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  • Publication
    Long wavelength stacking induced shift of the near-infrared photoluminescence from unintentional MOVPE grown InGaSb/GaSb quantum wells
    (2018-07-01) Chinedu, Christian; Tile, Ngcali; Johannes, Reinhardt; Navarro de Mesa, Amalia; Galiana Blanco, Beatriz; Ministerio de Economía y Competitividad (España)
    The effect of stacking on the near-infrared photoluminescence (NIR-PL) of InGaSb/GaSb quantum wells (QWs) which were inadvertently formed during an attempt to fabricate stacked InSb/GaSb quantum dots (QDs) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) are investigated in this work. The morphology of uncapped dots was studied by means of scanning probe microscopy (SPM) which shows a significant deviation in the shape and density of dots grown directly on the buffer compared to those that terminated an "embedded-dot" sample. Cross-sectional scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) of the capped structures clearly revealed the formation of QWs in the capped structures. An increase in the number of InSb QD-layers, which metamorphosed into QWs, was observed to cause an increase in the luminescence spectral line width and a long-wavelength shift of the QW PL lines, together with an enhancement in the strength of PL emission. Variations in layer thicknesses and alloy composition introduced as a result of inter-diffusion of Ga and In which is enhanced by the prolonged annealing time of the QDs (during spacer/cap layer deposition) and In adatom migration is suggested to alter the morphology of the capped dots and induce a change in PL peak positions and the spectral linewidth of the NIR low energy lines. (C) 2018 Author(s).
  • Publication
    Linear arrays of InGaAs quantum dots on nanostructured GaAs-on-Si substrates
    (Elsevier B.V., 2023-04-15) Tejedor, Paloma; García-Tabarés Valdivieso, Elisa; Galiana Blanco, Beatriz; Vázquez Burgos, Luis; Garcia, Basilio J.; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    Linear arrays of high-quality quantum dots (QD) integrated in Si are an ideal platform in exploring the manipulation and transmission of quantum information. Understanding QD self-organization mechanisms on substrates compatible with Si technology is therefore of great practical importance. Here we demonstrate the epitaxial growth of linear arrays of InAs and InGaAs QDs from As2 and In molecular beams on bare and GaAs-coated Si(0 0 1) substrates, patterned by high-resolution laser interference nanolithography. Atomic force microscopy, in combination with high-resolution scanning and transmission electron microscopies, show that these arrays exhibit an improvement in growth selectivity, lateral order and size uniformity of the QDs when a pseudomorphic 1 nm-thick GaAs buffer layer is grown prior to InAs deposition. In addition, preferential nucleation of InxGa1-xAs QDs along the 〈1 1 0〉 -oriented edges of the nanostructured GaAs-on-Si(0 0 1) substrate results from In adatom migration from (1 1 1) to (0 0 1) nanofacets and the erosion of the wetting and buffer layers caused by the Ga-In intermixing at the step edge during the Stranski-Krastanov transition. These are key elements in the formation of linear arrays of coherent QDs, which differ in morphology and structure from those obtained on both GaAs(0 0 1) and Si(0 0 1) planar surfaces.
  • Publication
    First accelerator test of vacuum components with laser-engineered surfaces for electron-cloud mitigation
    (American Physical Society, 2017-11-20) Calatroni, Sergio; García-Tabarés Valdivieso, Elisa; Neupert, Holger; Nistor, Valentin; Pérez Fontela, Ana Teresa; Taborelli, Mauro; Chiggiato, Paolo; Malyshev, Oleg; Valizadeh, Reza; Wackerow, Stefan; Zolotovskaya, Svetlana A.; Gillespie, W. Allan; Abdolvand, Amin
    Electron cloud mitigation is an essential requirement for high-intensity proton circular accelerators. Among other solutions, laser engineered surface structures (LESS) present the advantages of having potentially a very low secondary electron yield (SEY) and allowing simple scalability for mass production. Two copper liners with LESS have been manufactured and successfully tested by monitoring the electron cloud current in a dipole magnet in the SPS accelerator at CERN during the 2016 run. In this paper we report on these results as well as the detailed experiments carried out on samples-such as the SEY and topography studies-which led to an optimized treatment in view of the SPS test and future possible use in the HL-LHC.
