Russian scientists suggest a unique technology for making high-quality synthetic opals. As 3D structures these opals can make our women ever more beautiful, and as films are perfect as base for photonic crystals. Synthetic opal matrixes have pre-determined nanostructure, being ideal materials for optoelectronics, magnetic recording systems and other fields of science and technology.

Brand new technology is based on creating very small silicon dioxide spheres with diameter varying from tens of nanometer to several thousands of nanometers. Silicon dioxide is an ordinary material – sand, from which people make bricks, glass and various semiconductors. These items have well arranged atoms of silicon and oxygen in their structure. New materials are based on amorphous silicon dioxide, where atoms are in “disorder”. However, little silicon spheres have equal diameter and are densely packed in a periodic face-centered cubic lattice, like separate atoms in a crystal.

Of course, scientists had to develop a technique, allowing production of “bricks” – spheres made of amorphous silicon dioxide and having set diameter. Chemists applied a well-known technology – so-called sol-gel processing. During synthesis a solution of an organosilicon compound is poured into another solution, which under given conditions and catalytic agent results in a silicon dioxide sphere suspension.

The trick here is in getting spheres of a set diameter, because separating small spheres according their size is a very difficult task. Russian chemists triumphantly solved this problem. They have sorted out synthesis conditions so that reaction results in many identical (monodispersed or equal-grained) spheres. Moreover, their diameter can vary from 10 nanometers to two thousand nanometers.

After “bricks” are synthesized, they can be arranged in structures of any desired shape and size – from one-layer and multilayer films to three-dimensional structures. All you need to do is allowing spheres to sink to the bottom under gravity, and they will form necessary densely packed structure due to self-assembly. Such phenomenon has two reasons – first is that all spheres have same size, and the second is that each sphere carries a small negative charge, equal for all spheres. These two forces lead to formation of a tightly packed structure from monodispersed amorphous silicon dioxide spheres. Between said spheres there are empty pores of same size, which later can be left empty for a great sorbent for chromatography or filled with a metal, for instance, resulting in a system for magnetic recording. Thus the material obtains various unique properties, which allow using it as a basis for many optoelectronic devices.

Among possible applications for brand new technology (apart from synthesis of gem-quality opals) there is creating nanostructures for distributed-feedback lasers. Simply speaking, these structures look like flat “sandwiches”, made of three different layers. Lowest layer contains tightly-packed silicon dioxide spheres, covered with amorphous silicon dioxide (second layer). This eliminates surface irregularities, appearing due to single spheres. Third upper layer consists of another oxide – zinc oxide.

If we illuminate such surface with a laser beam (laser pumping), this beam will be trapped in a waveguide or, in other words, cause excitonic radiation, directed horizontally in zinc oxide. Resonance enhancement will multiply said radiation, because no beam is able to leave this layer due to reflection from its inner surface. This is the way to get much more powerful lasers with less energy consumption.

Varying size of spheres, one can affect wavelengths for opals to absorb or to reflect light, also in visible light range. Moreover, reflected light wavelength also depends on the angle of laser beam incidence on the surface. This means that when we turn an opal, we can see fascinating flashes of various colours.

Source:
    Russian Science News

Kizilova Anna