Highly efficient pc-LED phosphors Sr 1-xBaxSi 2O2N2:Eu2+ (0≤x≤1) - Crystal structures and luminescence properties revisited

Critical Reviews in Solid State and Materials Sciences

Volumn 39, Issue 3, 2014, Pages 215-229

  Seibald, Markus ^a   Rosenthal, Tobias ^a   Oeckler, Oliver ^a,b   Schnick, Wolfgang ^a  

^a UNIVERSITY OF MUNICH   (Germany)

^b UNIVERSITY OF LEIPZIG   (Germany)

Author keywords

oxonitridosilicates; solid state lighting; Structure property relation

Indexed keywords

CRYSTAL STRUCTURE; LUMINESCENCE; LUMINESCENCE OF INORGANIC SOLIDS; PHOSPHORS; SILICATES; SILICON; X RAY DIFFRACTION;

ELECTRON PHONON COUPLINGS; GREEN-YELLOW SPECTRAL REGION; LUMINESCENCE PROPERTIES; OXONITRIDOSILICATES; REAL-STRUCTURE EFFECTS; SOLID STATE LIGHTING; STRUCTURE DETERMINATION; STRUCTURE PROPERTY RELATION;

LIGHT EMITTING DIODES;

EID: 84897414384     PISSN: 10408436     EISSN: 15476561     Source Type: Journal    
DOI: 10.1080/10408436.2013.863175     Document Type: Review

References (94)

