A New Blue-Emitting Oxohalide Phosphor Sr4OCl6:Eu2+ for

Article pubs.acs.org/JPCC

A New Blue-Emitting Oxohalide Phosphor Sr4OCl6:Eu2+ for Thermally Stable, Efficient White-Light-Emitting Devices under Near-UV Sun Joong Gwak, P. Arunkumar, and Won Bin Im* School of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea ABSTRACT: New blue-emitting Sr4OCl6:Eu2+ (SOC:Eu2+) phosphor was prepared by solid-state reaction. Structural properties including phase purity were analyzed through Rietveld analysis, using X-ray powder diffraction. The photoluminescence (PL) property of the SOC:Eu2+ phosphor was explored, for its successful application in the white light-emitting devices (WLED) industry. The SOC:Eu2+ phosphor exhibits broad excitation spectra ranging from 250 to 425 nm, and an intense broad blue emission band centered at 446 nm under λex = 370 nm. The optimum concentration of Eu2+ in Sr4−xEuxOCl6 was found with x = 0.02 (0.5 mol %). The temperature-dependent PL studies have been investigated, and the phosphor exhibits strong thermal quenching resistance, retaining the luminance of ∼91% at 200 °C. The WLED device was fabricated by integrating the blue-emitting Sr3.98Eu0.02OCl6 with commercial red- and green-emitting phosphor, excited with near UV LED chip (λex = 395 nm), and shows excellent CIE chromaticity coordinates (x = 0.32, y = 0.33). The structural stability of the host, good thermal stability, and excellent CIE coordinates suggest that SOC:Eu2+ is a promising blue component for application in the white LED industry.

1. INTRODUCTION In this 50th anniversary year of the invention of the first lightemitting diode (LED) light source by Nick Holonyak of General Electric, we are reminded of the cornucopia that it has created in revolutionizing the lighting industry, with the emergence of solid-state lighting. The solid-state lighting for white LED (WLED) is considered to be the next-generation lighting source; because of its high efficiency, reduced energy consumption, safety, long life, low maintenance, and reliability, it outperforms incandescent and compact fluorescent lamps.1−4 A typical WLED was devised through dichromatic strategy by pumping the blue InGaN chip on a yellow-emitting Y3Al5O12:Ce3+ phosphor, known as phosphor-converted LEDs. Nevertheless, it has demerits, such as high correlated color temperature (CCT > 4500 K) and low color rendering (CRI) index (Ra < 75), due to the lack of red spectral component in the visible region.5−8 An alternative strategy to generate white light is by pumping the near-UV (NUV) LED chips (370−420 nm) with red-, green-, and blue-emitting phosphors, known as the trichromatic approach. This strategy produces excellent CRI values. However, it has an underlying bottleneck of poor efficiency, owing to the large Stokes shift between excitation and emission in the NUV excitable phosphor. Therefore, it is vital to develop a new phosphor with high efficiency and a structurally stable host, which can absorb in the NUV region and can transfer the energy efficiently to the activator. The most commonly used commercial blue phosphors, BaMgAl10O17:Eu2+ and (Sr, Ba, Ca)3MgSi2O8:Eu2+, are known to have high efficiency but suffer from poor thermal stability.9,10 Hence, it is vital to improve the thermal properties of these phosphors and to fabricate a new © 2014 American Chemical Society

