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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
Organically modified silicate (Ormosil) is an organic-inorganic hybrid material, in which organic functional groups are chemically bonded to an inorganic silica matrix [1]. By a suitable selection of the organic functional groups and an optimization of processing conditions, it is possible to control mechanical, electrical, and optical properties of an ormosil, such as dielectric constant, thermal stability, refractive index, surface roughness, and hydrophobicity [2–4]. Intermediate properties of ormosils between those of the inorganic silica and organic polymers have enlarged their practical applications. Scratch resistant hard coatings, laser photopatternalbe components for optics, and hosts for quantum dots, organic dyes, and metallic colloids have been major applications of ormosils [1,5]. In addition, lanthanide complexes incorporated ormosil phosphors have been studied for luminescent-solar-concentrators in photovoltaic cells, optical waveguides as well as integrated display or lighting devices [6,7]. Many researchers have synthesized luminescence films by using the phosphors and tried to improve their luminescence properties [6–8]. Because many ormosils are highly transparent in visible light region, it is believed that the phosphors would also be used as transparent luminescence materials for next generation transparent display devices.
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]. 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]. 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]. 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]. 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]. 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 [3,4]. 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
The methacrylate ormosil was prepared by following a previous study [3,4]. In short, methacryloxypropyl trimethoxysilane (MPTMS, Aldrich) and diphenylsilanediol (DPSD, TCI) were used as precursors without further purification, and barium hydroxide monohydrate [Ba(OH)2•H2O, 98 %, Aldrich] was also used as a catalyst to promote a condensation reaction between the two precursors. 2,2-dimethoxy-2-phenyl-acetophenone (BDK, 99 %, Aldrich) was used as a photo initiator. After mixing of the precursors and the catalyst, the mixture solution was kept at 80°C for 2 h to progress the condensation reaction. The total proportion of MPTMS and DPSD was 1:1 molar ratio. Methanol, a byproduct of the reaction, and the catalyst were removed by vacuum heating and filtering, respectively. After that, 1 mol% of the photo initiator to the total polymerizable methacryl group was added to the clear solution remained, the methacrylate ormosil. The synthesized ormosil was diluted with propylene glycol monomethyl ether acetate (PGMEA, 99 %, Aldrich). Eu(DBM)3phen (Aldrich) was dissolved in tetrahydrofuran (THF, 99.9 % Aldrich) by a sonification for 2 h, and the Eu complex solution was added to the dilute ormosil solution. The molar ratio of the ormosil : Eu(DBM)3phen was 9 : 1. Two solutions were homogeneously mixed by a sonification for 2 h, and the Ormosil:Eu(DBM)3phen phosphor was obtained.
2.2 Fabrication of the colloidal SiO2 photonic crystal template
The monodisperse spherical SiO2 particles were prepared by following the Stöber method [18]. 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%.
Figure 2(a) shows the transmittance spectrum of the film prepared by using the methacrylate ormosil. The ormosil matrix film was quite transparent in UV and visible, as well as infrared (IR) region. Figure 2(b) shows the transmittance spectrum of the plane film prepared with the Ormosil:Eu(DBM)3phen phosphor. The film was also transparent in visible and IR region, thus it is believed that the prepared phosphor would be used as a transparent luminescence material for next generation transparent display devices. An absorption band peaking at around 360 nm corresponds to the absorption band of DBM ligands [19]. Figure 2(c) shows the transmittance spectrum of the colloidal SiO2 photonic crystal template with the VF of 0.5 V%. A well-defined stopband centered at around 1068 nm was shown. The position of the stopband (λ) can be predicted by means of the Bragg equation [20],
where d111 is the interplanar spacing of (111) planes in fcc lattice, which is related to the mean diameter (D) of the colloidal particles by (2/3)0.5D, θ is an angle between the incident beam and the normal to the film (θ = 0°, in this study), and neff is the effective refractive index, which is determined as
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.
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]. 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]. 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]. 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.
