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A green-emitting phosphor, Ca14–xEuxMg2[SiO4]8 (CMS:Eu2+), has been synthesized as a component of white light emitting diodes (WLEDs). The emission spectrum is broad, with a maximum at about 505 nm under 400 nm excitation due to the transition from the 4f65d excited state to the 4f7-ground state of a Eu2+ ion. The dipole-dipole interaction was a dominant energy transfer mechanism of the electric multipolar character of CMS:Eu2+. The critical distance was calculated as 12.9 Å and 14.9 Å using a critical concentration of Eu2+ and Dexter’s theory for energy transfer. When CMS:Eu2+ and red phosphor are incorporated with an encapsulant on an ultraviolet (λmax = 395 nm) light emitting diodes (LEDs), white light with a color rendering index of 91 under a forward bias current of 20 mA was obtained. The structural and optical characterization of the phosphor is described.
© 2012 Optical Society of America
1. Introduction
Recently white light emitting diodes (WLEDs) have received great attention because they have several strong points such as low energy consumption, long operation time (> 100,000 h), and environmentally friendly characteristics. [1, 2] However, there have been many technical barriers to the use of WLEDs as replacements of conventional fluorescent lamps. Generally white light can be generated by a combination of blue light emitting diodes (LEDs) chips coated with the yellow-emitting phosphor. While this method, based on phosphor-down conversion, has high luminous efficiency (> 30 lm/W), but a poor color-rendering index (< 65) due to weak red emission is one of its main drawbacks. Moreover, most phosphors have no absorption band in the excitation wavelength of 440–460 nm, unlike a yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor for this type of WLED. [3] In this regard, a new class of WLEDs has been developed to guarantee excellent color-rendering properties compared to the conventional one (e.g. near-ultraviolet LEDs (near-UV LEDs) combined with multi-phase phosphors). [4] Although recent efforts to develop new LED phosphors include oxide, [5,6] oxyfluoride, [7–9] nitride, [10,11] and oxynitride host lattice [12,13] under a near-UV source have made progress, continued efforts are required to overcome relevant issues, especially, the need of high efficiencies, better thermal stability, and color properties.
The alkaline earth silicates phosphors are the most attractive candidates for the WLED applications because of their excellent thermal-, chemical stability, and their high efficiency. In particular, Eu2+-doped Sr2SiO4 phosphor has been highlighted as a yellow component for WLEDs, with an emphasis on its excellent performance. [14] Compounds with the general formula Sr2SiO4 crystallizing in the tetragonal space group Pmnb (S. G. No. 62) have been long known, and its structure has already been suggested. [15] On the basis of this structural information, we have studied the bredigite mineral, Ca14Mg2[SiO4]8 (CMS, S. G. No. 34, Pnn2), which has been an important phase in the crystal chemistry of cements, clinkers, slags, and fertilizers. Araki et al. were first to report on the powder X-ray data of CMS based on a new kind of pinwheel arrangement. [16] The study of Saalfeld et al. concluded that α-Ca2SiO4 and bredigite were distinct phases, and according to Midgley and Moore, larnite (β-Ca2SiO4) was a derivative of K2[SO4]. [17,18] The space group genealogy for bredigite yields Pmnn → Pnn2 because of tilting of the tetrahedral, which lowers the symmetry to Pnn2.
In this study, we report a green-emitting Ca14–xEuxMg2[SiO4]8 (CMS:Eu2+) phosphor, describing its structure and optical properties. In particular, we have investigated the mechanism of energy transfer in Eu2+ of critical concentration and Dexter’s theory for energy transfer. WLEDs based on a combination of an InGaN LED chip (λmax = 395 nm) with the CMS:Eu2+ phosphors have been fabricated and are discussed, along with the thermal quenching of the luminescence. In particular, a comparison with the behavior of a commercial Sr2SiO4:Eu2+ phosphor is made.
2. Experimental
Powder samples of Ca14–xEuxMg2[SiO4]8 (CMS:Eu2+) were prepared by solid-state reaction from CaCO3 (Aldrich, 99.99%), MgO (Aldrich, 99.9%), SiO2 (Aldrich, 99.9%), and Eu2O3 (Aldrich, 99.99%). The concentration of Eu was optimized. The powder reagents were intimately ground together and heated in the various temperature, ranging from 1100 to 1400°C in a reducing atmosphere of H2/N2 (5%/95%) for 4 h. Powder X-ray diffraction (XRD) data were obtained using Cu-Kα radiation (Philips X’Pert) over the angular range 10° ≤ 2θ ≤ 100° with a step size of 0.026°. The Rietveld method, as implemented in the General Structure Analysis System (GSAS) software suite, was used for crystal structure refinement. [19] Room temperature photoluminescence (PL) spectra were measured on a Hitachi F-4500 luminescence spectrophotometer scanning the wavelength range of 300 nm to 700 nm. The quantum efficiencies of the phosphor samples were measured using a quantum efficiency measurement system (Otsuka Electronics., Model QE-1000) with 400 nm excitation. Diffuse reflectance absorption spectra were recorded using a Varian Cary 500 Scan UV-visible-NIR spectrophotometer in the wavelength range of 200–600 nm.