blue phosphor host with excellent structural and thermal stability. The most common blue-emitting activator used in phosphor is Eu2+ ion, which has broad NUV excitation and an emission band due to the dipole that allows 4f−5d transitions. The emission of Eu2+ ions strongly depends on the crystal field influenced by the nature of host and the ligand−activator interaction, which may lead to red shift of the emission band due to the low energy difference between 5d−4f states.11 The most interesting aspects of the Eu2+ ion are its small Stokes shift and short decay time. Recently, researchers have explored new blue-emitting phosphor host with Eu2+ for WLED applications that include Na 0.34 Ca 0.68 Al 1.66 Si 2.34 O 8 :Eu 2+ , Ca2PO4Cl:Eu2+, LiCaPO4:Eu2+, SrMg2Al16O27:Eu2+, and Ba2Ca(PO4)2:Eu2+, but they have relatively low emission intensity, which remains unusable in the LED industry.12−16 In recent years, phosphor host with oxohalides has received great attention due to its high thermal, structural, and chemical stability. The high electronegativity of halides causes a strong anion polarizability, which invariably enhances the emission performance. Some of the new oxohalide phosphor hosts are investigated by many researchers, which include LaOCl:Ln3+ with different emission based on the activators used,17,18 Sr3−xAxAlO4F (A = Ca, Ba),19 and X5SiO4Cl6:Tb3+ (X = Sr, Ba).20 Hence, our search for a new and efficient blue phosphor host has led to the emergence of the new oxochloride host, with improved PL emission and conversion efficiency. In the current Received: September 5, 2013 Revised: January 15, 2014 Published: January 17, 2014 2686

dx.doi.org/10.1021/jp408901c | J. Phys. Chem. C 2014, 118, 2686−2692

The Journal of Physical Chemistry C

Article

explored by Frit et al.21 In the crystal structure of M4OX6, the metal ions form M4O tetrahedra, with slight distortion around the oxygen ion, while halogen atoms are four- and fivecoordinated with the central metal ion.22−24 The unit cell representation of the SOC and the existence of two crystallographic sites of strontium are depicted in Figure 1a,b,

study, we report blue emitting Sr4OCl6 phosphor host with Eu2+, which was designed, prepared, and explored for its potential suitability in the WLED industry. The samples showed intense broad excitation band from 250 to 425 nm that overlaps with the emission of the NUV LED chip, while a broad asymmetric emission band was exhibited, with a maxima at ∼446 nm. The WLED device was fabricated by integrating the InGaN LED chip (λmax = 395 nm) with SOC:Eu2+ phosphor, along with commercial CaAlSiN3:Eu2+ red and Ba2SiO4:Eu2+ green phosphor. The fabricated WLED possesses excellent thermal and structural stability of SOC:Eu2+, with ideal whitecolor CIE coordinates and improved PL efficiency.

2. EXPERIMENTAL PROCEDURE 2.1. Synthesis. Powder samples of the general formula Sr4−xEuxOCl6 (x = 0.01, 0.01, 0.03, 0.04, 0.05, and 0.07) were prepared by solid-state reaction. The starting materials are SrCl 2 anhydrous (Aldrich, ≥99.99%), SrCO3 (Aldrich, 99.995%), and Eu2O3 (Aldrich, 99.99%). SrCO3 and SrCl2 precursors in 1:3 mol ratio were thoroughly ground for 1 h, with acetone as the dispersing medium. The powder was then annealed at 900 °C in reducing atmosphere of H2/N2 (5%/ 95%) for 6 h. Finally, the samples were allowed to cool to room temperature. 2.2. Structural and Optical Properties. Structural information of the synthesized samples was explored through the General Structure Analysis System (GSAS) program, from the X-ray diffraction pattern, using Cu Kα radiation (Philips X′ Pert) over the angle range 10 ≤ 2θ ≤ 100°. The optical properties, including room-temperature photoluminescence (PL) spectra, were measured using Hitachi F-4500 fluorescence spectrophotometer over the wavelength range 200−750 nm. Diffuse reflectance spectra were recorded using Thermo Scientific Evolution 220 UV−visible spectrophotometer over the wavelength range 220−600 nm. Commercial (Sr, Ba, Ca)3MgSi2O8:Eu2+ (SMS:Eu2+) phosphor was used as the stateof-the-art reference blue phosphor. The commercial red and green phosphors used for the fabrication of the WLED device were CaAlSiN3:Eu2+(CASN:Eu2+) and Ba2SiO4:Eu2+ (BAS:Eu2+), respectively. The thermal quenching characteristics were measured in the temperature range of 25−200 °C, with the integrated heater, temperature controller, and thermal sensor connected to the Hitachi F-4500 fluorescence spectrometer. 2.3. Fabrication of Prototype WLED. Prototype WLED devices were fabricated by integrating the SOC:Eu2+ with commercial CASN:Eu2+ red and BAS:Eu2+ green phosphors, with a transparent silicone resin on InGaN LED chip (λmax = 395 nm). For electroluminescence (EL) measurements, discrete LEDs grown on m-plane GaN were placed in silver headers, and gold wires were attached for the electrical operation. The device was then encapsulated in a phosphor/ silicone mixture, with the mixture placed directly on the headers and then cured at 70 and 150 °C for 1 h. EL measurements of the packaged WLED device were carried out using an integrating sphere under DC forward bias condition.

Figure 1. (a) Unit-cell representation of the crystal structure of Sr4OCl6 with red, blue, green and orange spheres as Sr1, Sr2, Cl and O atoms, respectively. (b) Two different Strontium sites are depicted with eight- and seven-coordination with chlorine and oxygen atoms.

respectively. The SOC structure shows the existence of two different strontium sites with high asymmetry, with eight (SrOCl7) and seven (SrOCl6) coordinations, as depicted in Figure 1b. On the basis of the effective ionic radii (r) of cations with different coordination numbers (CN), the proximity of the ionic radius of Eu2+ (r = 1.20 Å) to that of Sr2+ (r = 1.21 Å) when CN = 7 and Eu2+ (r = 1.25 Å) to that of Sr2+ (r = 1.25 Å) when CN = 8 induces favorable doping of the Eu2+ ion.25 Although the crystal structure of Sr4OCl6 was well-investigated by Reckeweg et al.,26 the influence of Eu2+ doping on the crystal structure of SOC was explored here through Rietveld refinement and is shown in Figure 2. The lattice parameters

Figure 2. Rietveld refinement of the X-ray diffraction pattern of SOC:Eu2+ with CuKα as the X-ray source, with data (points) and fit (lines) and the profile difference.

of hexagonal SOC:Eu2+ are tabulated in Table 1. The refined profile confirms the phase purity, regardless of Eu doping. Two crystallographic strontium sites in SOC occupy 6c and 2b Wyckoff positions and are tabulated in Table 2. 3.2. Photoluminescence Properties of Sr4−xEuxOCl6 Phosphor. The excitation and emission spectra of the optimized Sr4−xEuxOCl6 phosphor with x = 0.02 (hereafter this composition is referred as SOC:Eu2+) were measured

3. RESULTS AND DISCUSSION 3.1. Phase Formation and Structural Analysis. The Sr4OCl6(SOC) phosphor host crystallizes in hexagonal phase with S.G. P63mc (# 186) and is isostructural to M4OX6 compounds (M = Ca, Sr, Ba; X = Cl, Br), which were first 2687



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Table 1. Rietveld Refinement and Crystallographic Data of Sr3.98Eu0.02OCl6 formula

Sr3.98Eu0.02OCl6

X-ray source T/K symmetry space group a,b/Å c/Å volume/Å3 Z Rp Rwp χ2

CuKα 295 hexagonal P63mc (#186) 9.469 (2) 7.203 (2) 559.34 (2) 2 2.20% 3.26% 3.823

