Open Access
Add to BibSonomyAbstract
Hybrid materials composed of organically modified silica nanoparticles and oligosiloxanes exhibited homogeneous dispersion without any phase separation or aggregation and good solution processability. These hybrid materials exhibited good solubility of photoinitiator and high transmittance of above 90% in visible wavelength ranges. In particular, the materials showed high photocurability, with degrees of conversion under UV exposure above 90%. With high photosensitivity, the hybrid materials were used for direct photopatterning without the developing process used in the photomask method. The various optical surface structures on a micrometer scale were easily fabricated in the hybrid films upon UV exposure, and the samples exhibited good root-mean-square (RMS) roughness of around 1 nm.
©2012 Optical Society of America
1. Introduction
Photosensitive materials are crucial in applications of optical elements and optical fabrication processes. Thus, studies of various types of photosensitive materials, including solution-processable materials such as photopolymers [1–4], sol-gel materials [5–8], and inorganic materials [9,10], have been actively performed for using the materials in applications of optical elements and optical processes. Solution-processable materials can be easily employed for fabricating optical elements because of their high photosensitivity, the simple solution, and low-temperature processes. However, the optical elements fabricated from these materials could exhibit problems because of the weakness of mechanical and thermal properties in the solution-processed soft material, which could thus limit the application of the optical elements. On the other hand, inorganic materials have good optical stability against surrounding; however, the materials are required for rather complex and several steps to reveal the optical structures. Thus, inorganic materials also have limitations in terms of their application for the fabrication of optical elements. Thus, the studies of making up these demerits in above two categorized materials have been required.
Currently, sol-gel hybrid materials have been widely studied because of the synergetic effect of the organic and inorganic materials through the hybridization of the two materials [11–14]. The materials can be applied to the solution process and the weak properties of the hybrid materials can be compensated for through the controlled incorporation of inorganic nanoparticles and organic components.
In this study, we focused on the fabrication of photosensitive and transparent hybrid materials containing organically modified silica nanoparticles and oligosiloxanes without any phase separation or aggregation. In particular, we investigated the photocurability of the hybrid materials with silica nanoparticles and tried to fabricate various optical surface structures in photosensitive hybrid materials in a simple manner. In addition, we investigated the outstanding optical property and surface quality such as transmittance and surface roughness in the fabricated photosensitive hybrid materials.
2. Experiment
Photosensitive and transparent hybrid materials were prepared using organically modified silica nanoparticles and oligosiloxanes. The organically modified silica nanoparticles were prepared using colloidal silica dispersed in water (30 wt%, LUDOX# HS-30), with methyltrimethoxysilane (MTMS, Aldrich) and methacryloxypropyl-trimethoxysilane (MPTMS, Aldrich) as the main precursor and surface modification agents, respectively. The surface of the colloidal-silica nanoparticles was modified with MTMS (0.2 M). Then, MPTMS (0.2 M) was reacted with the MTMS-treated colloidal-silica nanoparticles for chemical polymerization with oligosiloxanes. After the synthesis of the methyl and the methacryl silane-treated colloidal-silica nanoparticles, any residual product (such as alcohol or water) was replaced with propylene glycol monomethyl ether acetate (PGMEA, Aldrich) at 40°C, with an evaporator for the hybridization of the organically modified colloidal-silica nanoparticles with oligosiloxanes. The oligosiloxanes were prepared using MTMS and MPTMS in a ratio of 3:7 and hydrolyzed with 0.01 N HCl. After stirring for 20 h to obtain a full sol-gel reaction, all residual products such as alcohol or water were replaced with PGMEA at 40°C. Then, the 30 wt% colloidal-silica nanoparticles that had been stabilized through surface modification were homogeneously embedded in the fabricated oligosiloxanes containing MTMS and MPTMS. Solid benzyldimethylketal (BDK, Aldrich) (3 wt%) was added as a photoinitiator to the hybrid solutions composed of 30 wt% organically modified colloidal-silica nanoparticles and oligosiloxanes. After stirring the solutions for 1 h at room temperature, homogeneous photosensitive and transparent hybrid materials were obtained. These photosensitive and transparent hybrid solutions were filtered and spin-coated onto clean glass substrates and wafers. The coated hybrid films were then subjected to UV-induced polymerization (500 W Hg Lamp, λ = 350–390 nm, Oriel 97453, optical power density = 100 mW cm−2) under a nitrogen atmosphere after pre-drying at 90°C for 1.5 min. The UV-exposed films were characterized directly or after baking at 150°C for 1 h. Then the transparent hybrid films were fabricated.
