National Institute Of Materials Physics - Romania

Publications

860 articles found

631. Nanostructured gold layers. I. Deposition by Vacuum evaporation at small angles of incidence

Authors: Beica, T; Frunza, S; Zgura, I; Frunza, L; Cotarlan, C; Negrila, C; Vlaicu, AM; Zaharia, CN

Published: FEB 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 353, DOI:

Gold layers were deposited at oblique incidence onto glass plates. A B30.2 Hochvakuum Bedampfungsanlage equipment was used in this aim. The deposition conditions were chosen in order to obtain nanostructured layers. Correlations were found among these conditions and the corresponding nomograms were then obtained. Examples of such depositions were given and the layers structure was characterized and additionally supported by observing the molecular alignment given in the liquid crystal cells obtained with these gold layers.

632. The influence of shape and potential barrier on confinement energy levels in quantum dots

Authors: Lepadatu, AM; Stavarache, I; Ciurea, ML; Iancu, V

Published: FEB 2010, JOURNAL OF APPLIED PHYSICS, 107, DOI: 10.1063/1.3284083

The influence of the shape of silicon quantum dots embedded in an amorphous silica matrix on the quantum confinement energy levels, as well as that of the Si/SiO2 potential barrier, are studied. The energy levels are computed using both the infinite and finite rectangular quantum well models for spherical quantum dots and the infinite rectangular quantum well for prolate spheroidal quantum dots. The results are compared with each other and also with the experimental activation energies obtained from the temperature dependence of the dark current. These activation energies are identified with the differences between the quantum confinement energies, subject to the selection rules. The finite rectangular quantum well model takes into account the experimental value of the finite potential barrier and the matrix-to-dot electron mass ratio. The energy levels are smaller than those for the infinite rectangular quantum well case; they decrease when the potential barrier decreases and the mass ratio increases. Different aspects of the models are discussed. All the errors are less than about 4%. The spheroidal shape lifts the degeneracy on the magnetic quantum number. The energy levels can decrease or increase with eccentricity as a consequence of the different quantum confinement effects along the major and minor axes. The supplementary information on the magnetic quantum number is beneficial for optical applications.

633. Organic Photovoltaic Cells Based on ZnO Thin Film Electrodes

Authors: Ghica, C; Ion, L; Epurescu, G; Nistor, L; Antohe, S; Dinescu, M

Published: FEB 2010, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 10, 1326, DOI: 10.1166/jnn.2010.1836

Due to its wide band-gap (ca. 3.4 eV), ZnO is a possible candidate material to be used as transparent electrode for a new class of photovoltaic (PV) cells. Also, an increased interest for the photovoltaic properties of several organic monomers and polymers (merocyanines, phthalocyanines and porphyrins) was noticed, because of their high optical absorption in the visible region of the spectrum allowing them to be used as potential inexpensive materials for solar cells. Preparation and properties of CuPc (copper phthalocyanine) based photovoltaic cells using ZnO thin films as transparent conductor electrodes are presented in this paper. ZnO layers are grown by pulsed laser deposition, while the organic layers are obtained by thermal evaporation. Structural characterization is performed by electron microscopy. Optical and transport properties of the mutilayered structures are obtained by electrical and spectro-photometric measurements. The influence of the ZnO-polymer interface on the external quantum efficiency (EQE) of the photovoltaic cell is clearly evidenced by our measurements.

634. Incorporation and localization of substitutional Mn2+ ions in cubic ZnS quantum dots

Authors: Nistor, SV; Stefan, M; Nistor, LC; Goovaerts, E; Van Tendeloo, G

Published: JAN 15 2010, PHYSICAL REVIEW B, 81, DOI: 10.1103/PhysRevB.81.035336

Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dots-QDs) doped with 0.2% mol Mn2+, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional Mn2+ ions are localized at Zn2+ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter vertical bar D vertical bar = 41 x 10(-4) cm(-1). The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional Mn2+ impurity ions localization model. Based on these results it is suggested that the high doping levels of Mn2+ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

635. STUDY OF THE INTERACTIONS OF IONS IN SILICON: TRANSIENT PROCESSES AND DEFECT PRODUCTION

Authors: Lazanu, S; Lazanu, I; Iordache, G; Stavarache, I; Lepadatu, A; Slav, A

Published: 2010, 2010 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 332, DOI:

The thermal spike model which takes into account both ionization and nuclear energy loss processes of the projectile as distinct electronic and atomic heat sources is used to describe transient processes induced by ions in silicon. The time and space dependencies of the lattice and electron temperatures near the projectile trajectory are calculated. The contribution of the rise in temperature on defect formation and annealing is considered.

636. INFLUENCE OF THE DEPOSITION CONDITIONS ON THE PROPERTIES OF TiO2-Ge NANOCOMPOSITE FILMS SYNTHESIZED BY MAGNETRON CO-SPUTTERING

Authors: Slav, A; Stan, GE; Galca, AC

Published: 2010, 2010 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 340, DOI:

A growing need for eco-friendly energy sources have led recently to a frantic search of new compositional systems for photovoltaic applications. The nanocomposite titania-germanium (TiO2-Ge) systems represent a new viable family of optoelectronic materials. Their structural, optical and electronic properties can be easily tailored by customizing the density and size of Ge dots in the TiO2 matrix. Early studies on TiO2-Ge nanocomposites have shown promises for their use as an alternative in photovoltaic applications. In this study we report the TiO2-Ge films synthesis by reactive magnetron co-sputtering. Their properties were evaluated by compositional (EDS), structural (XRD, FTIR) and optical (UV-Vis) characterizations.

