Open Access
Add to BibSonomyAbstract
Ultraviolet (UV)-cured organic–inorganic hybrid materials with low refractive index (<1.40) were prepared for multilayer film applications. The hybrid materials comprised hollow silica nanoparticles modified with trialkoxysilane-derived reactive coupling agents. The average number of modifier molecules on the surface of a hollow silica nanoparticle was 0.47 molecule/nm2. The modified hollow silica nanoparticles were homogeneously mixed with UV-curable resins, which induced radical photopolymerization. A UV-cured film containing 60 wt% hollow silica nanoparticles showed a transmittance of >99% in the visible and near-infrared regions and a low refractive index of 1.372 at 633 nm. A TiO2-containing UV-cured hybrid film with high refractive index was easily coated on the UV-cured low-refractive-index film.
©2013 Optical Society of America
1. Introduction
In recent years, organic–inorganic hybrid materials have attracted considerable attention for applications in fields such as magnetics and quantum photovoltaics [1–3]. Organic–inorganic hybrid materials are expected to exhibit non-conventional optical characteristics, which is particularly relevant in the field of optics. The refractive index of hybrid thin films can be tuned by adding inorganic nanoparticles into an organic matrix [4–7]. Monodisperse arrangement of nanosized inorganic particles in an organic matrix can suppress light scattering loss. Such hybrid materials are expected to be useful as flexible optical components including antireflection films, optical waveguides [3], and plastic lenses [4]. However, most studies concerning refractive index-controlled hybrid materials have investigated materials with high refractive indices [5–7]. Few studies have focused on materials with low refractive indices, despite the need for layer materials in applications such as antireflection films and multilayer filters [8,9]. Fluorinated resins have generally been used as low-refractive-index materials (refractive index lower than 1.40) [10]. However, fluorinated resin surfaces often repel water, which makes the stacking process difficult.
Suzuki et al. [11] reported a low-refractive-index hybrid material consisting of MgF2 nanoparticles and polyimide that exhibited a refractive index of 1.477 at 1324 nm. However, in this organic–inorganic material, the similar indices of MgF2 and polyimide restricted further reduction of the refractive index of the hybrid material.
Hollow silica nanoparticles consist of an air core and silica shell. The refractive index of hollow silica nanoparticles can be tuned in the low-index region (1.0–1.45) by controlling the volume fraction of the silica shell and air core [6,12]. Recent advances in nanotechnology mean that it is now possible to obtain commercially available hollow silica nanoparticles. If hollow silica nanoparticles are hybridized with organic matrices, low-refractive-index hybrid materials can be produced that meet the requirements of multilayer film applications.
In this study, we prepare hybrid thin films with low refractive index for multilayer film applications using modified hollow silica nanoparticles and acrylate- and methacrylate-type UV-curable resins that induce radical photopolymerization. The UV-cured thin films show unique optical properties of high transmittance and low refractive index (lower than 1.40). A stacked film composed of a layer with high refractive index on the UV-cured low-refractive-index layer was fabricated simply by spin-coating and UV irradiation.
2. Experimental section
Figure 1 shows a schematic illustration of the process used to prepare the hybrid materials. The process can be categorized into two steps: (a) surface modification of hollow silica nanoparticles, and (b) preparation of hybrid materials. Details of each step are described below.
Fig. 1 Schematic illustrations of (a) surface modification of hollow silica nanoparticles, and (b) preparation of hybrid materials. In (a), the process involves (i) adding silane coupling agent (RSi(OMe)3) and heating under reflux for 4 h, (ii) dispersing modified nanoparticles in 1-methoxy-2-propanol using a centrifugal separator, and (iii) adding UV-curable resin. In (b), the process involves (i) coating the substrate with UV-curable hybrid resin, (ii) annealing at 80 °C for 2 min, and (iii) exposure to UV light (100 mW/cm2 monitored at 365 nm) for 10 s.
2.1 Surface modification of hollow silica nanoparticles
To homogeneously disperse high concentrations of hollow silica nanoparticles in UV-curable resins, we used two silane coupling agents, 3-trimethoxysilyl propyl acrylate (A, >93.0%, TCI, Japan) and 3-trimethoxysilyl propyl methacrylate (M, >98.0%, TCI). Hollow silica nanoparticles with an average particle diameter of 45 nm (JGC Catalysts and Chemicals, Japan) were dispersed in ultrapure water. The silane coupling agent was added to the dispersion, which was then heated under reflux for 4 h. The surface-modified nanoparticles were dispersed in 1-methoxy-2-propanol (≥99.0%, Sigma Aldrich, Japan) using a centrifugal separator. Transmission electron microscopy (TEM, Hitachi, HF-2000) and dynamic light scattering (DLS, Malvern, Zetasizer Nano ZS) measurements were performed to determine the size and distribution of nanoparticles in the solutions. Elemental analysis, thermogravimetric analysis (TGA, Shimadzu, DTG-60), and Fourier-transform infrared (FTIR) spectroscopy (Perkin Elmer, Spectrum One) were performed to characterize the surface modification of the nanoparticles.
