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A facile approach of fabricating hollow cylindrical inverse opals and inverse opal columns by sol-gel co-assembly method was proposed. Polystyrene (PS) colloidal suspension added with hydrolyzed silicate precursor solution was used to self-assemble composite colloidal crystals which consist of PS colloidal crystal template and infiltrated silica gel in the interstitial of microspheres. Continuous hollow cylindrical composite colloidal crystal films have been produced on capillaries’ outside and internal surface. Composite colloidal crystal columns which filling up the interior of a capillary were fabricated by pressure assisted sol-gel co-assembly method. Hollow cylindrical inverse opals and inverse opal columns were obtained after removing PS colloidal crystal from the composite colloidal crystal. Optical properties of the silica hollow cylindrical inverse opals were characterized by transmission spectrum and a stop band was observed. Structure and optical properties of the inverse opal columns were investigated.
©2011 Optical Society of America
1. Introduction
Microstructure optical fibers (MOFs) have exhibited many novel optical properties in recent years. Photonic band gap fibers (PBGFs) with hollow core guiding structures are a kind of microstructure optical fibers (MOFs) to overcome the limitations of traditional silica optical fibers [1]. Two dimensional photonic crystal fibers have been commercially available and widely used as sensing detector, high power optical waveguide and nonlinear optical devices. Three dimensional (3D) PBGFs were proposed subsequently as its unique structure enables various applications in optical sensing and signal processing. Many efforts have been made to fabricate 3DPBGFs: E. Valdivia et al. coated a standard optical fiber with three dimensional colloidal crystals which could be used as a template to form inverse opal hollow core PBGFs [1]; S.-M. Yang et al. created polymer hollow cylindrical structures with three dimensionally interconnected air cavities by infiltrating polymer precursor into a colloidal crystal template on fibers which was fabricated by dip-coating method [2]; Y. K. Lin et al. assembled colloidal crystals inside a hollow fiber to form a stop band in fiber longitudinal direction [3,4]. It is challenging to fabricate silica air core cylindrical inverse opal fibers or waveguides both for template fabricating and infiltration materials to realize inverse opal structure. Conventional methods [5–8] have difficulties in producing high quality inverse opals with good interconnectivity over large area or in complex shapes, such as over layers on the top surface and interior incomplete infiltration. Recently Hatton et al. developed a sol-gel co-assembly method to overcome these problems [9]. Multilayered composite colloidal crystal (CCC) films were generated via evaporative deposition of polystyrene (PS) colloidal spheres suspended in a hydrolyzed silicate sol-gel precursor solution by one step, as shown in part (A) of Fig. 1 . The composite colloidal crystal consists of ordered PS colloidal crystals and infiltrated silica gel in the interstitial of PS spheres. After removing PS colloidal crystals by calcination, large area, high ordered, crack free silica inverse opals were produced [9]. We apply this method to produce inverse opals on cylindrical surface (capillary outside surface), and propose an approachable way to fabricate silica air core cylindrical inverse opal [10,11] waveguides with robust mechanical stability on capillary’s internal wall. Silica inverse opal column (IOC) which filling up the interior of a capillary was also produced by pressure assisted sol-gel co-assembly method. Structural and optical characters of the silica hollow cylindrical inverse opal (HCIO) and IOC were studied. HCIOs on the inside capillary wall with good mechanical stability could form one kind of hollow core 3DPBGs. Silica IOCs could be used as fiber filtering, fiber sensing and column chromatography [12].
Fig. 1 Part (A) is a schematic diagram of composite colloidal crystal growth on a capillary internal wall with sol-gel co-assembly method [9]. Part (B) is a schematic diagram which shows the process of fabricating silica HCIO on the internal wall of a capillary: (a)–(c) are cross section view, (d) is longitudinal section view of the capillary internal wall coated with inverse opals.
