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Zagonel L.F.,Laboratorio Nacional Of Nanotecnologia | Zagonel L.F.,University Paris - Sud | Rigutti L.,University Paris - Sud | Rigutti L.,CNRS Material Physics Group | And 4 more authors.
Nanotechnology | Year: 2012

The optical properties of a stack of GaN/AlN quantum discs (QDiscs) in a GaN nanowire have been studied by spatially resolved cathodoluminescence (CL) at the nanoscale (nanoCL) using a scanning transmission electron microscope (STEM) operating in spectrum imaging mode. For the electron beam excitation in the QDisc region, the luminescence signal is highly localized, with spatial extent as low as 5nm, due to the high band gap difference between GaN and AlN. This allows the discrimination between the emission of neighbouring QDiscs and evidencing the presence of lateral inclusions, about 3nm thick and 20nm long rods (quantum rods, QRods), grown unintentionally on the nanowire sidewalls. These structures, also observed by STEM dark-field imaging, are proved to be optically active in nanoCL, emitting at similar, but usually shorter, wavelengths with respect to most QDiscs. © 2012 IOP Publishing Ltd.

Tizei L.H.G.,University of Campinas | Tizei L.H.G.,University Paris - Sud | Craven A.J.,University of Glasgow | Zagonel L.F.,University Paris - Sud | And 6 more authors.
Physical Review Letters | Year: 2011

We have performed a detailed study of the lattice distortions of InP wurtzite nanowires containing an axial screw dislocation. Eshelby predicted that this kind of system should show a crystal rotation due to the dislocation induced torque. We have measured the twisting rate and the dislocation Burgers vector on individual wires, revealing that nanowires with a 10-nm radius have a twist up to 100% larger than estimated from elasticity theory. The strain induced by the deformation has a Mexican-hat-like geometry, which may create a tube-like potential well for carriers. © 2011 American Physical Society.

Giorgioni A.,University of Milan Bicocca | Pezzoli F.,University of Milan Bicocca | Gatti E.,University of Milan Bicocca | Cecchi S.,Polytechnic of Milan | And 5 more authors.
Applied Physics Letters | Year: 2013

The polarization of the direct gap emission from Ge/SiGe multiple quantum wells has been studied in the 4 K to 300 K temperature range, and in samples with different well thicknesses. Our results demonstrate that the polarization type and degree strongly depend on the excitation of electrons from the heavy hole and the light hole subbands, thus providing an effective degree of freedom to control the polarization of the direct interband emission. In addition, the analysis of the temperature dependence of the polarization degree highlights spin depolarization mechanisms. © 2013 American Institute of Physics.

Diaz B.,Brazilian Synchrotron Light Laboratory (LNLS) | Malachias A.,Federal University of Minas Gerais | Montoro L.A.,Laboratorio Nacional Of Nanotecnologia | Rappl P.H.O.,National Institute for Space Research | Abramof E.,National Institute for Space Research
Nanotechnology | Year: 2012

Stacked EuTe magnetic quantum dots (QDs) separated by SnTe spacers of increasing thickness were grown and studied using x-ray diffraction (XRD) and electron microscopy. Grazing incidence XRD indicated that the EuTe QDs are under compressive in-plane strain. Both XRD analysis and microscopy images demonstrated that the EuTe QDs are vertically aligned, as a result of the strain field produced by buried QDs. The width of the lateral error distribution in the QDs' vertical alignment from layer to layer decreases for thinner SnTe spacers, corresponding to more stressed SnTe matrices. The system can be, therefore, tuned to explore magnetic interactions between QDs. The results are discussed in the light of previous elastic strain models in anisotropic matrices from the literature.

Ferraz C.P.,University of Sao Paulo | Garcia M.A.S.,University of Sao Paulo | Teixeira-Neto E.,Laboratorio Nacional Of Nanotecnologia | Rossi L.M.,University of Sao Paulo
RSC Advances | Year: 2016

Gold nanoparticles have shown excellent catalytic properties in selective oxidation of alcohols in the presence of base; however, the influence of the nature and concentration of the base on gold catalyst activity and selectivity is not completely understood. We here present a study of the effect of using weak and strong bases on the conversion and selectivity of PVA-stabilized gold nanoparticles supported on titania (AuPVA/TiO2) for benzyl alcohol oxidation. The increase in the concentration of base had little effect on conversion when a weak base was used (K2CO3, Na2B4O7 and Na(CH3COO)), due to the buffer effect, but strongly affected selectivity. The bases with higher pKa values provided higher conversions and increased production of benzoic acid. For the strong base NaOH, benzoic acid was always the major product, although conversion decreases in excess of base. The formation of benzoic acid is avoided by using K2CO3 in non-aqueous media; benzaldehyde is the main product in cyclohexane whereas benzyl benzoate is also formed in significant amounts in solvent-free conditions. The promotion effect observed in the presence of base was discussed in terms of reaction mechanism. © The Royal Society of Chemistry 2016.

Solovev A.A.,Leibniz Institute for Solid State and Materials Research | Xi W.,Leibniz Institute for Solid State and Materials Research | Gracias D.H.,Leibniz Institute for Solid State and Materials Research | Gracias D.H.,Johns Hopkins University | And 6 more authors.
ACS Nano | Year: 2012

We describe nanoscale tools in the form of autonomous and remotely guided catalytically self-propelled InGaAs/GaAs/(Cr)Pt tubes. These rolled-up tubes with diameters in the range of 280-600 nm move in hydrogen peroxide solutions with speeds as high as 180 μm s -1. The effective transfer of chemical energy to translational motion has allowed these tubes to perform useful tasks such as transport of cargo. Furthermore, we observed that, while cylindrically rolled-up tubes move in a straight line, asymmetrically rolled-up tubes move in a corkscrew-like trajectory, allowing these tubes to drill and embed themselves into biomaterials. Our observations suggest that shape and asymmetry can be utilized to direct the motion of catalytic nanotubes and enable mechanized functions at the nanoscale. © 2012 American Chemical Society.