2.2 Preparation of hybrid materials
The modified hollow silica nanoparticles dispersed in 1-methoxy-2-propanol were mixed with two kinds of UV-curable resin monomers, 1,6-hexanediol diacrylate (AC, Shin-Nakamura Chemical, Japan) and 1,6-hexanediol dimethacrylate (MC, TCI). The concentration of radical photopolymerization initiator (Irgacure 907, BASF Japan) added to the UV-curable resins was 5 wt%. Table 1 shows the four hybrid combinations that were prepared. To tune the refractive index of the UV-cured hybrid thin films, the mixing ratio of the modified nanoparticles to monomer was varied. Using a filter with a pore diameter of 0.20 μm, the mixture was added dropwise onto silica or silicon (Si) substrates and then spin-coated under the following conditions: slope/5 s, 1000 rpm/10 s, slope/5 s, 3000 rpm/30 s, slope/5 s. The coated film was annealed at 80 °C for 2 min and then cured by exposure to UV light (100 mW/cm2 monitored at 365 nm) for 10 s to give a UV-cured hybrid thin film. The refractive index of each film was measured using a prism coupler (Metricon, Model 2010) at 633 nm in transverse electric (TE) mode [13]. The transmittance of each film was characterized using a UV–visible–near IR spectrophotometer (Shimadzu, UV-3100PC). The surface roughness of each UV-cured hybrid film was measured using an atomic force microscope (AFM, Hitachi High-Tech Science, Nano Navi II).
Table 1. UV-cured hybrid thin films prepared from four combinations of monomers in UV-curable resins with silane coupling agents for surface modification of hollow silica nanoparticles
3. Results and discussion
3.1 Surface modification of hollow silica nanoparticles
Figure 2 presents the size distribution of the hollow silica nanoparticles before and after surface modification in 1-methoxy-2-propanol measured by DLS. A TEM image of the hollow silica nanoparticles is also shown in the inset. The size of the hollow silica nanoparticles did not change upon surface modification with a silane coupling agent, suggesting that the surfaces of the nanoparticles were successfully modified without much aggregation. Because the chain length of the silane coupling agent was shorter than 1 nm, the difference in size between the modified and unmodified particles is negligible. We investigated the content of the surface modifier in the modified nanoparticles; the results for the carbon component measured by elemental analysis are listed in Table 2. The content of the surface modifiers A and M in the modified hollow silica nanoparticles was estimated to be 7.15 wt% and 8.85 wt%, respectively. TGA curves of modified and unmodified nanoparticles are depicted in Fig. 3. The weight loss of the modified nanoparticles was larger than that of the unmodified ones. According to the weight loss observed in the TGA curves at 550 ○C, the organic component in the modified nanoparticles NP(A) and NP(M) was 7 wt% and 9 wt%, respectively. These values are consistent with the results of elemental analysis. Therefore, the number of modifier molecules on the surface of a silica nanoparticle was considered to be 0.47 molecule/nm2 for NP(A) and 0.40 molecule/nm2 for NP(M), assuming that each hollow nanoparticle had an outer diameter of 45 nm, was non-porous with a core-shell structure, and its outermost surface was modified. These values are consistent with those in other reports [14,15], confirming effective surface modification occurred.
Fig. 2 Size distribution of hollow silica nanoparticles (a) unmodified and modified with (b) 3-trimethoxysilyl propyl acrylate, A, and (c) 3-triethoxysilyl propyl methacrylate, M. Inset: TEM image of unmodified hollow silica nanoparticles.
Table 2. Carbon component ratio and weight fraction of surface modifiers determined by elemental analysis
Fig. 3 TGA curves of unmodified hollow silica nanoparticles, NP, and modified hollow silica nanoparticles NP(A) and NP(M) measured at a heating rate of 10 °C/min under air atmosphere.
The surface modification of the hollow silica nanoparticles was further studied by FTIR spectroscopy. Figure 4 shows the characteristic bands for the unmodified nanoparticles, A and modified nanoparticles NP(A). Peaks for surface-modified NP(A) appeared between 2850 and 2980 cm−1 and around 1700 cm−1. The former correspond to the asymmetric and symmetric molecular vibrational frequencies, νas and νs, of CH2, respectively. The latter was attributed to νs of C=O. The band at around 3300–3800 cm−1 (νs, of OH) observed for the unmodified nanoparticles disappeared after surface modification. This indicates that the surface silanol groups of the hollow silica nanoparticles were consumed by the formation of chemical bonds with hydrolysates from the trimethoxysilane-derived coupling agent. The FTIR results further confirm successful surface modification of the hollow silica nanoparticles by the silane coupling agents.