2. Experiment
2.1 Fabrication of silica hollow cylindrical inverse opals on the internal or outer surface of capillaries and silica inverse opal columns embedded in capillaries
Glass capillaries in 1.8mm, 850μm, 670μm diameters and in 10cm length were selected as an ideal cylindrical substrate. Aqueous plain PS microspheres solution with solids content ~10 wt. % was bought from Bangs Laboratories, Inc, and was only diluted to a certain concentration without any other treatment before use. The surface of the PS microspheres was covered with negatively-charged sulfate groups. Monodispersity of the PS microspheres is less than 10%. Capillaries were cleaned in an ultrasonic bath first using acetone and then deionized (DI) water for 10 minutes respectively. Then cleaned capillaries were dried with nitrogen before use. To fabricate HCIOs on the internal surface of capillaries, PS colloidal/TEOS solution [9] dispersed in DI water with concentration of ~1vol % (PS spheres) and ~2vol % (TEOS solution [9]) was injected into a capillary. The height of the solution inside the capillary is 10-95mm and can be precisely controlled by a micro-pump. Length of the HCIOs was affected by the height of the inside solution. One end of the capillary was sealed to control the evaporation direction of solvent. Figure 1 (A) shows a schematic diagram of sol-gel co-assembly CCC on a capillary internal wall in a single step and (B) shows the process of fabricating silica HCIO on the internal wall of a capillary. To assemble cylindrical CCC on its outer surface, cleaned capillaries were vertically fixed in a vial containing PS colloidal/TEOS (~0.5vol % / ~1vol %) solution, which is similar to assembling HCIOs on the internal wall. The schematic model of the HCIOs on the capillaries’ outer wall is shown in Fig. 2 (A) . Both the vial with a capillary and the capillary contained solution inside were set in a 50°C oven on a pneumatic vibration free table when crystallization was proceeding. Cylindrical CCCs of 20mm length on capillary outer and internal surface were formed in ~10 and ~15hours, respectively.
Fig. 2 Part (A) is a schematic diagram showing the cross section, longitudinal section of composite colloidal crystal and inverse opal on capillaries’ outer wall. Part (B) is a schematic diagram showing the cross section and front view of composite colloidal crystal column and inverse opal column fabricated in a capillary.
To fabricate silica IOCs, a 10cm length capillary was vertically immersed with one end in sealed PS colloidal/TEOS (~2vol % / ~2.5vol %) solution. Solvent in the vial can only escape through the other end of the capillary, and vapor pressure inside the vial along with capillary force built up and drove the mixed colloidal fluid to the top end of the capillary [4]. The whole setting was placed in an oven at 50°C. A CCC column with a length of 5mm was formed in ~6 hours. The capillary was moved to an empty vial to stop the crystal growth, and the capillary was still kept vertical until all the solvent in the capillary was gone. Schematic model of the ICOs embedded in capillaries was shown in Fig. 2 (B).
Capillaries with cylindrical CCCs on their outer, internal surface or embedded CCC column were fired at 500°C in a chamber oven for 2 hours with a 4-hour ramp time and gradually cool down to room temperature in 3 hours to remove the PS colloidal template and finally obtained the silica HCIO and IOC.
2.2 Characterization
Optical microscope (NIKON ECLIPSE LV150) with a CCD was used to record the optical images of prepared samples at low magnification. Scanning Electron Microscope (JEOL, JSM-6360LV) at 15kV was used to view images of prepared inverse opals. Transmission spectra were measured with a YOKOGAWA optical spectral analyzer (AQ6370). The YOKOGAWA white light source (AQ4305) was coupled into a multimode optical fiber and focused to inverse opals at normal incidence with a couple of collimating lens. Transmitted lights were focused to the optical spectral analyzer by a lens. Reflection spectra were measured by a miniature spectrometer (HR2000) of Ocean Optics.
