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Photonic crystal heterostructures fabricated by TiO2 and ZnO inverse opals using colloidal crystal template with single kind of microspheres
Optical Materials 34 (2012) 1758–1761
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Photonic crystal heterostructures fabricated by TiO2 and ZnO inverse opals using colloidal crystal template with single kind of microspheres Yongna Zhang, Ming Fu ⇑, Jigang Wang, Dawei He, Yongsheng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2011 Received in revised form 6 April 2012 Accepted 21 April 2012 Available online 17 May 2012 Keywords: Heterostructure Photonic crystal Electrochemical deposition Sol–gel Broad band gap
a b s t r a c t The fabrication of photonic crystal heterostructures is important for the applications in the fields of integrated photonic crystal chips, multi-frequency optical Bragg filters or mirrors. However, multiple steps of self-assembly process of microspheres are always employed in the fabrication of photonic crystal heterostructures, which may produce lattice mismatches of colloidal crystals. Therefore, photonic crystal heterostructures fabricated by using colloidal crystal template with single kind of microspheres were investigated in this paper. A colloidal crystal template with uniform periodicity was firstly formed by monodispersed polystyrene microsphere. Then ZnO was electrodeposited into the interstices of the template. The thickness of ZnO was controlled to be less than the thickness of the template by varying the deposition time. After the TiO2 precursor was filled into the top voids in the template, the polystyrene colloidal crystal template was removed and photonic crystal heterostructures fabricated by ZnO and TiO2 were formed. Both the dielectric constant and the periodicity of the two parts of the heterostructures are different due to the shrinkage of the sol–gel process. The ZnO/TiO2 heterostructures have a broad photonic stop band which is the superposition of photonic stop bands of ZnO inverse opals and TiO2 inverse opals. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Photonic crystal is a three-dimensional (3D) periodic arrangement of the ordered structure which is composed of two or more kinds of dielectric media. Photonic crystal (PC) heterostructures are composed of PCs with different lattice constants or refractive index [1,2]. Many of the attractive features of semiconductor heterostructures can be extended into the optical domain for the photonic crystal heterostructures [3]. The study on PC heterostructures have attracted considerable amount of interests because of their possible applications, including integrated photonic crystal chips, multi-frequency optical Bragg filters or mirrors [4]. There are many different methods to prepare photonic crystal heterostructures. In the inward growing self-assembling method, photonic crystal heterostructures are made by a sequential growth of one photonic crystal over another within a total time span of 6 h [5]; many high quality photonic crystals have been prepared by convective self-assembling method [6–8]. Photonic heterocrystals are also prepared by sandwiching films of self-assembled opals and force-assembled Langmuir–Blodgett colloidal crystals [9]. High-quality 3D PC heterostructures with deep photonic bandgaps
⇑ Corresponding author. E-mail address: [email protected] (M. Fu). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.04.014
(PBGs) and deep photonic band edges (PBEs) are successfully achieved by means of the modified self-assembly method [10]. Ma fabricated silicon 2D–3D photonic crystal heterostructures using glancing angle deposition via depositing method [11]. Wang fabricated a photonic crystal heterostructures composed of two TiO2 inverse opal films with different filling factors through the sequential deposition [12]. Colloidal photonic crystal heterostructures, which are composed of two opaline photonic crystal films of silica spheres with different diameters, were fabricated through a two-step spin-coating method [13]. However, monodispersed colloidal spheres with different sizes were always used in these methods. Therefore, multiple steps of self-assembly process must be employed in the process which may produce lattice mismatches of colloidal crystals in nearby layers with different microsphere diameters [14]. Therefore, it is important to develop a method for the fabrication of PC heterostructures in one complete colloidal crystal template with single kind of microspheres [15–17]. Therefore, in this paper, ZnO/TiO2 heterostructures are made by colloidal crystal template with single kind of microspheres by different fabrication methods. Due to its short experimental period and low cost, it is comparably easier to be implemented in the laboratory. Furthermore, the fabrication of heterostructures by using colloidal crystals with single kind of colloidal microspheres is helpful for avoiding lattice mismatch before removing the microspheres [18].