  • Publication
    Advanced transmission electron microscopy investigation of defect formation in movpe-growth of gap on silicon using arsenic initial coverage
    (Elsevier, 2023-02-01) Navarro de Mesa, Amalia; García-Tabarés Valdivieso, Elisa; Ramasse, Quentin M.; Caño, Pablo; Rey-Stolle, Ignacio; Galiana Blanco, Beatriz
    Integration of GaP layers on silicon substrates using AsH3 pre-exposure followed by a PH3-based GaP epitaxial growth allows the development of very promising processes for the photovoltaic industry, although many of the growth routines using this approach suffer from reproducibility issues when transferred to a new epitaxial system, leading to poor quality layers. This fact reveals a lack of knowledge on the mechanisms behind the formation of the most common planar defects (stacking faults and microtwins) and their dynamics for GaP/Si Metal Organic Vapor Phase Epitaxy using AsH3 and PH3. Therefore, in this work, a set of GaP/Si samples with a similarly high defect density grown between 700 °C and 725 °C, are analyzed by means of high-resolution scanning transmission electron microscopy and electron energy loss spectroscopy. The results presented show contaminant-free Si surfaces for temperatures above 725 °C, ruling out the hypothesis of contaminant as the origin of these planar defects. Regarding the interface Si/GaP, the GaP growth starts, in all the samples, with Gasingle bondSi bonds. Additionally, no traces of As are found, which reinforces the hypothesis of an effectively displacement of As on Si surface by Ga atoms at high temperature. Finally, it is observed complex chemical structures in the origin of the microtwins and the cause of the origin of these defects seems to be a localized gallium depletion at the GaP/Si interface.
  • Publication
    Negative-epsilon conditions in the dispersive LiNbO3-Ag nanoparticles composites
    (IOP Science, 2017-02) Cruz Fernández, Rosa María de la; Kanyinda-Malu, C; Muñoz Santiuste, Juan Enrique; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
    The silver (Ag)-embedded lithium niobate (LiNbO3) composites are theoretically analyzed under the effective medium Maxwell-Garnett approximation to account on the optimal conditions through which such composites present negative epsilon conditions. The dielectric function of Ag nanoparticles (NPs) is described by Drude theory with an additional Lorentz oscillator term to take into account the interband electronic transitions which typically occur in noble metals. The LiNbO3 dielectric function is evaluated through the Sellmeier equations. Once the effective dielectric function (epsilon(eff)) is evaluated, we investigate the negative epsilon condition (epsilon'(eff) < 0) as a function of the frequency. The results showed that, for given volumen fraction values, the negative epsilon (NE) condition is satisfied for critical sizes of Ag NPs. This condition defines an interval of energies, called NE range. That NE range enlarges for increasing radius and becames narrower for decreasing volume fractions. Furthermore, the calculated Frohlich frequency is nearly close to the lower-energy limit of NE range. In addition, the calculated extinction spectra of the composite are analyzed in terms of the radius of Ag NPs.
  • Publication
    Effect of strain rate on tensile mechanical properties of high-purity niobium single crystals for SRF applications
    (Elsevier, 2020-10-21) Croteau, J. F.; Pai Kulyadi, E.; Kale, C.; Siu, D.; Kang, D.; Perez Fontenla, A. T.; García-Tabarés Valdivieso, Elisa; Bieler, T. R.; Eisenlohr, P.; Solanki, K. N.; Balint, D.; Hooper, P. A.; Atieh, S.; Jacques, N.; Cantergiani, E.; European Commission
    An investigation of the mechanical properties of high-purity niobium single crystals is presented. Specimens were cut with different crystallographic orientations from a large grain niobium disk and uniaxial tensile tests were conducted at strain rates between 10-4 and 103 s-1. The logarithmic strain rate sensitivity for crystals oriented close to the center of a tensile axis inverse pole figure (IPF) is ~0.14 for all strain rates. The strain at failure (ranging from 0.4 to 0.9) is very sensitive to crystal orientation and maximal at ~10-2 s-1 for crystals oriented close to the center of an IPF. The high anisotropy observed at quasi-static strain rates decreased with increasing strain rate. The activation of multiple slip systems in the dynamic tests could account for this reduction in anisotropy. A transition from strain hardening to softening in the plastic domain was observed at strain rates greater than approximately 6 × 10-2 s-1 for crystals oriented close to the center of a tensile axis IPF. Shear bands were observed in specimens with orientations having similarly high Schmid factors on both {110} and {112} slip families, and they are correlated with reduced ductility. Crystal rotations at fracture are compared for the different orientations using scanning electron microscopy images and EBSD orientation maps. A rotation toward the terminal stable [101] orientation was measured for the majority of specimens (with tensile axes more than ~17° from the [001] direction) at strain rates between 1.28 × 10-2 and 1000 s-1.
