National Institute Of Materials Physics - Romania

Here we report a ferroelectric capacitor structure obtained by alternating ferroelectric and insulator thin-film layers which allows an increase of up to 2(n) polarization states, with n the number of ferroelectric layers. Four and up to eight distinct, stable and independently addressed polarization states are experimentally demonstrated in this work. The experimental findings are supported by a theoretical model based on the Landau-Ginzburg-Devonshire theory. The key parameter is the change in the strain conditions of ferroelectric layers induced by the insulating separator. Notably, the 2(n) increase in the storage capacity can be achieved without major changes in the present technology used for FeRAM devices. The test structures demonstrate very good memory characteristics such as retention and fatigue, opening the way towards the design of high density ferroelectric memories.

Thin films of chalcogenide, Cu(In, Ga) Se-2 have been obtained using a single quaternary target by radio frequency magnetron sputtering method, with thickness in the range 750 nm to 1200 nm. X-ray photoelectron spectroscopy investigations showed, that the composition of Cu(In, Ga) Se-2 thin films was very similar to that of the used target CuIn0.75Ga0.25Se2. Identification of the chemical composition of Cu(In, Ga) Se-2 thin films by XPS performed in high vacuum, emphasized that the samples exhibit surface features suitable to be integrated into the structure of solar cells. Atomic Force Microscopy and Scanning Electron Microscopy investigations showed that surface morphology was influenced by the increase in thickness of the Cu(In, Ga) Se-2 layer. From X-Ray Diffraction investigations it was found that all films were polycrystalline, having a tetragonal lattice with a preferential orientation along the (112) direction. The optical reflectance as a function of wavelength was measured for the studied samples. The increase in thickness of the Cu(In, Ga) Se-2 absorber determined a decrease of its optical bandgap value from 1.53 eV to 1.44 eV. The results presented in this paper showed an excellent alternative of obtaining Cu(In, Ga) Se-2 compound thin films from a single target. (C) 2016 Elsevier B.V. All rights reserved.

Using a fast and eco-friendly deposition method, ITO thin films with different thicknesses (0.5 mu m-0.7 mu m) were deposited on glass substrates by radio frequency magnetron sputtering technique. A comparative analysis of these oxide films was then carried out. AFM investigations showed that the deposited films were smooth, uniform and having a surface roughness smaller than 10 nm. X-ray diffraction investigations showed that all samples were polycrystalline and the grain sizes of the films, corresponding to (222) cubic reflection, were found to increase with the increasing film thickness. The optical properties, evaluated by UV-VIS-NIR (190-3000 nm) spectrophotometer, evidenced that the obtained thin films were highly transparent, with a transmission coefficient between 90 and 96%, depending on the film thickness. Various methods (Swanepoel and Drude) were employed to appreciate the optimal behaviour of transparent oxide films, in determining the dielectric optical parameters and refractive index dispersion for ITO films exhibiting interference patterns in the optical transmission spectra. The electrical conductivity also increased as the film thickness increased. (C) 2017 Elsevier B.V. All rights reserved.

Green compacts of B4C or B4C added with 1 vol% graphite were infiltrated with molten Si and subsequently were subject of processing by floating zone partial re-melting (FZPR). In FZPR only the low temperature fusible component, in this case Si, is melted. A fully dense B4C-based ceramic is obtained. It contains free-Si, SiC and B4C. In the center of the FZPR ceramic without graphite addition, the amount of Si is decreased when compared to the infiltrated material. Some impurity elements such as Al, Fe, or Ti detected in the raw B4C powder are preferentially gathered at the edges of the sample. In the sample added with graphite, formation of a high amount of SiC in the infiltrated material hinders Si shift from the center to the edges. The pulling rate and the particle size of the B4C raw powders are also important. It is recognized that sintering of powders larger than 10-20 gm is usually difficult: our approach is demonstrated to be suitable for processing of B4C powders with a very different particle size, from 10 to 250 gm. The FZPR ceramic had a Vickers hardness of 9-38 GPa depending on location of the indentation imprint and on the sample. A tensile strength of 114-188 MPa that is up to about 2-3 times higher than for the infiltrated material was recorded. Work indicates that the proposed processing approach offers extended control possibilities towards fabrication of new composite materials not available by traditional technologies.