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Fig. 1. SEM images of: (a) heterostructured inverse opal structures fabricated by ZnO and TiO2 (Area M: TiO2 inverse opals; Area N: ZnO inverse opals; Area N and P: colloidal crystal template); (b) pure TiO2 inverse opals and (c) pure ZnO inverse opals.
2. Methods 2.1. The preparation of colloidal crystal templates The colloidal crystals were made by the vertical deposition method via self-assembly process with monodispersed microspheres which were 370 nm in diameter. The glass sides covered with ITO films were ultrasonically cleaned with acetone and ethanol solvent. After that, glass slides were placed vertically into a bottle which contains 2 vol.% of monodispersed microsphere solution. Highly-ordered colloidal crystal templates were formed on the glass slides as the solvent evaporated at 50 °C and 30% moisture in the electric thermostat. After the solvent was evaporated, ITO glasses covered with colloidal crystals were formed. 2.2. Preparation of double layers of inverse opal structures fabricated by ZnO and TiO2 ZnO inverse opal structure was prepared by the electrochemical deposition method. ZnO films were deposited on the ITO substrate covered with colloidal crystal template in 0.1 mol/l Zn(NO3)2 aqueous solution at 70 °C. ITO substrates covered with colloidal crystal templates were used as the working electrode, while a platinum sheet was used as the counter electrode. A saturated calomel electrode was used as the referencing electrode. [19] The potentiostatic method at 1 V was employed for 10–30 min. The interstices of the colloidal crystal template were filled by ZnO from the bottom of the substrate. Subsequently, TiO2 inverse opal structure was prepared by sol– gel method in the interstices of the prepared ZnO-filled colloidal crystal template. Acetic acid (8.5 ml) was slowly dropped into a mixed solution of ethanol (7.5 ml) and tetrabutyl titanate (4.0 ml). After stirring for 30 min, deionized water (3 ml) was added. Then the solution was stirred for another 2 h at room temperature. After stirring, a few drops of sol were drawn out and dropped into the colloidal crystals containing deposited ZnO films. After the dropped sol precursors were slowly infiltrated into the interstice of the colloidal crystals, the residual sols out of the templates surface were imbibed by filter paper [20]. Then the spheres and precursor composites were annealed at 450–600 °C for 2 h. In the thermal process, the polystyrene colloidal crystal templates were removed and heterostructured inverse opal structure fabricated by ZnO and TiO2 was formed. 3. Results and discussion The ZnO/TiO2 inverse opal heterostructures were fabricated on the ITO substrate by using colloidal crystal template with monodispersed microspheres of 370 nm in diameter as shown in Fig. 1a. The ZnO inverse opal structures were formed from the bottom of colloidal crystals which contact with the ITO substrate. TiO2 inverse opals structures were fabricated in the template which is
partly-filled with ZnO structures [21]. However, the TiO2 inverse opals structure did not cover all the area due to the shrinkage during the sol–gel process. The surface of ZnO inverse opals in right area (Area N) are covered by TiO2 inverse opals structures in Fig. 1a. 3D periodic pore structure can be seen in the cross-section view of the TiO2 structure (Area M). However, the TiO2 structure is much thicker than the colloidal crystal template (Area N to P). Therefore, only a part of the TiO2 sol precursors were filled into the interstice of the colloidal crystal template. Other precursors were remained on the surface of the colloidal crystal template, resulting in solid TiO2 films covering the surface of TiO2 inverse opals. To investigate the different shrinkage properties of TiO2 and ZnO structures prepared by the sol–gel method and electrodeposition method, TiO2 and ZnO inverse opals were fabricated independently by using the colloidal crystal template, as shown in Fig. 1b and c, respectively. The pore periodicity of TiO2 inverse opals is 277 nm which is smaller than the diameters of colloidal microspheres. The diameter of TiO2 in the inverse opals structure has about 25% shrinkage, comparing the pore periodicities of inverse opals structure and the diameters of the microspheres, which is due to the sol–gel process. On the other hand, ZnO inverse opal structure is prepared by the electrochemical deposition method, and the pore periodicity of ZnO inverse opals is 334 nm which is close to the diameter of colloidal microspheres. The elemental line distribution of the heterostructures was analyzed by EDS as shown in the yellow line of Fig. 2a.1 The detailed