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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 5 — Mar. 2, 2009
  • pp: 3732–3740
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Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)3phen phosphor and its enhanced photoluminescence

Hyoung Sun Yoo, Ji Yeon Han, Sung Wook Kim, Duk Young Jeon, and Byeong Soo Bae  »View Author Affiliations


Optics Express, Vol. 17, Issue 5, pp. 3732-3740 (2009)
http://dx.doi.org/10.1364/OE.17.003732


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Abstract

Luminescence films were prepared by infiltration of the tris(dibenzoylmethane) mono(1, 10-phenanthroline) europium incorporated ormosil into colloidal SiO2 photonic crystal templates. Because a stopband of the template was not overlapped with the PL excitation and emission bands, the stopband did not suppress the PL intensity. The PL intensity of the infiltrated film into the template was about 13.1 times higher than that of the plane film prepared without the template. Three major terms, which are the mass term, the scattering term, and the crystallinity term, were considered as factors that improve the PL intensity. The relative ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2.

© 2009 Optical Society of America

1. Introduction

In general, only a small portion of light generated inside a luminescent layer present in thin-film emitting devices, such as light emitting diodes (LEDs), organic light emitting diodes (OLEDs), and inorganic electroluminescent devices (IELDs) comes out toward a viewing side. The low extraction efficiency is mainly due to the total internal reflection at the interfaces, at which there are significant differences in refractive indices [9

9. L. Jones, D. Kumar, R. K. Singh, and P. H. Holloway, “Luminescence of pulsed laser deposited Eu doped yttrium oxide films,” Appl. Phys. Lett. 71, 404–406 (1997). [CrossRef]

]. One of the simple methods to improve the extraction efficiency of a luminescent thin-film or a luminescent layer in thin-film emitting devices is an increase of the surface roughness at the interfaces. Rough surfaces induce some of the light, which would be internally reflected, to be scattered at different angles and thus help them to escape from the surfaces. E. F. Schubert et al. have introduced textured surface in GaAs LEDs and demonstrated 30% external efficiency [10

10. I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, “30% external quantum efficiency from surface textured, thin-film light-emitting diodes,” Appl. Phys. Lett. 63, 2174–2176 (1993). [CrossRef]

]. Recently, Y. R. Do et al. have introduced two dimensional phonic crystals in OLEDs and IELDs, and observed improvements of light extraction efficiency [11–13

11. Y. R. Do, Y. C. Kim, Y. W. Song, C. O. Cho, H. Jeon, Y. J. Lee, S. H. Kim, and Y. H. Lee, “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Adv. Mater. 15, 1214–1218 (2003). [CrossRef]

]. In addition, they have introduced two dimensional photonic crystals both on and under the thin-film phosphors to enhance the photoluminescence (PL) and cathodoluminescence (CL) extraction efficiency of the film and investigated the effects of the dimensions of the photonic crystals on the extraction efficiency [14–16

14. Y. K. Lee, J. R. Oh, Y. R. Do, and Y. D. Huh, “Strong perturbation of the guided light within Y2O3:Eu3+ thin-film phosphors coated with two-dimensional air-hole photonic crystal arrays,” Appl. Phys. Lett. 91, 231908 (2007). [CrossRef]

]. The improvements of the extraction efficiency were mainly attributed to the liberation of the light to be trapped within the high-index guiding layer.

In this study, a self-assembled SiO2 colloidal photonic crystal was used as a template of an organic-inorganic hybrid phosphor. Luminescence films were prepared by infiltration of a tris(dibenzoylmethane) mono(1, 10-phenanthroline) europium [Eu(DBM)3phen] incorporated ormosil phosphor [Ormosil:Eu(DBM)3phen] into the template. Eu(DBM)3phen is a well known red-emitting Eu complex, which could be used as a luminescent material in OLEDs [17

17. L. R. Melby, N. J. Rose, E. Abramson, and J. C. Caris, “Synthesis and fluorescence of some trivalent lanthanide complexes,” J. Am. Chem. Soc. 86, 5117–5125 (1964). [CrossRef]

