National Institute Of Materials Physics - Romania

Publications

5781 articles found

2661. Polymer-assisted crystallization of low-dimensional lead sulfide particles

Authors: Preda, N; Rusen, E; Enculescu, M; Matei, E; Marculescu, B; Enculescu, I

Published: AUG 2011, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 43, 1832, DOI: 10.1016/j.physe.2011.06.019

PbS micro- and nanoparticles were synthesized by a simple precipitation reaction of lead nitrate with thioacetamide in hydrosoluble polymer water solutions. The effects of four water soluble polymers: polyacrylamide (PAM), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and poly-N-vinyl pyrrolidone (PVP) on the PbS crystallites morphology and structural properties were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that for the PbS particles obtained in the PVA. PEG and PVP, the (2 0 0) diffraction peak of the nanocrystals becomes dominant. The highest texture in the [2 0 0] direction was observed for the crystallites obtained in the presence of PVP. Polydisperse PbS particles with cubic morphology and size ranging from 100 nm to several microns are obtained in the case of PAM and PEG. Monodisperse cubic PbS crystallites with an average size of 200 nm are formed in the presence of PVA and PVP. (C) 2011 Elsevier B.V. All rights reserved.

2662. Optical spectra of nitride quantum-dot systems: From tight-binding states to many-body effects

Authors: Seebeck, J; Lorke, M; Schulz, S; Schuh, K; Gartner, P; Jahnke, F

Published: AUG 2011, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 248, 1878, DOI: 10.1002/pssb.201147160

A microscopic theory for optical properties of self-assembled quantum-dot systems is presented. For nitride-based material systems, the single-particle states are determined from atomistic tight-binding (TB) calculations. This includes not only localized quantum-dot states but also delocalized wetting-layer states, since both contribute to the optical properties and are coupled via interaction processes. The TB wave functions are used to calculate matrix elements for optical dipole transitions as well as Coulomb and LO-phonon interaction of carriers. Absorption and gain spectra are investigated via a solution of the semiconductor Bloch equations including Coulomb interaction effects as well as dephasing due to carrier interaction with LO-phonons. Numerical results are presented for InN/GaN QD-WL systems and allow to investigate the excitation-density dependence of the optical spectra for this particular system. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2663. Spectroscopic Analysis of the Interstitial Anions in Some Layered Double Hydroxide Materials

Authors: Frunza, L; Gheorghe, N; Iova, F; Ganea, P; Neatu, F; Parvulescu, VI

Published: AUG 2011, REVISTA DE CHIMIE, 62, 772, DOI:

Layered double hydroxide materials with Zn2+ and Mg2+ as bivalent cations and Al3+ and Ga3+ as trivalent cations were studied. The anions to balance the positive charge of the network were carbonate and/or nitrate. The samples were prepared in laboratory and routinely characterized. Here the investigations by Fourier transform infrared spectra and thermogravimetric measurements are detailed. On the basis of calibration curves for carbonate and nitrate ions obtained from IR spectra of the corresponding salts, the sample analysis of the interlayer anions was performed quantitatively.

2664. Modification of AlN thin films morphology and structure by temporally shaping of fs laser pulses used for deposition

Authors: Ristoscu, C; Ghica, C; Papadopoulou, EL; Socol, G; Gray, D; Mironov, B; Mihailescu, IN; Fotakis, C

Published: JUL 29 2011, THIN SOLID FILMS, 519, 6387, DOI: 10.1016/j.tsf.2011.04.065

We studied the effect of temporally pulse shaping upon the properties of thin layers synthesized by pulsed laser deposition with fs laser pulses generated by a Ti-sapphire laser source. We showed that the film morphology and structure can be gradually modified when applying mono-pulses of different duration or passing to a sequence of two pulses of different intensities. (C) 2011 Elsevier B.V. All rights reserved.

2665. The study of the electric and magnetic properties of PbZr0.2Ti0.8O3-BiFeO3 multilayers

Authors: Stancu, V; Dragoi, C; Kuncser, V; Schinteie, G; Trupina, L; Vasile, E; Pintilie, L

Published: JUL 29 2011, THIN SOLID FILMS, 519, 6277, DOI: 10.1016/j.tsf.2011.03.136

The results of the electric and magnetic measurements performed on PbZr0.2Ti0.8O3-BiFeO3 symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO3 films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments. (C) 2011 Elsevier B.V. All rights reserved.

2666. On the cotunneling regime of interacting quantum dots

Authors: Cornean, HD; Moldoveanu, V

Published: JUL 29 2011, JOURNAL OF PHYSICS A-MATHEMATICAL AND THEORETICAL, 44, DOI: 10.1088/1751-8113/44/30/305002

Consider a bunch of interacting electrons confined in a quantum dot. The later is suddenly coupled to semi-infinite biased leads at an initial instant t = 0. We identify the dominant contribution to the ergodic current in the off-resonant transport regime, in which the discrete spectrum of the quantum dot is well separated from the absolutely continuous spectrum of the leads. Our approach allows for arbitrary strength of the electron-electron interaction while the current is expanded in even powers of the (weak) lead-dot hopping constant tau. We provide explicit calculations for sequential tunneling and cotunneling contributions to the current. In the interacting case, it turns out that the cotunneling current depends on the initial many-body configuration of the sample, while in the non-interacting case, it does not and coincides with the first term in the expansion of the Landauer formula w.r.t. tau.

