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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 6 — Mar. 12, 2012
  • pp: 6248–6257
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Bredigite-structure orthosilicate phosphor as a green component for white LED: the structural and optical properties

Kyoung Hwa Lee, Seung Hyok Park, Ho Shin Yoon, Yong-Il Kim, Ho Gyeom Jang, and Won Bin Im  »View Author Affiliations


Optics Express, Vol. 20, Issue 6, pp. 6248-6257 (2012)
http://dx.doi.org/10.1364/OE.20.006248


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Abstract

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 OSA

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

1. S. Nakamura, “Current status of GaN-based solid-state lighting,” MRS Bulletin 34, 101–107 (2009). [CrossRef]

, 2

2. J. S. Speck and S. F. Chichibu, “Nonpolar and semipolar group III nitride-based materials,” MRS Bulletin 34, 304–312 (2009). [CrossRef]

] 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

3. E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308, 1274–1278 (2005). [CrossRef] [PubMed]

] 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

4. Y. Uchida and T. Taguchi, “Lighting theory and luminous characteristics of white light-emitting diodes,” Opt. Eng. 12, 124003–124009 (2005). [CrossRef]

] Although recent efforts to develop new LED phosphors include oxide, [5

5. Y. Shimomura, T. Kurushima, and N. Kijima, “Photoluminescence and crystal structure of green-emitting phosphor CaSc2O4:Ce3+,” J. Electrochem. Soc. 154, J234–J238 (2007). [CrossRef]

,6

6. A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava, R. G. Chandran, and M. V. Shankar, “Crystal chemistry and luminescence of Ce3+-doped Lu2CaMg2(Si, Ge)3O12 and its use in LED based lighting,” Chem. Mater. 18, 3314–3322 (2006). [CrossRef]

] oxyfluoride, [7

7. W. B. Im, S. Brinkley, J. Hu, A. Mikailovsky, S. P. DenBaars, and R. Seshadri, “Sr2.975–xBaxCe0.025AlO4F: a highly efficient green-emitting oxyfluoride phosphor for solid state white lighting,” Chem. Mater. 22, 2842–2849 (2010). [CrossRef]

9

9. W. B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikailovsky, J. Hu, B. F. Chmelka, S. P. DenBaars, and R. Seshadri, “Efficient and color-tunable xxyfluoride solid solution phosphors for solid-state white lighting,” Adv. Mater. 23, 2300–2305 (2011). [CrossRef] [PubMed]

] nitride, [10

10. V. Bachmann, C. Ronda, O. Oeckler, and W. Schnick, “Color point tuning for (Sr, Ca, Ba) Si2O2N2:Eu2+ for white light LEDs,” Chem. Mater. 21, 316–325 (2009). [CrossRef]

,11

11. C. Hecht, F. Stadler, P. J. Schmidt, J. S. auf der Gunne, V. Baumann, and W. Schnick, “SrAlSi4N7:Eu2+ a nitridoalumosilicate phosphor for warm white light (pc) LEDs with edge-sharing tetrahedra,” Chem. Mater. 21, 1595–1601 (2009). [CrossRef]

] and oxynitride host lattice [12

12. Y. Q. Li, A. C. A. Delsing, G. de With, and H. T. Hintzen, “Luminescence properties of Eu2+-activated alkaline-earth silicon-oxynitride MSi2O2–δ N2+2/3δ (M = Ca, Sr, Ba): a promising class of novel LED conversion phosphors,” Chem. Mater. 17, 3242–3248 (2005). [CrossRef]

,13

13. R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, and M. Mitomo, “Eu2+-doped Ca–α–SiAlON: A yellow phosphor for white light-emitting diodes,” Appl. Phys. Lett. 84, 5404–5406 (2004). [CrossRef]

] 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

14. J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, “White light-emitting diodes of GaN-based Sr2SiO4:Eu and the luminescent properties,” Appl. Phys. Lett. 82, 683–685 (2003). [CrossRef]

] 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

15. G. G. M. Catti, G. Ivaldi, and G. Zanini, “The βα phase transition of Sr2SiO4. I. order-disorder in the structure of the α form at 383 K,” Acta Crystallogr. B , 39, 674–679 (1983). [CrossRef]

] 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

16. P. B. Moore and T. Araki, “The crystal structure of bredigite and the genealogoy of some alkaline earth orthosilicates,” Am. Mineral. 61, 74–87 (1976).

