National Institute Of Materials Physics - Romania

Publications

5714 articles found

3111. Evanescent channels and scattering in cylindrical nanowire heterostructures

Authors: Racec, PN; Racec, ER; Neidhardt, H

Published: APR 2009, PHYSICAL REVIEW B, 79, DOI: 10.1103/PhysRevB.79.155305

We investigate the scattering phenomena produced by a general finite-range nonseparable potential in a multichannel two-probe cylindrical nanowire heterostructure. The multichannel current scattering matrix is efficiently computed using the R-matrix formalism extended for cylindrical coordinates. Considering the contribution of the evanescent channels to the scattering matrix, we are able to put in evidence the specific dips in the tunneling coefficient in the case of an attractive potential. The cylindrical symmetry cancels the "selection rules" known for Cartesian coordinates. If the attractive potential is superposed over a nonuniform potential along the nanowire then resonant transmission peaks appear. We can characterize them quantitatively through the poles of the current scattering matrix. Detailed maps of the localization probability density sustain the physical interpretation of the resonances (dips and peaks). Our formalism is applied to a variety of model systems such as a quantum dot, a core/shell quantum ring, or a double barrier embedded into the nanocylinder.

3112. Critical current density and pinning potential in YBa2Cu3O7-delta thick films ablated from a BaZrO3-doped nanocrystalline target

Authors: Crisan, A; Kechik, MMA; Mikheenko, P; Dang, VS; Sarkar, A; Abell, JS; Paturi, P; Huhtinen, H

Published: APR 2009, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 22, DOI: 10.1088/0953-2048/22/4/045014

We report on the pulsed laser deposition and characterization of thick (1.27 mu m) films ablated from a nanocrystalline YBa2Cu3O7-delta (YBCO) target doped with 4 wt% BaZrO3 (BZO), nYBCO/BZO. DC magnetization measurements have shown a four-to five-fold increase in the critical current density J(c) and a three- to four-fold increase in the maximum pinning force in comparison with 1 mu m thick films grown from a commercial microcrystalline YBCO target, at both 77.3 and 65 K. We have found that the frequency-dependent critical current densities J(c)(f(0)/f) estimated from susceptibility measurements are linear in a double-logarithmic plot ( where f is the frequency of the AC field and f(0) is a macroscopic attempt frequency of about 10(6) Hz), consistent with a logarithmic dependence of the pinning potential on the current density, and are again several times higher in nYBCO/BZO thick films. From the slopes of the J(c)(f(0)/f) dependences we have also estimated the values of pinning potentials in fields between 3 and 5 T.

3113. Charge carrier transport phenomena in some organic heterojunctions

Authors: Rasoga, OL; Socol, M; Stanculescu, F

Published: APR 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 514, DOI:

This paper presents investigations on the electrical transport phenomena in sandwich type inorganic/single (double) organic layer/inorganic such as silicon; ITO/TPyP, Alq3, alpha-NPD; ZnPc; perylene" PTCDA/silicon and inorganic (metal)/organic/metal such as silicon (A/)/ZnPc; alpha-NPD; TPyP/copper, Al, structures, prepared by vacuum evaporation. For most of these heterostructures the IN characteristics in the static regime have a near ohmic behaviour only at low voltages (<1 V) that changes in a power dependence with a coefficient n=2 or n=3 at higher voltages because of the space charge or trapped charge phenomena. For the same applied voltage, the n type Si electrode injects more charge carriers in TpyP than in Alq3. For higher voltages (>10V) an increased number of charge carriers are injected from n type Si in Alq3 compared to TpyP. The injection properties of the interface ITO, Cu, Al electrode/organic layer have also been investigated. The best injection has been obtained through ITO/ZnPc interface. For a grid configuration of the Al electrodes the transport phenomena are determined by the resistivity of the structure.

3114. Photoluminescence and thermoluminescence of ZnO nano-needle arrays and films

Authors: Secu, CE; Sima, M

Published: APR 2009, OPTICAL MATERIALS, 31, 880, DOI: 10.1016/j.optmat.2008.10.025

ZnO nano-needle arrays and films have been prepared by anodic etching of ZnO foil at constant current in potassium zincate solution and sodium hydroxide alcoholic solution, respectively. Photoluminescence (PL) spectra recorded on ZnO nano-needles have shown a broad band at 585 nm due to the native oxygen defects incorporated during the synthesis processes; in a ZnO film-type sample this band is replaced by another one at 432 nm. Thermoluminescence (TL) glow curves recorded on ZnO nano-needle arrays have shown a broad peak at about 360 degrees C similar to that observed in ZnO polycrystalline powder. A very strong TL peak at about 325 degrees C superimposed on a broad one has been observed in a ZnO film-type sample. TL recorded on ZnO nano-needles has tentatively been attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centres; the TL mechanism is supported by a recovery effect of the PL and TL after storage in air. (C) 2008 Elsevier B.V. All rights reserved.