Magnetic nanoparticles embedded in the active layer of the Organic Light Emitting Diodes (OLEDs) significantly increases the electroluminescence and the charge transport without influencing the transparency of these devices. A brief comparison was done in order to identify which parameter influences these properties, by comparing the CoFe2O4 magnetic nanoparticles with CoFe2 metallic magnetic nanoparticles, the latter one being obtained by thermal reduction in hydrogen of cobalt ferrite nanoparticles. CoFe2 have shown a better efficiency of the metallic nanoparticles where probably the main advantage is the higher magnetization property instead of the coercive field. Concerning the charge transport across the OLEDs, these nanoparticles reduce the electron injection, acting as filling traps, which directly increases the electroluminescence and the current at the same voltage.

We have developed a thin film structure with a maximum magnetoresistance effect (MRE) at room temperature, which is one of the operating requirements for many applications. It is shown that La0.67Ba0.33Ti0.02Mn0.98O3 epilayers obtained by pulsed laser deposition onto (001) SrTiO3 single crystal substrates exhibit the highest MRE, Delta R/R(H) approximate to 150% or Delta R/R(0) approximate to 60% under 5 T, at 300 K, a temperature near to the corresponding Curie temperature (T-C). Both doping with a tiny amount of titanium and induced stress due to lattice mismatch between the thin film and the substrate contribute to a decrease in T-C as compared to the pristine compound and therefore to the decrease in the temperature where the highest MRE is recorded. Published by AIP Publishing.

Nanocomposites have been obtained by dispersing various amounts of vapor grown carbon nanofibers within isotactic polypropylene. Thermal investigations done by differential scanning calorimetry and dynamic mechanical analysis revealed the effect of the vapor grown carbon nanofibers on the melting, crystallization, alpha, and beta relaxations. Direct current electrical features of these nanocomposites have been investigated and related to the thermal features of these nanocomposites. The effect of the loading with carbon nanofibers on the electrical properties of these nanocomposites is discussed within the percolation theory. The percolation threshold was estimated at about 5.5% wt carbon nanofibers. The temperature dependence of the direct current conductivity is analyzed in detail and it is concluded that the electronic hopping is the dominant transport mechanism. A transition from one-dimensional hopping towards a three-dimensional hopping was noticed as the concentration of carbon nanofibers was increased from 10% wt to 20% wt carbon nano-fiber. The possibility of a differential negative resistivity is suggested. (C) 2017 Wiley Periodicals, Inc.

A novel nanocomposite was obtained through the covalent immobilization of lipoic acid on the surface of silver nanoparticles-decorated silica nanoparticles (SiO2@Ag). The hybrid organic inorganic material obtained was characterized by Fourier transform infrared spectroscopy, thermal analysis, scanning and transmision electron microscopy, X-ray photoelectron spectroscopy and UV-Visible spectroscopy. Its antioxidant, cytotoxic, antimicrobial activity and influence on mammalian cells cycle were evaluated. The results of this study have shown that the functionalization of SiO2@Ag with lipoic acid resulted in a composite with increased specificity of interaction with different mammalian cell lines and antioxidant activity, but with decreased cytotoxicity and antimicrobial properties. Therefore, the SiO2@Ag functionalized with lipoic acid could be successfully used in certain concentrations to modulate the cell cycle, in order to obtain the desired anti-proliferative or stimulatory therapeutic effect. (C) 2017 Elsevier B.V. All rights reserved.

Lowering the temperature of crystallization by deposition of thin films on a heated substrate represents the easiest way to find new means to develop and improve new working devices based on nanocrystals embedded in thin films. The improvements are strongly related with the increasing of operation speed, substantially decreasing the energy consumption and reducing unit fabrication costs of the respective semiconductor devices. This approach avoids major problems, such as those related to diffusion or difficulties in controlling nanocrystallites size, which appear during thermal treatments at high temperatures after deposition. This article reports on a significant progress given by structuring Ge nanocrystals (Ge-NCs) embedded in silicon dioxide (SiO2) thin films by heating the substrate at 400 degrees C during co-deposition of Ge and SiO2 by magnetron sputtering. As a proof-of-concept, a Si/Ge-NCs:SiO2 photo-sensitive structure was fabricated thereof and characterized. The structure shows superior performance on broad operation bandwidth from visible to near-infrared, as strong rectification properties in dark, significant current rise in the inversion mode when illuminated, high responsivity, high photo-detectivity of 10(14) Jones, quick response and significant conversion efficiency with peak value reaching 850% at -1 V and about 1000 nm. This simple preparation approach brings an important contribution to the effort of structuring Ge nanocrystallites in SiO2 thin films at a lower temperature for the purpose of using these materials for devices in optoelectronics, solar cells and electronics on flexible substrates.