National Institute Of Materials Physics - Romania

Small particles of Pbl(2) embedded in transparent polymer matrix have been studied by Raman scattering, UV-VIS absorption spectroscopy, low temperature photoluminescence (PL) and Scanning Electron Microscopy (SEM). The Pbl(2) crystallites were prepared by two different methods: i) by cooling to the room temperature a boiling saturated aqueous Pbl(2) solution containing polyacrylamide and ii) by chemical reaction of Kl and Pb(NO3)(2) in aqueous polyacrylamide solution. In both cases, beside the hexagonal platelets of Pbl(2) Were identified rods whose luminescence and Raman signature is quite different than of Pbl(2) platelets. Regardless the rods nature, as distinct KPbl(3) particles or lead iodide low-dimensional particles embedded in the polymer matrix, their photoluminescence is featured by an intense green emission (550 nm) appearing at low temperature at excitation wavelengths less then 350 nm.

In-plane current-voltage (I-V) characteristics of high-temperature superconductors in a zero applied magnetic field exhibit intrinsic two-dimensional (2D) behavior at relatively high transport-current densities. This allowed us an accurate determination of the temperature variation of the zero-field I-V exponent a(T) (V proportional to I-a) at the superconducting (CuO2)(2) layer level in the case of highly anisotropic Bi2Sr2CaCu2O8+delta films in the vicinity of the Kosterlitz-Thouless-Berezinskii transition temperature T-KT. The mean-field critical temperature and T-KT were determined from the zero-field resistive transition in the framework of the 2D Coulomb gas model. It was found that a(T-KT)approximate to 3, pointing toward a dynamic critical exponent z(T-KT)=2. The a(T) dependence observed by us is in good agreement with Langevin simulations of the 2D Coulomb gas model.

Band-pass filters with dielectric resonators of (Zr-0.8, Sn-0.2)TiO4 ceramic materials (ZST) were developed for X-band applications. The ZST materials exhibit a dielectric constant of about 36.8 and low loss in microwave range. A Qxf product higher than 50,000 was achieved for the ZST samples with 0.2 % wt NiO sintering addition. The external coupling was performed using microstrip lines on a substrate of 9.22 dielectric constant and 0.635 mill height. Filters with a minimum insertion loss between 0.46 dB and 1.74 dB, the bandwidth between 40 MHz and 170 MHz and with the central frequency around 10 GHz were manufactured.

This paper presents the investigation of the optical properties of bisphenol A polycarbonate organic matrix/CdS clusters composite material in correlation with the method for the films' preparation using as precursors cadmium acetate (Cd(CH3COO)(2)), respectively ammonium thiocyanate (NH4CNS), and as general solvent for all the components, is dimethyformamide. The effect of the heat treatment temperature (90 degrees C or 150 degrees C) on the position and shape of the fundamental absorption edge was investigated for given substrate, drying conditions and duration (1.5 h or 0.5 h) of the heat treatment. A large blue shift of the absorption edge was evidenced in films prepared in vertical configuration and thermally treated at 90 degrees C for 1.5 h, which can be associated with a small dimension of the CdS crystallites and in consequence with quantum confinement effects. Using the effective mass theory we have deduced a crystallite dimension in the range 8.8-15 nm. We also have studied the interaction of the light with this composite material evaluating an optical band gap between 2.92 and 3.69 eV (depending on the preparation and annealing conditions) and an absorption mechanism in CdS clusters closer to an indirect transition and deviated from the normal direct absorption process in bulk CdS semiconductor.

Optical transmittance studies were carried out on some doped titanium dioxide thin films, obtained by r.f. magnetron reactive sputtering, using glass and ITO/glass substrates. We have observed that, while TiO2 films have a good transparency till about 2000 nm, the ensemble TiO2 and ITO (indium tin oxide) films gives a decrease of the optical transmittance in near infraread region, being still transparent in the visible range. The mentioned decrease depends on the dopant used. This is an important observation, since TiO2 films are already used as coatings for many buildings. So, for economic reasons, the necessary energy used for the inside cooling of the air can be reduced, by getting a good visible transparent window, together with a NIR shielding.

Nanostructures of gold embedded in potassium chloride matrices were obtained after the doped crystals growth, by electrolytical colouring, and by thermal treatment. When the TEM images of the nanostructures were analyzed using fractal geometry it means was proved that the self assembling of the metallic structures is governed by the diffusion limited aggregation (DLA) mechanism. The values of the fractal dimension, which were in all cases between 1.7 and 1.9, calculated using two methods, sandbox and box-counting, proved the DLA mechanism.

The electron transfer processes induced by the ultraviolet exposure of the Fe and Sb doped polyvinyl alcohol (PVA) thin films were studied in comparison with those appeared in the correspondent mono (Fe or Sb) doped ones. Samples with different thickness and subjected to different exposures were analysed by optical absorption as well as by Fe-57 and Sb-121 Mossbauer spectroscopy. The influence of each element from the pair on the electronic mechanism activated by the ultraviolet exposure was investigated. Accordingly, the (Fe+Sb) doped PVA presents improved electronic characteristics, in respect to the specific behaviour of single metal doped polymer recording media for real time holography.

Two samples of large-grained commercial superabrasive cubic boron nitride crystalline powders in different shades of amber coloration have been investigated by low frequency X (9.4 GHz)-band electron paramagnetic resonance (EPR) spectroscopy at room and low (T < 15 K) temperatures. Both samples contain three spectra component lines, A1, A2 and A3, in the same proportion, and the total concentration of the corresponding paramagnetic centers being proportional to the intensity of the amber coloration. After in situ broad-band UV illumination at low temperature, the intensity of the A1 component line becomes dominant. The observed effects are attributed to electron trapping at EPR silent A1(+) precursor centers, which seem to coexist with the A1 centers and to the eventual ionization of the paramagnetic A3 centers responsible for the A3 component line.

This paper presents the optical absorption properties in crystalline meta-clinitrobenzene (m-DNB) and benzil. The optical band gap as an intrinsic property of these crystalline compounds has been studied using UV-VIS Spectroscopy on thin film. Analysing absorption near the fundamental absorption edge we have deduced the wide band gap semiconductor behaviour of these aromatic derivatives. Processing the experimental transmission data using a function obtained by the superposition of a linear function and a power function we have evaluated the band gap energy in m-DNB, E-g=2.90 eV and emphasised the same two edges patterns of the light absorption spectrum in benzil deposited on glass as for benzil deposited on quartz, with energetic thresholds slowly moved through lower energies, E-g1=2.79 eV and E-g2=3.49 eV. We have also remarked no significant change in the absorption spectrum in benzil induced by the dopant (organic and inorganic).