3. Results
The sol-gel co-assembly method provided uniform PS-silica gel CCCs of several centimeters in length both on the outer and internal surface of capillaries. After removing the PS colloidal crystal template at 500°C in air, large area uniform silica HCIOs were obtained. Scanning electron microscopy (SEM) images of silica HCIO on a capillary’s outer surface are shown in Fig. 3 . Diameter of the capillary is 670μm, and the coated inverse opals were co-assembled from 580nm PS microspheres. A continuous, uniform and highly-ordered hexagonal close-packed inverse opal structure with [111] crystallographic direction normal to the curved surface can be clearly observed. No large scale defects were discernable upon inspection of the coated capillary by rotating it around its axis. Figures 3 (a)–(c) show the HCIO with increased magnification. A few round shape defects can be found in Fig. 3 (b), we considered that they probably formed because of few bigger PS spheres in the solution. Figure 3 (c) shows the {111} plane of the HCIO surface and three small pores can be seen symmetrically on the bottom of every big pore. And Fig. 3 (d) is the {100} surface of FCC lattice. It can be seen that there are four small pores in each big pore and confirms the fabricated structure is FCC lattice. In Figs. 3 (e) and (f), cross section of the HCIO on a capillary indicates the produced silica inverse opal with high order and good connectivity from its top surface to interior. As well as the small pores in the middle of each big pore observed in the cross section (as magnified in inset in (f)), we confirm that the fabricated silica HCIOs are of highly-ordered air pores connected with its all neighbor pores from its top surface to interior.
Fig. 3 SEM images of glass capillaries in 670μm diameter coated with silica inverse opals co-assembled from 580nm PS spheres. (a) and (b) images of the capillary coated with silica inverse opals with increased magnification, (c) {111} plane of the inverse opal, (d) {100} plane of the inverse opal, (e) and (f) cross section of the silica inverse opal films of ~50 layers on the capillary surface.
The critical factors that have effect on the quality of produced silica HCIOs are concentrations of PS microspheres in colloidal suspension and volume ratio of the PS colloidal suspension and hydrolyzed TEOS solution. Over adding hydrolyzed TEOS solution results in over layered inverse opals, and less adding leads to incomplete infiltration. By varying microsphere concentration in the mixed dispersion, the self-assembled thickness could be controlled ranging from a monolayer to tens of microsphere layers. However, the CCC tends to fracture at a threshold thickness of approximately 5~10μm, which means a thresh concentration of the colloidal suspension [9]. Large area HCIOs with a thickness of 5~10μm of high quality on cylinder surface are accessible with sol-gel co-assembly method.
Silica inverse opals fabricated on a capillary’s internal wall are the same to it coated on capillaries’ outer surface. Figures 4 (a), (b), (c) exhibit optical microscope images observed in reflection mode: (a) a bare capillary, (b) a capillary with PS-silica gel CCC coated on its internal wall and (c) the capillary with silica inverse opals preserved on the internal wall after sintering. The axial uniformity together with the symmetric color distribution on the capillary internal edges attests to the high quality of the silica HCIO, which spectrally shifts the stop band with the viewing angle around the capillary curved surface. The color on the internal edge of the capillary has a blue shift after removing the PS colloidal crystal template as the effective refraction index decreased, as seen in Figs. 4 (b) and (c). Figures 4 (d)–(g) are SEM images of the longitudinal-section of the capillary in Fig. 4 (c). Image Fig. 4 (d) is a top view at low magnification and Fig. 4 (e) is longitudinal cross section of the silica inverse opals on the edge of the capillary internal wall with 7 layers. The robust inverse opals on the capillary internal wall are with good connectivity with the capillary substrate. Figures 4 (f) and 4 (g) are curved surface of the HCIO with increased magnification. High uniform and ordered structure and small pores connected the big pores which reveal good connectivity of the structure can be observed. Figures 4 (h) and (i) show the top view of the cross section of a capillary with silica inverse opals coated on its internal wall. The internal diameter of capillaries could range from ~100μm to several millimeters as the ratio PS diameter to capillary diameter is very large [4]. The average pore size of the fabricated inverse opals are 615nm, 500nm and 430nm, which made from PS spheres in 490nm, 580nm and 690nm diameters, with a contraction of 12.2%, 13.8% and10.8%, respectively.
