National Institute Of Materials Physics - Romania

A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L1(0) phase. A Co48Pt28Ag6B18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400 degrees C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473 degrees C promotes the formation of L1(0) phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670 degrees C (943 K) shows full formation of L1(0) CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L1(0) CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L1(0) CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L1(0) phase.

We have undertaken a temperature-dependent, investigation of the thermodynamics, structure, morphology and magnetism of two MnAl nanocomposite alloys (Mn60Al40 and Mn55Al45). Differential scanning calorimetry (DSC) studies allowed the determination of the exo-effects occurring in the structural phase transformation of MnAI, while by using temperature-dependent X-ray diffraction of synchrotron radiation we were able to monitor the phase transformation effects and the evolution with temperature of various structural phases occurring in the samples. We have shown that slight changes in stoichiometry (5 at.%) give rise to different phase structure in the as-cast state. While in Mn60Al40 only hcp epsilon phase can be found, in Mn55Al45 as-cast alloy, there is a quite complex phase structure with a mixture of gamma(2) (Al8Mn5) and epsilon as well as ferromagnetic MnAI T-phase. Activation energies of about 165 kJ/mol and 290 kJ/mol have been calculated from the Ozawa-Flynn-Wall analysis of DSC experimental data. These findings are in good agreement with the results obtained from XRD. For the Mn55Al45 as cast alloy, we have confirmed by temperature-dependent synchrotron XRD that the hcp e phase decomposes through the migration of interphase interfaces with the transformation rate controlled by boundary diffusion processes. High-resolution transmission electron microscopy have confirmed the phase structure obtained by XRD, while magnetic properties, obtained for the as-cast alloys are consistent with the multiphase character of the samples and in good agreement with previously reported results. The magnetization does not saturate for the maximum applied field of about 4 x 10(6) A/m and a marked coercivity of about 160 kA/m is obtained for the Mn55Al45 as-cast alloy.

The K beta x-ray spectra of the elements from Ca to Ge have been systematically investigated using a high resolution antiparallel double-crystal x-ray spectrometer. Each K beta(1,3) natural linewidth has been corrected using the instrumental function of this type of x-ray spectrometer, and the spin doublet energies have been obtained from the peak position values in K beta(1,3) x-ray spectra. For all studied elements the corrected K beta(1) x-ray lines FWHM increase linearly as a function of Z. However, for K beta(3) x-ray lines this dependence is generally not linear in the case of 3d elements but increases from Sc to Co elements. It has been found that the contributions of satellite lines are considered to be [KM] shake processes. Our theoretically predicted synthetic spectra of Ca, Mn, Cu, and Zn are in very good agreement with our high-resolution measurements, except in the case of Mn, due to the open-shell valence configuration effect (more than 7000 transitions for diagram lines and more than 100 000 transitions for satellite lines) and the influence of the complicated structure of the metallic Mn.

In the present work, we study the hysteretic behavior in the electric-field-dependent capacitance and the current characteristics of 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 (BCZT)/ZnO bilayers deposited on 0.7 wt % Nb-doped (001)-SrTiO3 (Nb:STO) substrates in a metal-ferroelectric-semiconductor (MFS) configuration. The X-ray diffraction measurements show that the BCZT and ZnO layers are highly oriented along the c-axis and have a single perovskite and wurtzite phases, respectively, whereas high-resolution transmission electron microscopy revealed very sharp Nb:STO/ BCZT/ZnO interfaces. The capacitance-electric field (C-E) characteristics of the bilayers exhibit a memory window of 47 kV/cm and a capacitance decrease of 22%, at a negative bias. The later result is explained by the formation of a depletion region in the ZnO layer. Moreover, an unusual resistive switching (RS) behavior is observed in the BCZT films, where the RS ratio can be 500 times enhanced in the BCZT/ZnO bilayers. The RS enhancement can be understood by the barrier potential profile modulation at the depletion region, in the BCZT/ZnO junction, via ferroelectric polarization switching of the BCZT layer. This work builds a bridge between the hysteretic behavior observed either in the C-E and current-electric field characteristics on a MFS structure.

Enhancing the long term stable performance of silicon detectors used for monitoring the position and flux of the particle beams in high energy physics experiments requires a better knowledge of the nature, stability, and transformation properties of the radiation defects created over the operation time. We report the results of an electron spin resonance investigation in the nature, transformation, and long term stability of the irradiation paramagnetic point defects (IPPDs) produced by high fluence (2 x 10(16) cm(-2) ), high energy (27 MeV) electrons in n-type, P-doped standard floating zone silicon. We found out that both freshly irradiated and aged (i.e., stored after irradiation for 3.5 years at 250 K) samples mainly contain negatively charged tetravacancy and pentavacancy defects in the first case and tetravacancy defects in the second one. The fact that such small cluster vacancy defects have not been observed by irradiation with low energy (below 5 MeV) electrons, but were abundantly produced by irradiation with neutrons, strongly suggests the presence of the same mechanism of direct formation of small vacancy clusters by irradiation with neutrons and high energy, high fluence electrons, in agreement with theoretical predictions. Differences in the nature and annealing properties of the IPPDs observed between the 27 MeV electrons freshly irradiated, and irradiated and aged samples were attributed to the presence of a high concentration of divacancies in the freshly irradiated samples, defects which transform during storage at 250 K through diffusion and recombination processes. Published by AIP Publishing.

