National Institute Of Materials Physics - Romania

Publications

5714 articles found

2811. Dynamics of energy absorption versus crystallization in Bi4Ge3O12 (BGO) amorphous materials

Authors: Polosan, S

Published: OCT 2010, MATERIALS RESEARCH BULLETIN, 45, 1495, DOI: 10.1016/j.materresbull.2010.06.017

Theoretical and experimental determinations show that the intrinsic luminescence of BGO materials is strongly influenced by GeO4- tetrahedra around Bi3+ ions. Besides Bi3+ transitions, the excitonic spectra mask the BGO bandgap. Theoretical computations give the bandgap at 5.19 eV compared with the experimental one at 5.17 eV in BGO crystals and a significant decrease at 3.26 eV in the amorphous materials. This is due to the high disordering of the GeO4- tetrahedral which also reduces the refractive index of the amorphous materials. Formation of excitonic states and their overlap with the Bi3+ transitions suggests that the energy absorption takes place via the p-electrons of the O-2(-) ions and is then transferred to the Bi3+ p-electrons (excited states) close to the conduction band. Light emission appears after de-excitation to the ground state. (C) 2010 Elsevier Ltd. All rights reserved.

2812. Non-centro-symmetric superconductors Li2Pd3B and Li-2(Pd0.8Pt0.2)(3)B: amplitude and phase fluctuation analysis of the experimental magnetization data

Authors: Badica, P; Salem-Sugui, S; Alvarenga, AD; Jakob, G

Published: OCT 2010, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 23, DOI: 10.1088/0953-2048/23/10/105018

We report on magnetization data obtained as a function of temperature and magnetic field in Li-2(Pd0.8Pt0.2)(3)B and Li2Pd3B non-centro-symmetric superconductors. Reversible magnetization curves were plotted as M-1/2 versus T. This allows study of the asymptotic behavior of the averaged order parameter amplitude (gap) near the superconducting transition. Results of the analysis show, as expected, a mean field superconducting transition for Li2Pd3B. By contrast, a large deviation from the mean field behavior is revealed for Li-2(Pd0.8Pt0.2)(3)B. This is interpreted as due to the strength of the non-s-wave spin-triplet pairing in this Pt-containing compound which produces nodes in the order parameter and, consequently, phase fluctuations. The diamagnetic signal above T-c(H) in Li2Pd3B is well explained by superconducting Gaussian fluctuations and agrees with the observed mean field transition. For Li-2(Pd0.8Pt0.2)(3)B the diamagnetic signal above T-c(H) is much higher than the expected Gaussian values and appears to be well explained by three-dimensional critical fluctuations of the lowest-Landau-level type, which somehow agrees with the scenario of a phase mediated transition.

2813. Structure and Mossbauer measurements on SnSe2 bulk and thin films

Authors: Bibicu, I; Lorinczi, A; Velea, A; Sava, F; Popescu, M

Published: OCT 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1571, DOI:

Thin films samples of SnSe2 deposited by PLD and PED have been prepared and studied by XRD and Mossbauer spectroscopy. The films are crystalline with a major phase of SnSe2 and minor phase of amorphous SnSe2. An oxidation process lead to the appearance of a thin layer of SnO2 on the surface of the PLD samples.

2814. Multifrequency ESR Characterization of Paramagnetic Point Defects in Semiconducting Cubic BN Crystals

Authors: Nistor, SV; Stefan, M; Ghica, D; Goovaerts, E

Published: OCT 2010, APPLIED MAGNETIC RESONANCE, 39, 101, DOI: 10.1007/s00723-010-0136-x

Low-frequency (X-band) electron spin resonance (ESR) investigations on commercially available large-grained cubic boron nitride (cBN) superabrasive powders of various coloration, combined with high-frequency (W-band) ESR measurements on oriented submillimeter-size single crystallites selected from the same powder samples, resulted in a clear identification of several types of paramagnetic point defects. The resulting spin Hamiltonian parameters describing the ESR spectra observed in the 3-293 K temperature range and the photosensitivity of the paramagnetic defects observed in amber-colored cBN samples are reported. It is shown that the nature of the paramagnetic centers depends on the color of the investigated samples and that, in many cases, uncontrolled impurities seem to be involved in their structure.

2815. Preparation and patterning of nanoscale hybrid materials for micro-optics

Authors: Obreja, P; Cristea, D; Teodorescu, VS; Dinescu, A; Obreja, AC; Comanescu, F; Rebigan, R

Published: OCT 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 2013, DOI:

The refractive index of organic polymers can be modified by incorporation of inorganic nanoparticles with higher or lower refractive index. The paper presents our experimental results regarding the preparation of hybrid nanocomposite films with controlled refractive index for micro-optics and integrated optics. These materials were obtained by chemical routes and sol-gel processes from commercial polymers (PMMA, PVA), metal (Ag), metal oxide nanoparticles (TiO2, ZrO2) or organic materials (Rhodamine, Alq3). The properties of these materials depend on the composition, metal/oxide concentration, particle size and dispersion homogeneity. The hybrid nanocomposite films were characterized using spectroellipsometry, UV-VIS spectroscopy, AFM, TEM and the optical properties were correlated with processing parameters. Micro/nano pattering techniques based on combination of electron beam lithography and replication techniques have been developed to obtain structures with applications in micro-optics. Components like micro-lenses, diffractive optical elements and optical waveguides have been obtained and characterized using SEM and AFM.