1. C. Feldmann, Technical revolution underway-Nitride-based phosphors for LED application, Z. Anorg. Allg. Chem. 638, 2169 (2012).
2. M. Mikami, H.Watanabe, K. Uheda, S. Shimooka, Y. Shimomura, T. Kurushima, and N. Kijima, New Phosphors for white LEDs: Material design concepts, IOPConf. Ser.Mater. Sci. Eng. 1, 012002 (2009).
3. S. Ye, F. Xiao, Y. X. Pan, Y. Y. Ma, and Q. Y. Zhang, Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties, Mater. Sci. Eng. R, 71, 1 (2010).
4. C. C. Lin and R.-S. Liu, Advances in Phosphors for Light-emitting Diodes, J. Phys. Chem. Lett., 2, 1268 (2011).
5. J. M. Phillips, M. E. Coltrin, M. H. Crawford, A. J. Fischer, M. R. Krames, R. Mueller-Mach, G. O. Mueller, Y. Ohno, L. E. S. Rohwer, J. A. Simmons, and J. Y. Tsao, Research challenges to ultra-efficient inorganic solid-state lighting, Laser Photon. Rev. 1, 307 (2007).
6. C. Sommer, P. Hartmann, P. Pachler, H. Hoschopf, and F. P.Wenzl, White light quality of phosphor converted light-emitting diodes: A phosphor materials perspective of view, J. Alloys Compd. 520, 146 (2012).
7. R.-J. Xie, N. Hirosaki, Y. Li, and T. Takeda, Rare-earth activated nitride phosphors: Synthesis, luminescence and applications, Materials 3, 3777 (2010).
8. D. P. Dutta, and A. K. Tyagi, Inorganic phosphor materials for solid state white light generation, Solid State Phenomena 155, 113 (2009).
9. R.-J. Xie, and N. Hirosaki, Silicon-based oxynitride and nitride phosphors for white LEDs-A review, Sci. Technol. Adv. Mater. 8, 588 (2007).
10. R.-J. Xie, N. Hirosaki, T. Takeda, and T. Suehiro, On the Performance Enhancement of Nitride Phosphors as Spectral Conversion Materials in Solid State Lighting, ECS J. Solid State Sci. Technol. 2, R3031 (2013).
11. C.-C. Yang, C.-M. Lin, Y.-J. Chen, Y.-T. Wu, and S.-R. Chuang, Highly stable three-band white light from an InGaN-based blue light-emitting diode chip precoated with (oxy)nitride green/red phosphors, Appl. Phys. Lett., 90, 123503 (2007).
12. R.-J.Xie,N.Hirosaki, and T. Takeda, Highly ReliableWhite LEDs using nitride phosphors, J. Kor. Ceram. Soc. 49, 375 (2012).
13. H. A. H?oppe, Recent developments in the field of inorganic phosphors, Angew. Chem. 121, 3626 (2009); Angew. Chem. Int. Ed. 48, 3572 (2009).
14. X.-H. He, N. Lian, J.-H. Sun, and M.-Y. Guan, Dependence of luminescence properties on composition of rare-earth activated (oxy)nitrides phosphors for white-LEDs applications, J. Mater. Sci. 44, 4763 (2009).
15. R.-J. Xie, and H. T. (Bert) Hintzen, Optical Properties of (Oxy)Nitride Materials: A Review, J. Amer. Ceram. Soc. 96, 665 (2013).
16. G. Blasse and B. C. Grabmaier, Luminescent Materials, 1st ed., Springer Berlin Heidelberg, Germany (1994).
17. C. R. Ronda, Luminescence, 1st ed.,Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany (2007).
18. R.-J. Xie, Y. Q. Li, N. Hirosaki, and H. Yamamoto, Nitride Phosphors and Solid-State Lighting, Taylor&Francis, Inc. Boca Raton, FL, USA (2011).
19. P. F. Smet,A. B. Parmentier, andD. Poelman, Selecting conversion phosphors for white light-emitting diodes, J. Electrochem. Soc. 158, R37 (2011).
20. M. Zeuner, S. Pagano, andW. Schnick, Nitridosilicates and oxonitridosilicates: From ceramic materials to structural and functional diversity, Angew. Chem. 123, 7898 (2011); Angew. Chem. Int. Ed. 50, 7754 (2011).
21. V. Bachmann, C. Ronda,O. Oeckler,W. Schnick, and A. Meijerink, Color point tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+, Chem. Mater. 21, 316 (2009).