Fig. 4 (a)–(c) are optical microscope images of capillaries in 350μm internal diameter. (a) bare capillary, (b) internal wall coated with composite colloidal crystal, (c) internal wall preserved with silica inverse opals after sintering. (d)–(i) are SEM images of inverse opals on the internal wall of a capillary, (d) longitudinal section of the capillary whose internal wall was coated with silica inverse opals, (e) longitudinal cross section of inverse opals, (f)surface of the inverse opals on the internal curved wall, (g) high magnification of the surface in (f), (h) and (i) are cross section of a capillary with silica inverse opals on its internal wall.
Transmission spectra of silica HCIOs coated on capillaries’ outer and internal surface are shown in Fig. 5 (a) : (A) presents a typical normalized transmission spectrum of HCIO on internal surface of a capillary in 670μm diameter from 490nm PS microspheres, (B) and (C) are transmission spectra of HCIO co-assembled from 580nm and 690nm PS microspheres on outer surface of a capillary in 850μm diameter at 50°C. The number of layers of the silica HCIOs measured are about 10~20. White source beam with a radius of 100μm was projected on the inverse opals at normal incidence and perpendicular to the capillary axis. The transmission gap centers at 1100nm, 922nm and 769nm respectively. Stop band center linearly changed with PS microsphere diameter, as shown in inset in Fig. 5 (a). Bandwidth of the stop band is broader because incident light is not strictly perpendicular to the inverse opal surface for the cylindrical geometry. The Fabry-Perot fringes on the pass band indicate a uniform and complete coat of silica inverse opals on the capillary. In Figs. 5 (b) and (c), we present the variance of transmission gap centre in different places along the capillary’s axis and along different radial directions. A HCIO co-assembled from 490nm PS microspheres on a capillary’s internal surface was measured. Different places along the axial direction, transmission gap centers have a variance about ± 7nm with the average gap centre at 769nm (Fig. 5 (b)). Around the axis, we take three measurements by rotate the capillary 120° each time, and got a maximum difference between the three measurements. The maximum differences in different places along the axial direction were plotted in Fig. 5 (c). The maximum difference is less than 5nm in 10cm length in our measurements. Considering the diameter deviation of PS microspheres we used is less than 10% of its average diameter, the deviations of the gap centre wavelength are reasonable. Optical properties of HCIOs on capillaries’ outer surface are almost the same as they are on internal surface. And these optical properties attest to a good quality of the HCIOs both on internal and outer surface of capillaries.
Fig. 5 Optical properties of silica HCIOs. (a) transmission spectra of silica HCIOs: (A) transmission spectrum of HCIOs on internal surface of a capillary in 670μm diameter made from 490nm PS microspheres, (B) and (C) transmission spectra of HCIOs on outer surface of capillaries in 850μm diameter made from 690nm and 580nm PS microspheres: (B) 580nm, (C) 690nm. Inset on right bottom corner is a graph of PS microsphere diameter to measured photonic stop band centre. (b) and (c) are optical properties of a silica HCIO made from 490nm PS microspheres on a capillary’s internal surface in 300μm diameter. (b) transmission gap centre wavelength dependence on axial translation. (c) maximum transmission gap centre wavelength difference of measurements in three times along different radial directions dependence on axial translation.
IOCs co-assembled from 390nm PS microspheres embedded in capillaries were examined by optical microscope and SEM. Its top cross section is {111} plane of FCC lattice with multi-domain, and its outer cylindrical surface was hexagonal close-packed like air pores, this surface structure is preserved for more than 50 layers inside the column; The most internal parts of the IOC are air pores stacked in FCC lattice with multi-domain upon inspection of the entire cross section, as shown in Fig. 6 .
