Color-Tunable and Highly Luminous N - ACS Publications

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Color-Tunable and Highly Luminous N3--doped Ba2-xCaxSiO4#N2/3#:Eu2+ (0.0 # x # 1.0) Phosphors for White NUV-LED Donghyeon Kim, Jong-Seong Bae, Tae Eun Hong, Kwun Nam Hui, Sungyun Kim, Chang Hae Kim, and Jung-Chul Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02778 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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ACS Applied Materials & Interfaces

Color-Tunable and Highly Luminous N3--doped Ba2-xCaxSiO4-δN2/3δ:Eu2+ (0.0 ≤ x ≤ 1.0) Phosphors for White NUV-LED Donghyeon Kim,† Jong-Seong Bae,§ Tae Eun Hong,§ Kwun Nam Hui,⊥ Sungyun Kim,∥ Chang Hae Kim,¶ Jung-Chul Park†,‡,* †

Graduate School of Advanced Engineering, Silla University, Busan 46958, Republic of Korea

‡

Center for Green Fusion Technology and Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 46958, Republic of Korea

§

Busan Center, Korea Basic Science Institute, Busan 46742, Republic of Korea

⊥

Institute of Applied Physics and Materials Engineering, University of Macau, Avenida da Universidade, Taipa, Macau, China

∥

¶

Institute of NT.IT Fusion Technology, Ajou University, Suwon 16499, Republic of Korea

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141, Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT: In search for well-defined phosphor materials for white NUV-LEDs, the highly enhanced luminous efficacy by N3- doping as well as color-tunability via Ca substitution has been successfully obtained in Ba2-xCaxSiO4-δN2/3δ:Eu2+ (x = 0.0, 0.5, 0.8, 1.0) phosphors. With increasing Ca-substitution rate, the crystal structures of the phosphor materials are changed from the primitive orthorhombic structure to the hexagonal one, so that the CIE coordinates move from bluish-green (at Ca = 0.0) to blue (at Ca = 0.5), and finally to near white region (at Ca = 0.8 and 1.0) in these materials. In combination with the results from X-ray photoelectron spectroscopy (XPS) and infra-red (IR) spectroscopy, the elemental distribution of the phosphor materials found from secondary ion mass spectrometry (SIMS) directly indicates that the N3ions are partially substituted for O2- ions into the crystal lattice of alkaline-earth orthosilicates, and thus critically improves the color-tunable photoluminescence (PL) and electroluminescence (EL) efficiency of the phosphor materials for white NUV-LEDs. The newly found “the N3doping and color-tunable effect” on large PL and EL enhancement may provide a platform in discovery of new efficient phosphors for solid state lighting. KEYWORDS: Ba2SiO4:Eu2+, Ca substitution, Color-tunable, N3- ion doping, Highly luminous PL and EL ■

INTRODUCTION

To realize white light illumination, the blue-emitting InGaN chips coated with a YAG:Ce3+ phosphor which emits yellow light, were initially developed.1 However, because these LEDs exhibit low efficiency and a low color rendering index (CRI), it is necessary to develop the new phosphor materials with high luminous efficacy for generating human-friendly white light. Thus, the development of new phosphors for white LEDs has been given much attention in the last


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decade. The general requirements for LED phosphors are as follows:2,3 i) excitation spectra overlapping with emission spectra of blue LED or near UV LED, ii) emission spectra lying in the green and red region for blue LED or RGB region for near UV LED, iii) high quantum efficiency, ⅳ) optimal grain size of phosphors, ⅴ) high thermal stability of luminescence, ⅵ) high chemical stability, ⅶ) low production cost, ⅷ) environmental friendly composition. The silicate-based phosphor materials have been extensively studied as expressed in the following formulae, Sr3Si2O4N2:Eu2+,4 Ba1.55Ca0.45SiO4:Eu2+/Mn2+,5 MSi2O2-δ N2+2/3δ :Eu2+/Ce3+ (M = Ca, Sr, Ba),6 CaAlSiN3:Eu2+,7 NaxCa1-xAl2-xSi2+xO8:Eu2+.8 It is well-known that Ce3+ and Eu2+ activators have been widely used to develop the luminescent properties of the color conversion LED phosphors because of the advantages of their PL properties, i.e., the excitation and emission bands formation due to d→ f transitions with the more intense emission than those originating from f→ f transitions. As the d-orbital is more sensitive to its environment than the f-orbital, the wavelength tunability of the emission can be obtained by controlling the strength of the crystal field splitting of the d-orbitals in the activator ion, more especially by changing the chemical environments of cation sites with other cations and changing anion sites (O2-) with halide, nitride, and sulfide ions etc. In the calcium orthosilicate (Ca2SiO4), the five crystalline phases exist ranging from room temperature to the melting point: γ (orthorhombic), β (monoclinic), αL′(orthorhombic), αH′ (orthorhombic) and α-phase (trigonal/hexagonal).9 Barium orthosilicate (Ba2SiO4) is isostructural with β-K2SO4 having a Pmcn space group10,11 and the only stable phase. The luminescence of Eu2+-activated barium and strontium orthosilicates was studied for the first time by Barry and Blasse et al.12,13 It is generally accepted that the green-emitting Ba2xMxSiO4:Eu

2+

(M = Ca, Sr) phosphors were known to be suitable for UV-LEDs because of high

luminous efficacy and its short decay time under UV light excitation.14 In particular, RE ions-


