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Coating process of ZnO thin film on macroporous silica periodic array
Thin Solid Films 504 (2006) 41 – 44 www.elsevier.com/locate/tsf
Coating process of ZnO thin film on macroporous silica periodic array Y.H. Cheng a,*, L.K. Teh a, Y.Y. Tay a, H.S. Park a, C.C. Wong a, S. Li b a
School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, Nanyang Avenue, 639798, Singapore b School of Materials Science and Engineering, Faculty of Science, University of New South Wales, Australia Available online 13 October 2005
Abstract ZnO was deposited onto a 3D ordered macroporous SiO2 structure using sol-gel method with reverse dip-coating. It was observed that a very thin layer of ZnO film formed on the surface of the macroporous SiO2 using field emission scanning electron microscopy (FESEM). This thin layer was approximately 70 nm thick. The layer was near transparent, clearly revealing the macroporous SiO2 structure underneath. The formation of this ZnO thin film suggests that miniscule air spheres trapped within the pores of the SiO2 structure prevent the ZnO precursor sol from entering into these preoccupied positions. This unique structure can probably serve as an embedded planar defect for photonic crystals in the future and is believed to exhibit unusual photonic and luminescence properties which hold novel future applications. D 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Thin film; Silica periodic array; Sol-gel; Photonic crystals
1. Introduction Photonic crystals have recently emerged as a prospective powerful platform for manipulating light. They are dielectric materials with alternating low and high refractive index areas [1]. The feature sizes of these crystals are comparable to the wavelength of photons thus exhibiting photonic bands of allowed and forbidden frequency regions for light propagation. A photonic band gap (PBG) may enhance or suppress spontaneous emission and achieve localization of light [2]. Three-dimensionally (3D) macroporous silica (SiO2) arrays fits this description. Such structures can be fabricated via a colloidal crystal templating technique [3]. The structure has a periodic arrangement of solid SiO2 walls of high refractive index of 1.455 and air voids of low refractive index of 1.0. These crystals are used in optical sensors, light filters, catalysts, bioactive scaffolds and other optoelectronic applications [4,5]. Changes to physical structures of photonic crystals may radically alter their optical properties. This is true where defect engineering (planar, line or point defects)
* Corresponding author. Tel.: +65 6790 6161; fax: +65 6790 9081. E-mail address: [email protected] (Y.H. Cheng). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.113
are necessary in photonic crystals for practical device applications [6]. Recently, it was reported that engineered planar defects could be embedded in the photonic crystals [7,8]. This embedded planar defect can act as photonic microcavities for photonic applications [7]. In addition, it has a potential in controlling light in the visible and nearinfrared wavelength range within colloidal photonic crystals [8]. On the other hand, zinc oxide is a versatile semiconductor material with unique optical and electronic properties [9]. It possesses a characteristic electronic band gap of 3.3 eV as well as photoluminescence qualities. This novel structure might hold unprecedented breakthroughs in photonics. It is discovered that ZnO thin film can act as a waveguide, which holds the key to the success of random lasing when the film is deposited on a ZnO nanorod array [10]. Such a film can strongly confine the light within the structure and also amplify the optical signals. Therefore we believed that the layer of ZnO thin film playing the role of an embedded planar defect, would modify the PBG of the SiO2 structure, enhance scattering and localization of photons and possibly enable light to be controlled and manipulated in ways not previously possible or even imaginable. In this work, we report a fabrication technique
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Y.H. Cheng et al. / Thin Solid Films 504 (2006) 41 – 44
Fig. 1. Top view of SEI (SEM) micrograph at 40 000 magnification of PS spheres in a close pack FCC array.