The changes in the chemical structures as well as the photocurability of the hybrid materials were examined using Fourier transform infrared (FT-IR, JASCO 680 Plus) spectroscopy before and after UV irradiation. The refractive index of the hybrid materials were measured using a prism coupler (Metricon 2010) at a wavelength of 632.8 nm. The optical transparency and surface properties of the hybrid materials were examined through ultraviolet visible near-infrared (UV/Vis/NIR) spectroscopy and atomic force measurement (AFM, SFI 3800N, SEIKO), respectively. The images of direct photofabricated surface structures were investigated using optical microscope (OEM, LV 100, Nikon Eclipse) and scanning electron microscope (SEM, S-4800, Hitachi).
3. Results and discussion
Figure 1 shows (a) the FT-IR spectra in the 1800-1600 cm−1 range and (b) the conversion degree (black square line) and the C = C peak decrease (red circle line) in the photosensitive hybrid films with the 30 wt% organically modified colloidal-silica nanoparticles and 70 wt% oligosiloxanes as a function of UV exposure time. The FT-IR shows that the polymerization or crosslinking of the methacryl groups of organically modified colloidal-silica nanoparticles and oligosiloxanes in the photosensitive hybrid films occurred during the UV exposure. The FT-IR shows that the intensity of the C = C peak at 1638–1615 cm−1 was reduced and that the C = O peak at 1725 cm–1 shifted to longer wavenumbers as the UV exposure time increased. This indicates the consumption of the C = C bonds and the loss of the conjugation with the C = C bond because of the photopolymerization in the photosensitive hybrid films. On the other hand, the integrated area of the C = O bond at around 1725 cm−1 remained constant [15]. The conversion degree of the C = C bond calculated from the integrated peak intensities of the C = C and C = O bonds was plotted as a function of UV exposure time, as shown Fig. 1(b). The conversion degree of the C = C bond increased to around 91% in the hybrid films with 30 wt% the organically modified colloidal-silica nanoparticles in oligosiloxanes. The conversion degree of our fabricated hybrid films indicated that the extent of the polymerization and photocurability, reached above 90%, which is high enough to easily cure and photofabricate our fabricated hybrid materials. Therefore, the photosensitive hybrid films could be applied to fabricating the various surface optical structures through simple UV irradiation because of the highly efficient photocurability of the photosensitive hybrid materials solutions developed herein.
Fig. 1 (a) FT-IR spectra of 1800–1600 cm−1 range and (b) the conversion degree (black square line) and C = C peak decrease (red circle line) in the photosensitive hybrid films depending on UV exposure time.
Figure 2 shows the optical transparency of the fabricated hybrid films with 30 wt% organically modified colloidal-silica nanoparticles and 70 wt% oligosiloxanes, as well as the oligosloxane films without colloidal-silica nanoparticles. The analysis of the optical transmission of the fabricated hybrid films coated on the quartz substrate was carried out.
Fig. 2 Optical transparency of the fabricated hybrid films with 30 wt% organically modified colloidal-silica nanoparticles and 70 wt% oligosiloxanes and the oligosloxane films without colloidal-silica nanoparticles (red line: hybrid films, black line: oligosiloxane films).
The fabricated hybrid films were optically transparent despite the incorporation of 30 wt% colloidal-silica nanoparticles in oligosiloxanes. The spectra show that the hybrid films with 30 wt% colloidal-silica nanoparticles in oligosiloxanes are highly transparent, with around 90% transmittance. This transparency is almost the same as that of oligosiloxanes without colloidal-silica nanoparticles. This is due not only to the homogeneous dispersion of the organically modified colloidal-silica nanoparticles in oligosiloxanes without phase separation but also to its high photodurability, without transparency decrease or discoloration, upon UV irradiation. Also, the refractive index of this hybrid films is 1.4756.
