National Institute Of Materials Physics - Romania

We study the intensity dependence of single-photon emission from a single semiconductor quantum dot after pulsed excitation. A semiconductor model is introduced that accounts for various configurations resulting from the occupation of the localized single-particle states with carriers. A detailed account is given on how the photon correlation dynamics and the antibunching can be calculated. We predict a novel effect, where, for strong excitation, the formation of multiexciton configurations during the excitation pulse leads to an improved antibunching signature in g((2)). (C) 2012 Optical Society of America

The mechanico-chemical interaction of a polyaniline-emeraldine base (PANI-EB) with ZnS in the cubic and wurtzite phases is studied by Raman spectroscopy and photoluminescence (PL). The results demonstrate that such an interaction leads to the formation of a PANI-salt and metallic Zn. Regardless of the structural form of the ZnS, the formation PANI-salt is indicated by a band in the Raman spectrum that shifts from 1162 to 1176 cm(-1) and the appearance of a new band at 1330 cm(-1) that indicates the protonated structure of a PANI-salt. The presence of the second product is determined by comparative PL studies performed on ZnS that has interacted mechanico-chemically with PANI-EB and metallic Zn powder. The variations of the PL spectra and their associated excitation spectra are explained as resulting from the charge collection processes that occur in the composite materials produced by the mechanico-chemical interaction between ZnS and PANI-EB or metallic Zn. (C) 2011 Elsevier Inc. All rights reserved.

Structural aspects and atomic intermixing processes in Be/W bilayers deposited on Si(0 0 1) substrates with Fe buffer layers enriched in the (57)Fe Mossbauer isotope have been studied via atomic force microscopy, grazing incidence X-ray diffractometry, X-ray reflectometry, X-ray photoelectron spectroscopy and conversion electron Mossbauer spectroscopy. The mentioned investigations allowed a full sequential characterization of the involved interfaces. Various ionic configurations appeared for Fe or W, while an amorphous state was observed in the case of Be. It has been proven that the Be layer has a negative influence on the roughness of the whole structure, which however presents an oxidation gradient from more oxidized elements at the surface towards more reduced elements in deeper layers. A strong diffusion of the W atoms inside the Be layer, induced by the deposition method, as well as of the Fe atoms inside the Be layer, induced by thermal annealing, has been evidenced. (C) 2011 Elsevier B.V. All rights reserved.

Triblock copolymers poly(ethylene oxide) (PEO)-poly(propylene oxide) (PPO)-poly(ethylene oxide) (PEO) Pluronic F127 was employed as capping agent for the Ag(0) metallic nanoparticles synthesized by the chemical reduction of silver nitrate, AgNO3, with excess of sodium borohydride, NaBH4. The morphology and the kinetics of Ag(0) nanoparticles were studied by transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic light scattering (DLS), UV-vis absorption and fluorescence probe technique. Spherical polydisperse particles of 12 +/- 4 nm mean diameter at varying the Pluronic concentration in the range of 0.13-13 wt.% were reported. Apart from the stabilizing role, the triblock copolymer promoted the agglomeration of individual Ag(0) nanoparticles in small assemblies whose plasmon band features differed from those of the individual nanoparticles. The formation of Ag(0) nanoparticles-F127 complex took place after full reduction of the Ag ions and it was controlled by the F127 concentration. The nanoparticle morphology and the fluorescence probe technique suggested that the excess PI uronic drove indirectly the Ag(0) nanoparticles association. Free F127 polymeric micelles encountered above critical micellar concentration (cmc) promoted the partial desorption of the adsorbed triblock copolymer and subsequently weakened the stability of the individual nanoparticles. (C) 2011 Elsevier B.V. All rights reserved.

Magnetization reversal in nanoscale (Sm-Co)/Fe (hard/soft) bilayer exchange-spring magnets with in-plane uniaxial magnetic anisotropy was investigated by magnetometry, conversion-electron Mossbauer spectroscopy (CEMS) and atomistic Fe spin-structure calculations. Magnetization loops along the easy direction exhibit signatures typical of exchange-spring magnets. In-field CEMS at inclined gamma-ray incidence onto thin (2 nm) Fe-57 probe layers embedded at various depths in the 20-nm-thick natural (soft) Fe layer provides depth-dependent information (via the line-intensity ratio R-23 as a function of the applied field H) about the in-plane rotation of Fe spins. A minimum in the R-23-vs-H dependence at (H-min, R-min) determines the field where Fe magnetic moments roughly adopt an average perpendicular orientation during their reversal from positive to negative easy-axis orientation. A monotonic decrease of H-min with distance from the hard/soft interface is observed. Rotation of Fe spins takes place even in the interface region in applied fields far below the field of irreversible switching, H-irr, of the hard phase. Formation of an Fe-Co alloy is detected in the interface region. For comparison, the noncollinear Fe spin structure during reversal and the resulting R-23 ratio were obtained by electronic-structure calculations based on a quantum-mechanical Hamiltonian for itinerant electrons. The coupling at the hard/soft interface is described by the uniaxial exchange-anisotropy field, h(int), as a parameter. Our calculated R-23 ratios as a function of the (reduced) applied field h exhibit similar features as observed in the experiment, in particular a minimum at (h(min), R-min). R-min is found to increase with h(int), thus providing a measure of the interface coupling. Evidence is provided for the existence of fluctuations of the interface coupling. The calculations also show that the Fe spin spiral formed during reversal is highly inhomogeneous. In general, our simulation of the Fe spin structure is applicable for the interpretation of experimental results on layered exchange-spring magnets.