that successfully coat a layer of ZnO thin film on a threedimensionally macroporous SiO2 structure by using sol-gel technology. This is promising as we believe that this ZnO thin film could serve as an embedded planar defect in photonic crystals in the future. 2. Experimental procedure Monodisperse polystyrene (PS) colloids (¨ 200 nm in diameter) were prepared by surfactant-free emulsion polymerization [11]. The PS colloids were coated on a Silicon substrate by means of vertical self-assembly. The substrate which serves as a platform for the array to form was precleaned with Piranha solution (1 H2O2 : 3 NaOH). The substrate with dimension, 2.5 cm 0.5 cm was placed vertically into a vial containing a suspension of PS colloidal solution (0.45 wt.% in DI water). The vial was then heated in a standard oven at 70 -C, close to the boiling point of water but below the glass transition temperature of PS. As the solvent was evaporated, the meniscus was swept vertically down the substrate while layers of PS were deposited onto the substrate due to capillary forces. The vial was carefully designed to ensure a slow deposition rate in order to yield a high quality PS template for subsequent filling of SiO2. After the solvent had completely evaporated, the substrate was removed from the vial. SiO2 precursor fluid was formed by mixing 4 ml of ethanol (99.8%, Merck), 6 ml of tetraethyl orthosilicate (TEOS 98%, Aldrich) and 3 ml of DI water. Concentrated hydrochloric acid (HCl 36%, Univar) was added drop wise to the sol as a catalyst for hydrolysis [12]. The entire mixture was stirred with constant speed of 330 rpm for 2 h. The SiO2 macroporous structure was fabricated by filling the interstices of closepacked arrays of PS spheres with the SiO2 precursor fluid which formed a solid skeleton around the spheres. Then, the PS template was placed vertically against a glass slide and balanced in a filter funnel. A certain volume of precursor sol was measured using a micropipette and wetted carefully onto the PS template. The template was left to dry at room temperature and subsequently placed into a furnace for calcination according to the heating profile: ramping at 2 -C
for 135 min, holding at 300 -C for 2 h, subsequently ramping at 2 -C for 10 h then followed by cooling. As the solvent and organic products evaporate, the precursor eventually gel to form a continuous network solid within the PS template. As the temperature further increased, the PS spheres will decompose completely to leave behind a porous SiO2 structure. ZnO precursor sol was prepared by mixing Zinc Acetate (C2H3O2)2Zn powder (ZnAc, 99.99%, Aldrich) with 2-propanol (IPA, Baker) by molar ratio of 1 : 55 [9]. The mixture was stirred thoroughly for 15 min on a magnetic stirrer after which 0.5733 ml of diethanolamine (DEA, C4H11NO2) was added to further dissolve ZnAc. The mixture was stirred for 2 h. A reverse dip-coating technique was employed to coat the ZnO precursor onto the macroporous silica structure. In this technique, the substrate was placed vertically against a beaker whereby ZnO precursor sol was added, covering the entire substrate. After a short duration the remaining solution was drawn out by a Peristaltic Pump at a prefixed speed. The substrate was subsequently heated in the furnace by first ramping up at 2 -C/min to 400 -C and held for 1 h for calcination of ZnO thin film. Electron micrographs were obtained on a Jeol SM 6340F Field Emission Scanning Electron Microscope (FESEM). Energy dispersive X-ray spectroscopy (EDX) was used to perform chemical analysis qualitatively. 3. Results and discussion 3.1. Preparation of Polystyrene spherical template on Silicon substrate The use of varying weight percents of PS emulsion solution allows manipulation of the thickness of PS spherical layers. By decreasing the weight percentage, the number of PS layers coated onto the Silicon substrate will effectively decrease. Each layer of PS is closely packed. The FESEM micrograph of PS template is as seen in Fig. 1. The PS colloids are monodisperse and approximately 200 nm in diameter. This size is specially chosen as the photonic crystal effects can be easily validated in porous silica [3].
Fig. 2. Top view of SEI (FESEM) micrograph at 27 000 magnification of macroporous silica structure. The pores are occupied by air spheres.
Y.H. Cheng et al. / Thin Solid Films 504 (2006) 41 – 44
43
Fig. 3. Top view of SEI (FESEM) micrograph at 14 000 magnification showing near transparent ZnO thin film on top of macroporous silica structure.