2667. Annealing of hydrogen-induced defects in RF-plasma-treated Si wafers: ex situ and in situ transmission electron microscopy studies

Authors: Ghica, C; Nistor, LC; Vizireanu, S; Dinescu, G

Published: JUL 27 2011, JOURNAL OF PHYSICS D-APPLIED PHYSICS, 44, DOI: 10.1088/0022-3727/44/29/295401

The smart-cut (TM) process is based on inducing and processing structural defects below the free surface of semiconductor wafers. The necessary defects are currently induced by implantation of light elements such as hydrogen or helium. An alternative softer way to induce shallow subsurface defects is by RF-plasma hydrogenation. To facilitate the smart-cut process, the wafers containing the induced defects need to be subjected to an appropriate thermal treatment. In our experiments, (0 0 1) Si wafers are submitted to 200 and 50 W hydrogen RF-plasma and are subsequently annealed. The samples are studied by transmission electron microscopy (TEM), before and after annealing. The plasma-introduced defects are {1 1 1} and {1 0 0} planar-like defects and nanocavities, all of them involving hydrogen. Many nanocavities are aligned into strings almost parallel to the wafer surface. The annealing is performed either by furnace thermal treatment at 550 degrees C, or by in situ heating in the electron microscope at 450, 650 and 800 degrees C during the TEM observations. The TEM microstructural studies indicate a partial healing of the planar defects and a size increase of the nanometric cavities by a coalescence process of the small neighbouring nanocavities. By annealing, the lined up nanometric voids forming chains in the as-hydrogenated sample coalesced into well-defined cracks, mostly parallel to the wafer surface.

2668. Carbon layers cleaning from inside of narrow gaps by a RF glow discharge

Authors: Stancu, C; Teodorescu, M; Galca, AC; Dinescu, G

Published: JUL 25 2011, SURFACE & COATINGS TECHNOLOGY, 205, S438, DOI: 10.1016/j.surfcoat.2011.03.090

The paper focuses on the utilization of radiofrequency discharges at intermediate pressures for plasma removal of carbon residuals or layers from narrow gaps and channels. It is shown that by proper handling of the discharge power and pressure it is possible to control the development of plasma inside narrow gaps, in the proximity of surfaces to be cleaned. The discharge operation inside gaps having the width in the range 0.6-2 mm was demonstrated. The cleaning effectiveness is exemplified for castellated surfaces having rectangular gaps coated with hydrogenated amorphous carbon layers. Cleaning of large surfaces can be approached with a movable "plasma shower" type device. (C) 2011 Elsevier B.V. All rights reserved.

2669. (Ti,Cr,Nb)CN coatings deposited on nitrided high-speed steel by cathodic arc method

Authors: Braic, M; Braic, V; Balaceanu, M; Vladescu, A; Zoita, CN; Lungu, CP; Grigorescu, CEA; Grigore, E; Logoftu, C

Published: JUL 25 2011, SURFACE & COATINGS TECHNOLOGY, 205, S213, DOI: 10.1016/j.surfcoat.2011.03.030

The combined processes of plasma nitriding and cathodic arc deposition of (Ti,Cr,Nb)CN coatings were applied to HSS substrates. The nitrided layers, obtained in a mixture of H-2 (70%) and N-2 (30%) at two different temperatures (480 degrees C and 510 degrees C), were examined for the microhardness depth profiles. Characterization of the duplex coatings was performed by investigating elemental and phase composition, texture, hardness, friction and wear. XRD and XPS analyses revealed the formation of a mixture of a carbonitride fcc solid solution, in a dominant proportion, and metallic chromium. The film hardness was measured to be similar to 34 GPa. The duplex (Ti,Cr,Nb)CN coatings exhibited superior tribological behavior as compared to both nitrided layers and non-duplex coatings. (C) 2011 Elsevier B.V. All rights reserved.

2670. New structural features of non-crystalline tetrahedrally bonded networks A modeling approach

Authors: Sava, F; Popescu, M

Published: JUL 1 2011, JOURNAL OF NON-CRYSTALLINE SOLIDS, 357, 2554, DOI: 10.1016/j.jnoncrysol.2011.02.056

A large model of amorphous silicon (2052 atoms) with 0.5% dangling bonds has been built and investigated. The refinement of the coordinates evidenced the presence of small domains with advanced ordering. These domains preclude the formation of crystallization nuclei and play an essential role in the redistribution of the defects in the material with homogenization of the free energy and stabilization against aging. An effect of amorphization of the ordered nuclei due to free energy redistribution is assessed. The glass relaxation of a-Si (a-Ge) occurring during heating below T(g) receives a natural explanation as a structural change from local quasi-ordering to homogeneous disordering. (C) 2011 Elsevier B.V. All rights reserved.