Figure 3 shows the optical micrographs of the various shaped surface structures with (a) a Fresnel-type circle, (b) a Fresnel-type circle array, and (c) a circle array directly photofabricated in the highly photosensitive and transparent hybrid films.
Fig. 3 Optical micrographs of the various-shaped surface structures with (a) a Fresnel-type circle, (b) a Fresnel-type circle array, and (c) a circle array directly photofabricated in the highly photosensitive and transparent hybrid films.
Figure 4 shows the scanning electron micrographs of the various-shaped surface structures with (a) a Fresnel-type circle, (b) a circle, (c) a circle array-1, and (d) a circle array-2 directly photofabricated in highly photosensitive and transparent hybrid films.
Fig. 4 Scanning electron micrographs of the various-shaped surface structures with (a) a Fresnel-type circle, (b) a circle, (c) a circle array-1, and (d) a circle array-2 directly photofabricated in highly photosensitive and transparent hybrid films.
As shown in Figs. 3 and 4, various types of surface structures could be directly photofabricated in the highly photosensitive and transparent hybrid films using the simple photomask method without a post-developing process. In order to use the high photosenstivity of hybrid films, the wet films before heat treatment were used for the photofabrication of surface structures. These simple fabricated surface structures can be applied to the formation process of optical elements and optoelectrical devices using the photolithography technique. These photopatterning effects are highly related to the photosensitivity of materials, which are in turn related to various photoinduced reactions, such as photopolymerization of the radicals in the matrix, a photolocking of photoinitiators into the matrix and a photomigration of matrix due to the concentration gradient between the unexposed and exposed areas [12]. These photoinduced reactions occur in all UV ranges and lead to the increase in refractive index and volume of the films. Thus, in this experiment, we could expect that BDK and oligosiloxanes, including surface modifiers of colloidal-silica with the photocurable moieties such as methacryl, in the exposed areas were fixed with the hybrid matrix by the various abovementioned photoinduced reactions and then not removed during heating and drying. On the other hand, BDK and oligosiloxanes in the unexposed areas were not involved in any of the photoinduced reactions. This means that the photoactive moieties such as BDK in the unexposed area were not fixed with the hybrid matrix. Therefore, the photoactive moieties such as BDK in the unexposed area were volatile and could be easily removed by baking. After baking, these different phenomena of photoactive moieties between the UV exposed area and unexposed area led to the large changes in both refractive index and volume. These differences became much larger with the increase in the UV exposure time. These highly photosensitive and transparent hybrid materials are very good candidates for the photopatterning of various surface structures, including optical and optoelectrical elements.
In order to investigate the surface quality of the fabricated surface structures in the photosensitive and transparent hybrid films, the surface roughness was observed through AFM. Figure 5 shows the AFM images of the photofabricated surface structures in the photosensitive and transparent hybrid films.
Fig. 5 AFM images of the photofabricated surface structures in photosensitive and transparent hybrid films (left: 3D image, right: 2D image).
The AFM images shown in Fig. 5 reveal that the surface structures that had been directly photofabricated through the incorporation and chemical networking of organically modified colloidal-silica nanoparticles with oligosiloxanes within hybrid materials have a very smooth and homogeneous surface, with a low root-mean-square (RMS) roughness of around 1.099 nm. This good RMS roughness is because the incorporation and photoinduced chemical networking of the organically modified colloidal-silica nanoparticles with oligosiloxanes could reduce the shrinkage of hybrid films during UV curing. The good surface properties with the abovementioned outstanding properties play an important role in enhancing the efficiency of the optical and optoelectrical devices.
4. Conclusion
In conclusion, highly photosensitive hybrid materials with silica nanoparticles and oligosiloxanes were successfully synthesized without any phase separation or aggregation. These hybrid materials exhibited a high transmittance of above 90% in visible wavelength ranges and a high photocurability of above 90% conversion degree by UV exposure. With the high photosensitivity, the hybrid materials were easily fabricated into the films and were also used for direct photopatterning without a developing process using the photomask method. The various surface structures with micrometer scales could be easily fabricated on the hybrid films by UV exposure and they exhibited the good RMS roughness of below 1 nm. Because of the good optical properties of hybrid materials, the hybrid materials can be good candidates for the application of optical elements.