]. Methacrylate ormosil was used as a matrix for the Eu complex, because the ormosil shows excellent optical properties such as a high transparency in visible light region and a photo-curability [3

3. Y. J. Eo, T. H. Lee, S. Y. Kim, J. K. Kang, Y. S. Han, and B. S. Bae, “Synthesis and molecular structure analysis of nano-Sized methacryl-grafted polysiloxane resin for fabrication of nano hybrid materials,” J. Polym. Sci. Part B 43, 827–836 (2005). [CrossRef]

,4

4. Y. J. Eo, J. H. Kim, J. H. Ko, and B. S. Bae, “Optical characteristics of photo-curable methacryl-oligosiloxane nano hybrid thick films,” J. Mater. Res. 20, 401–408 (2005). [CrossRef]

]. The PL properties of the infiltrated films were compared with the plane film prepared without the template. The factors responsible for an enhanced photoluminescence of the infiltrated films were divided and clarified, in detail.

2. Experimental

2.1 Synthesis of the Ormosil:Eu(DBM)3phen phosphor

2.2 Fabrication of the colloidal SiO2 photonic crystal template

The monodisperse spherical SiO2 particles were prepared by following the Stöber method [18

18. W. Stober, A. Fink, and E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” J. Colloid Interf. Sci. 26, 62–69 (1968). [CrossRef]

]. Tetraethyl orthosilicate (TEOS 99 %, Aldrich), aqueous NH4OH (ammonia 28 wt%, Junsei Chemical Co.) solution, ethanol (99.9 %, Merck), and deionized water were used as raw materials. 0.2 M of TEOS was hydrolyzed in an ethanol solution containing 10 M of water and 1 M of NH4OH solution. After TEOS was hydrolyzed at 22°C for 2 h, the SiO2 particles obtained were washed and separated from the solution by a centrifugation and dried at 60°C for 10 h. The colloidal solution was prepared by dispersing of the prepared SiO2 particles in ethanol (99.9 %, Merck). In order to control the thickness of the photonic crystal templates, the volume fraction of the SiO2 particles (VF) was varied from 0.1 V% to 3 V%. Slide glass cleaned by using a chromic-sulfuric acid and purified water were placed in the colloidal solutions. During the evaporation of the solvent, the templates were fabricated on the slide glass.

2.3 Synthesis of the Ormosil:Eu(DBM)3phen infiltrated photonic crystal

The Ormosil:Eu(DBM)3phen phosphor was spin-coated on the clean slide glass and the templates at 1000 rpm for 30 s. The thickness of the plane films prepared without the templates was controlled by changing the spin speed from 1000 rpm to 200 rpm. The thickness of the infiltrated films prepared by infiltration of the phosphor into the templates was mainly controlled by changing the thickness of the templates. Both the plane and the infiltrated films were photo-cured by using an ultraviolet (UV) lamp (500W, Hg lamp, λ: 365 nm, Oriel 97453) in N2 gas atmosphere. The UV dose was 1720 mJ/cm2.

2.4 Characterizations

The morphology and size of the prepared samples were observed by a Philips XL30SFEG scanning electron microscope (SEM). The average diameter and the standard deviation of the prepared SiO2 particles were confirmed by using a dynamic light scattering equipment (ZetaPlus, Brookhaven Instruments Corporation) at a wavelength of 674 nm with a normal incident angle. The normal incidence transmittance spectra of the prepared samples were obtained by using a Shimadzu UV-3101 PC spectrophotometer. The measurement range was ranging from 300 nm to 1500 nm. The refractive index (nD 22) of the prepared sample was determined by using a Abbe refractometer (Bellingham Stanley Ltd. 60/ED) at a wavelength of 589.6 nm. The PL spectra of the prepared samples were recorded by a standard spectrometer setup from a DARSA PRO 5100 PL spectrometer (Professional Scientific Instrument Co., Korea) using a Xe lamp as an excitation source. The excitation spectrum was corrected by a sodium salicylate.

