Article pubs.acs.org/crystal
Fabrication of a Crack-Free Large Area Photonic Crystal with Colloidal Silica Spheres Modified with Vinyltriethoxysilane Byoung-Ju Kim and Kwang-Sun Kang* Department of New & Renewable Energy, Kyungil University, 8-212-1, 33 Buhori, Hayangup, Kyeongsan, Kyeongbuk 712-701, South Korea
ABSTRACT: Crack-free large area photonic crystals have been fabricated using colloidal silica spheres modified the surface with vinyltriethoxysilane (VTES) and an in situ photo-cross-linking (ISPL) process during the self-assembly of the surface modified spheres (SMS). The VTES has been covalently attached on the surface of the silica spheres by hydrolysis and condensation reactions. The weight ratios of the VTES and the spheres are 1:7 (VTES-A), 2:7 (VTES-B), and 3:7 (VTES-C). The Fourier transform infrared (FTIR) absorption peak attributed to the −OH bond stretching vibration shifts from 3230 to 3480 cm−1 with increasing the amount of VTES. The characteristic absorption peaks of −CH and −CH2 appeared at 2853, 2956, and 3023 cm−1 for VTES-C. The absorption peat at 1409 cm−1 representing the −CH deformation vibration increased with the increase of the amount of VTES. The stop bands are shown at 655, 693, and 708 nm for VTES-A, VTES-B, and VTES-C, respectively. The stop band shift toward longer wavelength is mainly due to the increase of the effective refractive index. The characteristic absorption peak −CH2−CH3 increased drastically after the ISPL process and the washing process due to the stronger absorption intensity of the −CH2−CH3 compared with the absorption intensity of −CHCH2. Although the photonic crystal constructed with pure silica spheres has cracks between the clusters, the photonic crystal fabricated with SMS with VTES and the ISPL process during the self-assembly of the silica spheres shows no crack in large area. This result provides the fabrication method of a large area and stable photonic crystal without cracks.
■
INTRODUCTION Photonic crystals consist of periodically arranged materials with different refractive indices, are a novel architecture with unique optical properties, and have received great attention due to their potential applications in various optical devices.1 The crystal structures exhibit a photonic band gap under light illumination conditions, which indicates that a certain range of optical frequencies is forbidden in the crystal structure.2 Control of light emission and propagation in the photonic crystal is possible with this photonic band gap. Fabrication of a large area photonic crystal is essential to real application and can be achieved by colloidal crystallization,3 two-photon polymerization,4,5 semiconductor microfabrication,6 a modified vertical deposition method of tetraethoxysilane,7 and laser interference lithography.8 However, most of these methods have achieved various success levels. Therefore, fabrication of the large area photonic crystal is still a challenge. Fabrication of the photonic crystals with a less than 350 nm periodic structure requires obtaining a band gap in the visible © 2012 American Chemical Society
range and has been an important work for band gap engineering.9 For this reason, electron beam lithography has been employed and widely used to achieve the visible range band gap.9,10 However, this method is expensive and timeconsuming for large area photonic crystals. Combined focused ion beam and laser interference lithography9 has been employed to improve the fabrication speed and achieved a periodic submicrometer pattern over a very large area on the order of a square centimeter. In this paper, we report the fabrication process of a large area photonic crystal by modifying the surface of the colloidal silica spheres with vinyltriethoxysilane (VTES) and an in situ photocross-linking process (ISPL) during the self-assembly process. Fourier transform infrared (FTIR) spectra of the surface modified spheres and UV-cured spheres, characteristic visible Received: April 16, 2012 Revised: May 31, 2012 Published: June 15, 2012 4039
dx.doi.org/10.1021/cg3005147 | Cryst. Growth Des. 2012, 12, 4039−4042
Crystal Growth & Design
Article
Figure 1. Graphical representation of uniform size SiO2 sphere fabrication, with surface modification using base catalyst.
CH deformation vibration absorption at 1409 cm−1 increased as the amount of VTES increased. The CCH symmetric and asymmetric stretching vibration absorption peaks centered at 3023, 2956, and 2853 cm−1 increased with the increase of the amount of the VTES. Irgacure-184 is a commercially available photoinitiator and has the chemical name 1-hydroxy-cyclohexyl-phenyl-ketone. The chemical structure is shown inside Figure 2. It is a highly efficient photoinitiator which is used to initiate the photopolymerization of chemically unsaturated carbon−carbon bonds. Figure 2 shows the FTIR spectra for the bare spheres,
range transmittance of the modified spheres, and field emission scanning electron microscope (FESEM) images are also presented.
