluminum oxide (Al2O3) as a standard. The photoluminescence excitation (PLE) and photoluminescence (PL) spectra were obtained using a Spex Fluorolog-3 spectrofluorometer (Jobin Yvon Inc./Specx) with a 450 W xenon lamp. The X-ray absorption near-edge structure (XANES) spectra were measured with the BL17C beamline of the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. The quantum efficiency (QE) was measured by an integrating sphere whose inner face was coated with Spectralon® equipped with a spectrofluorometer (Horiba JobinYvon Fluorolog 3–2-2). The time resolved measurement of phosphors were observed by the FS5 Fluorescence Spectrometer (Edinburgh Instruments) with TCSPC (Time Correlated Single Photon Counting) module in combination with EPLED-360 picosecond pulsed light emitting diode laser system as the excitation source and MCS (Multi-Channel Scaling) method change over between the standard continuous light source and pulsed xenon flash lamp controlled by software. The thermal luminescence (TL) performance was analyzed using a heating device (THMS-600) equipped with PL equipment. The electroluminescence (EL) spectra were performed by SphereOptics integrating sphere with LED measurement starter packages (Onset, Inc.) recording at different currents in the range of 100−800 mA. Structural Characterizations and Crystallographic Parameters of the (La0.90Ce0.10)3Br(SiS4)2 phosphor. The Rietveld analysis was performed to ensure the phase purity and to obtain the detailed crystal structure information of

Figure 1. SXRD profiles for Rietveld refinement results of La3Br(SiS4)2:Ce3+. Observed intensities (cross), calculated patterns (red line), Bragg positions (tick mark), and difference plot (blue line) are presented.

Figure 2. (a) Schematic crystal structure of La3Br(SiS4)2 and coordination polyhedron around (b) BrLa3 (c) La1S8Br and (d) La2S8Br viewed down the c-axis. Light green, green, brown, yellow, and blue sphere balls describe La1, La2, Br, S and Si atoms. 411996) was applied as an initial model for Ce3+-doped lanthanum bromothiosilicate to reach a reliable approximation of the actual crystal structure. Figure 1 shows the results from the obtained SXRD patterns refined by using GSAS19,20 software for (La0.9Ce0.1)3Br(SiS4), which reveals the final converged weighted-profile R-index (Rwp) is 10.05%. The (La0.9Ce0.1)3Br(SiS4)2 synthesized in this work crystallized in the space group C2/c (No. 15) with Z ＝ 4 of the monoclinic system isotypical to the A-type lanthanum chlorosilicate La3Cl(SiO4)2.27 The crystal structure of La3Br(SiS4)2 viewed down the c-axis and the coordination polyhedron composed of La atomic site, as presented in Figure 2. The compounds possess two crystallographic positions (8f, 4e) of La3+ ions (La1 and La2) both coordinated by one Br- and eight S2- ions in the LaS8Br tricapped


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trigonal prisms, which linked to the SiS4 tetrahedral. Each Br- ion is also surrounded by three La3+ ions to form an unusual BrLa3 isosceles triangle.31,32 The refinement data, structural parameters, selected bond distances, and bond angles of this compound were summarized in Table S1 and Table S2 can be found in the Supporting Information (SI). The morphology and grain size distribution of crystalline phosphors were examined by SEM, indicating that the irregular granular microcrystals constitute the assynthesized phosphor. Furthermore, the nominal stoichiometry was also proved by EDS analysis in the SI (see Figure S1). Spectroscopic Study of the La3Br(SiS4)2:Ce3+ and La3Br(SiS4)2:Eu2+ phosphors. Figure 3 shows the DR spectra of as-synthesized polycrystalline La3Br(SiS4)2, (La0.90Ce0.10)3Br(SiS4)2, and (La0.97Eu0.03Na0.03)3Br(SiS4)2. For the La3Br(SiS4)2 host, in the wavelength ranging from 500 to 800 nm the DR spectrum shows high reflection, and a decrease in the reflection intensity from 250 to 500 nm. The band gap of the La3Br(SiS4)2 host can be investigated using the following equation:33

[ F ( R∞)hν ]n = A(hν '− E g )

(3)

where h stands for the Planck’s constant, ν’ the frequency of radiation, Eg the value of band gap, A the proportional constant, n = 2 represents the direct allowed transition. The measured reflectance (R) constitutes the Kubelka– Munk absorption coefficient (K/S) relation used to calculate the absorption edge through the following relationship:34 F (R∞) =

K (1− R)2 = S 2R

that of Eu2+ (6975 eV) increases compared with that of (La0.97Eu0.03)3Br(SiS4)2. Furthermore, when (La0.97Eu0.03Na0.03)3Br(SiS4)2 phosphor was excited at different excitation wavelengths, the shape of emission band

Figure 3. (a) DR of La3Br(SiS4)2 host. (b) hν − (F(R∞)hν)2 curve of La3Br(SiS4)2 host. DR spectrum, PLE, and PL spectra of (c) (La0.90Ce0.10)3Br(SiS4)2 and of (d) (La0.97Eu0.03Na0.03)3Br(SiS4)2.