] 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

17. P. B. Moore, “Bracelets and pinwheels: a topological-geometrical approach to the calcium orthosilicate and alkali sulfate structures,” Am. Mineral. 58, 32–42 (1973).

,18

18. C. M. Midgley, “The crystal structure of dicalcium silicate,” Acta Crystallogr. 5, 307–312 (1952). [CrossRef]

] The space group genealogy for bredigite yields PmnnPnn2 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

19. A. C. Larson and R. B. Von Dreele, “General structure analysis system (GSAS),” Los Alamos National Laboratory Report LAUR , 86–748 (1994).

] 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

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

16. P. B. Moore and T. Araki, “The crystal structure of bredigite and the genealogoy of some alkaline earth orthosilicates,” Am. Mineral. 61, 74–87 (1976).

] 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.

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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

21. S. Shionoya and W. M. Yen, Phosphor Handbook (CRC Press, New York, 1998).

] 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

22. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994). [CrossRef]

] 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

23. P. D. Rack and P. H. Holloway, “Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescent ZnS:Mn by codoping with potassium chloride,” Mater. Sci. Eng. Rev. 21, 171–219 (1998). [CrossRef]

]
Δ=Dq=Ze2r46R5
(1)
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

24. G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A. 24, 131–144 (1969).

]
Rc2(3V4πXcN)1/3
(2)
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

25. D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21, 836–850, (1953). [CrossRef]

] Blasse suggests the following formula for Rc: [26

26. G. Blasse, “Energy transfer between inequivalent Eu2+ ions,” J. Solid State Chem. 62, 207–211 (1986). [CrossRef]

]
PSA=2πh|S,A*|HSA|S*,A|2gs(E)gA(E)dE
(3)
Rc6=0.63×10284.8×1016PE4fS(E)FA(E)dE
(4)
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

24. G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A. 24, 131–144 (1969).

,26

26. G. Blasse, “Energy transfer between inequivalent Eu2+ ions,” J. Solid State Chem. 62, 207–211 (1986). [CrossRef]

] 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

25. D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21, 836–850, (1953). [CrossRef]

] 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

25. D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21, 836–850, (1953). [CrossRef]

] The emission intensity (I) per activator ion follows the equation given below [27

27. L. G. Van Uitert, “Characterization of energy transfer interactions between rare earth ions,” J. Electrochem. Soc. 114, 1048–1053 (1967). [CrossRef]

, 28

28. L. Ozawa, Cathodoluminescence: Theory and Applications (VCH, 1990).

]
Ic=k[1+βCθ3]
(5)
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 CXc, the non-radiative losses are attributable to multipolar transfer, and Eq. (5) can be simplified as shown below
Ic=k1βCθ3
(6)
where k1 is a constant. [27

27. L. G. Van Uitert, “Characterization of energy transfer interactions between rare earth ions,” J. Electrochem. Soc. 114, 1048–1053 (1967). [CrossRef]

, 28

28. L. Ozawa, Cathodoluminescence: Theory and Applications (VCH, 1990).

] 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

29. P. Kubelka and F. Munk, “Ein beitrag zur optik der farbanstriche,” Z. Tech. Phys. 12, 593–601 (1931).

]
KS=(1R)22R
(7)
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

21. S. Shionoya and W. M. Yen, Phosphor Handbook (CRC Press, New York, 1998).

, 22

22. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994). [CrossRef]

] 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

30. S. Bhushan and M. V. Chukichev, “Temperature dependent studies of cathodoluminescence of green band of ZnO crystals,” J. Mater. Sci. Lett. 7, 319–321 (1988). [CrossRef]

, 31

31. Y. Chen, B. Liu, C. Shi, G. Ren, and G. Zimmerer, “The temperature effect of Lu2SiO5:Ce3+ luminescence,” Nucl. Instrum. Methods Phys. Res. A 537, 31–35 (2005). [CrossRef]

]
I(T)=I01+Aexp(EkT)
(8)
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.

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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.

References and links

1.

S. Nakamura, “Current status of GaN-based solid-state lighting,” MRS Bulletin 34, 101–107 (2009). [CrossRef]

2.

J. S. Speck and S. F. Chichibu, “Nonpolar and semipolar group III nitride-based materials,” MRS Bulletin 34, 304–312 (2009). [CrossRef]

3.

