¿Sueña la juventud vulnerable con trabajos precarios? la toma de decisiones en los itinerarios de (In/Ex)clusión educativa

Este trabajo aborda el análisis de los discursos de jóvenes que han tenido dificultades en su itinerario educativo y que han participado en 'itinerarios de segunda oportunidad'. A partir de entrevistas a jóvenes, pretendemos aproximarnos a los argumentos desde los que dan sentido a sus itinerarios académicos y a las decisiones a las que se han enfrentado. Estos argumentos muestran fuertes implicaciones de la posición social de los actores, y están influidos por agentes de la institución educativa y familiar. En ese marco los jóvenes van configurando unas expectativas de futuro. Estas expectativas se debaten entre tres vías: salidas a espacios laborales poco cualificados y precarios, aunque funcionales al sistema productivo; salidas a entornos protegidos con una elevada dependencia; o salidas a vías marginales que les devuelven a redes de proximidad de alta vulnerabilidad. Estos itinerarios evidencian la tensión entre la reproducción social y la resistencia. This work analyses young people's discourses characterized by problematic educational tracks, and who have been part of "second opportunity itineraries". Based on interviews to the young, we try to understand the arguments they use to make sense of their own academic itineraries and the key decisions they have taken in this process. Their arguments highly relate with the actor's social position, and are influenced by agents such as the family and the educational institution. Within this framework, youngsters construct expectations for the future. These expectations follow three different paths: low-qualified and precarious work arenas, which remain functional in the productive system's context; protected settings with high dependency levels; and marginal paths which takes them to proximity networks with high vulnerability. These tracks show the tension between social reproduction and resistance

Atomic structure of Ge quantum dots on the Si(001) surface

In situ morphological investigation of the {105} faceted Ge islands on the Si(001) surface (hut clusters) have been carried out using an ultra high vacuum instrument integrating a high resolution scanning tunnelling microscope and a molecular beam epitaxy vessel. Both species of hut clusters--pyramids and wedges--were found to have the same structure of the {105} facets which was visualized. Structures of vertexes of the pyramidal clusters and ridges of the wedge-shaped clusters were revealed as well and found to be different. This allowed us to propose a crystallographic model of the {105} facets as well as models of the atomic structure of both species of the hut clusters. An inference is made that transitions between the cluster shapes are impossible.Comment: 6 pages, 6 figures. Accepted to JETP Letters (publication date 2010-03-25

Reflectance anisotropy spectroscopy assessment of the MOVPE nucleation of GaInP on Germanium (100)

This work summarizes the observations made on the variation and time evolution of the reflectanceanisotropy signal during the MOVPE growth of GaInPnucleation layers on Germanium substrates. This in situ monitoring tool is used to assess the impact of different nucleation routines and reactor conditions on the quality of the layers grown. This comparison is carried out by establishing a correlation between reflectanceanisotropy signature at 2.1 eV and the morphology of the epilayers evaluated by atomic force microscopy (AFM). This paper outlines the potential of reflectanceanisotropy to predict, explore, and therefore optimize, the best growth conditions that lead to a high quality III–V epilayer on a Ge substrat

STM and RHEED study of the Si(001)-c(8x8) surface

The Si(001) surface deoxidized by short annealing at T~925C in the ultrahigh vacuum molecular beam epitaxy chamber has been in situ investigated by high resolution scanning tunnelling microscopy (STM) and reflected high energy electron diffraction (RHEED). RHEED patterns corresponding to (2x1) and (4x4) structures were observed during sample treatment. The (4x4) reconstruction arose at T<600C after annealing. The reconstruction was observed to be reversible: the (4x4) structure turned into the (2x1) one at T>600C, the (4x4) structure appeared again at recurring cooling. The c(8x8) reconstruction was revealed by STM at room temperature on the same samples. A fraction of the surface area covered by the c(8x8) structure decreased as the sample cooling rate was reduced. The (2x1) structure was observed on the surface free of the c(8x8) one. The c(8x8) structure has been evidenced to manifest itself as the (4x4) one in the RHEED patterns. A model of the c(8x8) structure formation has been built on the basis of the STM data. Origin of the high-order structure on the Si(001) surface and its connection with the epinucleation phenomenon are discussed.Comment: 26 pages, 12 figure

Ge quantum dot arrays grown by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface: nucleation, morphology and CMOS compatibility

Issues of morphology, nucleation and growth of Ge cluster arrays deposited by ultrahigh vacuum molecular beam epitaxy on the Si(001) surface are considered. Difference in nucleation of quantum dots during Ge deposition at low (<600 deg C) and high (>600 deg. C) temperatures is studied by high resolution scanning tunneling microscopy. The atomic models of growth of both species of Ge huts---pyramids and wedges---are proposed. The growth cycle of Ge QD arrays at low temperatures is explored. A problem of lowering of the array formation temperature is discussed with the focus on CMOS compatibility of the entire process; a special attention is paid upon approaches to reduction of treatment temperature during the Si(001) surface pre-growth cleaning, which is at once a key and the highest-temperature phase of the Ge/Si(001) quantum dot dense array formation process. The temperature of the Si clean surface preparation, the final high-temperature step of which is, as a rule, carried out directly in the MBE chamber just before the structure deposition, determines the compatibility of formation process of Ge-QD-array based devices with the CMOS manufacturing cycle. Silicon surface hydrogenation at the final stage of its wet chemical etching during the preliminary cleaning is proposed as a possible way of efficient reduction of the Si wafer pre-growth annealing temperature.Comment: 30 pages, 11 figure

