National Institute Of Materials Physics - Romania

Silicon wafers have been submitted to hydrogen RF-plasma treatment in various experimental conditions. Hydrogen RF-plasma treatment induced two kinds of effects on Si wafers, depending on the treatment conditions: surface corrugation and formation of structural defects below the free surface. Atomic force microscopy (AFM) investigations showed that the surface roughness significantly increased with the treatment duration, leading to the formation of pyramidal humps on the surface. The structural defects resulting after the plasma treatments were investigated by conventional and high-resolution transmission electron microscopy (CTEM and HRTEM) techniques. The specificity of the induced extended defects due to hydrogen decoration was emphasized. Three types of extended defects were identified and characterized: planar defects in the {111} and {100} planes and nanometric voids. Point defects related to the hydrogenation process were investigated by electron paramagnetic resonance (EPR) in correlation with the electron microscopy results.

A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce3+ impurities in PbWO4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g(parallel to) and g_, are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g_-parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S-4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce3+ ions with tetragonal symmetry S-4 in the PbWO4 scintillator. (c) 2009 Elsevier BM. All rights reserved.

Amorphous thin films of SnSe2 and Ge2Sb2Te5 doped by different amount of silver (0.1, 0.2, 0.5 and 1 Ag atoms per formula unit) have been prepared by pulsed laser deposition (PLD) starting from solid polycrystalline targets. The films were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The good gas sensing properties for CO, as well as the sensitivity for CH4 and NO of the Ag; doped SnSe2 films have been demonstrated in the composition SnSe2Ag0.2. The structural effect of silver introduced in Ge2Sb2Te5 matrix has been investigated. The freshly deposited thin films doped by various amount of Ag develop three phases: an amorphous one, and two crystalline phases consisting of a major fcc cubic phase of AgSbTe2 and a minor cubic phase of composition Ag2Te. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Gold layers were deposited at oblique incidence onto glass plates. A B30.2 Hochvakuum Bedampfungsanlage equipment was used in this aim. The deposition conditions were chosen in order to obtain nanostructured layers. Correlations were found among these conditions and the corresponding nomograms were then obtained. Examples of such depositions were given and the layers structure was characterized and additionally supported by observing the molecular alignment given in the liquid crystal cells obtained with these gold layers.

The influence of the shape of silicon quantum dots embedded in an amorphous silica matrix on the quantum confinement energy levels, as well as that of the Si/SiO2 potential barrier, are studied. The energy levels are computed using both the infinite and finite rectangular quantum well models for spherical quantum dots and the infinite rectangular quantum well for prolate spheroidal quantum dots. The results are compared with each other and also with the experimental activation energies obtained from the temperature dependence of the dark current. These activation energies are identified with the differences between the quantum confinement energies, subject to the selection rules. The finite rectangular quantum well model takes into account the experimental value of the finite potential barrier and the matrix-to-dot electron mass ratio. The energy levels are smaller than those for the infinite rectangular quantum well case; they decrease when the potential barrier decreases and the mass ratio increases. Different aspects of the models are discussed. All the errors are less than about 4%. The spheroidal shape lifts the degeneracy on the magnetic quantum number. The energy levels can decrease or increase with eccentricity as a consequence of the different quantum confinement effects along the major and minor axes. The supplementary information on the magnetic quantum number is beneficial for optical applications.

Due to its wide band-gap (ca. 3.4 eV), ZnO is a possible candidate material to be used as transparent electrode for a new class of photovoltaic (PV) cells. Also, an increased interest for the photovoltaic properties of several organic monomers and polymers (merocyanines, phthalocyanines and porphyrins) was noticed, because of their high optical absorption in the visible region of the spectrum allowing them to be used as potential inexpensive materials for solar cells. Preparation and properties of CuPc (copper phthalocyanine) based photovoltaic cells using ZnO thin films as transparent conductor electrodes are presented in this paper. ZnO layers are grown by pulsed laser deposition, while the organic layers are obtained by thermal evaporation. Structural characterization is performed by electron microscopy. Optical and transport properties of the mutilayered structures are obtained by electrical and spectro-photometric measurements. The influence of the ZnO-polymer interface on the external quantum efficiency (EQE) of the photovoltaic cell is clearly evidenced by our measurements.

Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dots-QDs) doped with 0.2% mol Mn2+, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional Mn2+ ions are localized at Zn2+ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter vertical bar D vertical bar = 41 x 10(-4) cm(-1). The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional Mn2+ impurity ions localization model. Based on these results it is suggested that the high doping levels of Mn2+ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

The thermal spike model which takes into account both ionization and nuclear energy loss processes of the projectile as distinct electronic and atomic heat sources is used to describe transient processes induced by ions in silicon. The time and space dependencies of the lattice and electron temperatures near the projectile trajectory are calculated. The contribution of the rise in temperature on defect formation and annealing is considered.

A growing need for eco-friendly energy sources have led recently to a frantic search of new compositional systems for photovoltaic applications. The nanocomposite titania-germanium (TiO2-Ge) systems represent a new viable family of optoelectronic materials. Their structural, optical and electronic properties can be easily tailored by customizing the density and size of Ge dots in the TiO2 matrix. Early studies on TiO2-Ge nanocomposites have shown promises for their use as an alternative in photovoltaic applications. In this study we report the TiO2-Ge films synthesis by reactive magnetron co-sputtering. Their properties were evaluated by compositional (EDS), structural (XRD, FTIR) and optical (UV-Vis) characterizations.

Iron/iron oxide-based nanocomposites were prepared by IR laser sensitized pyrolysis of Fe(CO)(5) and methylmethacrylate (MMA) mixtures. The morphology of nanopowder analyzed by TEM indicated that mainly core-shell structures were obtained. X-ray diffraction techniques evidence the cores as formedmainly by iron/iron oxide crystalline phases. A partially degraded (carbonized) polymeric matrix is suggested for the coverage of the metallic particles. The nanocomposite structure at the variation of the laser density and of the MMA flow was studied. The new materials prepared as thick films were tested for their potential for acting as gas sensors. The temporal variation of the electrical resistance in presence of NO2, CO, and CO2, in dry and humid air was recorded. Preliminary results show that the samples obtained at higher laser power density exhibit rather high sensitivity towards NO2 detection and NO2 selectivity relatively to CO and CO2. An optimum working temperature of 200 degrees C was found.