  • Publication
    Process design for the manufacturing of soft X-ray gratings in single-crystal diamond by high-energy heavy-ion irradiation
    (Springer, 2022-10) García, G.; Martín, M.; Ynsa, M. D.; Torres-Costa, V.; Crespillo, M. L.; Tardío López, Miguel Modesto; Olivares, J.; Bosia, F.; Peña-Rodríguez, O.; Nicolas, J.; Tallarida, M.; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    This paper describes in detail a novel manufacturing process for optical gratings suitable for use in the UV and soft X-ray regimes in a single-crystal diamond substrate based on highly focused swift heavy-ion irradiation. This type of grating is extensively used in light source facilities such as synchrotrons or free electron lasers, with ever-increasing demands in terms of thermal loads, depending on beamline operational parameters and architecture. The process proposed in this paper may be a future alternative to current manufacturing techniques, providing the advantage of being applicable to single-crystal diamond substrates, with their unique properties in terms of heat conductivity and radiation hardness. The paper summarizes the physical principle used for the grating patterns produced by swift heavy-ion irradiation and provides full details for the manufacturing process for a specific grating configuration, inspired in one of the beamlines at the ALBA synchrotron light source, while stressing the most challenging points for a potential implementation. Preliminary proof-of-concept experimental results are presented, showing the practical implementation of the methodology proposed herein.
  • Publication
    Contribution to the understanding of the colour change in bluish-grey limestones
    (MDPI, 2022-09) Silva, Teresa P.; Oliveira, Daniel De; Veiga, João P.; Lisboa, Vitor; Carvalho, Jorge; Barreiros, M. Alexandra; Coutinho, Mathilda L.; Salas Colera, Eduardo; Vigário, Rogério; European Commission
    Bluish-grey limestones have been extensively used as ornamental stones for decoration purposes in buildings, as well as in works of art, and accordingly, have been the target of intense exploration. In Portugal, the Jurassic limestone massif known as the Maciço Calcário Estremenho (MCE), has been the source of grey-coloured ornamental stones, namely the Azul Valverde (one of the most well-known bluish-grey limestones) and Atlantic Blue varieties, both of which may undergo colour changes in outdoor environments. In this sense, it is important to understand the sudden colour change from bluish-grey to yellow/beige in the same limestone block in a quarry, or even, what happens to the colour when polished limestone is placed outdoors. This study was undertaken using various techniques, namely XRF (X-ray fluorescence spectrometry), XRD (X-ray diffraction), SEM (scanning electron microscopy), DTA-TG (differential thermal analysis/thermogra-vimetry) and colourimetry. Synchrotron radiation was also used at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) where XANES (X-ray Absorption Near Edge Structure) spectra at Fe K-edge were collected to ascertain the speciation state of Fe in different coloured zones of the limestone, previously checked by EDXRF (energy dispersive X-ray fluorescence). The presence of Fe2+ and Fe3+ are responsible for the greyish and yellow/brown colour, respectively. On the other hand, the UV radiation from the sun causes a quickened and severe bleaching/fading on the dark blue/grey polished limestone.
  • Publication
    Development of luminescent Nd-doped LaNbO compound thin film growth by magnetron sputtering for the improvement of solar cells
    (MDPI, 2023-02) Salas Colera, Eduardo; Tardío López, Miguel Modesto; García-Tabares, Elisa; Perea, Belén; Crespillo, Miguel L.; Muñoz Santiuste, Juan Enrique; Galiana Blanco, Beatriz; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España); Universidad Carlos III de Madrid
    Nd3+-doped LaNbO phosphor thin films were prepared by radio-frequency magnetron sputtering on Si substrates. The effects of a 1% Nd-doping concentration, after annealing at 1200 °C for 12 h, on the light-emitting properties of the sputtered thin films were characterized via several experimental techniques and deeply discussed. Photoluminescence characterization showed strong emission peaks typical of Nd3+ centers at 880 nm and 1060 nm when a 325 nm wavelength laser source was applied. Similar responses were detected in Nd3+-doped La3NbO7 powder samples fabricated by the solid-state reaction method. The coexistence of two phases (LaNbO4 and La3NbO7) in the thin films with higher nominal thickness was clearly identified based on different structural analyses. The promising results open the possibility for developing phosphor substrates as a preliminary step for the improvement of solar cells based on photon recycling mechanisms.