elemental distribution of oxygen, titanium, and zinc are shown in Fig. 2b–d, respectively. The oxide distribution is much even in Fig. 2b. The titanium distribution in the right part of the line is much higher than in the left. Therefore, the ratio of TiO2 in the right side is higher than in the left, which is consistent with the geometrical analysis of the macroporous patterns. In contrast, zinc has the opposite distribution. Therefore, the formation of ZnO/TiO2 inverse-opal heterostructures was confirmed by the elemental distribution analysis. Though the TiO2 is not covered all the surface of the structure by the shrinkage of TiO2 precursor, the induced differences in pore periodicities of TiO2 and ZnO have advantage on the broadening of photonic band gaps. However, the fabrication process is sensitive to the thickness of the colloidal crystal structure. When the thickness of ZnO is similar to that of colloidal crystal template, it is difficult to form ordered porous structures of TiO2. Fig. 3 is a typical SEM image of ZnO/ TiO2 heterostructure in that case. The ordered porous structures in the bottom are ZnO which are covered by disconnected TiO2 films without pores on the top. The photonic stop band of the photonic crystal heterostructures fabricated by TiO2 and ZnO by using colloidal crystal template with
1 For interpretation of color in Figs. 2 and 5, the reader is referred to the web version of this article.
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Fig. 2. The EDS analyses of heterostructure by using scanning electronic microscope. (a) The SEM image of the selective line in the heterostructures. (b–d) The elemental distribution of the oxygen, titanium and zinc, respectively.
Fig. 3. SEM image of ZnO/TiO2 inverse opals in which the deposition of ZnO is thicker.
monodispersed microspheres of 370 nm in diameter were characterized by reflection spectra as shown in Fig. 4a. The reflection spectra of inverse opals fabricated by pure ZnO and TiO2 are also shown in Fig. 4b and c, respectively. The ZnO inverse opals structure fabricated by the electrodeposition method has a photonic stop band at about 670 nm, which is matched with the referenced band position [22–24]. The pure TiO2 inverse opals have the stop band at 448 nm [25,26]. Though the difference of the refractive index of ZnO and TiO2 is not large and the porous structures are formed from the same template, the pore periodicities of TiO2 inverse opals are much smaller than those of ZnO inverse opals. Therefore, ZnO/TiO2 heterostructures have a widened photonic stop band from 448 to 670 nm, which have double reflection peaks
at 448 nm and 670 nm in Fig. 4a. The reflection peaks are consisted of those from the individual TiO2/ZnO photonic crystals shown in Fig. 4b and c. The band gap width of ZnO/TiO2 heterostructures is the superposition of the single photonic band gap. There is also a reflection valley formed at 570 nm between the two photonic stop band peaks which makes the heterostructures work as an excellent filter or selector. Furthermore, the reflection stop band is narrow, deep and sharp-shaped, which confirms the high quality and ordering of the internal structure of the samples and indicates that the samples have excellent properties of photonic band gap. The peak from TiO2 inverse opals in the heterostructure has small red shift compared with the one from pure TiO2 inverse opals due to the restricted shrinkage induced by the formed ZnO porous structure. The photonic crystal heterostructures can also be confirmed by the optical microscope. Fig. 5a gives the optical reflection photograph of inverse-opal heterostructures fabricated by ZnO and TiO2 using colloidal crystals with monodispersed microspheres of 370 nm in diameter. Fig. 5b is an optical image of ZnO inverse opals and Fig. 5c is an optical image of TiO2 inverse opals. The bottom of heterostructure shown in Fig. 5a is ZnO inverse opals, and the top is TiO2 inverse opals. The image is similar to the pure inverse opals structures in Fig. 5b and c. The ZnO inverse opal structure is continuously bright red and TiO2 inverse opals are divided into many tens of micro-sized pieces with blue color owning to the shrinkage during sintering process. Besides the cracks, the TiO2 inverse opal structures are arranged uniformly in the whole substrate. Therefore, the TiO2 inverse opal in the heterostructure also contains many micro-cracks. Furthermore, only single kind of polystyrene microspheres were used to fabricate the colloidal crystal templates. Afterwards, ZnO/TiO2 inverse opal
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Fig. 4. The reflection spectra of inverse opals: (a) ZnO/TiO2, (b) ZnO and (c) TiO2.