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activated (RE = Ce3+, Eu2+, Yb3+ etc.) alkaline-earth silicon-oxynitride phosphors have been investigated in recent years as the potential candidate materials to improve the low luminescent efficiency and low color rendering index for white LED applications.6,15-19 The N3- ions partially substituted for O2- ions in the alkaline-earth silicate lattice could alter the electronic structure of Eu2+ due to the difference of the electronegativity, the formal charge, the crystal field splitting, and the nephelauxetic effect in two ions,20 which results in the improvement of the luminescence properties of the alkaline-earth silicon-oxynitride phosphors. Herein, we report the highly enhanced photoluminescence (PL) and electroluminescence (EL) of the N3- ion doped Ba22+ xCaxSiO4-δN2/3δ:Eu xCaxSiO4:Eu

2+

(BSON:Eu2+ and BCSON:Eu2+) phosphors comparing with Ba2-

(BSO:Eu2+ and BCSO:Eu2+) phosphors (x = 0.0, 0.5, 0.8, 1.0). In particular, the

N3- contents are determined by secondary ion mass spectrometry (SIMS) and the N3- ion doping effect on the enhanced PL and EL intensity is discussed in these phosphors. ■

EXPERIMENT SECTION

Ba2-xCaxSiO4:Eu2+ (BSO:Eu2+ and BCSO:Eu2+) phosphors (x = 0.0, 0.5, 0.8, 1.0) were prepared from a stoichiometric mixture of BaCO3, CaCO3, SiO2, and Eu2O3 under 4% H2-Ar atmosphere at 1250°C for 6 h. N3--doped Ba2-xCaxSiO4-δN2/3δ:Eu2+ (BSON:Eu2+ and BCSON:Eu2+) phosphors (x = 0.0, 0.5, 0.8, 1.0) were prepared from a stoichiometric mixture of BaCO3, CaCO3, α-Si3N4, and Eu2O3 under NH3 atmosphere at 1150°C for 6 h. Powder X-ray diffraction (XRD) profiles were characterized using a X-ray diffractometer (XRD 6000 model, Shimadzu) using Cu-Kα radiation at 30 kV and 30 mA. The unit-cell parameters were obtained using a nonlinear least-squares cell refinement program (UnitCell). SIMS (CAMECA IMS-6f, France) was performed to determine the elemental composition of the N3--doped BSON:Eu2+ and BCSON:Eu2+ phosphors. The SIMS standard was prepared with

14

N isotope implanted as N+ at


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100 keV into SiO2 film at a dose of 5 × 1014 ions/cm2. The relative sensitivity factor (RSF) is a factor for conversion from secondary ion intensity to atom density. In order to determine the RSF of each element, the secondary ion intensity of each element was calibrated by the SIMS results of the standard sample. For high precision measurements, focused Cs+ primary ion beam was used with an electron neutralizer for charge compensation (net impact energy = 15 keV, beam current = 20 nA). Fourier-transform infrared (FT-IR) spectroscopy was performed using a FT-IR spectrophotometer (IRTracer-100, Shimadzu) in the range 400-2000 cm-1 using a KBr medium (KBr 200 mg + sample 1 mg) with the resolution range of ± 0.5 cm-1. The photoluminescence (PL) spectra were measured at room temperature using a fluorescent spectrophotometer with a 150 W Xenon lamp under an operating voltage of 350 V (Fluorometer FS-2, Scinco). The temperature-dependent luminescent properties were measured on a PSI Model-150 spectrophotometer equipped with a temperature-controlled electric furnace. The reflectance spectra of phosphors were recorded using UV-Visible spectrophotometer (UV-2600, Shimadzu) with BaSO4 as a reference. PL quantum yields of phosphor materials were measured using a PL spectrometer with an integrating sphere (Hamamatsu Photonics: Quantum Yield Measurement system C9920-02) at room temperature. ■ RESULTS AND DISCUSSION Structural Characterization. It is well-known that the color-tunability of the orthosilicate phosphors is closely related to the structural aspects depending on their versatile polymorphs and chemical compositions. Chen et al. reported that the crystal structures of Ca2-xSrxSiO4:Ce3+ are divided into two groups, namely β phase (0 ≤ x ≤ 0.15) and α´ phase (0.18 ≤ x ≤ 2), and the phase transition from β to α´ mechanism was originated from the different chemical compositions, which finally resulted in the emission peak shift of Ce3+ from 417 nm to 433 nm in


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the range of 0 ≤ x ≤ 0.8.21 Furthermore, Matković et al. reported that in the Ba2SiO4-Ca2SiO4 solid-solution region, the six distinct phases (Ba2SiO4, T, X, α´, β, and γ-Ca2SiO4) exist as a function of the mole fraction of Ca2SiO4.22 Based on their results, it is evident that among the six phases, the hexagonal T phase exists in the Ba-rich region in the composition range between Ba1.0Ca1.0SiO4 and Ba1.6Ca0.4SiO4. Thus, in order to examine the structural effect on the luminescent property, the solid-solution range between Ba2SiO4 and Ca2SiO4 was selected in Ba2-xCaxSiO4-δN2/3δ:Eu0.02 compounds (x = 0.0, 0.5, 0.8, 1.0). XRD patterns of Ba2xCaxSiO4:Eu0.02