  • Publication
    Towards blue long lasting luminescence of Eu/Nd doped calcium aluminate nanostructured platelets via the molten salt route
    (MDPI, 2019-10) Rojas-Hernandez, Rocío E.; Rubio-Marcos, Fernando; Serrano, Aida; Salas Colera, Eduardo; Hussainova, Irina; Fernandez, José Francisco; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    Calcia-alumina binary compounds doped with rare earths and some transition metals cations show persistent luminescence from the visible to the infrared range. Specifically, the blue light can be obtained through the Eu2+ activator center in a potential host, such as dodecacalcium hepta-aluminate (Ca12Al14O33) and monocalcium aluminate (CaAl2O4). By doping with Nd3+, the persistent luminescence can be substantially prolonged; for this reason, the Eu/Nd pair is a potential choice for developing long-lasting blue luminescence. Herein, the phase evolution of the calcia-alumina system via molten salt synthesis is reported as a function of the synthesis temperature and the atmospheric environment. The fraction of CaAl2O4 phase increases when the temperature is higher. Synthesized microparticles of platelet-type morphology represent isolated nanostructured ceramic pieces. Under visible light, the particles are white. This indicates that the followed process solves the dark-gray coloring of phosphor when is synthesized in a reduced atmosphere at high temperature. As regards the synthesis mechanism, which is assisted by the molten flux, the dissolution−diffusion transport process is promoted at the surface of the alumina microparticles. In fact, the emission intensity can be modulated through the phase of the Eu-doped calcium-aluminate discrete platelets synthesized. Consequently, the photoluminescence intensity depends also on the oxidation state of the Eu ion. X-ray absorption near-edge structure and photoluminescence measurements corroborate the Eu reduction and the grain coarsening with the enhancement of the blue emission. The doped phosphors with Eu/Nd show a broad and strong absorption in the region of 320–400 nm and a broad emission band at around 440 nm when they are excited in this absorption range. From a broader perspective, our findings prove that the Ca12Al14O33 and CaAl2O4 phases open new opportunities for research into the design of blue long-lasting emitters for a wide range of fields from ceramic to optoelectronic materials.
  • Publication
    A combined micro-Raman, X-ray absorption and magnetic study to follow the glycerol-assisted growth of epsilon-iron oxide sol-gel coatings
    (Elsevier, 2022-02-05) López Sánchez, Jesús; Serrano, Aída; Del Campo, Adolfo; Muñoz-Noval, Alvaro; Salas Colera, Eduardo; Cabero, Mariona; Varela, María; Abuin, Manuel; Castro, German R.; Rubio-Zuazo, Juan; Rodriguez De La Fuente, Oscar; Carmona, Noemi; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
    Epsilon iron oxide (ε-Fe2O3) coatings on Si(100) substrates are obtained by an easy one-pot sol-gel recipe assisted by glycerol in an acid medium. Glycerol, given its small dimensions, enables the formation of ε-Fe2O3 nanoparticles with a size of a few nanometers and the highest purity is reached in coatings after a densification treatment at 960 °C. The structural and compositional evolution up to 1200 °C is studied by confocal Raman microscopy and X-ray absorption spectroscopy techniques, correlating the existing magnetic properties. We report a novel characterization method, which allows monitoring the evolution of the precursor micelles as well as the intermediate and final phases formed. Furthermore, the inherent industrial technology transfer of the sol-gel process is also demonstrated with the ε-Fe2O3 polymorph, impelling its application in the coatings form.