Fig. 5. (a) Optical images of ZnO/TiO2 inverse opals; (b) optical images of pure ZnO inverse opals; and (c) optical images of pure TiO2 inverse opals.
heterostructures are formed. It is helpful for avoiding lattice mismatch in the process. 4. Conclusions In conclusion, a photonic crystal heterostructures fabricated by TiO2 inverse opals and ZnO inverse opals were developed. Both the dielectric constant and the periodicity of the two parts in the heterostructures are different owing to the distinction between the filling processes of different materials during the templating process. The photonic heterocrystals present excellent properties of photonic band gap. Many attractive features of semiconductor heterostructures can be extended into the optical domain by using the photonic crystal heterostructures. They may show many potential applications in the fields of integrated photonic crystal chips, multi-frequency optical Bragg filters and mirrors. Acknowledgements We gratefully acknowledge the financial supports of the National Natural Science Fund Project under Contract Nos. 50902008, 91123025, 60825407, Doctoral Fund of Ministry of Education of China No. 20090009120033, the Fundamental Research Funds for the Central Universities Nos. 2011JBM297, 2010JBZ006, and the National Basic Research Program of China (973Program) No. 2011CB932703. The Financial Support of the Excellent Doctor’s Science and Technology Innovation Foundation of Beijing JiaoTong University, the Fundamental Research Funds for the Central Universities No. 2011YJS283.
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Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Photonic crystal heterostructures fabricated by TiO2 and ZnO inverse opals using colloidal crystal template with single kind of microspheres Yongna Zhang, Ming Fu ⇑, Jigang Wang, Dawei He, Yongsheng Wang Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2011 Received in revised form 6 April 2012 Accepted 21 April 2012 Available online 17 May 2012 Keywords: Heterostructure Photonic crystal Electrochemical deposition Sol–gel Broad band gap
a b s t r a c t The fabrication of photonic crystal heterostructures is important for the applications in the fields of integrated photonic crystal chips, multi-frequency optical Bragg filters or mirrors. However, multiple steps of self-assembly process of microspheres are always employed in the fabrication of photonic crystal heterostructures, which may produce lattice mismatches of colloidal crystals. Therefore, photonic crystal heterostructures fabricated by using colloidal crystal template with single kind of microspheres were investigated in this paper. A colloidal crystal template with uniform periodicity was firstly formed by monodispersed polystyrene microsphere. Then ZnO was electrodeposited into the interstices of the template. The thickness of ZnO was controlled to be less than the thickness of the template by varying the deposition time. After the TiO2 precursor was filled into the top voids in the template, the polystyrene colloidal crystal template was removed and photonic crystal heterostructures fabricated by ZnO and TiO2 were formed. Both the dielectric constant and the periodicity of the two parts of the heterostructures are different due to the shrinkage of the sol–gel process. The ZnO/TiO2 heterostructures have a broad photonic stop band which is the superposition of photonic stop bands of ZnO inverse opals and TiO2 inverse opals. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Photonic crystal is a three-dimensional (3D) periodic arrangement of the ordered structure which is composed of two or more kinds of dielectric media. Photonic crystal (PC) heterostructures are composed of PCs with different lattice constants or refractive index [1,2]. Many of the attractive features of semiconductor heterostructures can be extended into the optical domain for the photonic crystal heterostructures [3]. The study on PC heterostructures have attracted considerable amount of interests because of their possible applications, including integrated photonic crystal chips, multi-frequency optical Bragg filters or mirrors [4]. There are many different methods to prepare photonic crystal heterostructures. In the inward growing self-assembling method, photonic crystal heterostructures are made by a sequential growth of one photonic crystal over another within a total time span of 6 h [5]; many high quality photonic crystals have been prepared by convective self-assembling method [6–8]. Photonic heterocrystals are also prepared by sandwiching films of self-assembled opals and force-assembled Langmuir–Blodgett colloidal crystals [9]. High-quality 3D PC heterostructures with deep photonic bandgaps