Fig. 4 FTIR spectra of hollow silica nanoparticles (NP), 3-trimethoxysilyl propyl acrylate (A), and modified hollow silica nanoparticles (NP(A)) in KBr pellets.
3.2 Optical properties and stacked-film formation of low-refractive-index hybrid thin films
The refractive indices of the four types of UV-cured hybrid thin films at 633 nm as a function of content of modified nanoparticles are presented in Fig. 5. As the concentration of modified nanoparticles increased, the refractive index of all cured hybrid thin films decreased. The refractive index determined at a modified nanoparticle concentration of 60 wt% was 1.383 for AC-NP(A), 1.372 for AC-NP(M), 1.381 for MC-NP(A), and 1.391 for MC-NP(M). These refractive indices lie below the lower limit of 1.4 for the refractive index of conventional photonic polymers, and are almost comparable to those of fluorinated polymers. The surface roughness (Ra) of the UV-cured hybrid thin film containing 60 wt% nanoparticles measured by AFM was 1.5 nm. This low Ra suggests that homogeneous hybrid films were prepared.
Fig. 5 Refractive indices (TE mode, 633 nm) of AC-NP(A), AC-NP(M), MC-NP(A), and MC-NP(M) hybrid thin films (thickness: 1–2 μm) as a function of content of modified nanoparticles.
Figure 6 illustrates the transmittance spectra for the cured hybrid thin films with a nanoparticle concentration of 60 wt%. All of the cured hybrid thin films showed excellent transmittance in the visible and IR regions. Transmittance >99% was observed at a data communication wavelength of 850 nm after compensating for Fresnel reflection.
Fig. 6 Transmittance spectra of AC-NP(A), AC-NP(M), MC-NP(A), and MC-NP(M) hybrid thin films with a nanoparticle concentration of 60 wt%.
To investigate the ability to form stacked films using the hybrid films, we attempted to fabricate a double layer film composed of high- and low-refractive-index hybrid thin films. Here, the UV-curable hybrid resin MC-NP(A) containing 60 wt% NP(A) was used to prepare the low-refractive-index layer. A UV-curable acrylate resin containing TiO2 nanoparticles (NTT Advanced Technology) was used to fabricate a UV-cured high-refractive-index layer with a refractive index of 1.830 at 633 nm. First, the MC-NP(A) layer was prepared on a silicon substrate by spin-coating and UV irradiation. The high-refractive-index layer was then formed by spin-coating and UV curing. For comparison, a low-refractive-index layer of a perfluorinated polymer (CYTOP®) was used instead of the MC-NP(A) layer.
Figures 7(a) and 7(b) show photographs of TiO2-containing UV-curable resin spin-coated onto low-refractive-index layers of CYTOP® and MC-NP(A), respectively. The TiO2-containing UV-curable resin was uniformly coated on the UV-cured MC-NP(A) layer. In contrast, dewetting readily occurred during spin-coating of the high-refractive-index layer onto the perfluorinated polymer surface.
Fig. 7 Photographs of a UV-curable TiO2-containing resin used to prepare a high-refractive-index layer after spin-coating onto a low-refractive-index layer of (a) perfluorinated polymer, and (b) UV-cured MC-NP(A).
A cross-sectional scanning electron microscope (SEM) image of a double-layer film consisting of a UV-cured TiO2-containing high-refractive-index layer on a low-refractive-index MC-NP(A) layer with 60 wt% NP(A) on a silicon wafer is presented in Fig. 8. A clear interface was observed between low- and high-refractive-index layers, suggesting that no intermixing occurred during the fabrication process. Therefore, the UV-curable low-refractive-index hybrid resin should be useful for fabricating many types of multilayer optical devices.
Fig. 8 Cross-sectional SEM image of a double-layer film composed of a TiO2-containing high-refractive-index layer and low-refractive-index MC-NP(A) layer.
4. Conclusion
In summary, we successfully prepared surface-modified hollow silica nanoparticles using a silane coupling agent without inducing marked aggregation of nanoparticles. Surface modification was confirmed by TGA, elemental analysis, and FTIR spectroscopy. UV-cured hybrid thin films containing the modified nanoparticles exhibited high transmittance over a wide range of wavelengths from 400 to 1600 nm. The refractive indices of UV-cured hybrid thin films containing 60 wt% modified hollow silica nanoparticles were less than 1.40, and the UV-cured AC-NP(M) hybrid film showed the lowest refractive index of 1.372 at 633 nm. A UV-cured high-refractive-index layer was fabricated on a low-refractive-index MC-NP(A) layer without intermixing to form a uniform double-layer film simply by spin-coating and UV exposure without additional surface treatment. Because the UV-cured hybrid thin films exhibit low refractive index and high transmittance, and tolerate subsequent film formation, they are suitable for various multilayer optical device applications.