and Ba2-xCaxSiO4-δN2/3δ:Eu0.02 compounds (x = 0.0, 0.5, 0.8, 1.0) are shown in

Figure 1. XRD pattern of Ba2SiO4:Eu0.02 (BSO:Eu) compound has a primitive orthorhombic cell (space group = Pmcn).10 The refined unit cell parameters for Ba2SiO4:Eu0.02 are as follows: a = 5.805 Å, b = 10.210 Å, c = 7.499 Å. As shown in the three XRD patterns for Ba2-xCaxSiO4:Eu0.02 (BCSO:Eu) compounds (x = 0.5, 0.8, 1.0), it is evident that the primitive orthorhombic structure of BSO:Eu compound is transformed into the hexagonal one (space group = P3m1)23 via the Ca2+ substitution for Ba2+, which is probably due to the difference in ionic radii between Ba2+ (1.47 Å at CN = 9) and Ca2+ (1.18 Å at CN = 9)24 in these compounds. All the observed peaks for BCSO:Eu compounds are coincident well with JCPDS file (# 48-0210). Additionally, the lattice parameters (a and c) are somewhat slightly decreased with increasing Ca content in BCSO:Eu compounds (Ca = 0.5, 0.8, 1.0), and there is no considerable change of the lattice parameters (a and c) before and after N3- ion doping in BCSO:Eu compounds (Ca = 0.5, 0.8, 1.0): a = 5.777 Å and c = 14.731 Å for BCSO (Ca = 0.5); a = 5.778 Å and c = 14.732 Å for BCSON (Ca = 0.5); a = 5.743 Å and c = 14.606 Å for BCSO (Ca = 0.8); a = 5.758 Å and c = 14.655 Å for BCSON (Ca = 0.8); a = 5.760 Å and c = 14.647 Å for BCSO (Ca = 1.0); a = 5.760 Å and c = 14.662 Å for BCSON (Ca = 1.0), implying that there is not enough N3- ion content to


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induce the considerable change in the lattice parameters of BCSON:Eu compounds relative to BCSO:Eu compounds. Figure 2 shows the SEM images of BCSO:Eu and BCSON:Eu compounds. The pseudo-spherical and regular shape of particles smaller than 1 ㎛ are observed in BCSO:Eu compounds before N3- ion doping, while for BCSON:Eu compounds after N3- ion doping, the pseudo-spherical and regular particles with a diameter of 2-3 ㎛ are observed, probably due to the sintering effect enhanced by Si3N4 addition in the synthetic step even the firing temperature of BCSON:Eu compounds (at 1150°C) is lower than that of BCSO:Eu (at 1250°C). Secondary Ion Mass Spectrometry: N3--ion Content. SIMS provides the best detection limit to identify elements with the extremely low concentrations that other surface-analytical techniques cannot detect. The elemental composition in SIMS can be quantified using an ion implanted standard with accurately known dose.25-27 The atomic concentration and intensity versus the sputtered depth of BCSON:Eu2+ phosphors are shown in Figure 3 (a, b, c). There is no difference in the secondary ion intensities (for Si, Eu, and O atom) among three phosphors, whereas the secondary ion intensities for Ba and Ca atom are changed because Ca-ions occupy Ba sites in the Ba2SiO4 crystal lattice with an orthosilicate-type structure as depicted in Ba2-xCaxSiO4 chemical formula. Additionally, the secondary ion intensities of the elements are constant from near the surface to the interior (ca. 1400 nm) of the grains for all the samples, indicating that the individual particles are atomistically homogeneous throughout the body, as desired. The SIMS profiles of the N atom concentrations are monitored as a function of sputtered depth. Figure 3-d) presents the calculated N3--ion contents based on the unit cell volume of the phosphors. The average N3--ion contents of the phosphors are estimated from 200 nm to 1400 nm depth of grain; 0.06 for BSON:Eu2+ (no-Ca), 0.06 for BCSON:Eu2+ (Ca = 0.5), 0.08 for BCSON:Eu2+ (Ca =


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0.8), 0.12 for BCSON:Eu2+ (Ca = 1.0). It is remarkable that for the Ca-doped BCSON:Eu2+ phosphors, the N3--ion content is increased with the higher Ca-doping rate, implying that BCSON:Eu2+ phosphor with the higher Ca-doping rate can accommodate more N-ions. This trend can be clearly explained by the difference in the ionic radii between Ba2+ (1.47 Å at CN = 9) and Ca2+ (1.18 Å at CN = 9) as well as in the chemical bond distance between SiO2 (SiO, 1.62 Å) and α-Si3N4 (SiN, 1.73 Å).28,29 In the Ca-doped BCSON:Eu2+ phosphors, the Ca2+ ions partially occupy the interstices in which they are surrounded by a number of O2- ions of the 2+ 2+ discrete SiO ions. As a consequence of the chemical bonding, Ca ---[O − Si − N] ---Ba ,

the structural stress due to the shortening of CaO bond length relative to BaO one may be relieved by the introduction of N3- ions, what is often called, the competition of chemical bonding, which results in the higher N3- content with increasing Ca-doping rate in the Ca-doped BCSON:Eu2+ phosphors. Evidence of N3--ions Incorporated in BCSO Host Lattice: IR and XPS Measurement. The infrared spectra of Ca-doped BCSO:Eu2+ phosphors before and after N3- ion doping (see Figure 4) corroborate our discussion as mentioned above. Notably, in all the IR spectra, the absorption bands around 1400 cm-1 were found to be exactly same wavenumber (at 1384.9 cm-1) associated 30-32 with the CO antisymmetric stretching of CO Furthermore, Du et al. performed in situ FT .