⇑ Corresponding author. E-mail address: [email protected] (M. Fu). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.04.014
(PBGs) and deep photonic band edges (PBEs) are successfully achieved by means of the modified self-assembly method [10]. Ma fabricated silicon 2D–3D photonic crystal heterostructures using glancing angle deposition via depositing method [11]. Wang fabricated a photonic crystal heterostructures composed of two TiO2 inverse opal films with different filling factors through the sequential deposition [12]. Colloidal photonic crystal heterostructures, which are composed of two opaline photonic crystal films of silica spheres with different diameters, were fabricated through a two-step spin-coating method [13]. However, monodispersed colloidal spheres with different sizes were always used in these methods. Therefore, multiple steps of self-assembly process must be employed in the process which may produce lattice mismatches of colloidal crystals in nearby layers with different microsphere diameters [14]. Therefore, it is important to develop a method for the fabrication of PC heterostructures in one complete colloidal crystal template with single kind of microspheres [15–17]. Therefore, in this paper, ZnO/TiO2 heterostructures are made by colloidal crystal template with single kind of microspheres by different fabrication methods. Due to its short experimental period and low cost, it is comparably easier to be implemented in the laboratory. Furthermore, the fabrication of heterostructures by using colloidal crystals with single kind of colloidal microspheres is helpful for avoiding lattice mismatch before removing the microspheres [18].
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Fig. 1. SEM images of: (a) heterostructured inverse opal structures fabricated by ZnO and TiO2 (Area M: TiO2 inverse opals; Area N: ZnO inverse opals; Area N and P: colloidal crystal template); (b) pure TiO2 inverse opals and (c) pure ZnO inverse opals.
2. Methods 2.1. The preparation of colloidal crystal templates The colloidal crystals were made by the vertical deposition method via self-assembly process with monodispersed microspheres which were 370 nm in diameter. The glass sides covered with ITO films were ultrasonically cleaned with acetone and ethanol solvent. After that, glass slides were placed vertically into a bottle which contains 2 vol.% of monodispersed microsphere solution. Highly-ordered colloidal crystal templates were formed on the glass slides as the solvent evaporated at 50 °C and 30% moisture in the electric thermostat. After the solvent was evaporated, ITO glasses covered with colloidal crystals were formed. 2.2. Preparation of double layers of inverse opal structures fabricated by ZnO and TiO2 ZnO inverse opal structure was prepared by the electrochemical deposition method. ZnO films were deposited on the ITO substrate covered with colloidal crystal template in 0.1 mol/l Zn(NO3)2 aqueous solution at 70 °C. ITO substrates covered with colloidal crystal templates were used as the working electrode, while a platinum sheet was used as the counter electrode. A saturated calomel electrode was used as the referencing electrode. [19] The potentiostatic method at 1 V was employed for 10–30 min. The interstices of the colloidal crystal template were filled by ZnO from the bottom of the substrate. Subsequently, TiO2 inverse opal structure was prepared by sol– gel method in the interstices of the prepared ZnO-filled colloidal crystal template. Acetic acid (8.5 ml) was slowly dropped into a mixed solution of ethanol (7.5 ml) and tetrabutyl titanate (4.0 ml). After stirring for 30 min, deionized water (3 ml) was added. Then the solution was stirred for another 2 h at room temperature. After stirring, a few drops of sol were drawn out and dropped into the colloidal crystals containing deposited ZnO films. After the dropped sol precursors were slowly infiltrated into the interstice of the colloidal crystals, the residual sols out of the templates surface were imbibed by filter paper [20]. Then the spheres and precursor composites were annealed at 450–600 °C for 2 h. In the thermal process, the polystyrene colloidal crystal templates were removed and heterostructured inverse opal structure fabricated by ZnO and TiO2 was formed. 3. Results and discussion The ZnO/TiO2 inverse opal heterostructures were fabricated on the ITO substrate by using colloidal crystal template with monodispersed microspheres of 370 nm in diameter as shown in Fig. 1a. The ZnO inverse opal structures were formed from the bottom of colloidal crystals which contact with the ITO substrate. TiO2 inverse opals structures were fabricated in the template which is
partly-filled with ZnO structures [21]. However, the TiO2 inverse opals structure did not cover all the area due to the shrinkage during the sol–gel process. The surface of ZnO inverse opals in right area (Area N) are covered by TiO2 inverse opals structures in Fig. 1a. 3D periodic pore structure can be seen in the cross-section view of the TiO2 structure (Area M). However, the TiO2 structure is much thicker than the colloidal crystal template (Area N to P). Therefore, only a part of the TiO2 sol precursors were filled into the interstice of the colloidal crystal template. Other precursors were remained on the surface of the colloidal crystal template, resulting in solid TiO2 films covering the surface of TiO2 inverse opals. To investigate the different shrinkage properties of TiO2 and ZnO structures prepared by the sol–gel method and electrodeposition method, TiO2 and ZnO inverse opals were fabricated independently by using the colloidal crystal template, as shown in Fig. 1b and c, respectively. The pore periodicity of TiO2 inverse opals is 277 nm which is smaller than the diameters of colloidal microspheres. The diameter of TiO2 in the inverse opals structure has about 25% shrinkage, comparing the pore periodicities of inverse opals structure and the diameters of the microspheres, which is due to the sol–gel process. On the other hand, ZnO inverse opal structure is prepared by the electrochemical deposition method, and the pore periodicity of ZnO inverse opals is 334 nm which is close to the diameter of colloidal microspheres. The elemental line distribution of the heterostructures was analyzed by EDS as shown in the yellow line of Fig. 2a.1 The detailed elemental distribution of oxygen, titanium, and zinc are shown in Fig. 2b–d, respectively. The oxide distribution is much even in Fig. 2b. The titanium distribution in the right part of the line is much higher than in the left. Therefore, the ratio of TiO2 in the right side is higher than in the left, which is consistent with the geometrical analysis of the macroporous patterns. In contrast, zinc has the opposite distribution. Therefore, the formation of ZnO/TiO2 inverse-opal heterostructures was confirmed by the elemental distribution analysis. Though the TiO2 is not covered all the surface of the structure by the shrinkage of TiO2 precursor, the induced differences in pore periodicities of TiO2 and ZnO have advantage on the broadening of photonic band gaps. However, the fabrication process is sensitive to the thickness of the colloidal crystal structure. When the thickness of ZnO is similar to that of colloidal crystal template, it is difficult to form ordered porous structures of TiO2. Fig. 3 is a typical SEM image of ZnO/ TiO2 heterostructure in that case. The ordered porous structures in the bottom are ZnO which are covered by disconnected TiO2 films without pores on the top. The photonic stop band of the photonic crystal heterostructures fabricated by TiO2 and ZnO by using colloidal crystal template with
1 For interpretation of color in Figs. 2 and 5, the reader is referred to the web version of this article.
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Fig. 2. The EDS analyses of heterostructure by using scanning electronic microscope. (a) The SEM image of the selective line in the heterostructures. (b–d) The elemental distribution of the oxygen, titanium and zinc, respectively.
