National Institute Of Materials Physics - Romania

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472 articles found

461. X-ray photoelectron spectroscopy study on n-type GaAs

Authors: Ghita, RV; Negrila, C; Manea, AS; Logofatu, C; Cernea, M; Lazarescu, MF

Published: DEC 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 863, DOI:

A surface characterization has been performed on n-type GaAs (Si: GaAs) samples by using X-ray photoelectron spectroscopy (XPS) technique in order to get information on the degree of surface contamination during the device processing steps. The GaAs samples were etched by "in situ" ion sputtering (Ar plasma) before and after carrying out the measurements. According to XPS measurements the native oxide layer on as-received surface contains As2O3, AS(2)O(5) and Ga2O3. Large amounts of C and O are also present at surface before plasma cleaning of surface: C-C bonds, chemisorbed oxygen atoms. The XPS spectra recorded on the surface sample sputtered with 5 keV Ar+ ions were analyzed using SDP subroutines program facilities. After the first sputtering step it was observed different shifts of the principal peaks of GaAs bonding and compounds together with a modification of the relative peak intensities. The C1s and O1s peak size decreased but remained still significant. The following sputtering steps lead to a drastically decrease of C and 0 peaks down to the noise level.

462. Bar-configuration in hall measurements with GaAs

Authors: Ghita, RV; Logofatu, C; Negrila, C; Lazarescu, MF; Manea, AS; Ciupina, V

Published: MAR 2003, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 5, 363, DOI:

The electrical and transport properties of GaAs crystals such as resistivity (rho), mobility (mu) and carrier concentration (n) depend on the purity and defects of a particular crystal. These parameters are normally obtained by performing a simple Hall effect analysis in a bar configuration. The technical procedure is connected with the design of appropriate contacts for Hall bar configuration. In order to ensure good ohmic contacts oil GaAs, thin metal layers of Au-Ge-Ni have been deposited in high vacuum followed by heat treatment in low vacuum. Hall effect measurement is important for checking the quality of the GaAs wafers. We analyzed various GaAs wafers with (100) and (110), orientation obtained by LEC and HB methods. The resistivity of the wafers varied from low resistivity, n-type doped crystals (n = 10(18) cm(-3)), to high resistivity crystals (n = 10(14) cm(-3)). This paper presents the data of Hall characterization of wafers cut from GaAs crystals grown by different methods (LEC, HB).

463. Reactivity and magnetism of Fe/InAs(100) interfaces

Authors: Teodorescu, CM; Chevrier, F; Brochier, R; Richter, C; Ilakovac, V; Heckmann, O; De Padova, P; Hricovini, K

Published: AUG 2002, EUROPEAN PHYSICAL JOURNAL B, 28, 313, DOI: 10.1140/epjb/e2002-00234-6

Interface reaction and magnetism of epitaxially-grown Fe on InAs(100) are studied by core-level photoemission (As 3d and In 4d) and Fe 2p X-ray magnetic circular dichroism using synchrotron radiation. The reactivity of Fe/InAs(100) is relatively low compared to that of other interfaces involving deposition of 3d metals on III-V semiconductors. As a consequence, we observe a magnetic signal at Fe L-2,L-3 edges for the lowest thicknesses studied (1 ML). The atomic magnetic moment reaches a value close to that of the bulk alpha-Fe (2.2 mu(B)) for Fe coverages exceeding 5 ML. A ferromagnetic compound with approximate stoichiometry of FeAs is formed at the interface. The orbital magnetism represents between 12 and 20% of the total momentum, due to 3d density of states depletion and to crystal-field modification of the electronic levels. These properties make the Fe/InAs(100) interface very promising for spin-tunneling devices.

464. Reducibility of ruthenium in relation with zeolite structure

Authors: Parvulescu, VI; Coman, S; Palade, P; Macovei, D; Teodorescu, CM; Filoti, G; Molina, R; Poncelet, G; Wagner, FE

Published: MAR 1999, APPLIED SURFACE SCIENCE, 141, 176, DOI: 10.1016/S0169-4332(98)00619-9

Ru-zeolites prepared via deposition of aqueous solution of ruthenium chloride on X, L, ZSM-5 and APO-54 were investigated. The impregnated solids were characterized by the following techniques: N-2 sorption, H-2 chemisorption, TPR, XRD, XPS, EXAFS and Mossbauer spectroscopy. It was found that deposition of Ru is influenced by the pore structure and the chemical composition of the supports, in particular the concentration and strength of the protonic sites, with the formation of the different species. The packing of Ru as well as the degree of the reduction and the metal particle size are different according to the penetration depth of the metal and the nature of the zeolite. The amount of chlorine possibly remaining as adatoms of Ru is very low or inexistent. In calcined and reduced catalysts no chloride peak could be evidenced by XPS. The experiments confirmed that the use of large amounts of Ru chloride precursor leads to rather large metal particles. (C) 1999 Elsevier Science B.V. All rights reserved.

