National Institute Of Materials Physics - Romania

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5781 articles found

3951. The SANS facility at the Pitesti 14MW TRIGA reactor

Authors: Ionita, I; Grabcev, B; Todireanu, S; Constantin, F; Shvetsov, V; Anghel, E; Popescu, G; Mincu, M; Datcu, A

Published: 2006, CRYSTALLOGRAPHY REPORTS, 51, S31, DOI: 10.1134/S1063774506070066

The SANS facility existing at the Pitesti 14MW TRIGA reactor is presented. The main characteristics and the preliminary evaluation of the installation performances are given. A monochromatic neutron beam with 1.5 angstrom <= lambda <= 5 angstrom is produced by a mechanical velocity selector with helical slots. A fruitful partnership was established between INR Pitesti (Romania) and JINR Dubna (Russia). The first step in this cooperation consists in the manufacturing in Dubna of a battery of gas-filled positional detectors devoted to the SANS instrument.

3952. Mesoscopic Fano effect in Aharonov-Bohm rings with an embedded double dot

Authors: Tanatar, B; Moldoveanu, V; Tolea, M; Aldea, A

Published: 2006, LOW TEMPERATURE PHYSICS, PTS A AND B, 850, +, DOI:

We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Buttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano, lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano, dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.

3953. Newly developed Fe-Fe2O3/polyoxocarbosilane core-shell nanocomposite prepared by laser pyrolysis: Characterization and sensing properties

Authors: Morjan, I; Pola, J; Alexandrescu, R; Dumitrache, F; Tomescu, A; Birjega, R; Gavrila-Florescu, L; Soare, I; Voicu, I; Galikova, A; Ciupina, V; Bastl, Z

Published: 2006, NANOSTRUCTURED MATERIALS AND HYBRID COMPOSITES FOR GAS SENSORS AND BIOMEDICAL APPLICATIONS, 915, +, DOI:

Nanocomposites of iron /iron oxide/polyoxocarbosilane were prepared by the IR co-pyrolysis of Fe(CO)(5) and HMDSO. Their morphologies, chemical content and thermal behavior were studied by different analytical techniques. For examining the sensing capabilities of the low-polymer content nanocomposite thick films, the variation of the electrical resistance to CO and CH4 gases was tested. A promising behavior as concerning the sensitivity and the selectivity was found.

3954. Fano interference in mesoscopic rings with quantum dots

Authors: Moldoveanu, V; Aldea, A; Tolea, M

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 2, 3, +, DOI: 10.1002/pssc.200562736

We propose a general formalism for describing the coexistence of coherent and resonant transport in hybrid mesoscopic structures. The approach is based on Landauer- Buttiker formula for the electronic transmittance and on an old formula of Feshbach. The latter gives the complete Green function of coupled subsystems in terms of effective Green functions of the disconnected parts and provides informations about the individual contributions of each subsystem to transport. Motivated by the experiments of Kobayashi et al.(Phys. Rev. Lett. 88, 256806 (2002)) and Holleitner et al. (Phys. Rev. Lett. 87, 256802 (2001)) we apply the formalism to study transport in Aharonov-Bohm interferometers (ABI) containing one or two coupled two-dimensional quantum dots (QD). In the single dot case, we reproduce and explain the magnetic field control of the Fano interference and investigate the interaction effects in a self-consistent approach. In the double dot case, we obtain the charging diagrams and establish precise criteria for the observation of mesoscopic Fano effect and AB oscillations. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3955. Effect of dopant on the physical properties of polymer films for microphotonics

Authors: Obreja, P; Cristea, D; Budianu, E; Rebigan, R; Kuncser, V; Bulinski, M; Filoti, G

Published: 2006, PROGRESS IN SOLID STATE CHEMISTRY, 34, 109, DOI: 10.1016/j.progsolidstchem.2005.11.051

This paper presents experimental results concerning the influence of the additives on the physical properties of polymer films. Metal oxides/inorganic salts were used as additives in vinyl-polymers solutions. The physical properties of the metal doped polymer films can be significantly modified by the composition of the doping elements and the curing conditions of the polymer. Morphologic, electronic, magnetic and optic properties of the doped polymers were analysed by AFM. SEM. Mossbauer spectroscopy and optical measurements. The film composition and the deposition processes were optimized to allow a better control of the optical parameters (refractive index. transmission). to reduce the processing temperatures and to improve the chemical sensitivity of the films for sensor applications. These compounds can be easily spin coated onto a variety of directly patterned semiconductor substrates. (C) 2005 Elsevier Ltd. All rights reserved.