Structural and magnetic properties of exchange spring magnets consisting of hard magnetic (FePt) and soft magnetic (Fe and Co) bilayers. prepared by ion beam sputtering method are studied via X-ray diffraction (XRD), magneto-optic Kerr effect (MOKE) and vibrating sample magnetometry (VSM). Thin tracer layers of Fe-57 were introduced in the soft layer in order to observe the Fe spin structure and interfacial diffusion by Conversion Electron Mossbauer Spectroscopy (CEMS). The observed in-plane exchange spring behavior extends also to the magnetic hard layer, whose switching field can be tuned in an unexpected manner via the top soft magnetic layer. To explain the observed phenomenon it is suggested that the increased switching field, found in the system with a Co/Fe bilayer acting as a single soft magnetic layer, is compatible with a peculiar behavior of the stiffness coefficient of the heterogeneous soft magnetic layer. According to this observation, possibilities to maximize the exchange spring effects via suitably chosen non-homogeneous soft magnetic layers are open. (C) 2011 Elsevier B.V. All rights reserved.

High quality nonlinear optical Gd0.84Lu0.11Nd0.05Ca4O(BO3)(3) (Nd: GdLuCOB) crystal has been grown by the Czochralski method. The high resolution and polarized spectroscopic data of Nd3+ in GdLuCOB crystal were carried out and analyzed in the framework of the Judd-Ofelt theory. The polarized emission cross-sections for the F-4(3/2) -> I-4(9/2) and F-4(3/2) -> I-4(11/2) Nd3+ transitions of special interest for laser application were determined. Near-infrared laser emission at 1061 nm was achieved under cw Ti: sapphire end pumping at 812 nm. For propagation along the Z-axis, laser emission polarized parallel to the Y-axis was obtained for the first time. This result offers the opportunity to achieve type I SFD of Nd3+ emission in the ZX plane which presents a non-linearity approximately three times larger than XY plane. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3673801]

(Na0.5Bi0.5)TiO3 doped with 5 mol% BaTiO3 (NBT-BT0.05) nanopowder was prepared by sol-gel method. Structural and microstructural investigations revealed a powder crystallized in the Na0.5Bi0.5TiO3 rhombohedral phase form. Ceramics processed from this powder show good dielectric (epsilon(r) = 3940 and tan delta = 0.016) and piezoelectric characteristics (d(33) = 77 pC/N, k(p) = 0.091, epsilon(T)(33) = 1080 and Q(m) = 197.5). The as-obtained NBT-BT0.05 ceramic appears effective for piezoelectric resonator and high power applications. (C) 2011 Elsevier Ltd. All rights reserved.

The influence of Au nanoparticles dimension on the photocatalytic performances of the TiO2 aerogels-Au composites was evaluated. Structural and morphological peculiarities of the TiO2 aerogel, unloaded and loaded with 5 and 22 nm Au nanoparticles, were studied by transmission electron microscopy technique, X-ray diffraction and N-2-sorption measurements. UV-Vis diffuse reflectance measurements were performed to determine the band gap energies. The photocatalytic activity was evaluated by monitoring the salicylic acid photodegradation. It was found that the Au nanoparticles promote the anatase phase crystallization and produce the decrease of the specific surface area and band gap energy values. The porous composite with the smallest Au nanoparticles dimension exhibits the best photocatalytic performances.

Iron-carbon nanocomposites have gained interest due to their new engineering and biomedical applications. Carbon coated iron nanoparticles (Fe@C) were obtained continuously and in a single step using the laser pyrolysis method. The continuous wave CO2 laser beam was used to continuously heat a sensitized (with ethylene) precursor gas mixture, in which iron pentacarbonyl (vapor) and acetylene were the iron and carbon donors, respectively. The effect of varying the residence time in the reaction zone through the variation of the internal nozzle diameter was explored in order to improve the particle size and the phase distributions. At increased nozzle diameter, (i) the particle mean diameter increases (from about 3.5 to 10.5 nm), (ii) higher ordering of the crystallographic network seems to occur, (iii) the dominance of the alpha-Fe and iron carbide phases is revealed. Onion-like graphenic layers often cover the buried iron cores. Magnetic measurements and temperature dependent Mossbauer spectroscopy were used in order to find correlations concerning the magnetic behavior and the Fe phase composition of samples. Preliminary experiments for obtaining stable water-based magnetic nanofluids are discussed. (C) 2011 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.