Fig. 5. EDX measurement of ZnO thin film on macroporous silica structure confirms the presence of ZnO.
3.2. Preparation of SiO2 macroporous spherical structure
surface of the macroporous structure after calcination. EDX measurement performed on the thin film layer has confirmed the presence of ZnO as shown in Fig. 5. At this region, the SiO2 porous structure is observed to be distorted and slightly broken. This is attributed to the process of cross-sectioning which could have caused slight damage to the structure. It is seen clearly that the thin layer of film is being supported on top of the macroporous layer only; there is no sign of ZnO gel infiltrating into the macroporous structure. Such a structure defies the natural phenomenon of capillary actions. By this, the minute pores ought to perform attractive forces in pulling the ZnO sol inwards and away from the surface. However, with the stringent tailored conditions mentioned earlier, forces that can be controlled to be much greater than capillary forces are in play here. During the sol-gel reverse dip-coating process, the ZnO precursor is concentrated on the porous SiO2 surface by gravitational draining with concurrent evaporation and condensation reactions [14]. These actions, with the assistance of surface tension, allow the progressive wetting and adherence of the ZnO sol on the topmost surface of the SiO2 macroporous structure. It is believed that the ZnO sol was prevented from entering the SiO2 pores because the pores were occupied by air spheres that are not readily displaced due to pressure differences of the inner pores and the ambient. In addition, this thin ZnO film reflects a high evaporation rate of the solvent, increasing the viscosity of the precursor sol rapidly. This allows the cohesive forces between the dispersed particles within the ZnO precursor sol as a result of sol-gel nature to be much greater than the adhesive forces between the ZnO sol and the inner walls of the porous structure, thus enhancing the attraction forces of the sol at the surface and allowing the sol to adhere to the top surface. Past work done on the photoluminescence of sol-gel ZnO and SiO2 composites shows that the optical band gap of the final composite structure ranged from 4.23 to 4.29 eV [12]. This marked an increase in band gap energy of pure ZnO or SiO2 alone which suggest that even if the silica structure might not exhibit full photonic band gap due to the low dielectric contrast a full photonic band gap could also possible be achieved when it is coated with the high dielectric ZnO thin film.
500 Al of SiO2 precursor sol is fixed as the standard volume for infiltration into the PS close packed array. As seen in Fig. 2, well-ordered air spheres and interconnected inorganic walls create a ‘‘honeycomb’’ pore structure in three dimensions. The macropores are monodisperse and each pore diameter is approximately 195 nm. The whole structure is built upon layers of interconnected SiO2 pores. The silica array is simply the reverse structure of its predecessor PS array and is classified as corresponding to planes of the FCC structure, such as (111) or (011) by previous work done by our team [13]. 3.3. Reverse dip-coating of ZnO thin film on macroporous SiO2 structure A layer of near transparent film is observed to have formed on the top surface of the SiO2 structure in Fig. 3. The pores are clearly visible beneath this layer of thin film which displays homogeneity throughout the entire surface. A cross-section was performed on this structure and it is shown in Fig. 4. The thin film has a thickness of about 70 nm. As seen, an ultrathin layer of film has been formed on the top
Fig. 4. SEI (FESEM) micrograph at 70 000 magnification showing crosssectioned area of thin film supported on macroporous silica structure. The silica structure is slightly distorted due to cross-sectioning.