Prototype LED devices were fabricated by applying an intimate mixture of CMS:Eu2+, red phosphor, and transparent silicone resin on a InGaN LED (λmax = 395 nm). For electroluminescence measurements, discrete LEDs grown on m-plane GaN were placed on silver headers and gold wires were attached for electrical operation. The device was then encapsulated in a phosphor/silicone mixture, with the mixture placed directly on the headers, and then cured. After packaging was completed, the device with phosphor was measured in an integrating sphere under DC bias forward conditions.
3. Results and discussion
3.1. Structural and optical properties of the Ca14–xEuxMg2[SiO4]8 phosphor
Figure 1(a) displays the unit cell representation of CMS which is an ordered Ca:Mg compound with the ideal polyhedral formula, X[12]X2[9]Y4[10]M[6][TO4]4, where X and Y are large polyhedra, M is the octahedron and T the tetrahedron. [16] The ideal upper limit of Mg content for bredigite is Ca14Mg2[SiO4]8. In the bredigite structure, MgO6 octahedra link by the tetrahedra to form chains that run parallel to the a axis. The 2a and 4c sites in this structure are occupied by Ca, and the 2b site is occupied by Mg. Si occupies the 4c site and the 4c site is also occupied by O. Eight Ca sites corresponding to polyhedra with 9-, 10- and 12-coordination are believed to substitute by Eu ions. When Eu2+ ions are incorporated into the crystal structure of CMS, Eu2+ ions may substitute for all cationic sites, that is, Ca2+, Mg2+, and Si4+. However, considering their respective ionic radius and allowed oxygen-coordination number (n), Ca2+ (1.12 Å, n = 8), Mg2+ (0.72 Å, n = 6), and Si4+ (0.26 Å, n = 4), it is difficult for Eu2+ ions (1.25 Å, n = 8) to substitute for Mg2+ and Si4+ ions. [20]
Fig. 1 (a) Unit cell representation of the crystal structure of Ca14Mg2[SiO4]8 (CMS). Black, red, blue, and orange spheres represent Ca, Mg, Si, and O atoms, respectively. The polyhedral geometry of MgO6 and SiO4 are depicted by blue and red polyhedral, respectively. (b) Rietveld refinement of the powder X-ray diffraction profile of Ca13.7Eu0.3Mg2[SiO4]8. Data (points) and fit (lines), the difference profile, and expected reflection positions are displayed.
Figure 1(b) displays the results of the Rietveld refinement of the X-ray diffraction (XRD) data profiles of Ca13.7Eu0.3Mg2[SiO4]8, obtained with Rwp = 4.25% and goodness of fit parameters (χ2) = 1.993. From the Rietveld refinement results, no impurity phases were identified in any of samples, regardless of the Eu content. Moore et al. [16] reported that a larger Ba2+ cation would prefer a Y site in the ideal chemical formula according to detailed analysis of bond distances. Thus, larger Eu2+ ions should be accommodated in Ca2+ sites from Ca5 to Ca8 among eight Ca sites. However, we are also in the process of acquiring microscopic structural properties on these, which should allow a more accurate mechanism for the preference sites by the Eu doping. The compound is orthorhombic in space group Pnn2 (S.G. #34), and the cell parameters were a = 10.915(1) Å b = 18.402(2) Å c = 6.753(3) Å. Structural parameters are listed in Table 1.
Table 1. Rietveld refinement and crystal data of Ca13.7Eu0.3Mg2[SiO4]8.
Figure 2(a) shows the excitation and emission spectra of the optimized CMS:Eu2+ (x = 0.3) under a 400 nm excitation at room temperature. In the excitation spectrum, direct excitation bands appeared from 300 nm to 450 nm. The emission spectrum is broad with a maximum at about 505 nm, which the emission band corresponds to the transition from the 4f65d excited state to the 4f7 ground state of a Eu2+ ion. [21] The full-width at half-maximum (FWHM) of the PL spectrum of CMS:Eu2+ is around 70 nm, somewhat narrower than that of typical Ce3+-activated phosphor; YAG:Ce3+ is about 104 nm. In general, emission spectra of Ce3+-activated phosphor are obtained because of transitions from the 5d excited state to the spin orbit split in the ground state, 2F5/2 and 2F7/2. [22] This explains the reason why the CMS:Eu2+ displayed narrower spectrum, unlike Ce3+-activated phosphors.