2816. Synthesis of Eu-doped bismuth germanate nano-ceramic powder

Authors: Polosan, S; Matei, E; Logofatu, C

Published: OCT 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 1508, DOI:

Europium doped Bi(4)Ge(3)O(12) nanocrystals were successfully obtained by solid state reaction between precursor oxides at 800 degrees C. The XRD and XPS measurements shows an increasing of the lattice constant in the case of Eu:BGO nanocrystals, due to the electropositive character of Eu(III) ions compared with the host cations, which induces an ionic character for Eu-O bound and a more covalent one for the Ge-O bounds. The photoluminescence of Eu:BGO due to (5)D(0) -> (7)F(2) transition have a lower splitting compared with Eu:BGO monocrystals suggesting a lower crystal field in the sintered nanocrystals, due to a mixing between the crystalline and amorphous phases.

2817. New analytical approximation of diffraction size broadened peak profile for spherical crystallites with a lognormal distribution

Authors: Popa, NC; Teodorescu, CM; Frunza, S

Published: OCT 2010, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 43, 1030, DOI: 10.1107/S0021889810024829

A new analytical approximation of the diffraction peak size profile due to the lognormal distribution of spherical crystallites is derived. In comparison with the existing approximation constructed with the condition of conserving the volume-averaged column length of the exact profile [Popa & Balzar (2002). J. Appl. Cryst. 35, 338-346], the new approximation conserves both volume- and area-averaged column lengths. Moreover, it is a better approximation for large distribution dispersions. Like the old approximation, the present one is appropriate for implementation in the whole powder pattern fitting codes, Rietveld included.

2818. Bioactive glass thin films deposited by magnetron sputtering technique: The role of working pressure

Authors: Stan, GE; Marcov, DA; Pasuk, I; Miculescu, F; Pina, S; Tulyaganov, DU; Ferreira, JMF

Published: SEP 15 2010, APPLIED SURFACE SCIENCE, 256, 7110, DOI: 10.1016/j.apsusc.2010.05.035

Bioglass coatings were prepared by radio frequency magnetron sputtering deposition at low temperature (150 degrees C) onto silicon substrates. The influence of argon pressure values used during deposition (0.2 Pa, 0.3 Pa and 0.4 Pa) on the short-range structure and biomineralization potential of the bioglass coatings was studied. The biomineralization capability was evaluated after 30 days of immersion in simulated body fluid. SEM-EDS, XRD and FTIR measurements were performed. The tests clearly showed strong biomineralization features for the bioglass films. The thickness of the chemically grown hydroxyapatite layers was more than twice greater for the BG films deposited at the highest working pressure, in comparison to those grown on the films obtained at lower working pressures. The paper attempts to explain this experimental fact based on structural and compositional considerations. (C) 2010 Elsevier B.V. All rights reserved.

2819. Structural and photoluminescence characterization of mesoporous silicon-phosphates

Authors: Tiseanu, C; Parvulescu, V; Parvulescu, V; Cotoi, E; Gessner, A; Kumke, M; Simon, S; Vasiliu, F

Published: SEP 5 2010, JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 215, 24, DOI: 10.1016/j.jphotochem.2010.07.015

Two different types of mesoporous silicon-phosphate supports using different surfactants (a mixture of (CH3)(3)C13H27NBr with an organophosphorus coupling molecule (HO-PO(i-C3H7)(2)) and with a co-surfactant ((C2H5)(3)(C6H5)PCl), respectively) were synthesized. Trivalent europium (Eu) ions were immobilized via ion-exchange on these supports. The resulting materials were characterized using nitrogen adsorption isotherms at -196 degrees C, thermogravimetric analysis, SEM, TEM, FT-IR, PXRD, CP/MAS. (HSi)-H-1-Si-29 and P-31 NMR, DR-UV-vis as well as steady-state and time-resolved photoluminescence spectroscopy. The results evidenced that the co-polymerization of silicon and phosphorous yielded a unique morphology in these materials. Following calcination at 450 and 900 degrees C europium-exchanged silicon-phosphates with great surface area (BET=600-705 m(2) g(-1)) and 3.4 nm sized mesopores were obtained. The differences among the optical properties of the non-calcined europium materials such as the emission lifetimes, local environment at the europium sites or the relative contribution of the upper excited levels to the total photoluminescence were assigned to the surfactants used in the synthesis. Calcination of the silicon-phosphates at higher temperatures than 450 degrees C did not induce major changes in the structural properties: in contrast, photoluminescence properties of europium were markedly improved in terms of intensity and average lifetime. (C) 2010 Elsevier B.V. All rights reserved.

2820. Optical and structural studies on Ba(Mg1/3Ta2/3)O-3 thin films obtained by radiofrequency assisted pulsed plasma deposition

Authors: Scarisoreanu, ND; Galca, AC; Nedelcu, L; Ioachim, A; Toacsan, MI; Morintale, E; Stoica, SD; Dinescu, M

Published: SEP 1 2010, APPLIED SURFACE SCIENCE, 256, 6530, DOI: 10.1016/j.apsusc.2010.04.041

Single-phase Ba(Mg1/3Ta2/3)O-3 thin films were prepared by radiofrequency plasma beam assisted pulsed laser deposition (RF-PLD) starting from a bulk ceramic target synthesized by solid state reaction. Atomic force microscopy, X-ray diffraction and spectroscopic ellipsometry were used for morphological, structural and optical characterization of the BMT thin films. The X-ray diffraction spectra show that the films exhibit a polycrystalline cubic structure. From spectroscopic ellipsometry analysis, the refractive index varies with the thin films deposition parameters. By using the transmission spectra and assuming a direct band to band transition a band gap value of approximate to 4.72 eV has been obtained. (C) 2010 Elsevier B.V. All rights reserved.