-----------------------------------------------------------

(4)

The fundamental band gap energy (absorption edge) of La3Br(SiS4)2 host was denoted to be approximately 3.73 eV as presented in Figure 3b (the enlarged view provided in Figure S2 in SI). A typical PLE/PL spectrum of (La0.90Ce0.10)3Br(SiS4)2 and (La0.97Eu0.03Na0.03)3Br(SiS4)2 is indicated in Figure 3c and Figure 3d. The cyan-emitting (La0.90Ce0.10)3Br(SiS4)2 phosphor generates a broad band peaking at 466 nm upon excitation at 375 nm, whereas the (La0.97Eu0.03Na0.03)3Br(SiS4)2 phosphor could be excited with extremely broad range from UV to blue region (300 to 600 nm) and generates a reddish-orange broadband emission centered at 640 nm. Eu-L3 XANES was utilized to analyze the valence state of Eu ion in the bromothiosilicate phosphor. The Eu L3-edge XANES spectra of (La0.97Eu0.03)3Br(SiS4)2, (La0.97Eu0.03Na0.03)3Br(SiS4)2, BaMgAl10O17:Eu2+ (BAM:Eu2+), and Eu2O3 are summarized in Figure S3 (see in SI). The latter two samples are used as references for Eu2+ and Eu3+ ions, respectively. Every spectrum shows two absorption peaks at 6975 and 6983 eV (white lines), which are assignable to the 2p3/2 → 5d transition in Eu2+ and Eu3+, respectively.35 Based on the observation, there are two valence states 2+ and 3+ of Eu ions coexist in the tested samples. As Na+ ion used as the charge compensator in (La0.97Eu0.03Na0.03)3Br(SiS4)2, the intensities of absorption by Eu3+ ions (peak at 6983 eV) decrease and

Figure 4. (a) PL spectra (λex ＝ 375 nm) of (La1−xCex)3Br(SiS4)2 with different Ce3+ concentration x. (c) PL spectra (λex ＝ 450 nm) of (La1−xEuxNax)3Br(SiS4)2 with different Eu2+ concentration x. PL deconvolution of (b) (La0.90Ce0.10)3Br(SiS4)2 and (d) (La0.97Eu0.03Na0.03)3Br(SiS4)2. The insets show photographs of the prepared phosphor taken under normal light (left) and 365 nm UV light (right). ---------------------------------------------------------------------almost remains the same and the emission wavelength had a slight redshift (see Figure S4 in SI). The results depict that although there are Eu3+ ions exist in (La0.97Eu0.03Na0.03)3Br(SiS4)2 phosphor, Eu2+ ions still dominate the spectrum. The PL spectra of (La1−xCe)3Br(SiS4)2 (0.01 ≤ x ≤ 0.15) and (La1−xEuxNax)3Br(SiS4)2 (0.01 ≤ x ≤ 0.05) are shown in Figure 4, respectively. The relative intensity in PL spectra varies with the dopant concentrations, and an optimal value of x = 0.03 (ca. 3 mol %) and x = 0.10 (ca. 10 mol %) are obtained for Eu2+ and Ce3+ dopant, respectively. The appearance of both as-prepared thiosilicates turns out to be greyish color, which is considered to be detrimental to the efficiency and may be related to the reabsorption observed in the visible region of the emission spectra. Meanwhile, energy transfer such as exchange interaction, radia-



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tion reabsorption, or multipole−multipole interaction between two nearest activators will cause concentration quenching. The critical energy transfer distance (Rc) can be estimated by following formula:  3V   Rc ≈ 2   4π xc N 

1 3

(5)