Fig. 2 (a) Excitation and emission spectra of the Ca13.7Eu0.3Mg2[SiO4]8 under 400 nm excitation source with varying Eu2+ concentration x. (b) Position of the emission maximum and (c) relative emission intensity as a function of Eu2+ substitution x.
The dependence of peak position and emission intensity on Eu2+ substitution is displayed in panels (b) and (c) of Fig. 2. With increasing Eu2+ concentration, the emission band (λex = 400 nm) shifts to a longer wavelength. This is attributed to reabsorption between Eu2+ ions rather than changes in the crystal field around Eu2+. In general, the crystal field around Eu2+ ions has been suggested as obeying: [23]
where, Dq (= Δ) is the crystal field for octahedral symmetry, R is the distance between the central ion and its ligands, Z is the charge or valence of the anion, e is the charge of the electron, and r is the radius of the d wavefunction. Thus, the crystal field splitting is not a main reason for the red shift due to the smaller ionic radius of 8-coordinate Ca2+ compared to Eu2+; 1.12 Å and 1.25 Å, respectively. From Fig. 2(c), we observe that optimum substitution of Eu2+ in CMS:Eu2+ is x = 0.3. When x exceeds 0.3, a drop in relative emission intensity is observed due to well-known concentration quenching.The critical distance for energy transfer (Rc) in Ca14–xEuxMg2[SiO4]8 can be calculated from the structural parameters with unit cell volume (V) and number of total Eu2+ sites per unit cell (N), together with the critical concentration (Xc). [24]
Here Rc corresponds to the mean separation between the nearest Eu2+ ions at the Xc. Using V = 1356.5 Å3, N = 8, and Xc = 0.3, the critical transfer distance of Eu2+ in Ca14–xEuxMg2[SiO4]8 is ≈ 12.9 Å.We also use the Dexter formula which represents the transfer of the electric dipole-dipole interaction since we are dealing with symmetry allowed transitions within Eu2+. [25] Blasse suggests the following formula for Rc: [26]
Here, P is the oscillator strength of the Eu2+ ion; E the energy of maximum spectral overlap; and ∫ fS(E)FA(E), the spectral overlap integral, which represents the product of normalized spectral shapes of emission and excitation. The values of E and ∫ fS(E)FA(E) can be derived from the spectral data in Fig. 2. For P corresponding to the broad 4f7 → 4f65d absorption band, a value of 10−2 is taken. [24,26] The value of E and ∫ fS(E)FA(E) are obtained from the spectra, which are 2.66 eV and 1.88 × 10−2 eV−1 (0.5 × height × width = 0.5 × 0.2 × 0.188), respectively. From 4, the value of Rc for the energy transfer in CMS:Eu2+ was calculated as 14.9 Å.Non-radiative energy transfer from a Eu2+ ion to another Eu2+ ion may occur by exchange interaction, radiation reabsorption, or multipole-multipole interaction. Dexter reported that exchange interaction is responsible for energy transfer for forbidden transition, and that typical critical distances are about 5 Å. [25] As a consequence, it can be infered that the mechanism of exchange interaction plays no role in energy transfer between Eu2+ ions in CMS:Eu2+ phosphor. Therefore, the energy transfer mechanism of Eu2+ of CMS:Eu2+ is the 4f7 → 4f65d allowed electric-dipole transition and the process of energy transfer should be controlled by electric multipole-multipole interaction according to Dexter theory. [25] The emission intensity (I) per activator ion follows the equation given below [27, 28]
where C is the activator concentration that is involved in self-concentration quenching, and k and β are constants for each interaction in the same excitation conditions for a given host lattice. In the case of C ≫ Xc, the non-radiative losses are attributable to multipolar transfer, and Eq. (5) can be simplified as shown below where k1 is a constant. [27, 28] Among the θ = 6, 8 or 10 are dipole-dipole, dipole-quadrupole or quadrupole-quadrupole interactions, respectively. The value of θ can be determined from the slop −θ/3 of the linear line in Fig. 3, which plots log (I/C) vs. log(C) on a logarithmic scale of I/C. The value of −θ/3 is found to be −2.16. Therefore, the value of θ is calculated as approximately 6, which indicates that the dipole-dipole interaction is the concentration quenching mechanism of Eu2+ emission in the CMS:Eu2+ phosphor.Fig. 3 Logarithm of the emission intensity per activator ion (log I/CEu) as a function of logarithm of the Eu2+ concentration (log CEu) in CMS:Eu2+ phosphor (λex = 400 nm).