Fig. 1. (a) The SEM image of the colloidal SiO2 photonic crystal template, (b) the cross sectional SEM image of the plane film coated on the slide glass, (c) the cross sectional image of the infiltrated film with the VF of 0.1 V%, (d) the tilted SEM image of the infiltrated film with the VF of 0.3 V%, (e) the infiltrated film into the template with the VF of 0.3 V%, (f) the schematic of the infiltrated film.

3. Results and discussion

3.1. Synthesis of the Ormosil:Eu(DBM)3phen infiltrated photonic crystal

The morphologies of the prepared samples in each step were observed by SEM. Figure 1(a) shows the SEM image of the template prepared by using the colloidal SiO2 particles, which have a mean diameter of 487 nm and a standard deviation of 8.6 nm (about 1.8 % of the mean diameter). The triangular arrangement can correspond to (111) surface of a face-centered-cubic (fcc) lattice. Figure 1(b) shows the cross sectional image of the plane film coated on the slide glass. The film was dense and uniform, and the adhesion to the substrate was quite strong. The thickness of the film was around 400 nm. Figure 1(c) shows the cross sectional image of the infiltrated film with the VF of 0.1 V%. Figure 1(d) shows the tilted SEM image of the infiltrated film with the VF of 0.3 V%. Figure 1(e) shows the SEM image of the infiltrated film with the VF of 0.3 V%. The Ormosil:Eu(DBM)3phen phosphor was infiltrated well into the monolayer and multilayer photonic crystal templates. Figure 1(f) shows the schematic of the infiltrated film.

Fig. 2. The transmittance spectra of (a) the film prepared by using the methacrylate ormosil, (b) the plane film coated on the slide glass, (c) the photonic crystal template with the VF of 0.5 V%, and (d) the infiltrated film into the template with the VF of 0.5 V%.

λ=2d111(neff2sinθ2)0.5.
(1)

neff=nspΦ+nair(1Φ).
(2)

where Φ is the fractional volume of the SiO2 particles (0.74 for fcc lattice), nsp and nair are the refractive indices of the SiO2 particles and air, respectively. As the nsp and nair are 1.45 and 1, respectively, the neff is 1.35. When the D is 487 nm, the calculated λ is 1071 nm, which is well matched with the observed λ (1068 nm). Figure 2(d) shows the transmittance spectrum of the infiltrated film with the VF of 0.5 V%. The measured refractive index (nphosphor) of the plane film was around 1.50. Although a difference between nsp and nphopshor was not much, a weak and broad stopband centered at around 1136 nm was observed. Assuming a full infiltration of the phosphor into the template, the λ calculated from Eq. (1) was 1161 nm. From this discrepancy, it was calculated that about 6 % of volume to be infiltrated was emptied during the spin-coating. Figure 3(a) shows the thickness of the infiltrated film depending upon the VF. The thickness of the films was linearly increased from 400 nm to 82 μm with increase of the VF from 0 V% (without template) to 3.0 V%. Figure 3(b) shows the transmittance value of the films measured at 360 nm depending upon the VF. The transmittance value was dramatically decreased from 52.4 % to 0.18 % with increase of the VF from 0 V% to 0.5 V%, and subsequently it was stabilized.

Fig. 3. (a) The transmittance value measured at 360 nm of the infiltrated films and (b) their thickness depending upon the VF.
Fig. 4. The PL excitation and emission spectra of the plane film coated on the slide glass.

3.2 Effects of the template thickness on the PL properties

Figure 4 shows the PL excitation and emission spectra of the plane film coated on the slide glass. The position of the excitation band was well matched with the absorption band in the transmittance spectrum [Fig. 2(b)], and the band corresponds to the absorption of DBM ligands. The energy absorbed by the ligands transferred to Eu3+ ion, and thus the characteristic red emission peaks of Eu3+ ion were observed. As indicated in the PL emission spectrum, the emission peaks can be assigned to the electronic transitions from 5D0 to 7Fj (j = 0, 1, 2, 3, 4) levels of Eu3+ ion. The main emission wavelength was around 613 nm due to the forbidden electric dipole 5D0-7F2 transition, which indicates that Eu3+ ions occupy a site without a center of inversion. In addition, the presence of only one 5D0-7F0 line emission means that Eu3+ ions occupy only a single site, and a single chemical environment exists around them [21

21. H. Liang, Q. Zhang, Z. Zheng, H. Ming, Z. Li, J. Xu, B. Chen, and H. Zhao, “Optical amplification of Eu(DBM)3Phen-doped polymer optical fiber,” Opt. Lett. 29, 477–479 (2004). [CrossRef] [PubMed]

]. The Commission International de l’Eclairge (CIE) 1931 chromaticity coordinates of the film was x = 0.665, y = 0.321.

