Publications

The electrical conductivity of new nanocomposite systems liquid-ctystal/montmorillonite clay is performed using impedance spectroscopy. The dependence of the electrical conductivity on the frequency, temperature and bias voltage is obtained. The results referring to the alternative voltage influence allow getting more information on the conduction mechanism than the previous ones obtained under frequency and temperature influence. An important role has the polarization at the electrode-sample interface, both in continuous and alternative current. The electrode polarization effect on the electrical conductivity in dependence on the above mentioned parameters is also examined.

Electrical and photoconductive properties of films consisting of amorphous Ge nanoparticles uniformly distributed in amorphous SiO2 were studied. These films were prepared by sol-gel method and treated by rapid thermal annealing technique. Measurements of current-voltage and conductance-temperature characteristics, spectral and bias dependences of the photocurrent on samples with coplanar geometry of electrodes, were performed. The current-voltage characteristics have a weak rectifying behaviour. The variable range hopping transport mechanism, described by the Mott law, in amorphous materials, in the absence of dominant Coulomb interactions, was evidenced in the temperature dependence of the dark current. The samples exhibit very good photoconductive properties, explained by taking into account the Ge clusters and defects, produced by the rapid thermal annealing.

GeSiO nanostructures obtained by using two different preparation methods, sol-gel and magnetron-sputtering were studied. They are formed from Ge nanoparticles dispersed in amorphous matrix, with different morphology depending on the preparation method. Electrical investigations (current-voltage and current-temperature measurements) were performed and experimental results were modelled. It was shown that preparation conditions induce the electrical behaviour of GeSiO films, so that the electrical transport in sputtered films is governed by a hopping mechanism, while in sol-gel ones it is dominated by the junction formed at the interface with Si substrate. Also, the temperature dependence of current shows different hopping mechanisms.

High quality granular ferromagnetic thin films on glass and silicon wafer substrates were obtained using the original thermionic vacuum arc (TVA) method developed at National Institute for Laser, Plasma and Radiation Physics. We report obtaining of 77-200 nm thick Cu/Ni/Fe structures. An experimental set-up, with three simultaneously discharges, was made to obtain the desired thin film composition and thickness. The structural and morphological properties of the prepared nanostructured films were analyzed by AFM (Atomic Force Microscopy), XRD (X-ray Diffraction) and SEM (Scanning Electron Microscopy). Magneto-Optical Kerr Effect studies were made in order to infer the changes in polarization of an optical radiation due to the magnetic field influence. In order to obtain concluding electrical measurements results, cooper electrodes were deposited using the same method before and after the actual three material thin structures. In this way significant changes in the electrical resistance behavior were noticed and were correlated with TVA plasma parameters used for film preparation.

Europium doped hydroxyapatite (Eu:HAp) nanocristalline powders was synthesized by co-precipitation method by setting the atomic ratio of Eu/[Eu + Ca] at 0% and 2% and [Ca+Eu] /P at 1.67. The structural properties were characterized by X-ray diffraction (XRD). Nitrogen adsorption-desorption isotherms were obtained on different samples using BET method. The X-ray diffraction analysis revealed that hydroxyapatite is the unique crystalline constituent of all the samples, indicating that Eu has been successfully inserted into the HAp lattice. Based on N-2 adsorption-desorption isotherms investigation, the pore size, surface area and pore volume of europium doped hydroxyapatite are 16.57 nm, 115.06 m(2)/g and 0.48 cm(3)/g.

Arrays of multisegment Pd/(Zn,Cu)O:Mn/Pd submicron wires have been grown electrochemically into the pores of nuclear track polycarbonate membranes. Their transport properties were investigated, by measuring current-voltage (I-V) characteristics in a temperature range from 300 K down to 40 K. I-V characteristics of Pd/(Zn,Cu)O:Mn/Pd wires exhibited a symmetric, non-linear shape on applied bias voltage ranging from -20 V to +20V. The ohmic region corresponds to polarization voltage under 3.5V. Comparatively, I-V characteristics of Pd/ZnO/Pd array of wires with diameter about 500 nm are linear in the entire range of applied voltages (-20 V to +20V).

A multi-purpose gas-flow proportional counter for surface Mossbauer spectroscopy is described. The main improvements obtained are: the height of the detection volume can be changed in large limits from 1 to 38 mm, the detection volume can be symmetrical or not in respect with anode plan, the anode changing is easily and different anode configuration can be used. It is suitable for studies with (57)Fe, (119)Sn and (151)Eu isotopes. The performances of the detector are presented.

The electrical behavior of multi-walled carbon nanotube network embedded in amorphous silicon nitride is studied by measuring the voltage and temperature dependences of the current. The microstructure of the network is investigated by cross-sectional transmission electron microscopy. The multi-walled carbon nanotube network has an uniform spatial extension in the silicon nitride matrix. The current-voltage and resistance-temperature characteristics are both linear, proving the metallic behavior of the network. The I-V curves present oscillations that are further analyzed by computing the conductance-voltage characteristics. The conductance presents minima and maxima that appear at the same voltage for both bias polarities, at both 20 and 298 K, and that are not periodic. These oscillations are interpreted as due to percolation processes. The voltage percolation thresholds are identified with the conductance minima.

Doped ceria powders, Ce-0.8 Ln(0.2)O(2-x) (Ln = Gd3+, Sm3+ and Yb3+) were sintered to nearly full density by Spark Plasma Sintering at 1100 and 1200 degrees C. The starting powders were obtained via a sol-gel method and had nanometric dimensions: 26.4 nm (Gd), 19.3 nm (Sm) and 31.4 nm (Yb). Samples characterization was performed by SEM, TEM and XRD to determine their structural and microstructural characteristics. The ionic conductivities of the sintered samples were determined by impedance measurements. The influence of the chemical composition, sintering temperature and microstructural characteristics on the total ionic conductivity were evaluated and discussed.

For the first time we presented the preliminary results of biocompatibility studies on bioceramic hydroxyapatite powders doped with europium Ca10-xEux(PO4)(6)(OH)(2), with 0.01 <= x(Eu) <= 0.2 prepared at low temperature by a simple coprecipitation approach. The X-ray diffraction (XRD) studies revealed the characteristic peaks of hydroxyapatite in each sample. No evidence for additional crystalline phases was found, proving the complete substitution of Eu in the HAp lattice in the whole range of concentrations. The scanning electron microscopy (SEM) observations suggest that these materials present a little different morphology, which reveals a homogeneous aspect of the synthesized particles for all samples. The PL investigations shown the PL intensity changed considerably by varying the x(Eu). The value of x(Eu) in the hydroxyapatite formula has almost no effect on the wavelength of emission peaks. In order to test Hap_Eu biocompatibility, the effect of europium substituted hydroxyapatite nanocristalline powders with different x(Eu) on cell viability and proliferation of HEK293 cell line were evaluated. The in vitro investigation showed no significant decrease of viability of HEK293 cell line and low levels of intracellular lipid peroxidation in the Hap_Eu treated cells. In conclusion, the results suggest that Eu doped HAp has low toxicity and exhibit a good biocompatibility.