IR spectroscopy to study carbonate transformations during adsorption and desorption of CO2 in the hydrotalcite-like compounds. They mentioned that during subsequent CO2 adsorption and desorption, the gaseous CO2 molecules were chemisorbed on the various active sites (highly basic metal-bound unsaturated oxygen atoms) forming unidentate, bidentate, and bridged carbonates with the absorption band at 1385 cm-1.33 Thus, it is presumed that the absorption band at 1384.9 cm-1 in our study, is a good internal standard one to estimate in detail the variation of


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the wavenumber of the IR modes before and after N3- ion doping in these compounds. It is wellknown that in the alkaline-earth orthosilicate-type structures, SiO complex anions are not only linked, but are surrounded by Me2+ ions and the chemical bond distance of SiO (mean value = 1.665 Å) is shorter than that of MeO (for Ba2+, mean value = 2.903 Å). Thus, the [SiO4] internal modes are predominantly manifested in the range of 1200–400 cm-1. From the IR spectra as shown in Figure 4, it is clearly seen that the IR frequencies of [SiO4] modes between 1000 and 800 cm-1 (υ3 stretching)34 are somewhat decreased after N3- ion doping in the Ca-doped BCSON:Eu2+ phosphors; for BCSO at Ca = 0.5, 983.7 cm-1, 883.4 cm-1; for BCSON at Ca = 0.5, 970.2 cm-1, 877.6 cm-1; for BCSO at Ca = 0.8, 985.6 cm-1, 885.3 cm-1; for BCSON at Ca = 0.8, 983.7 cm-1, 881.5 cm-1, implying that the N3- ions are partially incorporated into the Ca-doped BCSON host lattice. XPS results corroborate the variation of the SiO stretching modes before and after N3- ion doping observed from IR measurement in these compounds. Figure 5 presents Si 2p (ref. SiO2 and α-Si3N4, BCSO, and BCSON) and N 1s (BCSO and BCSON) XPS spectra. All the XPS spectra were fitted after a Shirley background correction. As shown Figure 5, the binding energy of Si 2p in ref. SiO2 (α-quartz) is 102.8 eV with a single Gaussian component (FWHM = 1.9 eV), whereas the binding energies of Si 2p in ref. α-Si3N4, 101.4 eV and 102.8 eV. It might be assured that in the Si 2p XPS peak of ref. α-Si3N4, the latter one (at 102.8 eV) is ascribed to the superficial oxidation induced by air or X-radiation because its intensity is very lower than that of the former one (at 101.4 eV). It is well-known that in barium orthosilicate (Ba2SiO4) structure, the isolated SiO4 tetrahedrons are linked with Ba atoms via O atoms, which results in two distinct Ba sites (I and II) with the increased structural disorder compared with SiO2. Thus, it is evident that the Si 2p XPS spectrum of Ba2SiO4 (BSO) compound exhibits an asymmetric and broad signal which can be deconvoluted by two Gaussian components of Si 2p


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binding energy, for example, the peak positions at 100.4 eV and 102.4 eV as previously reported,35,36 which is very similar to our results (100.5 eV and 102.4 eV) for BSO:Eu2+ compound. Furthermore, in the case of the Ca-doped BCSO compounds composed of five kinds of Ba/Ca sites (one 6-fold coordination, two 10-fold coordination, and two 12-fold coordination),23 the Si 2p XPS spectrum with an asymmetric and broad signal which can be deconvoluted by multi-Gaussian components, would be obtained. As shown in the middle of Figure 5, all the XPS peaks are nicely deconvoluted by three Gaussian components with higher binding energy around 102 eV and lower ones between 101 eV and 100 eV. The corresponding XPS fitting parameters for BCSO:Eu2+ and BCSON:Eu2+ compound are indicated in Table 1. After N3- ion doping, the remarkable changes in the peak areas (%) of the Gaussian components are observed without the variation of the binding energies as shown in Table 1. After N3- ion doping, the peak areas (%) are exclusively increased in the Gaussian components with lower binding energy, while the peak areas (%) largely decreased in the Gaussian component with higher binding energy around 102 eV. Thus, it is presumed that the N3- ions are definitely incorporated into the BCSON host lattice, which is confirmed by the N 1s binding energy around 397 eV in the N3- ion doped BCSON:Eu2+ compound (at the right of Figure 5). Photoluminescence and Electroluminescence Properties of Ba2-xCaxSiO4N:Eu2+. Figure 6-a) presents the PL spectra of BSO: Eu . (no-Ca) and BSON: Eu. (no-Ca) phosphors. The

optimal Eu content for N3--doped BSON:Eu . (no-Ca) phosphor was determined to be 0.02 and its PL property was compared with that of BSO:Eu . (no-Ca) phosphor. The excitation spectra of BSO:Eu . and BSON:Eu. (no-Ca) phosphors monitored at 503 nm, consist of broad

bands between 250 nm and 475 nm, which is associated with the allowed 4f7→ 4f65d transitions of Eu2+.37-41 The emission spectra monitored under 370 nm excitation, show symmetric bands


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centered at 503 nm corresponding to the green-emission. Notably, the maximum emission intensity of N3--doped BSON: Eu . (no-Ca) is about 5.0 times higher than that of 3BSO:Eu . (no-Ca). This fact implies that the N ions introduced into the Ba2SiO4 crystal lattice

play an important role in the increase of the emission intensity. Additionally, there is no the emission wavelength shift for the N3--doped BSON: Eu . (no-Ca) compared with 3BSO:Eu . (no-Ca) phosphor, which probably means that the N ion content is not enough to

induce the chemical shifts of the emission wavelength in the N3--doped BSON:Eu . (no-Ca) 3 phosphor. For BCSO:Eu . (Ca = 0.5) and N -doped BCSO:Eu. (Ca = 0.5) phosphors as

shown in Figure 6-b), the shape of the emission spectra is somewhat different from those of BSO: Eu . (no-Ca) and BSON: Eu. (no-Ca) phosphors, especially, in the emission

wavelength with maximum intensity; 443 nm for the N3--doped BCSON:Eu . (Ca = 0.5), 503 nm for the N3--doped BSON:Eu . (no-Ca) phosphor. It is presumed that the different shape of the PL spectra before and after Ca-doping may be ascribed to the distinct crystallographic sites of the Eu2+ in the host lattices. Furthermore, it is remarkable that the PL intensity of the N3- doped BCSON:Eu . (Ca = 0.5) is about 3.3 times higher than that of BCSO:Eu. (Ca = 0.5) phosphor. For BCSO:Eu . (Ca = 0.8) and BCSON:Eu. (Ca = 0.8) phosphors, the maximum

emission intensity of N3--doped BCSON:Eu . (Ca = 0.8) phosphor is about 1.7 times higher than that of BCSO:Eu . (Ca = 0.8) phosphor (Figure 6-c). For BCSO:Eu. (Ca = 1.0) and 3BCSON: Eu . (Ca = 1.0) phosphors, the maximum emission intensity of N -doped