E. F. Schubert and J. K. Kim, “Solid-state light sources getting smart,” Science 308, 1274–1278 (2005). [CrossRef] [PubMed]

4.

Y. Uchida and T. Taguchi, “Lighting theory and luminous characteristics of white light-emitting diodes,” Opt. Eng. 12, 124003–124009 (2005). [CrossRef]

5.

Y. Shimomura, T. Kurushima, and N. Kijima, “Photoluminescence and crystal structure of green-emitting phosphor CaSc2O4:Ce3+,” J. Electrochem. Soc. 154, J234–J238 (2007). [CrossRef]

6.

A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava, R. G. Chandran, and M. V. Shankar, “Crystal chemistry and luminescence of Ce3+-doped Lu2CaMg2(Si, Ge)3O12 and its use in LED based lighting,” Chem. Mater. 18, 3314–3322 (2006). [CrossRef]

7.

W. B. Im, S. Brinkley, J. Hu, A. Mikailovsky, S. P. DenBaars, and R. Seshadri, “Sr2.975–xBaxCe0.025AlO4F: a highly efficient green-emitting oxyfluoride phosphor for solid state white lighting,” Chem. Mater. 22, 2842–2849 (2010). [CrossRef]

8.

W. B. Im, Y. Fourré, S. Brinkley, J. Sonoda, S. Nakamura, S. P. DenBaars, and R. Seshadri, “Substitution of oxygen by fluorine in the GdSr2AlO5:Ce3+ phosphors: Gd1–xSr2+xAlO5–xFx solid solutions for solid state white lighting,” Opt. Express 17, 22673–22679 (2009). [CrossRef]

9.

W. B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikailovsky, J. Hu, B. F. Chmelka, S. P. DenBaars, and R. Seshadri, “Efficient and color-tunable xxyfluoride solid solution phosphors for solid-state white lighting,” Adv. Mater. 23, 2300–2305 (2011). [CrossRef] [PubMed]

10.

V. Bachmann, C. Ronda, O. Oeckler, and W. Schnick, “Color point tuning for (Sr, Ca, Ba) Si2O2N2:Eu2+ for white light LEDs,” Chem. Mater. 21, 316–325 (2009). [CrossRef]

11.

C. Hecht, F. Stadler, P. J. Schmidt, J. S. auf der Gunne, V. Baumann, and W. Schnick, “SrAlSi4N7:Eu2+ a nitridoalumosilicate phosphor for warm white light (pc) LEDs with edge-sharing tetrahedra,” Chem. Mater. 21, 1595–1601 (2009). [CrossRef]

12.

Y. Q. Li, A. C. A. Delsing, G. de With, and H. T. Hintzen, “Luminescence properties of Eu2+-activated alkaline-earth silicon-oxynitride MSi2O2–δ N2+2/3δ (M = Ca, Sr, Ba): a promising class of novel LED conversion phosphors,” Chem. Mater. 17, 3242–3248 (2005). [CrossRef]

13.

R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, and M. Mitomo, “Eu2+-doped Ca–α–SiAlON: A yellow phosphor for white light-emitting diodes,” Appl. Phys. Lett. 84, 5404–5406 (2004). [CrossRef]

14.

J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, “White light-emitting diodes of GaN-based Sr2SiO4:Eu and the luminescent properties,” Appl. Phys. Lett. 82, 683–685 (2003). [CrossRef]

15.

G. G. M. Catti, G. Ivaldi, and G. Zanini, “The βα phase transition of Sr2SiO4. I. order-disorder in the structure of the α form at 383 K,” Acta Crystallogr. B , 39, 674–679 (1983). [CrossRef]

16.

P. B. Moore and T. Araki, “The crystal structure of bredigite and the genealogoy of some alkaline earth orthosilicates,” Am. Mineral. 61, 74–87 (1976).

17.

P. B. Moore, “Bracelets and pinwheels: a topological-geometrical approach to the calcium orthosilicate and alkali sulfate structures,” Am. Mineral. 58, 32–42 (1973).

18.

C. M. Midgley, “The crystal structure of dicalcium silicate,” Acta Crystallogr. 5, 307–312 (1952). [CrossRef]

19.

