National Institute Of Materials Physics - Romania

Publications

490 articles found

331. Structure, reactivity, electronic configuration and magnetism of samarium atomic layers deposited on Si(001) by molecular beam epitaxy

Authors: Gheorghe, NG; Lungu, GA; Husanu, MA; Costescu, RM; Macovei, D; Teodorescu, CM

Published: FEB 15 2013, APPLIED SURFACE SCIENCE, 267, DOI: 10.1016/j.apsusc.2012.08.016

The surface structure, interface reactivity, electron configuration and magnetic properties of Sm layers deposited on Si(0 0 1) at various temperatures are investigated by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and magneto-optical Kerr effect (MOKE). It is found that metal Sm is present on samples prepared at low temperature, with an interface layer containing SmSi2 and Sm4Si3. When samples are prepared at high temperature, much less metal Sm is found, with an increasing amount of SmSi2. Room temperature ferromagnetism is observed for all prepared layers, with a decrease of the saturation magnetization when samples are prepared at high temperature. It is found that ferromagnetism implies mostly a compound with approximate stoichiometry Sm4Si3. Also, the decrease in the intensity of the XAS 2p(3/2) -> 3d white lines with the corresponding increasing amount of SmSi2 may be explained by assuming a higher occupancy of Sm 5d orbitals (5d(2) configuration), most probably due to hybridation effects. (C) 2012 Elsevier B. V. All rights reserved.

332. Room Temperature Ferromagnetic, Anisotropic, Germanium Rich FeGe(001) Alloys

Authors: Lungu, GA; Apostol, NG; Stoflea, LE; Costescu, RM; Popescu, DG; Teodorescu, CM

Published: FEB 2013, MATERIALS, 6, DOI: 10.3390/ma6020612

Ferromagnetic FexGe1-x with x = 2%-9% are obtained by Fe deposition onto Ge(001) at high temperatures (500 degrees C). Low energy electron diffraction (LEED) investigation evidenced the preservation of the (1 x 1) surface structure of Ge(001) with Fe deposition. X-ray photoelectron spectroscopy (XPS) at Ge 3d and Fe 2p core levels evidenced strong Fe diffusion into the Ge substrate and formation of Ge-rich compounds, from FeGe3 to approximately FeGe2, depending on the amount of Fe deposited. Room temperature magneto-optical Kerr effect (MOKE) evidenced ferromagnetic ordering at room temperature, with about 0.1 Bohr magnetons per Fe atom, and also a clear uniaxial magnetic anisotropy with the in-plane [110] easy magnetization axis. This compound is a good candidate for promising applications in the field of semiconductor spintronics.

333. Thermal analysis of corrosion products formed on carbon steel in ammonium chloride solution

Authors: Tutunaru, B; Samide, A; Negrila, C

Published: FEB 2013, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, 111, 1154, DOI: 10.1007/s10973-011-2187-0

The influence of different ions NO3 (-) and SO4 (2-) on the carbon steel corrosion in ammonium chloride was investigated using mass loss measurements and potentiodynamic polarization. Corrosion products were analyzed using X-ray photoelectron spectroscopy (XPS) and simultaneous thermal and differential scanning calorimetry (TG/DSC). XPS analysis shows that the main product of corrosion is a non-stoichiometric Fe3+ oxyhydroxide, consisting of a mixture of FeO(OH) and FeO(OH) containing inclusions of these anions, species such as Fe3+O(OH,Cl-); Fe3+O(OH,SO4 (2-)); and Fe3+O(OH,NO3 (-)). TG/DSC confirms the decomposition of the rusty products formed by chemical corrosion, compounds like Fe3+ oxyhydroxides, with beta-FeOOH as the major phase, crystal structure of which may contain Cl-, NO3 (-), and SO4 (2-)-e.g., akaganeite [Fe3+O(OH,A)].