Controlling the semiconductor-to-metal transition temperature in epitaxial VO2 thin films remains an unresolved question both at the fundamental as well as the application level. Within the scope of this work, the effects of growth temperature on the structure, chemical composition, interface coherency and electrical characteristics of rutile VO2 epitaxial thin films grown on TiO2 substrates are investigated. It is hereby deduced that the transition temperature is lower than the bulk value of 340 K. However, it is found to approach this value as a function of increased growth temperature even though it is accompanied by a contraction along the V4+-V4+ bond direction, the crystallographic c-axis lattice parameter. Additionally, it is demonstrated that films grown at low substrate temperatures exhibit a relaxed state and a strongly reduced transition temperature. It is suggested that, besides thermal and epitaxial strain, growth-induced defects may strongly affect the electronic phase transition. The results of this work reveal the difficulty in extracting the intrinsic material response to strain, when the exact contribution of all strain sources cannot be effectively determined. The findings also bear implications on the limitations in obtaining the recently predicted novel semi-Dirac point phase in VO2/TiO2 multilayer structures.

This paper reports the sensing mechanism of SnO2-CuWO4 mixed oxides towards H2S detection revealed through Pulsed Temperature Mode (PTM). After thick film deposition onto commercial substrates, the final annealing temperature enhances the material reliability through the changes occurred during the experimental conditions. PTM approach represents a tradeoff in terms of sensing mechanism versus sensing performance, in order to get insights about the fundamental aspects. The measurements have been done under dynamic gas flow conditions, in the presence of 50% relative humidity (RH). The gas sensors based on SnO2-CuWO4 exhibit sensor signals higher than 100 to 10 ppm H2S, low cross-sensitivity to common gaseous interfering agents (CO, CH4, NH3, SO2), average selective sensitivity of 8.64 towards the main interfering NO2 gas, moderate response (4 min)/recovery (5 min) transients and power consumption. In order to emphasize the role of moisture towards H2S detection mechanism, the photoacoustic spectroscopic insights were correlated with Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy results. Such approach has shown promising aspects towards further operational sensitive devices. (c) 2017 Elsevier B.V. All rights reserved.

This work is focused on a novel class of hybrid materials exhibiting enhanced optical properties and high surface areas that combine the morphology offered by the vinyl substituted silica host, and the excellent absorption and emission properties of 5,10,15,20-tetrakis(N-methyl-4-pyridyl) porphyrin-Zn(II) tetrachloride as a water soluble guest molecule. In order to optimize the synthesis procedure and the performance of the immobilized porphyrin, silica precursor mixtures of different compositions were used. To achieve the requirements regarding the hydrophobicity and the porous structure of the gels for the successful incorporation of porphyrin, the content of vinyltriacetoxysilane was systematically changed and thoroughly investigated. Substitution of the silica gels with organic groups is a viable way to provide new properties to the support. An exhaustive characterization of the synthesized silica samples was realised by complementary physicochemical methods, such as infrared spectroscopy (FT-IR), absorption spectroscopy (UV-Vis) and photoluminescence, nuclear magnetic resonance spectroscopy (Si-29-MAS-NMR) transmission and scanning electron microscopy (TEM and SEM), nitrogen absorption (BET), contact angle (CA), small angle X ray and neutron scattering (SAXS and SANS). All hybrids showed an increase in emission intensity in the wide region from 575 to 725 nm (Q bands) in comparison with bare porphyrin. By simply tuning the vinyltriacetoxysilane content, the hydrophilic/hydrophobic profile of the hybrid materials was changed, while maintaining a high surface area. Good control of hydrophobicity is important to enhance properties such as dispersion, stability behaviour, and resistance to water, in order to achieve highly dispersible systems in water for biomedical applications.

Gold nanoparticles were decorated on/into the SBA-15 channels covalently functionalized by new class of symmetrical tridentate NNN-pincer ligand. This provides a unique support for stabilization of Au NPs preventing agglomeration due to the strong ligation of pincer ligand. The catalyst proved to be highly active for the reduction of nitroarenes. The kinetics study showed that the rate constant for the current protocol are comparable or larger than the reported ones in literature. The catalyst also showed high recoverability and reusability up to ten times without significant loss of activity and leaching or aggregation of Au NPs (overall TOF = 8798).

Si and Ge nanocrystals in oxides are of a large interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in nanocrystals. In this work, dense Ge nanocrystals suitable for enhanced photoconduction were fabricated from 60% Ge in TiO2 amorphous layers by low temperature rapid thermal annealing at 550 degrees C. An exponential increase of the photocurrent with the applied voltage was observed in coplanar structure of Ge nanocrystals composite films deposited on oxidized Si wafers. The behaviour was explained by field effect control of the Fermi level at the Ge nanocrystals-TiO2 layer/substrate interfaces. The blue-shift of the absorption gap from bulk Ge value to 1.14 eV was evidenced in both photocurrent spectra and optical reflection-transmission experiments, in good agreement with quantum confinement induced bandgap broadening in Ge nanocrystal with sizes of about 5 nm as found from HRTEM and XRD investigations. A nonmonotonic spectral dependence of the refractive index is associated to the Ge nanocrystals formation. The nanocrystal morphology is also in good agreement with the Coulomb gap hopping mechanism of T-1/2 -type explaining the temperature dependence of the dark conduction.