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Y.H. Cheng et al. / Thin Solid Films 504 (2006) 41 – 44
4. Conclusion We report an innovative and simple method of supporting a thin layer of ZnO on a three-dimensionally ordered SiO2 macroporous structure by means of an appropriate reverse dipcoating technique. This thin ZnO film can be used as a potential embedded planar defect in photonic crystals in the future. It is believed that the combined actions of surface tension coupled with various adhesive forces allow the formation of a layer of ZnO thin film on the porous substrate. We propose that reverse dip-coating conditions, parameters as well as wetting and solidification of the ZnO thin film, are critical requirements. As these issues are addressed, the structure will be improved to meet the requirements of possible future applications. Such a structure may possibly emerge as a new class of photonic crystal and may form the basis of a new kind of laser or be applied in optoelectronic applications such as grating, optical switches and biosensors. Acknowledgements The authors wish to thank Dr Tan Thiam Teck from Singapore Institute of Manufacturing Technology (SIMTech) for his valuable advice and support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. J.D. Joannopoulos, Nature 414 (2001) 257. R.C. Schroden, A.D. Mohammed, A. Stein, Chem. Mater. 13 (2001) 2945. B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795. N.A. Kotov, Y. Liu, S. Wang, C. Cunming, E. Mohammad, G. Vargas, M. Motamedi, J. Nichols, J. Cortiella, Langmuir 20 (2004) 7887. Q. Yan, Z. Zhou, X.S. Zhao, S.J. Chua, Adv. Mater. 17 (2005) 1917. E. Palacios-Lidon, J.F. Galisteo-Lopez, B.H. Juarez, C. Lopez, Adv. Mater. 16 (2004) 341. N. Tetreault, A. Mihi, H. Miquez, I. Rodriquez, G.A. Ozin, Adv. Mater. 16 (2004) 346. M. Izaki, T. Omi, J. Electrochem. Soc. 144 (1997) 1949. S.F. Yu, C. Yuen, S.P. Lau, W.I. Park, G.C. Yi, Appl. Phys. Lett. 84 (2004) 3241. G.T.D. Shouldice, G.A. Vandezande, A. Rudin, Eur. Polym. 30 (1994) 179. S. Chakrabarti, D. Ganguli, S. Chaudhuri, J. Phys., D, Appl. Phys. 36 (2003) 146. L.K. Teh, N.K. Tan, C.C. Wong, S. Li, Appl. Phys. A 81 (2005) 1399. C.J. Brinker, G.C. Frye, A.J. Hurd, C.S. Ashley, Thin Solid Films 201 (1991) 97.
Coating process of ZnO thin film on macroporous silica periodic array Y.H. Cheng a,*, L.K. Teh a, Y.Y. Tay a, H.S. Park a, C.C. Wong a, S. Li b a
School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, Nanyang Avenue, 639798, Singapore b School of Materials Science and Engineering, Faculty of Science, University of New South Wales, Australia Available online 13 October 2005
Abstract ZnO was deposited onto a 3D ordered macroporous SiO2 structure using sol-gel method with reverse dip-coating. It was observed that a very thin layer of ZnO film formed on the surface of the macroporous SiO2 using field emission scanning electron microscopy (FESEM). This thin layer was approximately 70 nm thick. The layer was near transparent, clearly revealing the macroporous SiO2 structure underneath. The formation of this ZnO thin film suggests that miniscule air spheres trapped within the pores of the SiO2 structure prevent the ZnO precursor sol from entering into these preoccupied positions. This unique structure can probably serve as an embedded planar defect for photonic crystals in the future and is believed to exhibit unusual photonic and luminescence properties which hold novel future applications. D 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Thin film; Silica periodic array; Sol-gel; Photonic crystals
1. Introduction Photonic crystals have recently emerged as a prospective powerful platform for manipulating light. They are dielectric materials with alternating low and high refractive index areas [1]. The feature sizes of these crystals are comparable to the wavelength of photons thus exhibiting photonic bands of allowed and forbidden frequency regions for light propagation. A photonic band gap (PBG) may enhance or suppress spontaneous emission and achieve localization of light [2]. Three-dimensionally (3D) macroporous silica (SiO2) arrays fits this description. Such structures can be fabricated via a colloidal crystal templating technique [3]. The structure has a periodic arrangement of solid SiO2 walls of high refractive index of 1.455 and air voids of low refractive index of 1.0. These crystals are used in optical sensors, light filters, catalysts, bioactive scaffolds and other optoelectronic applications [4,5]. Changes to physical structures of photonic crystals may radically alter their optical properties. This is true where defect engineering (planar, line or point defects)