BCSON:Eu . (Ca = 1.0) phosphor is about 1.7 times higher than that of BCSO:Eu. (Ca =

1.0) phosphor (Figure 6-d). For clarity of the spectral variation, the normalized PL spectra are totally presented in Figure S1 and S2 (Supporting Information). It is generally accepted that in the silicone-based oxynitride phosphors, its PL properties are influenced by the O/N ratio


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because of the difference of the electronegativity, the formal charge, the crystal field splitting, and nephelauxetic effect between N3- and O2- ion, more especially which results in the longer emission wavelengths through the N3- ion doping.6,42 It is well-known that there are two types in the silicone-based oxynitride phosphors, i.e., oxygen-rich and nitrogen-rich one. Thus, in the oxygen-rich (nitrogen-poor) one, the red-shifts of the emission peak maxima are not always manifested depending on the N3- ion contents. Additionally, in the case that there are the crystallographically distinct sites in the host lattices, it is difficult to apply simply the N3--ion doping effect because of the many factors affecting the strength of the crystal field splitting. As the crystal structure of Ba1.3Ca0.7SiO4 is composed of five kinds of Ba/Ca sites (one 6-fold coordination, two 10-fold coordination, and two 12-fold coordination),23 it is presumed that the characteristics of the emission spectra can be varied via the site selectivity of the activator ion and N3- ion in oxygen-rich BCSON:Eu2+ phosphors. As shown in Figure 6, the red-shift of the emission peak maximum in N3- ion doped BSON:Eu2+ (no-Ca) phosphor is not observed, whereas the emission intensity is highly enhanced relative to that before N3- ion doping in these compounds. Regarding the origin of the enhanced PL intensity in BSON:Eu2+ (no-Ca) phosphor, the N3--ion doping effect, for the present, may be explained in two different ways. The first explanation is that a great number of the N atoms in Si3N4 are displaced with the O atoms dissociated from BaO, Eu2O3, and CO2 in the preparation step for Ba1.98Eu0.02SiO3.91N0.06 phosphor under NH3 at 1150℃ as depicted in equation (1)

1.98BaCO3 + 0.01Eu2O3 + 1/3Si3N4 → Ba1.98Eu0.02SiO3.91N0.06 + 0.02O2 + 0.63N2 + 1.98CO

(1)


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From equation (1), it might be assured that the preparation step of BSON:Eu2+ phosphor reflects a higher degree of ∆S (the change in entropy) compared with that of BSO:Eu2+ one. The Gibbs free energy change (∆G) can be used as a criterion of spontaneity for a particular reaction as the following equation:

∆G = ∆H – T∆S

(2)

where ∆G is the change in free energy, ∆H is the change in enthalpy, ∆S the change in entropy, and T the absolute temperature. If ∆G is negative, the chemical reaction might spontaneously proceed, and at high temperature, the T∆S term becomes increasingly important.43 As the more negative ∆G is very closely related to the well-occupied Eu2+ in the Ba2+ sites of the Ba2SiO3.91N0.06 host lattice, the highly enhanced luminescence property of Ba2SiO3.97N0.02:Eu2+ phosphor may be explained, even though the content of N3- ions is very low. As a second explanation, the change of local symmetry from higher to lower one induced by N3- ions doped in BSO:Eu2+ lattice may force the total eigenfunction of Eu2+ (activator) to be distorted, which results in the increased dipole moment. Thus, the enhanced PL intensity after N3- ion doping may be explained by the following equation (3):44

Im→n ∝│Mm→n│2 (Em ＞ En)

(3)

where Im→n and Mm→n are the emission intensity and dipole moment, respectively. It should be mentioned that the second explanation warrants the further study on the direct evidence of the enhanced emission intensity in phosphor materials after N3- ion doping, for example, the


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theoretical calculation of the electric dipole transition probability by LCAO, which is beyond the scope of present study. For N3--doped BCSON:Eu . (Ca = 0.5, 0.8, 1.0) phosphors, it is interesting that the emission peak maxima gradually change to the red-shift region with increasing Ca content; 443 nm for Ca = 0.5, 478 nm for Ca = 0.8, 485 nm for Ca = 1.0. The correlation between the crystal field splitting and the shape and size of the polyhedron is given in the following equation:45,46

Dq = 3Ze2r4/5R5

(4)

where Dq represents crystal field strength, Z represents the valence of the anion ligand, e represents the charge, r represents radius of frontier d wave function, and R represents the bond length between a center ion and ligands. According to the equation (4), the crystal field strength 2+ of BCSON:Eu . phosphor with the higher Ca content increases because the ionic radius of Ca

(1.18 Å at CN = 9) is smaller than that of Ba2+ (1.47 Å at CN = 9).24 Thus, the emission peak maxima of BCSON:Eu . phosphor with the higher Ca content appear in the longer-wavelength region. Figure 7 shows the diffuse reflectance spectra of BSO-host lattice, BSON-host lattice, BSON:Eu . , and BCSON:Eu. (Ca = 0.5, 0.8, 1.0) phosphors. The BSON host (no-Ca and

no-Eu) shows the host absorption between 200 nm and 260 nm. The broad bands appear between 300 nm and 480 nm associated with the 4f→ 5d absorption of Eu2+ ions when Eu2+-activator ions are doped into the host of BSON and BCSON, which is well coincident with the excitation spectra as previously presented in Figure 6. The thermal stability of the phosphors is one of the important parameters for its application in high-power LEDs. Temperature-dependent emission spectra of BCSON:Eu2+ (Ca = 0.0, 0.5, 0.8, 1.0) phosphors under 365 nm excitation between


