National Institute Of Materials Physics - Romania

We study the dependence of photo-spectral intensity on tri- and multilayers of SiO2/[SiGe [ dSiGe]/SiO2]N with repetitions N = 1 to 10 and thicknesses dSiGe=5-100 nm. Photocurrent analysis reveals a bimodal spectral feature. A comparison of the photocurrent analysis between tri- and multilayers shows that in the multilayer structures, the photo-spectral intensity increases with increasing repetition N. The change in intensity could then be further tuned by changing the thickness of the SiGe layers dSiGe. We attribute the change in intensity to an increase in tensile strain, along with increased Ge atomic concentration and reduced SiGe-nano cluster size.

15 at.% Yb3+:YAG/YAG transparent composite ceramics with a coaxial geometry were synthesized by a ceramic forming method combined with the reactive sintering at 1800 degrees C. Densification peculiarities, microstructure, optical properties, and laser characteristics of composite ceramic samples were studied. Powder mixtures of Yb3+:YAG and YAG stoichiometric compositions demonstrate almost the same densification enabling uniform shrinkage of composite without differential sintering. It was shown that in-line optical transmittance of 15 at.% Yb3+:YAG/YAG composite ceramics reaches 80 % at 1030 nm wavelength. The effective diffusion coefficient of Yb3+ ions in garnet structure has been determined. Efficient laser emission was generated from Yb3+:YAG/YAG composite ceramics with a slope efficiency of eta(sa) = 0.30.

Blending carbon particles (CPs) and nanoscale bioactive cerium dioxide is a promising approach for designing composites for biomedical applications, combining the sorption and antioxidant potentials of each individual component. To address this issue, it is crucial to assess the correlation between the components' ratio, physicochemical parameters, and biofunctionality of the composites. Thus, the current research was aimed at fabricating C@CeO2 composites with different molar ratios and the examination of how the parameters of the composites affect their bioactivity. XRD, X-ray photoelectron spectroscopy, and electron microscopy data verified the formation of C@CeO2 composites. CeO2 nanoparticles (NPs) of 4-6 nm are highly dispersed on the surfaces of amorphous CPs. The presence of CeO2 NPs on the carbon surface decreased its adsorption potential in a dose-dependent manner. Besides, the coexistence of carbon and CeO2 in a single composite promotes some redox interactions between O-functionalities and Ce3+/Ce4+ species, resulting in changes in the chemical state of the surface of the composites. These observations suggest the strong connection between these parameters and the biofunctionality of the composites. The presence of CeO2 NPs on the surface of carbon led to a significant increase in the stability of the prepared composites in their aqueous suspensions. The enhancement of bioactivity of the newly prepared C@CeO2 compared to bare carbon and CeO2 was validated by testing their pseudomimetic (catalase/peroxidase-like and superoxide dismutase-like), antiamyloid, and radioprotective activities.

The ca. 480 Ma Albesti granite (Southern Carpathians, Romania) is characterized by the presence of color zoned blue quartz grains, and is part of the rather extensive European Cambro-Ordovician blue quartz landscape. The color is heat sensitive, fading at temperatures as low as 300degree celsius, inconsistent with the thermally stable, light scattering, nanometric rutile/ilmenite inclusions cited in literature. Extensive X- and Q-band electron paramagnetic resonance (EPR) investigations were carried out, searching for distinctive features of the Albesti quartz that are directly or indirectly involved in the generation of the blue coloration. The analyzed quartz grains were extracted from three granite samples of varying coloration and anisotropy, and the quartz from each rock sample was further separated into colored and colorless fractions. The paramagnetic E' and [AlO4]0 centers, as well as Mn2+ ions localized in traces of amorphous associated minerals at grain boundaries or fissure planes, were observed in all quartz samples. Broad EPR lines associated with the presence of magnetic clusters were observed in the spectra of the white quartz sample and the corresponding colorless one. Isochronal annealing up to 500degree celsius induced the correlated recombination of the E' and [AlO4]0 centers, the strong decrease of the Mn2+ spectrum and the formation of a minority iron oxide phase at the grain boundaries and/or fissure planes. The EPR signature was similar for the colored and the corresponding colorless quartz samples, before and after annealing, showing that the heat sensitive coloration of the Albesti quartz does not directly involve the presence of paramagnetic defects and/or minority magnetic phases.