A. C. Larson and R. B. Von Dreele, “General structure analysis system (GSAS),” Los Alamos National Laboratory Report LAUR , 86–748 (1994).

20.

R. D. Shannon, “Revised effective ionic-radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr . B32, 751–767 (1976).

21.

S. Shionoya and W. M. Yen, Phosphor Handbook (CRC Press, New York, 1998).

22.

G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994). [CrossRef]

23.

P. D. Rack and P. H. Holloway, “Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescent ZnS:Mn by codoping with potassium chloride,” Mater. Sci. Eng. Rev. 21, 171–219 (1998). [CrossRef]

24.

G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A. 24, 131–144 (1969).

25.

D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys. 21, 836–850, (1953). [CrossRef]

26.

G. Blasse, “Energy transfer between inequivalent Eu2+ ions,” J. Solid State Chem. 62, 207–211 (1986). [CrossRef]

27.

L. G. Van Uitert, “Characterization of energy transfer interactions between rare earth ions,” J. Electrochem. Soc. 114, 1048–1053 (1967). [CrossRef]

28.

L. Ozawa, Cathodoluminescence: Theory and Applications (VCH, 1990).

29.

P. Kubelka and F. Munk, “Ein beitrag zur optik der farbanstriche,” Z. Tech. Phys. 12, 593–601 (1931).

30.

S. Bhushan and M. V. Chukichev, “Temperature dependent studies of cathodoluminescence of green band of ZnO crystals,” J. Mater. Sci. Lett. 7, 319–321 (1988). [CrossRef]

31.

Y. Chen, B. Liu, C. Shi, G. Ren, and G. Zimmerer, “The temperature effect of Lu2SiO5:Ce3+ luminescence,” Nucl. Instrum. Methods Phys. Res. A 537, 31–35 (2005). [CrossRef]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(230.3670) Optical devices : Light-emitting diodes
(250.5230) Optoelectronics : Photoluminescence

ToC Category:
Optical Devices

History
Original Manuscript: January 17, 2012
Revised Manuscript: February 18, 2012
Manuscript Accepted: February 20, 2012
Published: March 2, 2012

Citation
Kyoung Hwa Lee, Seung Hyok Park, Ho Shin Yoon, Yong-Il Kim, Ho Gyeom Jang, and Won Bin Im, "Bredigite-structure orthosilicate phosphor as a green component for white LED: the structural and optical properties," Opt. Express 20, 6248-6257 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-6-6248