  • Publication
    Searching for relevant criteria to distinguish natural vs. anthropogenic TiO2 nanoparticles in soils
    (Royal Society of Chemistry, 2018-12-01) Pradas Del Real, Ana Elena; Castillo Michel, Hiram; Kaegi, Ralf; Larue, Camille; De Nolf, Wout; Reyes Herrera, Juan; Tucoulou, Remi; Findling, Nathaniel; Salas Colera, Eduardo; Sarret, Geraldine
    Nanosized TiO2 is one of the most produced nanomaterials. Agricultural soils are a major compartment of accumulation of TiO2-NPs after release from consumer products into the sewer system, due to the use of sewage sludge as a soil amendment. TiO2 is naturally present in soils, and distinguishing between anthropogenic and natural TiO2 in soils is thus important to assess the risks associated with the increased use of nano-TiO2. Methods to distinguish these materials in complex matrices such as soils are currently lacking. The purpose of this study was, therefore, to search for characteristic physical and chemical properties of natural and engineered nano-TiO2, based on a combination of bulk, micro and nanofocused X-ray fluorescence and X-ray absorption spectroscopy, transmission electron microscopy, X-ray diffraction and chemical analyses. Digested sewage sludge, agricultural soil and sludge-amended soil were studied by these techniques. The particle size distribution was not a relevant criterion since the sludge and the sludge-amended soil contained a variety of nanometer- and micrometer sized Ti-containing particles. Both the sludge and the soil contained a mixture of rutile and anatase, with a minor proportion of amorphous TiO2. In the sludge, there was no trend relating particle size and Ti mineralogy. The morphology of the TiO2 particles proved to be different in the two matrices, with smooth faceted particles in the sludge and rough irregular ones in the soil. In addition, natural TiO2 particles were included in micro and macroaggregates of the soil and formed intricate assemblages with minerals and organic compounds. In the sludge, TiO2 formed homo and heteroaggregates of simpler structure, richer in organic matter. Thus, the study of the morphology of TiO2 particles and their status in unperturbed mineral–organic assemblages may provide some insights into their origin. The observed differences may attenuate over time, due to the incorporation of the sludge material within the soil structure.
  • Publication
    In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-X
    (National Academy of Sciences, 2017-01-10) Perez-Muñoz, Ana M.; Schio, Pedro; Poloni, Roberta; Fernández Martínez, Alejandro; Rivera-Calzada, Alberto; Cezar, Julio C.; Salas Colera, Eduardo; Castro, German R.; Kinney, Joseph; León, Carlos; Santamaria, Jacobo; Garcia-Barriocanal, Javier; Goldman, Allen M.; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
    These results underscore the debate on the true nature, electrostatic vs. electrochemical, of the doping of cuprates with ionic liquids. Here, we address the doping mechanism of the high-temperature superconductor YBa2Cu3O7-X (YBCO) by simultaneous ionic liquid gating and X-ray absorption experiments. Pronounced spectral changes are observed at the Cu K-edge concomitant with the superconductor-to-insulator transition, evidencing modification of the Cu coordination resulting from the deoxygenation of the CuO chains, as confirmed by first-principles density functional theory (DFT) simulations. Beyond providing evidence of the importance of chemical doping in electric double-layer (EDL) gating experiments with superconducting cuprates, our work shows that interfacing correlated oxides with ionic liquids enables a delicate control of oxygen content, paving the way to novel electrochemical concepts in future oxide electronics.
  • Publication
    Fabrication and characterization of Cu reinforced with Y-enriched particles following a novel powder metallurgy route
    (Elsevier, 2021-12) Muñoz Castellanos, Ángel; Savoini Cardiel, Begoña; Monge Alcázar, Miguel Ángel; Ortega Villafuerte, Yanicet; Dura, O. J.; Comunidad de Madrid; Ministerio de Ciencia, Innovación y Universidades (España)
    Dispersion strengthened copper alloys have been produced following an innovative powder metallurgy route. Copper and yttrium acetate powders have been mechanically alloyed and posteriorly thermal treated at 923 K for 3 h and 15 h under a hydrogen atmosphere in order to transform the yttrium acetate into Y2O3. Subsequently, the powders were consolidated by hot isostatic pressing. It has been concluded that the duration of the thermal treatment of the powder is a determining factor in the degree of densification of the alloy. The study of the microstructure by Scanning Electron Microscopy and Electron Backscatter Diffraction has revealed the presence of micrometer and submicrometer grains and nanometric Y-O enriched Cu particles embedded in the copper matrix, the mean grain size being smaller for the sample produced from the powder thermal treated for 15 h. Transmission Electron Microscopy investigations concluded that the nanoparticles exhibit a spherical shape with a size up to 25 nm and correspond to monoclinic Y2O3. Annealing twins have been also observed, especially in the material produced from thermal treated powder for longer. The mechanical properties have been inferred from Vickers microhardness measurements and compression tests. Below 473 K the yield strengths of the produced materials are greater than that of pure copper and above 473 K are close to them. From the study of the thermal properties of the densest material it has been found that its thermal conductivity remains nearly constant in the temperature range 300–773 K, and its value is around 85% the thermal conductivity of CuCrZr, the reference material for ITER.