14

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room temperature and 200°C, are presented in Figure 8. For BCSO:Eu2+ and BCSON:Eu2+ (Ca = 0.0) phosphors, the emission intensity at 150°C considerably decreases to about 40% of the initial one at room temperature, as previously reported.47,48 On the contrary, for Ca-doped BCSO:Eu2+ and BCSON:Eu2+ phosphors, the emission intensities at room temperature still remain without the considerable deterioration even after 150°C treatment: 80% of initial one for BCSO:Eu2+ (Ca = 0.5), 87% for BCSON:Eu2+ (Ca = 0.5); 77% for BCSO:Eu2+ (Ca = 0.8), 87% for BCSON:Eu2+ (Ca = 0.8); 76% for BCSO:Eu2+ (Ca = 1.0), 89% for BCSON:Eu2+ (Ca = 1.0). Notably, the thermal stability of BCSON:Eu2+ phosphors is superior to that of BCSO:Eu2+ phosphors, implying that N3- ions play an important role in enhancement of the thermal stability of these materials. The results also indicate that BCSON:Eu2+ phosphors could be a promising candidate for high-power LED applications. The Commission International de l’Eclairage (CIE) coordinates of BSO:Eu2+, BSON:Eu2+, BCSO:Eu2+, and BCSON:Eu2+ phosphor, are presented in Figure 9. In Ca = 0.5, the CIE coordinate moves from bluish-green to blue region, and in Ca = 0.8 and 1.0 move to near white region. To estimate the potential of the practical use of phosphor materials, LEDs were fabricated by coating phosphor powder-silica gel mixture onto the 375 nmemitting InGaN LED caps. Figure 10 shows the electroluminescence (EL) spectra of BSO:Eu2+, BSON:Eu2+, BCSO:Eu2+, and BCSON:Eu2+ phosphor. The EL spectra were obtained under forward bias currents from 10 mA to 50 mA. The UV light around 375 nm emitted from the InGaN LED is absorbed by the phosphors and down-converted into the intensive wide-band emitting light. As the luminous output has a similar rising tendency and the position and shape of the EL bands do not change significantly, it might be assured that the phosphor materials exhibit a stable EL property. It is well known that photoluminescence is not equivalent to electroluminescence as confirmed when we compare the PL spectra (in Figure 6) with EL spectra


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(in Figure 10). The discrepancy between PL and EL is originated from the distinct two techniques related to the different excitation principles and instrumentations: PL for light excitation and EL for electrical excitation. In the PL process, the excess carriers are photoexcited by a sufficiently intense light source, and the luminescence emitted from the radiative recombination of these photo-excited carriers, while in the EL process, the excess carriers are produced by an applied forward current and it is usually measured on finished device. Furthermore, the EL is determined by several factors, such as the optical properties and physical structures of the optically active layers, the electrical properties of two conductive regions in the cathode and anode. For the Ca non-doped BSO:Eu2+ phosphors, the maximum EL intensity (IEL) after N3- ion doping is found to be 4.8 times higher than that before N3- ion doping as presented in Figure 10-(a) and (b). For the Ca doped BCSO:Eu2+ phosphors, the maximum EL intensities are also increased after N3- ion doping; 3.4 times increase at Ca = 0.5; 1.5 times at Ca = 0.8; 1.7 times at Ca = 1.0, which is well coincident with the trend in the PL intensity before and after N3ion doping as previously presented in Figure 6. The obtained parameters of the phosphor-LEDs under applied forward current are summarized in Table 2. Luminescent properties of Ba2xCaxSiO4-δN2/3δ:Eu.

phosphor materials are summarized in Table 3. The maximum IPL and IEL

ratio associated with the radiative process have the similar values, while the QY values related with the radiative and non-radiative process are somewhat different because of the energy losses due to the non-radiative process. The CRIs for the selected phosphors, BCSO:Eu and BCSON:Eu (Ca = 0.8 and 1.0) were calculated using Osram Sylvania Color Calculator:49 CRI = 65 for BCSO:Eu (Ca = 0.8); CRI = 62 for BCSON:Eu (Ca = 0.8); CRI = 64 for BCSO:Eu (Ca = 1.0); CRI = 62 for BCSON:Eu (Ca = 1.0). The CRIs of the compounds are slightly lower when we compare with CRI of YAG:Ce LED (~70),50 because of its weak emission intensity in red


16

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spectral region. It should be noted that in this system, the comparable CRIs to YAG:Ce LED are meaningful except the large PL and EL enhancement by N3- doping. Furthermore, CRIs may be improved more than 90 using the co-activators such as Pr3+ and Mn2+ etc. as well as adding the small amount of red-emitting phosphor. The luminescent properties of Ba2-xCaxSiO4-δN2/3δ:Eu . phosphor materials reveal that the enhanced luminous efficacy by N3--doping as well as colortunability by Ca-doping allows us to explore highly luminous phosphor materials for white NUV-LEDs. ■