Design of composite support materials based on Sn-doped TiO2 and carbon is one of the strategies to develop corrosion-resistant and CO-tolerant Pt electrocatalysts for polymer electrolyte membrane (PEM) fuel cells. As the synthesis methodology may have crucial influence on the structural and functional properties of the composites, different preparation routes for the novel support materials are explored and compared. Ti(1-x)SnxO2-C (x: 0.1-0.3) composites with different mixed oxide/carbon ratios were prepared by two sol-gel-based synthesis routes, namely (i) the introduction of a Sn precursor after the formation of the TiO2-rutile nuclei on the carbon backbone (route A), and (ii) simultaneous introduction of Ti and Sn precursors, resulting in good mixing of the Sn- and Ti-sol before the addition of the carbon (route B). The bulk and surface microstructure of the composites and the electrocatalysts obtained by their Pt-loading were investigated in detail. The incorporation of tin into the TiO2-rutile unit cell was confirmed by X-ray powder diffraction and Raman spectroscopy; the results indicated doping levels in good accordance with the amount of tin precursor. The advantages of composites and Pt electrocatalysts obtained via synthesis route B were that they do not contain segregated Sn-0 or SnO2 phases, have a more homogeneous/uniform mixed oxide distribution over the carbon backbone, and the electrochemically active surface area values (similar to 60-80 m(2)/g(Pt)) are twice as high as those of catalysts with similar compositions synthesized by method A. A common feature of the composites prepared by routes A and B was the presence of a tin oxide-rich overlayer identified by X-ray photoelectron spectroscopy. As a consequence, the electrocatalytic behavior of the catalysts was not influenced by the Ti/Sn ratio and was mainly dependent on the synthesis method used in the preparation of composite support materials. Elemental maps confirmed the formation of areas where Pt and the Sn doping element were in atomic proximity to each other, which means a favorable interaction either for the bifunctional mechanism or the electronic ligand effect. An increase in carbon content in composite materials led to an increase in both catalytic activity and long-term stability. The results of electrochemical studies showed that Sn-containing Pt catalysts with a high carbon content (75 wt%) are the most promising for potential use both as an anode and a cathode for PEM fuel cells.

This study investigates the influence of synthesis methods and electrode geometry on the physico-chemical properties of 5%Gd-doped SnO2. Two distinct synthesis routes, co-precipitation and hydrothermal growth, were employed, resulting in powders denoted as SnO2: Gd 5%-CP and SnO2: Gd 5%-HT. Morpho-structural and textural analyses reveal a uniform morphology consisting of quasi-spherical nanoparticles with dimensions of similar to 6 nm and mesoporosity for CP and a non-uniform morphology with larger nanoparticles of similar to 42 nm, with irregular shapes and macroporosity for the HT sample, respectively. The powders were deposited onto alumina substrates equipped with platinum interdigital electrodes with alternative gaps of 200 mu m and 100 mu m. The back-side heater allows for variation in the temperature of the layer. Sensing properties assessed under in-field-like atmospheres simulated by a computer-controlled Gas Mixing System reveal higher sensitivity to methane compared to carbon dioxide. Although the sensor signals did not differ quantitatively, they exhibited distinct saturation tendencies with an increasing methane concentration, attributed to the morpho-structure and porosity induced by the synthesis method. Differentiation was achieved by varying the interdigital gap of the electrodes, highlighting different sensor signals and conduction mechanisms, determined by the specific size of the crystallites.

SiGe-SiO2-based structures present high interest for their high photosensitivity from visible to short-wavelength infrared. Herein, two postdeposition annealing procedures, that is, rapid thermal annealing (RTA) and rapid-like furnace annealing (FA), are compared. Both RTA and FA are performed at 600 degrees C for 1 min for SiGe nanocrystals (NCs) formation in SiO2 matrix in Si/SiO2/SiGe/SiO2 structures deposited by magnetron sputtering. The FA imitates RTA resulting in enhanced spectral response. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy are carried out showing Ge-rich SiGe NCs with 11.3 +/- 1.2 nm size for RTA and 9.4 +/- 0.8 nm for FA. Photocurrent spectra for both structures show several peaks that are annealing dependent. The photocurrent intensity for FA samples is approximate to 7 times higher than RTA samples while cutoff wavelengths are slightly different, that is, 1365 nm for FA and 1375 nm for RTA. The FA structures show (at -1.5 V) over 4 A W-1 responsivity at 730 nm, 6.4 x 10(7) Jones detectivity at 735 nm, and 2.2 x 10(7) Jones at about 1210 nm. FA structures contain small SiGe NCs with incorporated residual strain, while RTA ones are formed of columnar SiGe NCs separated by SiGeOx amorphous regions and show increased tensile strain in the SiGe.

