A. O. Rybaltovskii

Synthesis of silver nanocomposites by SCF impregnation of matrices of synthetic opal and Vycor glass by the Ag(hfac)COD precursor

Silver-containing nanocomposites were prepared by impregnating Vycor glass (a pore diameter of 4 nm) and synthesized opal matrices (an interstitial void size of 40 nm) with cyclooctadiene complex of silver hexafluoroacetylacetonate (Ag(hfac)COD), a silver precursor, dissolved in supercritical carbon dioxide and were examined by optical absorption spectroscopy, atomic force microscopy, and electron spin-resonance spectroscopy. It was demonstrated that the absorption spectra of Vycor glass and opal matrices impregnated with Ag(hfac)COD molecules and subjected to thermal treatment in air at temperatures above 50°C exhibit plasmon resonances characteristic of Ag nanoparticles at 420–430 nm. The peculiarities of the plasmon resonance band for both types of samples were attributed to the morphology of the pore space in which silver particles are formed. Paramagnetic Cu(hfac)2 molecules (copper hexafluoroacetylacetonate) were used as a spectroscopic probe for estimating the distribution of the precursor in the pores of Vycor glass and opal matrices during supercritical fluid impregnation.

Spectral Features of Composite Oil-in-Water Emulsions Containing Silicon Nanoparticles

A complex spectroscopic investigation of oil-in-water emulsions containing silicon nanoparticles synthesized by plasma chemical vapor deposition has been performed for the first time. It is established by electron microscopy and Raman and IR spectroscopy that nanoparticles synthesized by this method have a crystalline structure, sizes of about 10–15 nm, and an outer shell whose chemical composition depends on the powder synthesis atmosphere. Comparative measurements of the transmission spectra of silicon-containing emulsions showed that their transmission, taking into account scattering, decreases with a decrease in wavelength in the range below 450 nm. The wavelength dependences for particles with an oxynitride outer shell and particles having an oxide shell are significantly different. This result indicates a contribution of the outer shell of silicon nanoparticles to the transmission spectra of emulsions. This factor must be taken into account in design of UV protectors based on silicon powder. In addition, calculations performed for transparent media containing silicon nanoparticles predict the possibility of enhancement of the protective properties of such emulsions in the UV range with increasing sizes of particles above 10 nm.

Immobilization of luminescent nanosilicon in a microfine polytetrafluoroethylene matrix by means of supercritical carbon dioxide

With the use of supercritical carbon dioxide (SC-CO2), the matrix immobilization of photoluminescent silicon nanocrystals (nc-Si) in polytetrafluoroethylene microparticles (mp-PTFE) is performed, which leads to the formation of mp-PTFE/nc-Si photoluminescent nanocomposite containing ∼103–104 nc-Si particles per mp-PTFE particle (1–2 μm in size). This approach is based on the effect of polymer swelling in SC-CO2, efficient SC-CO2-assisted transport of nanoparticles into the internal free volume of the polymer, and contraction of the nanocomposite after the release of CO2, an effect that prevents the subsequent agglutination of nanoparticles. Particles of nc-Si photoluminescent in the visible spectrum were synthesized from silicon suboxide powder (SiOx, x ≈ 1) heated at various temperatures within 25–950°C and then etched in concentrated hydrofluoric acid. The hydrosilylation procedure was used to graft 1-octadecene molecules to the surface of nc-Si particles. As a result, the photoluminescence intensity of nc-Si increased substantially. According to TEM images and small angle X-ray scattering data, the maximum size of nc-Si particles did not exceed 5 nm and 7 nm, respectively, and the core of these nanoparticles consisted of crystalline silicon. The structure and spectral properties of the initial nc-Si particles and synthesized mp-PTFE/nc-Si photoluminescent nanocomposite microparticles were studied.

Spectroscopic Features of Silica Glasses Doped with Tin

The absorption, photoluminescence, and photoluminescence excitation spectra are investigated for tin-doped silica glasses synthesized by different methods. In all the glasses studied, two new centers belonging to the tin dopant in the SiO2 network are revealed in addition to the well-known oxygen-deficient center with the absorption band at 4.9 eV. One new center is associated with the absorption band at 5.9 eV and the photoluminescence bands at 2.7 and 3.6 eV, whereas the other center is characterized by the absorption band in the range of 4.56 eV and the photoluminescence band at 2.95 eV. Both new centers are identified as oxygen-deficient centers. The latter center is an analog of the centers observed in pure silica glasses synthesized in a nitrogen atmosphere and in germanosilicate glasses prepared by the MCVD and SPCVD methods. It is revealed that the formation of the oxygen-deficient center is one of the main channels of incorporating tin into the SiO2 network in all tin-containing silica glasses. Consideration is given to the reasons for the lower efficiency of the photodecay of oxygen-deficient centers in the SiO2 network in tin-containing glasses as compared to that in germanosilicate glasses and for the relatively high efficiency in the formation of the photoinduced refractive index in optical fibers with a core consisting of tin-containing silica glass.

