T. Petkova

Structural investigations of the Se-Ag-I system

Glasses from the Se–Ag–I system have been investigated. Structural information is gathered based on results collected by a combination of several types of diffraction measurements. First coordination sphere at r ¼ 2:3–2:5 A termined by radial distribution function calculations can be assumed to be composed by Se–Ag and Ag–I correlations in the network units andSe–Se correlation in the Se cluster units. The interatomic d istances andaverage bondangles decrease with the introduction of additives to selenium. The Raman spectra reveal that the introduction of silver and iodine, in particular, brings about a shift of the Se chain stretching mode towards lower wave numbers (251–236 cm � 1 ). Apparently, the newly formed structure after the introduction of additives into selenium is pretty compact and is becoming more covalent with a stronger interchain interaction. Depending on the concentration of additives the breathing modes of Ag–Se and Ag–I bonds also can be seen. 2003 Elsevier B.V. All rights reserved.

New Organic-Inorganic Hybrid Ureasil-Based Polymer Materials Studied by PALS and SEM Techniques

The new organic-inorganic materials, based on polyether chains covalently linked to a silica framework through urea bridges, referred as ureasilicates or ureasils, and semiconducting As2S3-ureasil composites are investigated using positron annihilation lifetime spectroscopy (PALS) and scanning electron microscopy (SEM) techniques. The results obtained show that incorporation of the As2S3 clusters into ureasil affects on the ortho-positronium (o-Ps) intensity or positronium formation probability and micro-/nanoscopic structure compared to the pure polymer, the effect is more essential as the loading fraction of As2S3 increases.

Nanovoids in Glasses and Polymers Probed by Positron Annihilation Lifetime Spectroscopy

Nanovoids in As2S3-based glasses (As2S3, (As2S3)85Ag15, and (As2S3)85(AgI)15), a polymer and a As2S3-polymer nanocomposite were studied using the positron annihilation lifetime spectroscopy (PALS) technique. After computer treatment of the PALS data recorded, it was found that only two components τ 1 (short-lived) near 0.2 ns and τ 2 (long-lived) near 0.4 ns are resolved for the As2S3-based glasses. At the same time, in the case of the polymer sample two components τ 2 near 0.3 ns and τ 3 (pick-off annihilation of ortho-positronium) near 2.8 ns were detected, while for the As2S3-polymer nanocomposite three components τ 1 near 0.2–0.3 ns, τ 2 near 0.4–0.5 ns and τ 3 near 2.4 ns were established. The volume of nanovoids in the materials studied was determined, and the fractional free volumes of the As2S3-polymer nanocomposite and the polymer matrix were compared. The results obtained are important to utilize As2S3-based glasses and polymer nanocomposites for advanced sensor applications.

Nanostructured Diamond Electrodes for Energy Conversion Applications

The subject of our work is the investigation of boron doped nanocrystalline diamond (NCD) layers functionalized with photosensitive molecules of manganese phthalocyanine. The functionalized NCD sample was used as a working electrode in a three electrode cell and electrochemically studied. The two methods applied were stationary volt-ampere (VA) measurements and cyclic voltammetry (CV) in dark and under illumination. The VA and CV measurements showed that the electrode has a wide potential range and a high photosensitivity. The results obtained are promising for future application of functionalized diamond electrodes in light conversion systems.

Photoinduced changes in As-Se-Ag amorphous films

Thin films from the As-Se-Ag system have been prepared on glass and silicon substrates by thermal vacuum evaporation from previously fabricated bulk glassy samples. The amorphous state of the samples has been proved by X-ray diffraction. Some optical properties of the amorphous As-Se-Ag thin films have been studied in relation to the Ag concentration in the sample. In order to investigate the photoinduced changes due to irradiation by with He-Ne laser light, transmission spectra of the thin films have been measured before and after irradiation. The optical characteristics as a function of the composition have been studied.

Ge-Chalcogenide Glasses – Properties and Application as Optical Material

The great interest toward chalcogenide materials is due to the simple technology of preparation in bulk forms and thin films; good thermal and mechanical properties; transparency and photo-sensibility in the IR spectral range. These advantages determine the possibilities for potential application of these materials like optical storage media, memory devices, optical elements (lenses, waveguides, gratings, etc). The idea of present study is to trace the impact of gallium or indium as metal introduction on the behaviors of the glasses from germanium - chalcogenide system.

Influence of the preparation method on the As-Se-AgI thin films behaviour

Bulk glasses of the (As 2 Se 3 ) 1-x (AgI) x and (AsSe)1-x(AgI)x systems, where x = 5, 10, 15 up to 35 mol.% have been prepared by the melt-quenched technique. The thin films have been deposited by means of vacuum thermal evaporation (VTE) and pulsed laser deposition (PLD). The XRD investigation reveals a generally amorphous structure; small peaks are only observed in the samples with the highest AgI. The film compositions have been determined by EDS (energy dispersive X-ray microanalysis). WDS (wavelength dispersive spectroscopy) studies have shown that the films do not contain oxygen within the accuracy of the method (1 %). The films are dense with smooth surface as revealed by using scanning electron microscopy (SEM) and atomic force microscopy (AFM).

Optical Behaviors of Novel Amorphous Ge – S – AgI Layers

The novel bulk glasses from the chalcohalide Ge-S-AgI system have been synthesized. From the as-prepared samples amorphous films have been deposited by vacuum thermal evaporation. The amorphous nature of the studied bulk and layered materials has been proved by Xray diffraction. The composition of the synthesized bulk chalcohalide glasses and corresponding amorphous thin films has been ascertained by means of Auger electron spectroscopy. The morphology and uniformity of the deposited layers have been investigated using scanning electron microscopy. The basic optical properties of the studied glassy films have been defined. Variations in the optical behaviors as a function of the composition have been derived. Experiments related to optical recording in the investigated Ge-S-AgI layers has been implemented. The diffraction efficiency as a function of various recording beam intensities has been obtained.

Ureasil-Based Polymer Matrices As Sensitive Layers for the Construction of Amperometric Biosensors

Ureasil and ureasil-chalcogenide glass composites of different history (fresh and aged during 1 year) were used for the immobilization of laccase and the construction of amperometric biosensors. A correlation between the microscopical free-volume of the polymer matrices as revealed by low-temperature positron annihilation lifetime spectroscopy and biosensor characteristics of the laccase-containing ureasil based biosensors is established. The observed findings could be applied for improvement of the operational parameters of the constructed biosensors, which may have potential for monitoring the level of pollution of wastewater.

Polymer Nanocomposites with Silver Nanoparticles Formed by Low-Energy Ion Implantation: Slow Positron Beam Spectroscopy Studies

Polymer nanocomposites formed by low-energy ion implantation were studied by means of positron annihilation spectroscopy with a variable-energy positron beam or slow positron beam spectroscopy. Silver ion implantation into polymethylmethacrylate (Ag:PMMA) and hybrid organic-inorganic ureasil (Ag:ureasil) was performed at different ion fluences with a constant energy of 30 keV and a current density of 1 μA/cm2 in order to prepare Ag nanoparticles in the near-surface region of polymer matrix. Contribution of Doppler broadening slow positron beam spectroscopy technique for understanding Ag nanoparticles formation in Ag:PMMA and Ag:ureasil nanocomposite films is demonstrated.