  • Publication
    Exploring CuCrFeVTi system to produce high entropy alloys for high heat flux applications
    (Elsevier, 2021-12) Rodríguez López, Álvaro; Savoini Cardiel, Begoña; Monge Alcázar, Miguel Ángel; Muñoz Castellanos, Ángel; Perez Zubiaur, Pablo; Comunidad de Madrid; Ministerio de Ciencia, Innovación y Universidades (España)
    Cu5Cr35Fe35V20Ti5, Cu10Cr35Fe35V10Ti10, Cu15Cr35Fe35V5Ti10 and Cu20Cr30Fe30V10Ti10 were produced by low-pressure arc-melting to evaluate the feasibility of developing reduced activation high entropy alloys containing copper. The materials present a dendritic microstructure and some Cu and Ti segregation in the as-cast condition, being the Cu5Cr35Fe35V20Ti5 the most homogenous alloy. Copper particles, with a mean size of about 10 nm, are found distributed inside the dendrites. The volume fraction of the copper particles is ~ 7 times higher in the alloy with 5 at.% Cu content that in the alloy with 20 at.%, with values ranging from 22 ± 5 to 3 ± 2 particles/¿m2. Combination of massive Berkovich nanoindentation, statistical analysis and analytical scanning electron microscopy, has been successful for determining the hardness and elastic modulus values of each phase and quantifying their contribution to microhardness values obtained by standard Vickers microindentation.
  • Publication
    Consequences of nitrogen doping and oxygen enrichment on titanium local order and photocatalytic performance of TiO2 anatase
    (American Chemical Society (ACS), 2017-03-30) Krivtsov, Igor; Ilkaeva, Marina; Salas Colera, Eduardo; Amghouz, Zakariae; García, José R.; Díaz, Eva; Ordoñez, Salvador; Villar-Rodil, Silvia; Ministerio de Economía y Competitividad (España)
    Extended X-ray absorption fine structure (EXAFS) investigation of the oxygen-rich titania formed via the thermal treatment of N-doped TiO2 has revealed that the removal of N-dopants is responsible for the creation of defect sites in the titanium environment, thus triggering at high temperatures (500-800 °C) the capture of atmospheric oxygen followed by its diffusion toward the vacant sites and formation of interstitial oxygen species. The effect of the dopants on Ti coordination number and Ti-Oint and Ti-Nint bond distances has been estimated. The photocatalytic p-cresol degradation tests have demonstrated that the interband states formed by the N-dopants contribute to a greater extent to the visible-light activity than the oxygen interstitials do. However, under the UV irradiation the oxygen-rich titania shows higher efficiency in the pollutant degradation, while the N-dopants in N-TiO2 play the role of recombination sites. The presence of the surface nitrogen species in TiO2 is highly beneficial for the application in partial photooxidation reactions, where N-TiO2 demonstrates a superior selectivity of 5-hydroxymethyl furfural (HMF) oxidation to 2,5-furandicarboxaldehyde (FDC). Thus, this work underlines the importance of a rational design of nonmetal doped titania for photocatalytic degradation and partial oxidation applications, and it establishes the role of bulk defects and surface dopants on the TiO2 photooxidation performance.
  • Publication
    Correlated effects of fluorine and hydrogen in fluorinated tin oxide (FTO) transparent electrodes deposited by sputtering at room temperature
    (Elsevier, 2021-01-30) Morán Pedroso, María; Gago Fernández, Raúl; Julin, Jaakko; Salas Colera, Eduardo; Jiménez, Ignacio; De Andrés, Alicia; Prieto de Castro, Carlos Andrés; Ministerio de Economía y Competitividad (España)
    The optical and electrical properties of fluorinated tin oxide (FTO) films deposited at room temperature by sputtering have been investigated varying the fluorine content and the hydrogen atmosphere. The complex behavior of the obtained films is disclosed using a wide set of characterization techniques that reveals the combined effects of these two parameters on the generated defects. These defects control the electrical transport (carrier density, mobility and conductivity), the optical properties (band gap and defects-related absorption and photoluminescence) and finally promote the amorphization of the samples. H2 in the sputtering gas does not modify the H content in the films but induces the partial reduction of tin (from Sn4 to Sn2) and the consequent generation of oxygen vacancies with shallow energy levels close to the valence band. A variation of up to four orders of magnitude in electrical conductivity is reported in samples with the appropriate fluorine doping and hydrogen fraction in the sputtering gas, maintaining excellent optical transparency. Optimized room temperature grown electrodes reach sheet resistance ~20 Omega and transparency 90%. This room temperature deposition process enables film preparation on flexible organic substrates, such as polyethylene terephthalate (PET), with identical performance of doubtless interest in flexible and large scale electronics.
