Publications

Alignment layers of polyvinylcarbazole (PVK) were obtained by withdrawing from different solutions in toluene onto glass plates having SiOx layers obliquely evaporated in vacuum at 82 degrees. The alignment direction of nematic liquid crystal molecules with cyan-end group imposed by bare SiOx layers is changed when these layers are coated with PVK; the same happens with over layers of polyvinylimidazole or of polyvinylcinnamate. The effect is inhibited by doping PVK with fullerene C60 (0.1%), when the liquid crystal orientation specific for bare SiOx obliquely evaporated substrates is obtained. Other nematic (with methoxy-end group) does not show change of the orientation properties by covering the SiOx obliquely deposited layers with the mentioned polymer over layers.

The aim of this article is in the first place to familiarize the readers with barrier-cluster-heating model of the non-crystalline semiconductors, then briefly and digestedly present some results achieved by physical phenomena explanation in non-crystalline semiconductors within the frame of this model. The barrier-cluster-heating model of disordered semiconductor gives new view on photoluminescence in disordered semiconductors, too. This model enables to explain Stokes shift, Street's empirical law, fatigue effect, PLE-characteristic, electric field influence on photoluminescence, and some other physical phenomena in disordered semiconductors, including the photoinduced structural changes.

GeSi-based nanostructures show unique properties which make them suitable for integrated circuit technology. The strong interest is to enhance their electronic properties in order to improve the device performance. In order to obtain fundamental knowledge on the electrical transport taking place in GeSi nanostructures we have investigated the effects of different microstructures on the electrical behavior of GeSi nanostructured films, by modifying the annealing conditions. We manufactured GeSi nanostructured films with equiatomic composition and different structures by co-sputtering followed by adequate annealing under different temperatures. For determining the electrical behavior we performed and modeled current-temperature I - T characteristics taking into account the films structures. We found that the electrical behavior changes with the film structure by evidencing a transition in conduction mechanism. In films that are almost crystallized, being formed of small GeSi nanocrystals separated by thin amorphous regions, the I - T dependence at low temperature is due to thermally activated tunneling of carriers between neighboring nanocrystals. In contrast, in the completely crystallized films with big GeSi nanocrystals and crystallized borders between them, the electrical behavior is a typical polycrystalline one. Our findings help to clarify the conduction mechanisms taking place in GeSi nanostructures and to provide a route to electronic devices with high performance based on these materials.

We report on realization of buried cladding waveguides in Nd:YAG single crystal and ceramic media by direct femtosecond (fs)-laser writing technique. A classical technique that moves the laser medium transversally to the fs-laser beam, as well as a new scheme in which the laser medium has a motion on a helical trajectory during the inscribing process was used. The waveguides laser emission performances at 1.06 and 1.3 mu m have been investigated under the pump at 807 nm with a fiber-coupled diode laser that was operated both in quasi-continuous wave (quasi-cw) and in cw regimes. Laser pulses with energy of 3.45 mJ at 1.06 mu m and of 1.05 mJ at 1.3 mu m (with overall optical-to-optical efficiency of 0.26 and 0.08, respectively) were obtained from a 50 mu m in diameter circular waveguide that was inscribed by the helical-moving techniques in a 5.0-mm long, 1.1-at.% Nd:YAG ceramic medium. Characteristics of the laser emission recorded in cw operation are discussed.

The different amount of Cu-doped Barium Strontium Titanate (BST) thick film materials have been tested for their gas-sensing performances towards NH3 and H2S under dry and 50% relative humidity (RH) background conditions. The optimum NH3 sensitivity was attained with 0.1mol% Cu-doped BST whereas the selective detection of H2S was highlighted using 5mol% Cu-doped BST material. No cross-sensitivity effects to CO, NO2, CH4 and SO2 were observed for all tested materials operated at their optimum temperature (200 degrees C) under humid conditions (50% RH). The presence of humidity clearly enhances the gas sensitivity to NH3 and H2S detection.

We propose a numerical procedure consisting of a simplified physical model and a numerical method with the aim of optimizing the performance parameters of dye-sensitized solar cells (DSSCs). We calculate the real rate of absorbed photons (in the dye spectral range) G(real)(x) by introducing a factor beta < 1 in order to simplify the light absorption and reflection on TCO electrode. We consider the electrical transport to be purely diffusive and the recombination process only to occur between electrons from the TiO2 conduction band and anions from the electrolyte. The used numerical method permits solving the system of differential equations resulting from the physical model. We apply the proposed numerical procedure on a classical DSSC based on Ruthenium dye in order to validate it. For this, we simulate the J-V characteristics and calculate the main parameters: short-circuit current density J(sc), open circuit voltage V-oc, fill factor FF, and power conversion efficiency eta. We analyze the influence of the nature of semiconductor (TiO2) and dye and also the influence of different technological parameters on the performance parameters of DSSCs. The obtained results show that the proposed numerical procedure is suitable for developing a numerical simulation platform for improving the DSSCs performance by choosing the optimal parameters.

