National Institute Of Materials Physics - Romania

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472 articles found

421. Capacitance-voltage characteristics of heterostructures with high leakage currents

Authors: Goldenblum, A; Stancu, V; Buda, M; Iordache, G; Pintilie, I; Negrila, C; Botila, T

Published: MAR 1 2008, JOURNAL OF APPLIED PHYSICS, 103, DOI: 10.1063/1.2844210

Leakage currents determine bumps in the capacitance-voltage characteristics of metal-oxide-semiconductor-type heterostructures with capacitance values larger than the insulator capacitance. A nearly parallel shift with frequency is also observed. These aspects are connected with the presence of a high density of interface states. We illustrate these effects for the case of nano-PbS/SiO2/Si heterostructures. The phenomena were simulated and we propose a method to quickly estimate the interface state density level of a heterostructure when leakage channels are present. (C) 2008 American Institute of Physics.

422. Photoluminescence properties of composites based on zinc oxyde and single-walled carbon nanotubes

Authors: Baibarac, M; Baltog, I; Husanu, M; Velula, T; Bucur, C; Mihut, L; Preda, N

Published: FEB 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 293, DOI:

Photoluminescence (PL) properties of the composites based on zinc oxide (ZnO) and single-walled carbon nanotubes (SWNTs) prepared by hydrothermal synthesis are studied in this paper. The emission and excitation spectra of nanometric ZnO powder are dramatically influenced by the adsorption of different molecules coming from the environment. Therefore, a special attention is given to reveal the influences of adsorption/de-sorption process on photoluminescence properties of zinc oxide nanoparticles. A quenching effect of the intrinsic PL is observed regularly when the ZnO is synthesized in the presence of SWNTs, i.e. when a ZnO/SWNTs composite is formed. A distinct feature of the ZnO/SWNTs composite, is an emission band with a maximum at 405-450 nm.

423. Raman and photoluminescence studies on intercalated lead iodide with pyridine and iodine

Authors: Preda, N; Mihut, L; Baibarac, M; Husanu, M; Bucur, C; Baltog, I

Published: FEB 2008, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 10, 322, DOI:

Intercalated PbI2 compounds were prepared by exposing crystalline films and crystalline powders of lead iodide to pyridine or iodine vapor at room temperature. Correlated studies of optical absorption, Raman scattering and low temperature photoluminescence (PL) reveal different properties in comparison with those of pure crystalline PbI2 powder. Depending of the nature of intercalated molecules (organic or inorganic) the basic semiconducting properties of PbI2 are dramatically modified. The main signature of iodine intercalated PbI2 consists in a luminescence band peaking at 2.24 eV appearing at 77 K under 350 nm excitation light. In the case of pyridine intercalated PbI2 a new intense band at about 3.3 eV in the absorption spectrum is observed. The the PL spectra of pyridine intercalated PbI2 at 77 K change substantially when the excitation wavelength. The Raman spectra confirm the presence of pyridine between the PbI2 layers.

424. Influence of bilayer period on the characteristics of nanornetre-scale ZrN/TiAIN multilayers

Authors: Vladescu, A; Kiss, A; Popescu, A; Braic, M; Balaceanu, M; Braic, V; Tudor, I; Logofatu, C; Negrila, CC; Rapeanu, R

Published: FEB 2008, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 8, 721, DOI: 10.1166/jnn.2008.D218

in the last decade, considerable research effort was directed to the deposition of multilayer films with layer thicknesses in the nanometer range (superlattice coatings), in order to increase the performance of various cutting tools and machine parts. The goal of the present work was to investigate the main microstructural, mechanical and wear resistance characteristics of a superlattice coating, consisting of alternate multilayer ZrN/TiAIN films, with various bilayer periods (5 divided by 20 nm). The coatings were deposited by the cathodic arc method on Si, plain carbon steel and high speed steel substrates to be used as wear resistance surfaces. The multilayer structures were prepared by using shutters placed in front of each cathode (Zr and Ti + Al). The characteristics of multilayer structures (elemental and phase composition, texture, Vickers microhardness, thickness, adhesion, and wear resistance) were determined by using various techniques (AES, XPS, XRD, microhardness measurements, scratch, and tribological tests). A comparison with the properties of ZrN and TiAIN single-layer coatings was carried out.