where V denotes the volume of the unit cell, xc the critical dopant concentration, and N the number of total dopant sites in the unit cell. Thus, the Rc was calculated to be 31.63 Å and 24.73 Å for Eu2+ and Ce3+ dopants, respectively. For the exchange interaction occurs typically in the Rc around 5 Å (forbidden transition) as well as the PLE and PL spectra do not overlap well. Based on the Dexter theory, we can deduce that electric multipolar interaction leads to the non-radiative concentration quenching between the two nearest activator centers.36 By Gaussian deconvolution, the PL spectra of (La0.97Eu0.03Na0.03)3Br(SiS4)2 can be decomposed into two Gaussian components, with peaks centering at 630 nm (15,872 cm-1) and 681 nm (14,684 cm-1), respectively. (Figure 4d dash line, also see an enlarge view in Figure S5) In addition, the PL spectra of (La0.90Ce0.10)3Br(SiS4)2 can be decomposed into four Gaussian components, with peaks centering at 455 nm (21,960 cm-1), 478 nm (20,886 cm-1), 501 nm (19,956 cm-1), and 529 nm (18,881 cm-1) (Figure 4c dash line, see an enlarge view in Figure S6), respectively. These components can be ascribed to the contribution of the transitions from the lowest 5d excited state to the two 2F7/2 and 2F5/2 ground states in La1 and La2 sites. The values of spin-orbit coupling were calculated to be 2004 cm-1 and 2005 cm-1 in (La0.90Ce0.10)3Br(SiS4)2 (usually ∼2000 cm-1). Moreover, (La1−xCe)3Br(SiS4)2 (0.01 ≤ x ≤ 0.15) exhibits a higher internal quantum efficiency (IQE) value than that of (La1−xEuxNax)3Br(SiS4)2 (0.01 ≤ x ≤ 0.05) (see Figure S7 in SI). The comparatively low absorption coefficients in (La1−xEuxNax)3Br(SiS4)2 (0.01 ≤ x ≤ 0.05) samples may be the major element in the rationalization of the low IQE. The stability against moisture and atmospheric gasses of the phosphor is a significant parameter to be concerned for LED lighting. Figure S8 in SI presents the dependence of the IQE on time for La3Br(SiS4)2:Ce3+ and La3Br(SiS4)2:Eu2+ exposed to ambient air. The IQE value of La3Br(SiS4)2: Ce3+ drops from 56.94% to 30.17%, whereas that of La3Br(SiS4)2: Eu2+ drops from 20.95% to 9.66% by revealing to ambient air after 5 weeks. According the results, the degradation of the bromothiosilicate phosphor happens under ambient conditions.

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(b) Decay curve of (La0.97Eu0.03Na0.03)3Br(SiS4)2 phosphor under 450 nm excitation and monitored at 640 nm. Both decay curves are fitted with a second-order equation. ------------------------------------------------------------------Time-Resolved Measurement and Thermal Luminescence Properties of the La3Br(SiS4)2:Ce3+ and La3Br(SiS4)2:Eu2+ phosphor. The decay curve of (La0.90Ce0.10)3Br(SiS4)2 phosphor monitored at 466 nm under 360 nm excitation and the decay curve of (La0.97Eu0.03Na0.03)3Br(SiS4)2 phosphor monitored at 640 nm under 450 nm excitation are illustrated in Figure 5a and 5b, respectively. The second-order exponential equation was utilized to fit the measured decay lifetime, as indicated in the following relationship:37 −t −t   + A2 exp   I = A1 exp  τ  τ  1  2

(6)

where I stands for the luminescence intensity at time t; A1 and A2 stand for constants; τ1 and τ2 stand for short and long lifetimes for exponential components; α1 and α2 stand for the corresponding amplitudes of the lifetime component, respectively. The values of τ1, τ2, α1, α2 are analyzed and summarized in the Figure 5, and the average decay times can be calculated with these parameters by the equations shown in the followings:37  α τ + α 2τ 2 < τ * > =  1 1  α1 + α 2

   

(7)

The average luminescence decay times were calculated to be 21.46 ns and 239.51 ns for (La0.90Ce0.10)3Br(SiS4)2 and (La0.97Eu0.03Na0.03)3Br(SiS4)2, respectively, both results are acceptable for the parity-allowed 4fn−1 → 5d14fn transitions of Ce3+ and Eu2+ and correspond to the normal conditions,38 and short enough for LED lighting applications. These results demonstrate that the activator ions occupy the two different La3+ ions crystallographic positions in the La3Br(SiS4)2 host, which is also in agreement with the deconvolution of the emission spectra shown in Figure 3.39

Figure 6. Temperature dependence of relative PL integrated intensity for La3Br(SiS4)2:Ce3+, La3Br(SiS4)2:Eu2+, CaS:Ce3+, and CaS:Eu2+ over the range 20 to 200 °C. The inset shows the fitted PL integrated intensity and the calculated thermal activation energy (Ea) as a function of temperature. Figure 5. (a) Decay curve of (La0.90Ce0.10)3Br(SiS4)2 phosphor under 360 nm excitation and monitored at 466 nm.

