Publications

A genuine GaAs surface is covered with a relatively thick layer (similar to nm) of native oxide pinning the Surface Fermi level within the band gap of semiconductor. The method presented in this work is related to sulfur passivation by treating n-GaAs in different solutions of alkane thiols. At the surface of GaAs it is developed an adherent layer of sulfur compound as a result of chemical interaction of sulfur ions with GaAs(100) face, that are putted in evidence by scanning electron microscopy (SEM) images and second harmonic generation (SHG) analysis. Using X-ray photoelectron spectroscopy (XPS) is presented a detailed analysis of XPS data at the surface of thiols on GaAs together with the presence of the covalent bond As-S. The electric characteristics of the AuGeNi/Thiol/GaAs structure are presented in I (V) curves recorded in the region of small currents.

Due to the extensive use of new technologies in agricultural and industrial field the soil and groundwater were severely polluted with elements that may pose a serious threat to the environment. For that purpose studies concerning new materials that can be successfully used for removal of heavy metals and other toxic elements from contaminated soil and water were conducted. The attention of the scientific studies were focused on the family of calcium phosphates, with a particular interest in hydroxyapatite (HAp), with general formula Ca-10(PO4)(6)(OH)(2), due to the exquisite ability of adsorbing heavy element ions in aqueous conditions. This study focuses on synthesizing nanocrystalline hydroxyapatite powders with controllable parameters and very good stoechiometry. The structure, morphology and optical properties were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR).

Ba(X1/3Ta2/3)O-3 (X = Zn, Mg) perovskites were prepared by using the solid state reaction method. X-ray diffraction and scanning electron microscopy were employed for structural and morphological characterization of Ba(X1/3Ta2/3)O-3 samples. The dielectric constant of the resonators was 28 for Ba(Zn1/3Ta2/3)O-3 and 24 for Ba(Mg1/3Ta2/3)O-3. A strong influence of the cation ordering on the dielectric loss has been found. The achieved values of Q x f product, ranging from 100 to 200 THz, make Ba(X1/3Ta2/3)O-3 dielectric resonators attractive for microwave and millimetre-wave applications.

The search of new alternative materials for bone reconstruction is a major objective in biomaterials engineering and reconstructive medicine. In this study the compact bone derived materials are proposed, their structure and composition being modified by thermal treatments performed in 200-1200 degrees C range, using 200 degrees C temperature steps. The monitoring of the bone modifications under heat-treatment was carried out using thermal analysis (TGA-DSC), microstructural (SEM), compositional (EDS) and structural (FTIR, XRD) methods. Tailoring the bone material's morphological, chemical and structural properties by heat-treatments, expressed by the HA/beta-TCP compositional ratio, degree of crystallinity and porosity control, could lead to the obtaining of good quality bone substitute materials suitable for the bone regeneration of large osseous defects.

Titanium oxide is one of the most promising candidate for relatively low cost, simple manufacture and high-performance new generation solar cells and the recent achievements in dye-sensitized solar cells (DSSCs) efficiencies and life time confirm the fact that these new generation solar cells becomes one of the future solutions in energy conversion. In DSSCs the presence of a dense titanium oxide layer is necessary in order to avoid short circuits between electrodes. Then, the interest to have a second porous TiO2 layer is determined by the fact that compared to a flat surface, a dye-sensitized porous surface area increase the absorption and hence conduct to the increase of solar cells efficiencies. Generally these two layers are prepared by successive depositions using two different methods. In this paper we present a simple technique to prepare both layers during the same spraying process. Films morphology and structure of titanium oxide deposited on glass and ITO substrate was investigated by AFM, SEM and XRD respectively.

The rapid evolution of GPS, mobile and wireless communication systems in the last years is based, substantially, on ceramic based dielectrics with effect on miniaturization of components (antennas, filters, oscillators), cost and temperature stability. The development of high dielectric constant materials was critical for the dimensions reduction of the equipment but also for their performance improvements. These materials have high dielectric constant, epsilon(r), between 20 and 90; low losses in microwave and millimetre wave domain and a low variation of dielectric constant with the temperature. The paper describes a high dielectric constant material, Barium Neodymium Titanate - (Ba,Pb)Nd2Ti4O12 (BNT), and presents some results obtained by using this ceramic based material to manufacture a dielectric resonator antenna.

The hydrogen storage material lithium borohydride was infiltrated from solvent solution into the pores of the carbon replica of MSU-H mesoporous silica by the incipient wetness method. Different amounts of lithium borohydride up to 40 % from the weight of the high surface area carbonic material were used. By this method was achieved a good dispersion of the hydride into the carbonaceous support. The hydrogen desorption starts at temperatures as low as 150 degrees C for the sample with 8 wt% LiBH4 dispersed onto the MSU-H carbon replica while pure LiBH4 does not release hydrogen below 300 degrees C. After rehydrogenation, a lower amount of hydrogen is desorbed due to the fact that increasing temperature up to 400 degrees C a part of hydride segregates outside the pores of the carbonaceous support.

In this paper, we want to present a simple and efficient numerical method for SHG analysis in one-dimensional photonic crystals (PhCs) based on full nonlinear system of equations. For solving the nonlinear SHG problem we used a simple method of finite elements coupled with fixed point iteration. Our model does not need additional analytic approximation compared with some existing methods, and it can be easily extended to study the SHG problem in two-dimensional photonic crystals. We used the FlexPDE Professional program to plot the diagrams varying the parameters. At the end we obtained two maximum intensities of the second harmonic wave within each high index layer, that being in contrast to the fundamental wave peak. This result can be found also in the literature. In addition, we have plotted the lattice using the Optiwave FDTD software and we observed the propagation of the field in time.

We present the fabrication, structural, and transport properties of MgB2-based ceramic composites with magnetic nanospheres fabricated by spark plasma sintering. The nanospheres are either carbon encapsulated iron or iron oxide. The former nanospheres have been prepared by laser pyrolysis whereas the iron oxide was obtained by the pyrolysis of the polysiloxane-based copolymers embedded into MgB2 matrix during the sintering process. The structural data show the shrinkage of the a-axis lattice constant as a result of the partial carbon substitution for boron. However, the transport data suggest that carbon diffusion is limited to the outer layer of the MgB2 grains in both cases.

We study the conductance G and Seebeck coefficient S of a side-coupled double quantum dot system. The transport properties are the result of the competition between the Fano interference and Kondo correlations, being also controlled by the Coulomb blockade of the multilevel side-dot. The external parameters are the gate potential V-g applied on the side-dot and the temperature. When V-g is varied continuously the blockade is switched on and off periodically, resulting in oscillations of the transport coefficients. The profile of the oscillations and the temperature dependence are studied. An extended Anderson model and the Keldysh transport formalism are used.