Inverse opal

1 Photonic Crystals via Self-assembly of Colloidal Spheres 

Whilst not showing full band gap capability self-assembled photonic oystals do show interesting incomplete bands, known as stop gaps. For instance, 3D crystalline arrays from differently sized PS beads show different colours in transmission 270 nm beads give a red colour (absorbance at /L = 650 nm), 220 nm beads a green colour (absorbance at A = 550 nm) and 206 nm beads a blue colour (absorbance at 7=460 nm). 

A better way of achieving a complete 3D photonic band gap is considered to be by constructing a lattice of air balls surrounded by an interconnecting matrix of material with a higher refractive index. The successful construction of such crystals by growth techniques is unlikely and likewise the build up by deposition techniques is no simple procedure. 

In one example, the colloidal structure, is made by sedimentation of polystyrene beads, giving voids in the range 120-1000 mn, and the voids are filled with TiO generated from titanium tetrapropoxide. The polystyrene bead lattice is then removed by calcining to give an iridescent material, but not with a full photonic band gap. In this case one of the controlling factors is the refractive index of the matrix, which needs to be greater than 2.8. 

However, recently, material constructed using silica sphere photonic crystals as the template has given a complete 3D-photonic band gap centred on 1.46 pm, which is a favoured wavelength for hbre-optic communications. Sintering of the silica array causes each sphere to be joined by short necks. Silicon is then grown in the 

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Scientists also have learned how to mimic the surface of a butterfly wing. Polystyrene beads and smaller silica nanoparticles are suspended in water and mixed thoroughly using ultrasound. When a glass slide is dipped into the suspension and slowly withdrawn, a thin film forms on the glass surface. This film is a regular array of beads encased in a matrix of nanoparticles. Heating the film destroys the polystyrene beads but leaves the silica web intact. The result is a silica inverse opal film. 

Somani, P. R. Dionigi, C. Murgia, M. Palles, D. Nozar, P. Ruani, G. 2005. Solid-state dye PV cells using inverse opal Ti02 films. Solar Energy Mater. Solar Cells 87 513-519. 

The porous membrane templates described above do exhibit three-dimensionality, but with limited interconnectedness between the discrete tubelike structures. Porous structures with more integrated pore—solid architectures can be designed using templates assembled from discrete solid objects or su-pramolecular structures. One class of such structures are three-dimensionally ordered macroporous (or 3-DOM) solids, which are a class of inverse opal structures. The design of 3-DOM structures is based on the initial formation of a colloidal crystal composed of monodisperse polymer or silica spheres assembled in a close-packed arrangement. The interconnected void spaces of the template, 26 vol % for a face-centered-cubic array, are subsequently infiltrated with the desired material. 

Figure 12. Inverse opal of vanadium oxide ambigel. The pores are formed by packing -fim styrene beads and infiltrating a vanadium sol. (Reproduced with permission from ref 100. Copyright 2002 The Royal Society of Chemistry.)...

CVD, and electrodeposition, depending on the desired composition. Removal of the colloidal templating spheres renders a negative replica (the inverse opal) structure of the active material, with an interconnected, 3-D array of pores, typically sized in the hundreds of nanometers. 

Figure 15.7 (a) The preparation of inverse opal photonic balls using polymer spheres and an inorganic... 

Nanoscale structures such as inverse opals have fascinating photonic properties related to Natural nanostructures with potential applications in optical computing. 

Swinerd, V. M., Collins, A. M., Skaer, N. J. V., Gheysens, T., Mann, S., Silk inverse opals from template-directed beta-sheet transformation of regenerated silk fibroin. Soft Matter 2007, 3, 1377-1380. 

FIGURE 8.12. SEM images of two inverse opal samples that were fabricated by templating a U V-curable prepolymer to the poly(acrylate methacrylate) copolymer against the opaline lattices of polystyrene beads with different orientations. 

Figure 41. The SEM micrograph images of an Er 1 Ti()2 inverse-opal structure templated using a colloidal crystal of 466-nm polystyrene beads by filling the interstitial volumes with colloidal 50-nm diameter Lr 1 Ti()2 nanocrystals followed by calcination to remove the poylystyrene. (a) Low magnification. (b) High magnification. [Adapted from (187).]...

FIGURE 15 (A) SEM image of PMMA opal photonic crystal (B) SEM image of ceria inversed opal photonic crystal made from the PMMA template. Reprinted with permission from Waterhouse and Waterland (2007). Copyright 2008 American Chemical Society. 

Ordered macroporous materials (OMMs) are a new family of porous materials that can be synthesized by using colloidal microspheies as the template. - The most unique characteristics of OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a three-dimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precursor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 30 periodicity of the materials, these structures have been extensively studied for photonic applications. In this paper, the synthesis and characterization of highly ordered macroporous materials with various compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorption/separation is analyzed and discussed. 

The opals obtained by self-assembly are mechanically unstable because there is only Van der Waals force between spheres. The subsequent infiltration process could easily destroy the ordered colloid arrays. So we annealed the opals of polymer sphere to increase their stability. As a result, there would form interconnections between spheres, which come from the slight melting of the sphere surfoces. These necks can provide the opal with necessary mechanical stability. In addition, they are important for producing inverse opal structure. After infiltration, when the samples are treated with calcinations, these necks can act as channels for the transport of the products formed during calcination like CO2. 

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