* Corresponding author. Tel.: +65 6790 6161; fax: +65 6790 9081. E-mail address: [email protected] (Y.H. Cheng). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.113
are necessary in photonic crystals for practical device applications [6]. Recently, it was reported that engineered planar defects could be embedded in the photonic crystals [7,8]. This embedded planar defect can act as photonic microcavities for photonic applications [7]. In addition, it has a potential in controlling light in the visible and nearinfrared wavelength range within colloidal photonic crystals [8]. On the other hand, zinc oxide is a versatile semiconductor material with unique optical and electronic properties [9]. It possesses a characteristic electronic band gap of 3.3 eV as well as photoluminescence qualities. This novel structure might hold unprecedented breakthroughs in photonics. It is discovered that ZnO thin film can act as a waveguide, which holds the key to the success of random lasing when the film is deposited on a ZnO nanorod array [10]. Such a film can strongly confine the light within the structure and also amplify the optical signals. Therefore we believed that the layer of ZnO thin film playing the role of an embedded planar defect, would modify the PBG of the SiO2 structure, enhance scattering and localization of photons and possibly enable light to be controlled and manipulated in ways not previously possible or even imaginable. In this work, we report a fabrication technique
42
Y.H. Cheng et al. / Thin Solid Films 504 (2006) 41 – 44
Fig. 1. Top view of SEI (SEM) micrograph at 40 000 magnification of PS spheres in a close pack FCC array.
that successfully coat a layer of ZnO thin film on a threedimensionally macroporous SiO2 structure by using sol-gel technology. This is promising as we believe that this ZnO thin film could serve as an embedded planar defect in photonic crystals in the future. 2. Experimental procedure Monodisperse polystyrene (PS) colloids (¨ 200 nm in diameter) were prepared by surfactant-free emulsion polymerization [11]. The PS colloids were coated on a Silicon substrate by means of vertical self-assembly. The substrate which serves as a platform for the array to form was precleaned with Piranha solution (1 H2O2 : 3 NaOH). The substrate with dimension, 2.5 cm 0.5 cm was placed vertically into a vial containing a suspension of PS colloidal solution (0.45 wt.% in DI water). The vial was then heated in a standard oven at 70 -C, close to the boiling point of water but below the glass transition temperature of PS. As the solvent was evaporated, the meniscus was swept vertically down the substrate while layers of PS were deposited onto the substrate due to capillary forces. The vial was carefully designed to ensure a slow deposition rate in order to yield a high quality PS template for subsequent filling of SiO2. After the solvent had completely evaporated, the substrate was removed from the vial. SiO2 precursor fluid was formed by mixing 4 ml of ethanol (99.8%, Merck), 6 ml of tetraethyl orthosilicate (TEOS 98%, Aldrich) and 3 ml of DI water. Concentrated hydrochloric acid (HCl 36%, Univar) was added drop wise to the sol as a catalyst for hydrolysis [12]. The entire mixture was stirred with constant speed of 330 rpm for 2 h. The SiO2 macroporous structure was fabricated by filling the interstices of closepacked arrays of PS spheres with the SiO2 precursor fluid which formed a solid skeleton around the spheres. Then, the PS template was placed vertically against a glass slide and balanced in a filter funnel. A certain volume of precursor sol was measured using a micropipette and wetted carefully onto the PS template. The template was left to dry at room temperature and subsequently placed into a furnace for calcination according to the heating profile: ramping at 2 -C