425. Experimental modelling of the radionuclides diffusion process in uranifer tailing dump

Authors: Bragea, M; Perju, D; Jinescu, G; Toro, L; Cristache, C

Published: JAN 2008, REVISTA DE CHIMIE, 59, 78, DOI:

Generally, the uranifer tailings dump are not protected. During the time appear chemical and physical modifications which favour spreading of the radioactive contaminant into the environment, because of the waters provided from precipitations with pH variation (acid rainfall, snow), solar radiance, variation of temperature and humidity, interaction of different minerals from the waste rock with infiltration water during the flow, leading to precipitations phenomenon, ionic exchange, adsorption and re-dissolving [2]. As a rule, the speed of radionuclides diffusion from radioactive tailings dump depends on each of these factors and is very specific to the hydro and geochemistry of the nearest rocks [7]. Because of that, the scientific research is very difficult and will obliges us to study through the simplified physical systems but maintaining the important parameters which are influencing the diffusion process of the contaminant in the environment. The main objective of this paper is to characterize hydro and geochemistry properties controlling the migration of radionuclides extracted from tailings dump into the environment, including the processes of sorption and adsorption and neglecting other physical processes as precipitation, ion exchange or some other geochemical interactions within the system. For this purpose we made studies on Ra-226 and U-nal extraction from uranifer tailings dump analyzed in pure water in dynamic system, studies about the influence of clay presence on speed of diffusion process in sorption and adsorption phenomenon. Finally we made a prediction for long term about variation of concentrations of Ra-226 and U-nal based on experimental data using some software programs from literature [8].

426. Structural and Magneto-Optical Properties of Co-doped ZnO Thin Films Prepared by Sol-Gel Method

Authors: Neamtu, J; Teodorescu, CM; Georgescu, G; Ferre, J; Malaeru, T; Jitaru, I

Published: 2008, NSTI NANOTECH 2008, VOL 1, TECHNICAL PROCEEDINGS: MATERIALS, FABRICATION, PARTICLES, AND CHARACTERIZATION, +, DOI:

Recent experiments evidenced room temperature ferromagnetism in ZnO:Co diluted magnetic semiconductors, however the diluted semiconductor nature and the ferromagnetism strongly depends on the preparation method employed. We have investigated the ability of the sol-gel route for the synthesis of cobalt doped ZnO films grown on Si (100) and glass substrates. A homogeneous and stable Zn (1-x) CoxO sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 degrees C. Local structure studies of the Zn1-xCoxO thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the co-existence of a few amount of small metal cobalt aggregates with Co in non-stoichiometric ConOm aggregates, with n > m, and m similar to 4. Low temperature (1.5 K) Kerr magnetometry give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n - m similar to 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of similar to 200 Oe is superimposed, most probably due to metal cobalt nanoclusters.

427. Particular signature of isolated and bundled carbon nanotubes in their Raman spectra

Authors: Husanu, M; Baibarac, M; Baltog, I

Published: 2008, ROMANIAN REPORTS IN PHYSICS, 60, 699, DOI:

The efficiency of different surfactants (anionic, cationic, neutral) as chemical agents favoring in the unbinding of nanotube bundles in isolated entities is investigated by Raman and UV-VIS-NIR spectroscopy. ne neutral surfactants Tween 20 - polyoxyethylene sorbitan monolaurate (T-20-PSM) and Tween 80 - polyoxyethylene (80) sorbitan monooleate (T-80-PSM) have been found the most efficient. The isolated single walled carbon nanotubes (SWNTs) nanotubes are apprised at lambda(exc) = 1064 nm by two simultaneous modifications of the Raman spectra: i) narrowing of the band associated to the transversal vibration modes, i.e., G band (similar to 1592 cm(-1)); ii) change of shape of the complex band associated to the radial vibration modes (RBM) consequent on the decrease of the intensity of the components associated with the bundled nanotubes. The individualization of nanotubes is evidenced also in the UV-VIS-NIR absorption spectra by the appearance of a fine structure in the wide absorption band (similar to 1400-1750 nm) that is associated with the transitions E-S(11) of semiconducting SWNTs. The fine structure grows gradually with the separation degree in isolated nanotubes entities.