under room temperature, and are depicted in Figure 3a. The sample showed a broad blue emission band, with maxima at ∼446 nm under λex = 370 nm, which corresponds to the allowed electronic transition of 4f65d to 4f7 of Eu2+ ion.27 The full width at half-maximum of SOC:Eu2+ on the emission spectrum was ∼50 nm. The excitation band consists of a broad unresolved band ranging 250 to 425 nm in the near-UV region, which is attributed to the direct excitation band that corresponds to the 4f7 → 4f65d1 transition of Eu2+ ions. Moreover, it was anticipated that the presence of two different strontium sites may lead to the degeneracy of 5d levels of Eu2+ ions, causing broadness of the excitation spectrum. A broad excitation band in the near-UV region ratifies the suitability of this phosphor for its application in near UV-pumped WLED lighting. The influence of doping concentration of Eu2+ ions on the emission property of the synthesized Sr4‑xEuxOCl6 phosphor is illustrated in Figure 3b. With the increasing Eu2+ concentration, the emission intensity (λex= 370 nm) of Sr4‑xEuxOCl6 increases, and reaches a maxima at x = 0.02. Beyond this value, the intensity tends to decrease gradually, due to the concentration quenching effect. Hence, the optimal concentration of Eu2+ in the hexagonal Sr4‑xEuxOCl6 matrix was x = 0.02. The concentration quenching arises due to the spontaneous energy transfer from one activator to another, leading to nonradiative transitions. For a better understanding of the concentration quenching characteristics of SOC:Eu2+, it is essential to know the critical distance (Rc) of the Eu2+ activator ion. The critical distance was calculated using two different techniques. The first technique was proposed by Blasse28 using the concentration quenching data, and the other was through the spectral overlap equation proposed by Dexter.29 The critical distance (Rc1) between Eu2+ ions for efficient energy transfer in the SOC host was calculated from Blasse’s concentration quenching method using the unit cell volume (V) and the number of Eu2+ sites per unit cell (N) together with the critical concentration (Xc)29 using eq 1.

Figure 3. (a) Excitation spectra of the SOC:Eu2+ at λem = 446 nm and emission spectra under λex = 370 nm and (b) relative emission intensity as a function of Eu2+concentration in the SOC host.

⎡ 3V ⎤1/3 R c1 ≈ 2⎢ ⎥ ⎣ 4πXc N ⎦

(1)

Rc1 (Blasse method) was calculated using the known structural parameter values of V = 559.34 Å3, N = 4, and Xc = 0.02 and found to be ∼23 Å. For achieving consistency of the critical distance calculation, an alternative critical distance R c2 calculation was employed, using Dexter’s formula,29 which assumes the electric dipole−dipole interaction between Eu2+ ions, by considering the symmetry allowed transitions within the Eu2+ ions, with the equation given as ⎡ 48 × 10−16 ⎤ R c2 ≈ 0.63 × 1028⎢ ·P ⎥ E4 ⎣ ⎦

∫ fs (E)fa (E) dE

(2)

where E is the energy of maximum spectral overlap, P is the oscillator strength of the Eu2+ ion, and ∫ fs(E)fa(E) dE is the spectral overlap integral from the normalized excitation and emission spectrum of SOC:Eu2+.The oscillator strength (P) for an allowed broad 4f7→4f65d transition is 0.01,30 whereas E and ∫ fs(E)fa(E) dE were calculated from the spectra and found to be 2.88 and 1.05 eV−1, respectively. The Rc2 for the energy transfer in SOC:Eu2+ through the spectral overlap method was calculated to be 27 Å. A collective analysis of these results reveals that the energy transfer between Eu2+ ions in SOC:Eu2+ phosphor was primarily due to 4f7→4f65d, which allowed an electric−dipole transition. The critical distance value of SOC:Eu2+ from the spectral overlapping method (Rc2) is 27

Table 2. Structural Parameters of Sr3.98Eu0.02OCl6 As Obtained from the Rietveld Refinement through Powder X-ray Diffraction at Room Temperature atom

Wyckoff site

x

y

Z

g

100 × Uiso/Å2

Sr(1) Sr(2) O Cl(1) Cl(2)

6c 2b 2b 6c 6c

0.8051(2) 1/3 1/3 0.1381(1) 0.5348(1)

0.1948(2) 2/3 2/3 0.8618(2) 0.4651(2)

