Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nanometer to micrometer and mosaicity effects

High-quality thermoelectric LaxSr1-xTiO3 (LSTO) layers (here with x = 0.2), with thicknesses ranging from 20 nm to 700 nm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1{\deg}), and present very low electrical resistivity (<5 x 10-4 ohm.cm at room temperature), one order of magnitude lower than commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of around -60 microV/K have been found for all films, accordingly. Finally, a correlation is given between the mosaicity and the (thermo)electric properties. These functional LSTO films can be integrated on Si in opto-microelectronic devices as transparent conductor, thermoelectric elements or in non-volatile memory structures

Structural and magnetic study of La0.7Sr0.3MnO3 nanotubes grown from chemical solutions in confined geometries

26-30 Mai 2014International audienceCompared with solid nanowires [1], metal oxide tubular nanostructures have attracted significant research interest because of their large specific surface areas, very narrow inner pores, and enhanced surface catalytic properties. Furthermore, 1D nanotubes (NTs) have showed improved performance in gas sensors, field-emission, photovoltaics, and batteries. However, the development of facile, mild and effective approaches for generating size controllable 1D NTs of complex oxides remains a significant challenge. We demonstrate that self standing La0.7Sr0.3MnO3 (LSMO) NTs with diameters ranging from 80 to 200nm can be successfully synthesized by template assisted chemical solution deposition using nanoporous anodized alumina membranes of varying pore size. The template synthetic strategy provides almost monodisperse size distribution in the fabricated NT dimensions. A sol-gel based polymer precursor route was used allowing a good control of the viscosity and stability of the precursor solution, which are crucial parameters for template aided synthesis. The porous membranes were filled with the precursor solution and subsequently heated at high temperature (700-900°C) for phase formation. We prove that the synthesized LSMO NTs are polycrystalline and ferromagnetic with a Curie temperature above 350K. These 1D nanostructures are good candidates to study the magnetic properties of reduced dimensionality systems. [1] A. Carretero-Genevrier et al. Chem.Soc.Rev. 10.1039/C3CS60288E (2013

Chemical solution growth of La0.7Sr0.3MnO3 nanotubes in confined geometries

International audienceSelf-standing La0.7Sr0.3MnO3 nanotubes with outer diameter ranging from 100 - 200 nm have been successfully synthesized by template assisted chemical solution deposition using nanoporous anodized alumina membranes of varying pore size. This template synthetic strategy provides rather monodisperse size distributed nanotubes. A sol-gel based polymer precursor route was used to fill the porous membranes and a subsequent heat treatment (700-1000ºC) enabled the phase formation and crystallization of the nanotubes. A good control over viscosity, stoichiometry and stability of the precursor solution were identified as crucial parameters for the template aided synthesis. The synthesized La0.7Sr0.3MnO3 nanotubes are polycrystalline and ferromagnetic with a Curie temperature of 350 K. Control over the nanowall thickness is attained by varying template filling time which is corroborated by magnetic moment results

Combining physical and chemical processes to add new functionalities in epitaxial oxides on silicon

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Soft-Chemistry-Based Routes to Epitaxial alpha-Quartz Thin Films with Tunable Textures

International audiencePiezoelectric nanostructured quartz films of high resonance frequencies are needed for microelectronic devices; however, synthesis methods have been frustrated by the inhomogeneous crystal growth, crystal twinning, and loss of nanofeatures upon crystallization. We report the epitaxial growth of nanostructured polycrystalline quartz films on silicon [Si(100)] substrates via the solution deposition and gelation of amorphous silica thin films, followed by thermal treatment. Key to the process is the combined use of either a strontium (Sr2+) or barium (Ba2+) catalyst with an amphiphilic molecular template. The silica nanostructure constructed by cooperative self-assembly permits homogeneous distribution of the cations, which are responsible for the crystallization of quartz. The low mismatch between the silicon and a-quartz cell parameters selects this particular polymorph, inducing epitaxial growth

PHOTOCATALYTIC TIO2-BASED COATINGS ON FLEXIBLE MATERIALS FOR BUILDING APPLICATIONS

International @ AIR+IBE:DAG:CGUInternational audienceThe heterogeneous photocatalysis is now recognized as a promising process for outdoor and indoor air treatment and self-cleaning applications. A lot of photocatalytic materials have been developed for exterior building elements (tiles, glass, tents, plastic ?lms, panels) or interior furnishing (tiles, wallpapers, window blinds, paints, ?nish)[1]. These materials are for the most part based on TiO2 which is the most widely used photocatalyst due to several key advantages. TiO2 is indeed relatively inexpensive, safe, chemically stable, exhibits high photocatalytic activity and is compatible with traditional construction materials[2]. However, for some of the substrates, the high photocatalytic activity of TiO2 may lead to significant damage and coating development on these substrates needs to include a way to protect them from unwanted degradation reactions. In this study, it is intended to obtain photocatalytic TiO2 based coatings on flexible organic materials for building applications (textile or paper) and the substrate protection is provided by two different technological innovations. On the one side, micrometric TiO2/SiO2 particles obtained by sol-gel chemistry are introduced into the existing organic coating of the substrates and on the other side, a new coating including TiO2 particles into a hybrid sol (organic/inorganic) is developed to replace the existing organic coating. Characterization includes surface analysis techniques (SEM, XPS and ToF-SIMS), UV light irradiation tests of the coated substrates and photocatalytic tests, i.e. photocatalytic oxidation of a model pollutant (formic acid) and the standardized methylene blue test (ISO 10678). Results showed that TiO2/SiO2 micrometric particles and the hybrid sol included TiO2 Degussa P25 particles exhibit significant photocatalytic activities. In parallel, coating surface characterization demonstrated the importance of TiO2 surface availability. Relevant examples will be displayed to illustrate the complementarity of the various characterization results, including the influence of pre-treatments (thermal or UV) (TiO2 particles into a hybrid sol) and the accessibility of pollutants to TiO2 through mesoporous silica (TiO2/SiO2 particles). References [1]Y. Paz, Applied Catalysis B: Environmental, 99 (2010) 448. [2]J. Chen and C-S. Poon, Building and Environment, 44 (2009) 1899

Crystallization of hollow mesoporous silica nanoparticles

International audienceComplex 3D macrostructured nanoparticles are transformed from amorphous silica into pure polycrystalline α-quartz using catalytic quantities of alkaline earths as devitrificants. Walls as thin as 10 nm could be crystallized without losing the architecture of the particles. The roles of cation size and the mol% of incorporated devitrificant on crystallization behavior are studied, with Mg2+, Ca2+, Sr2+ and Ba2+ all producing pure α-quartz under certain conditions