Acknowledgments
The authors thank Dr. Shoichi Kubo and Dr. Bin Cai for their technical assistance and fruitful discussion. This study was partially supported by the Strategic Promotion of Innovative Research and Development program, Japan Science and Technology Agency.
References and links
1. C. Goh, S. R. Scully, and M. D. McGehee, “Effects of molecular interface modification in hybrid organic-inorganic photovoltaic cells,” J. Appl. Phys. 101(11), 114503 (2007). [CrossRef]
2. Y. Vaynzof, D. Kabra, L. Zhao, P. K. H. Ho, A. T.-S. Wee, and R. H. Friend, “Improved photoinduced charge carriers separation in organic-inorganic hybrid photovoltaic devices,” Appl. Phys. Lett. 97(3), 033309 (2010). [CrossRef]
3. Y. K. Kwon, J. K. Han, J. M. Lee, Y. S. Ko, J. H. Oh, H. S. Lee, and E. H. Lee, “Organic–inorganic hybrid materials for flexible optical waveguide applications,” J. Mater. Chem. 18(5), 579–585 (2008). [CrossRef]
4. M. Russo, M. Campoy-Quiles, P. Lacharmoise, T. A. M. Ferenczi, M. Garriga, W. R. Caseri, and N. Stingelin, “One-pot synthesis of polymer/inorganic hybrids: toward readily accessible, low-loss, and highly tunable refractive index materials and patterns,” J. Polym. Sci., Part B: Polym. Phys. 50(1), 65–74 (2012). [CrossRef]
5. S. Kudo, K. Nagase, S. Kubo, O. Sugihara, and M. Nakagawa, “Optically transparent and refractive index-tunable ZrO2/photopolymer composites designed for ultraviolet nanoimprinting,” Jpn. J. Appl. Phys. 50(6), 06GK12 (2011). [CrossRef]
6. S. Lee, H. J. Shin, S. M. Yoon, D. K. Yi, J. Y. Choi, and U. Paik, “Refractive index engineering of transparent ZrO2–polydimethylsiloxane nanocomposites,” J. Mater. Chem. 18(15), 1751–1755 (2008). [CrossRef]
7. B. Cai, O. Sugihara, H. I. Elim, T. Adschiri, and T. Kaino, “A novel preparation of high-refractive-index and highly transparent polymer nanohybrid composites,” Appl. Phys. Express 4(9), 092601 (2011). [CrossRef]
8. J. Y. Kim, Y. K. Han, E. R. Kim, and K. S. Suh, “Two-layer hybrid anti-reflection film prepared on the plastic substrates,” Curr. Appl. Phys. 2(2), 123–127 (2002). [CrossRef]
9. T. Komikado, A. Inoue, K. Masuda, T. Ando, and S. Umegaki, “Multi-layered mirrors fabricated by spin-coating organic polymers,” Thin Solid Films 515(7-8), 3887–3892 (2007). [CrossRef]
10. J. U. Park, W. S. Kim, and B. S. Bae, “Photoinduced low refractive index in a photosensitive organic–inorganic hybrid material,” J. Mater. Chem. 13(4), 738–741 (2003). [CrossRef]
11. A. Suzuki and S. Ando, “Preparation and optical properties of fluorinated polyimides/MgF2 nanohybrid thin films exhibiting high transparency and low refractive indices,” Polymer Preprints, Japan. 57, 2 (2008).
12. Y. Du, L. E. Luna, W. S. Tan, M. F. Rubner, and R. E. Cohen, “Hollow silica nanoparticles in UV-visible antireflection coatings for poly(methyl methacrylate) substrates,” ACS Nano 4(7), 4308–4316 (2010). [CrossRef] [PubMed]
13. P. A. Barnes and D. P. Schinke, “Refractive index of a native oxide anodically grown on GaAs,” Appl. Phys. Lett. 30(1), 26–28 (1977). [CrossRef]
14. J. D. Miller and H. Ishida, “Quantitative monomolecular coverage of inorganic particulates by methacryl-functional silanes,” Surf. Sci. 148(2-3), 601–622 (1984). [CrossRef]
15. I. D. Sideridou and M. M. Karabela, “Effect of the amount of 3-methacyloxypropyltrimethoxysilane coupling agent on physical properties of dental resin nanocomposites,” Dent. Mater. 25(11), 1315–1324 (2009). [CrossRef] [PubMed]