3956. Optical properties of 3,4,9,10-perylenetetracarboxylic dianhydride and 8-hydroxyquinoline aluminum salt films prepared by vacuum deposition

Authors: Stanculescu, A; Stanculescu, F; Tugulea, L; Socol, M

Published: 2006, ADVANCED MATERIALS FORUM III, PTS 1 AND 2, 514-516, 960, DOI: 10.4028/www.scientific.net/MSF.514-516.956

The main purpose of this paper is to investigate the optical properties of PTCDA and Alq3 films, prepared by two steps, vacuum evaporation and deposition processes on platelets of glass, quartz, and indium-tin-oxide (ITO) coated glass. We have emphasised the bands structure of the absorption spectra with peaks situated at 358 nm, 374 nm, 475 nm and 552 nm in PTCDA, respectively 232 nm, 261 nm and 380 nm in Alq3 that confirms the dominant presence of Alq3 meridianal molecular isomer. For PTCDA films deposited on glass coated with ITO, the structure of the weak double peak at low wavelength is partially modified, but the positions of the two important absorption peaks situated at 2.25 eV and 2.61 eV are unchanged. The two different luminescence emission peaks obtained in Alq3 for different excitation wavelengths (gimel=360 nm and gimel=520 nm) suggest the existence of the facial isomer beside the meridianal one. We have evidenced a significant Stocks shift in the spectra (Delta E-PTCDA=0.40 eV; Delta E-Alq3=0.9 eV) and a large Frank-Condon shift (0.40-2.3 eV), suggesting important effect associated respectively with the solid state structure and important conformational differences between the ground and excited state.

3957. Spectroscopic studies on nanocomposites obtained by functionalization of carbon nanotubes with conducting polymers

Authors: Lefrant, S; Baibarac, M; Baltog, I

Published: 2006, MOLECULAR CRYSTALS AND LIQUID CRYSTALS, 447, 403, DOI: 10.1080/15421400500385332

Vibrational properties of composites based on single-walled carbon nanotubes (SWNTs) and conducting polymers of the type polyaniline (PANI) and poly (3,4-ethylene dioxythiophene) (PEDOT) are reported. For PANI-functionalized SWNTs, the intensity increase of the Raman band at 178 cm(-1) , associated with radial breathing modes of SWNTs bundles, indicates an additional roping of nanotubes due to the presence of the polymer. The interaction of this composite with NH4OH solution involves an internal redox reaction between PANI and SWNTs. Thus, the polymer chain undergoes a transition from the semi-oxidized state into a reduced one. The functionalization of SWNT side walls with PEDOT is invoked as well.

3958. Quantum dots and nanowires grown by metal-organic chemical vapor deposition for optoelectronic device applications

Authors: Tan, HH; Sears, K; Mokkapati, S; Fu, L; Kim, Y; McGowan, P; Buda, M; Jagadish, C

Published: NOV-DEC 2006, IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 12, 1254, DOI: 10.1109/JSTQE.2006.882663

This paper discusses the growth of In(Ga)As quantum dots (QDs) and nanowires by metal-organic chemical vapor deposition and their application to optoelectronic devices. The performance characteristics of QD lasers and QD photodetectors as well as the selective area growth of QDs for integrated devices are reported.

3959. Low temperature CL investigation of BN1 vibronic structure in c-BN

Authors: Cossio, R; Fizzotti, F; Vittone, E; Lo Giudice, A; Manfredotti, C; Nistor, S

Published: APR-AUG 2006, DIAMOND AND RELATED MATERIALS, 15, 1168, DOI: 10.1016/j.diamond.2005.11.048

A full vibronic spectrum has been measured for the first time by LNT Cathodoluminescence in HPHT c-BN amber-coloured microcrystalline samples. The related BN1 centre at 3.293 eV seems not created by electron irradiation in this case and the accurately determined phonon energy (141 +/- 3 meV) could be related to LO phonon at X point in Brillouin Zone (BZ) of c-BN, as determined by CL results obtained at indirect gap. Consequently, BN1 centre has not the full cubic symmetry of c-BN, as being due to N interstitials. The interpretation of the results is possibly twofold: either we are observing a vibronic spectrum with ZPL at BN1 centre, together with other two centres called PF-1 and PF-2 at 3.573 and at 3.412 eV, respectively, or, since all the 6 or 7 observed peaks are exactly equally spaced, the whole vibronic spectrum is related to a ZPL line at 3.573 eV. This new interpretation seems to be in better agreement with the general theory of colour centres. (c) 2005 Elsevier B.V. All rights reserved.

3960. Quantum kinetic theory of phonon-assisted carrier transitions in nitride based quantum-dot systems

Authors: Seebeck, J; Nielsen, TR; Lorke, M; Gartner, P; Jahnke, F

Published: 2006, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 3, NO 6, 3, 2033, DOI: 10.1002/pssc.200565227

A microscopic theory for the interaction of carriers with LO phonons is used to study ultrafast capture and relaxation dynamics in semiconductor quantum dots. Energy renormalization due to carrier-phonon interaction leads to quantum-dot polarons which are included in quantum-kinetic equations to describe scattering processes. Our results show efficient capture and relaxation processes. For an InGaN quantum-dot system the influence of the strong built-in electrostatic field on the carrier dynamics is investigated. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.