Fig. 3. SEM image of ZnO/TiO2 inverse opals in which the deposition of ZnO is thicker.
monodispersed microspheres of 370 nm in diameter were characterized by reflection spectra as shown in Fig. 4a. The reflection spectra of inverse opals fabricated by pure ZnO and TiO2 are also shown in Fig. 4b and c, respectively. The ZnO inverse opals structure fabricated by the electrodeposition method has a photonic stop band at about 670 nm, which is matched with the referenced band position [22–24]. The pure TiO2 inverse opals have the stop band at 448 nm [25,26]. Though the difference of the refractive index of ZnO and TiO2 is not large and the porous structures are formed from the same template, the pore periodicities of TiO2 inverse opals are much smaller than those of ZnO inverse opals. Therefore, ZnO/TiO2 heterostructures have a widened photonic stop band from 448 to 670 nm, which have double reflection peaks
at 448 nm and 670 nm in Fig. 4a. The reflection peaks are consisted of those from the individual TiO2/ZnO photonic crystals shown in Fig. 4b and c. The band gap width of ZnO/TiO2 heterostructures is the superposition of the single photonic band gap. There is also a reflection valley formed at 570 nm between the two photonic stop band peaks which makes the heterostructures work as an excellent filter or selector. Furthermore, the reflection stop band is narrow, deep and sharp-shaped, which confirms the high quality and ordering of the internal structure of the samples and indicates that the samples have excellent properties of photonic band gap. The peak from TiO2 inverse opals in the heterostructure has small red shift compared with the one from pure TiO2 inverse opals due to the restricted shrinkage induced by the formed ZnO porous structure. The photonic crystal heterostructures can also be confirmed by the optical microscope. Fig. 5a gives the optical reflection photograph of inverse-opal heterostructures fabricated by ZnO and TiO2 using colloidal crystals with monodispersed microspheres of 370 nm in diameter. Fig. 5b is an optical image of ZnO inverse opals and Fig. 5c is an optical image of TiO2 inverse opals. The bottom of heterostructure shown in Fig. 5a is ZnO inverse opals, and the top is TiO2 inverse opals. The image is similar to the pure inverse opals structures in Fig. 5b and c. The ZnO inverse opal structure is continuously bright red and TiO2 inverse opals are divided into many tens of micro-sized pieces with blue color owning to the shrinkage during sintering process. Besides the cracks, the TiO2 inverse opal structures are arranged uniformly in the whole substrate. Therefore, the TiO2 inverse opal in the heterostructure also contains many micro-cracks. Furthermore, only single kind of polystyrene microspheres were used to fabricate the colloidal crystal templates. Afterwards, ZnO/TiO2 inverse opal
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Fig. 4. The reflection spectra of inverse opals: (a) ZnO/TiO2, (b) ZnO and (c) TiO2.
Fig. 5. (a) Optical images of ZnO/TiO2 inverse opals; (b) optical images of pure ZnO inverse opals; and (c) optical images of pure TiO2 inverse opals.
heterostructures are formed. It is helpful for avoiding lattice mismatch in the process. 4. Conclusions In conclusion, a photonic crystal heterostructures fabricated by TiO2 inverse opals and ZnO inverse opals were developed. Both the dielectric constant and the periodicity of the two parts in the heterostructures are different owing to the distinction between the filling processes of different materials during the templating process. The photonic heterocrystals present excellent properties of photonic band gap. Many attractive features of semiconductor heterostructures can be extended into the optical domain by using the photonic crystal heterostructures. They may show many potential applications in the fields of integrated photonic crystal chips, multi-frequency optical Bragg filters and mirrors. Acknowledgements We gratefully acknowledge the financial supports of the National Natural Science Fund Project under Contract Nos. 50902008, 91123025, 60825407, Doctoral Fund of Ministry of Education of China No. 20090009120033, the Fundamental Research Funds for the Central Universities Nos. 2011JBM297, 2010JBZ006, and the National Basic Research Program of China (973Program) No. 2011CB932703. The Financial Support of the Excellent Doctor’s Science and Technology Innovation Foundation of Beijing JiaoTong University, the Fundamental Research Funds for the Central Universities No. 2011YJS283.
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