0.4018(2) 0.3581(1) 0.0017(2) 0.2957(1) 0.1164(2)

1.0 1.0 1.0 1.0 1.0

2.12(5) 2.65(7) 4.17(9) 0.23(1) 4.15(4)

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environment on the emission band of Eu2+. The empirical relation was given as:32

Å, and is consistent with the Rc1 (23 Å) calculated through the concentration quenching method. The broadness and the asymmetric nature of the SOC:Eu2+ emission band (as observed from Figure 3a) describes the presence of different emission bands due to the existence of two strontium crystallographic sites. The strontium atoms form two polyhedral units, with chlorine and oxygen atoms with eight- and seven-coordination, namely, Sr(x)OCl7 and Sr(y)OCl6 (x and y are the strontium atomic positions), which provide two sites for Eu2+ to accommodate. The luminescent property depends on the crystallographic orientation and the surrounding environment of the Eu2+ ion in the SOC host crystal lattice.31 Therefore, two emissions bands may be anticipated for the Eu2+ in SOC host, with two strontium crystallographic sites occupying Wyckoff positions 2b and 6c. The Eu2+ ions in different crystallographic Strontium sites experience a different crystal field effect on the 5d orbit and have different energy levels, resulting in the extension and overlapping of the emission bands into the wider wavelength region. Deconvolution of the emission spectra was used to identify the presence of two emission bands and identify the site occupancy of the Eu2+, corresponding to the particular emission spectra. Hence, the PL emission spectra of Sr3.98Eu0.02OCl6 were deconvoluted using two Gaussian equations with reasonable fitting values and are depicted in Figure 4. The inset in Figure 4 shows the presence of two

⎤ ⎡ ⎛V ⎞ E (cm−1) = Q ⎢1 − ⎜ ⎟1/ V × 10−(n * r * Ea)/80⎥ ⎝4⎠ ⎦ ⎣

(3) 2+

where E is the position of the emission band of Eu , Q is the position in energy for the lower d-band edge of the free Eu2+ ion (Q = 3400 cm−1), V is the valence state of the Eu2+ ion (V = 2), n relates to the number of anions in the immediate shell of Eu2+ ion, which is the coordination number of Eu2+, Ea is the electron affinity of the anions around the Eu2+ ion (in electronvolts), and r is the ionic radius of the host cation displaced by Eu2+ (in angstroms). It is very difficult to assign an emission band to a particular crystallographic site because any small change in crystal disorder, lack of charge neutrality, or charge compensation would influence the local environment of the Eu2+ ion, subsequently altering the position of the 4f−5d transition. However, the well-known empirical formula method was used to precisely calculate the site occupancy of Eu2+, with the deconvoluted emission peak position. The calculation of the energy of the emission bands was made by assigning specific coordination numbers into the empirical formula with the known n, Ea, and r parameters, and it was found that the deconvoluted Gaussian curves at ∼22 573 (443 nm) and ∼21 551 cm−1 (464 nm) are assigned to Sr(1)OCl7 (eightcoordination) and Sr(2)OCl6 (seven-coordination), respectively. The luminescence intensity and emission area of a phosphor are the important parameters for industrial application. Figure 5 shows a comparison of PL excitation

Figure 5. Comparison of PL excitation (λem = 446 nm) and emission spectra (λex = 370 nm) of SOC:Eu2+ with that of commercial blue reference phosphor under 370 nm excitation.

