National Institute Of Materials Physics - Romania

Abstract: The influence of temperature factor to crystal structure as well as magnetic and electric properties of strontium hexaferrite partially substituted with diamagnetic indium ions has been investigated. Ferroelectric properties have been found out in SrFe11.9In0.1O19 compound that contradicts to conventional opinion, which describe its crystal structure within the framework of centrosymmetric space group P6(3)/mmc (No. 194). For determination features of the crystal structure, which are responsible for ferroelectric properties of strontium hexaferrite, have been carried out neutron diffraction measurements with high resolution in temperature range from 1.5 to 740 K. The analysis of hexaferrite structure has been executed within the framework both centrosymmetric and non-centrosymmetric space group. Values of coefficients of magneto crystalline anisotropy at 5 and 300 K and influence of ambient temperature to linear size of magnetic regions have been determined from magnetic measurements. (C) 2019 Elsevier B.V. All rights reserved.

Abstract: In view of the HL-LHC upgrade, radiation-tolerant silicon sensors containing low-resistivity p-type implants or substrates, like LGAD or HV-CMOS devices, are being developed in the framework of ATLAS, CMS, RD50 and other sensor R&D projects. These devices are facing a particular problem - the apparent deactivation of the doping due to the irradiation, the so-called acceptor removal effect. In the present work proton and neutron fluence-dependent radiation damage effects, including the change in leakage current and effective doping concentration, space charge sign-inversion, but also introduction and annealing of point- and cluster-like defects have been studied in Si pad diodes fabricated from p-type EPI material of different resistivity (10-1000 Omega cm). Standard electrical characterisations (IV, CV), TCT (Transient Current Technique) and TSC (Thermally Stimulated Current) techniques were applied. A correlation between effective doping concentration obtained from CV measurements and defect concentration N-t extracted from TSC measurements for both - neutron and proton - irradiations was observed pointing towards the microscopic origin of the acceptor removal. A detailed analysis of the dominant TSC peaks - E(30), BiOi and three main deep acceptor levels H(116), H(140) and H(152) - responsible for the changes in the effective space charge is performed. The origin and annealing behaviour of E(30) and H(40) and other cluster-related defects are discussed as well.

Abstract: A one-step method assisted by hydrothermal treatment was approached to obtain nanocrystalline 1 and 10 mol. % In doped 2 mol.% Pd-SnO2 powders using a non-ionic surfactant Brij52 and Polyethylene glycol 6000 (PEG) as templates. Depending on In content, the samples were labeled as Pd1InSn and Pd10InSn. The obtained materials consist of nanosized crystallites packed into micrometric grains with a high porosity, as revealed by the morphological and structural investigations (SEM, TEM). A dependence of the grain size with respect to the In content has been revealed i.e. the sample Pd1InSn was showing an average grain size of around 10 nm, whilst for the sample Pd10InSn the average grain size was found to be around 5 nm. The XPS investigations highlighted the differences occurred in the surface chemistry in terms of surface hydroxylation as well as the chemical states of Pd. The sensing properties towards different CO concentrations have been examined under infield background conditions, at low operating temperature of 50 degrees C. The sensing mechanism model for CO was discussed in detail according to the possible interplay between oxygen and water related species based on the experimentally results acquired through simultaneous electrical resistance and work function measurements.

Abstract: Ba0.6Sr0.4TiO3 (BST) ferroelectric thick films were grown on MgO(001) and Al2O3 (0 0 0 1) single-crystal substrates by using a pulsed laser deposition method. Structural, morphological, optical, and terahertz characterization of the BST films were performed by X-ray Diffraction, Atomic Force Microscopy, Spectroscopic Ellipsometry (SE), and Terahertz Time-Domain Spectroscopy (THz-TDS). Single-phase samples with strong preferred (1 1 1) orientation and surface roughness lower than 1.5% of their thicknesses have been obtained for both types of substrates. SE was employed to extract the thickness and optical properties by using a 3-layer optical model (substrate/thin film/roughness). The inferred refractive index @630 nm is around 2.05, while the optical interference is visible until 3.3 eV. The THz-TDS measurements in transmission set-up were carried out one after the other on substrates before and after the BST film deposition. The standard THz-TDS analysis of double-layer samples proved difficult to complete in the cases in which a thin or thick film is deposited on a much thicker substrate of known dielectric properties. However, we have been able to extract the complex dielectric permittivity in the THz domain for BST samples with thicknesses of few microns, by developing a specific procedure of data analysis.

Abstract: A complex investigation of the morpho-structural, magnetic, magneto-optical and magneto-transport properties of amorphous Fe-Gd thin films crossing the magnetization compensation point is reported and the unexpected observed magneto-functionalities are discussed. A tendency of magnetic domain formation with increasing the Fe content over the compensation concentration is observed. The switch from a reversed Magneto-optical Kerr Effect loop to a direct loop when increasing the Fe content over the compensation point is explained via the specific contribution to the rotation of the polarization vector from each magnetic sublattice, belonging to Fe and Gd, respectively. Local atomic configurations and magnetic interactions ascertained the amorphous character and revealed an out-ofplane orientation of the magnetic moment of Fe above the compensation point. The thermomagnetic curves prove a concentration dependent behavior, explained by weakly coupled magnetization relaxation processes of the two magnetic sub-lattices. On the other hand, the magnetic hysteresis loops gave evidence of two exchange coupled magnetic phases with different coercive fields. According to structural and Fe-57 Mossbauer Spectroscopy results, the two phases correspond to definite nanosized volumes of two different average concentrations (one of them closer to the compensation point) which are randomly distributed in the film. The unexpected single step-like behavior of the magneto-resistivity curves was explained by dissimilar switching of the spins in these two magnetic phases distributed in nano-sized volumes.