■
EXPERIMENTAL SECTION
Monodisperse silica spheres have been fabricated via the Stöber method. A mixture of methanol and ammonium hydroxide (NH4OH) solution has been utilized as a solvent and a catalyst, respectively. Methanol (100 mL) and tetraethoxysilane (TEOS, 1.4 g) were charged to the 250 mL round-bottom flask. The NH4OH (100 mL) solution was added to the mixture of methanol and TEOS, and the solution was stirred for 6 h at room temperature. And then, 200 mg (VTES-A), 400 mg (VTES-B), or 600 mg (VTES-C) of VTES was added stepwise with approximately 200 mg at each step. The spheres were centrifuged with a spinning rate of 3000 rpm and washed with methanol. UV−visible and FTIR spectra were obtained with a UV visible spectrometer (Thermo scientific genesys 10S) and FTIR spectrometer (Nicolet iS5), respectively. The spheres were dispersed into the methanol, and approximately 2 wt % of Irgacure-184 (Ciba) with respect to the sphere weight was added to the sphere solution as a photoinitiator. In situ self-assembly with ISPL has been performed by dropping the surface modified spheres (SMS) solution onto the silicon wafer and UV-illumination (254 nm). FESEM images were obtained with a JEOL JSM-7401F microscope operated at 5 kV.
■
RESULTS AND DISCUSSION The VTES molecule has three ethoxy (−OCH2CH3) groups and one vinyl group (−CHCH2). Three ethoxy groups are hydrolyzed to become three silanol groups, Si(OH)3. The silanol groups react with silanol groups on the surface of the spheres or hydrolyzed VTES. A graphical representation of this process is shown in Figure 1. Each silanol group on the surface of the spheres can be attached to single or multiple hydrolyzed VTES molecules as shown in Figure 1. Walrafen and Samanta11 deconvoluted the infrared absorption band of Si−OH with four Gaussian components at 3690, 3665, 3605, and 3510 cm−1, which indicated that the four different types of hydroxyl environments were in the glass matrix. High water-content SiO2 has a strong absorption band at 3665 and 3450 cm−1, and liquid water exhibits 3450 and 3200 cm−1 absorption bands.12,13 Figure 2 shows the FTIR spectra of VTES-A, VTES-B, and VTES-C. The absorption peak at 3230 cm−1 representing the −OH stretching vibration caused by absorbed water on the surface of the spheres shifted toward 3480 cm−1 and reduced the intensity with the increase of the amount of VTES. This result shows that the amount of the adsorbed H2O decreases with the increase of the amount of the VTES. The absorption intensity of the characteristic C
Figure 2. FTIR spectra of (a) SMS with various amounts of VTES; (b) bare spheres, VTES-C, VTES-C + Irgacure, and VTES-C exposed to UV-light and washed with methanol. The inset is the structure of Irgacure-184. 4040
dx.doi.org/10.1021/cg3005147 | Cryst. Growth Des. 2012, 12, 4039−4042
Crystal Growth & Design
Article
420 nm for VTES-A, VTES-B, and VTES-C, respectively. These UV-range stop bands are due to the small spheres with the diameters of approximately 190 nm and show a similar trend compared with the longer wavelength stop bands, except VTES-C, which requires more investigation in this exceptional case. The cracks can be easily developed in the thin films fabricated with silica spheres. Since the stress and strain are accumulated in the silica sphere film as the silica sphere film dries, the cracks are propagated in the film. The fabrication of the silica sphere film with bare spheres produces a large number of cracks, as shown in Figure 4a. Jin et al.17 demonstrated
VTES-C, VTES-C mixed with Irgacure-184, and VTES-C exposed to UV-light and washed with methanol. Approximately 2 wt % of Irgacure-184 was added to the VTES-C before UVexposure. The −OH absorption peak increased after mixing Irgacure-184, due to the −OH group of the Irgacure, and the CO absorption peak appeared in 1690 cm−1. After UVexposure and washing with methanol, the CO absorption peak disappeared, and the −OH absorption peak drastically reduced. Moreover, the characteristic −CH absorption peaks drastically increased after washing with methanol. These results provide that the increase of the −CH absorption is due to the change from −CHCH2 to −CH2−CH3, the absorption of −CH2−CH3 is much stronger than that of −CHCH2, and the impurity of the reaction product can be efficiently removed by washing and centrifuging the reaction product. The ordered periodic structures with two different refractive indices exhibit a stop band. The wavelength (λ) of the stop band can be estimated by14 λ = 1.633dnavg
(1)
where d is the center-to-center distance between two neighboring spheres and naverage is the average refractive index. As the refractive index or diameter of the spheres increases, the stop band shifts toward longer wavelength,15 as shown in eq 1. The photonic stop band can be proved experimentally by measuring its transmission spectra with different wavelengths. Figure 3 shows the differential trans-
Figure 3. Differential UV−visible spectra of VTES-A, VTES-B, and VTES-C.