22. Y. Q. Li, A. C. A. Delsing, G. De With, and H. T. Hintzen, Luminescence properties of Eu2+-activated alkaline-earth siliconoxynitride MSi2O2-N2+2/3 , Chem. Mater. 17, 3242 (2005).
23. B. Y. Han, and K.-S. Sohn, Ternary combinatorial library of (Sr,Ba,Ca)Si2N2O2 phosphors in terms of photoluminescence and color chromaticity, Electrochem. Solid-State Lett. 13, J62 (2010).
24. H. A. H?oppe, F. Stadler, O. Oeckler, andW. Schnick, Ca[Si2O2N2]-A novel layer silicate, Angew. Chem. 116, 5656 (2004); Angew. Chem. Int. Ed. 43, 5540 (2004).
25. G. Pilet, H. A. H?oppe, W. Schnick, and S. Esmaeilzadeh, Crystal Structure and Mechanical Properties of SrSi7N10, Solid State Sci. 7, 391 (2005).
26. J. Grins, Z. Shen, S. Esmaeilzadeh, and P. Berastegui, The structures of the Ce and La N-phases RE3Si8-xAlxN11-xO4+x (x 1.75 for RE = Ce, x 1.5 for RE = La), determined by single-crystal X-ray and time-of-flight neutron powder diffraction, respectively, J. Mater. Chem. 11, 2358 (2001).
27. J. A. Kechele, O. Oeckler, P. J. Schmidt, and W. Schnick, (Sr1-xCax)(11+16y-25z)/2(Si1-yAly)16(N1-zOz)25 (x 0.24, y 0.18, z 0.19)-A novel sialon with a highly condensed silicate framework, Eur. J. Inorg. Chem. 3326 (2009).
28. C. Hecht, F. Stadler, P. J. Schmidt, J. Schmedt auf Der G?unne, V. Baumann, and W. Schnick, SrAlSi4N7:Eu2+-A nitridoalumosilicate phosphor for warm white light (pc)LEDs with edge-sharing tetrahedra, Chem. Mater. 21, 1595 (2009).
29. J. A. Kechele, C. Hecht, O. Oeckler, J. Schmedt auf Der G?unne, P. J. Schmidt, andW. Schnick, Ba2AlSi5N9-A new host lattice for Eu2+-doped luminescent materials comprising a nitridoalumosilicate framework with corner-and edge-sharing tetrahedra, Chem. Mater. 21, 1288 (2009).
30. O. Oeckler, J. A. Kechele, H. Koss, P. J. Schmidt, and W. Schnick, Sr5Al5+xSi21-xN35-xO2+x:Eu2+ (x0)-Anovel green phosphor for white light pcLEDs with disordered intergrowth structure, Chem. Eur. J. 15, 5311 (2009).
31. H. Yamane, S. Shimooka, and K. Uheda, Synthesis, crystal structure and photoluminescence of a new Eu-doped Sr containing sialon (Sr0.94Eu0.06)(Al0. 3Si0.7)4(N0.8O0.2)6, J. Solid, State Chem. 190, 264 (2012).
32. R. H?ubenthal, Madlung Part of Lattice Energy, version 4, University of Gie?en, Germany (1993).
33. R. Hoppe, Madelung Constants, Angew. Chem. 78, 52 (1966); Angew. Chem. Int. Ed. 5, 95 (1966).
34. R. Hoppe, The coordination number-an "inorganic chameleon", Angew. Chem. 82, 7 (1970); Angew. Chem. Int. Ed. 9, 25 (1970).
35. W. H. Zhu, P. L. Wang, W. Y. Sun, and D. S. Yan, Phase relationships in the Sr-Si-O-N system, J. Mater. Sci. Lett. 13, 560 (1994).
36. R. Mueller-Mach, G. Mueller, M. R. Krames, H. A. H?oppe, F. Stadler, W. Schnick, T. Juestel, and P. Schmidt, Highly efficient all-nitride phosphor-converted white light emitting diode, Phys. Status Solidi A 202, 1727 (2005).
37. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomie distances in halides and chaleogenides, Acta Crystallogr. Sect. A: (Sr,Ca,Ba)Si2O2N2:Eu2+ Found. Crystallogr. 32, 751 (1976).
38. Y. Q. Li, K. V. Ramanujachary, S. E. Lofland, G. De With, and H. T. Hintzen, Optical and magnetic properties of EuSi2O2N2, J. Mater. Res. 21, 396 (2006).
39. F. Stadler, O. Oeckler, H. A. H?oppe, M. H. M?oller, R. P?ottgen, B. D. Mosel, P. Schmidt, V. Duppel, A. Simon, and W. Schnick, Crystal Structure, Physical Properties and HRTEM investigation of the new oxonitridosilicate EuSi2O2N2, Chem. Eur. J. 12, 6984 (2006).