465. EXAFS CHARACTERIZATION OF DY AND PD-DY ON ALUMINA CATALYSTS

Authors: MACOVEI, D; PARVULESCU, V; TEODORESCU, CM

Published: APR 1994, REACTION KINETICS AND CATALYSIS LETTERS, 52, 86, DOI: 10.1007/BF02129853

Two Pd-Dy/gamma-Al2O3 catalysts, with 1 wt.% Dy and various Pd loadings (0.1 and 0.5 wt.%) have been investigated by EXAFS at the Dy L(III)-edge, and compared with 1 % Dy/gamma-Al2O3. The Dy-carrier interaction was found to change with the Pd dispersity.

466. MULTIPHOTON IONIZATION OF RUBIDIUM

Authors: COLLINS, CB; CURRY, SM; JOHNSON, BW; MIRZA, MY; CHELLEHMALZADEH, MA; ANDERSON, JA; POPESCU, D; POPESCU, I

Published: 1976, PHYSICAL REVIEW A, 14, 1671, DOI: 10.1103/PhysRevA.14.1662

467. FINE-STRUCTURE MEASUREMENTS OF 2-PHOTON TRANSITIONS IN ATOMIC CESIUM WITH A TUNABLE DYE LASER

Authors: CURRY, SM; COLLINS, CB; MIRZA, MY; POPESCU, D; POPESCU, I

Published: 1976, OPTICS COMMUNICATIONS, 16, 255, DOI: 10.1016/0030-4018(76)90229-7

468. MULTIPHOTON EXCITATION OF ATOMIC RUBIDIUM WITH A TUNABLE DYE LASER

Authors: COLLINS, CB; CURRY, SM; JOHNSON, BW; MIRZA, MY; POPESCU, D; POPESCU, I

Published: 1975, BULLETIN OF THE AMERICAN PHYSICAL SOCIETY, 20, 246, DOI:

469. DOUBLET LINE-STRENGTH RATIOS FOR PRINCIPAL SERIES OF CESIUM

Authors: POPESCU, D; POPESCU, I; MAURER, J; COLLINS, CB; JOHNSON, BW

Published: 1975, PHYSICAL REVIEW A, 12, 1431, DOI: 10.1103/PhysRevA.12.1425

470. Sonogashira Synthesis of New Porous Aromatic Framework- Entrapped Palladium Nanoparticles as Heterogeneous Catalysts for Suzuki-Miyaura Cross-Coupling

Authors: Cata, L; Terenti, N; Cociug, C; Hadade, ND; Grosu, I; Bucur, C; Cojocaru, B; Parvulescu, VI; Mazur, M; Cejka, J

Published: , ACS APPLIED MATERIALS & INTERFACES, DOI:

Palladium nanoparticles entrapped in porous aromatic frameworks (PAFs) or covalent organic frameworks may promote heterogeneous catalytic reactions. However, preparing such materials as active nanocatalysts usually requires additional steps for palladium entrapment and reduction. This paper reports as a new approach, a simple procedure leading to the self-entrapment of Pd nanoparticles within the PAF structure. Thus, the selected Sonogashira synthesis affords PAF-entrapped Pd nanoparticles that can catalyze the C-C Suzuki-Miyaura cross-coupling reactions. Following this new concept, PAFs were synthesized via Sonogashira cross-coupling of the tetraiodurated derivative of tetraphenyladamantane or spiro-9,9 '-bifluorene with 1,6-diethynylpyrene, then characterized them using powder X-ray diffraction, diffuse reflectance infrared Fourier transform spectroscopy, X-ray photoelectron spectroscopy, high-resolution scanning transmission electron microscopy, and textural properties (i.e., adsorption-desorption isotherms). The PAF-entrapped Pd nanocatalysts showed high catalytic activity in Suzuki-Miyaura coupling reactions (demonstrated by preserving the turnover frequency values) and stability (demonstrated by palladium leaching and recycling experiments). This new approach presents a new class of PAFs with unique structural, topological, and compositional complexities as entrapped metal nanocatalysts or for other diverse applications.