National Institute Of Materials Physics - Romania

Photoluminescence properties of the glass-ceramics microrods containing Eu3+-doped LiYF4 nanocrystals have been studied and characterized. Judd-Ofelt parameters and quantum efficiency has been computed from luminescence spectra and discussed by comparison to the glass ceramic bulk and pellet. The radiative decay rate A(rad) is higher in the glass ceramic rods (221 s(-1)) than in the glass ceramic bulk (130 s(-1)) but the quantum efficiency computed is very low (21%) compared to the glass-ceramic bulk (97%). There are effective non-radiative decay channels that might be related to an influence of the dimensional constraints imposed by the membrane pores during xerogel formation and subsequent glass ceramization. (C) 2015 Elsevier B.V. All rights reserved.

Hetero-epitaxial TiC thin films were deposited at 100 degrees C on MgO(001) by DC reactive magnetron sputtering in a mixture of Ar and CH4. The 62 nm thick films were analyzed for elemental composition and chemical bonding by Auger electron spectroscopy, X-ray photoelectron spectroscopy and micro-Raman spectroscopy. The crystallographic structure investigation by high resolution X-ray diffraction revealed that the films consist of two layers: an interface partially strained epilayer with high crystalline quality, and a relaxed layer, formed by columnar grains, maintaining the epitaxial relationship with the substrate. The films presented smooth surfaces (RMS roughness similar to 0.55 nm), with circular equi-sized grains/crystallites, as observed by atomic force microscopy. The Hall measurements in Van der Pauw geometry revealed relatively high resistivity value similar to 620 mu Omega cm, ascribed to electron scattering on interfaces, on grain boundaries and on different defects/dislocations. (C) 2015 Elsevier B.V. All rights reserved.

The half-metallic properties of a new and promising full-Heusler compound, Zr2CoSn, are investigated by means of ab initio calculations within the Density Functional Theory framework. It was shown that the ferromagnetic ordered Hg2CuTi-type crystal structure is energetically the most favorable for this compound. The total magnetic moment is 3 mu(B)/f.u. and follows a typical Stater-Pauling dependence. The half metallicity disappears if the unit cell volume is contracted by more than 5%. 2015 Elsevier B.V. All rights reserved.

The time-dependent transport through single-molecule magnets coupled to magnetic or nonmagnetic electrodes is studied in the framework of the generalized master equation method. We investigate the transient regime induced by the periodic switching of the source and drain contacts. If the electrodes have opposite magnetizations the quantum turnstile operation allows the stepwise writing of intermediate excited states. In turn, the transient currents provide a way to read these states. Within our approach we take into account both the uniaxial and transverse anisotropy. The latter may induce additional quantum tunneling processes which affect the efficiency of the proposed read-and-write scheme. An equally weighted mixture of molecular spin states can be prepared if one of the electrodes is ferromagnetic.

The present study reveals the synthesis of metallic particles at atmospheric pressure using a radiofrequency low-temperature argon plasma jet. Copper bulk material of the powered electrode acted as solid precursor in the process and the metallic particles were obtained directly in gaseous environment. The particles were collected onto Si (100)-oriented substrates downstream of the atmospheric plasma jet and were ex situ characterized via optical microscopy, scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. We obtained copper particles, reddish brown in color, either of nano-or micro-metric sizes. Additionally, the plasma species and gas temperature were determined by OES. In conclusion, it was demonstrated that conversion of bulk metals into particles is possible at atmospheric pressure using a radiofrequency plasma jet and the development of several applications is recommended.

Previous studies have shown that the magnetic parameters of the Romanian loess/paleosol sequences have recorded Quaternary glacial/interglacial cycles over the last 600 kyr. Previous rock magnetic investigations pointed out that the magnetic enhancement in Romanian loess sequences is mainly due to pedogenic magnetite close to superparamagnetic/stable single domain boundary. We report the first reconstruction of grain size distribution (GSD) of the superparamagnetic (SP) particles for two Romanian loess deposits, Costinesti and Mircea Voda, located in the Dobrogea Plateau between the Danube River and the Black Sea. The GSDs were obtained using both the wide-band frequency spectrum of magnetic susceptibility (FSMS) method and low temperature Mossbauer spectroscopy. The FSMS method shows that the SP particles are present both in loess and paleosols. The GSDs in loess are shifted to slightly higher diameters with respect to paleosols. The concentration of SP particles has an opposite trend. The largest value is reached in the forest paleosol and the lowest in the loess samples. Both loess and paleosols indicated almost the same dominant grain size (about 17 nm in paleosols and 18 nm in loess samples). However the GSD shapes in loess are different than those found in paleosols. in the paleosols, the shape of GSDs seems to be independent of climate and degree of pedogenesis. These factors control only the concentration of SP particles present in each paleosol. Temperature dependent Mossbauer spectroscopy also supports the presence of SP particles both in loess and paleosol with a mean diameter around 13-14 nm. Dispersion of the diameters in the case of Mossbauer spectroscopy varies from sample to sample being probably affected by random surface defects. (C) 2015 Elsevier B.V. All rights reserved.