Fig. 6 (a) and (b) are optical images of a silica IOC co-assembled from 390nm PS microspheres with colors, (a) top cross section view, (b) side view. (c)–(j) are SEM images of the silica IOC co-assembled inside capillaries. (b) top view of the IOC embedded in a capillary, inset shows its top surface, (d) the outer cylindrical surface of IOC, (e) longitudinal cross section of the IOC and the top cross section surface, (f) and (g) top view of the longitudinal cross section of a silica IOC, insets in (f) and (g) with increased magnification show the details, (h) is edge of the internal cross section, (i) and (j) interior of the internal cross section at different magnification. Inset in (i) shows {100} plane and inset in (j) is {111} plane.
Figures 6 (a) and (b) are optical images of a silica IOC with bright colors. (a) is top view of its top cross section and (b) is side view of the IOC. SEM image (c) shows its top flat surface with an inset which exhibits its magnified top surface. Multi-domain can be observed on the top flat surface which is similar to the inverse opals co-assembled on planar substrate. To investigate interior structure of the IOC, it was ejected from the capillary by a wood stick and was break into several parts. Figure 6 (d) exhibits the outer round surface of the silica IOC in different magnifications. We can see hexagonal close-packed like air pores due to the physical confinement of internal capillary wall which is same to co-assembly inverse opals on curved surface. As the outmost layer of the IOC was stick to the capillary wall, the round surface was slightly damaged when it was ejected from the capillary. Figure 6 (e) is image of longitudinal cross section of the IOC while the top flat surface also can be observed. Inset is a schematic model. A layer by layer structure vertical to the axis can be seen on the longitudinal cross section. Images (f) and (g) exhibit the whole and details of longitudinal cross section of the silica IOC. There are no cracks run through a long distance more than 50μm while there are some micro-cracks, typically 10–15μm, distributed in the interior of the entire IOC. We considered this is mainly because of the shrinkage of the silicate gel when it was calcined in air. Cross section images of the IOC are shown in Figs. 6(h)–6(j). Image (h) exhibits cross-section edge which is close to the capillary wall. The outside ordered structure was preserved for more than 50 layers, and then the inverse opal structure has a smooth transformation from the edge toward the center as the ratio PS spheres to capillary internal diameter is very large [4]. (i) is interior of the cross section and inset is a high magnification image of {100} plane of FCC, Image (j) is typical {111} plane in interior of the cross section vertical to axis of the capillary and inset shows the magnified {111} plane. In the inset uniform small pores on the bottom of the big pores can be seen obviously which show the good connectivity of the air spheres.
Optical properties of the silica IOC were investigated. Reflection spectra along and vertical to its axis were measured by a miniature spectrometer with a spot size in ~500μm diameter, as shown in Figs. 7 (a) and 7(b). And we examined the reflection spectra in different places and along different directions vertical to the IOC’s axis, shown in Figs. 7 (c) and (d). The measured silica IOC was co-assembled from 390nm PS microspheres, in 500μm diameter, and it was pull out of the capillary before measuring. The reflection peak along the IOC’s axis located at 687.3nm, compared to 695nm ± 5nm on planar substrate, we considered this difference may be caused by a little curvature of the IOC’s top cross section result from the micro-pressure alone the capillary during the evaporation. From top cross section to the other end, along axis of the IOC, reflection peaks along radial direction in different length have a distribution which was shown in Fig. 7 (c). Along different radial direction in same length we measure three times while each time rotate the IOC by 120°, and average the three peak wavelength to get a mean value as reflection peak wavelength in this length. The maximum difference of three times measured peak wavelength in same length is plotted in Fig. 7 (d). As shown in (c), we observed that close to the top cross surface, reflection peak wavelength was started from 516.1nm, and with a ~300μm-length transition region the peak wavelength moved to 620nm ± 10nm, and then peak wavelength in place in length from ~300μm to ~5000μm moved around 625nm and slowly increased to ~635nm in ~5000μm, and then from 6000μm to close to the other end, peak wavelength was almost around 635nm ± 4nm. Near the ending end, about 500μm in length, the reflection spectrum peak cannot be obviously observed.
