National Institute Of Materials Physics - Romania

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847 articles found

821. Modified ruthenium exchanged zeolites for enantioselective hydrogenation

Authors: Parvulescu, VI; Parvulescu, V; Coman, S; Radu, C; Macovei, D; Angelescu, E; Russu, R

Published: 1995, PREPARATION OF CATALYSTS VI: SCIENTIFIC BASES FOR THE PREPARATION OF HETEROGENEOUS CATALYSTS, 91, 570, DOI:

Preparation of modified ruthenium molecular sieves has been investigated in two steps deposition of ruthenium and modifying of ruthenium molecular sieves in presence of ligands. As support sieve there were used two molecular sieves with large pore apertures and low acidity strength (zeolite L and APO-34). Correlation of in situ W-VIS ruthenium deposition measurements with catalyst characterisation revealed that ruthenium deposition takes place not only through ionic exchange but also through adsorption of ruthenium hydrolysed species. Modifying of the ruthenium molecular sieves catalysts with ligands has as effect a diminution of electronic charge on metal. The experimental data indicate that the presence of ligand favours an enantioselective hydrogenation of D-fructose to D-mannitol even if the yields are not very high.

822. EFFECTS OF ANNEALING ON THE CONDUCTIVITY OF C-60 THIN-FILMS

Authors: BELUMARIAN, A; MANAILA, R; STOICA, T; DRAGOMIR, A; MANCIU, M; DEVENYI, A; BRAUN, T

Published: 1995, FULLERENE SCIENCE AND TECHNOLOGY, 3, 509, DOI: 10.1080/153638X9508543803

Films of C-60, at different stages of annealing of T-c = 200 degrees and 300 degrees C have been electrical characterized over the temperature domain from -130 degrees C to T-c. X-ray diffraction revealed a random polycrystalline fee structure with stacking defects of an intrinsic nature, due to deposition conditions. The value of room-temperature conductivity was found to be in the range (6.3-1.0)*10(-10) (Omega cm)(-1). In the stable annealed state the conductivity showed an activated temperature dependence above 423 K and a non-activated dependence below 330-280 K. The activation energies E(2) 0.8 eV (film thickness 0.70 mu m) and E(2) = 1.0 eV (film thickness 2.40 mu m) were in good agreement with the energy gap values (1.63 eV and 2.08 eV) which were deduced from the al;sorption spectral dependence. Annealing decreased the non-activated contribution to conduction, extending the intrinsic conduction temperature range.

823. HIGH POWER OPERATION OF LOW CONFINEMENT LASER DIODE STRUCTURE

Authors: PETRESCUPRAHOVA, IB; BUDA, M; IORDACHE, G; KAROUTA, F; SMALBRUGGE, E; VANDEROER, TG; KAUFMANN, LMF; WOLTER, JH; VANDERVLEUTEN, WG

Published: 1995, FOURTH CONFERENCE IN OPTICS, ROMOPTO '94, 2461, 9, DOI: 10.1117/12.203458

824. CONFINEMENT FACTOR MEASUREMENT IN LASER DIODES BY ABSORPTION LOSS MODULATION WITH INJECTED CARRIERS

Authors: CENGHER, CD; VLAICU, AM; IORDACHE, G; PETRESCUPRAHOVA, IB

Published: 1995, FOURTH CONFERENCE IN OPTICS, ROMOPTO '94, 2461, 64, DOI: 10.1117/12.203503

825. MAGNETOTRANSPORT AND PHOTOLUMINESCENCE OF 2-DIMENSIONAL HOLE GASES IN SI/SI1-XGEX/SI HETEROSTRUCTURES

Authors: LOO, R; VESCAN, L; HARTMANN, A; APETZ, R; ZASTROW, U; SCHAPERS, T; LEUTHER, A; DIEKER, C; LUTH, H; GARTNER, P; STOICA, T

Published: DEC 15 1994, PHYSICAL REVIEW B, 50, 18123, DOI: 10.1103/PhysRevB.50.18113

826. TRAPPING LEVELS IN BI12SIO20 CRYSTALS

Authors: PETRE, D; PINTILIE, I; BOTILA, T; CIUREA, ML

Published: AUG 15 1994, JOURNAL OF APPLIED PHYSICS, 76, 2219, DOI: 10.1063/1.357637

Localized levels play a major role in the electro-optic properties of Bi12SiO2O (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2-1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.

827. HIGH GAP SPUTTERED DLC LAYERS

Authors: MOROSANU, CO; STOICA, T; DEMARTINO, C; DEMICHELIS, F; TAGLIAFERRO, A

Published: APR 1994, DIAMOND AND RELATED MATERIALS, 3, 816, DOI: 10.1016/0925-9635(94)90275-5

Adherent and transparent unhydrogenated DLC layers were deposited by means of r.f. and d.c. magnetron sputtering at low deposition temperatures. The optical gap value increases from 0.7 to 2.2 eV when deposition pressure is increased from 10(-2) to 10(-1) Torr. In high gap materials, large amounts of sp3 hybridized carbon atoms are present. When Al contacts are evaporated in coplanar configuration on these high gap materials, rectifying and photoelectrical effects are observed.

828. LINE-SHAPE MODEL FOR THE BROAD PHOTOLUMINESCENCE BAND FROM SI1-XGEX/SI HETEROSTRUCTURES

Authors: HARTMANN, A; VESCAN, L; DIEKER, C; STOICA, T; LUTH, H

Published: DEC 15 1993, PHYSICAL REVIEW B, 48, 18279, DOI: 10.1103/PhysRevB.48.18276

Photoluminescence studies on Si1-xGex layers exceeding the critical thickness show not only dislocation-related peaks D1,...,D4, but also in addition a broad band (T band), centered at similar to 110 meV below the strained alloy band gap. Depth profiling studies show that the T band is due to centers in the Si1-xGex. The excitation power dependence and high quantum efficiency of the T band can be explained by assuming that it is due to an isoelectronically trapped exciton. T-band line shapes were fitted by a model based on statistical fluctuations of the Ge content and their effect on the local gap energy.

829. MEYER-NELDEL CORRELATION IN SEMICONDUCTORS AND MOTT MINIMUM METALLIC CONDUCTIVITY

Authors: POPESCU, C; STOICA, T

Published: DEC 15 1992, PHYSICAL REVIEW B, 46, 15071, DOI: 10.1103/PhysRevB.46.15063

The paper outlines conditions under which the statistical shift of the Fermi level within the density-of-states (DOS) distribution may result in the Meyer-Neldel correlation. Sufficient conditions have been found: (i) The DOS should have, around the Fermi level, two competing exponential slopes; (ii) The temperature range for the statistical shift should correspond to kT values higher than at least one of these two characteristic energies associated with the slopes. Under such conditions, the Fermi level is no longer the dominant energy for carrier-concentration changes. Relating the Mayer-Neldel correlation to DOS parameters is an essential step in a correct deduction of Mott's minimum-metallic-conductivity value from experiment.

830. TRANSFORMATIONS OF AMINIC ANTIOXIDANTS IN THE INHIBITED OXIDATION OF HYDROCARBONS

Authors: NEGOITA, N; HERDAN, JM; GRECU, N; IORDACHE, G

Published: FEB 1988, REVUE ROUMAINE DE CHIMIE, 33, 194, DOI: