National Institute Of Materials Physics - Romania

Publications

5781 articles found

3891. Effect of Auger recombination on the performance of p-doped quantum dot lasers

Authors: Mokkapati, S; Buda, M; Tan, HH; Jagadish, C

Published: APR 17 2006, APPLIED PHYSICS LETTERS, 88, DOI: 10.1063/1.2193433

Experimental results on spontaneous emission rates from InGaAs quantum dot lasers that can be explained theoretically by considering the influence of nonradiative mixed state recombinations in the quantum dot-wetting layer system are presented. Our model qualitatively explains the experimental results such as an increase in the threshold current density, temperature stability, and a narrower gain spectrum due to doping the quantum dot active region with the acceptors. Our model also predicts that moderate acceptor concentrations can improve the laser performance at higher carrier injection densities; but high acceptor concentrations deteriorate the laser performance due to the nonradiative Auger recombination that counteracts the benefits of increased spontaneous emission rates.

3892. Highly efficient, 0.84 slope efficiency, 901 nm, quasi-two-level laser emission of Nd in strontium lanthanum aluminate

Authors: Lupei, V; Aka, G; Vivien, D

Published: APR 15 2006, OPTICS LETTERS, 31, 1066, DOI: 10.1364/OL.31.001064

The possibility of using direct pumping into the emitting level of the Nd3+ ion in magnesium-compensated strontium lanthanum. aluminate (Sr1-xLax-yNdyMgxAl12-xO19) to improve 900 nm F-4(3/2) -> I-4(9/2) laser emission is discussed. Selection of the composition parameters x and y for optimization of laser emission and reduction of heat generation is based on the spectroscopic and crystal growth characteristics. Pumping in the 865.5 nm absorption band I-4(9/2)(Z(1)) F-4(3/2)(R-1) transforms the laser process into a quasi-two-level scheme of very low (below 4%) quantum defects. A very high slope efficiency (over 84%) for 901 nm continuous-wave laser emission is demonstrated with Ti:sapphire laser pumping in this band for a crystal with x = 0.4 and y = 0.05. (c) 2006 Optical Society of America.

3893. Synthesis and electron spin resonance of La2/3Ca1/3MnO3 nanowires

Authors: Toloman, D; Mihailescu, G; Darabont, A; Pop, CVL; Olenic, L; Popa, A; Raita, O; Jivanescu, M; Grecu, MN; Giurgiu, LM; Idziak, S; Hoffmann, SK

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 469, DOI:

La2/3Ca1/3MnO3 (LCMO) nanowires were prepared using a sol-gel process and an anodically oxidized aluminum template. Scanning electron microscopy (SEM) shows that the nanowires are almost parallel and their diameter is around 40 nm. The temperature dependence of the electron spin resonance (ESR) linewidth is similar to that observed for the case of nanostructured LCMO. From this dependence, the determined magnetic transition temperature, T-C, of the LCMO nanowires, is of the order of 120-130 K.

3894. FexOy-SiO2 nanocomposites studied by Mossbauer spectroscopy

Authors: Predoi, D; Kuncser, V; Zaharescu, M; Jitianu, A; Crisan, M; Keune, W; Sahoo, B; Filoti, G; Raileanu, M

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 522, DOI:

FexOy-SiO2 nanocomposites were obtained via the sol-gel method, by using three different precursors for the silica matrix. The morphology and the iron phase composition is strongly influenced by the precursors. The presence of magnetically ordered goethite and fayalite is evidenced by low temperature Mossbauer spectroscopy. Subsequent annealing treatments performed on the initial samples have induced the formation of hematite nanoparticles in the silica matrix, mainly on the account of the goethite decomposition. The total amount and the size distribution of the hematite nanoparticles can be controlled via the initial precursors and the subsequent annealing conditions. The simultaneous analysis of the iron phase composition and of the local magnetic interactions in the analysed samples is performed.

3895. Growth of ZnO micro and nanowires using the template method

Authors: Sima, M; Enculescu, I; Vasile, E

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 828, DOI:

The electrochemical deposition in ion track nanoporous membranes was employed for ZnO micro and nanowires preparation. A ZnNO3 bath was employed for the deposition, performed at -800 mV (vs. SCE) and at temperatures around 90 degrees C. Scanning and transmission electron microscopy were used to characterize the wires from a morphological point of view. Electron and X-ray diffraction were employed for the characterization of materials from the point of view of structure. The technique allowed the fabrication of nanowire arrays with good morphological and structural quality.

3896. Radiological impact of C-14

Authors: Bucur, C; Olteanu, M; Olteanu, C; Pavelescu, M

Published: APR 2006, REVISTA DE CHIMIE, 57, 367, DOI:

Near surface disposal is considered a suitable option for the disposal of short-lived low and intermediate level waste (LILW) containing radionuclides which decay to insignificant radiation levels within a few decades or centuries. LILW containing limited concentrations of long-lived radionuclides may also be disposed of in near surface facilities. The main radionuclides in short lived LILW are Cs-137 and Sr-90 and therefore, a period of 300 years for the post-closure institutional controls would correspond to about ten half-lives of these radionuclides. The longer-lived radionuclides, such as C-14, I-129, Tc-99 will continue to be released even after that period due to the engineering barriers degradation. The paper presents the post closure radiological impact of LIL waste disposal due to the C-14, one of the long life radionuclides presented in the LILW generated at Cernavoda Nuclear Power Plant.

3897. Electron microscopy of gamma-irradiated polyethylene

Authors: Bercia, R; Tirnovan, M; Bercia, C; Popescu-Pogrion, N

Published: APR 2006, SURFACE AND INTERFACE ANALYSIS, 38, 556, DOI: 10.1002/sia.2275

In this paper, we have studied the structure and the phase transformations in thin films (obtained by evaporation from gamma-irradiated polyethylene bulks - ageing polymer) before and after laser irradiation. We made a comparison between the following samples: A - thin films obtained from gamma-irradiated polyethylene bulk and not subjected to laser irradiation; B - thin films obtained from gamma-irradiated polyethylene bulk and subjected to laser irradiation. We have revealed many modifications in the dielectric and structural properties of the thin films, depending on the dose, dose rate and the time of laser irradiation (in the case of samples B). The thickness of all deposited films was 90 nm. The samples (non-irradiated or irradiated) were analysed by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), in combination with selected area electron diffraction (SAED). We have also presented the structure of non-aged and non-irradiated thin polyethylene films (control polyethylene film), in which we observed a particularly puzzling phase between an incipient spherulitic phase and a fine structure of the spherulitic phase (specifically for thin PE films structure). After gamma- and laser irradiations, we observed a strong modification of the structure. The spherulitic phase was transformed. We observed large, amorphous, transparent crystals containing many defects, carbon precipitates, the beginning of crystallisation revealed by electron diffraction, treeing and breakdown figures (usually after laser irradiation). Copyright (C) 2006 John Wiley & Sons, Ltd.

3898. Angstrom-science, angstrom-technology, angstrom-devices: A new challenge

Authors: Popescu, M

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 768, DOI:

Beyond nano-science is angstrom-science, beyond nano-technology is angstrom-technology, beyond nano-devices are angstrom-devices. Beyond these new emerging fields in materials physics is to ride on electron, but this is almost pure phantasy.

3899. Investigation of chemical and grain boundary effects in highly ordered Sr2FeMoO6: XPS and Mossbauer studies

Authors: Raekers, M; Kuepper, K; Hesse, H; Balasz, I; Deac, IG; Constantinescu, S; Burzo, E; Valeanu, M; Neumann, M

Published: APR 2006, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 8, 460, DOI:

We have studied the oxidation states of Fe and Mo and the presence of grain boundaries in the magneto resistive (MR) compounds Sr2FeMoO6 by means of x-ray photoelectron spectroscopy (XPS), Mossbauer spectroscopy and electrical resistivity measurements. XPS of the Mo 3d and Fe 3s core levels is indicating a mixed valence state involving around 30% Fe3+ - Mo5+ and 70% Fe2+ - Mo6+ states. Mossbauer studies confirm the presence of a valence fluctuation state and an essential amount of grain boundaries in the present Sr2FeMoO6 crystals. Resistivities and magnetoresistance studies evidenced strong grain boundary effects.

3900. Mossbauer spectroscopical investigation of the exchange biased Fe/MnF2 interface

Authors: Sahoo, B; Macedo, WAA; Keune, W; Kuncser, V; Eisenmenger, J; Nogues, J; Schuller, IK; Felner, I; Liu, K; Rohlsberger, R

Published: APR 2006, HYPERFINE INTERACTIONS, 169, 1377, DOI: 10.1007/s10751-006-9453-8

Two different Fe/MnF2 samples have been prepared by e-beam evaporation on MgO(001) substrates. The Fe layer in the samples includes a 10 angstrom thick Fe-57 probe layer either at the Fe/MnF2 interface (interface sample) or 35 angstrom away from the interface (center sample). The samples are characterized by X-ray diffraction, conversion electron Mossbauer spectroscopy (CEMS) and SQUID magnetometry. Fe-57 CEMS has been employed to study the depth dependent hyperfine interactions in Fe/MnF2 as a function of temperature between 18 K to 300 K. The hyperfine field B-hf has been obtained for the interfacial and off-interfacial Fe-57 layers. At the interface, besides B-hf of bcc-Fe, the presence of a component with a distribution P(B-hf ) is observed. The latter is assigned to interfacial Fe-57 atoms, indicating some (similar to 15%, equivalent to similar to 1 Fe atomic layer) intermixing at the Fe/MnF2 interface and a decrease of the average < B-hf > by 21%. The influence of the interface disappears as the Fe-57 probe layer is placed away from the interface. The temperature dependence of the average < B-hf > of the interface has been measured. The Fe spins, at remanence, are found to lie in the film plane.