Abstract: Single-phase Ba1-xSrxTiO3 (BST) perovskite ceramics with 0.3 <= x <= 0.4 were prepared from powders synthesized via sol-gel route. The compositions have the ferroelectric-paraelectric phase transition close to room temperature. At 20 degrees C the BST ceramics are ferroelectric for 0.3 <= x <= 0.35 and paraelectric for x = 0.375 and x = 0.40. The study follows the relation between the structural changes produced when increasing the Sr content and the dielectric properties at low intensity electric fields. It is found that the grain size and tetragonality decreases as the Sr content increases. Analyses of complex permittivity and impedance spectroscopy reveal the temperature and frequency dependencies of the dielectric properties. The phase transitions seem to be of first order for all compositions, with a thermal hysteresis that decreases with increasing the Sr content, fact attributed to the corresponding increase of the grain boundaries weight allowing a more efficient stress relaxation in the structure during the change of the symmetry from cubic to tetragonal. The diffusiveness degree during the phase transition is increasing with Sr content, suggesting some relaxor-type contribution attributed to smaller grain size. The ac conductivity follows the universal Jonscher law, with an ac component having the power parameter s independent of Sr content, and a dc component that it is thermally activated with an activation energy of about 0.7-0.77 eV attributed to oxygen vacancies acting as donor-like defects. The fit of impedance spectra at different temperatures and frequencies is obtained by using an equivalent circuit accounting the grains, grain boundaries, electrode interfaces and the local contributions produced by reorientation of defect dipoles or defect clusters. All the component circuits have significant variations around phase transitions. These are discussed in relation to structural changes occurring during transition and considering the changes in the distribution of various charges when polarization vanishes.

Abstract: In the present work, a magnetodielectric flexible thick film composite with a quaternary Ba12Fe28Ti15O84 ferrite filler (<= 9 vol%) embedded into PVDF polymer matrix was investigated. The role of filler volume on the macroscopic dielectric, magnetic and ferroelectric properties as well as on the nanoscale magnetic and piezoelectric response was analyzed. The formation of small amounts of polar phases besides the majority alpha-phase of PVDF was shown by XRD and FTIR combined analysis. The electrical properties are dominated by the polymer response, while magnetic order is derived as sum property from the ferrite ones, having a predominant soft magnetic character with small coercivity of H-c similar to 60 Oe and high saturation magnetization of M-s = 2.6 emu/g for the highest concentration of 9 vol%. Permittivity and losses slightly increase with ferrite filler addition, and the composite maintains a dielectric character for all the compositions. Local PFM and MFM investigations have shown a combined ferro/piezoelectric character and magnetic order, with magnetoelectric coupling demonstrated by the reorientation of filler particles and modifications of local piezoresponse when applying a static magnetic field.

Abstract: The time evolution of a quantum dot exciton in Coulomb interaction with wetting layer carriers is treated using an approach similar to the independent boson model. The role of the polaronic unitary transform is played by the scattering matrix, for which a diagrammatic, linked cluster expansion is available Similarities and differences to the independent boson model are discussed. A numerical example is presented.

Abstract: Hybrid organic-inorganic thin films based on zinc phthalocyanine (ZnPc) and ZnO nanoparticles were deposited by Matrix Assisted Pulsed Laser Evaporation (MAPLE). Synthesized by a simple wet chemical precipitation method, the ZnO nanoparticles were featured by a hexagonal wurtzite structure, a band-gap value of similar to 3.3 eV and emission bands typical for this semiconductor. The hybrid films containing ZnPc and various amounts of ZnO nanoparticles were evaluated from morphological, compositional, structural, optical and electrical point of view. No chemical decomposition of the organic compound was observed in the FTIR spectra of the deposited layers. The transmittance and photoluminescence spectra recorded on hybrid films disclose the optical signature of both organic (ZnPc) and inorganic (ZnO) components. The electrical measurements carried out under illumination emphasized the importance of the quantity of the inorganic component on the performance parameters of the structures prepared with the hybrid films. Our study provides new insight in the MAPLE deposition of the organic-inorganic hybrid films with potential applications in the photovoltaic cells area.

Abstract: Aminopropyl-silica nanoparticles S0 were obtained and further functionalized with eight halogen-reactive organic compounds 1-8, leading to the S1-S8 hybrid organic-inorganic materials, with spherical morphology and having a 50-200 nm size, as showed TEM analysis. These were further characterized by IR and DLS. Biological assays were performed using five microbial strains, belonging to species frequently encountered in the aetiology of human infections (Enterococcus faecalis, Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia toll and Candida albicans). The obtained materials exhibited promising antimicrobial properties, being active against planktonic, but particularly on biofilm-embedded cells. Among the tested hybrid nanomaterials, S6 seemed to exhibit the most promising antimicrobial profile.