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References

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  5. Y. Shimomura, T. Kurushima, and N. Kijima, “Photoluminescence and crystal structure of green-emitting phosphor CaSc2O4:Ce3+,” J. Electrochem. Soc.154, J234–J238 (2007). [CrossRef]
  6. A. A. Setlur, W. J. Heward, Y. Gao, A. M. Srivastava, R. G. Chandran, and M. V. Shankar, “Crystal chemistry and luminescence of Ce3+-doped Lu2CaMg2(Si, Ge)3O12 and its use in LED based lighting,” Chem. Mater.18, 3314–3322 (2006). [CrossRef]
  7. W. B. Im, S. Brinkley, J. Hu, A. Mikailovsky, S. P. DenBaars, and R. Seshadri, “Sr2.975–xBaxCe0.025AlO4F: a highly efficient green-emitting oxyfluoride phosphor for solid state white lighting,” Chem. Mater.22, 2842–2849 (2010). [CrossRef]
  8. W. B. Im, Y. Fourré, S. Brinkley, J. Sonoda, S. Nakamura, S. P. DenBaars, and R. Seshadri, “Substitution of oxygen by fluorine in the GdSr2AlO5:Ce3+ phosphors: Gd1–xSr2+xAlO5–xFx solid solutions for solid state white lighting,” Opt. Express17, 22673–22679 (2009). [CrossRef]
  9. W. B. Im, N. George, J. Kurzman, S. Brinkley, A. Mikailovsky, J. Hu, B. F. Chmelka, S. P. DenBaars, and R. Seshadri, “Efficient and color-tunable xxyfluoride solid solution phosphors for solid-state white lighting,” Adv. Mater.23, 2300–2305 (2011). [CrossRef] [PubMed]
  10. V. Bachmann, C. Ronda, O. Oeckler, and W. Schnick, “Color point tuning for (Sr, Ca, Ba) Si2O2N2:Eu2+ for white light LEDs,” Chem. Mater.21, 316–325 (2009). [CrossRef]
  11. C. Hecht, F. Stadler, P. J. Schmidt, J. S. auf der Gunne, V. Baumann, and W. Schnick, “SrAlSi4N7:Eu2+ a nitridoalumosilicate phosphor for warm white light (pc) LEDs with edge-sharing tetrahedra,” Chem. Mater.21, 1595–1601 (2009). [CrossRef]
  12. Y. Q. Li, A. C. A. Delsing, G. de With, and H. T. Hintzen, “Luminescence properties of Eu2+-activated alkaline-earth silicon-oxynitride MSi2O2–δ N2+2/3δ (M = Ca, Sr, Ba): a promising class of novel LED conversion phosphors,” Chem. Mater.17, 3242–3248 (2005). [CrossRef]
  13. R. J. Xie, N. Hirosaki, K. Sakuma, Y. Yamamoto, and M. Mitomo, “Eu2+-doped Ca–α–SiAlON: A yellow phosphor for white light-emitting diodes,” Appl. Phys. Lett.84, 5404–5406 (2004). [CrossRef]
  14. J. K. Park, M. A. Lim, C. H. Kim, H. D. Park, J. T. Park, and S. Y. Choi, “White light-emitting diodes of GaN-based Sr2SiO4:Eu and the luminescent properties,” Appl. Phys. Lett.82, 683–685 (2003). [CrossRef]
  15. G. G. M. Catti, G. Ivaldi, and G. Zanini, “The β ⇌ α phase transition of Sr2SiO4. I. order-disorder in the structure of the α form at 383 K,” Acta Crystallogr. B, 39, 674–679 (1983). [CrossRef]
  16. P. B. Moore and T. Araki, “The crystal structure of bredigite and the genealogoy of some alkaline earth orthosilicates,” Am. Mineral.61, 74–87 (1976).
  17. P. B. Moore, “Bracelets and pinwheels: a topological-geometrical approach to the calcium orthosilicate and alkali sulfate structures,” Am. Mineral.58, 32–42 (1973).
  18. C. M. Midgley, “The crystal structure of dicalcium silicate,” Acta Crystallogr.5, 307–312 (1952). [CrossRef]
  19. A. C. Larson and R. B. Von Dreele, “General structure analysis system (GSAS),” Los Alamos National Laboratory Report LAUR, 86–748 (1994).
  20. R. D. Shannon, “Revised effective ionic-radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. B32, 751–767 (1976).
  21. S. Shionoya and W. M. Yen, Phosphor Handbook (CRC Press, New York, 1998).
  22. G. Blasse and B. C. Grabmaier, Luminescent Materials (Springer, Berlin, 1994). [CrossRef]
  23. P. D. Rack and P. H. Holloway, “Improved brightness, efficiency, and stability of sputter deposited alternating current thin film electroluminescent ZnS:Mn by codoping with potassium chloride,” Mater. Sci. Eng. Rev.21, 171–219 (1998). [CrossRef]
  24. G. Blasse, “Energy transfer in oxidic phosphors,” Phys. Lett. A.24, 131–144 (1969).
  25. D. L. Dexter, “A theory of sensitized luminescence in solids,” J. Chem. Phys.21, 836–850, (1953). [CrossRef]
  26. G. Blasse, “Energy transfer between inequivalent Eu2+ ions,” J. Solid State Chem.62, 207–211 (1986). [CrossRef]
  27. L. G. Van Uitert, “Characterization of energy transfer interactions between rare earth ions,” J. Electrochem. Soc.114, 1048–1053 (1967). [CrossRef]
  28. L. Ozawa, Cathodoluminescence: Theory and Applications (VCH, 1990).
  29. P. Kubelka and F. Munk, “Ein beitrag zur optik der farbanstriche,” Z. Tech. Phys.12, 593–601 (1931).
  30. S. Bhushan and M. V. Chukichev, “Temperature dependent studies of cathodoluminescence of green band of ZnO crystals,” J. Mater. Sci. Lett.7, 319–321 (1988). [CrossRef]
  31. Y. Chen, B. Liu, C. Shi, G. Ren, and G. Zimmerer, “The temperature effect of Lu2SiO5:Ce3+ luminescence,” Nucl. Instrum. Methods Phys. Res. A537, 31–35 (2005). [CrossRef]

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