for 135 min, holding at 300 -C for 2 h, subsequently ramping at 2 -C for 10 h then followed by cooling. As the solvent and organic products evaporate, the precursor eventually gel to form a continuous network solid within the PS template. As the temperature further increased, the PS spheres will decompose completely to leave behind a porous SiO2 structure. ZnO precursor sol was prepared by mixing Zinc Acetate (C2H3O2)2Zn powder (ZnAc, 99.99%, Aldrich) with 2-propanol (IPA, Baker) by molar ratio of 1 : 55 [9]. The mixture was stirred thoroughly for 15 min on a magnetic stirrer after which 0.5733 ml of diethanolamine (DEA, C4H11NO2) was added to further dissolve ZnAc. The mixture was stirred for 2 h. A reverse dip-coating technique was employed to coat the ZnO precursor onto the macroporous silica structure. In this technique, the substrate was placed vertically against a beaker whereby ZnO precursor sol was added, covering the entire substrate. After a short duration the remaining solution was drawn out by a Peristaltic Pump at a prefixed speed. The substrate was subsequently heated in the furnace by first ramping up at 2 -C/min to 400 -C and held for 1 h for calcination of ZnO thin film. Electron micrographs were obtained on a Jeol SM 6340F Field Emission Scanning Electron Microscope (FESEM). Energy dispersive X-ray spectroscopy (EDX) was used to perform chemical analysis qualitatively. 3. Results and discussion 3.1. Preparation of Polystyrene spherical template on Silicon substrate The use of varying weight percents of PS emulsion solution allows manipulation of the thickness of PS spherical layers. By decreasing the weight percentage, the number of PS layers coated onto the Silicon substrate will effectively decrease. Each layer of PS is closely packed. The FESEM micrograph of PS template is as seen in Fig. 1. The PS colloids are monodisperse and approximately 200 nm in diameter. This size is specially chosen as the photonic crystal effects can be easily validated in porous silica [3].
Fig. 2. Top view of SEI (FESEM) micrograph at 27 000 magnification of macroporous silica structure. The pores are occupied by air spheres.
Y.H. Cheng et al. / Thin Solid Films 504 (2006) 41 – 44
43
Fig. 3. Top view of SEI (FESEM) micrograph at 14 000 magnification showing near transparent ZnO thin film on top of macroporous silica structure.
Fig. 5. EDX measurement of ZnO thin film on macroporous silica structure confirms the presence of ZnO.
3.2. Preparation of SiO2 macroporous spherical structure
surface of the macroporous structure after calcination. EDX measurement performed on the thin film layer has confirmed the presence of ZnO as shown in Fig. 5. At this region, the SiO2 porous structure is observed to be distorted and slightly broken. This is attributed to the process of cross-sectioning which could have caused slight damage to the structure. It is seen clearly that the thin layer of film is being supported on top of the macroporous layer only; there is no sign of ZnO gel infiltrating into the macroporous structure. Such a structure defies the natural phenomenon of capillary actions. By this, the minute pores ought to perform attractive forces in pulling the ZnO sol inwards and away from the surface. However, with the stringent tailored conditions mentioned earlier, forces that can be controlled to be much greater than capillary forces are in play here. During the sol-gel reverse dip-coating process, the ZnO precursor is concentrated on the porous SiO2 surface by gravitational draining with concurrent evaporation and condensation reactions [14]. These actions, with the assistance of surface tension, allow the progressive wetting and adherence of the ZnO sol on the topmost surface of the SiO2 macroporous structure. It is believed that the ZnO sol was prevented from entering the SiO2 pores because the pores were occupied by air spheres that are not readily displaced due to pressure differences of the inner pores and the ambient. In addition, this thin ZnO film reflects a high evaporation rate of the solvent, increasing the viscosity of the precursor sol rapidly. This allows the cohesive forces between the dispersed particles within the ZnO precursor sol as a result of sol-gel nature to be much greater than the adhesive forces between the ZnO sol and the inner walls of the porous structure, thus enhancing the attraction forces of the sol at the surface and allowing the sol to adhere to the top surface. Past work done on the photoluminescence of sol-gel ZnO and SiO2 composites shows that the optical band gap of the final composite structure ranged from 4.23 to 4.29 eV [12]. This marked an increase in band gap energy of pure ZnO or SiO2 alone which suggest that even if the silica structure might not exhibit full photonic band gap due to the low dielectric contrast a full photonic band gap could also possible be achieved when it is coated with the high dielectric ZnO thin film.