References and links
1. N. Zettsu, T. Ubukata, T. Seki, and K. Ichimura, “Soft crosslinkable azo polymer for rapid surface relief formation and persistent fixation,” Adv. Mater. 13(22), 1693–1697 (2001). [CrossRef]
2. N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, “Surface relief structures on azo polymer films,” J. Mater. Chem. 9(9), 1941–1955 (1999). [CrossRef]
3. T. J. Trout, J. J. Schmieg, W. J. Gambogi, and A. M. Weber, “Optical photopolymers: design and application,” Adv. Mater. 10(15), 1219–1224 (1998). [CrossRef]
4. V. Weiss, A. A. Friesem, and V. A. Krongauz, “Holographic recording and all-optical modulation in photochromic polymers,” Opt. Lett. 18(13), 1089–1091 (1993). [CrossRef] [PubMed]
5. D. J. Kang, W. S. Kim, B. S. Bae, H. K. Park, and B. H. Jung, “Direct photofabrication of refractive-index-modulated multimode optical waveguide using photosensitive sol-gel hybrid materials,” Appl. Phys. Lett. 87(22), 221106 (2005). [CrossRef]
6. W. Yu and X.-C. Yuan, “Variable surface profile gratings in sol-gel glass fabricated by holographic interference,” Opt. Express 11(16), 1925–1930 (2003), http://www.opticsexpress.org/abstract.cfm?URI=OPEX-11-16-1925. [CrossRef] [PubMed]
7. B. S. Bae, O. H. Park, R. Charters, B. Luther-Davies, and G. R. Atkins, “Direct laser writing of self-developed waveguides in benzyl dimethyl ketal doped sol-gel hybrid glass,” J. Mater. Res. 16(11), 3184–3187 (2001). [CrossRef]
8. P. Äyräs, J. T. Rantala, S. Honkanen, S. B. Mendes, and N. Peyghambarian, “Diffraction gratings in sol-gel films by direct contact printing using a UV-mercury lamp,” Opt. Commun. 162(4-6), 215–218 (1999). [CrossRef]
9. R. Sramek, R. Smektala, W. X. Xie, M. Douay, and P. Niay, “Photoinduced surface expansion of fluorozirconate glasses,” J. Non-Cryst. Solids 277(1), 39–44 (2000). [CrossRef]
10. A. Akella, T. Honda, A. Y. Liu, and L. Hesselink, “Two-photon holographic recording in aluminosilicate glass containing silver particles,” Opt. Lett. 22(13), 967–969 (1997). [CrossRef] [PubMed]
11. D. J. Kang, D. H. Han, D. P. Kang, J. W. Yoo, and B. U. Kim, “Fabrication of surface-structure controlled photopatterns upon transparent silica-acryl nanohybrid films with enhanced thermal and mechanical properties,” Opt. Express 16(22), 18320–18325 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-22-18320. [CrossRef] [PubMed]
12. D. J. Kang and B. S. Bae, “Photo-imageable sol-gel hybrid materials for simple fabrication of micro-optical elements,” Acc. Chem. Res. 40(9), 903–912 (2007). [CrossRef] [PubMed]
13. D. J. Kang, J. P. Jeong, and B. S. Bae, “Direct photofabrication of focal-length-controlled microlens array using photoinduced migration mechanisms of photosensitive sol-gel hybrid materials,” Opt. Express 14(18), 8347–8353 (2006), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-18-8347. [CrossRef] [PubMed]
14. A. H. O. Kärkkäinen, J. T. Rantala, J. M. Tamkin, and M. R. Descour, “Photolithographic processing of hybrid glasses for microoptics,” J. Lightwave Technol. 21(3), 614–623 (2003). [CrossRef]
15. W. S. Kim, R. Houbertz, T. H. Lee, and B. S. Bae, “Effect of photoinitiator on photopolymerization of inorganic-organic hybrid polymers (ORMOCER),” J. Polym. Sci. B 42(10), 1979–1986 (2004). [CrossRef]