40. O. Oeckler, F. Stadler, T. Rosenthal, andW. Schnick, Real structure of SrSi2O2N2, Solid State Sci. 9, 205 (2007).
41. R.-S. Liu, Y.-H. Liu, N. C. Bagkar, and S.-F. Hu, Enhanced luminescence of SrSi2O2N2:Eu2+ phosphors by codoping with Ce3+, Mn2+, and Dy3+ ions, Appl. Phys. Lett. 91, 061119 (2007).
42. M. Seibald, T. Rosenthal, O. Oeckler, C. Maak, A. T?ucks, P. J. Schmidt, D. Wiechert, and W. Schnick, New polymorph of the highly efficient LED-phosphor SrSi2O2N2:Eu2+ polytypism of a layered oxonitridosilicate, Chem. Mater. 25, 1852 (2013).
43. M. Seibald, O. Oeckler, V. R. Celinski, P. J. Schmidt, A. T?ucks, and W. Schnick, Real structure and diffuse scattering of Sr0.5Ba0.5Si2O2N2:Eu2+-A highly efficient yellow phosphor for pc-LEDs, Solid State Sci. 13, 1769 (2011).
44. G. Anoop, I. H. Cho, D. W. Suh, and J. S. Yoo, Luminescence characteristics of Sr1-xBaxSi2O2N2:Eu2+ phosphors for white light emitting diodes, Phys. Status Solidi A 209, 2635 (2012).
45. J. A. Kechele, O. Oeckler, F. Stadler, and W. Schnick, Structure elucidation of BaSi2O2N2-A host lattice for rare-earth doped luminescent materials in phosphor-converted (pc)-LEDs, Solid State Sci. 11, 537 (2009).
46. L. Wang, H. Ni, Q. Zhang, and F. Xiao, Electronic structure and linear optical property of BaSi2N2O2 crystal, J. Alloys Compd. 509, 10203 (2011).
47. B.-G.Yun, T. Horikawa,H. Hanzawa, and K. Machida, Preparation and luminescence properties of single-phase BaSi2O2N2:Eu2+, a bluish-green phosphor for white light-emitting diodes, J. Electrochem. Soc. 157, J364 (2010).
48. J. Botterman, K. Van Den Eeckhout, I. De Baere, D. Poelman, and P. F. Smet, Mechanoluminescence in BaSi2O2N2:Eu, Acta Mater. 60, 5494 (2012).
49. I. H. Cho, G. Anoop, D. W. Suh, S. J. Lee, and J. S. Yoo, On the stability and reliability of Sr1-xBaxSi2O2N2:Eu2+ phosphors for white LED applications, Opt. Mater. Express 2, 1292 (2012).
50. B.-G. Yun, Y. Miyamoto, and H. Yamamoto, Luminescence properties of (Sr1-uBau)Si2O2N2:Eu2+, yellow or orange phosphors for white LEDs, synthesized with (Sr1-uBau)2SiO4:Eu2+ as a precursor, J. Electrochem. Soc. 154, J320 (2007).
51. B. Lei, K. Machida, T. Horikawa, and H. Hanzawa, Preparation of (Sr0.5Ba0.5)Si2N2O2:Eu2+ phosphor and its luminescence properties, Chem. Lett. 40, 140 (2011).
52. M. Seibald, T. Rosenthal, O. Oeckler, F. Fahrnbauer, A. T?ucks, P. J. Schmidt, andW. Schnick, Unexpected luminescence properties of Sr0.25Ba0.75Si2O2N2:Eu2+-A narrow blue emitting oxonitridosilicate with cation ordering, Chem. Eur. J. 18, 13446 (2012).
53. B.-G. Yun, T. Horikawa, H. Hanzawa, and K. Machida, Effect of oxygen content in raw materials on the synthesis of a single-phase EuSi2O2N2, J. Electrochem. Soc. 157, J97 (2010).
54. S. D. Jee, K. S. Choi, and J. S. Kim, Luminescence properties of (Sr0.95-x-yMgxYy)Si2O2-yN2+y:Eu2+ 0.05 for novel LED conversion phosphors, Met. Mater. Int. 17, 655 (2011).
55. X. Yang, H. Song, L. Yang, and X. Xu, Reaction mechanism of SrSi2O2N2:Eu2+ phosphor prepared by a direct silicon nitridation method, J. Amer. Ceram. Soc. 94, 28 (2011).
56. K. Y. Jung, and J. H. Seo, Preparation of fine-sized SrSi2O2- N2+2/3:Eu2+ phosphor by spray pyrolysis and its luminescent characteristics, Electrochem. Solid-State Lett. 11, J64 (2008).
57. Y. Zhang, X. Liu, B. Lei, H. Wang, and Q. Sun, Effect of C3N6H6 on luminescent properties of SrSi2N2O2:Eu2+ yellow phosphors prepared by microwave reaction method, Energy Procedia 16, 391 (2012).