Figure 4. Deconvoluted PL emission spectra of SOC:Eu2+under excitation at 370 nm representing two crystallographic strontium sites, with inset depicting strontium sites with eight- and sevencoordination, where red, blue, green, and orange spheres are Sr1, Sr2, Cl, and O atoms, respectively.

and emission spectra of the SOC:Eu2+, with the commercial reference blue phosphor. When SOC:Eu2+ phosphor was excited at 370 nm, the emission performance calculated by integrating the emission area was measured to be ∼57% that of the commercial (Sr, Ba, Ca)3MgSi2O8:Eu2+ blue phosphor. Although SOC:Eu2+ is yet to be fully optimized, the initial results suggest that this new phosphor is a promising candidate for WLED application. For detailed comparison of the emission property, the broadness of the emission peaks has been taken into consideration. SOC:Eu2+ phosphor was found to have a broader emission band compared with the commercial

different strontium crystallographic sites, with eight- and sevencoordination. Deconvolution of the emission bands discloses the presence of two peaks: one high intense emission peak is centered at ∼443 nm, while the other feeble band is centered at ∼464 nm. The site occupancy of Eu2+ in each of the two strontium sites may be identified using the empirical formula proposed by Van Uitert using the lower d-band edge of free Eu2+ ion, which involves the strong influence of the local 2689



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reference blue phosphor, and its calculated fwhm values are 46 and 37 nm, respectively, which is due to the presence of luminescent centers in different environments in the host material. A broader emission band is an essential parameter for obtaining high CRI value and PL efficiency for WLED applications. 3.3. Thermal Quenching Properties of the SOC:Eu2+ Phosphor. The thermal quenching properties of SOC:Eu2+ were studied in comparison with the commercial reference blue phosphor in the temperature ranging from room temperature to 200 °C and are depicted in Figure 6. A typical phosphor

I (T ) =

Io 1 + A ·e(−E / kT )

(4)

where Io is the initial intensity, I(T) is the intensity at a given temperature T, A is a constant, E is the activation plot for thermal quenching, and K is the Boltzmann constant. Figure 6c depicts a plot of ln[(Io/I) − 1] versus 1/(kT). Through the best fit using the Arrhenius equation, the activation energy (E) was obtained as 0.1378 and 0.1736 eV for SOC:Eu2+ and commercial reference blue phosphor, respectively. The influence of operating temperature on the emission intensity of SOC:Eu2+ phosphor was well understood and has been previously discussed. Apart from the emission intensity, another parameter that is greatly affected by temperature is the broadness of the emission spectra. The role of the temperature in the broadness of the emission spectra was explained on the basis of the Huang−Rhys parameter, which describes the electron−phonon interaction. In principle, at higher temperature there is an increase in the population density of phonons due to the electron−phonon interaction, which results in the broadening of the emission spectrum.2 In corroboration with this understanding, an increase in the broadening of the emission intensity was observed for the SOC:Eu2+ when excited at 370 nm, as depicted in Figure 6a. Typically, a phosphor suffers from a decrease in conversion efficiency as the temperature increases, which is due to an increase in the nonradiative transition probability in the configurational coordinate diagram. Hence, the fwhm of the emission spectra is affected by thermally active phonon modes.35 3.4. Fabrication of Prototype of WLED. EL spectra and CIE chromaticity coordinates of the prototype WLED were measured for the fabricated LED using an InGaN LED source (λmax = 395 nm) with the synthesized blue-emitting SOC:Eu2+, commercial BAS:Eu2+ green, and commercial CASN:Eu2+ red phosphor, under different forward bias currents (20 to 100 mA), as shown in Figure 7. The WLED was fabricated without SOC:Eu2+ in the presence of commercial BAS:Eu2+ green and commercial CASN:Eu2+ red phosphor, under 20 mA, as depicted in Figure 7a. The emission wavelength of the InGaN LED chip used for WLED fabrication is ∼395 nm, as shown in Figure 7b. LED chips were operated in the voltage range of 3.0 to 3.6 V. The EL spectra for the fabricated WLED using SOC:Eu2+ with commercial green and red phosphor (Figure 7c,d) reveals that ideal CIE color coordinates (0.32, 0.33) were obtained under 20 mA at a cool white-lightcorrelated color temperature of 5956 K, with color rendering index Ra = 82. The decreased intensity of the blue emission of SOC:Eu2+ at 446 nm for fabricated WLED is attributed to the partial absorption of its blue emission by commercial CASN:Eu2+ red and BAS:Eu2+ green phosphor. This was corroborated by the fact that CASN:Eu2+ red phosphor absorbs over the complete UV−visible range 200−550 nm,36 while BAS:Eu2+ green phosphor absorbs in the range 350−470 nm.37 The optical parameters for the WLED prototype, with and without blue-emitting SOC:Eu2+, are tabulated in Table 3. The CIE color coordinates of fabricated WLED without SOC:Eu2+ are (0.34, 0.30) under 20 mA at warm white CCT of 4486 K with color-rendering index, Ra = 42. The CIE color coordinates reveal that blue deficiency was observed for the WLED without SOC:Eu2+, whereas an ideal white light was achieved by the inclusion of SOC:Eu2+. Moreover, the color-rendering index was much higher for WLED fabricated with SOC:Eu2+ (Ra = 82), than without SOC:Eu2+ (Ra = 42), which confirms the