3115. XPS analysis of n-GaP(111) native and etched surfaces

Authors: Cotirlan, C; Logofatu, C; Negrila, CC; Ghita, RV; Manea, AS; Lazarescu, MF

Published: APR 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 390, DOI:

The obtaining of a good ohmic contacts on the semiconductor material surfaces is strongly depending on the ability of the cleaning procedures to remove the native oxides and also, in the case of the compounds, to keep the surface stoichiometry. In this work, the XPS technique has been used in order to compare the efficiency of both, chemical etching and the Ar+ ion sputtering methods as to be suitable cleaning procedures in the case of n-GaP semiconductor compound surfaces preparation for the optoelectronic devices obtaining. The results have revealed some differences in the final surface composition/stoichiometry. The atomic ratio P/Ga within the outer layer of the single crystalline sample (similar to 100 angstrom) has been evaluated. The presence of residual of surface oxides was recorded after performing the chemical cleaning.

3116. Optical properties of pulsed-laser deposited ZnO thin films

Authors: Bazavan, R; Ion, L; Socol, G; Enculescu, I; Bazavan, D; Tazlaoanu, C; Lorinczi, A; Mihailescu, IN; Popescu, M; Antohe, S

Published: APR 2009, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 11, 428, DOI:

ZnO thin films were deposited by pulsed laser deposition (PLD) technique on optical glass substrate. Structural analysis by X-ray diffraction revealed a wurtzite phase ZnO, highly textured, with (002) planes perpendicular to the growth direction. They were optically characterized by absorption and transmission spectrometry. The wavelength dependence of the refractive index was extracted from transmission data by using a numerical fitting procedure.

3117. Total oxidation of toluene on ferrite-type catalysts

Authors: Florea, M; Alifanti, M; Parvulescu, VI; Mihaila-Tarabasanu, D; Diamandescu, L; Feder, M; Negrila, C; Frunza, L

Published: MAR 30 2009, CATALYSIS TODAY, 141, 366, DOI: 10.1016/j.cattod.2008.05.005

Ferrite catalysts were prepared following two routes, i.e. the hydrothermal one and the calcination of an oxide mixture. In the first route sodium hydroxide, ferrous sulfate and the sulfate of the substituting ion (Mn, Ni) were used. The ferrites obtained using this route was NixFe3-xO4 (x = 0.5) and MnxFe3-xO4 (x similar to 0.65). Following the second route were prepared three samples: NiFe2O4, Ni0.5Zn0.5Fe2O4 and MnFe2O4. Two of them contain the corresponding ferrites while the latter is a presintered oxide mixture (Fe2O3-Mn2O3). All the samples were fully characterized using chemical analysis, X-ray fluorescence spectroscopy and EDX, nitrogen adsorption-desorption isotherms at -196 degrees C, X-ray diffraction (XRD), FTIR, scanning electron microscopy and XPS. The catalytic activity evaluation was made using a mixture of 1700 ppm vol. of toluene and air flowing at 100 ml min(-1) raising the temperature up to 600 degrees C in steps of 25 degrees C. Among these systems the sample representing the presintered Fe2O3-Mn2O3 mixture showed the highest activity. (C) 2008 Elsevier B.V. All rights reserved.

3118. Synthesis and sensing properties of mixed oxide magnetic nanoparticles

Authors: Sorescu, M; Diamandescu, L; Tomescu, A; Krupa, S

Published: MAR 22 2009, ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, 237, DOI:

3119. MAPLE prepared polymeric thin films for non-linear optic applications

Authors: Socol, G; Mihailescu, IN; Albu, AM; Antohe, S; Stanculescu, F; Stanculescu, A; Mihut, L; Preda, N; Socol, M; Rasoga, O

Published: MAR 1 2009, APPLIED SURFACE SCIENCE, 255, 5614, DOI: 10.1016/j.apsusc.2008.07.206

This paper presents a study of some functionalised copolymers thin films prepared on silicon and quartz substrates by matrix assisted pulsed laser evaporation ( MAPLE) method. Two polymeric structures have been synthesised by the copolymerisation of maleic anhydride and methyl methacrylate, respectively, maleic anhydride and vinyl benzyl chloride and their subsequent functionalisation with 2,4-dinitroaniline. UV-Vis, FTIR, Raman and photoluminescence spectroscopy have been used to investigate the influence on the properties of the films of different substrate temperature ( 150 degrees C and 250 degrees C), background N-2 pressure (5-30 Pa) and polymer concentration in target (2% and 3%). We have evidenced that this deposition process does not damage the chemical structure of the polymers. SEM investigations revealed the droplets type morphology of the polymeric films with thickness between 41 nm and 105 nm calculated from ellipsometric measurements. (C) 2008 Elsevier B. V. All rights reserved.

3120. Radiofrequency plasma beam deposition of various forms of carbon based thin films and their characterization

Authors: Vizireanu, S; Stoica, SD; Mitu, B; Husanu, MA; Galca, A; Nistor, L; Dinescu, G

Published: MAR 1 2009, APPLIED SURFACE SCIENCE, 255, 5381, DOI: 10.1016/j.apsusc.2008.08.042

The characteristics of carbonic materials obtained by downstream deposition in a low pressure argon plasma beam injected with acetylene are reported. The influence of substrate temperature, presence of Ni catalyst and hydrogen in gas composition on the material properties is described. By increasing the substrate temperature, an enhanced order in the material is revealed by Raman spectroscopy, while FTIR measurements show a decreasing of the hydrogen content and the disappearing of sp(1) hybridized carbon in the deposit. The SEM and Raman investigation show a clear tendency of crystalline phases formation when hydrogen is assisting the deposition. (C) 2008 Elsevier B. V. All rights reserved.