58. K. Machida, Synthesis and materials design for heteroanion compounds, IOP Conf. Ser. Mater. Sci. Eng. 18, 052001 (2011).
59. B.-G. Yun, K. Machida, and H. Yamamoto, Preparation and luminescence properties of SrSi2O2N2:Eu2+ phosphors for white LEDs, J. Ceram. Soc. Jpn. 115, 619 (2007).
60. X.-K. Meng, J. Tan, T. Lei, W. Zhang, W. Yan, C.-M. Li, and X.-W. Sun, preparation and properties characterization of SrSi2O2N2:Eu2+ phosphors, Chin. J. Inorg. Chem. 28, 893 (2012).
61. J. Ruan, R.-J. Xie, N. Hirosaki, and T. Takeda, Photoluminescence properties and energy transfer in Eu2+-Yb2+ codoped SrSi2O2N2 oxynitride phosphor, J. Electrochem. Soc. 159, H66 (2012).
62. H. Yamamoto, M. Yamauchi, J. Iizuka, B. G. Yun, K. Machida, and N. Kijima, Effect of fluxes on particle morphology and phase formation of (Sr,Ba)Si2O2N2:Eu2+ phosphors, ECS Trans. 33, 109 (2011).
63. F. Stadler, and W. Schnick, The new layer-silicates Ba3Si6O9N4 and Eu3Si6O9N4, Z. Anorg. Allg. Chem. 632, 949 (2006).
64. F. Stadler, Neue Nitridosilicate und ihre Verwendung f?ur phosphorkonvertierte LEDs, Doctoral thesis, LMU Munich (2006).
65. M. Zhang, J. Wang, Z. Zhang, Q. Zhang, and Q. Su, A tunable green alkaline-earth silicon-oxynitride solid solution (Ca1xSrx)Si2O2N2:Eu2+ and its application in LED, Appl. Phys. B Lasers Opt. 93, 829 (2008).
66. V. Bachmann, T. J?ustel, A. Meijerink, C. Ronda, and P. J. Schmidt, Luminescence properties of SrSi2O2N2 doped with divalent rare earth ions, J. Lumin. 121, 441 (2006).
67. J.-Y. Tang, X.-F. Yang, C. Zhan, L.-Y. Hao, X. Xu, and W.-H. Zhang, Synthesis and luminescence properties of highly uniform spherical SiO2@SrSi2O2N2:Eu2+ core-shell structured phosphors, J. Mater. Chem. 22, 488 (2012).
68. J. Botterman, K. Van Den Eeckhout, A. J. J. Bos, P. Dorenbos, and P. F. Smet, Persistent luminescence in MSi2O2N2:Eu phosphors, Opt. Mater. Express 2, 341 (2012).
69. C.-W. Yeh, Y. Li, J.Wang, and R.-S. Liu, Appropriate green phosphor of SrSi2O2N2:Eu2+,Mn2+ for AC LEDs, Opt. Express 20, 18031 (2012).
70. A. Kirakosyan, A. Mnoyan, S. H. Cheong, G. Y. Lee, and D. Y. Jeon, Double-color emitting SrSi2O2N2:Eu2+,Yb2+ oxynitridosilicate phosphor for warm white-light emitting diode with highly stable color chromaticity, ECS J. Solid State Sci. Technol. 2, R5 (2013).
71. X. Song, R. Fu, S. Agathopoulos, H. He, X. Zhao, and R. Li, Luminescence and energy-transfer mechanism in SrSi2O2N2:Ce3+, Eu2+ phosphors for white LEDs, J. Electrochem. Soc. 157, J34 (2010).
72. Y. Song, X. Xu, H. Zou, Y. Sheng, and H. You, MSi2O2-N2+2/3:Eu (M = Sr, Ba) phosphors for field emission displays, J. Alloys Compd. 513, 86 (2012).
73. X. Song, H. He, R. Fu, D. Wang, X. Zhao, and Z. Pan, Photoluminescent properties of SrSi2O2N2:Eu2+ phosphor: Concentration related quenching and red shift behaviour, J. Phys. D: Appl. Phys. 42, 065409 (2009).
74. X. Song, R. Fu, S. Agathopoulos, H. He, X. Zhao, and J. Zeng, Luminescence and energy transfer of Mn2+ co-doped SrSi2O2N2:Eu2+ green-emitting phosphors, Mater. Sci. Eng. B 164, 12 (2009).
75. Y.-C. Fang, P.-C. Kao, Y.-C. Yang, and S.-Y. Chu, Two-Step Synthesis of SrSi2O2N2:Eu2+ green oxynitride phosphor: Electronphonon coupling and thermal quenching behavior, J. Electrochem. Soc. 158, J246 (2011).
76. H. Ju, X. Su, B.Wang, D. Deng, S. Zhao, and S. Xu, Luminescent properties of SrSi2N2O2:Ce3+,Tb3+-a potential phosphor forwhite light emitting diodes, J. Rare Earth 30, 97 (2012).
