Inverted opal

Scott, R.W.J. et al.. Tin dioxide opals and inverted opals near-ideal microstructures for gas sensors, Adv. Mater., 13, 1468, 2001. 

Figure 3. SEM image of a vanadium dioxide photonic crystal with inverted opal structure [10]. 

Figures. SEM surface images of the titania-opal composites (left) and the corresponding titania inverted opals (right) with different withdrawal rates (a) 500, (b) 2000, (c) 6000 (immersion time 5 min., 10 dips).

Sakamoto, J. and B. Dunn, Hierarchical battery electrodes based on inverted opal structures. J. Mater. Chem. 2002, 72, 2859. 

R. Zentel, et al.. Modification of the spontaneous emission of CdTe nanocrystals in Ti02 inverted opals, J. Appl. Rhys. 94 (2003) 1205. 

Ceramic monolithic structures made from inverted opal silicon carbonitride (see Figure 6.6) were presented by Mitchell et al. [460]. The inverse opal structure was achieved using polystyrene templates. They prepared monoliths of 74% porosity, typically 350-pm wide, 100-pm high and 3-mm long. Propane steam reforming was then successfully performed in the reactor (see Section 7.1.2). 

Figure 6.6 Top, inverted opal silicon carbide monoliths with 7,2-pm pore size middle, fracture profile of a ruthenium coated silicon carbide monolith bottom, integrated ceramic monolith with alumina housing five ceramic monoliths are integrated and alumina inlet and outlet tubes connected [460],...

The ceramic monolithic stmctures made from inverted opal silicon carbonitride (see Section 6.2), which had been developed by Mitchell et al. [460], were applied for propane steam reforming. A 5 wt.% mthenium catalyst was coated onto the monoliths. Full conversion was achieved at fairly high reactor temperatures of 900 °C. Despite the low S/C ratio of 1.33, stable operation of the catalyst and reactor at 800 °C and 60% conversion was achieved through several hours of operation and more than 15 thermal cycles without apparent coke formation. 

S Coombs, N Tetreault, N., Masuura, N., Ruda, H.E., and Ozin, G.A. (2002) Barium titanate inverted opals - synthesis, characterization, and optical properties. Adv. Funct. Mater, 12, 71-77. 

Figure 13-8. Schematic illustration of a procedure for electrochemically producing inverted opal photonic crystals (Braun, 2001a).

Templates used in the fabrication of photonic crystals basically need to be removed in the final stage of the process. There are many organic solvents that can dissolve polymers that are generally used as template, and it is also possible to remove a polymer template by biuning it out. Silica template, which is also widely used particularly in fabricating inverted opals, can be removed by dissolving it with hydrofluoric acid. 

Most of the 3D photonic crystals fabricated via solution-based approaches are inverted opals, since the fabrication of inverse-opal photonic crystals and the evaluation of their optical properties both are relatively simple and easy. There have been a large number of studies that dealt with opal and inverse-opal photonic crystals in the literature, and it is impossible to cover most of the contents of the studies reporting inverse-opal (and opal) photonic crystals concerning their fabrication processes, characterization of the crystals, optical properties, and applications (Chen, 1999 Sun, 2000a Muller, 2000 McComb, 2001 Stein, 2001 Scroden, 2001 Astratov, 2002). Therefore, the contents of only a few of such studies as dealt with metal oxide inverted opals fabricated by the sol-gel process are presented here. 

The use of photonic crystals for the enhancement of solar cells by utilizing PBG structure instead of the high-reflection Bragg mirrors has been reported by Bermel et al. [292]. They proved that a six-period triangular two-dimensional PBG structure made of air holes in silicon increases power generation more than 2 %, even more if a combination of a Bragg mirror and 2D PBG is used. A similar improvement is obtained if an eight-period inverted opal photonic crystal [293] is utilized. 

Fig. 2.53 Schematic presentation of cavity enhancement in a photodetector using photonic crystal. The front half of the PEG structure is removed to show the photodetector slab. The photonic crystal is an inverted opal, but this is for illustration purpose only sinee it can be any other type of PEG structure...

A thin film of an inverted opal has been obtained by infiltrating an opal (made from 200 nm silica spheres) with a photopolymer. This inverted opal was then filled with a liquid crystal. Figure 11.14a shows the electro-optical response in which the reflection peak (corresponding to the stop band) shifts due to the change of the refractive index of the liquid crystal [72]. Figure 11.14b shows the electron microscope image of the film itself. The discontinuous peak shift is due to the resolution of the degeneracy of the band caused by the introduction of the anisotropic medium. Furthermore, a memory effect due to the interface between liquid crystal and the opal pores can also be seen. 

---