Sol-gel Based Nanoceramic Materials: Preparation, Properties and Applications by Ajay Kumar Mishra

Author:Ajay Kumar Mishra
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

In order to assemble an inverse opal structure, one has to infiltrate the voids of the opal structure with different materials, which can be dielectrics, semiconductors or even metals. Depending on the nature of the spheres that are forming the opal template [silica, polystyrene (PS) or polymethylmethacrylate (PMMA)], one can finally get rid of these spheres either by selective etching or by heat treatment. The resulting structure is an FCC lattice made of spherical voids in a (sparse) surrounding dielectric, semiconducting or even metallic matrix [86]. Figure 5.4b shows a SEM photograph of an inverse opal structure obtained starting from an opal structure with nanoparticles of size 236 nm.

The spherical voids of the inverse opal can be selectively filled with other materials again, and thus lead to photonic waveguides. In such a structure, light with a frequency in the band gap of the inverse opal can only propagate along regions where the spherical voids are filled with an additional dielectric material.

In general, the voids of both the opal and the inverse opal can be filled with plasmonic silver or gold nanoparticles, which add plasmonic features to the functionality of these basic photonic materials [85].

It might not come as a big surprise that opal and inverse opal layers are also one of our preferred systems to enhance the efficiency of solar cells, as described in the previous section. Opal and inverse opal structures are relatively easy to synthesize on the basis of a well-defined sol–gel synthesis protocol, where the glass spheres can subsequently be deposited on a photovoltaic substrate using simple techniques like dip coating. Furthermore, the glass spheres can easily be doped with all sorts of rare-earth ions, and the inverse opal structure even provides omnidirectional photonic band gaps to mould the flow of photons within a solar cell.

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