77. X. Xu, K. Fu, L. Hu, Z. Lu, X. Zhang, and C. Tang, Synthesis and photoluminescence properties of two-bands excited SrSi2O2N2:Yb2+ phosphors, Adv. Mater. Res. 430-432, 315 (2012).
78. C.-H. Hsu, and C.-H. Lu, Microwave-hydrothermally synthesized (Sr1-x-yCexTby)Si2O2-N2+ phosphors: Efficient energy transfer, structural refinement and photoluminescence properties, J. Mater. Chem. 21, 2932 (2011).
79. Q.-N. Fei,Y.-H. Liu, T.-C.Gu, andD.-J.Wang, Color improvement of white-light through Mn-enhancing yellow-green emission of SrSi2O2N2:Eu phosphor for white light emitting diodes, J. Lumin. 131, 960 (2011).
80. R. Li, R. Fu, X. Song, H. He, X. Yu, B. He, and Z. Shi, Green emission from Tb-doped SrSi2O2N2 phosphors under ultraviolet light irradiation, J. Phys. Chem. Solids 72, 233 (2011).
81. C.-H. Hsu, B.-M. Cheng, and C.-H. Lu, Structure and novel optical characteristics of SrSi2O2N2:Ce3+/Tb3+ oxynitride phosphors, J. Am. Ceram. Soc. 94, 3256 (2011).
82. S.-H. Lee, K.-B. Kim, J.-M. Kim, Y.-K. Jeong, and J.-G. Kang, Highly enhanced luminescence of SrSi2O2N2:Eu2+ phosphors by Mg2+ substitution and their application in pc-LED, Phys. Status Solidi A 210, 1093 (2013).
83. Y. Q. Li, G. De With, and H. T. Hintzen, Luminescence of a new class of UV-blue-emitting phosphorsMSi2O2-N2+2/3:Ce3+ (M = Ca, Sr, Ba), J. Mater. Chem. 15, 4492 (2005).
84. L. Wang, H. Ni, and Q. Zhang, Luminescence and encapsulation properties of BaSi2O2N2 doped with divalent europium, Chin. J. Luminescence, 33, 465 (2012).
85. M. Wang, X. Zhang, Z. Hao, X. Ren, Y. Luo, H. Zhao, X. Wang, and J. Zhang, Long-lasting phosphorescence in BaSi2O2N2:Eu2+ and Ba2SiO4:Eu2+ phases for X-ray and cathode ray tubes, J. Electrochem. Soc. 157, H178 (2010).
86. Y. Gu, Q. Zhang, Y. Li, and H. Wang, Nitridation from coreshell oxides for tunable luminescence of BaSi2O2N2:Eu2+ LED phosphors, J. Mater. Chem. 20, 6050 (2010).
87. X. Song, R. Fu, S. Agathopoulos, H. He, X. Zhao, and X. Yu, Synthesis of BaSi2O2N2:Ce3+, Eu2+ phosphors and determination of their luminescence properties, J. Am. Ceram. Soc. 94, 501 (2011).
88. R.-J. Xie, N. Hirosaki, N. Kimura, K. Sakuma, and M. Mitomo, 2-phosphor-converted white light-emitting diodes using oxynitride/nitride phosphors, Appl. Phys. Lett. 90, 191101 (2007).
89. R.-J. Xie, N. Hirosaki, K. Sakuma, and N. Kimura, White lightemitting diodes (LEDs) using (oxy)nitride phosphors, J. Phys. D: Appl. Phys. 41, 144013 (2008).
90. M. Mikami, S. Shimooka,K.Uheda,H. Imura, andN.Kijima,New green phosphor Ba3Si6O12N2:Eu for white LED: Crystal structure and optical properties, Key Eng. Mater. 403, 11 (2009).
91. S. Shimooka, K. Uheda, M. Mikami, N. Kijima, H. Imura, and K. Horibe, Complex oxynitride phosphor, light-emitting device using same, image display, illuminating device, phosphor-containing composition and complex oxynitride, PCT Int. Appl. WO088966, A1 (2007).
92. C. Braun, M. Seibald, S. L. B?orger, O. Oeckler, T. D. Boyko, A. Moewes, G. Miehe, A. T?ucks, andW. Schnick,Material properties and structural characterization of M3Si6O12N2:Eu2+ (M = Ba, Sr)-A comprehensive study on a promising green phosphor for pc-LEDs, Chem. Eur. J. 16, 9646 (2010).
93. Z. Zhang, J. Wang, Y. Yin, J. Zhou, S. Zhou, and Y. Pan, A novel green-to-yellowemitting phosphor:BaSi2SN2.67:Eu2+ for potential application in UV-LEDs, Opt. Mater. 35, 1273 (2013).
94. M. Mikami, and N. Kijima, 5d Levels of rare-earth ions in oxynitride/nitride phosphors: TO WHAT IS THE IDEA COvalency reliable Opt. Mater. 33, 145 (2010).