334. Phytosynthesis of gold nanoparticles using caesalpinia pulcherrima (peacock flower) flower extract and evaluation of their antimicrobial activities

Authors: Nagaraj, B; Divya, TK; Barasa, M; Krishnamurthy, NB; Dinesh, R; Negrila, CC; Predoi, D

Published: MAR-APR 2013, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 15, 304, DOI:

Caesalpinia pulcherrima (Linn) popularly known as Peacock flower in India belongs to family Caesalpiniace. In this work, we describe environment friendly technique for green synthesis of gold nanoparticles from AuCl4 solution using the Caesalpinia pulcherrima (Peacock flower) flower extract as reducing agent. The gold nanoparticles were characterized using UV-visible spectroscopy and transmission electron microscopy (TEM). The UV- visible spectra indicate a strong Plasmon resonance that is located at similar to 450 nm. The TEM analysis shows that products have spherical morphology with size ranging between 1.0-50 nm. The study also indicates that gold nanoparticles show good antimicrobial activities when compared to the standard antibiotics.

335. TWO DIMENSIONAL PHOTONIC CRYSTALS WITH DIFFERENT SYMMETRIES FOR WAVEGUIDES AND RESONANT CAVITIES APPLICATIONS

Authors: Popescu, DG

Published: 2013, UNIVERSITY POLITEHNICA OF BUCHAREST SCIENTIFIC BULLETIN-SERIES A-APPLIED MATHEMATICS AND PHYSICS, 75, 252, DOI:

In this paper we investigate the properties of two-dimensional photonic crystals (2D PhCs) with both hexagonal and square symmetry for applications in optoelectronics and telecommunication. An efficient way of manipulating the light is to make use of the defects introduced within the periodic structure, which consequently break the symmetry of the crystal, resulting in linear waveguides or resonant cavities. We calculate using finite-difference time-domain and finite-difference frequency-domain methods the properties associated with several kinds of defects, conveniently selected, performed in 2D PhCs with square and hexagonal symmetry.

336. Analysis of even and odd modes of a two-dimensional photonic crystal at Si/SiO/Cu interface

Authors: Popescu, DG; Sterian, P

Published: JUL-AUG 2013, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 15, 614, DOI:

We present a careful analysis of field characteristics in two-dimensional photonic crystals with square and hexagonal symmetry designed in a Si/SiO2/Cu heterostructure. A special attention is devoted to frequencies close to photonic band gap. Following the symmetry classification in odd and even modes, their field characteristics are studied. The material prepared in a dedicated deposition chamber in ultra-high vacuum by annealing the Si/SiO2 substrate and subsequent deposition of 10 angstrom copper in order to prevent the sample oxidation, allows the study of the photonic characteristics of the copper/silicon oxide/silicon interface. The chemical state of the Cu/SiO/Si system is addressed by photoelectron spectroscopy which allows us to deduce the amount of crystalline Si and amorphous oxide. In this manner a precise estimation of the dielectric constant of the materials is possible. It is expected that at the interface with copper, a strong confinement of radiation should appear due to high reflectivity of copper.

337. FERROMAGNETISM AND REACTIVITY OF FE DEPOSITED ON GaAs(001) BY MAGNETRON SPUTTERING

Authors: Vasilache, V; Lungu, GA; Logofatu, C; Medianu, RV; Teodorescu, CM

Published: JAN-MAR 2013, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 8, DOI:

Iron layers are grown on GaAs(001) single crystal wavers by magnetron sputtering and characterized by magneto-optical Kerr effect (MOKE) and X-ray photoelectron spectroscopy (XPS). It is found that all deposited layers, starting with 2 nm thickness, are ferromagnetic at room temperature with a relatively high coercitive field (500 to 750 Oe). The average Fe magnetic moment is larger for the thinnest Fe film deposited, of about 0.52 Bohr magnetons per Fe atom. The average Fe magnetic moment decreases to 0.26 Bohr magnetons, then to 0.17 Bohr magnetons for 4 and 8 nm, resepctively. XPS evidenced that the outermost Fe layers are oxidized; however, the thinnest Fe film presented the lower oxidation state. We may connect this reaction with Fe-As bonds, resulting in a nonferromagnetic interface compound. Indeed, XPS depth profiling of the 8 nm sample evidenced the formation of a very thin (about 1.4 nm) Fe metal layer immediately at the interface with GaAs. In this layer, Fe may reach an atomic magnetic moment as high as 1 Bohr magneton.

