National Institute Of Materials Physics - Romania

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5714 articles found

2591. Structural characterization and photoluminescence of nanocrystalline Ho-doped BaTiO3 derived from sol-gel method

Authors: Secu, M; Cernea, M; Secu, CE; Vasile, BS

Published: AUG 2011, JOURNAL OF NANOPARTICLE RESEARCH, 13, 3128, DOI: 10.1007/s11051-011-0224-3

Nanocrystalline Ho-doped BaTiO3, with average nanocrystals size of 20 nm, have been prepared using a sol-gel combustion technique. The structural and morphological properties of the powders have been investigated by X-ray powder diffraction and high resolution transmission electron microscopy. Chemical states of the holmium on the Ba0.97Ho0.03TiO3 ceramic surface were analyzed using X-ray photoelectron spectroscopy. Furthermore, their photoluminescence properties were analyzed.

2592. Step-shape angular spin distribution in layered systems by Fe-57 Mossbauer spectroscopy: A general treatment

Authors: Kuncser, V; Keune, W

Published: AUG 2011, JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 323, 2201, DOI: 10.1016/j.jmmm.2011.03.030

In the so-called 'step-shape' angular spin distribution model for layered systems, the non-collinear directions of the atomic magnetic moments are confined to the film plane and form a homogeneous fan spanning inside an (in-plane) angular interval Delta phi centered at an angle phi(0). A general approach for deriving the two parameters phi(0) and Delta phi via Fe-57 Mossbauer spectroscopy measurements is discussed. The analysis extends our previously reported treatment, which assumed that the angular aperture Delta phi develops symmetrically versus a fixed direction phi(0) (e.g., the in-plane easy axis of magnetization) oriented either along or perpendicular to the in-plane projection of the Mossbauer gamma-ray direction. The proposed approach is also applicable for those cases when not only the spin aperture Delta phi is changing but also the aperture center phi(0) is rotating under the influence of different external parameters, such as applied field, temperature, stress, etc. The method is suitable for applications to nanoscale layered heterostructures with in-plane uniaxial or unidirectional magnetic anisotropy. The method is applied to experimental data obtained on a 2-nm thick defected Fe layer with in-plane magnetic texture. (C) 2011 Elsevier B.V. All rights reserved.

2593. Significantly different contamination of atomically clean Si(001) when investigated by XPS and AES

Authors: Gheorghe, NG; Lungu, GA; Costescu, RM; Teodorescu, CM

Published: AUG 2011, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 248, DOI: 10.1002/pssb.201147220

Clean Si(001) single crystal surfaces provided different surface reconstructions: p(1 x 2) and c(n x 2) (n = 4, 6) at room temperature. The in situ oxidation of these surfaces was followed by Auger electron spectroscopy and by X-ray photoelectron spectroscopy. It is found that, in similar ultrahigh vacuum conditions, much faster contamination (about 500 times) occurs when the samples are investigated by AES than by XPS, owing to the interaction of the electron beam with the sample surface. With the sample subject to the AES investigation, the contamination occurs by forming >Si2C=O complexes based on the Si dimers. During XPS, reaction with water molecules from the residual gas should also be taken into account.

2594. 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.

2595. 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

2596. 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.

2597. 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.

2598. 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.

2599. 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.

2600. 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.