The absorption and photoluminescence (PL) spectra, PL decays, Raman spectrum, cyclic voltammetry (CV) and nuclear magnetic resonance of heteroleptic Ir-compound IrQ(ppy)(2) compound with two phenylpyridine (ppy) ligands and one quinoline (Q) ligand have been investigated experimentally and theoretically. Two very weak absorption bands due to the transitions to the triplet states are found at about 560 and 595 nm in IrQ(ppy)(2) doped in CH2Cl2 solution. IrQ(ppy)(2) exhibits a dual emission of red and green phosphorescence bands. The red emission intensity is much higher than the green one in IrQ(ppy)(2) powder, but much lower than the green one in lightly IrQ(ppy)(2)-doped CH2Cl2 solution and PMMA film. The intensity ratio of the red emission to the green emission, however, is observed to increase with increasing the IrQ(ppy)(2) concentration in CH2Cl2 solution and PMMA film. The enhancement of the red emission is suggested to be caused by the Forester energy transfer from Ir-ppy component to Ir-Q components between two neighboring IrQ(ppy)(2) molecules. The HOMO energy is estimated to be -4.865 eV from the CV measurement, which is close to the HOMO energy of -4.844 eV calculated using the time dependent density function theory (TD-DFT). The LUMO energy is estimated as -2.856 eV from the HOMO energy and the long-wavelength absorption edge found at 617 nm in the absorption spectrum. The absorption spectrum of IrQ(ppy)(2) is calculated by the TD-DFT. Discussion is given on a deviation of the calculated spectrum from the measured spectrum. (C) 2015 Elsevier B.V. All rights reserved.

The recent observation of a fundamental direct bandgap for GeSn group W alloys and the demonstration of low temperature lasing provide new perspectives on the fabrication of Si photonic circuits. This work addresses the progress in GeSn alloy epitaxy aiming at room temperature GeSn lasing. Chemical vapor deposition of direct bandgap GeSn alloys with a high Gamma- to L-valley energy separation and large thicknesses for efficient optical mode confinement is presented and discussed. Up to 1 mu m thick GeSn layers with Sn contents up to 14 at. % were grown on thick relaxed Ge buffers, using Ge2H6 and SnCl4 precursors. Strong strain relaxation (up to 81%) at 12.5 at. % Sn concentration, translating into an increased separation between Gamma- and L-valleys of about 60 meV, have been obtained without crystalline structure degradation, as revealed by Rutherford backscattering spectroscopy/ion channeling and transmission electron microscopy. Room temperature reflectance and photoluminescence measurements were performed to probe the optical properties of these alloys. The emission/absorption limit of GeSn alloys can be extended up to 3.5 mu m (0.35 eV), making those alloys ideal candidates for optoelectronics in the mid-infrared region. Theoretical net gain calculations indicate that large room temperature laser gains should be reachable even without additional doping.

Homogeneous hybrids in which iron oxide nanoparticles are entrapped within polymer structure are of interest for their potential applications in biomedical field, such as diagnostic, therapeutic and theranostic purposes. For this reason, hybrid nanomaterials based on branched polyethyleneimine (PEI) and iron oxide with different ratios were synthesized in a single step by hydrothermal procedure at high pressure and low temperature. Iron oxide is formed in the presence of branched PEI and the interaction between them takes place in the reaction medium. The influence of synthesis parameters on the hybrid formation, as well as chemical and structural properties was studied by means of FTIR, DSC-TG, HRTEM, electron paramagnetic resonance (EPR), magnetic measurements (SQUID) and Fe-57 Mossbauer analyses. It has been shown that synthesis parameters influence thermal stability and morphology of the hybrids. FeO(OH) crystallites of 2-5 nm are formed. Iron oxyhydroxide nanoparticles strongly entrapped in PEI structure are obtained. The low and distributed values of the specific spontaneous magnetisation in samples prepared under the same pressure conditions support the presence of very fine FeO(OH) nanoparticles, which formation and magnetic properties are depending on the mass ratio between iron oxide and PEI. (C) 2015 Elsevier B.V. All rights reserved.