By choosing the appropriate mean diameter of SiO2 particles (487 nm), the PL excitation and emission bands were not overlapped with the stopbands of either the template or the infiltrated film into the template. J. Siver et al. and R. Withnall et al. have infilled rare-earth ions doped inorganic oxide phosphors into photonic silica templates, and observed the effects of stopband positions on the luminescence intensity [22–24

22. J. Silver, T. G. Ireland, and R. Withnall, “Facile method of infilling photonic silica templates with rare earth element oxide phosphor precursors,” J. Mater. Res. 19, 1656–1661 (2004) [CrossRef]

]. They have reported that the luminescence intensity could be substantially suppressed, when the stopbands of the photonic phosphors were overlapped with their PL excitation and emission bands. Y.-S. Lin et al. have also reported that the luminescence intensity of the photonic crystal fabricated by using the Tb(OH)3@SiO2 core/shell particles was much reduced, when the stopband was overlapped with the emission band [25

25. Y. S. Lin, Y. Hung, H. Y. Lin, Y. H. Tseng, Y. F. Chen, and C. Y. Mou, “Photonic crystals from monodisperse lanthanide-hydroxide-at-silica core/shell colloidal spheres,” Adv. Mater. 19, 577–580 (2007). [CrossRef]

]. In this study, the stopbands of either template (1068 nm) or the infiltrated film (1136 nm) were located far from both the emission bands (613 nm) and the excitation bands (360 nm). Therefore, the stopbands did not suppress the PL intensity of the infiltrated films.

Fig. 5. The PL spectra of the infiltrated films with the various VFs of 0 V%, 0.2 V%, 0.4 V%, and 0.5 V%. The inset shows the relative PL intensity of the infiltrated films depending upon the VF.

3.3 Effects of the template thickness on the PL properties

The PL intensity of the infiltrated film with the VF of 0.5 V% (sample D) was about 13.1 times higher than that of the plane film prepared without the template (sample A). The sample A and D were spin-coated at 1000 rpm. Three major terms can be considered as factors responsible for the improvement of the PL intensity. The first term is a mass term. The thicknesses of the sample A and D were around 400 nm and 9.5 μm, respectively. Assuming a full infiltration of the phosphor into the template, the mass of the phosphor in the sample D corresponds to the thickness of around 2.5 μm. Therefore, there was a difference between the mass of the phosphor used in the sample A and that in the sample D. In order to access an effect of the mass term, plane films, which have varied thicknesses, were prepared without the template. The thickness of the plane films was controlled by changing the spin speed. When the spin speed of the spin-coating step was 200 rpm, a plane film with the thickness of around 2.5 μm (sample B) was obtained. The PL intensity of the sample B was about 2.1 times higher than that of the sample A.

Fig. 6. The transmittance spectra of the ordered template and the disordered one with the VF of 1 V%. The inset shows the SEM image of the disordered template.
Fig. 7. The PL spectra of the sample A (the plane film prepared without the photonic crystal template at 1000 rpm), B (the plane film prepared without the photonic crystal template at 200 rpm), C (the infiltrated film into the disordered template), and D (the infiltrated film into the ordered template).

4. Conclusion

Acknowledgments

This research was supported by WCU(World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (Grant No. R32-2008-000-10051-0).

References and links

1.

J. D. Mackenzie and E. P. Bescher, “Structures, properties and potential applications of ormosils,” J. Sol-Gel Sci. Techn. 13, 371–377 (1998). [CrossRef]

2.