Figure 4 shows the diffuse reflectance absorption spectra for Ca14–xEuxMg2[SiO4]8 (x = 0, 0.3) samples, where the Kubelka-Munk absorption coefficient (K/S) was calculated from the measured reflectance (R) following the relation: The absorbances (A) of CMS and CMS:Eu2+ were calculated from the measured reflectances (R) using the equation given below, [29]
Through the above calculation, the band gap energy (Eg) of CMS:Eu2+ was turned out as ≈ 2.56 eV. According to the Eg value of CMS:Eu2+, we could estimate easily that the excitation energy of near-UV (400 nm, 45,000 cm−1) could not transfer from valence band to conduction band directly. The undoped phase, CMS, exhibits no absorption in the wavelength range below 380 nm and accordingly, was white in body color. As a consequence, electrons in the 4f7 level of Eu2+ could be excited by near-UV light in the 4f65d level.Fig. 4 Diffuse-reflectance spectra for (a) CMS and (b) Ca13.7Eu0.3Mg2[SiO4]8 (CMS:Eu2+) under 400 nm excitation.
The quantum efficiency (QE) was measured using the excitation source of 400 nm. The QE from CMS:Eu2+ was obtained as ∼23% at room temperature, as compared with ∼85% from the reference Sr2SiO4:Eu2+ (commercial).
3.2. Thermal quenching of the Ca13.7Eu0.3Mg2[SiO4]8 phosphor
Figure 5 shows the temperature quenching characteristics of the powders of the CMS:Eu2+ sample and commercial Sr2SiO4:Eu2+ (Force4 Corp.) phosphor in the temperature range from RT to 200°C. In general, phosphors suffer a decline in conversion efficiency as the temperature of operation of the LED is increased, as a result of an increase in the non-radiative transition probability in the configurational coordinate diagram. [21, 22] As the temperature increases from RT to 150°C, the PL intensities of these phosphor decreased by 11% and 18% of the initial PL intensity, corresponding to Sr2SiO4:Eu2+ and CMS:Eu2+, as shown in Fig. 5(a). Further, in order to investigate temperature quenching characteristics, the activation energy was calculated using the Arrhenius equation, which has been suggested as obeying: [30, 31]
Here, I0 is the initial intensity; I(T) is the intensity at a given temperature T ; A is a constant; E is the activation energy for thermal quenching; and k is Boltzmann’s constant. Figure 5(b) displays a plot of ln[(I0/I)−1] vs. 1/(kT). Through the best fit using the Arrhenius equation, the activation energy (E) was obtained as 0.348 and 0.158 eV for Sr2SiO4:Eu2+ and CMS:Eu2+.Fig. 5 (a) Temperature-dependent emission intensities and (b) activation plots for thermal quenching of commercial Sr2SiO4:Eu2+ (Force4 Corp.) and Ca13.7Eu0.3Mg2[SiO4]8 phosphor using the Arrhenius equation.
3.3. WLED fabrication and electroluminescence spectra
Figure 6(a), 6(b) shows electroluminescence (EL) spectra and CIE chromaticity coordinates from a device fabricated with the CMS:Eu2+ phosphor (x = 0.3) and red phosphor (commercial) on an InGaN LED (λmax = 395 nm) under different forward bias currents in the range of 10 mA to 70 mA as indicated. The measured luminous efficacy was 10 lm/W to 15 lm/W depending on the current. From observed CIE chromaticity coordinates (0.33, 0.38) and color temperature 5500 K at 20 mA, we obtain color rendering index (Ra) of about 91. The full set of the Ra and the average Ra are listed in Table 2.
Fig. 6 Luminescence of the InGaN LED + phosphor, under different forward bias currents (indicated): (a) InGaN (λmax = 395 nm) + CMS:Eu2+ + red phosphor. The inset is a photograph of the actual device under self-illumination. (b) CIE chromatic coordinates of the device under different forward bias currents [as in panel (a) and (b)]. The Planckian locus line and the points corresponding to color temperatures of 3500 K and 6500 K are indicated.
Table 2. Full set of 9 components of the Ras and the average Ra of a UV LED pumped with CMS:Eu2+ + red phosphor.
4. Conclusion
We have prepared a green-emitting phosphor, Ca14–xEuxMg2[SiO4]8, and investigated its structural and luminescent properties by Rietveld refinement and optical measurements. The critical transfer distance for this phosphor is 12 Å and 14.9 Å and the energy transfer between Eu2+ ions was found to be an electric dipole-dipole interaction. Applying CMS:Eu2+ (x = 0.3) on InGaN LEDs (λmax = 395 nm), we obtain WLEDs outputting 13 lm/W at 20 mA, with a 91 Ra and 5500 K color temperature.
Acknowledgments
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology ( 2011-0009611). And the authors also appreciate the financial support from the Joint Research Project.
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