National Institute Of Materials Physics - Romania

Thin films and crystalline powder of PbI2 intercalated with ammonia or pyridine have been studied by optical absorption, Raman scattering, photoluminescence, FTIR spectroscopy and thermo-gravimetric analysis. Ammonia intercalated PbI2 shows an increased optical band gap, of about 0.6 eV, with an intense broad emission band peaking at about 2.3 eV as signature in the photoluminescence spectrum. The intercalation of PbI2 with ammonia is noticed in the Raman spectrum by the appearance of new lines situated in low frequency range. The D-3d coordination geometry of Pb2+ in the PbI2 crystal is reduced by compression to orthorhombic one, the distribution of the electronic states in valence band is also changed. Thus, the top of the valence band undergo a deformation inducing a weakness of the interaction between the lead and iodine ions within a layer. Also the PbI2 intercalated with pyridine is featured by optical and vibration properties different that of pure crystalline powder. Finally, the intercalation of PbI2 with different molecules changes the basic semiconducting properties of the crystal. Thermal analyses and infrared absorption spectra have been used to study the desorption of guests species from PbI2 in order to evaluate the stability range and morphological changes with temperature.

Fe(III)-doped TiO2 aerogels are prepared by acid catalyzed sol-gel method followed by supercritical drying, and then heat treatment. Raman spectra together with X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns of the iron-doped TiO2 aerogel samples revealed the existence of both anatase and brookite crystalline phases. It was found that the brookite phase formation is favored by the increase of the iron content in the dried samples. XRD measurements show that the lattice constant c of anatase phase decreases with the dopant addition, while the value of a remains essentially unchanged. The microstructure of the investigated samples is relatively compact with small mesopores as revealed from transmission electron microscopy (TEM). The most enhanced photocatalytic activity was exhibited by the TiO2 aerogel sample with 1.8 at.% Fe(III) whose apparent rate constant of the salicylic acid photodegradation was found to be of almost six times higher than that of Degussa P25.

Commercial wires of Nb3Sn and NIgB2 were inspected through X-ray cone-beam micro-tomography (mu CT). Details of the architecture of the wires of different topologies were visualized. In the case of MgB2 wires this allowed to compare the geometrical perfection of the component elements between the wires with 18 and 7 superconducting sub-elements heat treated for different conditions. Defects such as 3D voids and interruption of the diffusion barriers were also visualized. High resolution 3D-mu CT observations of the Nb3Sn wire architecture allowed also direct and non-invasive determination of the twist-pitch parameter. 3D tomography shows important advantages over 2D traditional microscopy methods.

Imaging experiments at the XFEL pose unprecedented requirements to the detectors in terms of radiation tolerance: Fluxes of up to 10(16) (12 keVphotons/cm(2)) corresponding to approximately 10(9) Gy in silicon, are expected. An irradiation station has been set up in the DORIS beam line F4, MOS test structures have been irradiated, and first results on the dose dependence of the CN-characteristics, surface current density, and interface trap density have been obtained.

This paper presents the investigation of electrical properties of carbon nanotubes (CNT). A sandwich configuration, quartz substrate/Cr/Al/CNT (partially immersed in SiN)/Cr/Al was investigated. The CNT are mainly oriented parallel with the electrodes. Current - voltage characteristics were taken at 20 K and room temperature and a current - temperature characteristic was taken at constant voltage (20 mV). The I - V characteristics are almost linear, while the G - V characteristics present some peaks and dips, interpreted as voltage percolation thresholds. The I - T and R - T characteristics are also linear (except at low temperatures). The investigated structures have a high electrical polarizability.

In this paper a novel configuration of microwave planar filters, with multiple cross-couplings and with a number of attenuation poles equal to the order of the filter, is investigated. The location of the poles on the frequency axis can be controlled, allowing the design of band-pass filters with improved selectivity with respect to the adjacent channels. A microstrip bandpass filter with the novel configuration was designed, verified by em-field simulation, fabricated and tested. The response of the designed filter are in good agreement with the specification, confirming the possibilities of realizing microwave band-pass filters of a relatively low order with moderate losses and with improved selectivity performances.

This article presents a versatile gas-flow proportional counter for surface and transmission Mossbauer spectroscopy, suitable for studies with Fe-57, Sn-119 and Eu-151 isotopes. The main advantages obtained by new design are: (1) the height of the detection volume can be changed in large limits from 0 to 38 mm, (2) the detection volume can be choose symmetrical or not in respect with anode plan, (3) the anode replacement is easily (4) and different anode configuration can be used. The characteristics of the detector, operating at room temperature, are reported.

We report the synthesis, by a surfactant-assisted liquid-liquid reaction, of nanocrystalline ZnS doped with 0.2 mol% Mn2+ ions self-assembled in a mesoporous structure. The XRD measurements demonstrate the formation of a sponge-like mesoporous material with a tight distribution of pores of 1.8 nm mean diameter built from cubic ZnS nanocrystals of 1.8 nm average size. TEM investigation confirms the formation of the mesoporous structure with walls of 3.1 nm mean thickness built from nanocrystallites of cubic ZnS. The ordering effect of self-assembling, which is reflected in the tight size distribution of crystallites and pores, might be also responsible for the well resolved EPR spectra, attributed to the presence of three types of isolated Mn2+ paramagnetic centers. (C) 2008 Elsevier Ltd. All rights reserved.

Fe / Fe3O4 (magnetite) powders obtained by ball milling at room temperature, undergo an incomplete redox reaction with formation of FeO. This reaction is favoured due to the high energy developed during the milling and alloying. Concurrent effects of the milling, such as grain refinement down to the nanometric scale lead at the end of the milling processes to a mixed multiphased nanopowder, with a homogeneous dispersion of Fe and Fe oxide grains. Such ferromagnetic - antiferromagnetic systems are extensively studied due to their exchange bias properties, extremely useful in technological applications. We study the phase transformation that leads to a multiphased metal / oxide microstructure with an energy-dispersive in-situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray spectra after few minutes exposure, at selected temperatures, ranging between 20 degrees C and 1000 degrees C. Magnetic behavior has been studied for as-milled and annealed samples and the obtained magnetic parameters are correlated to the microstructure and phase composition at each stage of annealing. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanogranular powder.

Exchange bias and spin valve structures with Fe as ferromagnetic layers selectively enriched in Fe-57 and Ir-Mn of different compositions as antiferromagnetic pinning layers have been prepared by r.f. sputtering. Conversion Electron Mossbauer Spectroscopy and Magneto Optic Kerr Effect have shown that the local structure and interactions in the whole ferromagnetic layers, with direct influence on their magnetic reversal processes, depend on growing order of the layers and composition of the AF layer. Interfacial atomic diffusion is the main reason of perturbing the local structure of the ferromagnet. Mossbauer Spectroscopy is a powerfool tool which allows a detailed analysis of local configurations, in order to optimize the magnetic and giant magnetoresistive parameters of such multilayers of important technological applications.