W. S. Kim, K. S. Kim, Y. J. Eo, K. B. Yoon, and B. S. Bae, “Synthesis of fluorinated hybrid material for UV embossing of a large core optical waveguide structure,” J. Mater. Chem. 15, 465–469 (2005). [CrossRef]

3.

Y. J. Eo, T. H. Lee, S. Y. Kim, J. K. Kang, Y. S. Han, and B. S. Bae, “Synthesis and molecular structure analysis of nano-Sized methacryl-grafted polysiloxane resin for fabrication of nano hybrid materials,” J. Polym. Sci. Part B 43, 827–836 (2005). [CrossRef]

4.

Y. J. Eo, J. H. Kim, J. H. Ko, and B. S. Bae, “Optical characteristics of photo-curable methacryl-oligosiloxane nano hybrid thick films,” J. Mater. Res. 20, 401–408 (2005). [CrossRef]

5.

H. Schmidt and H. Wolter, “Organically modified ceramics and their applications,” J. Non-Cryst. Solids 121, 428–435 (1990). [CrossRef]

6.

T. Jin, S. Inoue, K. Machida, and G. Adachi, “Photovoltaic cell characteristics of hybrid silicon devices with lanthanide complex phosphor-coating film,” J. Electrochem. Soc. 144, 4054–4058 (1997) [CrossRef]

7.

P. Escribano, B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo, B. Viana, and C. Sanchez, “Photonic and nanobiophotonic properties of luminescent lanthanide-doped hybrid organic-inorganic materials,” J. Mater. Chem. 18, 23–40 (2008). [CrossRef]

8.

T. Jin, S. Tsutsumi, Y. Deguchi, K. Machida, and G. Adachi, “Luminescence characteristics of the lanthanide complex incorporated into an ORMOSIL matrix using a sol-gel method,” J. Electrochem. Soc. 143, 3333–3335 (1996). [CrossRef]

9.

L. Jones, D. Kumar, R. K. Singh, and P. H. Holloway, “Luminescence of pulsed laser deposited Eu doped yttrium oxide films,” Appl. Phys. Lett. 71, 404–406 (1997). [CrossRef]

10.

I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, “30% external quantum efficiency from surface textured, thin-film light-emitting diodes,” Appl. Phys. Lett. 63, 2174–2176 (1993). [CrossRef]

11.

Y. R. Do, Y. C. Kim, Y. W. Song, C. O. Cho, H. Jeon, Y. J. Lee, S. H. Kim, and Y. H. Lee, “Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals,” Adv. Mater. 15, 1214–1218 (2003). [CrossRef]

12.

Y. R. Do, Y. C. Kim, S. H. Cho, D. S. Zang, Y. D. Huh, and S. J. Yun, “Influence of a two-dimensional SiO2 nanorod structure on the extraction efficiency of ZnS:Mn thin-film electroluminescent devices,” Appl. Phys. Lett. 84, 1377–1379 (2004). [CrossRef]

13.

Y. J. Lee, S. H. Kim, G. H. Kim, Y. H. Lee, S. H. Cho, Y. C. Kim, and Y. R. Do, “A high-extraction-efficiency nanopatterned organic light-emitting diode,” Appl. Phys. Lett. 82, 3779–3781 (2003). [CrossRef]

14.

Y. K. Lee, J. R. Oh, Y. R. Do, and Y. D. Huh, “Strong perturbation of the guided light within Y2O3:Eu3+ thin-film phosphors coated with two-dimensional air-hole photonic crystal arrays,” Appl. Phys. Lett. 91, 231908 (2007). [CrossRef]

15.

Y. K. Lee, J. R. Oh, and Y. R. Do, “Enhanced extraction efficiency of Y2O3:Eu3+ thin-film phosphors coated with hexagonally close-packed polystyrene nanosphere monolayers,” Appl. Phys. Lett. 91, 041907 (2007). [CrossRef]

16.

K. Y. Ko, Y. K. Lee, H. K. Park, Y. C. Kim, and Y. R. Do, “The variation of the enhanced photoluminescence efficiency of Y2O3:Eu3+ films with the thickness to the photonic crystal layer,” Opt. Express 16, 5689–5696 (2008), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-16-8-5689. [CrossRef] [PubMed]

17.