mission spectra of an ordered sphere film with VTES-A, VTESB, and VTES-C. The attenuation bands were located at 672, 693, and 704 nm for the VTES-A, VTES-B, and VTES-C, respectively. As the amount of the VTES increased, the stop band shifted to the longer wavelength. According to the high resolution FESEM result, the diameter of the spheres is approximately 310 nm for all three samples. Since, according to the Stöber synthetic process,16 the average geometric standard deviation is approximately 1.06, which implies the deviation is less than 1%, and the used spheres are the same batch, we assume the average diameter may not significantly affect the band gap shift. The refractive indices for the VTES-A, VTES-B, and VTES-C are 1.327, 1.368, and 1.398. Since the thermal process is not performed for the spheres, the low refractive index may be due to the low density of the spheres. The increase of the refractive index should be due to the vinyl group of VTMS. The second stop bands appeared at 405, 418, and
Figure 4. FESEM images of (a, top) photonic crystal fabricated with bare spheres and (b, bottom) photonic crystal fabricated with SMS and ISPL.
fabrication of a crack-free photonic crystal with a modified template assisted colloidal self-assembly with a patterned substrate. Figure 4b shows a crack-free large area photonic crystal and a stable structure. The process with SMS and in situ ISPL during the self-assembly process is a promising method to fabricate a crack-free and stable large area photonic crystal. 4041
dx.doi.org/10.1021/cg3005147 | Cryst. Growth Des. 2012, 12, 4039−4042
Crystal Growth & Design
■
Article
(17) Jin, C.; McLachlan, M. A.; McComb, D. W.; De La Rue, R. M.; Johnson, N. P. NanoLett 2005, 5, 2646.
CONCLUSION The surface of the monodisperse silica spheres was modified with VTES using base catalyst. The large area photonic crystal was fabricated with SMS and ISPL during the self-assembly process. The FTIR absorption peak position shifts from 3230 to 3480 cm−1 with increasing the amount of VTES. The absorption peaks at 2853, 2956, and 3023 cm−1 representing the −CH and −CH2 stretching vibration, and the absorption peak at 1409 cm−1 characterizing −CH deformation vibration increased as the amount of VTES increased, which indicates the existence of the VTES on the surface of the silica spheres. The absorption peaks between 2800 and 3000 cm−1, representing −CH2−CH3 absorption, increased drastically after ISPL and the washing process due to the stronger absorption of the −CH2−CH3 with respect to −CHCH2. The stop bands of VTES-A, VTES-B, and VTES-C exist at 672, 693, and 708 nm, respectively. The stop band shift toward longer wavelength is mainly due to the increase of the refractive index. Although the photonic crystal constructed with pure silica spheres has cracks between the clusters, the photonic crystal fabricated with SMS and ISPL during the self-assembly shows no crack in a large area. This result provides the fabrication method of a large area and stable photonic crystal without a crack.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS This research is performed under the support of Kyungil University. REFERENCES
(1) John, S. Phys. Rev. Lett. 1987, 58, 2486. (2) MacDiarmid, A. G.; Chiang, J. C.; Richter, A. F.; Epstein, A. J. Synth. Met. 1987, 18, 285. (3) Miguez, H. Appl. Phys. Lett. 2003, 81, 2493. (4) Zhou, W.; Kuebler, S. M.; Braun, K. L.; Yu, T.; Cammack, J. K.; Ober, C. K.; Perry, J. W.; Marder, S. R. Science 2002, 296, 1106. (5) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich, J. E.; Erskine, L. L.A.; Heikal, A.; Kuebler, S. M.; Sandy Lee, I.Y.; McCord-Maughon, D.; Qin, J.; Rockel, H.; Eumi, M.; Wu, X. L.; Marder, S. R.; Perry, J. W. Nature 1999, 398, 51. (6) Cheng, C. C. J. Vac. Sci. Technol., B 1997, 82, 2764. (7) Wang, L.; Zhao, X. S. J. Phys. Chem. C 2007, 111, 8538. (8) Berger, V.; Gauthier-Lafaye, O.; Costard, E. Electron. Lett. 1997, 33, 425. (9) Vogelaar, L.; Nijdam, W.; van Wolferen, H. A. G. M.; de Ridder, R. M.; Segerink, F. B.; Fluck, E.; Kuipers, L.; van Hulst, N. F. Adv. Mater. 2001, 13, 1551. (10) Charlton, M. D. B.; Zoorob, M. E.; Parker, G. J.; Netti, M. C.; Baumberg, J. J.; Cox, S. J.; Kemhadjian, H. Mater. Sci. Eng., B 2000, 74, 17. (11) Walrafen, G. E.; Samanta, S. R. J. Chem. Phys. 1978, 69, 493. (12) McMillan, P. F.; Remmele, R. L. Am. Mineral. 1986, 71, 772. (13) Navarra, G.; Boscaino, R.; Leone, M.; Boizot, B. J. Non-Cryst. Solids 2007, 353, 555. (14) Gu, Z. Z.; Kubo, S.; Qian, W.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. Langmuir 2001, 17, 6751. (15) Cho, G.; Jung, M.; Yang, H.; Lee, B.; Song, J. H. Mater. Lett. 2007, 61, 1086. (16) Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62−69. 4042
dx.doi.org/10.1021/cg3005147 | Cryst. Growth Des. 2012, 12, 4039−4042