Polymer based silver nanocomposites as versatile solid film and aqueous emulsion SERS substrates

Nanocomposites containing Ag nanoparticles (average diameter similar to 11 nm) dispersed in poly(tertbutylacrylate) were prepared by in situ polymerization via miniemulsions and constitute active and versatile SERS substrates. The use of this synthetic strategy enables the dual use of the final composites as SERS substrates, both as aqueous emulsions and as cast films, shown here by several measurements using thiosalicylic acid as the testing analyte. The main advantage of these types of materials is related to the potential to scale up and the widespread use of handy substrates, using technology already available. This requires homogeneous composite substrates with SERS activity and this was demonstrated here by means of confocal Raman microscopy. Finally, a series of experiments were carried out on Ag/polymer nanocomposites submitted to temperature variations below and above the polymer glass transition temperature (T(g)) in order to conclude about the effect of temperature processing conditions on the composites' SERS activity.FCT- SFRH/BD/66460/2009FCT- SFRH/BPD/66407/2009FCT- PTDC/QUI/67712/ 2006RNME-Pole UA-FCT Project REDE/1509/RME/200

Bio-inspired nanocatalysts for the oxygen reduction reaction

Electrochemical conversions at fuel cell electrodes are complex processes. In particular, the oxygen reduction reaction has substantial overpotential limiting the electrical power output efficiency. Effective and inexpensive catalytic interfaces are therefore essential for increased performance. Taking inspiration from enzymes, earth-abundant metal centres embedded in organic environments present remarkable catalytic active sites. Here we show that these enzyme-inspired centres can be effectively mimicked in two-dimensional metal-organic coordination networks self-assembled on electrode surfaces. Networks consisting of trimesic acid and bis-pyridyl-bispyrimidine coordinating to single iron and manganese atoms on Au(111) effectively catalyse the reduction and reveal distinctive catalytic activity in alkaline media. These results demonstrate the potential of surface-engineered metal-organic networks for electrocatalytic conversions. Specifically designed coordination complexes at surfaces inspired by enzyme cofactors represent a new class of nanocatalysts with promising applications in electrocatalysis.Fil: Grumelli, Doris Elda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentina. Max Planck Institute for Solid State Research; AlemaniaFil: Wurtser, Benjamin. Max Planck Institute for Solid State Research; AlemaniaFil: Stepanow, Sabastian. Max Planck Institute for Solid State Research; AlemaniaFil: Kern, Klaus. Max Planck Institute for Solid State Research; Alemania. Ecole Polytechnique Federale de Lausanne; Suiz

Electron Beam-Induced Writing of Nanoscale Iron Wires on a Functional Metal Oxide

Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO2(110)-(1 × 1) in ultrahigh vacuum. The wires have a width down to ∼20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe(CO)5. On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring

Supramolecular networks stabilise and functionalise black phosphorus

The limited stability of the surface of black phosphorus (BP) under atmospheric conditions is a significant constraint on the exploitation of this layered material and its few layer analogue, phosphorene, as an optoelectronic material. Here we show that supramolecular networks stabilised by hydrogen bonding can be formed on BP, and that these monolayer-thick films can passivate the BP surface and inhibit oxidation under ambient conditions. The supramolecular layers are formed by solution deposition and we use atomic force microscopy to obtain images of the BP surface and hexagonal supramolecular networks of trimesic acid and melamine cyanurate (CA.M) under ambient conditions. The CA.M network is aligned with rows of phosphorus atoms and forms large domains which passivate the BP surface for more than a month, and also provides a stable supramolecular platform for the sequential deposition of 1,2,4,5-tetrakis(4-carboxyphenyl)benzene to form supramolecular heterostructures

Mechanism of periodic height variations along self-aligned VLS-grown planar nanostructures

In this study we report in-plane nanotracks produced by molecular-beam-epitaxy (MBE) exhibiting lateral self-assembly and unusual periodic and out-of-phase height variations across their growth axes. The nanotracks are synthesized using bismuth segregation on the GaAsBi epitaxial surface, which results in metallic liquid droplets capable of catalyzing GaAsBi nanotrack growth via the vapor–liquid–solid (VLS) mechanism. A detailed examination of the nanotrack morphologies is carried out employing a combination of scanning electron and atomic force microscopy and, based on the findings, a geometric model of nanotrack growth during MBE is developed. Our results indicate diffusion and shadowing effects play significant roles in defining the interesting nanotrack shape. The unique periodicity of our lateral nanotracks originates from a rotating nucleation “hot spot” at the edge of the liquid–solid interface, a feature caused by the relative periodic circling of the non-normal ion beam flux incident on the sample surface, inside the MBE chamber. We point out that such a concept is divergent from current models of crawling mode growth kinetics and conclude that these effects may be utilized in the design and assembly of planar nanostructures with controlled non-monotonous structure