L. R. Melby, N. J. Rose, E. Abramson, and J. C. Caris, “Synthesis and fluorescence of some trivalent lanthanide complexes,” J. Am. Chem. Soc. 86, 5117–5125 (1964). [CrossRef]

18.

W. Stober, A. Fink, and E. Bohn, “Controlled growth of monodisperse silica spheres in the micron size range,” J. Colloid Interf. Sci. 26, 62–69 (1968). [CrossRef]

19.

L. N. Sun, H. J. Zhang, L. S. Fu, F. Y. Liu, Q. G. Meng, C. Y. Peng, and J. B. Yu, “A new sol-gel material doped with an erbium complex and its potential optical-amplification application,” Adv. Func. Mater. 15, 1041–1048 (2005). [CrossRef]

20.

H. Miguez, C. Lopez, F. Meseguer, A. Blanco, L. Vazquez, R. Mayoral, M. Ocana, V. Fornes, and A. Mifsud, “Photonic crystal properties of packed submicrometric SiO2 spheres,” Appl. Phys. Lett. 71, 1148–1150 (1997). [CrossRef]

21.

H. Liang, Q. Zhang, Z. Zheng, H. Ming, Z. Li, J. Xu, B. Chen, and H. Zhao, “Optical amplification of Eu(DBM)3Phen-doped polymer optical fiber,” Opt. Lett. 29, 477–479 (2004). [CrossRef] [PubMed]

22.

J. Silver, T. G. Ireland, and R. Withnall, “Facile method of infilling photonic silica templates with rare earth element oxide phosphor precursors,” J. Mater. Res. 19, 1656–1661 (2004) [CrossRef]

23.

R. Withnall, M. I. Martinez-Rubio, G. R. Fern, T. G. Ireland, and J. Silver, “Photonic phosphors based on cubic Y2O3:Tb3+ infilled into a synthetic opal lattice,” J. Opt. A: Pure Appl. Opt. 5, S81–S85 (2003) [CrossRef]

24.

R. Withnall, T. G. Ireland, M. I. Martinez-Rubio, G. R. Fern, and J. Silver, “Rare-earth element anti-stokes emission from three inverse photonic lattices,” J. Mod. Opt. 49, 965–976 (2002). [CrossRef]

25.

Y. S. Lin, Y. Hung, H. Y. Lin, Y. H. Tseng, Y. F. Chen, and C. Y. Mou, “Photonic crystals from monodisperse lanthanide-hydroxide-at-silica core/shell colloidal spheres,” Adv. Mater. 19, 577–580 (2007). [CrossRef]

OCIS Codes
(160.5690) Materials : Rare-earth-doped materials
(250.5230) Optoelectronics : Photoluminescence
(160.5298) Materials : Photonic crystals

ToC Category:
Photonic Crystals

History
Original Manuscript: December 19, 2008
Revised Manuscript: February 19, 2009
Manuscript Accepted: February 22, 2009
Published: February 25, 2009

Citation
Hyoung Sun Yoo, Ji Yeon Han, Sung Wook Kim, Duk Young Jeon, and Byeong Soo Bae, "Self-assembled SiO2 photonic crystal infiltrated by Ormosil:Eu(DBM)3phen phosphor and its enhanced photoluminescence," Opt. Express 17, 3732-3740 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-5-3732