Fig. 7 Optical properties of silica IOC in 500μm diameter co-assembled from 390nm PS microspheres. (a) reflection spectrum along its axis direction. (b) reflection spectra along different radial directions. (c) reflection peak wavelength dependence on axial translation. (d) maximum reflection peak wavelength difference of measurements in three times around the axis dependence on axial translation.
In Figs. 6 (a) and 6(b), we can see the top cross surface is slightly curved, which result in a blue shift in reflection spectrum in the first ~300μm length long region. As to reflection peak wavelength has a weak increase in length between ~300μm to ~5000μm, we guess it probably because that the precursor solution is still slowly in reaction in the capillary when the CCC is growing. And this needs further investigation. When the evaporation is going to finish, the last few amount of PS microspheres tend to have less motivity, and cause to a ~500μm length long random ordering in the end of IOC. We noticed that the reflection peak wavelength along axial direction is 52nm longer than that along the IOC’s radial direction. We think this is result from different structure of the two directions: top surface is planer and the side surface is high curvature. Along different radial direction in same length reflection peak wavelength have a deviation less than 4nm (Fig. 7 (d)), typically ~2.5nm, which demonstrate the silica IOC with a good round symmetry. Figure 7 (b) presents several typical reflection spectra along IOC’s radial direction.
4. Discussion
We have fabricated silica HCIO and IOC in different diameters in several centimeters with sol-gel co-assembly method. As the precursor solution was added in the PS suspension, the mixed solution became a little viscous, so when the composite cylindrical film was co-assembled on capillaries’ surface, it was not tend to crack easily. sol-gel co-assembly method proposed by Hatton et al [9] also provided an approach to fabricate IOCs. With conventional infiltration method, IOC with several centimeters can hardly be produced with good interior structure, such as inter-connected big pores. The precursor solution worked both as glue and precursor. Using this sol-gel co-assembly method we can produce HCIOs with less challenge. Both HCIOs and IOCs of different materials can be made through adding different precursor solution in PS colloidal solution, and their pore size could vary in a wide range from tens to hundreds nanometers by using PS microspheres in different diameters. Silica inverse opals do not possess a complete photonic band gap as the dielectric constant is below 2.8, however, through explore materials whose dielectric constant is large enough and have a sol-gel precursor exist suitable for the sol-gel co-assembly method, we will produce a complete photonic band gap HCIOs which can traps and guide light within completely. Also by infiltration the macro-porous structure with high dielectric constant material, a complete photonic band gap can be formed either. Considering HCIO’s macro-porous wall whose refractive index is sensitive to its surroundings, such as gas, liquid, and nano-particulates, HCIOs of various materials process many promising applications in device fabricating, sensing, engineering materials. As silica occupies less than 26vol % in inverse opal structure, it will be more sensitive than opal structure to be a refractive index sensor. Besides glass capillary, metal tube and many other material tubes can also be used as substrate to coat inverse opals. HCIOs fabricated on the internal wall of a capillary with good mechanical stability can be used as a new kind of air core 3DPBGFs which could be used as high power laser waveguide, fluids refractive index sensing components, catalytic, separation and micro-reactor supports [13] which lights were involved in. The IOCs could be used as fiber filter, sensing device, column chromatography, separation for its ordered structure in a long length and it incorporates two kinds of extremely uniform pores in the whole volume.
5. Conclusion
In conclusion, with sol-gel co-assembly method silica HCIOs coated on capillaries’ outer or internal wall and silica IOCs embedded in capillaries were obtained. The fabricated silica HCIOs and IOCs were structurally and optically characterized. Additionally, other materials HCIOs and IOCs can be made by this method, and tubes of many materials such as metal can be used as substrate to coat cylindrical inverse opals. These silica HCIOs with ordered macro-pores structure can form a new kind of air core 3DPBGFs. And IOCs can be applied to photonic devices such as optical fiber filter, catalytic, column chromatography, separation and micro-reactor supports.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 61178044), Jiangsu province Scientific and Technical Supporting project (BE2008138).We also appreciate the generous help from professor Zhong-Ze Gu, State Key Laboratory of Bioelectronics, Southeast University, and his research group.
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