We explored the possibility to guide the forming process in a Ta/TiO2/Pt memristive device using an X-ray nanopatterning procedure, which enables the manipulation of the oxygen content at the nanoscale. The irradiation of selected areas of the sample by a 65 x 58 nm2 synchrotron X-ray nanobeam locally generated oxygen vacancies which resulted in the formation of a conductive filament in the desired position in the material. The subsequent application of an electric field between the electrodes was exploited to achieve reversible bipolar resistive switching. A multitechnique characterization was then performed, highlighting a local increase in the height of the crystal and the formation of a dislocation network, associated with the presence of Wadsley defects. Our results show that X-ray nanopatterning could open new avenues for a more deterministic implementation of electroforming in oxide-based memristive devices. We tuned the oxygen content in a Ta/TiO2/Pt memristive device at the nanoscale by a synchrotron X-ray nanobeam. We obtained a conductive filament of oxygen vacancies in the desired position in the material to achieve a controlled resistive switching.

Fabrication and extensive characterization of hard-soft nanocomposites composed of hard magnetic low-temperature phase LTP-MnBi and amorphous Fe70Si10B20 soft magnetic phase for bulk magnets are reported. Samples with compositions Mn55Bi45 + x center dot(Fe70Si10B20) (x = 0, 3, 5, 10, 20 wt.%) were prepared by spark plasma sintering of powder mixtures. Characterization has been performed by X-ray diffraction, scanning and transmission electron microscopy, magnetometry and Fe-57 Mossbauer spectroscopy. It was shown that samples contain crystallized and nanometric LTP-MnBi phases with various elemental compositions depending on the degree of Bi clustering. Complex correlations between starting compositions, processes during fabrication, and functional magnetic characteristics were observed. Unexpected special situations of the relation between microstructure and magnetic coupling mechanisms are discovered. Exchange spring effects of different strengths occur, being very sensitive to morpho-structural and compositional features, which in turn are controlled by processing conditions. An in-depth analysis of related microscopic characteristics is provided. Results of this work suggest that fabrication by powder metallurgy routes, such as spark plasma sintering of hard and soft magnetic powder mixtures, of MnBi-based composites with exchange spring phenomena have a high potential in designing and optimization of suitable materials with tunable magnetic properties towards rare-earth-free permanent magnet applications.

Tri-layered heterojunction devices based on oxide thin films are attracting significant attention for ultra-fast visible photodetection. However, the responsivity of these devices is still low. In this work, high performance photodetectors based on a tri-layered heterojunction of n-Si/p-Co3O4/n-ZnO were fabricated. Under no applied bias, a maximum responsivity and detectivity of 14.2 mA W-1 and 1.34 x 10(12) Jones were achieved respectively, for a power density of 9.35 mW cm(-2). Remarkably, a significant increase in the responsivity of approximately 864% was found when the device was biased at -2 V. This effect is understood based on the coupling of the photovoltaic and pyroelectric effects. Also, upon applying an external bias of -2 V, at a laser power density of 9.35 mW cm(-2) and at a chopper frequency of 10 Hz, the device exhibits a detectivity and sensitivity of 3.4 x 10(11) Jones and 2.2, respectively, together with a rise and fall time of 4 and 2 mu s, respectively. Compared to high performance Al/Si/SnO/ZnO/ITO and Au/pCuI/ZnO devices, our voltage-biased Al/Si/Co3O4/ZnO/ITO devices exhibit a >40% increase in R and >10x higher D*. Furthermore, an important advantage of our PDs is the p-type component, Co3O4, which is more stable and stoichiometric than CuI and SnO, ensuring PD performance that is stable with time. Therefore, n-Si/p-Co3O4/n-ZnO heterojunction devices shows great promise for ultrafast visible photodetection.