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References

  1. J. D. Mackenzie and E. P. Bescher, "Structures, properties and potential applications of ormosils,"J. Sol-Gel Sci. Techn. 13, 371-377 (1998). [CrossRef]
  2. W. S. Kim, K. S. Kim, Y. J. Eo, K. B. Yoon, and B. S. Bae, "Synthesis of fluorinated hybrid material for UV embossing of a large core optical waveguide structure," J. Mater. Chem. 15, 465-469 (2005). [CrossRef]
  3. Y. J. Eo, T. H. Lee, S. Y. Kim, J. K. Kang, Y. S. Han, and B. S. Bae, "Synthesis and molecular structure analysis of nano-Sized methacryl-grafted polysiloxane resin for fabrication of nano hybrid materials," J. Polym. Sci. Part B 43, 827-836 (2005). [CrossRef]
  4. Y. J. Eo, J. H. Kim, J. H. Ko, and B. S. Bae, "Optical characteristics of photo-curable methacryl-oligosiloxane nano hybrid thick films," J. Mater. Res. 20, 401-408 (2005). [CrossRef]
  5. H. Schmidt and H. Wolter, "Organically modified ceramics and their applications," J. Non-Cryst. Solids 121, 428-435 (1990). [CrossRef]
  6. T. Jin, S. Inoue, K. Machida, and G. Adachi, "Photovoltaic cell characteristics of hybrid silicon devices with lanthanide complex phosphor-coating film," J. Electrochem. Soc. 144, 4054-4058 (1997) [CrossRef]
  7. P. Escribano, B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo, B. Viana, and C. Sanchez, "Photonic and nanobiophotonic properties of luminescent lanthanide-doped hybrid organic-inorganic materials," J. Mater. Chem. 18, 23-40 (2008). [CrossRef]
  8. T. Jin, S. Tsutsumi, Y. Deguchi, K. Machida, and G. Adachi, "Luminescence characteristics of the lanthanide complex incorporated into an ORMOSIL matrix using a sol-gel method," J. Electrochem. Soc. 143, 3333-3335 (1996). [CrossRef]
  9. L. Jones, D. Kumar, R. K. Singh, and P. H. Holloway, "Luminescence of pulsed laser deposited Eu doped yttrium oxide films," Appl. Phys. Lett. 71, 404-406 (1997). [CrossRef]
  10. I. Schnitzer, E. Yablonovitch, C. Caneau, T. J. Gmitter, and A. Scherer, "30% external quantum efficiency from surface textured, thin-film light-emitting diodes," Appl. Phys. Lett. 63, 2174-2176 (1993). [CrossRef]
  11. Y. R. Do, Y. C. Kim, Y. W. Song, C. O. Cho, H. Jeon, Y. J. Lee, S. H. Kim, and Y. H. Lee, "Enhanced light extraction from organic light-emitting diodes with 2D SiO2/SiNx photonic crystals," Adv. Mater. 15, 1214-1218 (2003). [CrossRef]
  12. Y. R. Do, Y. C. Kim, S. H. Cho, D. S. Zang, Y. D. Huh, and S. J. Yun, "Influence of a two-dimensional SiO2 nanorod structure on the extraction efficiency of ZnS:Mn thin-film electroluminescent devices," Appl. Phys. Lett. 84, 1377-1379 (2004). [CrossRef]
  13. Y. J. Lee, S. H. Kim, G. H. Kim, Y. H. Lee, S. H. Cho, Y. C. Kim, and Y. R. Do, "A high-extraction-efficiency nanopatterned organic light-emitting diode," Appl. Phys. Lett. 82, 3779-3781 (2003). [CrossRef]
  14. Y. K. Lee, J. R. Oh, Y. R. Do, and Y. D. Huh, "Strong perturbation of the guided light within Y2O3:Eu3+ thin-film phosphors coated with two-dimensional air-hole photonic crystal arrays," Appl. Phys. Lett. 91, 231908 (2007). [CrossRef]
  15. Y. K. Lee, J. R. Oh, and Y. R. Do, "Enhanced extraction efficiency of Y2O3:Eu3+ thin-film phosphors coated with hexagonally close-packed polystyrene nanosphere monolayers," Appl. Phys. Lett. 91, 041907 (2007). [CrossRef]
  16. K. Y. Ko, Y. K. Lee, H. K. Park, Y. C. Kim, and Y. R. Do, "The variation of the enhanced photoluminescence efficiency of Y2O3:Eu3+ films with the thickness to the photonic crystal layer," Opt. Express 16, 5689-5696 (2008), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-16-8-5689. [CrossRef] [PubMed]
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  18. W. Stober, A. Fink, and E. Bohn, "Controlled growth of monodisperse silica spheres in the micron size range," J. Colloid Interf. Sci. 26, 62-69 (1968). [CrossRef]
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