CONCLUSIONS

Ba2-xCaxSiO4:Eu2+ phosphor materials (x = 0.0, 0.5, 0.8, 1.0) before and after N3- ion doping have been successfully synthesized and characterized. The primitive orthorhombic structure of Ba2SiO4 is transformed into the hexagonal one by the Ca substitution for Ba between Ca = 0.5 and Ca = 1.0 in these compounds, which is probably due to the difference in ionic radii between Ba2+ and Ca2+. From the results of the PL and EL measurements, it is evident that the colortunability of these materials is mainly dependent on the Ca-substitution rate; at Ca = 0.0, bluishgreen; at Ca = 0.5, blue; at Ca = 0.8 and 1.0, nearly white. Furthermore, the enhanced luminescent efficiency (PL and EL intensity, and QY) is attributed to the N3- ion doping. Additionally, the evidences of the N3- ion doped into the crystal lattice of phosphor materials are confirmed by IR, XPS, and SIMS. It should be mentioned that it is necessary to develop the new phosphor materials with highly luminous efficacy as well as color tenability for white NUVLEDs. The newly found the N3- doping and color-tunable effect on large PL and EL enhancement may provide a platform in discovery of new efficient phosphors for white NUVLEDs.


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FIGURES

Figure 1. XRD patterns of BCSO:Eu (a) and BCSON:Eu (b) phosphors.


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Figure 2. SEM images of BCSO:Eu and BCSON:Eu (Ca = 0.0, 0.5, 0.8, 1.0) phosphors; BCSO:Eu, Ca = 0.0 (a); BCSON:Eu, Ca = 0.0 (b); BCSO:Eu, Ca = 0.5 (c); BCSON:Eu, Ca = 0.5 (d); BCSO:Eu, Ca = 0.8 (e); BCSON:Eu, Ca = 0.8 (f); BCSO:Eu, Ca = 1.0 (g); BCSON:Eu, Ca = 1.0 (h).


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Figure 3. Atomic concentration and intensity versus sputtered depth of BCSON:Eu; Ca = 0.5 (a), 0.8 (b), 1.0 (c) phosphors, and the calculated N ion contents (d) obtained by SIMS measurements. The inset shows N ion content of BSON:Eu (no-Ca) compound.


20

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Figure 4. IR spectra of BCSO:Eu and BCSON:Eu (Ca = 0.5 and 0.8).

Figure 5. Si-2p and N-1s XPS binding energies of SiO2, α-Si3N4, and BCSON:Eu (Ca = 0.5 and 0.8).


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Figure 6. Excitation and emission spectra of BCSO:Eu; Ca = 0.0 (a), 0.5 (b), 0.8 (c), 1.0 (d) phosphors before and after N3- ion doping.


22

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Figure 7. Diffuse reflectance spectra of BSON and BCSON:Eu (Ca = 0.0, 0.5, 0.8, 1.0) phosphors.


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Page 24 of 47

Figure 8. Temperature-dependent emission spectra of BCSON:Eu; Ca = 0.0 (a), 0.5 (b), 0.8 (c), 1.0 (d) phosphors.


24

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Figure 9. CIE chromaticity of BCSO:Eu phosphors before and after N3- ion doping monitored under 365 nm UV light; BCSO:Eu, Ca = 0.0 (a); BCSON:Eu, Ca = 0.0 (b); BCSO:Eu, Ca = 0.5 (c); BCSON:Eu, Ca = 0.5 (d); BCSO:Eu, Ca = 0.8 (e); BCSON:Eu, Ca = 0.8 (f); BCSO:Eu, Ca = 1.0 (g); BCSON:Eu, Ca = 1.0 (h).


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Figure 10. EL spectra of BCSO:Eu phosphors before and after N3- ion doping monitored with a 375 nm NUV chip; BCSO:Eu, Ca = 0.0 (a); BCSON:Eu, Ca = 0.0 (b); BCSO:Eu, Ca = 0.5 (c); BCSON:Eu, Ca = 0.5 (d); BCSO:Eu, Ca = 0.8 (e); BCSON:Eu, Ca = 0.8 (f); BCSO:Eu, Ca = 1.0 (g); BCSON:Eu, Ca = 1.0 (h).


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TABLES Table 1. Corresponding Si 2P XPS Fitting Parameters for BCSO:Eu2+ and BCSON:Eu2+ (Ca = 0.5 and 0.8) Phosphors. Compound BCSO-Ca0.5

BCSON-Ca0.5

BCSO-Ca0.8

BCSON-Ca0.8

Peak position (eV) 100.0

FWHM (eV) 1.93

Peak area (%) 21.5

101.3

1.58

39.4

102.2

1.99

39.1

100.0

2.01

29.0

101.2

1.59

44.4

102.2

2.01

26.6

99.7

1.71

17.6

101.0

1.65

38.8

102.0

2.08

43.6

99.9

1.79

22.4

101.0

1.51

46.1

101.9

2.00

31.5


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Table 2. Parameters of the phosphor-LEDs under applied forward current. 10 mA (Appl.) Out CIE put (x, y) (mV)

20 mA (Appl.) Out CIE put (x, y) (mV)