Pitthan E.,Federal University of Rio Grande do Sul | Gobbi A.L.,Laboratorio Nacional Of Nanotecnologia | Stedile F.C.,Federal University of Rio Grande do Sul
Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms | Year: 2016

Phosphorus detection and quantification were obtained, using the 31P(α,p)34S nuclear reaction and Rutherford Backscattering Spectrometry, in deposited silicon oxide films containing phosphorus and in carbon substrates implanted with phosphorus. It was possible to determine the total amount of phosphorus using the resonance at 3.640 MeV of the 31P(α,p)34S nuclear reaction in samples with phosphorus present in up to 23 nm depth. Phosphorous amounts as low as 4 × 1014 cm-2 were detected. Results obtained by nuclear reaction were in good agreement with those from RBS measurements. Possible applications of phosphorus deposition routes used in this work are discussed. © 2015 Elsevier B.V. All rights reserved.

Vega-Baudrit J.,National University of Costa Rica | Vega-Baudrit J.,Laboratorio Nacional Of Nanotecnologia | Delgado-Montero K.,National University of Costa Rica | Madrigal-Carballo S.,National University of Costa Rica
Cellulose Chemistry and Technology | Year: 2011

In a previous work, it was demonstrated that 70% of the molasses isolated from the biowaste generated by the agro-industrialization of sugar cane allow obtaining composite materials with adequate mechanical properties and bulk density. In the present research, these results were applied in the preparation of composite polyurethanes based on sugar cane molasses reinforced with bagasse fiber from the same biowaste. Initially, milled fibers from bagasse were characterized both physically and chemically, according to their content of a-cellulose, moisture, ashes, hemicelluloses and lignin, as well as to their extractability in organic and aqueous solvents. The most representative values were a-cellulose (47.3%), lignin (24.4%) and holocellulose(66.1%). Subsequently, polyurethane foams (PUF) were prepared using the natural fiber as a reinforcement, with 4 different percentages of fiber ground (5, 10, 15, 20% w/w) with an average particle size of 60 mesh; an extra PUF without fiber reinforcement was also synthesized as a reference. PUF synthesis was carried out with a PEG-molasses mixture in a 30:70% (w/w) ratio, diphenylmethane diisocyanate (MDI) and dibutyltin dilaurate as catalysts. Composite PUFs were characterized according to their bulk density, thermal (thermogravimetry-TGA) and mechanical (compression modulus) properties. The results showed that the composite PUF sample with 10% sugar cane fiber added as reinforcement had the lowest bulk density. Also, its TGA thermal analysis showed a general trend towards reducing both the initial temperature of decomposition, as well as the temperature at the maximum rate of degradation, compared to the reference PUF without fiber added. Finally, its mechanical properties showed a maximum value for both effort to compression and compression modulus. It has been concluded that the use of agro-industrial biowaste from sugar cane permits to develop composite materials with suitable properties for applications as foams. The PUF synthesized by partial replacement of PEG for molasses and reinforced with sugar cane bagasse fiber can be used in the packaging industry. In this case, the materials are used for short periods, and are usually very bulky, less dense. Thus, this novel type of PU foam could be useful in reducing the production costs and in improving the potential biodegradability of the polymeric matrix, becoming an interesting value-added application for this important source of agricultural biowastes, especially in Latin America.

Herklotz A.,Martin Luther University of Halle Wittenberg | Herklotz A.,Leibniz Institute for Solid State and Materials Research | Kataja M.,Leibniz Institute for Solid State and Materials Research | Nenkov K.,Leibniz Institute for Solid State and Materials Research | And 5 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

SrRuO3 films have been grown in the tetragonal, structurally single-domain state under 1% of biaxial tensile strain. The angular dependencies of the magnetization and the magnetoresistance reveal an upright orientation of the tetragonal unit cell and biaxial magnetic in-plane anisotropy with 110 t easy axes. Reversible biaxial strain from piezoelectric Pb(Mg 1/3Nb2/3)0.72Ti0.28O3 (PMN-PT) substrates has been applied to probe the direct strain response of the magnetization and the electrical resistance. At 1% tensile strain, the Curie temperature (TC) and the ordered magnetic moment (mS) at low temperatures are found to substantially decrease with further growing tensile strain. This suggests a suppression of mS resulting from distortions of the RuO6 octahedra, in line with reported density-functional calculations. Reversible strain has also been applied to a film under weak tensile strain revealing the opposite response, i.e., an enhancement of TC and mS with tensile strain. Structural and magnetic properties of SrRuO3 films in several static strain states (compressive, weak and strong tensile strain) are compared. © 2013 American Physical Society.

Coelho P.M.,Federal University of Minas Gerais | Ribeiro G.A.S.,Federal University of Minas Gerais | Malachias A.,Federal University of Minas Gerais | Pimentel V.L.,Laboratorio Nacional Of Nanotecnologia | And 4 more authors.
Nano Letters | Year: 2013

Topological insulators such as Bi2Se3 and Bi 2Te3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi2Se3, one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi 2Te3. X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior. © 2013 American Chemical Society.

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