The shapes and positions of the PL excitation and emission spectra were not significantly changed with the thickness of the templates, but only the PL intensity was varied. Figure 5 shows 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 excitation wavelength was 360 nm. The inset of Fig. 5 shows the relative PL intensity of the infiltrated films depending upon the VF. As shown in Fig. 5, the PL intensity was suddenly increased by using the templates. The PL intensity was increased with increase of the VF from 0 V% to 0.5 V%, and the PL intensity of the infiltrated film with the VF of 0.5 V% was about 13.1 times higher than that of the plane film. However, above the 0.5 V%, the PL intensity was saturated. The saturation is related to the penetration of the excitation source. As shown in Fig 3(a), the transmittance value of the films measured at 360 nm was dramatically decreased and stabilized above 0.5 V%. Therefore, the Eu(DBM)3phen located below around 9.5 μm from the top surface could not be excited by the excitation source and as a result it does not contribute to the luminescence.
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).
In the infiltrated film, some part of the light emitted from the Eu(DBM)3phen, which can be trapped in the film by the total internal reflection, was scattered by the regularly ordered SiO2 particles. The scattering makes the light trapped within the film free, and thus the extraction efficiency was enhanced. The scattering can be affected by a crystallinity of SiO2 particles in a template. Although SiO2 particles are not regularly ordered in a template, the light could be scattered by disordered SiO2 particles. Therefore, a scattering term, originated from the scattering by SiO2 particles can be distinguished from a crystallinity term related to the crystallinity of the template. In order to distinguish the scattering term from the crystallinity term, a colloidal SiO2 template, in which SiO2 particles were not regularly ordered, was additionally prepared. SiO2 particles with different mean diameters such as 253 nm, 487 nm, and 625 nm were used to prepare the disordered template. Weight ratio of the particles with mean diameters of 253 nm : 487 nm : 625 nm was 2 : 2: 1. The SEM image of the disordered template is shown in the inset of the Fig. 6. Because the particle size was not uniform, a regularly ordered photonic crystal was not obtained. Figure 6 shows the transmittance spectra of the ordered template and the disordered one with the VF of 1 V%. The stopband was clearly shown in the spectrum of the ordered template; however, it was not shown in that of the disordered one. Although the SiO2 particles were not regularly ordered, the PL intensity of the infiltrated film into the disordered template (sample C) was about 5.8 times higher than that of the sample A. The thickness of the disordered template was around 12.3 μm, and the transmittance value at 360 nm was 0.14 %. Therefore, it is believed that the thickness of the disordered template did not seriously affect the PL intensity. Figure 7 shows the PL spectra of the sample A, B, C, and D. As shown in Fig. 7, it is possible to clearly divide the effects of the three terms on the large improvement of the PL intensity. The three terms played important roles to improve the PL intensity. The ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2. Although the contributions of the three terms were similar, that of the scattering term was distinguished. In this study, the photonic crystal template acted as both a reservoir of the Ormosil:Eu(DBM)3phen phosphor and a ordered scattering center of the light to be trapped within the film.
4. Conclusion
The luminescent film was prepared by spin-coating of the Ormosil:Eu(DBM)3phen phosphor on the clean glass slide. The plane film was dense and uniform, and the adhesion to the substrate was quite strong. In addition, the plane film was quite transparent in visible and IR region, thus it is believed that the prepared phosphor would be used as a transparent luminescence material for next generation transparent display devices.
The PL intensity of the film was largely improved by the introduction of the colloidal SiO2 photonic crystal template. The PL intensity of the phosphor 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 responsible for the improvement of the PL intensity. The ratio of the effects of the mass term : the scattering term : the crystallinity term was 2.1 : 2.8 : 2.2. The photonic crystal template acted as both a reservoir of the phosphor and a ordered scattering center of the light to be trapped within the film. It is believed that the hybrid luminescence materials embedded photonic crystal could be one of the promising either transparent or opaque luminescence films for next generation displays.
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).
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