Bare LED

0.046

-

0.189

-

0.419

-

0.923

-

2.629

-

BSO-noCa

0.446

0.152, 0.506

1.013

0.163, 0.480

1.596

0.168, 0.468

2.178

0.168, 0.465

2.751

0.170, 0.457

BSON-noCa

1.335

0.153, 0.559

2.681

0.158, 0.550

3.922

0.161, 0.545

4.978

0.163, 0.540

5.922

0.165, 0.536

BCSO-Ca a0.5

0.559

0.162, 0.264

1.570

0.173, 0.269

2.057

0.184, 0.277

2.810

0.177, 0.196

3.498

0.193, 0.278

BCSON-Ca0.5

1.069

0.157, 0.179

2.162

0.167, 0.189

3.314

0.173, 0.193

4.283

0.177, 0.196

5.367

0.180, 0.199

BCSO-Ca0.8

0.758

0.258, 0.401

1.579

0.267, 0.388

2.524

0.270, 0.382

3.452

0.270, 0.378

4.293

0.271, 0.377

BCSON-Ca0.8

0.855

0.231, 0.366

1.837

0.238, 0.355

2.837

0.242, 0.352

3.848

0.243, 0.351

4.667

0.243, 0.350

BCSO-Ca1.0

0.645

0.226, 0.360

1.507

0.235, 0.350

2.243

0.235, 0.356

3.105

0.243, 0.343

3.924

0.244, 0.342

BCSON-Ca1.0

1.059

0.246, 0.388

2.173

0.252, 0.377

3.266

0.254, 0.374

4.300

0.255, 0.372

5.023

0.255, 0.371

Sample


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Table 3. Luminescent Properties of Ba2-xCaxSiO4-δN2/3δ:Eu . Phosphor Materials Maximum IPL ratio (%)a

Maximum IEL ratio (%)a, 50 mA

Quantum Yield (%)

BSO-noCa

19.2

20.7

21.2

BSON-noCa

100

100

71.2

BCSO-Ca a0.5

29.9

30.9

39.1

BCSON-Ca0.5

100

100

81.2

BCSO-Ca0.8

24.3

41.1

38.4

BCSON-Ca0.8

41.6

59.9

69.0

BCSO-Ca1.0

24.1

38.6

34.1

BCSON-Ca1.0

41.3

65.0

58.7

Compound

a

The intensity of PL and EL of BSON:Eu . is adjusted to 100% compared with BSO:Eu. , and for Ca-doped compounds, that of BCSON:Eu. (Ca = 0.5) is adjusted to 100%.

■

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (J.-C. Park).

■

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TABLE OF CONTENTS/ABSTRACT GRAPHIC

The highly luminous efficacy by N3- doping as well as color-tunability via Ca substitution has been successfully obtained in Ba2-xCaxSiO4-δN2/3δ:Eu2+ (x = 0.0, 0.5, 0.8, 1.0) phosphors. The newly found the N3- doping and color-tunable effect on large PL and EL enhancement may provide a platform in discovery of new efficient phosphors for solid state lighting.


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Figure 1. XRD patterns of BCSO:Eu (a) and BCSON:Eu (b) phosphors. 190x336mm (300 x 300 DPI)



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Figure 2. SEM images of BCSO:Eu and BCSON:Eu (Ca = 0.0, 0.5, 0.8, 1.0) phosphors; BCSO:Eu, Ca = 0.0 (a); BCSON:Eu, Ca = 0.0 (b); BCSO:Eu, Ca = 0.5 (c); BCSON:Eu, Ca = 0.5 (d); BCSO:Eu, Ca = 0.8 (e); BCSON:Eu, Ca = 0.8 (f); BCSO:Eu, Ca = 1.0 (g); BCSON:Eu, Ca = 1.0 (h). 12x22mm (600 x 600 DPI)


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Figure 3. Atomic concentration and intensity versus sputtered depth of BCSON:Eu; Ca = 0.5 (a), 0.8 (b), 1.0 (c) phosphors, and the calculated N ion contents (d) obtained by SIMS measurements. The inset shows N ion content of BSON:Eu (no-Ca) compound. 294x191mm (300 x 300 DPI)



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Figure 4. IR spectra of BCSO:Eu and BCSON:Eu (Ca = 0.5 and 0.8). 226x203mm (300 x 300 DPI)


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Figure 5. Si-2p and N-1s XPS binding energies of SiO2, α-Si3N4, and BCSON:Eu (Ca = 0.5 and 0.8). 267x187mm (300 x 300 DPI)



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Figure 6. Excitation and emission spectra of BCSO:Eu; Ca = 0.0 (a), 0.5 (b), 0.8 (c), 1.0 (d) phosphors before and after N3- ion doping. 252x188mm (300 x 300 DPI)


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Figure 7. Diffuse reflectance spectra of BSON and BCSON:Eu (Ca = 0.0, 0.5, 0.8, 1.0) phosphors. 252x203mm (300 x 300 DPI)



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Figure 8. Temperature-dependent emission spectra of BCSON:Eu; Ca = 0.0 (a), 0.5 (b), 0.8 (c), 1.0 (d) phosphors. 280x203mm (300 x 300 DPI)


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Figure 9. CIE chromaticity of BCSO:Eu phosphors before and after N3- ion doping monitored under 365 nm UV light; BCSO:Eu, Ca = 0.0 (a); BCSON:Eu, Ca = 0.0 (b); BCSO:Eu, Ca = 0.5 (c); BCSON:Eu, Ca = 0.5 (d); BCSO:Eu, Ca = 0.8 (e); BCSON:Eu, Ca = 0.8 (f); BCSO:Eu, Ca = 1.0 (g); BCSON:Eu, Ca = 1.0 (h). 18x19mm (600 x 600 DPI)



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Figure 10. EL spectra of BCSO:Eu phosphors before and after N3- ion doping monitored with a 375 nm NUV chip; BCSO:Eu, Ca = 0.0 (a); BCSON:Eu, Ca = 0.0 (b); BCSO:Eu, Ca = 0.5 (c); BCSON:Eu, Ca = 0.5 (d); BCSO:Eu, Ca = 0.8 (e); BCSON:Eu, Ca = 0.8 (f); BCSO:Eu, Ca = 1.0 (g); BCSON:Eu, Ca = 1.0 (h). 291x361mm (300 x 300 DPI)


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ABSTRACT GRAPHIC 244x182mm (300 x 300 DPI)


Color-Tunable and Highly Luminous N - ACS Publications

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