Figure 6. (a) Temperature-dependent PL emission intensity of SOC:Eu2+ phosphor, (b) temperature-dependent normalized emission intensities of SOC:Eu2+ and commercial blue phosphor, and (c) activation plots for thermal quenching of commercial reference blue phosphor and SOC:Eu2+ phosphor using the Arrhenius equation.

suffers from the decrease in PL conversion efficiency, with the increase in the operating temperature of the LED device, due to the increase in nonradiative transition probability in the configurational coordinate diagram.33 As the temperature increases from RT to 200 °C, the PL intensities decreased by 9 and 19% at 200 °C from the initial emission intensity of SOC:Eu2+ and commercial blue phosphor, respectively, as shown in Figure 6b. This reveals that the SOC:Eu2+ possesses better thermal stability than the commercial reference blue phosphor, which confirms it as a promising blue phosphor for the LED industry. Moreover, for deep understanding of the temperature-quenching characteristics, the activation energy was calculated using the Arrhenius equation from the equation given as:34 2690



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distance for Eu2+ ions in the Sr4OCl6 host was calculated to be 27 Å, using Dexter’s spectral overlap method, and the energytransfer mechanism occurs through dipole−dipole interaction. Two different emissions were observed for SOC:Eu2+ at 444 and 463 nm, which affirms the presence of two different strontium sites, which occupy 6c and 4b sites in the SOC crystal lattice. The thermal stability of the SOC:Eu2+ was much higher than that of the commercial reference blue phosphor, with only 9% emission loss for SOC:Eu2+ compared with 19% for commercial phosphor at 200 °C. WLED fabricated with the synthesized SOC:Eu2+ on InGaN LEDs showed ideal CIE chromaticity (0.32, 0.33) at the applied current of 20 mA, with the highest CRI value of 82. SOC:Eu2+ phosphor may be a promising blue component in the white LED industry due to its excellent structural and thermal stability, together with ideal white CIE coordinates.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-62-530-1715. Fax: +82-62-530-1699. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Strategic Key-Material Development and the Materials and Components Research and Development bodies, funded by the Ministry of Knowledge Economy (MKE, Korea).

Figure 7. Electroluminescence of InGaN LED + phosphor under different forward bias currents represents: (a) commercial green + commercial red without SOC:Eu2+, (b) InGaN LED alone, (c) InGaN LED (λmax = 395 nm) + SOC:Eu2+ + commercial red + commercial green phosphor, and (d) CIE chromaticity coordinates of the device under different forward bias current. The Planckian locus line and the points corresponding to the color temperatures of 3500 and 6500 K are indicated.