Thermal luminescence quenching property of a phosphor is a critical factor to be deliberated for application in


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high-power WLEDs. Figure 6 shows temperature dependence of relative PL integrated intensity for (La0.90Ce0.10)3Br(SiS4)2, (La0.97Eu0.03Na0.03)3Br(SiS4)2, CaS:Ce3+, and CaS:Eu2+ over the range of 20 to 200 °C. The thermal stability of the as-prepared (La0.90Ce0.10)3Br(SiS4)2 is higher than that of the commercial binary sulfide CaS:Ce3+; on the other hand, (La0.97Eu0.03Na0.03)3Br(SiS4)2 also has higher thermal stability than the commercial CaS:Eu2+, which may be attributed to the stiff (SiS4) tetrahedral network that makes the host more stable. The lower left inset of Figure 6 presents the calculated thermal activation energy (Ea), which can be expressed by the following equation:

I (T ) =

Io

 E  1+ Aexp  − a   kT 

(8)

where I0 and I(T) represent the PL integrated intensity at room temperature and testing temperature (20−200 °C), respectively, and k the Boltzmann constant. The values of Ea for (La0.90Ce0.10)3Br(SiS4)2 and (La0.97Eu0.03Na0.03)3Br(SiS4)2 were estimated to be 0.291 and 0.240 eV, respectively.

Figure 7. (a) EL spectra of the device using 450 nm LED chip with green-emitting (Ba,Sr)2SiO4:Eu2+, and redemitting La3Br(SiS4)2:Eu2+ phosphors and (b) variation in CIE chromaticity coordinates of the WLED operated under different currents (100 to 800 mA). (c) CIE chromaticity coordinates of the used phosphors and the fabricated LED are presented. The insets show the used phosphors and the LED device photographs recorded under 365 nm excitation.

La3Br(SiS4)2:Eu2+ phosphor has outstanding color stability. Besides, the CRI and LER values were obtained decreasingly from 94.60 to 92.49 and 23.95 lm-1W to 14.59 lm-1W, respectively, and the CCT values increased from 5,120 K to 5,365 K with raised currents. The competition of CCT and CRI of the exhibited white light between the a conventional method (combing commercial Y3Al5O12:Ce3+ phosphor with a blue LED)40 and this work indicates that the latter has lower color temperature and higher CRI value.

CONCLUSION In summary, we have prepared and investigated two novel Ce3+- and Eu2+-doped lanthanum bromothiosilicate phosphors with compositions of La3Br(SiS4)2:Ce3+ and La3Br(SiS4)2:Eu2+ by the solid-state method in an evacuated and sealed quartz glass ampoule. The detailed crystal structure, morphology, chromaticity, overall luminescence performance (i.e., PL/PLE spectra, QE, thermal luminescence properties), luminescence decay, and the WLED device performance were presented. The results show that La3Br(SiS4)2:Ce3+ is excitable over a broad range from 350 to 420 nm, and generates an intense cyan emission with the IQE of 56.9% under excitation of 375 nm. On the other hand, the emission wavelength of La3Br(SiS4)2:Eu2+ is a broadband reddish-orange emission with the IQE of 20.9% under excitation of 450 nm, and can be excited from n-UV to blue spectral region. Applying a blend of La3Br(SiS4)2:Eu2+ and green-emitting (Ba,Sr)Si2O4:Eu2+ phosphors and pumped with a blue LED chip, we can receive a WLED device with higher CRI value of ~95 and lower CCT value of 5,120 K. Our investigation results indicate that these materials can potentially serve as conversion phosphors for pc-LEDs. ASSOCIATED CONTENT Supporting Information. Further details are illustrated in Figure S1-S9 and Table S1-S2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

CIE chromaticity Coordinates and Performance of LED Devices Based on La3Br(SiS4)2: Eu2+ Phosphor. In order to illustrate the potential of La3Br(SiS4)2:Eu2+ for pc-WLED application, we have integrated the La3Br(SiS4)2:Eu2+ phosphor with green-emitting (Ba,Sr)2SiO4:Eu2+ commercial phosphor, and a 450 nm blue InGaN-based LED chip to fabricate a WLED device. Figure 7a shows the EL spectra of a fabricated WLED device driven under the forward-biased current increasing from 100 to 800 mA. While increasing the current, the EL intensity of the WLED device increased, but the saturation phenomenon was not observed even at a high driven current of 800 mA. As illustrated in the Figure 7b (the enlarged zoom-in view provided in Figure S9 in SI), the CIE chromaticity coordinates shifted a little from (0.346, 0.365) to (0.339, 0.374) with raised currents, indicating that the

ACKNOWLEDGMENT This research was financially supported by Ministry of Science and Technology of Taiwan (R.O.C.) under Contract No. MOST104-2113-M-009-018-MY3. We would like to thank Dr. Ting-Shan Chan (NSRRC, Hsinchu, Taiwan) for assistance in crystal structure refinement, Dr. Chien-Hao Huang (ITRI, Hsinchu, Taiwan) for WLEDs fabrication, Dr. Hung-Yu Hsu (NCTU, Hsinchu, Taiwan) for luminescence decay lifetime measurements, and Mr. Shuang-De Liu for support on SEM morphological analysis.

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Synthesis and Luminescence Properties of Novel Ce3+- and Eu2+-

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