The structure of GeSiO films resulted from deposition and annealing conditions draws their electrical behaviour. GeSiO films prepared either by magnetron sputtering or sol-gel method and subsequently annealed are formed of Ge nanocrystals and/or amorphous Ge nanoparticles embedded in amorphous SiO2 matrix. Firstly, the size effect which is the main effect in these systems produces specific quantum confinement energy levels in the enlarged forbidden energy band gap in nanocrystals. Secondly, these films are percolative systems, so that the main conduction mechanisms which govern the electrical behaviour are the tunnelling and hopping between neighbouring Ge nanocrystals or amorphous nanoparticles. Accordingly, the charge transport is strongly determined by the structure and morphology of films.

Aerobic oxidative condensation of benzylamine to N-benzylidenebenzylamine was carried out on new gold-based catalysts using as support bimodal UVM-7-like mesoporous silica containing Ni and Ce (or Sn) as oxide nanodomains partially embedded inside the mesoporous UVM-7 silica walls. These nanodomains acted as effective and stable inorganic anchors favoring the nucleation, growth and stability of supported gold particles. Following the atrane method ( a "one-pot" strategy able to harmonize the hydrolytic reactivity of different heteroelements through the use of complexes containing triethanolamine-derived ligands (atranes) as precursors) the stability of the oxide nanodomains during oxidation and thermal treatments was ensured. The catalysts were prepared using a two-step synthesis in which gold was incorporated through impregnation of previously synthesized bimodal porous silicas containing different heteroelements trapped by the silica mesostructure. The versatility of the preparative approach allowed the high surface area and accessibility of mesoporous silica and also an easy and homogeneous inclusion of heteroelements in one pot. Aerobic oxidative condensation of benzylamine on these catalysts occurred selectively only with the formation of N-benzylidenebenzylamine. While selectivity was total, the productivities (mol(product) g(cat)(-1) h(-1)) varied in a wide range as a function of catalyst composition (chemical composition and atomic ratios of the constitutive support elements). The catalytic behavior was merely controlled by the size of gold particles, but the support also exerts an influence. The best results were obtained on the mesoporous Au/Ce-60-Ni-10-UVM-7 catalyst having a 0.97( wt.%) gold content ( in the form of nanoparticles of ca. 4 nm) and preserving a relatively high surface area ( 566 m(2) g(-1)) and pore volume ( 0.91 cm(3) g(-1)). The comparison with the individual or bicomponent catalysts led to the conclusion of a cooperative interaction between the catalyst components. However, gold itself exhibits a higher activity than the promoters or co-catalysts (i.e. phases containing Ni and/or Ce).

New findings on the formation and annealing of interstitial boron-interstitial oxygen complex (BiOi) in p-type silicon are presented. Different types of n(+)-p structures irradiated with electrons and alpha-particles have been used for DLTS and MCTS studies. Electronic excitation essentially changes the formation rate of BiOi. It has been found that the increase of oxygen content slows the BiOi annealing rate down. The activation energy of the BiOi dissociation has been determined and it was found that germanium doping does not change the activation energy.

We report on structure-property relationships in Pr-doped CeO2 and ZrO2 using X-ray diffraction (XRD), Raman, UV to Vis Diffuse Reflectance (DR-UV/Vis), X-ray Photoelectron (XPS), and luminescence (PL) spectroscopies. Both 3+ and 4+ valence states of Pr are evidenced, irrespective of the host and calcination temperature, T (T = 500 and 1000 degrees C) with consequences on absorption, surface, vibrational and luminescence properties. Only zirconia represents a suitable host for Pr3+ luminescence. The distinct trivalent Pr centers and their excitation mechanism are identified in relation to the tetragonal and monoclinic phases of ZrO2. A near-infrared to visible up-conversion (UPC) emission of Pr3+ is observed upon excitation at 959 nm which occurs, most probably, via a two-photon excited state process. By using a multi-wavelength, time-gated excitation, the UPC process is established as phase selective, i.e. only Pr3+ located in the monoclinic sites of the mixed phase, monoclinic and tetragonal ZrO2 (T = 1000 degrees C) contribute to the UPC emission. We believe that, besides the local symmetry, a key role in phase selective UPC is played by the presence of Pr3+ low-lying 4f 5d levels. To the best of our knowledge, this is the first report of phase selective up-conversion emission in a lanthanide doped multi-phase host.