500 Al of SiO2 precursor sol is fixed as the standard volume for infiltration into the PS close packed array. As seen in Fig. 2, well-ordered air spheres and interconnected inorganic walls create a ‘‘honeycomb’’ pore structure in three dimensions. The macropores are monodisperse and each pore diameter is approximately 195 nm. The whole structure is built upon layers of interconnected SiO2 pores. The silica array is simply the reverse structure of its predecessor PS array and is classified as corresponding to planes of the FCC structure, such as (111) or (011) by previous work done by our team [13]. 3.3. Reverse dip-coating of ZnO thin film on macroporous SiO2 structure A layer of near transparent film is observed to have formed on the top surface of the SiO2 structure in Fig. 3. The pores are clearly visible beneath this layer of thin film which displays homogeneity throughout the entire surface. A cross-section was performed on this structure and it is shown in Fig. 4. The thin film has a thickness of about 70 nm. As seen, an ultrathin layer of film has been formed on the top
Fig. 4. SEI (FESEM) micrograph at 70 000 magnification showing crosssectioned area of thin film supported on macroporous silica structure. The silica structure is slightly distorted due to cross-sectioning.
44
Y.H. Cheng et al. / Thin Solid Films 504 (2006) 41 – 44
4. Conclusion We report an innovative and simple method of supporting a thin layer of ZnO on a three-dimensionally ordered SiO2 macroporous structure by means of an appropriate reverse dipcoating technique. This thin ZnO film can be used as a potential embedded planar defect in photonic crystals in the future. It is believed that the combined actions of surface tension coupled with various adhesive forces allow the formation of a layer of ZnO thin film on the porous substrate. We propose that reverse dip-coating conditions, parameters as well as wetting and solidification of the ZnO thin film, are critical requirements. As these issues are addressed, the structure will be improved to meet the requirements of possible future applications. Such a structure may possibly emerge as a new class of photonic crystal and may form the basis of a new kind of laser or be applied in optoelectronic applications such as grating, optical switches and biosensors. Acknowledgements The authors wish to thank Dr Tan Thiam Teck from Singapore Institute of Manufacturing Technology (SIMTech) for his valuable advice and support.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. J.D. Joannopoulos, Nature 414 (2001) 257. R.C. Schroden, A.D. Mohammed, A. Stein, Chem. Mater. 13 (2001) 2945. B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795. N.A. Kotov, Y. Liu, S. Wang, C. Cunming, E. Mohammad, G. Vargas, M. Motamedi, J. Nichols, J. Cortiella, Langmuir 20 (2004) 7887. Q. Yan, Z. Zhou, X.S. Zhao, S.J. Chua, Adv. Mater. 17 (2005) 1917. E. Palacios-Lidon, J.F. Galisteo-Lopez, B.H. Juarez, C. Lopez, Adv. Mater. 16 (2004) 341. N. Tetreault, A. Mihi, H. Miquez, I. Rodriquez, G.A. Ozin, Adv. Mater. 16 (2004) 346. M. Izaki, T. Omi, J. Electrochem. Soc. 144 (1997) 1949. S.F. Yu, C. Yuen, S.P. Lau, W.I. Park, G.C. Yi, Appl. Phys. Lett. 84 (2004) 3241. G.T.D. Shouldice, G.A. Vandezande, A. Rudin, Eur. Polym. 30 (1994) 179. S. Chakrabarti, D. Ganguli, S. Chaudhuri, J. Phys., D, Appl. Phys. 36 (2003) 146. L.K. Teh, N.K. Tan, C.C. Wong, S. Li, Appl. Phys. A 81 (2005) 1399. C.J. Brinker, G.C. Frye, A.J. Hurd, C.S. Ashley, Thin Solid Films 201 (1991) 97.