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(1) Bergh, A.; Craford, G.; Duggal, A.; Haitz, R. The Promise and Challenge of Solid-State Lighting. Phys. Today 2001, 54, 42−47. (2) Im, W. B.; Fellows, N. N.; DenBaars, S. P.; Seshadri, R.; Kim, Y. I. LaSr2AlO5, a Versatile Host Compound for Ce3+-Based Yellow Phosphors: Structural Tuning of Optical Properties and Use in Solid-State White Lighting. Chem. Mater. 2009, 21, 2957−2966. (3) Brinkley, S. E.; Pfaff, N.; Denault, K. A.; Zhang, Z.; Hintzen, H. T.; Seshadri, R.; Nakamura, S.; DenBaars, S. P. Robust Thermal Performance of Sr2Si5N8:Eu2+: An Efficient Red Emitting Phosphor for Light Emitting Diode based White Lighting. Appl. Phys. Lett. 2011, 99, 241106. (4) Krings, M.; Montana, G.; Dronskowski, R.; Wickleder, C. αSrNCN:Eu2+ − A Novel Efficient Orange-Emitting Phosphor. Chem. Mater. 2011, 23, 1694−1699. (5) Lee, S.; Seo, S. Y. Optimization of Yttrium Aluminum Garnet:Ce 3+ Phosphors for White Light-Emitting Diodes by Combinatorial Chemistry Method. J. Electrochem. Soc. 2002, 149, J85−J88. (6) Setlur, A. Phosphors for LED-based Solid-State Lighting. Electrochem. Soc. Interface 2009, 16, 32−36. (7) Hao, Z.; Zhang, J.; Zhang, X.; Sun, X.; Luo, Y.; Lu, S.; Wang, X.-J. White Light Emitting Diode by using alpha-Ca2P2O7:Eu2+,Mn2+ Phosphor. Appl. Phys. Lett. 2007, 90, 261113. (8) Shi, Y.; Wang, Y.; Wen, Y.; Zhao, Z.; Liu, B.; Yang, Z. Tunable Luminescence Y3Al5O12:0.06Ce3+,xMn2+ Phosphors with Different Charge Compensators for Warm White Light Emitting Diodes. Opt. Express. 2012, 20, 21656−21664. (9) Zhu, P.; Zhu, Q.; Zhu, H.; Zhao, H.; Chen, B.; Zhang, Y.; Wang, X.; Di, W. Effect of SiO2Coating on Photoluminescence and Thermal Stability of BaMgAl10O17: Eu2+ under VUV and UV Excitation. Opt. Mater. 2008, 30, 930−934. (10) Im, W. B.; Kim, Y.-I.; Yoo, H. S.; Jeon, D. Y. Luminescent and Structural Properties of (Sr1−x,Bax)3MgSi2O8:Eu2+: Effects of Ba

Table 3. Optical Parameters for Fabricated WLED Prototype under InGaN LED (λmax = 395 nm) phosphor sample: commercial BAS:Eu2+green + commercial CASN:Eu2+red

CIE x

CIE y

CCT

Ra

without SOC:Eu2+ with SOC:Eu2+

0.34 0.32

0.30 0.33

4486 5950

42 82

REFERENCES

enhanced efficiency of the fabricated WLED in the presence of SOC:Eu2+. The performance of WLED depends on the operating current, maximum emission wavelength, ratio of epoxy resin to phosphor powder, and use of mixing agents, and it is foreseen that through optimization the optical properties of SOC:Eu2+ can be further improved. An ideal white emission was obtained for the fabricated LED with SOC:Eu2+, which confirms the promising role of SOC:Eu2+ phosphor as a blue component in the WLED industry.

4. CONCLUSIONS A new blue-emitting Sr4OCl6:Eu2+ phosphor was synthesized through solid-state reaction and investigated for its optical properties, including photo- and electroluminescence studies. The obtained Sr4−xEuxOCl6 phosphors have a broad excitation band ranging from 250 to 425 nm, which matches perfectly with the emission of NUV LED chips (λmax = 395 nm). Sr4−xEuxOCl6 phosphor emits intense blue light under λex = 370 nm, with optimal concentration of x = 0.02. The critical 2691



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A New Blue-Emitting Oxohalide Phosphor Sr4OCl6:Eu2+ for

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