  • Publication
    The role of silicon oxide in the stabilization and magnetoresistance switching of Fe3O4/SiO2/Si heterostructures
    (Elsevier, 2021-09-01) Arnay, Icíar; López Sánchez, Jesús; Salas Colera, Eduardo; Mompean, Federico; Del Campo, Adolfo; Castro, German R.; Rubio Zuazo, Juan; Ministerio de Ciencia e Innovación (España); Consejo Superior de Investigaciones Científicas (España)
    In this work we analyze the role of the SiO2 layer in the functionality of Fe3O4/SiO2/Si heterostructures, which have been proved to present a strong potential for spin-based applications. Nevertheless, a complete control of the interfaces properties is fundamental for application. In this work, high quality heterostructures are fabricated avoiding chemical exchange and achieving good quality interfaces. The chemical interaction between the Fe3O4 and SiO2 layers during the heterostructures manufacture is deeply analyzed and its role on the transport, magnetic and magneto transport response is revealed. It is proven that during Fe3O4 deposition a competitive interplay happens between the catalytic action of Fe atoms, the transport of dissociated oxygen through SiO2 and the stabilization of Fe3O4. A defective silicon oxide layer is found to grow on top of the native SiO2 enabling the formation of single phase Fe3O4 layer. Such a defective layer and the granular character of the Fe3O4 determine the magnetic and transport response of the heterostructures. The present results prove that the defects in the SiO2 layer induce the switching of the MR sign, so that anomalous positive MR at RT exceeding 17% at 80 kOe is obtained in heterostructures with 19 nm thick magnetite layer, while conventional negative MR response is obtained for thicker films.
  • Publication
    Local disorder and structure relation induced by magnetic exchange interactions in A2(Mo1-yMny)2O7 pyrochlores
    (Elsevier, 2021-06-05) Castellano, C.; Scavini, M.; Berti, G.; Rubio Marcos, F.; Lamura, G.; Sanna, S.; Salas Colera, Eduardo; Muñoz Noval, N.; Cimberle, M. R.; Demartin, F.; Agencia Estatal de Investigación (España)
    We present an extended X-ray absorption fine structure study at the Mo K-edge of A2(Mo1yMny)2O7 (A Gd, Ho; y 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A2Mo2O7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples Ho2(Mo1yMny)2O7 (y 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO(1)6 octahedron and of the MoHo and MoMo second shell. On the other hand, two Gd2(Mo1yMny)2O7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones.
  • Publication
    Ferromagnetic epitaxial Cr2O3 thin films grown on oxide substrates by Pulsed Laser Deposition
    (Elsevier, 2020-12-30) Vila, María; Rubio Zuazo, Juan; Lucas, Irene; Magén, César; Prados, Alicia; Salas Colera, Eduardo; Comunidad de Madrid; Ministerio de Economía y Competitividad (España)
    Single-phase and single-oriented epitaxial Cr2O3 thin film has been grown on SrTiO3 (1 1 1) substrate for the first time. The morphology, epitaxial growth mode and oxygen stoichiometry of the obtained film have been investigated by AFM, XRR, XRD and XPS, and compared to thin film grown on alfa-Al2O3 under equivalent conditions. The Cr2O3/SrTiO3 system presents a non-coincidence growth based on in&#-plane rotation of 30° of the Cr2O3 layer respect to the underlying SrTiO3 (1 1 1) substrate, while a coincidence growth based on axis-on-axis coupling is present for Cr2O3-Al2O3. However, in both cases an in-plane compression occurs in order to match the layer and substrate lattices. The formation of punctual defects in the form of oxygen vacancies have been observed by XPS for the layer grown on alfa-Al2O3, being the main mechanism for strain alleviation (4%). However, the 18 nm thick layer grown on SrTiO3 presents stoichiometric oxygen content maintaining an epitaxial strain (1.6%) accumulated on the lattice. Both epitaxial Cr2O3 layers show soft ferromagnetic response with coercive fields of 60 Oe and 90 Oe for the layer grown on SrTiO3 and alfa-Al2O3, respectively.