428. Vibrational and photoluminescence properties of the polystyrene functionalized single-walled carbon nanotubes

Authors: Baibarac, M; Baltog, I; Lefrant, S; Mevellec, JY; Bucur, C

Published: JUL-OCT 2008, DIAMOND AND RELATED MATERIALS, 17, 1388, DOI: 10.1016/j.diamond.2008.01.061

Using surface enhanced Raman scattering (SERS) and FTIR spectroscopy, a covalent functionalization of single wall carbon nanotubes (SWNTs) with polystyrene (PST) is demonstrated. For this, two types PST/SWNTs composites were used: one resulting from the radical polymerization reaction of styrene achieved at 90 degrees C in the presence of benzene and benzoyl peroxide, latter being the initiator and another obtained by mixing the two constituents. These compounds reveal different SERS and photoluminescence spectra. The main experimental facts supporting the covalent functionalization of SWNTs with PST are provided by FTIR spectroscopy. In this frame significant is the increase of the intensities of absorption bands at 1270 and 1721 cm(-1), which are associated with the tangential C-H bending+C-O stretching and C=O stretching vibration modes, respectively. The presence of these absorption bands in the FTIR spectra of the PST/C-60 composites proves the formation of a compound with a similar molecular structure. in the case of polystyrene functionalized SWNTs, the appearance of a new FTIR band at ca. 1635 cm(-1) attributed to the C-C in cyclic hydrocarbons ring di-substituted vibration mode indicates the formation of an additional reaction product. (C) 2008 Elsevier B.V. All rights reserved.

429. Photocatalytic and structural properties of mixed titania and zirconia aerogels

Authors: Danciu, V; Baia, L; Cosoveanu, V; Baia, M; Vasiliu, F; Diamandescu, L; Teodorescu, CM; Feder, M; Popp, J

Published: 2008, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 2, 80, DOI:

A series of TiO2-ZrO2 aerogels with different ZrO2 contents (3-10 %) were prepared by an acid catalyzed sol-gel method followed by low-temperature supercritical drying and heat treatment. The structure and morphology of mixed aerogels were studied by means of Raman spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), xenon sorption and elemental analysis. XRD spectra refinements show that the calcined TiO2-ZrO2 aerogels consist mainly in anatase phase accompanied by a small fraction of tetragonal ZrO2. The Raman spectra proof that the network of the calcined mixed aerogels mainly preserves an anatase structure similar to that of the calcined TiO2 aerogel. The TEM images reveal that the anatase particles present round shapes with a highly homogeneous size (about 3-8 nm). Areas containing chains of amorphous tetragonal zirconia were also identified by the TEM investigations. The addition of zirconia to titania significantly increased the surface area and stabilized the anatase phase of titania. The best catalytic efficiency for the salicylic acid photodecomposition was obtained for the aerogel with 9.39% zirconia that exhibited an apparent rate constant of about three times higher than that of the pure titania aerogel.

430. Nanostructured thin layers of vanadium oxides doped with cobalt, prepared by pulsed laser ablation: structure, chemistry, morphology, and magnetism

Authors: Teodorescu, CM; Socol, G; Negrila, C; Luca, D; Macovei, D

Published: 2008, NSTI NANOTECH 2008, VOL 1, TECHNICAL PROCEEDINGS: MATERIALS, FABRICATION, PARTICLES, AND CHARACTERIZATION, 438, DOI:

Cobalt-doped vanadium oxide thin layers prepared by pulsed laser ablation are investigated by (i) X-ray photoelectron spectroscopy, (ii) the local atomic order by X-ray absorption near-edge structure (XANES), (iii) the morphology of the films was investigated by atomic force microscopy (AFM), and (iv) magnetic properties were quantified by magneto-optical Kerr effect (MOKE). In most cases, the chemical composition of the host matrix was found to be the vanadium (5+) oxide V2O5 at the sample surface and lower ionization states (+2 and +3) in the bulk. Co ions are found either in high ionization state Co5+ (for samples synthesized in high vacuum condition, denoted by VO1), or with Co in lower ionization states Co4+ (for samples synthesized in a mixture of argon and oxygen atmosphere, denoted by VO2). Consistent information was obtained from chemical shifts from individual core level scans in XPS, compared with existent data in litterature, and the amplitude of the pre-edge peak in XANES, which is a sign of quadrupole 1s -> 3d dipole-forbidden transition and whose amplitude is proportional to the number of 3d vacancies per atom. AFM revealed big particles with sizes > 100 mu m for VO1 samples, whereas smaller nanoparticles with sizes ranging between 20 and 30 mu m were observed for VO2 samples. VO1 samples presented very high coercitive fields with a relatively low saturation magnetisation at room temperature, whereas VO2 samples presented double-loop hysteresis curves, indicating the coexistence of exchange bias between two kinds of magnetic moieties with strong anisotropy.