Polyvalent States of Chromium Ions in Silica Glasses Prepared by Plasma-Chemical Deposition

The spectral–luminescent properties of chromium-doped silica glass prepared by gas-phase plasma-chemical deposition are investigated. The absorption, photoluminescence, and EPR spectra are measured for bulk glass samples in the form of a core of optical fiber preforms. The influence of gamma irradiation and additional reducing thermochemical treatment on the spectral characteristics of the synthesized glasses is analyzed.

One photon and two photon process in photo-decomposition of germanium oxygen deficient centres

UV photon-induced transformation of germanium oxygen deficient centres (GODC) in germanium-doped silica glass have been studied using photocurrent measurements, absorption and fluorescence bleaching. It has been identified that the photocurrent are generated via a two photon effect. Evidence have been found suggesting that the UV photon-induced destruction of GODCs is achieved via two reaction pathways, a single photon pathway and a two photon pathway. The process is discussed.

Ultrafast laser-induced birefringence in various porosity silica glasses: from fused silica to aerogel

We compare a femtosecond laser induced modification in silica matrices with three different degrees of porosity. In single pulse regime, the decrease of substrate density from fused silica to high-silica porous glass and to silica aerogel glass results in tenfold increase of laser affected region with the formation of a symmetric cavity surrounded by the compressed silica shell with pearl like structures. In multi-pulse regime, if the cavity produced by the first pulse is relatively large, the subsequent pulses do not cause further modifications. If not, the transition from void to the anisotropic structure with the optical axis oriented parallel to the incident polarization is observed. The maximum retardance value achieved in porous glass is twofold higher than in fused silica, and tenfold greater than in aerogel. The polarization sensitive structuring in porous glass by two pulses of ultrafast laser irradiation is demonstrated, as well as no observable stress is generated at any conditions.

Plasmon resonances of silver nanoparticles in silica based meso-structured films

The process of fabrication of plasmon mesostructures based on opal-like films (films consisting of silica spheres of similar diameters) and homogeneous and quasi-homogeneous sol-gel SiO2 films has been elaborated. The kinetics of formation of silver nanoparticles (NPs) in such films in the course of thermal treatment has been studied. Two mechanisms of the formation of the plasmon absorption bands in such structures have been revealed. The results of the plasmon amplification of the Raman scattering signal of the Rhodamine 6G dye on opal-like films containing silver NPs have been reported.

Laser-induced formation of structures of silver nanoparticles in fluoracrylate films impregnated with Ag(hfac)COD molecules

Methods of absorption spectroscopy and transmission electron microscopy (TEM) are used to study the processes of the laser-induced formation of silver nanoparticles (Ag NPs) in films of fluoracrylate polymers (FAPs) impregnated with Ag(hfac)COD molecules in a medium of supercritical CO2. An observation of the behavior of plasmon resonance (PR) bands in the impregnated FAP samples reveals the presence of a few features in the formation of Ag NPs under exposure to a continuous laser emission in the visible light range. Firstly, the formation of Ag NPs depends on the properties of the matrix (degree of polymerization) and on the power and dose of the laser exposure; its mechanism differs from the thermochemical one. Secondly, only in this case (using TEM) did we find the effect of the formation of periodic layered structures of nanoparticle layers situated inside a film perpendicular to the direction of emission. In addition, while recording the absorption spectra for a long time (a few days) after laser exposure, we revealed the effect of the postradiation growth of PR bands in the samples kept under darkroom conditions at room temperature. The models and formation mechanisms of Ag NPs are discussed for each case under consideration.

Color Centers in Sulfur-Doped Silica Glasses: Spectroscopic Manifestations of an SO2 Interstitial Molecule

The absorption spectra of sulfur-doped silica glass are investigated under different external actions (laser irradiation, saturation of samples in a hydrogen atmosphere, and thermal annealing). It is found that the absorption band at 400 nm has a vibrational structure. A correlation between changes in the intensities of the absorption bands at 237 and 400 nm under different actions is revealed. Reasoning from analysis of the results obtained, a model is proposed according to which these bands are assigned to an S+2 interstitial molecular ion. It is demonstrated that none of the optical centers considered in this work is responsible for the observed ESR signal.