338. SATELLITES IN Ce 3d X-RAY PHOTOELECTRON SPECTROSCOPY OF CERIA

Authors: Radutoiu, N; Teodorescu, CM

Published: OCT-DEC 2013, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 8, 1549, DOI:

Ce 3d X-ray photoelectron spectroscopy (XPS) spectra is characterized by the presence of "shake-up" and "shake-down" satellites, which complicate the analysis of the XPS data and especially the identification of different ionization states. The present paper proposes a new model for interpreting the 3d XPS lines of cerium oxide (CeO2). This model supposes 6 pairs of doublets with spin-orbit splitting, three corresponding of trivalent cerium and three corresponding of tetravalent cerium. The three components used for each ionization state correspond to the "shake-down" process, to the "parent" (Koopman) line, and to the "shake-up" line, in order of increasing binding energy. From the time dependence of Ce 3d and C 1s XPS spectra, a model for the contamination process is derived and also the length of the alkyl chains of the hydrocarbons may be estimated. The present analysis allowed a more accurate computation of the Ce 3d atomic sensitivity factor (ASF), in accord with the values found for other rare earths elements.

339. ON THE PASSIVATION OF GaAs SURFACE BY SULFIDE COMPOUNDS

Authors: Ghita, RV; Negrila, CC; Cotirlan, C; Logofatu, C

Published: JUL-SEP 2013, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 8, 1344, DOI:

A genuine GaAs surface is covered with a layer (similar to few nm) of native oxide pinning the surface Fermi level within the band gap of semiconductor. In this work is presented a study of GaAs surface passivation by sulfur compounds inorganic and organic (alkane thiols), a method that combines both chemical and electronic passivation. At the surface of GaAs it is developed an adherent layer of sulfur compound as a result of chemical interaction of sulfur ions with GaAs surface, a compound putted into evidence by scanning electron microscopy (SEM) images, photoluminescence analysis, second harmonic generation (SHG) measurement and Raman spectroscopy. Using X-ray photoelectron spectroscopy (XPS) it is putted into evidence the presence of covalent bonds Ga-S and AsS as a result of chemical interaction with sulf ions. Electrical characteristic of AuGeNi/thiol/GaAs structure is presented in I (V) recorded in the region of small currents.

340. Surface free energy of smooth and dehydroxylated fused quartz from contact angle measurements using some particular organics as probe liquids

Authors: Zgura, I; Moldovan, R; Negrila, CC; Frunza, S; Cotorobai, VF; Frunza, L

Published: JUL-AUG 2013, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 15, 634, DOI:

The dispersion and the polar independent contributions to the surface free energy in the case of fused quartz plates were evaluated by contact angle measurements against a series of liquids like water, glycerol, ethylene glycol, dimethyl sulfoxide and a nematic mixture NP5 of azoxybenzene type compounds. The smooth fused quartz plates were thermally pretreated at 240 degrees C and 1000 degrees C to decrease the number of surface hydroxyl groups; their surface was characterized by X-ray photoelectron spectroscopy. The contact angle data were further processed applying either the approach of Owens-Wendt with geometric mean or the Wu's approach of harmonic mean of nonpolar and polar interactions. The liquids were chosen thus to fulfill Della Volpe and Siboni's criterion namely that these have the condition number of system matrix low enough. The values obtained for the components of surface free energy were discussed in comparison with the results obtained in literature for crystalline quartz or for other silica forms as bulk or films. The study has shown a decrease of the polar component of the surface tension by increase of the pretreatment temperature.