E. N. Samarov

Porous structure of synthetic opals

Some adsorption, structural, and annihilation characteristics of synthetic opals are measured. An analysis of these characteristics makes it possible to conclude that the total porosity in opals comprises macropores, which are in essence the voids between structure-forming spheres, and nanopores connected to the substructure of the spheres. The systems of macropores and nanopores are separated by narrow channels that collapse upon the thermal treatment of opals. The sizes of primary a-SiO2 particles and nanopores, which correspond to the voids between these particles, are estimated.

The Intrinsic Structure of Spherical Particles of Opal

The intrinsic structure of spherical SiO2 particles synthesized by hydrolysis of tetraethyl orthosilicate in an alcohol-water-ammonia medium was studied using transmission electron microscopy. It was established that the relatively large spherical silica particles were “tertiary” structures made up of smaller spherical particles (“ secondary” particles), which in turn consisted of even smaller primary spherical particles 5–10 nm in diameter. It was shown that, under the experimental conditions, the large SiO2 particles can contain a central core comprising primary particles surrounded by several layers of secondary particles smaller than the core diameter.

Photoluminescence of ZnO layers grown on opals by chemical deposition from zinc nitrate solution

The emission from ZnO layers grown on the surface of bulk opals using chemical deposition is studied under excitation by the 351.1 nm line of an Ar+ laser at different excitation power densities and temperatures. The emission spectrum exhibits narrow photoluminescence (PL) bands associated with the recombination of bound and free excitons as well a relatively broad band around 3.31 eV. The width of the excitonic lines (2–3 meV) along with their energy position are indicative of the high quality and strain-free state of the layer. The origin of the 3.31 eV PL band is discussed in connection with its dependence upon the excitation power density and temperature.

Structural modification of synthetic opals during thermal treatment

The density and porosity of synthetic opals with spheres 315 and 1000 nm in diameter were measured in relation to the annealing temperature. At annealing temperatures of up to 500°C, the seeming density and porosity remain almost unchanged. Then, at temperatures of up to 950°C, the density increases gradually and, accordingly, the porosity decreases due to the collapse of nanopores caused by the sphere substructure. As the annealing temperature increases further, the opal density increases sharply up to 2.22 g/cm3 (which corresponds to the density of amorphous silica) and the open microporosity due to the voids between spheres disappears. Differential thermal and thermogravimetric analyses showed that SiO2 powders with particles with average size of 315 and 1000 nm can have, respectively, two-and three-level systems of micro-and nanopores.

Ultraviolet Luminescence of ZnO Infiltrated into an Opal Matrix

Technology for the infiltration of zinc oxide into a three-dimensional opal lattice using chemical deposition from a solution was developed. Samples of ZnO-opal composites, whose luminescence at room temperature mainly occurs in the ultraviolet spectral range, were obtained. The filling ratio was monitored by two different techniques: (i) checking the increase in the mass of the sample and (ii) checking the shift of the peak in the optical reflection spectrum of samples filled with ZnO in comparison with the initial opal matrices. The results obtained by these two methods are consistent with each other. Optimum conditions for synthesizing ZnO-filled opals in order to attain the highest intensity of ultraviolet luminescence were determined. It was shown that using “raw” opals, whose voids are incompletely filled with the semiconductor material, leads to a severalfold increase in the intensity of the edge excitonic emission band at room temperature. The results obtained can be used in the development of efficient directed laser light sources in the ultraviolet spectral range based on the “photonic crystal” effect.

Photonic stop bands in opal films and crystalline liquids

We investigate the optical properties of thin films of two photonic systems: synthetic opal and crystalline liquid BPII. Diffraction related with three-dimensional periodic structure and interference related with light reflection from film surfaces were measured. We found a substantial change of refractive indices at the edges of the photonic bands in both materials. The relation between the frequency and the wave vector of light shows a nonlinear behavior near zone boundary.

Luminescence of zinc oxide nanorods

The spectra of spontaneous and stimulated emission of ZnO nanorods grown by two low-temperature procedures are studied. Stimulated emission at 385 nm is observed at room temperature in CVD ZnO nanocrystals pumped by the nitrogen laser radiation at the wavelength 337 nm. The threshold pumping power density for lasing exitonic recombination is ∼600 kW/cm2.

Opal-ZnO nanocomposites: structure and emission properties

The structure of the opal-ZnO composites has been studied by TEM and X-ray methods. It was found that the solid state reaction of the opal-ZnO interface interaction is occurring during the heat treatment of the infiltrated samples resulting in the formation of the zinc silicate β-Zn2SiO4 and its high temperature modification of willemite Zn2SiO4. Nanocomposite structure and emission properties have been studied in dependence on the filling degree. The blue luminescence at 430 nm stipulated for the β-Zn2SiO4 phase has been observed for the sample with 25 filling cycles. Angular dependences of the PL and Reflection spectra of the opal-ZnO composite with 4 filling cycles demonstrate the suppression effect of the ZnO spontaneous emission in the stop band.