637. Direct Production of a Novel Iron-Based Nanocomposite from the Laser Pyrolysis of Fe(CO)(5)/MMA Mixtures: Structural and Sensing Properties

Authors: Alexandrescu, R; Morjan, I; Tomescu, A; Simion, CE; Scarisoreanu, M; Birjega, R; Fleaca, C; Gavrila, L; Soare, I; Dumitrache, F; Prodan, G

Published: 2010, JOURNAL OF NANOMATERIALS, 2010, DOI: 10.1155/2010/324532

Iron/iron oxide-based nanocomposites were prepared by IR laser sensitized pyrolysis of Fe(CO)(5) and methylmethacrylate (MMA) mixtures. The morphology of nanopowder analyzed by TEM indicated that mainly core-shell structures were obtained. X-ray diffraction techniques evidence the cores as formedmainly by iron/iron oxide crystalline phases. A partially degraded (carbonized) polymeric matrix is suggested for the coverage of the metallic particles. The nanocomposite structure at the variation of the laser density and of the MMA flow was studied. The new materials prepared as thick films were tested for their potential for acting as gas sensors. The temporal variation of the electrical resistance in presence of NO2, CO, and CO2, in dry and humid air was recorded. Preliminary results show that the samples obtained at higher laser power density exhibit rather high sensitivity towards NO2 detection and NO2 selectivity relatively to CO and CO2. An optimum working temperature of 200 degrees C was found.

638. Effects of layer by layer deposition on the structural and optical characteristics of thin films

Authors: Prepelita, P; Medianu, R; Garoi, F; Moldovan, A; Vlaicu, AM

Published: 2010, ROMOPTO 2009: NINTH CONFERENCE ON OPTICS: MICRO- TO NANOPHOTONICS II, 7469, DOI: 10.1117/12.859695

CuIn1-xGaxS2 (CIGS2) thin-films for solar cells were prepared by rf-magnetron sputtering and were deposited on glass substrate. These films were prepared using a stepwise process consisting of succesive deposition of CuInGa (d = 1500 nm) and ZnS (d = 200 nm) layers. Each layer was structurally characterized by X-ray diffraction and atomic force microscopy. The microstructural and optical properties of CIGS2 component films of the solar cell in comparison with those of ZnS and CuInGa films separately deposited onto glass substrates under the same conditions, were studied. Transmission spectra of our thin films are strongly influenced by deposition conditions and nature of the support material and they were recorded for each component film and CIGS2 solar cell.

639. DIFFERENCES IN THE GAS SENSING PROPERTIES READOUT WITH N AND P-TYPE MOX MATERIALS

Authors: Simion, CE; Tomescu-Stanoiu, A

Published: 2010, 2010 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), VOLS 1 AND 2, 204, DOI:

The way in which surface changes are transduced into a variation of an electrical parameter (often electrical resistance) depends on the surface oxidation level, material morphology its semiconductor behaviour, etc. Therefore, if one wants to understand more about the way in which gas-surface interactions affect the gas sensing properties of material, complex phenomenological and spectroscopic investigations are needed. Herein, by simultaneous DC and relative work function investigations we could explain the differences induced by the MOX semiconductor character (n and p-type) to the surface phenomena transduction mechanism.

640. IMPROVING THE CUBIC ZnS NANOCRYSTALS QUALITY BY SELF-ASSEMBLING INTO A MESOPOROUS STRUCTURE

Authors: Nistor, SV; Nistor, LC; Stefan, M; Ghica, D; Mateescu, CD; Birjega, R

Published: 2010, ROMANIAN REPORTS IN PHYSICS, 62, 328, DOI:

Nanocrystals of cubic ZnS (cZnS) doped with 02 % mol Mn2+, self-assembled into a mesoporous structure, have been prepared at room temperature by a surfactant-assisted liquid-liquid reaction The X-ray diffraction measurements confirm the formation of a sponge-like mesoporous structure built from ZnS nanocrystals with cubic (sphalerite) structure and pores of similar diameter of (1 8 +/- 0 2) nm. The Transmission Electron Microscopy (TEM) images show that the mesoporous structure consists of nanocrystals of cZnS with a tight size distribution centered around the average diameter value (21 +/- 0 3) nm The analysis of the observed Election Paramagnetic Resonance (EPR) spectrum demonstrates the presence of the Mn2+ activating ions at isolated sites in the mesoporous material, resulting in three types of paramagnetic centers called Mn2+(I), Mn2+(II) and Mn2+(III) The EPR spectrum of the Mn2+(I) center, attributed to substitutional Mn2+ ions at Zn2+ cation sites in the ZnS nanocrystals, exhibits the smallest linewidth values reported so far, reflecting an increased lattice ordering The high quality of the nanocrystals forming the mesoporous cZnS Mn, as reflected in a tight nanocrystallites size distribution and reduced crystallites lattice disorder, is attributed to the restraining effect of the self-assembling